Zero-Voltage-Switching Two-Switch Forward Converter with Asymmetric Two Transformers

In this paper, a zero-voltage switching (ZVS) two-switch forward converter with asymmetric two transformers and an active-clamp circuit is proposed to satisfy wide ZVS range, wide input voltage range and high efficiency requirements. Each transformer acts not only the power transformer but also an inductor during different times of the switching period. The asymmetric two transformers can result in wide ZVS range and reduce the conduction losses in the primary side. The active-clamp circuit can help the transformer reset and recycle the energy stored in the leakage and magnetizing inductances of the transformers. The output capacitances of power switches and the leakage inductance are utilized to realize the ZVS operation for all the switches to reduce the switching losses. Since the proposed converter does not have the duty cycle limitation, the converter is suitable for wide input voltage range. The operating principles and design considerations of the proposed converter are given. Finally, the experimental results of a 300 W prototype are provided to verify the validity of the proposed converter.


Introduction
The forward converter is one of the most popular DC-DC converter topologies for low and medium power applications [1,2].In the conventional forward converters, a tertiary reset winding is necessary to avoid transformer core saturation.Forward converters with RCD clamp reset circuit can store and dissipate the transformer magnetizing energy in the RCD network.However, the power dissipation on the resistor reduces the efficiency.These two converters suffer from high voltage stresses on the power switches.Moreover, the power switches are operated in the hard switching such that the switching losses are considerable.
The active-clamp forward (ACF) converter is one of the most attractive topology to realize zero-voltage switching (ZVS) operation [3][4][5][6].The active-clamp circuit is composed of the clamp switch and the clamp capacitor to provide the flux reset of the transformer, and to recycle the energy stored in the leakage and magnetizing inductance of the transformer.The spike voltage at the transformer primary side can be suppressed such that the voltage stress of the main switch is limited.In order to achieve wide ZVS range, the converter is generally designed to use a small magnetizing inductance such that the magnetizing current has higher negative peak value to discharge the output capacitor of the main switch.It facilitates the ZVS turn-on for the main switch over wide load conditions.However, the method results in the high peak current in the primary side, which increases conduction losses by 30-50% [7].On the other hand, both switches of ACF converter have high voltage stresses, i.e., the sum of input voltage and reset capacitor voltage, such that it is not suitable for high input-voltage applications.
In practical high input voltage applications, the two-switch forward converter [8][9][10] is often applied in order to use lower voltage rating of power switch.However, 50% maximum duty ratio limitation is necessary to insure the flux reset of the transformer.Therefore, it is not suitable for wide input voltage range applications [11].
In order to have low-voltage stresses on the main switches and wide ZVS range for all the switches, a structure of two-switch forward converter with an active-clamp circuit and asymmetric two transformers is proposed.Each transformer acts as not only a power transformer but also an inductor such that there is no output inductor in the secondary side.The magnetizing inductances of two transformers are different.It makes the proposed converter have wide ZVS range for two main switches and reduce conduction losses in the primary side.
The output capacitances of power switches and resonant inductance are utilized to realize the resonance such that the ZVS operation for all the switches can be achieved to reduce the switching losses.The maximum duty cycle is not limited by 50%, so that the proposed converter is suitable for wide input voltage range applications.
The circuit configuration, operating principle and design considerations of the proposed converter are presented in this paper.Experimental results on a 300W prototype converter are provided to verify the validity of the proposed converter.

Circuit Configuration and Operating Principle
The circuit configuration of the proposed converter is shown in Fig. 1  C , i=1~3, represent the output capacitances of all the switches.With the same PWM control signal, two main switches are turned on and off simultaneously.On the other hand, the active-clamp switch is driven complementarily to the main switches with a small dead time to realize the ZVS turn-on operation.The key operating waveforms of the converter in the steady state are illustrated in Fig. 2.
The following assumptions are made in the analysis of the operating modes.1) All MOSFETs are considered to be ideal except for their body diodes and output capacitances.2).The turn ratios of 1 T and 2 T are identical of n : 1 .However, their magnetizing inductances are different with .
3) The clamp capacitor C C is large enough so that the clamp voltage C V are treated as a constant voltage.4).
o C is large enough to neglect the output voltage ripple.
According to the switching states of the switches and diodes, each switching period is divided into eight modes.The equivalent circuits are given in Fig. 3.The operating principle is described as follows.
In this mode, the currents Lr i and 2 Lm i are the same.This mode ends at 1 t when 1 S and 2 S are turned off.Clearly, the high inductance 2 m L can reduce the peak value and the root-mean-square (rms) value of Lr i in the primary side.Thus, the conduction loss in the primary side can be reduced.
. Since the resonant capacitances are very small, the duration of this mode is very short.At time 2 t , the primary total voltage across 1 T and 2 T , , is equal to zero when voltage where the equivalent capacitance The mode ends at time 3 t when and the voltage S is equal to zero such that 3 S should be turned on to realize ZVS operation before Lr i reverses its direction.In this mode, the current Lr i can be equated as The mode ends at time 4 t when T acts as an inductor to store the energy.In this mode, the current Lr i is the same with 1 Lm i and its direction will reverse to flow through 3 S .
1 Lm i and 2 Lm i can be expressed as This mode ends at time 5 t when the switch 3 S is turned off.

Mode 6 [
Since the capacitances ri C , i=1~3, are very small, the duration of this mode is very short.This mode ends at time This mode ends at time 7 t when S conduct so that the voltages across 1 S and 2 S are zero.The voltage across the resonant inductor r L is equal to in V , and thus Lr i is linearly increasing.Before Lr i reverses its direction, 1 S and 2 S can be turned on under ZVS operation.In this mode, Lr i can be expressed as The mode ends at time 8 t when the commutation between

Design Considerations
To simplify the steady-state analysis, we neglect the small dead time interval.Only the modes 1 and 5 are considered herein.During mode 1 and mode 5, the voltage across , respectively.Moreover, the voltage across , respectively.Applying the volt-second balance principle to the V can be obtained as where D is the duty cycle.Moreover, the voltage conversion ratio can be obtained as According to the operation of clamp capacitor current, the clamp voltage ripple can be derived as where the current ripple The voltage stresses of all the switches are given by The maximum currents of all the switches can be approximately expressed as where s T is the switching period.To ensure ZVS operation for switch 3 S , the initial energy stored in r L must be greater than the energy required to charge completely such that the ZVS range for 1 S and 2 S is wide.Based on Eq. ( 9), the ZVS condition for 1 S and 2 S is given by The larger inductance r L is selected in Eqs. ( 19) and (20) to ensure ZVS turn-on operation for all the switches.A feedback control system for the proposed converter is built to obtain good performance of output voltage regulation in spite of variations in the input or load.Based on the Bode plot measurement of converter control-to-output by the dynamic signal analyzer (Agilent 35670A), a Type II compensator is designed by the K factor method [12] such that the control loop gives a crossover frequency of 3 kHz and a phase margin of  53 .The compensator circuit is shown in Fig. 4 with Fig. 4. Type II compensator.
Table 1.Parameters of The Proposed Converter

Experimental Results
To verify the effectiveness of the proposed converter, a W 300 prototype converter with 350-400 V input and 24V output operating at 100 kHz is built and tested.The circuit parameters are listed in Table 1.
The waveforms of gating signals of all the switches at full load are shown in Fig. 5.The steady-state waveforms of gating signal of 1 S , input voltage and output voltage at full load are depicted in Fig. 6.The switching waveforms of all the switches at full load and half load are shown in Fig. 7.It can be seen that the drain-to-source voltages have become zero before the switches are turned on.Therefore, ZVS turn-on operation of each switch is achieved.From Fig. 9, it is revealed that the output voltage can be well regulated due to the compensator design.Moreover, the output voltage response under a step change in load from 300 W to 150 W and vice verse is shown in Fig. 10.It can be seen that the effect of load variations on the output voltage can also be well rejected.The control circuit adjusts the duty cycle to compensate for changes in operating conditions.The measured efficiency at different load conditions is list in the Table 2.At the rated output power, the efficiency is 89.7%.The highest efficiency of the experimental converter is 90.1% at the output power 210W.

Conclusion
A two-switch forward converter with asymmetric two transformers and an active-clamp circuit is proposed to be suitable for wide ZVS range, wide input voltage range and high efficiency applications.The asymmetric two transformers can result in wide ZVS range and reduce the conduction losses in the primary side.The active-clamp circuit can recycle the energy stored in the leakage and magnetizing inductances of the transformers.The output capacitances of switches and the leakage inductance are utilized to realize the ZVS operation of all the switches such that the switching losses are reduced.A Type II compensator is designed for output voltage regulation.The experimental results of a prototype with 350-400V input and 24V/12.5Aoutput verify the analysis and performance of the proposed converter.

1 D and 2 D 2 D i decreases to zero and 2 D
finishes.At this instant, turns off.After the eight modes, the next switching cycle starts.

L
over one switching period, the clamp voltage C

3 .
Based on Eq. (4), the ZVS condition for switch 3 S ensure ZVS operation for 1 S and 2 S , the initial energy stored in r L is required to discharge mode 7. From Eq. (6), it is clear that the low inductance 1 m L can result in high negative peak value of Lr i .It facilitates to release the energy stored in

Fig. 5 .
Fig. 5. Waveforms of gating signals of all the switches.

Fig. 6 Fig. 7 .
Fig. 6.Waveforms of 1 gs v , o V and in V at full load.

Fig. 10 .
Fig. 10.Output voltage regulation under the step load change.
which is the two-switch forward converter equipped with asymmetric two transformers and an active-clamp circuit.It contains of two main switches 1
Key waveforms of the proposed converter.

Table 2 .
Efficiency at Different Load Conditions