Review of Soft Switched Flyback Converters

Review of Soft Switched Flyback Converters

Bimlesh Tiwari

Electrical Engineering

Student of Department of Electrical Engineering, JIS College of Engineering. kalyani , India

Biswamoy Pal

Electrical Engineering

Asst. Professor, Department of Electrical Engineering, JIS College of Engineering. Kalyani , India

AbstractFly back converters are widely used in different low power domestic and industrial applications due to its simple structure and low component count. It is desirable that modern power electronic converters must have features like reduced size, high power density, high efficiency etc. By employing soft switching cells to existing converter structure, performance of the converter can be improved significantly. Soft switching enables converters to reduce switching loss and also enables the converter to operate at very high frequency. This facilitates reduction in size and volume of the converter, which improves power density and efficiency of the converter. This paper reviews detailed study of different soft switched fly back converters topologies to improve performance of the converters.

Keywords Flyback converters , Zero Voltage Switching (ZVS), Zero Current Switching (ZCS), Snubber circuit, Soft switching

I.INTRODUCTION

DC-DC converters are widely used in numerous applications. Reduction in converter size, weight and increase in efficiency are of main concern. Flyback converters among all DC-DC converters are widely used in low power applications [8-9] due to its simplicity, low component count, less control complexity. Flyback converters mostly suffer from the drawbacks such as low power conversion efficiency, high voltage spike of the main switch during turn off. To overcome these issues generally active clamping [1, 5 ,6] and soft switching [2- 6] methods are employed. Active clamping methods is generally used to reduce voltage spike across the main switch at turn off whereas soft switching methods reduce the switching losses of the converters significantly, which enables the converters to operate at very high frequency. This high frequency operation leads to reduction in converters size, weight and improves power density [24]. In many literature different soft switched flyback converters topologies such as ZCS operated flyback converters [4], ZVS operated flyback converter [2], ZVZCS flyback converter [3] have been presented.

This paper presents an overview of different soft switched flyback converter topologies with their detailed operating principles with their merits and demerits.

II. REVIEW OF DIFFERENT SOFT SWITCHED FLYBACK CONVERTERS

1. Transformer Assisted ZVS Scheme for Flyback Converter [1]

   There is a trend to operate the power supplies at high switching frequencies because of the high power density required. However, high switching frequency will bring high switching losses and lower the conversion efficiency. Soft switching techniques are normally employed to resolve

   this problem. The zero voltage switching (ZVS), as a type of soft switching.

   Figure 1 One type of ZVS flyback converter is a secondary side regenerating flyback converter [1]

   Magnetizing current of transformer is controlled bi- directional (positive and negative). The magnetizing current of the transformer and the current flowing through S1 and S2 are shown in the waveform below.

   OPERATIONAL PRINCIPLE:

   T is an auxiliary transformer; The magnetizing inductance of it is far smaller than that of main transformer

   Stage 1:

   Switch S1 is on, switch S2 and diode D is off. The magnetizing inductance Lm, is charged linearly just as normal flyback operation.

   Stage 2:

   When S1 is turned off, the diode D is forward biased, and resonance occurs between the magnetizing inductance Lm, and snubber capacitors (Cr1, Cr2,). Hence Cr1 is charged and Cr2 is discharged at a time.

   Stage 3:

   At this time the voltage across Cr2 becomes zero and the anti-parallel diode of S2 begins to conduct.

   Stage 4:

   S2 is turned on at zero voltage, and it begins to carry current. As the magnetizing inductance Lmr is comparatively small, its current decreases much faster than that of Lm. iLmr reduces to zero, then increases in opposite direction. When iLmr becomes (iLm, min) S2 is turned off . The imin is set by controller so as to make the energy stored in Lmr is

   Stage 5:

   S2 is turned off which makes resonance to occur between the magnetizing inductance Lmr and snubber capacitors (Cr1, Cr2). Once again it returns to stage 2 & again Capacitor Cr1 is discharged while Cr2 is charged.

   Stage 6 :

   The voltage across Cr1 becomes zero and the anti-parallel diode of S1 begins to conduct.

   The current in magnetizing inductance Lmr, increases very During fast at this stage.

   Stage 7 :

   Here S1 is turned on at zero voltage, and current is being carried by it . In this interval, the current in magnetizing inductance Lmr still increases very fast, and finally it will be equal to iLm so that the current iD becomes zero. & hence diode is off . The magnetizing inductance Lm and Lmr, begin to linearly charge again.

   Strength:

   • Soft switching of switching devices.

   • Reduced Voltage spikes.

   • High frequency operation which reduces the filter size.

   • large ripple in the magnetizing current (also in the current flowing through S1and S2).

   • A high ratio of the leakage inductor to magnetizing inductor of transformer.

   • A very high loss of leakage inductor is generated which in turn will lower the efficiency.

   • It needs large filter parameters.

   • Since all output filter capacitors have Equivalent Series Resistor (ESR), large ripple current will be generated hence high loss in the capacitors. Thus for good efficiency the magnetizing current must have low ripples.

     Weakness:

     Control complexity.

   • More magnetic materials are used.

   • Use of conventional transformer increases the no load loss hence use of electronic transformer can improve the efficiency and the overall performance for the low power application.

2. Operation principle of the ZVS-PWM flyback dc/dc converter with a simple auxiliary circuit [2]

   The power stage diagram of the proposed ZVS-PWM yback dc/dc converter is shown The circuit can be divided in two sections. The rst section is a conventional yback converter. It is contains switch Sm, diode D1, an isolated transformer, and output lter capacitor Co. Lm and LS are the magnetizing inductor and leakage inductor of the isolated transformer, respectively. This section performs the operation of the conventional yback dc/dc converter. Another section is a ZVS-PWM commutation cell to provide the zero-voltage-switching on all semiconductors in the circuit of the rst section [1]. It is composed of the auxiliary diodes D2, D3, the resonant inductors Lr, the resonant capacitors Cr, and auxiliary switch Sa, which are of small ratings.

   D2 Ls n1 : n 2 D1

   iLs

   Lm ILm Co

   Lr D3 Sa

   iLr

   VCr

   Sm Cr

   Fig2 ZVS-PWM flyback dc/dc converter with a simple auxiliary circuit [2]

   It is considered that ZVS-PWM yback converter is operating in a steady-state.

   Assumptions made in one cycle.

   1. Ideal components and device.

   2. Lm >>> Lr.

   3. o/p capacitor is high enough to have ripple free output voltage.

   4. Input voltage is const.

   5. Initially at t=to resonant voltage Vcr(t)= Vcro OPERATIONAL PRINCIPLE

      Stage 1:

      At t = t0, the switches Sm and Sa were in turn-off state.

      The energy stored in magnetizing inductor Lm is delivered to output lter capacitor Co and load through ideal transformer and diode D1.

      Sa is turned on with ZCS at t=to

      Lm and capacitor Cr is started following the path of Vin, Cr, Sa, D3, and Lr. The energy stored in magnetizing inductor Lm is delivered to output. Thus, the resonant current iLr(t) is increased and the resonant voltage vCr(t) is decreased and the diode D2 is turned on and this stage is ended here.

      Stage 2:

      It is intended that the supply of energy from the magnetizing inductor Lm is delivered continuously to the output terminal in this stage. Hence the voltage across the magnetizing inductor Lm is a constant voltage source nVo. Diode D2 is turned on which makes resonant circuit by D2, Ls, nV, Vin, Cr, Sa, D3, and Lr. The current iLs(t) on the leakage inductor is increased. When the resonant voltage vCr(t) is dropped to null, the body diode of the switch Sm is naturally turned on and this stage is nished.

      Stage 3:

      when the resonant voltage vCr(t) is dropped to null this stage is started and the body diode of the switch Sm is naturally turned on. the voltage across switch Sm switched to zero by its body diode in this stage, it is the best time to turn on the switch Sm under zero-voltage switching condition.

      Hence ZVS switching is achieved

      The leakage inductor Ls is linearly charged by voltage source Vin+nVo and the current iLs(t) is linearly increased. The resonant inductor Lr is linearly discharged by Vin hence resonant current iLr(t) also decreases and finally. D3 is turned off and this stage is ended.

      Stage 4:

      As iLr(t) is dropped to null, the diode D3 does not allow resonant inductor to discharge via input voltage source Vin. Therefore, the current through main switch Sm is blocked at zero by the diode D3 in this stage and hence once again ZCS is achieved & it is the high time to turn off the auxiliary switch Sa under ZCS condition [4]

      Stage 5:

      In this stage, the magnetizing inductor Lm and the leakage inductor Ls are linearly charged Vin. Hence it operates as the conventional PWM flyback dc/dc converter operating at turn-on state.

      Stage 6:

      In this stage the switch Sm is turned off as the voltage across resonant capacitor cannot be changed instantly, the voltage across the switch Sm equals to zero and the switch Sm turs off and hence ZVS is achieved

      In this stage iLm charges the resonant capacitor Cr and the resonant voltage vCr(t) is linearly increased and diode D1 is

      Mode 1:

      OPERATIONAL PRINCIPLE:

      turned on.

      STAGE 7:

      As D1 turns on, the energy stored in magnetizing inductor Lm supplies the output terminal again hence, the voltage across magnetizing inductor Lm is equal to nVo as before. The resonance of the Ls and the Cr is started following the path of Vin, Ls, nVo, and Cr. The resonant voltage vCr(t) is increased and the current iLs(t) in the leakage inductor Ls is decreased. When iLs(t) is dropped to zero, the diode D2 is turned off.

      STAGE 8:

      The energy stored in magnetizing inductor Lm continuously supplies the output terminal. This operating behavior is the same as the conventional PWM flyback dc/ dc converter operating at turn-off state. After this stage, the circuit operation is returned to the first stage.

      Strength:

      • Soft switched operation

      • Reduced Voltage spikes

      • High frequency operation

        Weakness:

        • Leakage energy can be recovered

        • Reverse recovery loss of diodes

3. Full Soft Switching ZVZCS Flyback Converter Using an Active Auxiliary Cell [3]

   A soft switching flyback DC-DC converter is shown. The auxiliary resonant cell added to the conventional flyback converter contains a switch (S2) a diode ( D3 ), an inductor

   1. and two capacitors ( C1 and C2 )[2]

      Assumptions:

      The non-ideal transformer having a magnetizing inductance, Lm and an ideal transformer.

      The Lm inductance is large.

      The output filter capacitor, Co is large

      C1 D1

      2

      1 S

      The switches of S1 and S2 are off, and the energy saved in

      the magnetizing inductance, Lm is transferred directly to the output through output diode, Do.

      The parameter a is the transformer turns ratio and is

      determined as n1/n2. Mode 2:

      At the end of the previous stage 1, switch S2 is on under ZVZCS, since the resonant inductor. In this mode, the L inductor and C2 capacitor starts resonating

      Mode 3:

      At the end of mode 2 diode D3 turns on under ZVZCS. During this mode, the voltage across inductor L achieves a constant value of aVo.

      Mode 4:

      Do diode turns off under ZCS and L inductor and C1 capacitor start to resonate. This resonance makes inductor current iL to increase and Vc1 decrease.

      Mode 5:

      the body diode of switch S1 and D1, turns on under ZVZCS and the inductor current (iL) decreases linearly.

      Mode 6:

      D diode turns off and S1 switch is turned on, both under ZVZCS. The inductor current (iL) continues to decrease .

      Mode 7:

      D diode turns off, D2 diode turns on and S2 switch is turned off all under ZVZCS.

      During this mode, L inductor and C2 capacitor resonate for half of a resonant period which reverses the polarity of the capacitor voltage (Vc2).

      Mode 8:

      D2 diode turns off under ZCZVS. Mode 9:

      S2 switch is turned on under ZVZCS. During this mode, L inductor and the C2 capacitor resonate, makes the inductor current (iL) and capacitor voltage (Vc2) to increase .

      Mode 10:

      Mode 9 ends when the switch current (iS1) decreases to zero. D1 turns on under ZVZCS and S1 switch turns off under ZVZCS.

      During this mode, the resonance between L and C2 makes Vc2 positive.

      Mode 11:

      D1 diode turns off under ZVZCS. During this mode, L inductor resonates with C1 and C2 capacitors.

      Mode 12:

      o

      V

      V

      2 diode D3

      i

      turns on under ZVZCS. The capacitor voltage

      i (Vc2) is equal to Vi and the inductor current ( iL) decreases

      Lm Co

      i

      i

      L

      D3

      and the capacitor voltage (V resonance between L and C1 Mode 13:

      c1) increases due to the

      Fig3 proposed soft switching flyback

      The features of switches are:

      the S1 switch is operated with a PWM control and S2 switch is activated only during transition of switch S1.

      D2 diode turns on under ZVZCS, due to the existence of L inductor in the current flow path of this diode, D3 diode turns off under ZVZCS and switch S2 can be turned off under ZVZCS.

      Mode 14:

      D2 diode turns off under ZVZCS. During this mode, the capacitor voltage (Vc1)

      is linearly increased by the constant magnetizing inductance current ILm.

      Strength:

      • Soft switched operation

      • Reduced Voltage spikes

      • High efficiency

      • High frequency operation ZVZCS switching

   Weakness:

   • Leakage energy can be recovered

   • No consideration of leakage inductance in converter analysis

4. A Novel ZCS-PWM Flyback Converter With a Simple ZCS-PWM Commutation Cell. [4] [7]

Fig 4 ZCS-PWM Flyback Converter With a Simple ZCS-PWM Commutation Cell [4]

It is composed of an isolated transformer, a switch Sm, a diode D1, and output filter Co. The second section is a ZCS- PWM commutation cell to provide the ZCS on the switch Sm. It is composed of the auxiliary diodes D2, D3, the resonant inductor Lr, the resonant capacitor Cr, and auxiliary switch Sa, which are rated for small power when compared to the output power.

The following assumptions are made during one switching cycle of operation

1. All components and devices are ideal.

2. The magnetizing inductor Lm is large enough to assume that the current IL on the inductor Lm is constant [5]

3. The output filter capacitor Co is large enough so as to assume that the output voltage Vo is constant and ripple-

   free.

4. Input voltage Vin is constant. Following parameters are defined: n = n1/ n2

   nL = Ls/Lr

   Mode 1:

   switches Sm and Sa maintain turn-off state. The energy stored in magnetizing inductor Lm is delivered to output filter capacitor Co and loaded through the ideal transformer anddiodeD1[6].

   The stage ends when the current in the leakage inductor Ls reaches ILm and diode D1 turns off with ZCS [4]

   Mode 2:

   iLs(t) in leakage inductor Ls reaches ILm and diode D1 is turned off with ZCS. The magnetizing inductor Lm and the leakage inductor Ls are linearly charged by Vin.This achieves a same operating behavior as like a conventional PWM flyback dc/dc converter.

   operating at turn-on state. Mode 3:

   Switch Sa is turned on as the initial value of resonant current iLr(t) is zero, ZCS in auxiliary switch Sa can be achieved. The resonance of resonant inductor Lr and capacitor Cr is started by Vin, Lr, Cr, D3, and Sa.

   This state ends when the resonant current iLr(t) drops to null again.

   Mode 4:

   In this stage, the resonant behavior in stage 3 is maintained, but the resonant route changes following Vin, Lr, Cr, D2, and Sm. The resonant voltage vCr(t) decreases and the resonant current iLr(t) rises negatively

   Mode 5:

   As the resonant current iLr(t) rises to ILm and its flow path is changed by Vin, Lr, Cr, D2, and the antiparallel diode of Sm hence no current flows through the main switch Sm along with the diode D3 is naturally closed at t = t3 hence no current flows through the auxiliary switch Sa. It is the best time to turn off the switches Sm and Sa under ZCS

   Mode 6:

   D1 is turned on with ZCS hence another resonant route is formed via Cr, Lr, Ls, nVo, and D2. The resonant voltage vCr(t) continuously decreases. The resonant current iLr(t) rises toward zero the current iLs(t) in the leakage inductor drops toward zero value. This stage ends when the energies stored in the resonant inductor Lr and the leakage inductor Ls are completely transferred to the resonant capacitor Cr. Mode 7:

   The resonant current iLr(t) and the current iLs(t) in leakage inductor Ls equals to zero, and the diodeD2 is naturally turned off with ZCS. The energy stored in magnetizing inductor Lm is continuously loaded through D1. This operating behavior is the same as the conventional PWM flyback dc/dc converter operating at turnoff state.

   After stage 7, the circuit operation returns to the first stage.

   Strength:

   • Soft switched operation

   • Reduced Voltage spikes

   • High efficiency

   • High frequency operation ZCS operation of power switches

     Weakness:

   • Leakage energy can be recovered

   • Voltage stress of output diode can be reduced

CONCLUSION:

A detailed study on different soft switched flyback converter topologies have been presented with their merits and demerits. All the topologies were aimed to performed switching transition of the semiconductor switches either in a ZCS or in ZVS condition employing different soft switching cells and active clamping methods .this soft

switching cells used auxiliary switch which is also operated under ZCS or ZVS . By doing this converters were operated with very high frequency, which improves converter efficiency, power density and reduces converter size and weight. In addition the leakage energy of flyback transformer was utilized properly or sent to the input side for further improvement of converter efficiency.

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2. Wang, C.-M & Su, C.-H & Yang, C.-H. (2006). ZVS-PWM flyback converter with a simple auxiliary circuit. Electric Power Applications, IEE Proceedings -.vol pp 116 – 122.

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