resonant soft switchching converters

8/12/2019 resonant soft switchching converters

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1. INTRODUCTION:

Advances in power electronics in the last few decades have led to not just improvements inpower devices, but also new concepts in converter topologies and control. In the 1970s,

conventional pulse width modulated (P!" power converters were operated in a switched

mode operation. Power switches have to cut off the load current within the turn#on and

turn#off times under the hard switching conditions. $ard switching refers to the stressful

switching behavior of the power electronic devices. %he switching trajector& of a hard#

switched power device is shown in 'ig.1.

'ig.1 %&pical switching trajectories of power switches

uring the turn#on and turn#off processes, the power device has to with# stand high voltage

and current simultaneousl&, resulting in high switching losses and stress. issipative

passive )nubbers are usuall& added to the power circuits so that the dv/dt and di/dt ofthe power devices could be reduced, and the switching loss and stress be diverted to the

passive snubbed circuits. $owever, the switching loss is proportional to the switching

fre*uenc&, thus limiting the ma+imum switching fre*uenc& of the power converters.

%&pical converter switching fre*uenc& was limited to a few tens of ilo#$ert- (t&picall& 0/

0 $-" in earl& 190s. %he stra& inductive and capacitive components in the power circuits

and power devices still cause considerable transient effects, which in turn give rise to electro#

magnetic interference (2!I" problems. 'igure. shows ideal switching waveforms and

t&pical practical waveforms of the switch voltage. %he transient ringing effects are major

causes of 2!I.

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'ig . %&pical (a" ideal and (b" practical switching waveforms

In the 190s, lots of research efforts were diverted towards the use of resonant

converters. %he concept was to incorporate resonant tans in the converters to create

oscillator& (usuall& sinusoidal" voltage and3or current waveforms so that -ero#voltage

switching (45)" 0 -ero#current switching (46)" conditions can be created for the power

switches. %he reduction of switching loss and the continual improvement of power

switches allow the switching fre*uenc& of the resonant converters to reach hundreds ofilo#$ert- (t&picall& 100/00 $-". 6onse*uentl&, the si-e of magnetic components

can be reduced and the power densit& of the converters increased. 5arious forms of

resonant converters have been proposed and developed. $owever, most of the resonant

converters suffer several problems. hen compared with the conventional P!

converters, the resonant current and the voltage of resonant converters have high pea

values, leading to higher conduction loss and higher V and I rating re*uirements for

the power devices. lso, man& resonant converters re*uire fre*uenc& modulation ('!" for

output regulation. 5ariable switching fre*uenc& operation maes the 8lter design and

control more complicated.

In late 190s and throughout 1990s, further improvements have been made in

converter technolog&. ew generations of soft#switched converters that combine the

advantages of conventional P! converters and resonant converters have been developed.

%hese soft#switched converters have switching waveforms similar to those of conventional

P! converters e+cept that the rising and falling edges of the waveforms are :smoothed;

with no transient spies. ther than that, the& behave just lie conventional P! converters. ith

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simple modi8cations, man& customi-ed control integrated circuits (I6s" designed for

conventional converters can be emplo&ed for soft#switched converters. ?ecause the

switching loss and stress have been reduced, soft#switched converter can be operated at the

ver& high fre*uenc& (t&picall& 00 $- to a few !ega#$ert-". )oft#switching converters

lso provide an effective solution to suppress 2!I and have been applied to 6/6,6/6, and 6/6 converters. %his chapter covers the basic technolog& of resonant and

soft#switching converters. 5arious forms of soft#switching techni*ues such as 45), 46),

voltage clamping, -ero#voltage transition methods, etc. are addressed. %he emphasis is

placed on the basic operating principle and practicalit& of the converters without using

much mathematical anal&sis.

1.1 RESONANCE:In an electrical circuit, the condition that e+ists when the inductive reactance and the

capacitive reactance are of e*ual magnitude, causing electrical energ& to oscillate between

the magnetic fieldof the inductor and the electric fieldof the capacitor.

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2. CLASSIFICATION:

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3. RESONANT SWITCH:

Prior to the availabilit& of full& controllable power switches, %h&ristors was the major

power devices used in power electronic circuits. 2ach %h&ristors re*uires a commutation

circuit, which usuall& consists of aLC resonant circuit, for forcing the current to -ero in the

turn#off process. %his mechanism is in fact a t&pe of -ero#current turn#off process. ith

the recent advancement in semiconductor technolog&, the voltage and current handling

capabilit&, and the switching speed of full& controllable switches have signi8cantl& been

improved. In man& high power applications, controllable switches such as gate turn#offs

(@%>s" and insulated gate bipolar transistors (I@?%s" have replaced %h&ristors. $owever,

the use of resonant circuit for achieving 46) and3or 45) has also emerged as a new

technolog& for power converters. %he concept of resonant switch that replaces

conventional power switch is introduced in this section.

resonant switch is a sub#circuit comprising a semiconductor switch ) and resonant

elements, Lr and 6r. %he switch ) can be implemented b& a unidirectional or

bidirectional switch, which determines the operation mode of the resonant switch.

%wo t&pes of resonant s w i t c h e s , including -ero#current (46" resonant switch

a n d -ero#voltage (45" resonant switches, are shown in 'igs.A and B, respectivel&.

Fig.3 Zero-curren !ZC" re#on$n S%ic&

Fig.' Zero-(o)$ge !Z(" re#on$n #%ic&

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3.1 ZC Re#on$n S%ic&:

In a 46 resonant switch, an inductor Lr isconnected in series with a power switch )

in order to achieve -ero#current switching (46)". If the switch ) is a unidirectional switch,

the switch current is allowed to resonate in the positive half c&cle onl&. %he resonant

switch is said to operate in half#wavemode. If a diode is connected in anti#parallel with

the unidirectional switch, the switch current cans Cows in both directions. In this case, the

resonant switch can operate in full#wavemode. t turn#on, the switch current will rise

slowl& from -ero. It will then oscillate, because of the resonance between Lr and Cr.

'inall&, the switch can be commutated at the ne+t -ero current duration. %he objective of

this t&pe of switch is to shape the switch current waveform during conduction time in

order to create a -ero#current condition for the switch to turn off

3.2. Z( Re#on$n S%ic&:

In a 45 resonant switch, a capacitor Cr isconnected in parallel with the switch ) for

achieving -ero#voltage switching (45)". If the switch ) is a unidirectional switch, the

voltage across the capacitor Cr can oscillate freel& in both positive and negative half#c&cle.

%hus, the resonant switch can operate in full#wavemode. If a diode is connected in anti#

parallel with the unidirectional switch, the resonant capacitor voltage is clamped b& the

diode to -ero during the negative half#c&cle. %he resonant switch will then operate in half#wavemode. %he objective of a 45 switch is to use the resonant circuit to shape the switch

voltage waveform during the off time in order to create a -ero#voltage condition for the

switch to turn on

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'. *UASI-RESONANT CON(ERTERS:

Duasi#resonant co nverters (D=6s" can be considered as a h&brid of resonant andP! converters. %he underl&ing principle is to replace the power switch in P!

converters with the resonant switch. large famil& of conventional converter circuits can

be transformed into their resonant converter counterparts. %he switch current and3or

voltage waveforms are forced to oscillate in a *uasi#sinusoidal manner, so that 46)

and3or 45) can be achieved.

'.1 ZCS-*RC

46)#D=6 designed for half-wave operation is illustrated with a buc t&pe 6/6converter. %he schematic is shown in 'ig.a. It is formed b& replacing the power switch in

conventional P! buc converter with the 46 resonant switch in 'ig.Aa. %he circuit

waveforms in stead& state are shown in 'ig.b. %he output 8lter inductor Lf is sufficientl&

large so that its current is appro+imatel& constant. Prior to turning the switch on, the output

current Io freewheels through the out# put diode F. %he resonant capacitor voltage

VCRe*uals -ero. t t0, the switch is turned on with 46). *uasi#sinusoidal current I)

Cows through Er and 6r, the output 8lter,andthe load. ) is then softl& commutated at

t1 wi t h 46) again. uring and after the gate pulse, the resonantcapacitor volt# age VCR

rises and then deca&s at a ratedepending on the output current. >utput voltage regulationis achieved b& con# trolling the switching fre*uenc&.>peration and characteristics of the

converter depend mainl& on the design of the resonant circuit Lr Cr. %he following

parameters are de8nedF voltage conversion ratio M, characteristic impedance Zr, resonant

fre*uenc&Fr,normali-ed load resistance r, normali-ed switching fre*uenc& .

(1)

(2)

(3)

(4)

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(5)

It can be seen from the waveforms that ifIo > Vi 3Zr,ISwillnot come bac to -ero

naturall& and the switch will have to be forced off, thus resulting in turn#off losses. %he

relationships between M and at different r are shown in 'ig.c. It can be seen that M is

sensitive to the load variation. t light load conditions, the unused energ& is stored in Cr,

leading to an increase in the output voltage. %hus, the switching fre*uenc& has to be

controlled, in order to regulate the output voltage.

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'ig. $alf#wave, *uasi# resonant buc converter with 46) (a" schematic diagram

(b" 6ircuit aveforms(c" relationship between ! G H

If an anti#parallel diode is connected across the switch, the converter will be

operating in full#wave mode. %he circuit schematic is shown in 'ig.a. %he circuit

waveforms in stead& state are shown in 'ig.b. %he operation is similar to the one in

half#wavemode. $owever, the inductor current i s allowed to reverse through the anti#

parallel diode and the duration for the resonant stage is lengthened. %his permits e+cess

energ& in the resonant circuit at light loads to be transferred bac to the voltage source

Vi %his signi8cantl& reduces the dependence of Vo on the output load. %he relationships

between M and at different r are shown in 'ig.c. It can be seen that !is insensitive toload variation.

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'ig. 'ull#wave, *uasi#resonant buc converter with 46)F (a" schematic diagramJ

(b" 6ircuit waveformsJ and (c" relationship between ! and

?& replacing the switch in the conventional converters, a famil& of D=6 with 46) is

shown in 'ig.7

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'ig.7 famil& of *uasi#resonant converter with 46)

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'.2 Z(S-*RC

In these converters, the resonant capacitor provides a -ero# voltage condit ion for t h e

switch to turn on and off.

*uasi#resonant buc converter designed for half#waveoperation is shown in 'ig.

a / using a 45 resonant switch in 'ig.Bb. %he steadstate circuit waveforms are shown

in 'i g. b. ?asic relations of 45)#D=6s are given in 2s*. (1". hen the switch ) is

turned on, it carries the output current Io.%he suppl& voltage Vi reverse biases the diode

f. hen the switch is -ero#voltage (45" turned off, the output current starts to Cows

through the resonant capacitor Cr. hen the resonant capacitor voltage VCr ise*ual to Vi ,

f turns on. %his starts the resonant stage. hen VCR e * u a l s -ero, the anti#parallel

diode turns on. %he resonant capacitor is shorted and the source voltage is applied to the

resonant inductor Lr. %he resonant inductor current ILr i n c reases linearl& until it reaches

Io. %hen F turns off. In order to achieve 45), ) should be triggered during the time

when the anti#parallel diode conducts. It can be seen from the waveforms that the pea

amplitude of the resonant capacitor voltage shouldbe greater or e*ual to the input voltage

(i.e. Io Zr > VIN.'rom 'ig. c, it can be seen that the voltage conversion ratio is load#

sensitive. In order to regulate the output voltage for different loads r, the switching

fre*uenc& should also be changed accordingl&.

45)converterscanbeoperatedinfull#wavemode.%he circuit schematic is shown in

'ig.9a. %he circuit waveforms in stead& state are shown in 'ig.9b. %he operation is similar to

half#wave mode of operation, e+cept that 56= ca n swing between positive and ne gative

voltages. %he relationships between ! and g at different r are shown in 'ig.9c. 6omparing

'ig.c with 'ig.9c, it can be seen that ! is load#insensitive in full#wave mode. %his is a

desirable feature. $owever, as the series diode limits the direction of the switch current,

energ& willbe stored in the output capacitance of the switch and will dissipate in the switch

during turn on. $ence, the full#wave mode has the problem of capacitive turn#on loss, and is

less practical in high fre*uenc& operation. In practice, 45)#D=6s are usuall& operated in

half#wave mode rather than full#wave mode.

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'ig.

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'ig.9

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?& replacing the 45 resonant switch in the conventional converters, various 45)#D=6s

can be derived. %he& are shown in 'ig.10.

'ig.10

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'.3 Co+,$ri#on# e%een ZCS $n Z(S

46) can eliminate the switching losses at turn off and reduce the switching losses at turn

on. s a relativel& large capacitor is connected across the output diode during resonance,

the converter operation becomes insensitive to the diodeKs junction capacitance. hen

power !>)'2%s are -ero#current switched on, the energ& stored in the deviceKs capacitance

will be dissipated. %his capacitive turn#on loss is proportional to the switching fre*uenc&.

uring turn on, considerable rate of change of voltage can be coupled to the gate drive

circuit through the !iller capacitor, thus increasing switching loss and noise. nother

limitation is that the switches are under high current stress, resulting in higher conduction

loss. $owever, it should be noted that 46) is particularl& effective in reducing switching

loss for power devices (such as I@?%" with large tail current in the turn#off process.

45) eliminates the capacitive turn#on l o s s . It is suitable for high#fre*uenc& operation.

'or single#ended con8guration, the switches could suffer from e+cessive voltage stress,

which is proportional to the load. It will be shown in )ection that the ma+imum voltage

across switches in half#bridge and full#bridge con8gurations is clamped to the input

voltage.

'or both 46) and 45), output regulation of the resonant converters can be achievedb&

variable fre*uenc& control. 46) operates with constant on#time control, while 45)

operates with constant off#time control. ith a wide input and load range, both

techni*ues have to operate with a wide switching fre*uenc& range, maing it not eas& to

design resonant converters optimall&.

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/. Z(S in Hig& Fre0uencA,,)ic$ion#

?& the nature of the resonant tan and 46), the pea switch current in resonant converters

is much higher than that in the s*uare#wave counterparts. In addition, a high voltage will

be established across the switch in the off state after the resonant stage. hen the switch is

switched on again, the energ& stored in the output capacitor willbe discharged through the

switch, causing a signi8cantl& power loss at high fre*uencies and high voltages. %his

switching loss can be reduced b& using 45).

45) can be viewed as s*uare#wave power utili-ing a constant off#time control. >utput

regulation is achieved b& controlling the > time or switching fre*uenc&. uring the off

time, the resonant tan circuit traverses the voltage across the switch from -ero to its pea

value and then bac to -ero again. t that 45 instant, the switch can be reactivated. part

from the conventional single#ended converters, some other e+amples of converters with

45) are illustrated in the following section.

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. Z( S %i& C)$+,e (o)$ge

%he high voltage stress problem in the single#switch con8guration with 45) can be

avoided in half#bridge ($?" and full#bridge ('?" con8gurations. %he pea switch volt# age

can be clamped to the dc suppl& rail, and thus reducing the switch voltage stress. In

addition, the series transformer leaage and circuit inductance can form parts of the

resonant path. %herefore, these parasitic components, which are undesirable in hard#

switched converter become useful components in 45) ones. 'igures.11 and1 show the

45) $? and '? circuits, respectivel&, together with the circuit waveforms.

%he resonant capacitor is e*uivalent to the parallel connection of the two capacitors

(Cr 3" across the switches. %he off#state voltage of the switches will not e+ceed the input

voltage during resonance because the& will be clamped to the suppl& rail b& the anti#

parallel diode of the switches.

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Fig.11

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'ig.1

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7 3u)i-re#on$n Con4erer# !RC"

%he 46)# and 45)#D=6s optimi-e the switching condition for either the active switch or

the output diode onl&, but not for both of them simultaneousl&. !ulti#resonant switch

concept, which is an e+tension of the concept of the resonant switch, has been developed toovercome such limitation. %he -ero#current multi#resonant (46#!=" and -ero#voltage multi#

resonant (45#!=" switches are shown in 'ig.1A.

Fig.13

%he multi#resonant c i r c u i t s incorporate all major parasitic components, including

switch output capacitance, diodejunction capacitance, and transformer leaage inductance

into the resonant circuit. In general, 45) (half-wave mode" is more favorable than 46) in

6/6 converters for high#fre*uenc& operation because the parasitic capacitance of the

active switch and the diode will form a part of the resonant circuit.

'ig.1B

n e+ample of a buc 45)#!=6 is shown in 'ig.1. epending on the ratio of the

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resonant capacitance C! 3CS , two possible topological modes, namel& mode I and mode

II, can be operated . %he ratio affects the time at which the voltages across the switch )

and the output diode F "e#o$e -ero. %heir waveforms are shown in 'igs.1a and b,

respectivel&. If diode voltage V! f a l l s to -ero earlier than the switch voltage VS, the

converter will follow mode I. >therwise, the converter will follow mode II.

Fig.15

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Fig.15

Instead of having one resonant stage, there are three in this converter. %he mode I

operation in 'ig.1a is described 8rst. ?efore the switch ) is turned on, the output diodef is conducting and the resonant inductor current ILr i s negative ('lowing through the

anti#parallel diode of )". ) is then turnedon with 45). %he resonant inductor current ILr

in#reases linearl& and F is still conducting. hen ILr rea#hes theoutput current Io,

the 8rst resonant stage starts. %he resonant circuit is formed b& the resonant inductor Lr

and the capacitor C!across the output diode. %his stage ends when S is turnedoff with

45). %hen, a second resonant stage starts. %he resonant circuit consists of Lr , C! , and

the capacitor across the switch Cs . %his stage ends when the output diode becomes

forward biased. 7 third resonant stage will then start. Lr and

Cs form the resonant circuit.

%his stage ends and completes oneoperation c&cle when the diode Cs " e #o$es forward

biased. %he onl& difference between mode I and mode II in'ig.1b is in the third

resonant stage, in which the resonant circuit is formed b& Lr and C! . %his stage ends

when F becomes forward biased. %he concept of the multi#resonant switch can be

applied to conventional converters 5ia famil& of !=6s is shown in 'ig.1.

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'ig.1

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lthough the variation of the switching fre*uenc& for regulation in !=6s is smaller than that

of D=6s, a wide#band fre*uenc& modulation is still re*uired. $ence, the optimal design of

magnetic components and the 2!I filters in !=6s is not eas&. It would be desirable to have

a constant switching fre*uenc& operation. In order to operate the !=6s with constant

switching fre*uenc&, the diode in 'ig.1A can be replaced with an active switch ) . constant#fre*uenc& multi#resonant (6'#!=" switch is shown in 'ig.1. %he output voltage is

regulated b& controlling the on#time of the two switches. %his concept can be illustrated with

the buc converter as shown in 'ig.1, together with the gate drive waveforms and operating

stages. )1 and ) are turned on during the time when currents flow through the anti#parallel

diodes of )1 and ). %his stage ends when ) is turned off with 45). %he first resonant stage

is then started. Lr and CS form the resonant circuit. second resonant stage begins. Lr

resonates with CS1 and CS. %he voltage across S1 oscillates to -ero. hen ILr becomes

negative, S1 will be turned on with 45). %hen, Lr resonates with CS. S will be turned on

when current flows through !S. s the output voltage is the average voltage across S,

output voltage regulation is achieved b& controlling the conduction time of S.

'ig.17

7ll switches in !=6s operate with 45), which reduces theswitching losses and switching

noise and eliminates the oscillation due to the parasitic effects of the components (such

as the junction capacitance of the diodes". $owever, all switches are under high current

and voltage stresses, resulting in an increase in the conduction loss.

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'ig.1

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5 Lo$ Re#on$n Con4erer#:

Eoad resonant converters (E=6s" have man& distinct fea# tures over conventional power

converters. ue to the soft commutation o f the switches, no turn#off loss or stress ispresent. E=6s are speciall& suitable for high#power applications b e c a u s e the& al low

high#fre*uenc& op er at io n for e*uipment si-e3weight reduction, without sacri8cing the

con# version efficienc& and imposing e+tra stress on the switches. ?asicall&, E=6s can be

divided into three different con8gurations, namel& series resonant converters, parallel

resonant converters, and series/parallel resonant converters.

5.1 Serie# Re#on$n Con4erer#

)eries resonant converters ()=6s" have their load connected in series with the resonant tan

circuit, which is formed b& Lr andCr .%he half#bridge con8guration is shown in 'ig.19.

hen the resonant inductor current iLr is positive, it Cows through %1 if %1 is >.

>therwise it Cows through thediode hen iLr isnegative, it Cows through % if% is

onJ otherwise it Cows through the diode 1. In the steadstate s&mmetrical operation,

both the active switches are operated in a complementar& manner. epending on the ratio

between the switching fre*uenc& %S and the converter resonant fre*uenc& %r, the

converter has severalpossible operating modes.

Fig.19

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5.2 6$r$))e) Re#on$n Con4erer#

Parallel resonant co nve rt er s (P=6s" have their lo ad connected in parallel with theresonant tan capacitor Cr &7/A0L. %he half#bridge con8guration is shown in 'ig.0 )=6

behaves as a current source, whereas the P=6 acts as a voltage source. 'or voltage

regulation, P=6 re*uires a smaller operating fre*uenc& range than the )=6 to compensate

for load variation.

'ig.0

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7. Re#on$n DC Lin8 In4erer %i& Lo%(o)$ge Sre##

'ig.1

resonant dc lin inverter with low voltage stress is shown in 'ig. 1 it consists of a front#

end resonant converter that can pull the dc lin voltage down just before an& inverter

switching. %his resonant dc circuit serves as an interface between the dc power suppl& and

the inverter. It essentiall& retains all the advantages of the resonant (pulsating" dc lin

inverters. ?ut it offers e+tra advantages such asF

o increase in the dc lin voltage when compared with conventional hard#switched

inverter. %hat is, the dc lin voltage is 1.0 per unit

%he -ero voltage condition can be created at an& time. %he 45) is not restricted

to the periodic -ero#voltage instants as in resonant dc lin inverter.

ell#established P! techni*ues can be emplo&ed.

Power devices of standard voltage ratings can be used.

%he timing program and the si+ operating modes of this resonant circuit are as

shown in 'igs.and1respectivel&.

Fig.22

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'ig.A

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!1"Nor+$) +oe:

%his is the standard P! inverter mode. %he resonant inductor curr en t iEr(t" and the

resonant voltage V#r (t" are given b&

here Vs is the nominal dc lin voltage

!2"Mode 1 (initiating mode): (t9t1)

t t0, mode 1 begins b& switching on % and %A on with -ero current. iLr (t" increases

linearl& with a di3dt of Vs 3Lr. If iLr (t" is e*ual to the initiali-ed currentIi , %1 is -ero#voltage

turned off. If (IsMIo" ' Ii , then the initiali-ation is ended when iLr (t" is e*ual toIi , whereIs

is the current flowing into the dc inductorLd#. If (IsMIo" > Ii , then this mode continues until

iLr (t" is e*ual to (IsMIo". %he e*uations in this interval are

!3"Mode 2(Resonant mode):(t1 Mt"

fter %1 is turned off under 4 5 ) condition, r e s o n a n c e between Lr and Cr

occurs. V#r (t" decreases from 5s to0. t t, iLr (t" r eaches the pea value in this

interval. %he e*uations areF

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!I("Mode 3(Freewheeling mode) (t MtA"

%he resonant inductor current Cows through two freewheel#ing paths (%#Er# and %A#

1#Er". %his duration is the -ero voltage periods created for 45) of the inverter, and should

be longer than the minimum on and off times of the inverterKs power switches.

(("Mode 4 (Resonant mode):(tA MtB"

%his mode begins when % and %A are switched off under 45). %he second half of the

resonance between Lr and Crstarts again. %he capacitor voltage VCR (t" increases bac

from0 to Vs and is clamped to Vs. %he relevant e*uations in thismode are

hereI on is the load current after the switching state.

!(I"Mode 5 (Discharging mode): (t':

t/"

In this period, %1 is switched on under 45 condition because V#r (t" N Vs. %he inductor

cur rent decr ease s linearl&. %his mode 8nishes when iLr (t" becomes -ero.

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CONCLUSIONS

=esonant converter topologies can be used to increase circuit switching speeds, allowing the

cost of circuit magnetic to be reduced, while still eeping switching losses to a minimum.

'ull wave rather than half wave topologies are generall& used, as the& generate less 2!I.

6apacitive switching losses when turning on with a high drain#source voltage means that

!>)'2%s are more suitable for 4ero #5oltage than 4ero#6urrent switches, while its poor

turn#off characteristics mean that the I@?% is more suited to 4ero#6urrent topologies.

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References:1. R. M. !avis, *ower !iode and +hristor Cir#its, I Monora0h,

Series 1, 2erts) *ererins, 3413.

5. M. 2. Rashid, *ower le#troni#s, Cir#its, !evi#es, and 600li#ations,

700er Saddle River, N8) *earson/*renti#e 2all, 599:.

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resonant soft switchching converters