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14/6/2004 221332 Diodes, Asst.Prof.Dr. MONTREE SIRIPRUCHYANUN 1

213332 Electronic Circuit and Devices I

AsstAsst.. Prof Prof. Dr.. Dr. Montree Montree SIRIPRUCHYANUN SIRIPRUCHYANUNDept. of Teacher Training in Electrical Engineering,King Mongkut’s Institute of Technology North Bangkok

Diode ApplicationsDiode Applications

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Applications of p-n Junction Diodes(a) Rectifiers (for converting ac signal into dc

signal)(b) Power Supplies (used in computers and

telecommunication ystems)

(c) Clippers (to generate desired signals by dropping off some portion of available signal)(d) Clampers (these are the diode circuit to

clamp a signals to different dc levels)(e) Voltage Multipliers (to generate signal that

has its amplitude integral multiple of original signals)

(d) Logic gates

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(a) RectifiersSinusoidal inputs half wave rectification

There are two conditions fordiode (i) ON and (ii) OFF

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Sinusoidal inputs half wave rectification (continued)(i) Diode is ON

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Sinusoidal inputs half wave rectification (continued)(ii) Diode is OFF

PIV rating of diode ? Vm

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Average output over a period of ac inputwave Vdc = (1/T)vo(t)dt

Limits of this integral are between 0 to T

Sinusoidal inputs half wave rectification (continued)

Input waveform

Output waveform

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(a) Rectifiers (continued)Sinusoidal inputs full wave rectification

D2 D4

D3D1

RLACinputvoltagesource

+

-

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The full wave rectifier which could be a bridgerectifier can also drawn like shown below;

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Sinusoidal inputs full wave rectification (continued)

Average output over a period of ac inputwave Vdc = (1/T)?vo(t)dt

Limits of this integral are between 0 to T

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PIV Rating of Diode in a Full Wave Rectifier

+

-

PIV rating of diode ? Vm

Vmsinwt

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(a) Rectifiers (continued)Rectifier with input isolation

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Rectifier with input isolation (continued)

Average output over a period of ac inputwave Vdc = (1/T)?vo(t)dt

Limits of this integral are between 0 to T

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PIV Rating of Diode in an Input Isolated FullWave Rectifier

PIV ? Vsecondary + VR = Vm+Vm

PIV ? Vm

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Diode Application to Power Supplies forComputers and Telecommunications Systems

The power supplies normally use a C and R-C filter.The IC regulator is a BJT or OPAMP based integratedcircuit which provides a constant dc voltage for aninput with ripples and variations within a range. Theseare 7805, 7812, 7824, 7905, 7912, 7924

Parts of a power supply system

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Capacitor Filter: Used often to filter outripples in rectified voltage.

Capacitor filter connected at the rectifier output and dcvoltage obtained across the capacitors terminals isused to feed load (computer load, telecommunicationload or any load which needs dc supply)

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Capacitor filter operation and its effectiveness

Filter (OFF) Filter (ON)

It is clear that filtered waveform is a dc voltage withsuperimposed ac variation. While, unfiltered wave is arectified ac wave with much more ripples.

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Capacitor filter and Diode Current

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Capacitor filter (Value of cap) & Diode Current

Larger the value of filter cap, higher the peak charging current butsmaller charging period. Contrary to this, smaller the value of filter cap,lower the peak charging current but larger the charging period.

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R-C filter and Ripple in Output VoltageIt is possible to further reduce the amount of ripple across afilter capacitor by using an additional RC filter section. Theadded RC filter will pass most of the dc component whileattenuating (reducing) as much of ac component as possible.

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R-C filter and Ripple in Output Voltage (cont.)

C2 attenuates ac ripples while passing dc signals withoutany attenuation and distortion to the load.

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Clippers: These are the diodecircuit used to clip off a portion ofinput signals without distorting theremaining part. Depending uponthe position of the diode in a circuit,the clippers are categorized as;(a) Series Clipper(b) Parallel Clipper

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(a) Series Clipper

The input voltage is a time varying signal and may havepositive and negative values with respect to ground terminal.During positive values of the input signal the diode will be inON position while during negative values of input signal thediode will have OFF position. Depending upon ON/OFFpositions of the diode the output voltage will have shape ofinput voltage or zero value.

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(a) Series Clipper (continued)

(a) Series Clipper

(b) Response withsquare wave input

(c) Response with triangular wave input

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(b) Parallel Clipper

The input voltage is a time varying signal and may have positive andnegative values. During positive values of the input signal the diode willbe in ON position while during negative values of input signal the diodewill have OFF position. When diode is ON the output voltage is zero(neglecting the threshold voltage), while diode is OFF, the output voltagewill be same as input voltage. Therefore, depending upon OFF/ONpositions of the diode the output voltage will have shape of input voltageor zero value.

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(b) Parallel Clipper (continued)

(a) Parallel Clipper(b) Response withsquare wave input

(c) Responsewith triangular

wave input

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Clamper: This is a diode circuit which clampsa signals to different dc levels. Unlike theclipping circuit (where diode, resistance, varioussources are used), the clamping circuit musthave a capacitor, a diode, and resistance. Theclamping circuit also employs an independent dcsupply to obtain an additional shift. Themagnitude of the R and C must be chosen suchthat time constant of the circuit ? = RC is largeenough to ensure that capacitor voltage does notdischarge significantly during thenonconducting intervals of the diode.

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Diode Clamper Circuit

However, the negligible resistance of the diode will allow thecapacitor to charge fully as ? = RdiodeC ? 0. During negativehalf cycle diode will be in OFF position and this will allowcapacitor and input source combined together to appear acrossoutput resistance. Therefore, the output voltage will have ashift (different level) as compared to input voltage.

The input voltage is a timevarying signals. When inputvoltage has its positive halfcycle, the diode will conductand (if VT and Rdc and rac areneglected) the diode willproduce a short circuit acrossoutput terminals.

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Clamper Circuit (continued)

Circuit

DiodeON

DiodeOFF

Input signal

Outputsignal

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Zener Diode and Waveform Generation (Clipper Circuits)

Zener diode eitherAvalanche designed orZener designed operates inthird quadrant of diode V-Icharacteristics. The zenerdiode has negligibly smallvalue of the thresholdvoltage (VT). The diode V-Icharacteristics enters inzener region moment thereversed biased voltageexceed VZ (Zener potentialor ionization potential)

V-I Characteristicsof Zener Diode

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Zener Diode Equivalent Circuit and ON/OFF Conditions

Complete EquivalentCircuit

ApproximateEquivalent Circuit

ON Condition OFF Condition

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Sinusoidal AC Regulation

The back to back connected zener diodes can be usedas an ac regulator. The output voltage will beregulated at 20 V peak value and if input voltage ismore than this value the zener circuit will clip off theexcess voltage from input wave.

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Different supply voltage levels from a single supply source

Using Zener diode, a 50 V DC supply source can providefour different supply voltage levels 0-50V, 0-10V, 0-20 Vand 0-30V. 0-50 and 0-30 have a common ground pointwhereas 0-10 and 0-20 have another common ground point.

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Generation of square wave signal. Same circuitcan be used as a clock signal generator in a

microprocessor, DSP and Computers.

A high frequency ac signals is clipped using back toback zener diodes for generation of a square wavesignals. This circuit can be used in a functiongenerators.

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Diode Applications in Voltage Multipliers

Voltage multiplier circuits are employed tomaintain a relatively low transformer peak voltagewhile stepping up the peak output voltage to two,three, or more times the peak rectified voltage.

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Half Wave Voltage Doubler

During positive half cycle the point A of transformer secondary winding is ahigher potential as compared to point B. This will led a forward bias for diode D1and reverse bias for diode D2. Therefore during positive half cycle Diode D1 canbe replaced by a short circuit thereby allowing capacitor C1 to charge to voltagelevel Vm. During negative half cycle of ac wave the point B will be at higherpotential as compared to point A. This will led the diode D1 to go OFF conditionand diode D2 to ON condition. Thus allowing the capacitor to C2 to charge tovalue 2Vm.

A

B

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Half Wave Voltage Doubler (continued)

Circuit condition Circuit conditionduring positive half cycle during negative half cycle

KVL in the loop duringnegative half cycle;-Vm -Vc1+Vc2=0-Vm -Vm+Vc2=0Vc2=2Vm

During subsequent positive half cyclesthe diode D2 is not conducting andcapacitor C2 will discharge through theload. If no load is connected both thecapacitor will retain their voltage the atVm (C1) and 2Vm (C2).

PIV Rating of diodes ? 2Vm

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Full Wave Voltage DoublerDuring positive half cycle oftransformer secondary voltage thediode D1 is conducting and anddiode D2 is reversed biased. This willallow the capacitor C1 to charge tovoltage level Vm. During negativehalf cycle diode D1 will be OFF anddiode D2 is ON this will allowcapacitor to C2 to charge to valueVm. If no load is connected thetotal voltage across C1 and C2will be 2Vm (constant). If the loadcurrent is drawn then the voltagewill be same as obtained from a fullwave rectifier with a filter capacitorconnected at dc output terminals.Both the capacitors will providepoorer filtering capacity ascompared to a capacitor alone.

Full wave voltage doubler circuiteliminates the need of center taptransformer

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Full Wave Voltage DoublerPIV Rating of diodes ? 2Vm

Circuit condition Circuit conditionduring positive half cycle during negative half cycle

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Full wave rectifier with a filter capacitor acrossDC load and centertapped input stage transformer

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Voltage Tripler and Quadrupler Circuit

This circuit is an extension of half-wave voltage doubler toobtain a output voltage which is three times and four times theinput voltage. On addition of more stages may results in anoutput voltage five, six, seven and so on times the inputvoltage.

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In operation Voltage Tripler and Quadrupler Circuit, capacitorC1 charges through diode Dl to a peak voltage, Vm, during the positivehalf-cycle of the transformer secondary voltage. Capacitor C2 chargesto twice the peak voltage 2Vm developed by the sum of the voltagesacross capacitor C1 and the transformer, during the negative half-cycleof the transformer secondary voltage.

During the positive half-cycle, diode D3 conducts and thevoltage across capacitor C2 charges capacitor C3 to the same 2Vm peakvoltage. On the negative half-cycle, diodes D2 and D4 conduct withcapacitor C3, charging C4 to 2Vm.

The voltage across capacitor C2 is 2Vm, across C1 and C3 it is3Vm and across C2 and C4 it is 4Vm. If additional sections of diode andcapacitor are used, each capacitor will be charged to 2Vm. Measuringfrom the top of the transformer winding (Fig. Voltage Tripler andQuadrupler circuit) will provide odd multiples of Vm at the output,whereas measuring the output voltage from the bottom of thetransformer will provide even multiples of the peak voltage, Vm.

The transformer rating is only Vm, maximum, and each diodein the circuit must be rated at 2Vm PIV. If the load is small and thecapacitors have little leakage, extremely high dc voltages may bedeveloped by this type of circuit, using many sections to step up the dcvoltage.

Working of Voltage Tripler and Quadrupler Circuit