IET Power Electronics Research Article Secondary-side phase-shifted full-bridge converter with reset winding ISSN 1755-4535 Received on 26th November 2019 Revised 26th February 2020 Accepted on 16th April 2020 E-First on 19th May 2020 doi: 10.1049/iet-pel.2019.1470 www.ietdl.org Wei Liu1, Qinglin Zhao1 , Deyu Wang1, Kunlun Li1, Yujie Wang1 1School of Electrical Engineering, Yanshan University, Qinhuangdao, People's Republic of China E-mail: powerzql@ysu.edu.cn Abstract: A resonant inductor and two clamping diodes are introduced into the secondary phase shift full-bridge converter (SPS-FBC), which can well suppress voltage spike across the rectifiers. However, the resonant inductor and two clamping diodes will bring some other problems. In this study, an auxiliary winding in series with the resonant inductor is added to the SPS-FBC to reset the clamp diode current rapidly. The auxiliary winding not only reduces the circulation losses of the switches and the clamping diodes, but also makes the clamping diodes naturally turn-off and avoids the reverse recovery even when the output filter inductance and magnetising inductance are relatively large. The principle of operation and performance are illustrated and verified on a 1.5 kW experimental circuit. In addition, the efficiency and losses of the converters with/without reset winding are compared and analysed. The efficiency of the converter with reset winding is significantly improved over the entire operating range and the peak efficiency is 96.60% at full load. 1 Introduction As global environmental issues are increasingly prominent and the fossil energy crisis intensifies, the development and promotion of new energy vehicles is imminent. As an important part of new energy vehicles, the battery charging system is mainly composed of the fronted-stage PFC and high-frequency isolated DC–DC converter [1–3]. To meet the requirements of various on-vehicle power battery specifications, the study of a wide output voltage range and high reliability DC/DC converter is very important for the performance of the entire battery charging system. The control method, operating characteristics and application occasion of primary phase-shifted full-bridge converter (PPS-FBC) and secondary phase shift full-bridge converter (SPS-FBC) are similar. The characteristics are introduced using PPS-FBC as an example. As one of the mainstream topologies of isolated DC/DC circuits, the traditional PPS-FBC has the characteristics of high efficiency, high power density and high reliability [4–6]. Utilising the transformer leakage inductance and switch intrinsic capacitance, it can achieve ZVS without any external passive components [7–10]. PPS-FBC has been widely used in medium and high power applications [11–14], but there are some major problems: (i) limited ZVS range of the lagging-leg switches; (ii) the circulating energy issue; (iii) the voltage spike of the output rectifiers; (iv) the severe loss of duty cycle. To overcome the above shortcomings, experts and scholars have conducted in-depth research work in recent years. The ZVS range can be extended by utilising a saturated inductor instead of a common inductor, meanwhile reducing the loss of duty cycle [15]. However, saturated inductor brings too much loss. An auxiliary circuit with lagging leg can improve the soft-switching range [16–18], but the auxiliary network energy has nothing to do with the load, and the conduction loss of heavy load is particularly serious. In [19, 20], the energy of the auxiliary circuit can be adaptively changed with the load. However, the current stress is relatively large because the auxiliary circuit is connected in series with the main circuit, which will decrease the reliability of the converter. On the other hand, the zero-voltage zero-current switching (ZVZCS) PPS-FBC not only retains all the advantages of the conventional PPS-FBC but also reduces the conduction loss of the primary side [21, 22]. In the ZVZCS full-bridge topology, to reset the primary current during the freewheeling time, two diodes are added in series with the lagging leg in [23]. After that, the lagging leg realises ZCS, which IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 reduces switching loss and circulating current loss. However, the on-state loss of the diodes is generated. The auxiliary winding of the transformer is utilised to reset the primary current, however, it increases the complexity of the transformer design [24]. An energy recovery buffer circuit was added on the secondary side in [25], but the control of the converter becomes complicated as the amount of the devices increases. Compared with the traditional PPS-FBC, the SPS-FBC has been developed and reported in [26–28]. The SPS-FBC has the advantages of wide soft-switching range, no circulating current loss, and small duty cycle loss. It can effectively solve the problems of traditional PPS-FBC such as the narrow ZVS range of the lagging leg and the primary circulating loss. Although the basic and conceptual research has been reported in [26–28], such as the analysis on the soft-switching range and the steady-state power regulation characteristics, the problem that the severe voltage overshoot and oscillation across the diode rectifier is caused by the interaction between junction capacitance of the rectifier diode and leakage inductance of the transformer has not been evaluated indepth in the past relevant work. To solve the above problem, a resonant inductor and two clamping diodes can be introduced into the primary side of the transformer. Then the excess energy of the resonant inductor can be fed back to the input power, which will suppress the voltage stress of the rectifiers. However, the conduction loss of the switches and the clamping diodes will increase, and the clamping diodes are hard turned-off if the output filter inductance and magnetising inductance are relatively larger. In this paper, an auxiliary winding in series with the resonant inductor is added to the SPS-FBC to reset the clamp diode current rapidly when the clamp diode conducts. The reset winding not only reduces the conduction losses, but also makes the diodes naturally turn-off and avoids the reverse recovery. This paper is organised as follows: Section 2 analyses the circuit configuration and the working principle of the circuit in detail. Section 3 gives the comparisons between the SPS-FBC with/without reset winding. In Section 4, a 1.5-kW hardware converter prototype has been designed, fabricated, and tested to verify the correctness of the theoretical analysis. Finally, the concluding remarks are drawn in Section 5. 2252 Fig. 1 SPS-PWM full bridge converters (a) Without reset winding, (b) With reset winding Fig. 2 Key waveforms of the SPS-PWM full-bridge converters (a) Without reset winding, (b) With reset winding 2 Circuit description and operation principle The proposed SPS-FBC with/without reset winding and the key waveforms for different stages are shown in Figs. 1 and 2, respectively. Different from [26–28], this paper considers the junction capacitance of the rectifiers when analysing the reverse recovery. As shown in Fig. 1, the SPS-FBC converter topology consists of four MOSFETs S1∼S4, the parasitic capacitances C1– C4, a series resonant inductor Lr, a tightly coupled transformer T with an air gap, the clamping diodes Dc1 and Dc2, secondary-side rectifiers (reverse blocking active switches Q5/Q6 and diodes D7/ D8), LC output filter (Lf and Co), the load R. In this control strategy, the output voltage is regulated by controlling the phaseshift angle between the primary and secondary switches. Compared with Fig. 1a, there is an additional auxiliary winding in series with the resonant inductor in Fig. 1b. As shown in Fig. 2, the clamping diodes conduct twice in a switching cycle and the voltage of reset winding can make the current of clamping diodes decay to zero rapidly. The operation principles of the SPS-FBC with/without reset winding are similar except during the periods [t4, t6] and [t8, t9]. Therefore, only the working principle of the SPS-FBC with reset winding is analysed in this paper. The converter with reset winding goes through ten stages during a half-switching cycle in SPS mode. And the analysis is based on the following assumptions: (i) all switches, diodes, inductors, IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 capacitors and transformers are ideal devices, except for the rectifier diodes D5–D8, which is equivalent to an ideal diode and a paralleled capacitor. (ii) The turns ratio of the transformer T is the primary winding: the reset winding: the secondary winding = n1:n3:n2, (iii) the output filter inductor Lf is large enough to be regarded as a constant current source in one cycle, (iv) the output capacitors of switches S1–S6 and the junction capacitance of the rectifiers are expressed as C1–C6 = Coss, CDR5–CDR8 = CDR. The equivalent circuits of different stages in a half switching cycle are shown in Figs. 3–6. Stage 1[t0, t1]: Since the gate signals for S2 and S3 are removed simultaneously at t0, the primary current ip starts to charge C2/C3 and discharge C1/C4. As the voltage VBA begins to decline, the voltage across the secondary-side transformer T winding terminals also begins to decreases. The intrinsic capacitance C5, CDR5 and CDR8 start to discharge. During this time, stage 1 will end once VBA reaches zero. Stage 2[t1, t2]: At t1, the intrinsic capacitance C5, CDR5 and CDR8 are discharged to zero. When VBA starts to increase reverse, ip and is decreases rapidly. The voltage across the secondary-side transformer T winding terminals also becomes zero, and D8 is naturally forward-biased. Hence, iLf is freewheeled through D7, Q6 and D8. 2253 Fig. 3 Equivalent circuits of different stages in a half switching cycle (a) Stage 1, (b) Stage 2, (c) Stage 3 Fig. 4 Equivalent circuits of different stages in a half switching cycle (a) Stage 4, (b) Stage 5, (c) Stage 6 Fig. 5 Equivalent circuits of different stages in a half switching cycle (a) Stage 7, (b) Stage 8, (c) Stage 9 Fig. 6 Equivalent circuits of different stages in a half switching cycle (a) Stage 10 Fig. 7 Further equivalent circuits (a) Stage 5, (b) Stage 10 Stage 3[t2, t3]: At t2, the output capacitors of S2 and S3 continuously are charged up to Vin and that of S1 and S4 are discharged down to zero. Therefore, the antiparallel diodes of S1 and S4 are conducting. The ZVS of S1 and S4 can be achieved at this moment. During this interval, the secondary side of 2254 transformer T is always shorted and ip keeps decreasing and approaching to iLm at t = t3. Stage 4[t 3, t 4]: At t3, is decreases to zero. The resonance occurs between Lr, C5 and CDR6, so VCB starts to increase. At t4, CDR6 is charged to Vinn2/n1, and the potential voltage of point C rise to Vin. Therefore, DC1 starts to conduct and VCB are clamped at Vin. Stage 5[t4, t5]: During this stage, the resonant inductor Lr is shorted, and iLr flows through S1 and DC1 with a constant value. ip decreases downward to the magnetising current iLm. The voltage of the reset winding VCD is Vinn3/n1 which acts on Lr to make the iLr decline rapidly. This stage will end when ip equals iLr at t5. Then DC1 turns off naturally. The further simplified equivalent circuit of this mode is shown in Fig. 7a. Stage 6[t5, t6]: When ip is equal to iLm at t5, the input voltage Vin acts on Lr and Lm. The primary and secondary sides of T are disconnected, so iLf is freewheeled through diodes D7 and D8. Stage 7[t6, t7]: The gate of S5 is supplied at t = t6. Since Q5 and D7 are on-state simultaneously, the secondary side of transformer T is shorted again. The secondary side current is starts to increase linearly and iD7 decreases gradually due to the effect of Lr. Stage 8[t7, t8]: At t7, iD7 decreases to zero, and is reaches iLf. The resonance occurs between Lr, CDR6 and CDR7, so the voltage VCB starts to increase. At t8, CDR6 and CDR7 is charged to Vinn2/n1, and the voltage of point C rise to Vin. So DC1 conducts again and VCB is clamped at Vin. Stage 9[t8, t9]: In this interval, the resonant inductor Lr is shorted again. iLr flows through S1, DC1, and remains constant. ip step downward to ip(t8). The voltage of the reset winding VCD is IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 Vinn3/n1, which acts on Lr to make the iLr decline rapidly. This stage will end when ip equals iLr at t9. Then DC1 turns off naturally. Stage 10[t9, t10]: During this stage, ip equals iLr, and the power is delivered from Vin to the load R. The reset winding n3 is in series with the primary winding n1, so the input voltage Vin acts on Lr and Lm. Then, the voltage VDB = Vin(n1 + n3)/n1, and the resonance occurs between Lr, Lm, CDR6 and CDR7. The further simplified equivalent circuit of this mode is shown in Fig. 7b. This stage ends at t10 when S1 and S4 are turned off. Then, the next half switching cycle starts. Hence, vrect can be expressed as: vrect t = 3.2 Magnetising inductance n2LriLm t9 ω22 n2 ω22 sin ω1 t − t9 2 V in − n1 + n3 ω1 n1 + n3 ω12 2 2 2 1 + n2 n2 ω − V cos ω1 t − t9 n1 n1 + n3 ω in ≃ n2 n n2 V + 2− V cos ω1 t − t9 n1 + n3 in n1 n1 + n3 in Fig. 8 Primary-side current waveforms without reset winding (1) where ω1 = 1/ LeqCeq ω2 = 1/ LrCeq Leq = LmLr /(Lm + Lr) 2 Ceq = 2CossCDR + CDR / Coss + CDR n2 / n1 + n3 2 Equation (1) shows that vrect have slight oscillation. In practice, vrect will finally stay at the average value Vinn2/(n1 + n3) since the parasitic resistance exists in the main circuit. According to Fig. 2a, the clamping diodes DC1 and DC2 conduct during the interval [t4, t5] and [t13, t14] on the SPS-FBC without the reset winding. Therefore, there are circulation losses of the switches and the clamping diodes. Furthermore, if the magnetising inductor Lm is relatively large, the clamping diodes are hard turned-off, causing significant reverse recovery loss. To solve the above problems, the value of Lm should be small. The transformer primary current ip, resonant current iLr and magnetising current iLm waveforms without reset winding are shown in Fig. 8. △I1 is the primary current spike caused by the resonance of the resonant inductor and secondary side parasitic capacitors △I1 = 3 Analysis of the proposed converter 3.1 Transformer turns ratio In contrast to [26–28], the reset winding is added to the SPS-FBC. Therefore, the transformer turns ratio is compared between the improved converter and the original one in this part. According to Fig. 2b, the time intervals [t6, t8] and [t10, t11] are considered to be negligible at a half switching cycle. Therefore, Vo can be expressed as: V o ≃ V rectD1 = V inn2 2 V inn2 t + t T s n1 89 n1 + n3 910 C′j = (CDR + Coss) Before the reset winding is introduced, the voltage conversion ratio of the SPS-FBC can be given by Vo n = 2D V in np 1 (4) where np is the primary turns of the transformer. Under the same working conditions, the conclusions can be obtained from (3) and (4): np = n1 + n3 (5) It means that the transformer primary winding of the SPS-FBC is divided into two parts, and one of them is used as the reset winding in the improved converter. Lm ≤ n2 np Lm diLm = V in dt Imp = V in T s Lr + Lm 4 IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 2 (7) (8) (9) In this interval, taking into account the voltage drops on the clamping diode and the switch, the rise slope of the magnetising current can be given by diLr 1 = − [V D + (Imp − △I1)R] dt Lr (10) where VD is the forward voltage drop of the clamping diode; R is the sum of the static drain-to-source on-resistance of the switch S1 and parasitic resistance of resonant inductance Lr. From (6) and (10), Δt1 is the conduction time of the clamping diode and can be obtained by: △t1 = △I1 V in /Lm + V D + (Imp − △I1)R)/Lr (11) To avoid reverse recovery of the clamping diode, Δt1 should be smaller than D2Ts/2. The maximum value of Lm without reset winding can be obtained by (4), (9), and (11) (see (12)) , where (V inn2 − V onp)T sLr2 ⋅ n2 A2 − 2/V inn22 (V inn2 − V onp)2 T sLr 2 The voltage across the magnetising inductor Lm and the peak value of the excitation current iLm can be expressed, respectively, as: If the appropriate turns of the reset winding are chosen, [t8, t9] is shorter than [t9, t10]. Equation (2) can be further simplified to: (3) (6) Based on the model analysis, C′j can be derived as: (2) Vo n2 t810 n2 ≃ = D V in n1 + n3 T s /2 n1 + n3 1 V in Lr /C′j 1 V D − △I1R − 2V inLr△I1n2 /(V inn2 − V onp)T s − A (12) 2255 A= diLr VD Ion2 V in = − − − Imp − Δt dt Lr np Lm 1 (V inn2 − V onp)D2T s 1 V D − △I1R + V in Lr + V inRT s 2V inn2 4 According to (12), Fig. 9 shows the relationship between the maximum excitation inductance Lm and the input voltage Vin. During the D2Ts/2 period, the clamping diode can turn off naturally by reducing the value of the transformer magnetising inductance Lm. However, the value of Lm is relatively small, so the magnetising current increases. The conduct loss of the primary switch and magnetic component is deteriorated, which is harmful to the improvement of efficiency. The waveforms of primary current ip, resonant current iLr and magnetising current iLm with reset winding are shown in Fig. 10. In contrast to the reset winding voltage, the voltage drops on the clamping diodes and the switches can be neglected during this interval. Hence, the rising slope of the resonant inductance Lr can be given by diLr V in n3 = dt Lr n1 (13) From (6), (8), (13), the conduction time of the clamping diode can be obtained by: Δt1′ = LmLrΔI1 /V in n3 /n1 Lm + Lr (14) In order to avoid reverse recovery of the clamping diodes, Δt1′ should be smaller than D2Ts/2. Lm is assumed to be large enough to be infinity. Therefore, (14) can be further simplified to: 9.36n1 V o(n1 + n3) + ≤1 n3V in n2V in (15) R Lr (19) where Io is the load current. To avoid reverse recovery of the clamping diode, Δt2 should also be smaller than D1Ts/2. From (16), (18), (19), The maximum value of Lf without reset winding can be given by (see (20)) . According to (20), Fig. 11 shows the relationship between the maximum output filter inductor Lf and input voltage Vin. During the D1Ts/2 period, the clamping diode can turn off naturally by reducing the value of the output filter inductor Lf. However, the above solution will increase the ripple current of output filter inductor. Thus, the filter capacitance will increase. On the transformer with reset winding, the conduction time of the clamping diodes can be derived based on (13), (18) and is given by Δt2′ = ΔI2LrLf n3 /n1 V inLf + n2 /np Lr V in n2 /np − V o (21) To avoid reverse recovery of the clamping diode, Δt2′ should also be smaller than D1Ts/2. Lf is assumed to be large enough to be infinity, so (21) can be further simplified as: 0.03n1n2 ≤1 n3(n1 + n3) (22) Substituting the converter parameters in this paper, (22) is permanently established. In other words, regardless of the value of filter inductor Lf on the SPS-FBC with the reset winding, the clamping diodes can turn off naturally if chosen the appropriate turns of the reset winding. Substituting the converter parameters in this paper, (15) is permanent established. In other words, regardless of the value of the magnetising inductance Lm on the transformer with the reset winding, the clamping diode can turn off naturally. The circulation losses of the clamping diodes and the switches are greatly reduced. 3.3 Output filter inductance According to Fig. 2a, due to the reverse recovery of the clamping diodes D7/D8, the clamping diodes DC1 and DC2 conduct again during the interval [t6, t7] and [t15, t16] on the SPS-FBC without the reset winding. If the output filter inductor Lf is relatively larger, Δt2 will be greater than D1Ts/2 which means that the clamping diodes are hard turned-off. Therefore, it is necessary to analyse the effect of the value of the output filter inductor Lf on the circuit. According to Fig. 8, ΔI2 and C′′ j can be expressed, respectively, as: ΔI2 = V in (16) Lr /C′′j C′′ j = 2CDR n2 np 2 Fig. 9 Relationship between the maximum value of Lm and the input voltage Vin without reset winding (17) The slopes of output filter inductor current iLf and resonant inductor current iLr is described, respectively, as diLf 1 V inn2 = − Vo dt Lf np Lf ≤ 2256 (18) Fig. 10 Primary-side current waveforms with reset winding n2 − V o Lr np − 1 − D1 /Lm + 2LrΔI2 /D1T − n2 /np IoR − V D V in 1/2 V inRT s 1/ 2 Lr + Lm (20) IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 Fig. 11 Relationship between the maximum value of Lf and the input voltage Vin without reset winding Table 1 Circuit parameters and specification of prototypes Parameter Value unit input voltage Vin 360–400 V output voltage Vo 250 V output power Po 1.5 kW switching frequency fs transformer T 130 kHz FerroxcubePQ35/35ferrite core; n1:n3:n2 = 19:3:18, Lm = 600 μH resonant inductor Lr FerroxcubePQ20/20ferrite core; N = 6; Lr = 12 μH output inductance Lf KEDA KS130-075A Sendust core; Lf = 180 μH output capacitor Co2 Electrolytic capacitor; Co2 = 200 μF primary/secondary switches S1–S6 rectifiers D5–D8 clamp diodes Dc1–Dc2 Infineon IPW60R070CFD7 650 V, 31 A Ultrafast recovery diode HFA15TB60 600 V, 15 A SiC Schottky diode C3D04060E 600 V, 4 A Fig. 12 Observed waveforms of the converter with reset winding at Po = 1.5 kW (a) Driving voltages Vg1–Vg6, (b) Driving voltages Vg2 and Vg4, drain-source voltages Vds2 and Vds4 4 Results and discussions To verify the working principle of the SPS-FBC with/without reset winding, a 1.5 kW experimental prototype was designed and built. Furthermore, the circuit parameters are illustrated in Table 1. Driving voltages Vg1–Vg6 of the converter with reset winding at Po = 1.5 kW are shown in Fig. 12a. Fig. 12b shows that S2 and S4 both achieve ZVS with the reset winding. Actually, compared with S2 and S4, S1 and S3 are easier to realise ZVS, so all the primary side switches achieve ZVS. Fig. 13 shows the experimental waveforms of VAB, VCB, ip and iLr without (left)/with (right) reset winding under the full load condition. It can be seen that each clamping diode conducts twice in a switching period before the reset winding is introduced. When the clamping diode is conducting, the resonant inductor Lr is shorted and iLr remains constant. Hence, the potential voltage of point C is clamped at 0/Vin. When the clamping diode conducts, iLr decay rapidly by introduced the reset winding. Therefore, the clamping diode average current is decreased, leading to a low circulation loss. IET Power Electron., 2020, Vol. 13 Iss. 11, pp. 2252-2259 © The Institution of Engineering and Technology 2020 Fig. 14 shows the experimental waveforms of VQ5, is and VD7 without/with reset winding under the full load condition. It can be seen that the voltage overshoot and oscillation across the rectifiers are suppressed effectively. After introducing the reset winding, the secondary-side active switch Q5 can realise ZCS turn-on and ZCS turn-off. However, the voltage stress of the secondary side power device increases. The efficiency with different loads under the input voltage Vin = 400 V is given in Fig. 15 and the power loss analysis of the SPSFBC with/without reset winding at full load is shown in Fig. 16. It can be seen that the efficiency of the improved SPS-FBC is higher than that of the converter without reset winding. That is because the circulation losses of the clamping diodes, the leading switches and the resonant inductor are decreased. What is more, the peak efficiency of SPS-FBC with reset winding is 96.60% and the efficiency is improved at light load. 5 Conclusion A secondary-side phase-shifted full-bridge converter with reset winding is proposed in this paper. The working principle and 2257 Fig. 13 Measured waveforms of VAB, VCB, ip and iLr at Po = 1.5 kW (a) Without reset winding, (b) With reset winding Fig. 14 Measured waveforms of VQ5, is and VD7 at Po = 1.5 kW (a) Without reset winding, (b) With reset winding circuit characteristics of the converter are analysed in detail. To verify the correctness of the structure, a 1.5 kW experimental prototype was built, and comparison of the main experiment results on the transformer with/without the reset winding was given. Introduced the reset winding in series with the resonant inductor, the clamping diodes can turn off naturally without reverse recovery. 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