Proc. Int. Conf. on Control System and Power Electronics, CSPE
An Improved Push-Pull Converter with ZVS-ZCS in
Active and Passive Switches for Low Voltage
Applications
K.Deepa1, Dr.M.Vijaya Kumar2
1
Assistant Professor, EEE Department,
Amrita Vishwa Vidyapeetham University,
Amrita school of engineering, Bangalore, Karnataka, India
2
Professor, EE Department,
JNTUA, Anantapur, Andhra Pradesh, India
1
deepa.kaliyaperumal@rediffmail.com
Abstract. Isolated dc-dc push-pull converter with soft switching in both active
and passive switches of primary and secondary circuits is proposed. The
proposed converter is well suited for low voltage high frequency applications as
the internal capacitance of the switch, diode and leakage inductance of the
transformer are engrossed in the resonance. The series resonant circuit in the
secondary accomplishes ZCS in the rectifying diodes thus removing the reverse
recovery problem. The primary series resonant circuit employs the energy
stored in the leakage inductance during resonance and pull off ZVS in the
active switches. Soft switching (ZVS/ZCS) realized lessen switching losses and
augment the efficiency of the converter. Maneuver of the converter, theoretical
design and analysis, PSIM simulation results and hardware results obtained
from the laboratory prototype operating at 50 kHz are presented.
Keywords: Push-pull Converter, Zero Voltage Switching, Zero Current
Switching.
1 Introduction
The requirements of high power density and miniaturization have enhanced the need
for converters with high switching frequency. Numerous advantages of high
switching frequency such as decrease in weight of passive components like inductor,
transformers; increased power density is limited by switching losses across the
switches. Hence high switching frequency is applicable only if switching losses are
reduced. This is accomplished by resonant techniques like zero-voltage switching
(ZVS) and zero-current switching (ZCS) [1]-[3]. Thereby several new topologies
arose as reported in [4] – [9]. In this paper, an improved ZVS QRC push-pull
converter is presented. The circuit shows many excellent features such as transformer
isolation, full exploitation of the transformer core high power density [5] – [9], [11]
and no ringing of the voltage across the switches. The operating principle and
theoretical study of the circuit are proposed. These converters have a drawback of
© Elsevier, 2012
519
high stress across the rectifying diodes in the secondary. Hence a series-resonant
circuit to remove the reverse-recovery problem of the rectifying diodes [10] is
proposed as an improvement to the original counterpart [8 – 9]. From the theoretical
study, the operating waveforms as well as from all necessary mode equations the
formulae required for the design are deducted. The theoretical results are compared
with the waveforms obtained from the proposed converters.
2 Improved Push Pull ZVS QRC
The modes of operation of improved ZVS – ZCS push-pull QRC shown in Fig.1. is
similar to the primary ZVS push-pull QRC [ 8] – [9], with an added feature of
secondary ZCS in the rectifying diodes(D1 and D2). The primary active switches (S1
and S2) are switched one after the other so that the magnetization of the inductor and
transformer core does not saturate and that the energy is transferred via the high
frequency transformer (isolation) to the load. The presence of magnetizing inductance
in the isolating transformer helps in maintaining continuous current in the inductor
even in light load conditions. The resonant inductor (Lr1 & Lr2) and the resonant
capacitor (Cr1 & Cr2) form a series resonant circuit to exhibit ZVS across the switches.
Fig. 1.Circuit diagram of a improved push- pull
ZVS-ZCS-QRC
Fig. 2 Idealised resonant waveforms of
SISO ZVS push-pull converter
3 CCM Topological States
The modes of operation in continuous conduction mode (CCM) of the proposed
converter for one cycle is as discussed below.
Mode 1 (0<t<t1 ) - Power transfer interval: - Upper limb: This mode Td1 starts with
an initial condition of ILr1 = Im/2. Both the active switches are off and the diodes D2 is
forward biased and D1 is reverse biased respectively. In this interval power is
transferred from primary to secondary. The resonant inductor with initial charge acts
as a constant current source and charges the resonant capacitor voltage from 0 to 2V S.
Lower limb: Same as upper limb in mode 4.The energy stored in the transformer
leakage inductance is discharged to charge the secondary resonant capacitor.
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Mode 2: (t1<t<t2 ) - Resonant upper transition interval 1:- Upper limb: Interval Td2
begins when S2 is turned ON, S1 is OFF, diode D2 is forward biased and D1 is reverse
biased. The resonant elements Lr1 and Crl form the resonant circuit. The resonant
inductor transfers its energy to the resonant capacitor which over charges the
capacitor from 2VS to a value of 2VS+ ImZo/2. The resonance still continues and the
resonant capacitor voltage, Vcr1 discharges from 2VS+ImZo/2 to 2Vs and hence called
as resonant upper transition interval 1. The resonance between Lr1 and Cr1 ends when
the voltage across VCr1 = 2Vs. The initial conditions are V cr1 (0) = 2 V S , ILr1(0) = Im/2.
Lower limb: Same as upper limb in mode 5.The secondary circuit is the same as in the
previous mode.
Mode 3: (t2 <t<t3) :- Upper limb: In interval Td3, S1 and S2 conditions are the same as
mode 2 , diode D1 is forward biased and D2 is reverse biased. The resonant capacitor
voltage, Vcr1 discharges from 2VS to 0, resonant inductor current, iLr1 charges
negatively (i.e.) it increases linearly to -Im/2 at t = t3. Lower limb: Same as upper limb
in mode 6. In the secondary circuit the energy stored in the capacitance is discharged
back to the transformer secondary.
Mode 4 (t3<t<t4) - Power transfer interval :- Upper limb: In this interval Td4, S1 is
OFF and S2 is OFF, D1 is forward biased and D2 is reverse biased. This interval is
known as power transfer interval and is similar to mode 1. The resonant capacitor
voltage, Vcr1 = 0. Resonant inductor current, i Lr1 discharges negatively through the
switch diode. The initial conditions V cr1(0) = 0 , ILr1(0) = - Im/2. Lower limb: Same as
upper limb in mode 1. The energy stored in the transformer leakage inductance is
discharged to charge the secondary resonant capacitor in the negative direction.
Mode 5: (t4<t<t5) - Resonant lower transition interval :- Upper limb: In interval
Td5, S1 is ON and S2 is OFF, D1 is forward biased and D2 is reverse biased. This
interval is known as resonant lower transition interval. Resonant inductor current, i Lr1
discharges negatively (i.e) increases linearly. The initial condition is ILr1(0) = (Im/2)cosα . This mode terminates when resonant capacitor (Cr2 ) voltage reaches 2VS.
Lower limb: - Same as upper limb in mode 2.The secondary circuit works in same
manner as that of mode 4.
Mode 6: (t5<t<t6): Upper limb: In interval Td6, S1 is ON and S2 is OFF, D2 is forward
biased and D1 is reverse biased. The resonant capacitor voltage, Vcr1 is 0, Resonant
inductor is positively charged and current ( iLr1) increases linearly to Im/2. Lower
limb: - Same as upper limb in mode 3. In the secondary circuit the energy stored in
the capacitance is discharged back to the secondary circuit.
At the end of this mode, next cycle starts and the working of the converter
continues.
4 Simulation Results
The open loop simulation circuit of the converter shown in Fig.1 is carried out in
PSIM. The output voltage and current obtained are 3.3V and 400mA and is shown in
Fig.3 (a). The peak voltage of 78V across both the resonant capacitor voltage is as
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shown in Fig.3 (b) and is similar to the theoretical waveform as shown in Fig.2 which
also confirms the ZVS turn on of the active switches. It is obvious from the fig. 3(d)
that at the zero crossing of the diode currents, the diode stops conducting (i.e.) ZCS
turn off of the passive switches. Fig. 3(c) shows the voltage across the rectifier diode
VD1 = VD2 = 5V and the rectifier diode currents iD1 = iD2 = 3A. From this it is observed
that there is no oscillation and voltage spike on the rectifier diodes.
5 Experimental Results
A 2W output power (3.3V, 400mA) prototype converter is built in the laboratory to
verify the operation principle of the improved ZVS-ZCS QRC Push-pull converter.
The parameters of the converter are: Vin = 15±5V; V 0 = 5V, S1 and S2 : IRF840; D1
and D2: Fast recovery schottky diodes BA159; n = 0.6; Lf = 4.7 µH; Cf = 220 µF;
switching frequency: fs = 50 kHz; Pulses are derived from dsPIC30f4011; Driver cum
isolation: TLP250.
5.1 Design Details
The formulae required for the design of the proposed converter are as given below
1) To obtain ZVS the condition to be satisfied is ImZ0 > 2VS => Z0 = 25(chosen)
2) The characteristic impedance Z0 = √(Lr/Cr)
3) The resonant frequency is fO = 1/ 2π√( LrCr)
Using the above equations for a resonant capacitance Cr1 = Cr2 = 0.048µF, the
resonant inductance Lr1 = Lr2 = 45µH.
5.2 Result Analysis
Fig. 4 shows the experimental results at full load. Fig. 4(a) & (b)gives the nominal
converter output voltage (VO), current (IO) and the gating pulses derived from
dsPICf4011.The resonant capacitor voltages Vcrl and Vcr2 are shown in Fig. 4(d) &
(e). It is observed from this figure that the switches S1 and S2 are turned on, when Vcrl
and Vcr2 becomes zero respectively and the waveforms are same as that of the
theoretical waveform shown in Fig 2 and the simulated waveform is shown in Fig
3(b). Fig. 4(c) shows the rectifier diode currents iD1 and iD2, from which it’s observed
that there is no oscillation and voltage spike across the rectifier diodes. From the
analysis of waveforms obtained as shown in Fig 4, it’s observed that the controller
produces a pulse of 31.3% and another pulse of 44.3% is obtained from the logic gates
with amplitude of 15V approximately. This pulses obtained triggers the voltage
controlled device (MOSFET IRF840). The resonant capacitor voltage obtained are
60V, 62V respectively for each switches and it is observed that the switches are
turned ON when the voltage across the capacitor crosses zero which reduces the
switching losses and thereby increases efficiency. The switching frequency of the
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prototype is 50 kHz and so the sizes of the components used reduces to ¼ of the
original size.
(a)
(b)
(c)
(d)
Fig 3. Simulation waveforms (a) Output voltage & current (b) Resonant capacitor voltages
with gating pulses (c) Diode current & voltages (d) Diode current with gating pulses.
(a)
(b)
(d)
(c)
(e)
Fig 4. Harware waveforms (a) Output voltage & current (b) Gating pulses (c) Diode currents
(d) and (e) Resonant capacitor voltages with gating pulses.
6 Conclusions
In this paper, design, simulation and analysis of a improved ZVS push pull converter
for high frequency low voltage application is carried out. The hardware results
obtained are similar to that of the simulated results of the converter from PSIM
software. The hardware waveform of the improved ZVS QRC push-pull converter has
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all the advantages of its original counterpart such as: (1) the switches realize ZVS in a
wide load range with the use of the energy stored in the filter inductances. (2) Less
switching losses and higher efficiency due to soft switching commutation technique
used. (3) Compact since the inductor and transformer occupies less size due to high
switching frequency selection. And in addition to these features the rectifier diodes
commute naturally without the oscillation and voltage spike, which reduces the stress
across the diodes and hence the losses.
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