May 93

Snubber Circuits:Theory , Design and Application

Philip C. Todd

Passive Snubber TypesThe basic function of a snubber is to absorb

energy from the reactances in the power circuit.The fIrst classification of snubber circuits is wheth-er they absorb energy in controlling a voltage or acurrent. A capacitor placed in parallel with othercircuit elements will control the voltage acrossthose elements. An inductor placed in series withother circuit elements will control the currentthrough those elements. Figure I shows this con-cept. A voltage snubber (Fig. la) has energy stor-age capacitors in it and a current snubber (Fig. Ib)has inductors for energy storage. The networksassociated with the inductor and capacitor shown inFigure I determine how energy is passed to thestorage element and how the energy is removedfrom it

All of the otherclassifications of snub-berg relate to the waysin which the energy istransferred to and fromthe snubber. If theenergy stored in thesnubber is dissipated ina resistor the snubber isclassed as dissipativebut if the energy ismoved back to the inputor ahead to the outputthe snubber is classedas non-dissipative eventhough there may besome small losses. Asnubber is classed aspolarized or non-polar-ized depending on

IntroductionSnubbers are an essential part of power electron-

ics. Snubbers are small networks of parts in thepower switching circuits whose function is tocontrol the effects of circuit reactances.

Snubbers enhance the performance of the switch-ing circuits and result in higher reliability, higherefficiency, higher switching frequency, smaller size,lower weight, and lower EMI. The basic intent ofa snubber is to absorb energy from the reactiveelements in the circuit. The benefits of this mayinclude circuit damping, controlling the rate ofchange of voltage or current, or clamping voltageovershoot. In performing these functions a snubberlimits the amount of stress which the switch mustendure and this increases the reliability of theswitch. When a snubber is properly designed andimplemented the switch will have lower averagepower dissipation, much lower peak power dissipa-tion, lower peak operating voltage and lower peakoperating current. This article describes some of thevarious types of snubbers, where they are used,how they function, how they are designed and whattheir limitations are.Snubbers may be either passive or active networks.This article is limited to the main types of passivesnubbers. Passive snubber network elements arelimited to resistors, capacitors, inductors and diodes.Active snubbers include transistors or other activeswitches, often entail a significant amount of extracircuitry and introduce another level of parasiticswhich must be dealt with (usually with a passivesnubber). However, active snubbers are appropriatein some applications. A good example of an activesnub is what you would like to say to your bosswhen he or she decides not to give you a raise.

Fig. lb

2-1Snubber Circuits

whether energy moves in or out of the snubber on

one edge of the switching waveform or both. The

last classification for snubber circuits is rate of rise

of voltage or voltage clamping. Current snubbers

are all rate of rise control types as there is no

passive current limiting device available yet. volt-

age snubbers l1Jay clamp a voltage to a fixed or

variable level or may control the dV/dt of the

voltage.Snubbers are often used in combinations and a

given application may have two or three snubbers

merged into one network to control both the current

and voltage of the switch.

Table I is an applications guide and it gives a

breakdown of the basic snubber types and their

uses. The simple damping snubbers are dissipative

by definition. Rate of rise control and voltage

clamp snubbers may be either dissipative or non-

dissipative. Non-dissipative snubbers are more

complex than dissipative snubbers but this complex-

ity is justified when the power dissipation is too

high or the efficiency is too low.

Table I -QUICK GUIDE TO SNUBBER CIRCUIT USAGE

Table 2 is an index to the snubber circuits de-scribed in this article and gives the page numberthat the description of that snubber begins on.

Duality in Snubber OperationSnubbers have a duality which is a drawback in

some applications. A snubber which controls theswitch voltage at turn off will create a current pulsein the switch at turn on. A snubber which controlsthe switch current at turn on will create a voltagepulse across the switch at turn off.

Converters with alternating switches, such as apush-pull converter, with a voltage snubber on oneswitch to control the voltage at turn off will have acurrent spike in the other switch when it turns on.The same is true for snubbers on the output diodesof a converter. Some diodes are driven off by theswitches and others are driven on so if a snubber isnot properly designed it will present a low imped-ance to the switches when they are turning on andresult in a large current spike.

Snubber FundamentalsEach snubber in this discussion will be shown in

an example circuit which is as generic as possible.Figure 2 shows the basic buck, boost and flybackconverter circuits drawn so that the switch is alwaysgrounded. The figures which follow will generallyshow a snubber in relation to a generic groundedswitch so Figure 2 may be used to apply the snub-ber to most other topologies. In this approach theswitch and the snubber may be thought of as asingle unit, a snubbed switch, which may be used

RATE OF R~gQ~JROL SNUBBERS

~--reduce power dissipa~on and stress in switch at turn-off--prevent overshoot and ringing by not exci~ng resonance--reduce EMI by reducing high frequency noise

Current--reduce power dissipanon and stress in switch at turn-on--reduce diode reverse recovery current

VOLTAGE CLAMP

Table II .INDEX TO SNUBBER CIRCUITS--reduce peak switch voltage--reduce peak switch power dissipauon at turn-off--reduce ringing at switch turn-off

Dissipa~ve Snubbers Simple RC Voltage Snubber

RCD Voltage Snubber. ...

Simple RL Current Snubber

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DAMPING

~--reduce overshoot and ringing at switch tum-olf

--reduce switch power dissipation

--reduce EMI

Current

10

10

11

12

13

13

15

Non-dissipanveSnubbers Two Terminal3D.2C-1L Voltage Snubber. Three TerminaI3D-2C-1L Voltage Snubber. Three Terminal Voltage Snubber with Intermediate Voltage

Flyback Reset Current Snubber. Resonant Recovery Current Snubbers TransformerResetVoltageClamp c c.

SnubbingDiodes c

--reduce overshoot and ringing at switch turn-on--reduce power dissipauon in switch--reduce EMI

Rate of rise control snubbers and voltage clamp snubbers may be eitherdissipative or non-dissipative. Non-dissipauve snubbers reduce the pow-er dissipauon of the snubber and increase the efficiency of the system. 15

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switch for a particular purpose. An inductor inseries with the switch, a comnt snubber, presentsthe switch with an inductive load at turn-on so thatit switches on with zero comnt. This is a modifi-cation of the load which would typically be some-what capacitive at turn-on. The same is true at turn-off with a voltage snubber. At turn-off the load onthe switch will typically look inductive so a capaci-tor in shunt with the switch, a voltage snubber, willchange the load line to be capacitive so that theswitch can turn off at zero voltage.

Dissipative Voltage SnubbersDissipative snubbers are those which dissipate

the energy they absorb in a resistor. Dissipativesnubbers may be either voltage or current snubbersand may be either polarized or non-polarized. Dissi-pative snubbers may be designed to control the rateof rise of voltage or current or be designed toclamp the voltage.

Simple RC Voltage Snubber: The simple RCsnubber shown in Figure 3A provides damping ofthe parasitic resonances in the power stage and isprobably the most widely used of all snubbercircuits. It is used on output inductors and thesecondaries of transformers as well as across diodesand switches. It is applicable both to rate of risecontrol and to damping. The simple RC snubber isone of few snubbers which is effective in theclassic push-pull switch configuration.

Figure 3B shows the RC snubber applied to thegeneric switch circuit. As discussed above thegeneric switch circuit is a clamped inductive loadso in the idealized form shown here there are noparasitic resonances to damp. In this case the RCsnubber may be used to reduce the peak powerdissipation in the switch. If the values of R and Care chosen correctly the switching losses can bereduced by up to 40% including both the loss in theswitch and the loss in the resistor over the complete

switching cycle[l].The main application of an RC snubber is damp-

ing the resonance of parasitic elements in the powercircuit. In applications where damping is requiredthe value of the resistor must be close to theimpedance of the parasitic resonance which it isintended to damp. The snubber capacitance must be

in almost any

switchingregulatortopology.

Not allsnubbers areapplicable toa particularproblem. Thecircuits inFigure 2 areall c lam pedinductiveloads andthese circuitsdo not need

voltage clampsnubbers sincethat functionis inherent inthe topology.The snubbers I

which are Iappropriate inthese topolo- Igies are volt-age and cur- I

rent rate of Lrise controlsnubbers.

C II .Fl YBACKontro mg

the rate of Fig 2a. b. c

rise of voltage and current and clamping the voltageand controlling resonances reduces the stress on theswitch. Snubbers can control the voltage and currentto the point where switching occurs at zero voltageand zero current and this raises the reliability of thepower stage significantly. Taken to the extreme,zero voltage and current switching becomes reso-nant power conversion. This becomes necessarywhen the circuit parasitics become large relative tothe power level. High voltage outputs are oneexample. In most cases, however, the optimumefficiency level is reached with small snubbers longbefore a full resonant approach is necessary .

The effect of a snubber on the switch may beviewed as changing, or shaping, the load line on the

2-3Snubber Circuits

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if the impedance of the parasitic resonant circuit isknown. The value of inductance is usually theleakage inductance of the transfonDer and can beestimated with some effort. The resistor value is setequal to the resulting characteristic impedance.

It is always good practice to estimate the valuesneeded fIrst since a calculated value which isgrossly in error indicates that either the circuit isnot built as designed or was not designed as built.

Once the circuit has been built and is operating,the values of the snubber components may be opti-mized experimentally. Start with a small value ofcapacitor and place it in the circuit in the snubberposition, often this is directly across the switch, andthen observe the voltage wavefonD with and with-out the capacitor in the circuit. Increase the value ofthe capacitor until the frequency of the ringing tobe damped has been halved. At this point the circuitcapacitance is four times the original value so theadditional capacitance is three times the originalcircuit value. This is a near optimum value for thecapacitor since it allows damping very near Q = 1.

The circuit inductance may be calculated from thetwo resonant frequencies and the two values ofcapacitance. The characteristic impedance of theparasitic resonant circuit may be calculated from theoriginal circuit capacitance and the inductance andthe value of the snubber resistance is equal to thisimpedance. These calculated values of resistanceand capacitance may be added to the circuit to fonDthe snubber.

Example: Assume that the primary of a forwardconverter transfonDer has an unclamped leakageinductance of 2 pH and the power mosfet has anoutput capacitance of 330 pF. One ampere flowingin the switch at turn off will produce a voltagespike of 78 volts above Vcc if there is no dampingin the circuit. The ringing frequency will be 6.2MHz. The characteristic impedance of the LC tankis 78 ohms. If a l000pF capacitor is added inparallel with the switch the ringing frequency willdecrease to 3.1 MHz. If 78 ohms are added inseries with the new capacitor the ringing willdisappear .

larger that the resonant circuit capacitance1but must be small enough so that the power ~ C1

dissipation of the resistor is kept to a mini-mum. The power dissipation in the resistorincreases with the value of capacitance.

Figure 3C shows an application whichhas an extra unclamped inductance. Thiscould be leakage inductance in an isolated 3aflyback converter, aforward converter or apush-pull converter. Itmight also be theinductance from acurrent snubber. Inthese cases there is anunclamped inductanceand a resonating ca-pacitance, the switch

output capacitance.When the switch turns Fig 3b

off, the energy storedin the inductance will ringwith the capacitance if thereis no snubber. For simplicitywe assume the switch with-stands the extra voltage.There is typically very littleloss in a parasitic resonantcircuit so many cycles ofringing normally occur.

The RC snubber willdamp the ringing and if thesnubber resistance is equalto the characteristic imped- Fig. 3cance of the resonant circuit[(L/C)~] then the resonant circuit will be criticallydamped and have no overshoot. The capacitor inseries with the resistor must be larger than thecircuit parasitic capacitance to reduce overshoot and

ringing.The value of the capacitor and resistor can often

be estimated from the other circuit components. Thedominant circuit capacitance is the output capaci-tance of the switching transistor and its value canbe obtained from the data sheet. The snubbercapacitance will generally be two to four times thisvalue. The snubber resistor value can be estimated

2-4 UNITRODE CORPORATION

may be found from the capacitance and the voltage.Power Dissipation of Simple RC VoltageSnubber: The power dissipation of the resistormust be found also. Finding an exact expression forthe power dissipation is a mathematically difficulttask but it may be estimated. The assumption is thatthe time constant of the snubber (t=RC) is shortcompared to the switching period but is longcompared to the voltage rise time. If the timeconstant is on the same order as the rise time thepower dissipation estimate will be too high. If thetime constant is too long the equations are not validand the estimated power will again be too high.

The capacitor in a snubber stores energy. In asimple RC snubber the capacitor charges anddischarges. By the principle of conservation ofcharge an amount of energy equal to that storedwill be dissipated for each charge and dischargecycle. This amount of power dissipation is inde-pendent of the value of the resistor. The powerdissipation may be calculated from the capacitance,the charging voltage and the switching frequency.The equation evolves as follows:

= 2CVf= L\Q'2f and L\Q = cy so

and

p = PR, so p = 4C2V2f2R

p = 2f(1/2 C V2) = f C V2

Where p is the power dissipation, F is the switchfrequency, C is the value of the snubber capacitorand V is the voltage that the capacitor charges toon each switching transition. This is a handyequation and is quite useful when designing thepower circuit. The dominant value of capacitance isusually the switch output capacitance so three timesthat is the snubber capacitance. The voltage that thecapacitor charges to is generally known so themaximum power dissipation of the snubber resistorcan be calculated. This allows the selection of aresistor wattage which will not flame at the firstapplication of power. Note that the frequencychosen is the switch frequency. In some singleended converters the output frequency is half of theclock frequency. In a push-pull converter the switchfrequency will be equal to the clock frequency.

It is also possible to calculate a minimum powerdissipation which is based on the average currentthrough the snubber resistor. The actual dissipationwill be greater than this. The equation is derivedfrom averaging the absolute value of the charge anddischarge currents over the time period. The current

This calculation is especially useful when thetime constant of the snubber is on the order of therise time of the voltage. In that case the powerdissipation will be at least equal to the FR losscalculated above.

Example: The inductance is 2pH and the capaci-tance is 33OpF as above. The snubber capacitor islOOOpF and the resistor is 78 ohms. The voltageacross the switch is 400V and the switching fre-quency is 100KHz. The power dissipation will be16.0 watts. The resistor selected must be non-inductive since it must handle very high frequen-cies. In general, even a non-inductive wire woundresistor will have too much inductance to be effec-tive. The minimum power dissipation will be 0.5

watts.

Polarized Voltage SnubbersThe objective of a polarized voltage snubber is

different from that of a non-polarized snubber. Thepolarized voltage snubber does not necessarilyprovide damping since is disconnected from thecircuit over much of the cycle. Its main functionsare rate of rise control or clamping.

The RCD Voltage Snubber: The circuit inFigure 4A applicable to either rate of rise control orclamping. The circuit variation shown in Figure 4B

is applicable only to the clamp operation.A typical application of a resistor-capacitor-diode

snubber is to control the rate of rise of voltage onthe drain or collector of a switching transistor in aforward, flyback or boost converter. At turn-off, thesnubber will carry a major portion of the switchcurrent (if not all of it) and this transfers the powerdissipation of the switch into the snubber. Thereliability of the switch increases since its peakpower dissipation is reduced and the controlled rateof rise of voltage also lowers the high frequencyEM! which the uncontrolled switching generates.

When the resistor-capacitor-diode snubber is usedto control the rate of rise of voltage, the RC timeconstant must be short compared to the switching

2-5Snubber Circuits

R1 ~

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Where I is the maximum peak switch current,.1 V is the peak voltage the capacitor will charge to,.1t is the rise time of the voltage and C is the valueof the capacitor. The resistor is then chosen to havea time constant which is small compared to theswitching period. A typical value for the timeconstant would be one tenth of the switch maxi-mum on time.

The power dissipation of the resistor is deter-mined by the size of the capacitor since the timeconstant is short compared to the switching period.All of the energy stored in the capacitor is dissipat-ed on each cycle but there is only one transition(the capacitor discharge) so the power dissipated bythe resistor is given by:

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frequency because the capaci- Itor must be charged and dis-charged on each cycle. Thecircuit in Figure 4C showshow the snubber would beconnected in the genericswitch circuit. When theswitch turns off, the currentfrom the inductor is divertedthrough the snubber diode tothe snubber capacitor until thecapacitor is charged to the railvoltage and the main diodeturns on to carry the inductorcurrent. The snubber is activeonly during the switchingtransition. When the switchturns on the snubber capacitoris discharged through theresistor andthe switch. Itmust be al-most fullydischarged oneach cycle tocontrol therate of rise ofvoltage on theswitch.

The designof the snubberbegins with thechoice of the risetime at maximuminductor current andthe supply voltage orthe peak voltage thecapacitor will chargeto. The relationshipbetween the voltageand current in acapacitor allows thecalculation of the

necessary capacitorvalue.

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Where p is the power dissipated, C is the capaci-tance value, V is the peak voltage the capacitorcharges to, and F is the number of discharge cyclesper second. Again, note that the power dissipatedby the resistor is independent of its value as long asthe time constant is short compared to the switchingperiod. If the time constant is longer than this, thesnubber is functioning in a different mode anddifferent equations apply.

A point to keep in mind about this kind ofsnubber is that when the switch turns on, thecurrent which is discharging the snubber is flowingthrough the switch. This current will add to thespike on the leading edge of the current waveform.Another point is that the discharge is sensitive tothe pulse width. If the pulse width becomes verynarrow, the capacitor will not be fully discharged.This usually happens only during an overcurrentcondition. When this happens the peak stress on theswitch will go way up but the average powerdissipation on the switch is generally reasonablebecause the duty factor is quite low.

Example: The switch current is 1.0 amps andthe maximum voltage is 400v. The voltage rise timeis to be 400ns. The capacitance necessary isl000pF. The time constant to discharge the capaci-tance is chosen to be 500ns, which is compatiblewith a looKHz switching frequency. The dischargeresistor value is 500 ohms. The power dissipated in

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2-6 UNITRODE CORPORATION

simple equation to define this relationship.

WL + WCl = WC2

Where WL is the energy stored in the parasiticinductance, W Cl is the initial energy stored on thecapacitor and WC2 is the final energy stored on thecapacitor. This equation may be expanded asfollows:

1/2LF + I/2CV2 = I/2C(V +Ll V)2

This can be rean-anged to solve for C:

LI2C=

LlV (LlV +2V )

Where L\. V is the change of voltage on thecapacitor and V is the initial voltage. L is theinductance to be discharged and I is the peakcurrent in the inductor at the time the switch turnsoff. This gives the minimum value for the capaci-tance. The value may be larger without compromis-ing performance and this may be desirable. Thevalue of V is determined by the voltage to whichthe discharge resistor is connected V is the differ-ence between the this voltage and the averagecapacitor voltage. The resistor may be connected tosome voltage in the circuit rather than ground sincethat will lower the power dissipation.

In a flyback converter, the capacitor will chargeto a voltage proportional to the turns ratio and theoutput voltage plus the input voltage. The energystored in the leakage inductance will add to thislevel. The energy that the resistance must dissipateis equal to !f2LI2F plus the dissipation from thevoltage due to the output voltage and transformerturns ratio.

Example: The unclamped inductance is 2pH andthe current is 1.0A. The switching frequency is100KHz. The change of the voltage on the capaci-tor is to be less than 2.0V so the capacitor will be0.5pF (the initial voltage is 0). A 0.1pF capacitorwill have a voltage change of 4.5V. The powerdissipation of the resistor will be 0.1 watt if theresistor is returned to the output of a boost or aflyback converter or the input of a buck converter.The time constant of the resistor and capacitor mustbe quite long compared to the switching frequency.200 ohms would be a reasonable value for the

the resistor is 8.0 watts. A wire wound resistor maybe used in this case since the time constant isrelatively long and the self inductance of theresistor will not be important.

The RCD Voltage Snubber in Clamp Mode:The purpose of a resistor<apacitor-diode (RCD)snubber used in clamp mode is different from oneused in the rate of rise mode. In clamp mode theobjective is to keep the switch from exceedingsome maximum voltage. The switch itself will haveto sustain the peak power dissipation of turning off.Only the peak voltage will be limited. The primecharacteristic of the snubber which distinguishes theclamp mode from the rate of rise mode is the RCtime constant which is much longer that the switch-ing period in the clamp mode.

The circuit shown in Figure 4A and 4C willwork for clamp mode but the clamp level will be afunction of the duty factor since the discharge time,which is long compared to the switch on time,changes as a function of duty factor. To remove thesame amount of charge on each switching cyclewith a fixed resistance the clamp voltage mustchange with the duty factor. The clamp voltage willbe high when the duty factor is small and low whenthe duty factor is large. This is normal for thistopology but is not always desirable.

The circuit variation shown in Figure 4B and 4Dis a better choice for the clamp mode RCD snubber.It cannot be used for rate of rise control since thereis a DC path through the snubber. To functionproperly the resistor value must be large. As shownin Figure 4D the clamp mode RCD snubber has theresistor going to ground but it is often used wherethe voltage to be clamped goes above the DC busand then the resistor may be returned to the DCbus. By returning the resistor to this bus the totalpower dissipation is reduced.

The value of the capacitor is based on theamount of energy stored in the parasitic inductance.The maximum stored energy must make only arelatively small change in the voltage on the capaci-tor since the capacitor voltage is the clamp level.The parasitic inductance, the leakage inductance ina flyback converter or the magnetizing inductancein a forward converter, must be discharged into thecapacitor on each switching cycle. We can write a

Snubber Circuits 2-7

resistor with the O.5pF capacitor. This gives a lOOpstime constant. The average voltage across theresistor is O.5V and the average current is 2.5mA.The capacitor must have low inductance to handlethe relatively high peak currents at switch turn-off.

Clamping may also be performed by zenerdiodes. The advantage of a zener is that it clampsat an absolute level. The zener must be rated forhigh peak power dissipation as well as the averagepower dissipation. The zener is not necessarily ahigh speed device when the package inductance istaken into account so care must be taken with thecircuit layout to insure that the stray inductance iskept low to avoid overshoot. If the zener is largethen it may not be possible to keep the inductancelow and it may be necessary to add a small highfrequency capacitor in parallel with the zener and toput a diode in series with both of them. The capaci-tor handles the very high frequency currents. Azener clamp may also be integrated into an RCDsnubber and the snubber handles the high frequencycurrents and the zener clamps the voltage on thesnubber capacitor and thereby the voltage across theswitch.

The simple flyback regulator shown in Figure 5is from Unitrode Application Note U-96A. C8, 05,and Rll form a rate of rise control voltage snubberand C9, 04 and R12 form a clamp to limit thedrain voltage. This is an example of using twosnubber circuits to accomplish different objectivesand control the voltage across the switch.

Dissipative Current SnubbersThe purpose of a current snubber is to control

the rate of rise of current in the switch and, con-versely, it is often used to control the rate ofdecrease of current in, and therefore turn- off of,the output diodes. The series inductance allows theswitch to be fully turned on by the time the currentreaches its operating value. This greatly reduces thepeak power dissipation in the switch, it reduces theaverage dissipation in the switch and it increasesthe reliability .

The current snubber for the switch also benefitsthe turn off of the diode on the output. In PWMconverters one of the output diodes is driven off bythe switch and high reverse currents result. Thecurrent snubber on the switch provides a controlled

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since the square loop core functions as a switch anddoes not provide a controlled dI/dt for the diode.However, in situations where the diode is not aconsideration the square loop cores can be effective.

Variations of tbe Simple RL Snubber: Oneway to reduce the power dissipation of the resistorin a simple RL snubber circuit is to add a diode inseries with the resistor. This makes it a polarizedsnubber which is often a desirable characteristic.This is shown in Figure 6B and a simple applica-tion is shown in Figure 6C. The inductor functionsnormally when the switch turns on and current onlyflows through the resistor when it is needed todissipate the energy which is stored in the inductor.The amount of power dissipated by the resistor isequal to lhLIZP where I is the peak current in theinductor and it includes the diode reverse recoverycurrent as well as the load current. This snubbercan be very effective in increasing the overallefficiency of the circuit and in increasing thereliability .

The variation shown in Figure 6D is actually acombination of the simple RC snubber and theinductor and has the lowest loss of the RL dissipa-tive snubbers. The capacitor eliminates DC losses inthe resistor, which are the major contributor topower loss in switching circuit applications, andallows the resistor value to be optimized for damp-ing. This configuration is especially useful in non-dissipative energy recovery networks because thereare usually low energy, high frequency resonanceswhich must be damped.

Example: A switch is turning on into 4OOV suchas in a boost converter. The circuit is Figure 6CSwitch current is 1.OA. An inductor of 4OpH willgive the diode lOOns to turn off. The inductor willstore 20pJ of energy. The power dissipation of theresistor is 2.0 watts with a lOOKHz switchingfrequency. 80 ohms will give a 500ns reset timeconstant which should be reasonable at lOOKHz.There will also be an 80V spike across the switchwhen it turns off which must be taken into account.A lower value of resistance would lower this peak

voltage.

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rate of change of current in thediode and the low dI/dt lowers thedissipation in the diode and reducesthe peak reverse current.

The Simple RL Current Snub-ber: The simple RL circuit shownin Figure 6A. This is the dual ofthe simple RC snubber circuit. It isnot often used in switching circuitsin this fonn because from a practi-cal standpoint the value of resis-tance tends to be small for gooddamping of the circuit. In a switch- lieing circuit this will have high I LI

power dissipation. This circuit israther common in input and outputfilters where the AC component ofthe current is relatively small andthe resistor provides damp-ing necessary to controlthe Q of the filter transferfunctionl2]. Other variationsof this snubber with lowerloss are useful for switch-ing circuits.

A ferrite bead is actual-Iy a simple RL circuit withboth the inductance andresistance incorporated intoa single device. It can bevery effective in low pow-er situations but the beadshave only a very limited

power dissipation capabili-ty. In switching circuits thebead will have high powerdissipation and this gener-ally makes it unusable inall but the lowest powersituations.

Square loop cores are ~ Qsometimes used for current ~snubbers so that the resis-tor may be eliminated. Thesquare loop core will allow F "

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2-9Snubber Circuits

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off, the induc-tor is conduct-

ing throughthe maindiode and thatthe two ca-pacitors are

discharged.The snubbermust be resetwhen theswitch turnson. As the Fig 7bswitch isturning on, diodes D I and D2 will turn off and thecapacitors will apply V cc across the snubber induc-tor because they are discharged. Current will flowthrough the inductor and it will ring with thecapacitors until the current reaches zero and thediode in series with the inductor turns off. At thispoint both capacitors are charged to V cc and thesnubber is ready for the switch turn-off. When theswitch turns off all of the current from the maininductor will flow into the two capacitors. The twodiodes in series with the capacitors now conduct sothat the capacitors are effectively in parallel. Thetwo capacitors control the rate of change of voltageacross the switch. The turn off dissipation of theswitch is very small since the capacitors take thefull inductor current. The main diode clamps thesnubber voltage when the capacitors are fullydischarged and the cycle is ready to begin again.

The first step in the design process is knowingwhat the peak switch current is to be, what themaximum value of V cc is and what the desiredswitch voltage rise time is. The value of the twocapacitors in parallel is found from the equation fora capacitor which has been rearranged into thefollowing form:

Non-dissipative SnubbersNon-dissipative snubbers are sometimes called

resonant snubbers although the class is more exten-sive than just resonant energy recovery. Non-dissip-ative snubbers are the class of snubbers whichrecycle the energy they collect either to the input orthe output or they circulate it to prepare for thenext cycle. They include both current and voltagesnubbers and both rate of rise and clamp types ofvoltage snubbers. The basic principles of non-dissipative snubbers are the same as for dissipativesnubbers but in general non-dissipative snubbers donot provide damping. Damping is by defmition adissipative function and small resistors are oftennecessary in non-dissipative snubbers to controltertiary resonances. All non-dissipative snubbers arepolarized snubbers.

Non-dissipative Voltage SnubbersA voltage snubber controls voltage by transfer-

ring energy into a capacitor and in a dissipativesnubber this energy is removed from the capacitorand turned into heat but in a non-dissipative snub-ber a way is found to transfer the energy eitherback into the source or forward into the load or tocycle it back and forth within the snubber. Thereare only a few basic types of non-dissipative orresonant energy recovery voltage snubbers. Theyare all polarized and they operate on only one edgeof the switching waveform and are reset on theother edge. In some applications they can becomevery complex, especially when combined withcurrent snubbers.

Two Terminal Voltage Snubber: One of thebasic circuits is a three diode, two capacitor, oneinductor (3D-2C-IL) circuit which is configured asa two terminal network. There are two versions of3D-2C-IL networks, the first of which is a twoterminal network. This network is shown in Figure7 A and the connection to the generic switch circuitis shown in Figure 7B. Note that this snubber isapplicable to all three converters, buck, boost andbuck-boost. This is a rate of rise control snubberand cannot be used as a clamp. The two capacitorsgenerally have equal values and the resonant fre-quency of the two capacitors and the inductor ismuch higher than the switching frequency.

2C=I.t./Vcc

2-10 UNITRODE CORPORATION

Where c is the value of one of the capacitors, Iis the peak current in the switch, ~ is the desiredmaximum rise time and V cc is the maximum

supply voltage.The length of time it takes to charge the capaci-

tors is half of one complete resonant period of thetwo capacitors in series with the inductor. This timeperiod must be less than the smallest expectedswitch on time. If the on time becomes less thanthis, the snubber will not be fully reset and theswitch dissipation will go up. Note that this nor-mally happens only when the power supply is incurrent limit. Under a short circuit condition thesnubber will be completely ineffective and caremust be taken to see that the switch will surviveduring this condition. Once the reset time is knownthe value of inductance may be calculated from the

following equation:

L = 2(2

L

stress and power dissipation in the switch.Example: The current in the switch is l.OA and

the switch voltage is 4OOV. The rise time is to be400ns. The value of the capacitors is SOOpF each.A l.Ops recovery time gives an inductor value of4OOpH. The peak current in the inductor will be3l6mA. Note that with this small a capacitance therecovered charge of the diodes becomes significant.The diodes must be very fast and have a very lowrecovered charge. The avemge current through themis very small so they do not have to be large. If therecovered charge in the diodes is too large thevoltage stored on the capacitors will not be suffi-cient to reset them for the next cycle.

Three Terminal Voltage Snubber: Anotherconfiguration of the three diode, two capacitor, oneinductor non-dissipative voltage snubber has threeterminals rather than two. In operation it is similarto the two terminal circuit The three terminalsnubber is shown in Figure SA. Figure SB is thesame circuit with the top and bottom terminalsreversed The circuit in Figure SC shows the snub-ber connected to the generic converter of Figure 2.This snubber may be used with either buck, boost

or flyback configurations. The~ ::1 snubber is a rate of rise control

type.C Referring to Figure SC, the1 operation of the circuit begins

with the switch off, the uppercapacitor, C 1 , discharged andthe lower capacitor, C2,charged to V cc. The maindiode, DO, is conducting thecurrent in the main inductor,LO. When the switch turns on,

the diodes in series with the snub-ber capacitors turn off and V cc isapplied across the snubber inductor,Ll, because Cl is discharged andC2 is charged to Vcc. Currentflows from C2 through Ll and D3to charge Cl. When the lowercapacitor is discharged, the uppercapacitor, Cl, is charged to Vcc,the inductor current is zero, anddiode D3, in series with the induc-

D3 L

l1 , It= ~

Fig 80

1

lie

C1t2

Where L is the value of the inductance, t is thereset time and C is the value of one of the snubber

capacitors.The peak current in the inductor must be found

so that the inductor and the diode in series with itcan be sized to handle the current. The peak currentis found by equating the inductive and capacitiveenergy when each is at its peak during the cycle.The equation reduces to the following:

22 CVcc

1=-

2L

Where I is the peak current in the inductor, C isthe value of one of the snubber capacitors, L is thevalue of the snubber inductor and V cc is the maxi-mum supply voltage. An important consideration isthat the half sine current pulse which resets thesnubber when the switch is on must flow throughthe switch. The switch must be able to handle thiscurrent in addition to the load current. The snubbercurrent should not flow through the current sensemechanism when the switch turns on.

Some designers make one of the two capacitors10-20% larger than the other to insure that at leastone of the two capacitors will be completelycharged to Vcc. They feel that this gives the lowest Fig 8b

Snubber Circuits 2-11

Vcc

i l

CI Lo

tor, is off. Theswitch turns offsome time later andthe current throughthe main inductor, D3 LLO, flows into thesnubber. This current

L '111-discharges the uppercapacitor through D Iand charges thelower capacitorthrough D2. Thiscontrols the rate ofrise of voltage across the switch. The two capaci-tors are effectively operating in parallel even thoughone is charging and the other is discharging.

The design of this snubber is exactly the same asfor the two terminal snubber discussed previously.

Voltage Snubber With Intermediate Voltage:The voltage snubber shown in Figure 9A requiresan intermediate voltage which makes this snubberespecially useful in forward and flyback converters.This is a three terminal network and itmay be used as either a rate of risecontrol snubber or as a clamp modesnubber. Figure 9B shows how thesnubber connects to the generic con-verters of Figure 2 and in this applica-tion it is operating as a rate of rise II ~ L 1

control snubber. In clamp mode of E

operation the circuit is the same but thevalues of the components are different.This mode is normally used in associa-tion with a current snubber for resonantenergy recovery so discussion of thismode of operation will be v,discussed later.

The operation of thesnubber begins with theswitch off and the capacitorcharged to some voltagewhich is the difference be-tween the diode anode volt-age (VI) and the right in-ductor terminal (V2). In abuck or flyback converterthis will be the output volt-

~1+--

., D2

ra

;C2 '~Fig Bc

age, while in a boost converter this will be thedifference between the input and the output voltag-es. When the switch turns on the capacitor willforce the voltage on the snubber inductor negativewhich will ring with the capacitor until the currentthrough the snubber reaches zero or until the diodeDl conducts to clamp the voltage. The capacitorvoltage will have reversed sign but will not belarger than V2 in Figure 9B. When the switch turnsoff, the current from the main inductor will flowinto the capacitor, through diode Dl and back toV2, and this controls the rate of rise of voltageacross the switch. When the voltage is high enoughto turn on, the main diode and capacitor will becharged to the initial state (VI-V2).

If VI-V2 in Figure 9B is less than V2, thecharge on the snubber capacitor at the end of thereset period will not be equal to V2. When thishappens, the switch will not turn off at zero voltagebut instead will turn off at some intermediatevoltage which is determined by the point at whichD 1 is conducting. This is still a dramatic improve-ment over not having a snubber at all but it must beconsidered in switch selection and heat sinking.

The design procedure for this snubber is similarto the procedure given above for the two terminalnon-dissipative snubber. The capacitor value, C, isfound from:

L

--i 1-C1

I = C dV/dt

Where I is the maximum switch current anddV /dt is the maximum rate of change of voltagedesired across the switch. The inductor value, L, isfound from the length of time available for the resetand the capacitor value. The equation is:

4(2L=-

C1t2

L

Fig 9a

.-DolO~ Where t is the length of time available for

resetting the capacitor and it is shorter than thesmallest pulse width under normal operation. Thepeak current in the inductor is given by:

[2 = CV2

-V2

~ DlI ~I

~I

CI

LIL

Q Where I is the peak inductor current and V is the

initial voltage on the capacitor and is equal to (V 1-

V2) in Figure 9B.Fig 9b

UNITRODE CORPORATION2-12

Non-dissipative Current SnubbersThe basic principle of a current snubber is that

energy is collected in an inductor as part of itsfunction of controlling the rate of rise of current inthe switch. Non-dissipative current snubbers arevery similar to their dissipative counterparts. Thebasic functionality is the same, an inductor isplaced in series with the switch to control the rateof rise of current. In a non-dissipative currentsnubber the energy stored in the inductor on eachcycle is transferred back to the input or ahead tothe output instead of being dissipated. There areseveral methods for removing the energy from theinductor on each cycle and some of the morecommon methods are discussed in this section.

Flyback Reset Current Snubber: One of themost obvious ways to get the energy out of theinductor is to put an extra }winding on it T~s allows the ! 11:NI~energy to be directed any- .~ ~where and provides a con-trolled overvoltage condition Fig lOaon the switch set by the turnsratio and the recovery volt-age. Figure lOA shows thebasic snubber and FigurelOB and Figure lOC showtwo ways of hooking thesnubber to the generic con-verter of Figure 2. The con-figuration chosen depends 1--'"on whether it is a buck, fly- 1-,. Q

back (Figure lOB) or boost ,(Figure lOC) converter. Inall cases the energy is trans-ferred to the load althoughother connections could trans-fer the energy back to the

inputThe design of this type of

snubber is rather simple. Thevalue of the primary induc-tance is the same as it wouldbe for a dissipative snubberand the voltage which isadded to the switch duringreset is determined by the

L

+--1:N

~11i;-!*-.:!II!:

Fig lOb

D2 L.

L Lo

turns ratio and either the source or load voltage.The major problem with this type of snubber is

the leakage inductance between the primary andsecondary of the inductor. This leakage inductancecan cause a large voltage spike across the switch.This type of snubber has been used effectively butit is generally used with high power converterswhere the rise and fall times are slow or where rateof rise voltage snubbers are also used. Simple RCdamping networks are usually needed across boththe primary and secondary of the inductor.

Resonant Recovery Current Snubbers: FigureIIA shows the generic converter of Figure 2 witha current snubber where the energy is being re-turned to one of the converter voltages. The energyrecovery in this snubber is being handled in theclamp mode and the energy is recovered when theswitch turns off. The voltage across the switch isclamped to the highest voltage in the converterwhether it is an input or an output voltage. Obvi-ously. a single diode could have been used in placeof the RLD network so there must be some justifi-cation for using the network. The object of thenetwork is to reduce the current in the snubberinductor to zero as rapidly as is practical. If asingle diode were used the switch would beclamped but there would be no voltage across thesnubber inductor so it would continue conductinguntil the switch turned on again and it wouldtherefore be ineffective as a current snubber.

The purpose of the snubber in Figure llA is toprovide voltage to reset the snubber inductor to zerocuITent on each cycle. The clamping of the switchvoltage at turn off is a secondary benefit. Thesnubber ca-pacitor needsto be small sothat it has a

significant l211~voltagechange due tothe energyfrom thesnubberinduc-tor. The volt-age change onthe capacitor

I~

~Do

--JTTn--.

1:N~II~

,..Dl

,--1

'1C1Q

Fig llaFig IOc

2-13Snubber Circuits

J

is what allows the inductor to reset to zero current.Again, the simple relationship between the energystored in the inductor and the energy in the capaci-tor is used to calculate the size of the capacitor.

lLo

~

+--

WL = Wc or l/].LP = l/].CV2

Ct

This equation may be rearranged to solve for the

capacitance:LI

c = Ll2

V2

QWhere c is the capacitance in the snubber, L isthe inductance of the snubber, I is the current in theswitch at turn-off and V is the change of voltage onthe capacitor. Note that the smaller the capacitor is,the greater the voltage will be and the faster theinductor will reset. The reset time is approximatelyone quarter resonant cycle and is given by:

t=~2

Where t is the reset time, L is the snubberinductor and C is the snubber capacitance.

The inductor which is used to discharge thecapacitor may be large or small. If the value issmall, that is, the resonant frequency is smaller thanthe switching frequency, the diode in series isnecessary to limit the resonance to a single halfcycle. The L and C will resonate to discharge thecapacitor and at the end of the discharge cycle thecapacitor will be as far below the nominal as it wasabove at the beginning. A small series resistor or alarge parallel resistor may be necessary to eliminateringing when the diode turns off. If the value of theinductor is large, that is, the resonant frequency ismuch greater than the switching frequency, thediode is not necessary since the inductor will be incontinuous conduction. The voltage waveform onthe capacitor will be similar but the discharge willbe a straight line. This is the most efficient config-uration of the snubber. The only problem to watchout for is that the resonant frequency of the dis-charge inductor and capacitor must be high enoughso that under transient conditions the peak voltageon the switch will not exceed its specifications. Ifthe resonant frequency of the capacitor and reset

inductor is largerthan the switching Dlfrequency the volt-age on the capacitor La II~can change with theload current. Caremust be taken toinsure that the volt- Dlage does not exceedthe switch rating.

Example: Thecircuit is shown inFigure llA. Theswitch current is Fig 11 bl.OA and the seriesinductor is 4OpH as was given in the example fordissipative current snubbers. The reset time of theinductor is chosen to be l.Ops for a lOOKHzswitching frequency. The capacitor will be O.OlpF.The change of voltage across the capacitor will be63V. The inductor in the recovery network may beeither large or small as discussed above.

The snubber circuit shown in Figure llB isintended to solve a somewhat different problemfrom that of Figure llA. Both snubber circuitscontrol the current in the switch when it turns onbut the energy recovery circuit shown in FigurellB recovers the diode tom off energy. The snub-ber inductor stores the reverse recovery charge fromthe diode and it could drive the diode into anovervoltage condition. The snubber in Figure llBrecycles this energy.

The energy recovery part of the snubber inFigure llB is the same as the voltage snubber inFigure 9B except that it is operating in the clampmode. The operation of the snubber in Figure llBbegins with the switch off and the main diodecarrying the inductor current. When the switch turnson the current in snubber inductor Ll will ramp upand the current through the main diode will rampdown. The diode current will eventually reach zeroand it will begin to turn off. The voltage across thediode will not change until it is completely turnedoff. The diode reverse recovery current as it turnsoff must flow into the snubber inductor since themain inductor current will not change significantlyin that short a time. Once the diode is fully turned

2-14 UNITRODE CORPORATION

off the snubber inductor will drive the voltage toground since its current is higher than the current inthe main inductor. The energy which was needed toturn off the diode is now stored in the snubberinductor. The snubber network in Figure IIB isdesigned to recycle the extra energy stored in theinductor. After the diode turns off and the voltagehas dropped to zero, the snubber capacitor beginscharging through diode D 1. The excess energy inthe snubber inductor will transfer into the capacitorand the diode will hold the energy on the capacitor.When the switch turns off the main diode willconduct again and the snubber discharge inductorL2 will reverse the charge on the snubber capacitorso that it is ready for the next cycle. Since theenergy recovery network is operating in clampmode the capacitor is relatively large and thevoltage across it will be small.

Example: The same example as above. Thecircuit is Figure IIB. The diode reverse recoverycurrent is assumed to be O.5A peak and the snubberinductor is 4OpH. The energy will be transferredinto the capacitor in I.Ops so the capacitor isO.OlpF. The extra voltage across the diode will be32V peak. The reset inductor may be small with areset time of 2Jls. Note that the reset requires a fullhalf cycle so the equation is t=1t.JLC. The resetinductor is 4OpH but it only carries O.5A peak.

Transformer Reset Non-dissipative Volt-

age ClampThere is one circuit which deserves special

mention and which is not particularly easy toclassify since it requires a transformer for operation.It is a voltage snubber and it operates in clampmode. It is particularly applicable to forwardconverters and push-pull converters, both of whichare notoriously difficult to snub. The forwardconverter version of the circuit is shown in Figure12.

verter to 50%. Thecircuit operation be-gins with the switchon and the capacitorcharged to V cc by thereset winding. When v..the switch turns off,both the transfonnerleakage inductanceand the magnetizinginductance will drivethe voltage on theswitch above Vcc. Fig 12When the voltageacross the switchreaches twice Vcc the diode will turn on and thecurrent in the transfonner leakage inductance willbe clamped by the capacitor and diode. The resetwinding will then conduct the magnetizing currentthrough the diode to reset the transfonner core.When the switch turns on, the capacitor, which isstill holding the energy from the leakage induc-tance, will discharge into the reset winding and isvoltage will again be equal to V cc. A small resistormay be needed in series with the reset winding ofthe transfonner to damp the resonances which mayoccur between the capacitor and the windings. Thecapacitor must be large enough to absorb the energyfrom the leakage inductance with only a smallvoltage change and the diode must handle the peakCUlTent from the spike and must be rated for at leasttwice Vcc.

Snubbing DiodesSome diodes are driven off by the switch and

others are driven on. Those driven off are usuallyturned on naturally by the energy storage elementsin the output circuit and those driven on are usuallyturned off by the energy storage elements. Somecare must be taken to insure that adding a snubberto the diodes will not add to the stress on theswitch. For example, adding a simple RC snubberto the output of a forward converter will add to thecurrent spike in the switch at turn-on. And yet, thesnubber may be needed to control ringing whichresults from the reverse recovery CUlTent of theshunt diode and the transfonner leakage inductance.

Winding Nl on the transfonI1er in Figure 12 isthe primary power winding. Winding N2 is thereset winding and it controls the voltage on thecapacitor and provides the core reset that theforward converter needs. Note that the two wind-ings have the same number of turns and that thislimits the maximum duty factor of the forward con-

2-15Snubber Circuits

Figure 13 shows a forward converter with twosimple RC snubbers and the transformer leakageinductance is shown explicitly. There are tworesonances in this output which must be damped.The fIrst is the resonant circuit formed by thetransformer leakage inductance and diode D l' scapacitance in parnllel with the output inductorcapacitance and the stray circuit capacitance. Thiscircuit is excited by the recovery current in D 1 asit turns off. This occurs when the switch turns onso it is important to minimize the value of C 1. Ingeneral, this resonant frequency will be the lowestone in the output section because the stray capaci-tance is largest. Snubbing this resonance with Cland Rl will result in minimum loss since thevoltage swing is smallest at this point.T l.I D2 ~..~--r;:+- -

.C2R2 L ~

RI

Yo

-CO.1 Dl

Ill"Fig 13

The other resonance in the forward converteroutput is due to the transformer leakage inductanceand the capacitance of D2. The anode of D2 willring due to the reverse recovery cun-ent. Thisresonance will be higher in frequency than the otherone because the capacitance is smaller. This re-quires a smaller capacitor for C2 so the powerdissipation will be minimized in R2. This circuit isexcited when the switch turns off.

It is possible to use a current snubber in serieswith Dl to control the turn-off of the diode. Thesnubber energy may be recovered using both of therecovery networks shown in Figure 11 or dissipa-tive networks may be used but two networks are

required.

Component ProblemsThe characteristics of the components used in

snubbers are very important and this is especiallytrue in clamping snubbers. The rate of change ofcurrent in a snubber is very large and very smallparasitics can make it almost completely ineffective.The author recalls a clamp snubber in a boostconverter which had one inch of track on the PWBover a ground plane. That provided enough induc-

tance to cause high amplitude ringing at 50 MHz.Good layout practices are extremely important. A

ground plane is a necessity and tracks which carryhigh frequency currents must be kept wide relativeto the board thickness to keep the inductance down.

The diodes used in snubbers generally do nothave to be large diodes. They must handle relative-Iy large peak currents but only low average cur-rents. They need low recovered charge, especiallyin non-dissipative snubbers as was mentioned in theexamples above. Small diodes are often the bestchoice and may need small heat sinks.

Inductors have parallel capacitance which mustbe minimized. The bandwidth of the inductor mustbe as high as possible for good snubber operation.The inductor itself may go into parallel resonanceand this cannot be damped electronically since it isa field resonance. It must be eliminated by chang-ing the winding configuration. Progressive windingand bank winding techniques will minimize thewinding capacitance. Layer winding will maximizethe capacitance and random winding will have arandom distribution of values.

Capacitors have series inductance which must beminimized. Capacitors are often paralleled to reducethe circuit inductance and this is effective up to apoint. The series inductance of a large capacitor isquite capable of resonating with a small capacitorplaced in parallel with it and the resonant circuitwill have high Q. This is especially true for theoutput capacitor in a boost converter, the inputcapacitor in a buck converter and all capacitors ina flyback converter.

The resistors used in RC damping networks mustbe low inductance types. Non-inductive wire woundresistors generally have too much inductance andwill cause ringing and overshoot at high frequenciesinstead of providing damping. If there is no otheralternative it is sometimes possible to parallel awire wound resistor with a series RC network todamp the inductance of the wire wound resistor.

General Approach to Snubbing

Switching RegulatorsThe first step in snubbing a switching regulator

takes place when the design is still on paper.Formulate an overall snubbing strategy and calcu-

2-16 UNITRODE CORPORATION

late the values for each snubber. Where is thegreatest need in the circuit. How important isefficiency? How important is cost?

In the laboratory there is a general approach tosnubbing and trouble shooting which is effective.We should point out here that it is sudden death toa project to snub your technician and we stronglyrecommend that you not even try .Statements ofappreciation are usually much more effective atpreventing spiking and flaming.

The switches are the most vulnerable part of thesystem and must be treated gently until the circuitis well behaved. The power stage can be observedwith only a small fraction of the rated voltage andcurrent so that measurements can be made withoutthe possibility of switch damage. In this mode theeffect of each snubber can be ascertained and itseffectiveness determined. Snubbing generallyproceeds from the input to the output of the circuitnot only because the switches are the most vulnera-ble part of the circuit but also because the problemsin a switching power supply are staggered in time.

When a switch is on there is no need for avoltage snubber to control the switch voltage. Theswitch is connecting the output network of thesupply directly to a voltage source. When theswitch turns off, however, there may be somereactances which are not controlled and which willring. In a forward converter, for example, when theswitch turns off the leakage inductance on theprimary will drive a spike across the switch. Theleakage inductance on the secondary will continueto supply current through the diode. The leakageinductance on the secondary will prevent the diodefrom turning off for a few tens of nanoseconds afterthe switch voltage has reached its clamp level. Thistime displacement is observable. This allows theindividual parasitic elements to be separated andeach one can be dealt an appropriate snubber. Itusually requires an oscilloscope with matched and

properly adjusted probes.Once a point has been identified as needing a

snubber the objective of the snubber must beconsidered. Is the objective to provide damping ofa resonant circuit, to clamp an overshoot, to controla rise or fall time or to increase efficiency. Thisdetermines the type of snubber to add. Always

remember that more than one snubber may berequired for any particular location.

ConclusionA properly snubbed circuit enhances system reli-

ability, is more efficient, and is quieter than anunsnubbed circuit A properly snubbed circuitperforms well over time, temperature and produc-tion tolerances. It is well worth the time to under-stand and use snubber circuits.

References

[ 1] W. McMurray, "Optimum Snubbers for PowerSemiconductors," IEEE Transactions on IndustryApplications, September/October 1972, pp 593-600.

[2] T. K. Phelps and W. S. Tate, "OptimizingPassive Input Filter Design," Proceedings ofPowercon 6, May 1979, paper G1.

[3] I. C. Fluke Sr., "The Flyback Characteristics ofFerrite Power Transformers In Forward Convert-ers, " Proceedings of the Power Electronics Show

and Conference, October 1986, pp128-133.

[4] E. C. Whitcomb, "Designing Non-dissipativeCurrent Snubbers for Switched Mode Convert-ers," Proceedings of Powercon 6. May 1979,paper B1.

[5] x. He, S. I. Finney, B. W. Williams and T. C.Green, "An Improved Passive Lossless Tum-onand Turn-off Snubber," IEEE Applied PowerElectronics Conference Proceedings. March

1993, pp385-392.

[6] E. T. Calkin and B. H. Hamilton, "Circuit Tech-niques for Improving the Switching Loci ofTransistor Switches in Switching Regulators,"IEEE Transactions On Industry Applications.Iuly/August 1976, pp364-369.

[7] W. R. Skanadore, "Load Line Shaping Consid-erations for the High Speed Switching TransistorIn Switching Regulators and Other HighlyInductive Environments," Proceedings of Power-con 4. May 1977, paper H-4.

[8] P. 0. Lauritzen and H. A. Smith, "A Nondissi-pative Snubber Effective Over a Wide Range ofOperating Conditions," IEEE Power ElectronicsSpecialist Conference Proceedings. Iune 1983,

pp345-354.

2-17Snubber Circuits

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