Abstract : A phase-shifted zero-voltage switching PWM three-level DC-DC converter with output saturated inductor is introduced. Compared with the traditional zero-voltage three-level converter, it has the characteristics of achieving zero-voltage soft switching in a wider load range; reducing secondary duty cycle loss; and reducing parasitic oscillation and voltage spikes of the output diode. Experimental prototype shows that the circuit has high overall efficiency, and the system has high stability after adopting current peak control. It is easy to realize medium and high power DC/DC conversion. Keywords: saturated inductor; zero-voltage switching; three-level 1 Introduction At present, in medium and high power communication DC/DC power supplies, three-level DC/DC converters have become a research hotspot [1][2]. This topology can make the voltage stress of the switching tube half of the input DC voltage, which is a great advantage in the case of three-phase PFC input (output DC is generally 760~850V). It can make low-voltage switching devices used on high voltage. In fact, the three-level circuit is an extension of the half-bridge circuit. However, compared with the hard switching of the half-bridge circuit, the three-level converter cleverly combines the characteristics of the phase-shifting circuit and uses the leakage inductance of the transformer (or the external resonant inductance) and the parasitic junction capacitance of the switching transistor to achieve ZVS of the switching transistor. Similar to the traditional phase-shifting full-bridge ZVS soft switching, for the hysteresis arm soft switching, the traditional phase-shifting ZVS three-level circuit is difficult to achieve ZVS under light load and has the problem of duty cycle loss. In view of the shortcomings of the ZVS three-level circuit, a zero-voltage zero-current (ZVZCS) three-level converter is proposed [3], in which two achieve ZVS of the switching transistor and the other two achieve ZCS of the switching transistor. However, the circuit will cause the output voltage to oscillate due to the reverse recovery problem of the output diode, which will cause the diode to withstand a very high voltage spike and be easily damaged. This paper proposes an improved ZVS three-level converter with output saturation inductance and freewheeling diode. The experimental results of the 250W prototype prove that it effectively overcomes the shortcomings of the ZVS circuit and combines the three-level converter and phase-shifting control well. Compared to the traditional ZVS three-level converter, the cost increase is not significant, and it is easy to implement for medium to high power conversion. This article will first explain its working principle, then propose the design concept of the saturated inductor, and analyze the stability of the peak current control mode. Finally, the experimental results and waveforms are presented. 2 Working Principle In Figure 1, Q1 and Q4 are the leading arms, Q2 and Q3 are the lagging arms, Cs is the flying capacitor, Dc1 and Dc2 are clamping diodes, Lr is the resonant inductor, Ls1 and Ls2 are the output saturated inductors, and Ds is the freewheeling diode. Compared to the traditional ZVS three-level converter, it adds secondary saturated inductors Ls1 and Ls2 and the output freewheeling diode Ds. [align=center] [img=366,248]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/42-1.jpg[/img] [/align] Generally, a phase-shifted three-level circuit has 12 switching states in a complete switching cycle. In addition to the two power output processes and two clamping freewheeling processes during the positive and negative half-cycles, there are also the resonance and commutation processes starting from the dead time during the operation of the leading arm, and the resonance and commutation processes starting from the dead time during the operation of the lagging arm. To achieve ZVS soft switching, both the leading and lagging arms must have sufficient inductance to absorb the charge on the parasitic capacitance of the switching transistor and the distributed capacitance of the transformer, as shown in the following equation: [img=321,38]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/42-2.jpg[/img] Where: L is the total inductance participating in resonance; [img=291,33]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/42-6.jpg[/img] C[sub]tr[/sub] is the equivalent value of the transformer distributed capacitance. Because the three-level zero-voltage leading arm has the magnetizing inductance and the equivalent value of the output inductance n2Lf participating in resonance, it has sufficient energy to achieve ZVS over a wide range. However, in a traditional ZVS three-level circuit, both output rectifier diodes are simultaneously in freewheeling mode during lagging arm resonance. Only the primary resonant inductor Lr participates in resonance, while the output inductor and magnetizing inductor do not participate in resonant commutation. Since Lr ≪ n²Lf, it is difficult to achieve ZVS over a wide range compared to the leading arm. Furthermore, the ZVS circuit experiences secondary duty cycle loss during commutation. Adding an output saturation inductor and a freewheeling diode alters the commutation process of the lagging arm, allowing the magnetizing inductor to participate in resonant commutation. Based on the characteristics of the saturation inductor, during commutation of the leading arms Q1 and Q4, the process is similar to a traditional ZVS three-level circuit. At this time, the saturation inductor is in a saturated, low-resistance conducting state due to the large current. When the lagging arms Q2 and Q3 commutate, the primary current drops to the magnetizing current, the output rectifier diode current approaches zero, and the saturation inductor quickly exits saturation, exhibiting a high blocking state. The entire output current flows through the freewheeling diode Ds, and the transformer transforms into a pure inductor state, thus initiating the magnetizing inductor to participate in the series resonance of the lagging arm. Since the equivalent resonant inductance is greatly increased, under the condition that the primary current is approximately constant, according to equation (1), the lagging arm will have enough energy to absorb parasitic charges and achieve a wide-range ZVS. At the same time, after adding the saturation inductance, since the lagging arm has the excitation inductance participating in the resonance, the resonance condition is not significantly related to the load. Therefore, when designing the primary resonant inductance, the inductance can be minimized. Thus, according to the discussion on duty cycle loss in reference [2], the duty cycle loss on the secondary side can be reduced. The characteristics of the saturation inductance are equivalent to a magnetic switch. When the current is less than Ic, the saturation inductance is not saturated and the inductance is very large. The magnetic switch turns off the output rectifier diode, which effectively blocks the reverse recovery current generated by the output diode, thereby reducing the voltage oscillation caused by the diode parasitic capacitance and the transformer output leakage inductance, and at the same time reducing the voltage spike. [b]3 Design of Saturated Inductor[/b] This paper discusses the VITROPERM6050Z cobalt-based amorphous magnetic core, which has a permeability of 2000-3000, very low core loss, and a very high aspect ratio, exhibiting a very large inductance when the current is close to zero. This large inductance effectively blocks the reverse recovery current generated by the diode. The core can saturate under relatively small current conditions. The hysteresis loop of the saturated inductor is shown in Figure 2, and its operation is as follows: When the operating point 1 is reached (current is on), the core is in a saturated state with a very low inductance. When the current is turned off, the operating point reaches the remanent magnetization point 2. The reverse recovery effect of the diode causes the current to decrease in the direction of less than zero. At this time, due to the very high permeability of the cobalt-based amorphous core, its inductance is very large, effectively suppressing the diode's peak current and achieving soft recovery of the diode. Due to the high inductance value, the core is prevented from operating at the operating point 3 and remains at the reverse remanent magnetization point 4, and then is magnetized to start the next cycle. [img=172,215]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/42-4.jpg[/img] Generally speaking, the magnetic flux φ of a saturated inductor must satisfy equation (2). [img=315,38]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-1.jpg[/img] Where: S[sub]a[/sub] is the cross-sectional area of ​​the magnetic core; J is the copper wire current density; F[sub]cu[/sub] is the copper wire fill factor, for wire-wound magnetic core inductors, F[sub]cu[/sub]=0.3～0.4; for copper wire diameter magnetic core inductors, F[sub]cu[/sub]=1; t[sub]rr[/sub] is the reverse recovery time of the output diode; V[sub]r[/sub] is the reverse voltage of the output diode; I[sub]o[/sub] is the output current. Therefore, for a copper wire core inductor with a medium line current density (2A/mm²), equation (2) can be simplified to [img=345,144]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-2.jpg[/img] 4 Stability Analysis of Peak Current Control 4.1 Peak Current Control Principle Peak current control is a new control mode that emerged in the 1980s. It has the characteristics of good dynamic performance, high output accuracy, large gain bandwidth, and instantaneous current limiting protection. Currently, many phase-shift control ZVS systems adopt this control system. Of course, phase-shift three-level ZVS is no exception. As shown in Figure 3, the system superimposes the dynamic signal VL representing the transient current of the switching transistor with a sawtooth wave signal Vm with a fixed switching frequency and a very small amplitude (the sawtooth wave is slope compensation, which will be discussed below), and then compares it with the reference voltage signal Ve to jointly determine the phase shift angle α of the conducting bridge arm. This forms a current-type control system with an outer voltage loop and an inner peak current loop. The duty cycle D of the four switching transistors is fixed at approximately 50%. The phase shift angle α of the conducting bridge arm determines the volt-second value of the transformer's energy conversion. This value is proportional to both the reference voltage and related to the instantaneous peak current of the switching transistors, ensuring that the instantaneous peak current of the switching transistors follows changes in the reference voltage. This control mode has the capability to combine with phase-shift control and achieve instantaneous current limiting adjustment, providing dynamic protection for the switching devices while automatically maintaining the dynamic balance of the high-frequency power transformer. [align=center][img=343,174]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-4.jpg[/img] [img=251,367]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-3.jpg[/img][/align] 4.2 Discussion on Slope Compensation for Peak Current Systems A prominent problem in peak current control is that when the phase shift duty cycle Dα > 50% in continuous mode, as the offset Δi changes, the offset will become increasingly larger over several state cycles. This will cause the closed-loop system to become unstable, leading to subharmonic oscillations, inductor current ringing, etc. To ensure system stability, slope compensation is needed. Although this sacrifices some system gain, it will keep the system stable when the phase shift duty cycle Dα > 50%. The relationship between system stability and slope compensation is shown in Figure 4. The following is its slope compensation stability analysis. From the geometric relationship, we know [img=372,50]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-5.jpg[/img] where: m is the rising slope of the compensation signal; m1 is the rising rate of the inductor detection current; m2 is the falling rate of the inductor detection current. Therefore, after one switching cycle, the change in current in the output inductor is [img=394,108]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/43-6.jpg[/img]. For the system to be stable, the offset current must approach zero, i.e. [img=281,35]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-1.jpg[/img]. Therefore, the necessary and sufficient condition for system stability is [img=292,38]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-2.jpg[/img]. Because under stable conditions, D·m1 = (1 - D)m2, After eliminating m1 and rearranging, the necessary and sufficient condition for the stability of the phase-shifted peak current control system is [img=255,40]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-3.jpg[/img] From equation (11), it can be seen that when there is no slope compensation, i.e., m=0, the phase-shifted duty cycle Dα must be <0.5. This means that theoretically, without compensation, the system will be unstable when the phase-shifted duty cycle Dα>0.5. In actual control engineering, the compensation slope m is generally taken as m=(0.7～0.8)m2, which ensures that the system meets the stability condition and also ensures the dynamic index of the system. 5 Experimental Results Considering the above analysis, a 250W three-level ZVS DC-DC converter with output saturated inductor was developed. The characteristics of adding and not adding saturated inductor were compared. The experimental parameters of the prototype are as follows: Vin = 300 V; f = 100 kHz; Vo = 47-50 V; Io = 5 A. [img=362,220]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-9.jpg[/img] From the experimental waveforms in Figures 5 and 6, it can be seen that under a light load condition of 5% Io, the leading and lagging bridge arms can basically achieve ZVS soft switching, and the waveforms are relatively clean, indicating that the switching interference is very small, and soft switching has been achieved. At the same time, the voltage stress of the switching transistor is only half of the input voltage, approximately 150V. This indicates that the converter successfully achieved ZVS three-level switching. Figures 7 and 8 compare the waveforms of the output diodes without and with saturated inductors (Io = 5A). As can be seen, the actual waveform matches the theoretical analysis. Adding a saturated inductor effectively reduces output voltage spikes and oscillations, stabilizes output characteristics, and prevents overshoot damage to the output diodes. Figure 9 shows the primary voltage waveform with a saturated inductor (Io=5A), which is consistent with the theoretical analysis of the ZVS three-level circuit. [align=center][img=323,150]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-4.jpg[/img] [img=323,151]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-5.jpg[/img] [img=323,153]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-6.jpg[/img] [img=320,155]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-7.jpg[/img] [img=323,159]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/44-8.jpg[/img][/align] The efficiency characteristics of this prototype were tested, as shown in Figure 10, comparing the efficiency curves with and without a saturated inductor. Adding an output saturated inductor improves the overall efficiency by 1% to 3%. This is due to the magnetic switch, which reduces the parasitic oscillation of the secondary diode and prevents the diode's reverse recovery current. Simultaneously, since the output relies on an external freewheeling diode for current flow, the resonant inductance can be reduced accordingly, decreasing the secondary duty cycle loss and thus reducing the primary-side commutation losses. Furthermore, it was demonstrated that the added cobalt-based amorphous magnetic core has very low losses. [img=313,225]http://www.bjx.com.cn/files/wx/dyjsyy/2003-3/45-1.jpg[/img] 6 Conclusion This paper proposes an improved ZVS three-level DC-DC converter. Theoretical and experimental results demonstrate that the novel circuit using an output saturated inductor to start the magnetizing inductor, combined with an input resonant inductor to achieve ZVS three levels, is a simple, reliable, economical, and practical zero-voltage DC-DC converter. 1) By combining zero-voltage soft switching and a three-level circuit, the advantages of both are integrated, achieving ZVS over a wide load range; 2) It effectively reduces voltage spikes and parasitic oscillations in the output diode, reduces secondary duty cycle loss, and improves efficiency; 3) It easily implements peak current-type phase-shift control, with a simple control strategy; appropriate slope compensation can improve system stability. [b]References[/b] [1] J. Renes Pinheriro and Ivo Barbi. The three-level ZVSPWM converter - A new concept in high-voltage DC-to-DC conversion [C]. IEEE IEC ON, 1992: 173-178. [2] Ruan Xinbo, Xu Dayu, Yan Yangguang. Phase-shifted zero-voltage switching three-level converter [J]. Journal of Electrical Engineering, 2001, (12): 36-40. [3] Franciso Canales, Peter Barbosa and Fred C. Lee. Azero Voltage and Zero-Curent Switching Three-Level DC/DC Converter [C]. VPEC, 1999. [4] Liu Shengli, Yan Yangguang. Detailed analysis of twelve working processes in one cycle of a soft-switching phase-shifting full-bridge converter [C]. Proceedings of the 13th National Conference on Power Supply Technology, 1999: 118-127. [5]G.Hua,FC LeeandM.M.Jovanovic.AnimprovedFullBridgeZeroVoltageswitchingPWMConverterUsingaSaturableInductor[J].IEEETransactiononPowerElectronics,1993,8(4).