A new zero voltage transition interleaved flyback converter

Received Nov 2, 2021 Revised Mar 13, 2022 Accepted Mar 27, 2022 The paper introduced a new zero voltage transition (ZVT) interleaved flyback converter which has two similar flyback converters. Two flyback converters are in parallel connection and auxiliary circuit in this converter provides ZVT condition for all of the main switches and also provides zero current switching and zero voltage zero current switching (ZVZCS) conditions for the auxiliary switch. Also, ZCS conditions are created for diodes turning off, so reverse recovery problem is solved. The auxiliary circuit in the suggested converter is modular, and by adding parallel branches to the flyback circuit, this circuit can provide soft switching conditions for all switches without significantly change. A complete analysis of the converter is provided and its operating intervals are explained. A 180 W laboratory prototype of the converter is made to approve the theoretical calculations. The experimental results show 7.7% increase in efficiency.

diodes are reduced, but the switch voltage is higher than 2Vin. A zero-voltage interleaved buck converter is offered in [21] in which there is an active switch in its auxiliary circuit structure. In suggested converter, each the semiconductor devices achieve soft switching operation but the converter has numerous auxiliary elements and the auxiliary switch has high voltage and current stresses. With two transformers in a ZVS interleaved flyback converter, the over-size problem of the transformer is eliminated and the converter efficiency is prospered. The major demerit of this converter is the dependency of soft switching condition to the load. In such a way that ZVS condition in light load is eliminated [22].
A new interleaved half-bridge flyback converter with ZCT technique in [23] is presented and the efficiency prospered and also the switching-off losses reduced. Parameter variations lead to a power imbalance problem in the proposed ZCS interleaved converter. A ZVS parallel interleaved current-double converter which reduces the current stress and the current ripple is recommended in [24]. Furthermore, on account of the coupling of the output inductors, the number of the output inductors and the output current ripple are decreased. Many numbers of magnetizing components, high conduction losses and duty cycle losses are the disadvantages of the converter. Types of non-isolated converters like as boost, buck, and buckboost are illustrated in [25], [26] in which a conventional auxiliary circuit has applied. The ZVT auxiliary circuit only uses an auxiliary switch which creates soft switching conditions for two main switches of interleaved structure. The most important problem in the suggested circuit [25] is the intense voltage stress in its auxiliary switch, which increases the RDS (on) of the auxiliary switch and as a result increases its conduction losses and converter [26] problems are that the number of auxiliary circuit components is numerous and the voltage stress on the auxiliary switch is measured also intensive.
In this study, a new auxiliary circuit for an interleaved flyback converter has presented so that it can be extended to more parallel branches and because of the low number of elements, soft switching of elements and low circulating current of the auxiliary circuit, this circuit does not inflict considerable losses on the converter. Since all converter diodes also turn off under ZCS conditions, there is no problem of reverse recovery in this converter. For this reason, the efficiency compared to previous converters has increased significantly. In part 2, the ZVT interleaved flyback converter operating analysis is provided. Design technique of the converter is proposed in part 3 and the control circuit of this converter is proposed in part 4. Experimental results of the suggested proposed ZVT interleaved flyback converter have exhibited in part 5. Part 6 compares the efficiency of the ZVT interleaved flyback converter with a conventional interleaved flyback converter.

CIRCUIT DESCRIPTION AND OPERATION 2.1. Circuit structure
The introduced converter has exhibited in Figure 1 which is composed of the main switches M 1 and M2, the output diodes Do1 and Do2, output filter capacitor Co, two isolating transformers which consist of primary windings L p1 and Lp2, leakage inductances Lk1 and Lk2, and secondary windings Ls1 and Ls2. The auxiliary circuit has an auxiliary switch Ma, auxiliary inductor La, auxiliary diodes Da and Db, and auxiliary winding Lb coupled to the main winding, a capacitor Cb, snubber diodes Ds1 and Ds2, and snubber capacitor Cs. The first isolating transformer includes the primary winding Lp1, the secondary winding Ls1 and the auxiliary winding Lb. The second isolating transformer includes the primary winding Lp2 and the secondary winding Ls2. The turn ratio of Ls1/Lp1 is ns1/np1=n and the turn ratio of Ls2/Lp2 is ns2/np2=n.

Operating of the suggested converte
The checking of the suggested converter can be simplified, therefore the following assumptions have presented: i) All elements design ideal; ii) The capacitor Co has a large value, in order that the output voltage can be fixed; iii) The Capacitor Cb has a large value, in order that its voltage can be fixed and identical to Vcb; and iv) Magnetizing inductances Lm1 and Lm2 are same and large, thus the current ILm is considered fixed: Lm1=Lm2=Lm.
To evaluate the suggested converter, the first 9 intervals of the converter are fully analyzed. The equivalent circuit of each of the 9 intervals has exhibited in Figure 2. Figure 3 indicates the key waveforms of the operating intervals. Before the interval 1: It can be presumed that the main switches M1 and M2 are off and diodes Do1, Do2 and Db are on and transmit current, and so the snubber capacitor voltage VCs is identical to Vin+Vo/n and the capacitor voltage VCb is identical to mVo.
Interval 1 [to-t1]: At the beginnig, the auxiliary switch Ma turns on under ZCS condition, because an auxiliary inductor La with the auxiliary switch Ma are in series. Because of the output diodes Do1 and Do2 conduct, the constant voltage mVo+Vo/n is placed across Lk1 and La and also across Lk2 and La. Therefore, the snubber circuit diodes Ds1 and Ds2 start to conduct and the auxiliary switch current IMa increases linearly. Since the values of Lk1 and Lk2 are small, in this interval snubber capacitor voltage VCs has considered constant and identical to Vin+Vo/n, therefore the auxiliary switch current IMa is calculated from (1).
By increasing the auxiliary switch current IMa, current in output diodes Do1 and Do2 is reduced and when the auxiliary switch current IMa achieves ILm1+ ILm2, the output diodes Do1 and Do2 turn off with ZCS and current mode finishes. Duration of this mode is: where: Because the anode voltage of Db is less than (Vin+Vo/n)/2, the resonance stops after half a cycle and at the finale of this interval the capacitor Cs has fully discharged and this mode ends. Then, the duration of current interval can be derived: Interval 3 [t2-t3]: With complete discharge of Cs, the body diodes of the main switches M1 and M2 (Db1 and Db2) start to conduct under ZV. Therefore, the constant voltage Vin-mVo is reversed across the auxiliary inductor La and the auxiliary switch current IMa reduces linearly. The main switch M1 can turn on with zerovoltage. This interval ends by turning the body diodes of the main switches M1 and M2 (Db1 and Db2) off. Whenever the auxiliary inductor current ILa is identical to ILm1+ILm2, the body diodes of the main switches M1 and M2 (Db1 and Db2) have turned off. The following (10)-(11) obtain the auxiliary switch current IMa and the duration of this interval: Interval 4 [t3-t4]: Current transmission from the body diode of the main switch M1 to the main switch M1 occurs and increases linearly with the same slope and the body diodes of the main switches M1 and M2 (Db1 and Db2) turn off with ZCS. While the current of the main switch M1 achieves ILm1, finally the snubber diode Ds1 turns off with ZCS. The main switch current IM1 and the time of this interval are derived from (12)-(13).
Interval 5 [t4-t5]: This interval starts as soon as the snubber diode Ds1 turns off and the ILm2 current charges the capacitor Cs linearly. Also, current of the auxiliary switch Ma achieves zero and the auxiliary switch Ma turns off under ZVZC condition on account of the presence of capacitor Cs and the auxiliary circuit entirely exits from the converter. Also the current of the main switch M1 is fixed and identical to ILm1.
Interval 6 [t5-t6]: In this mode, while the Cs capacitor voltage achieves to Vin+Vo/n, the snubber diode Ds2 turns off and the diode Do2 conducts, and the magnetizing inductance Lm2 starts discharging to the the output. Interval 7 [t6-t7]: Interval 7 occurs simultaneously as the main switch M1 is turned off, and the magnetizing inductance Lm1 charges the snubber capacitor Cs and the output diode Do1 has also turned on under ZCS condition. The equation of duration is expressed as: Interval 8 [t7-t8]: The auxiliary diode Db turning on occurs at the beginning of this interval. Because both of the main switches M1 and M2 are off, the magnetizing inductances are discharged to the output. In this mode, the resonance occurs between the leakage inductance of the transformer Lk1 and the snubber capacitor Cs and the voltage of the main switch M1 is increased resonating. This interval ends with the complete discharge of the leakage inductance Lk1. Finally, the main switch voltage (VM1) and transformer leakage inductance current (ILk1) expressions for this interval are as follows: Where: Duration of this interval is: Interval 9 [t8-t9]: by fully discharging the energy of the transformer leakage inductance Lk1 and turning snubber circuit diode Ds1 off under ZCS condition, this interval begins and therefore voltage across the main switch M1 decreases to a fixed amount and identical to Vin+Vo/n, and similar to a regular flyback converter in off switch mode, both magnetizing inductances are discharged to the output.

DESIGN METHOD
In this part, the design of the converter is discussed. The converter has prepared for 300V input voltage, 40V output voltage, and 180W output power. Switching frequency can be selected at 100kHz. The turn ratio of L b/Ls1 should be selected in such a way that the anode voltage of Db is less than (Vin+Vo/n)/2. If Lb turns is defined as nb and Ls1 turns as ns1 and Ls2 turns as ns2, then: The snubber capacitor Cs is selected like a turn-off snubber capacitor according to (22) It can be presummed that the switch current fall time is indicated by abbreviation tf, Isw is the switch current before the switch is turned off and Vsw is the switch voltage after the switch is turned off. To prove soft switching in practice, the snubber capacitor C s should be much greater than CSmin. To prove the snubber capacitor Cs discharge, the amount of Cb should be greater than Cs. La is designed like a turn-on snubber. For appropriate selection of La, there is a relationship between Ma maximum current and Dmax which can be indicated by: Lb should be much greater than La. The maximum current and voltage of auxiliary switch Ma are obtained as follows:

CONTROL CIRCUIT
The closed loop system digital control circuit of the suggested converter is exhibited in Figure 4. A SPARTAN-6 FPGA is selected as the PI digital controller hardware. The output voltage feedback is directed to the analog-to-digital converter (ADC), and then the output of the ADC converter is appraised with the reference voltage (Vref). The error voltage (Verror) is directed to the PI digital controller which produces the  Figure 4. The suggested interleaved flyback converter with implemented digital control circuit

EXPERIMENTAL RESULTS
A new ZVT interleaved flyback converter has demonstrated. The picture of the tested converter has exhibited in Figure 5. The design values and components of the converter are exhibited in Table 1. In the Figure 6(a), gating signals waveforms of main switches and auxiliary switch is displayed. The ZV conditions for the main switches are illustrated in Figures 6(b) and 6(c). At the turn on instant of the main switches, their currents are negative and the body diodes are conducting, therefore the main switches can turn on under ZV. The ZCS condition for the auxiliary switch is illustrated in Figure 6(d). The auxiliary switch current increases with the slope, thus the auxiliary switch can turn on with ZCS. The auxiliary switch current decreases with the slope and also the voltage has identical value to zero, therefore the auxiliary switch can turn off with ZVZCS technique. The output diodes are also soft switched as shown in Figures 6(e) and 6(f). The output diodes can turn on and off with ZCS. The operation of this converter is justified by the experimental results.   6. EFFICIENCY Figure 7 shows the suggested ZVT interleaved flyback converter efficiency diagram and as well as the hard switching interleaved converter efficiency diagram. According to Figure 7, both efficiencies are designed for 180 W. The efficiency has measured at 5 various loads and when compared to the hard switching interleaved converter, the efficiency has increased by 7.7%. In Table 2, the losses of the suggested ZVT interleaved flyback converter with a hard switching interleaved flyback sample have compared. In the presented Table, the rise and fall times of the converter switches currents are indicated by abbreviations tr and tf, respectively. Also, trr can be considered as reverse recovery time of the diodes. In addition, Cout can define as the switches output capacitance, Rds is supposed equivalent to the switches on state resistance, Iave can be considered as an average current of output and auxiliary diodes, VF can also be considered as a forward voltage of diodes, and fsw is switching frequency. Furthermore, all semiconductor elements can turn on and off with soft switching technique, accordingly switching losses have significantly declined.  converter is much less than the regular flyback converter due to its interleaved structure. Because of the a few numbers of components, and low circulating current in the auxiliary circuit, this circuit does not inflict considerable losses on the converter. The practical results of the suggested converter exhibit a 7.7% increment in efficiency at full load versus the hard switching counterpart.