A New 98% Soft-Switching Full-Bridge DC-DC Converter based on Secondary-Side LC Resonant Principle for PV Generation Systems

IEEE PEDS 211, Singapore, 5-8 December 211 A New 98% Soft-Switching Full-Bridge DC-DC Converter based on Secondary-Side LC Resonant Principle for PV Generation Systems Daisuke Tsukiyama*, Yasuhiko Fukuda*, Shuji Miyake*, Saad Mekhilef**, Soon-Kurl Kwon*** and Mutsuo Nakaoka*** * Dispersed Power System Division, Daihen Corporation, Osaka, Japan ** University of Malaya, Malaysia *** Kyungnam University, Republic of Korea/Yamaguchi University, Yamaguchi, Japan d-tsukiyama@daihen.co.jp Abstract This paper is mainly concerned with the state-of-the-art feasible development of a novel prototype high-efficiency phase-shift soft-switching pulse modulated full-bridge DC-DC power converter with a highfrequency power transformer, which is designed for utility-grid tied photovoltaic (PV) power inverters. The proposed (HFTR) link DC- DC converter topology is based upon a new conceptual secondary-side series resonant principle and its inherent nature. All the active power switches in the HFTR primary-side can achieve lossless capacitive snubberbased ZVS with the aid of transformer parasitic inductances. In addition to this, passive power switches in its secondary-side can also perform ZVS and ZCS transitions for input voltage and load variations. In the first place, the operation principle of the newlyproposed DC-DC converter and some remarkable features are described in this paper on the basis of the simulation analysis. In the second place, the 5 kw experimental setup of the DC-DC converter treated here is demonstrated and its experimental results are illustrated from a practical point of view. Finally, some comparative evaluations between simulation and experimental data are actually discussed and considered in this paper, together with its future works. Index Terms---DC-DC power converter, full bridge topology, highfrequency transformer link, secondary-side LC resonant principle, primary-side ZVS, secondary-side ZCZVS, photovoltaic generation system I. INTRODUCTION In recent years, a variety of collaborated developments on renewable, sustainable energy conversion devices technologies and advanced power electronics have been strongly required toward concrete realization of non-carbon society from a global point of view. Of these relating to energy electronics, effective utilizations of the clean PV generating power and the energy with the aid of the latest energy storage devices such as batteries and/or capacitors have attracted special interest in the fields of power electronic distributed power supply systems applications and DC feeding smart grid. Under such conditions mentioned above, next generation developments of high-efficiency, high-power Fig. 1. Overview of utility-grid tied photovoltaic (PV) system with highfrequency isolation and, circuit diagram of the new phase-shift soft-switching pulse modulated full-bridge DC-DC converter, which is based on the secondary-side series LC resonant principle. density and high performance solar converters have been practically needed so far, which include high frequency switching pulse modulated DC-DC converters and utility interactive sinewave modulated inverters on the basis of digital control schemes. In particular, the front-end high-efficiency, high frequency switching boost DC-DC power converters such as nonisolated and isolated circuit topologies have been considered and discussed for solar power converters in order to improve their efficiency, power density and control performances including noise issue. The authors have proposed several circuit topologies of high- frequency switching DC-DC highpower converter circuits operating under the conditions of soft commutation schemes for high power industrial applications [1-2]. In addition, the authors have also 1 978-1-4577-1-9/11/$26. 211 IEEE 1112

developed utility-grid tied three-phase sinewave inverter operating under the conditions of high efficiency control strategy [3]. To further decrease the power consumption, advanced high efficiency and high-power density softswitching DC-DC converter topology with high frequency transformer link is proposed for a new issue on power solution, which is based on its secondary-side LC resonant circuit principle for the front-end solar converter in PV generation systems. Also, some previous researches about a variety of DC-DC converter with are introduced to improve their efficiency and power density [4-14]. It includes series-resonant soft-switching DC-DC converter with [13-14]. In these researches, DC-DC converter utilizes the leakage inductance of transformer for energy storing and/or soft commutation. However, although it is useful for realizing soft switching DC-DC converter, a large leakage inductance more than 2 μh or presence of parasitic capacitance more than 1 μf have to decrease magnetic coupling coefficient of the transformer. This result leads to not only reducing the high-frequency transformer efficiency but also increasing the surge voltage applied to the high-frequency full bridge inverter circuit which is introduced into this DC-DC converter with highfrequency transformer. In addition to this, it is extremely difficult that some series resonant DC-DC converters which utilize the leakage inductance of transformer is adopted as high voltage and high power applications above 1 kw, due to the reduction of power transmission efficiency from primaryside to secondary-side. This paper presents an improved efficiency phase shift softswitching pulse modulated full-bridge DC-DC converter with a high frequency transformer stage and front-end boost converter cascade stage, which includes soft-switching full bridge diode rectifier operating on the basis of the resonant operating principle and inherent nature of the secondary-side LC series resonant circuit. This new DC-DC power converter suitable for solar converter can achieve not only soft-switching transition based on ZVS in the primary-side, but also ZVS and ZCS commutation for the full-bridge diode rectifier in secondary-side. The operating principle of this DC-DC converter in a periodic steady-state is described by using switching mode equivalent circuits and simulation analysis, along with its inherent remarkable features as compared with conventional ones. The simulated operating voltage and current waveforms are comparatively illustrated in experimental ones. The actual efficiency vs. output power characteristics and power loss analysis are demonstrated from an experimental point of view. The practical effectiveness of the proposed converter for PV generation systems are confirmed and verified by means of 5 kw setup implementation and simulation analysis. II. NEW SOFT-SWITCHING DC-DC CONVERTER A. Circuit Description Figure 1 shows a proposed link phase-shift soft-switching pulse modulated full-bridge DC- DC converter with the front-end boost converter cascade stage which incorporates soft-switching full bridge rectifier 2 operating at the secondary-side LC resonant principle. The LC resonant components are located across the output diode rectifier. This high frequency isolated DC-DC converter designed for PV generation systems can operate under ZVS condition of the primary-side active switches S 1 ~ S 4 in H- Bridge arms with the aid of lossless capacitor, leakage inductor L l and magnetizing inductance L m of the twowinding with ferrite core. On the other hand, the passive switches in the secondaryside full-bridge rectifier can also operate under a principle of ZVS and ZCS transitions due to the secondary-side LC resonant circuit property. B. Principle of Converter Operation Figure 2 illustrates the relevant voltage and current operating waveforms during a complete switching period for the gate driving pulse sequences. The switching operating modes of the soft-switching full-bridge DC-DC converter with a high frequency transformer are divided into 6 operations modes from mode 1 to mode 6 in accordance with operational timing points from t to t 6. As can be seen in Fig. 3, the operation principle is described with the equivalent circuits corresponding to each operating mode. Mode 1 (t ~t 1 ): During an active state the corresponding set of the primary switches and secondary rectifier diodes (S 2, S 3 and D 2, D 3 ) conduct simultaneously so that the secondary voltage and current have the same polarity. The output power is delivered from the DC source to the load. Positive voltage magnitude V in is applied to the. Both V S1 and V S4, the voltages across S 1 and S 4, respectively, are also equal to the source voltage V in. Mode 2 (t 1 ~t 2 ): The turn-off signal is applied to S 3 at time t 1. After S 3 is turned off, V S1 begins to decrease gradually because the capacitor paralleled with S 1, previously charged to source voltage V in, is linearly discharging until zero level is reached. At the same time, the capacitor paralleled with S 3, previously discharged to zero level, is linearly charging toward positive voltage. Therefore, low-side switch S 3 is able to be turned off with ZVS due to current flowing into the capacitor paralleled with S 3. On the other hand, diode currents (i D2, i D3 ) form sinusoidal wave approache to zero so that not only D 2 but also D 3 can be easily turned off with extremely soft recovery during this period. Mode 3 (t 2 ~t 3 ): After the capacitor voltage V S1 reaches to zero, the diode paralleled with S 1 (D S1 ) starts to conduct and clamps voltage on the high-side switch S 1 at zero. And consequently, the voltage across S 3 is clamped at source voltage V in. On the other hand, the turn-off gate pulse signal is applied to S 2 at time t 2. After S 2 is turned off, V S4 begins to decrease gradually because the capacitor paralleled with S 4, previously charged to source voltage V in, is linearly discharging until zero level is reached. At the same time, the capacitor paralleled with S 2, previously discharged to zero level, is linearly charging toward positive voltage. Therefore, high-side switch S 2 is turned off with almost ZVS due to current flowing into the capacitor paralleled with S 3. Although all primary switches (S 1 ~ S 4 ) are turned off, primary current is 1113

maintained Fig. 2. Operating voltage and current waveforms of soft-switching highfrequency link phase-shift full-bridge DC-DC converter based on the secondary-side LC resonant principle. 3 1114

Mode 1 (t t < t 1 ) Mode 4 (t 3 t < t 4 ) Mode 2 (t 1 t < t 2 ) Mode 5 (t 4 t < t 5 ) Mode 3 (t 2 t < t 3 ) Mode 6 (t 5 t < t 6 ) Fig. 3. Switching mode equivalent circuits of the phase-shift soft-switching full-bridge DC- DC power converter with two winding link and softswitching full-bridge rectifier. 4 1115

1. 8. 6. 4. 2 1. 8. 6. 4. 2 S W 1 S W 3 S W 2 S W 4. 1 9 9 9 4. 1 9 9 9 5. 1 9 9 9 6. 1 9 9 9 7. 1 9 9 9 8. 1 9 9 9 9 T i m e ( s ) maintained in the same direction during this state. Mode 4 (t 3 ~t 4 ): Similarly to Mode 3, the diode D S4 begins to flow current and clamps voltage on the bottom switch S 4 at zero after the capacitor voltage V S1 previously reaches to zero. On the other hand, the current through the secondary-side diodes (D 1 and D 4 ) start to flow after V D1 and V D2 are equal to zero at the same time when both V S2 and V S3, the voltage across S 2 and S 3, respectively, reach to the source voltage V in. At the time t 4, primary current is still flow through diodes paralleled with S 1 and S 4 (D S1 and D S4 ) while the turn-on signal is applied to S 1. Mode 5 (t 4 ~t 5 ): The current through the diodes of D S1 and D S4 decreases gradually toward zero as the secondary-side current of D 1 and D 4 increases. At the time t 5, Both S 1 and S 4 are turned on with ZCS simultaneously. Mode 6 (t 5 ~t 6 ): The sinusoidal resonant current flows through primaryside active switches S 1, S 4 and the winding in primary-side of. The current through secondary-side diodes D 1, D 4 and the secondary-side winding of the transformer also forms sinusoidal resonant wave as well. The current through the primary-side is lagging behind the voltage while the phase between the voltage and current through the secondary-side is close to zero, because the resonant capacitor is located between the transformer and the full-bridge diode rectifier acts to form a leading current. This nature is effective to improve the converter efficiency. In other words, there is no phase difference between the voltage and the current in the secondary-side so that diodes set in this side are able to be turned off without generating reverse recovery current. In addition to this, all the diodes paralleled with the primary-side active switches utilize the lagging current during free-wheeling states and can be turned off with zero reverse recovery current at time t 5. C. Unique Features The specific advantageous points of the proposed DC-DC converter in Fig. 1 are as follows; (1) Primary-side ZVS transition of all the active switches. (2) Secondary-side ZVS and ZCS transition of all the passive switches. (3) Minimum reverse recovery current switching loss at the diode full bridge stage. (4) Higher converter efficiency realization (98%, 38 V DC output). (5) Flexible design of high voltage conversion ratio. (6) Ground leakage current protection from PV panel to power conversion circuit portion. (7) Utilization of the high-efficiency transformer with less MOSFET Full-bridge circuit Diode rectifier Series resonant inductor and capacitor HF- Transformer (a) (b) (C) (d) (e) (f) 4 2 6 4 2-2 2-2 15 1 5-5 -1 ID1*2 V_D1 I_S1*2 V_S1 Itrans_pri I_Lout*5 Vcap S 3 S 1 S 2 S 2 S 4 S 4 V D1 (V) D1 i 2 (A) V S1 (V) S1 i 2 (A) tr i (A) Lout 5 i (A) V cap (V) Time (s).19994.19995.19996.19997.19998.19999 Time (s) Fig. 4. Whole exterior appearance of boost converter-fed soft-switching full-bridge high-frequency inverter link DC-DC converter with soft-switching full bridge rectifier based on the secondary-side series LC resonant principle. Fig. 3. Simulated results of (a) gate driving signal S 1 and S 3, (b) gate driving signal S 2 and S 4, (c) voltage and current of D 1, (d) voltage and current of S 1, (e) resonant current of high-frequency transformer and (f) inductor current and capacitor voltage of LC resonant circuit under the conditions of in 38 V and 5.5 kw. Fig. 5. Two winding high-frequency isolated power transformer is set to the DC-DC high power converter. 5 1116

leakage inductance and parasitic capacitance. (8) High DC voltage step-up due to front-end boost converter cascade topology. III. CONVERTER SIMULATION Real time-based computer simulations were carried out in PSIM version 9. to reveal the operation of this converter under constant load conditions. Figure 3 shows the waveform of gate driving pulse signal (a) (b), the output rectifier (c), the MOSFET primary switch (d), transformer secondary current (e) and voltage across the resonant capacitor connects series with the transformer secondary (V Cr ) and the inductor current which composes output filter connected to load (i Lout ) (f) with 5 kw output specifications. As shown in Fig. 3 (c), diode D 1 is turned off with ZVS and turned on with ZCS. In Fig. 3 (d), switch S 1 is turned off after i S1 reaches to zero due to the capacitor paralleled with it. The current through the transformer forms sinusoidal wave due to series LC resonant circuit is depicted in Fig. 3 (e). Figure 3 (f) indicates the energy commutation between the resonant capacitor (1/2CV 2 ) and the resonant inductor (1/2Li 2 ) which is connected to the output diode rectifier. IV. EXPERIMENTAL RESULTS AND EVALUATIONS A 38 V input DC-5 kw soft-switching phase-shift pulse modulation full bridge DC-DC converter with soft-switching full bridge diode rectifier operating on the basis of the secondary-side LC resonant topology was built and tested in experiment. Figure 4 shows the whole appearance of the proposed soft-switching full-bridge DC-DC converter based on the secondary-side LC resonant principle for PV generation systems. The two-winding high-frequency transformer with ferrite core which is introduced in the DC- DC converter is shown in Fig. 5. The design specifications and circuit parameters of the converter are listed in Table 1. The measured voltage and current waveforms of this prototype converter are illustrated in Fig. 6. Comparing Fig. 6 to Fig. 3 (c), it can be seen that these observed voltage and current waveforms have good agreements with the simulated ones. The actual efficiency vs. output power characteristic is illustrated in Fig. 7, under the conditions of constant output voltage and some varying specifications. Observing Fig. 7, the actual efficiency reaches maximum value of 98% around the 3-4 kw output power ranges indicated by white-diamond marks. Over higher output power range, the efficiency slightly decreases mainly because the total conduction loss of the converter increases in accordance with the load current increase. However, 97.8% efficiency can be still measured at 5.2 kw outputs. The actual efficiency above 97% is measured in the range of 1-5.2 kw outputs. Figure 7 also depicts the efficiency in black-circle marks. In this condition, terminals of these components are connected directly to the measuring instruments by lengthening cables for examining losses of each component which compose the soft-switching DC-DC converter with high-frequency transformer. The maximum decrease in efficiency of.3% was measured over 3 kw because of increasing actually conduction losses. Table 1. Design specifications and circuit parameters V G_S2 V D1 Item Symbol Value Unit DC input voltage (from boost converter) (1 V/Div) V S2 (1 V/Div) (25 V/Div) V in 38 ~388 DC output voltage V out 38 V Maximum input power P in 5.5 kw Switching frequency f sw 3.6 khz Switching period T s 16.33 μs Phase shift time T φ.95 μs Dead time T d 1.6 μs Leakage inductance of L l < 1.3 μh Magnetizing inductance of L m 64 μh Magnetic coupling coefficient of k.998 - Inductance of output filter (series resonant inductor) L out 2 μh Capacitance of capacitors paralleled with transistors Capacitance of input/output capacitors Capacitance of series resonant capasitor Turn ratio of ID1 (5 A/Div) Fig. 6. Experimental waveforms of gate-source voltage of S 2 (V G_S2, 1 V/div), drain-source voltage of S 2 (V S2, 25 V/div), diode D 1 current (I D1, 5 A/div) and anode-cathode voltage (V D1, 1 V/div). V C S1 ~C S4 5 nf C in, C out 5 μf C r.88 μf N 1 : N 2 1 : 1 - Item Symbol Product type Primary-side MOSFET switches Secondary-side rectifier diodes Core material of S 1 ~S 4 D 1 ~D 4 - FCA76N6N RHRP36 ferrite (PQ17) 6 1117

Efficiency (%) 99 98.5 98 97.5 97 96.5 96 95.5 95 94.5 94 93.5 93 V in = 38 V f sw = 3.6 khz L out = 2 µh Without wiring for measurement With wiring for measurement 1 2 3 4 5 6 7 Output Power (kw) Fig. 7. Measured actual efficiency of the trial-produced soft-switching full-bridge DC-DC power converter with secondary-side series LC resonant rectifier as a function of the output power. Loss (W) 16 14 12 1 8 6 4 2 V in = 38 V f sw = 3.6 khz L out = 2 µh 2.1 4.1 HF-Transformer with resonant capacitor MOSFET switches 2.4 4.7 3.2 12.8 3.2 15.3 24.5 28.2 26.7 31.3 Rectifier + Output filter.5 1.1 1.7 2.1 3. 4.1 5.1 5.6 7.7 19. 41.4 Output Power (kw) Fig. 9. Power loss analysis about, full-bridge inverter circuit which is consists of MOSFET primary switches, high-frequency transformer, and diode rectifier with output filter. 12.9 24. 59. 21. 31. 73. 27. 36. 81. 1 99.5 99 98.5 98 97.5 97 96.5 96 95.5 95 94.5 94 Efficiency (%) V in = 38 V f sw = 3.6 khz L out = 2 µh Rectifier + Output Filter HF-Transformer with resonant capacitor MOSFET switches Total system efficiency 1 2 3 4 5 6 7 Output Power (kw) Fig. 8. Measured actual efficiencies versus output power characteristics of, diode rectifier with output filter (black-circle marks), highfrequency-transformer (white-triangle marks), MOSFET fullbridge inverter circuit (black-square marks) and total efficiency of the proposed converter with cables length consideration (whitecircle marks). Loss (p. u.) 1.9.8.7.6.5.4.3.2.1 V in = 38 V f sw = 3.6 khz L out = 2 µh.5 1.1 1.7 2.1 3. 4.1 5.1 5.6 Output Power (kw) Rectifier + Output filter HF-Transformer with resonant capacitor MOSFET switches Fig. 1. Power loss analysis represented by p. u. of converter components for, full-bridge inverter circuit adopts power MOSFETs as the primary-side switches,, and diode rectifier with output filter. By connecting each input/output terminal of each power circuit component composes the phase shift soft-switching pulse modulated full-bridge DC-DC converter with a high frequency transformer to the measuring instrument, input and/or output power of each component can be measured directly so that efficiencies and losses of these components can also be investigated. Power efficiency of each component was measured are shown in Fig. 8 as a function of the output power. Total efficiency (white-circle marks) in Fig. 8 is the same as black-circle marks in Fig. 7. As shown in Fig. 8, all elements except full-bridge inverter which is constructed by MOSFET reach to high efficiency of 99.5% around 4 kw or more. Although the efficiency of the rectifier with the filter (black-circle marks) is extremely high in the area of.5-2 kw, it slightly decreases as output power becomes larger than 3 kw. The efficiency of the full-bridge inverter shows 98.6% maximum at 3 kw and appears to maintain almost the same value even when output power is more increased. 7 Power loss analysis about components of the converter was investigated, the result of which is illustrated in Fig, 9. As seen in Fig. 9, the sum of these losses is increased with increasing output power. In the experimental condition of V in = 38 V, total loss of the soft-switching full-bridge DC- DC converter with secondary-side resonant circuit is increased from 3.7 W to 144 W (about 4.7 times growth) with increasing output power from.5 kw to 5.5 kw (11 times growth). In the MOSFET full-bridge circuit part, the loss is increased from 24.5 W to 81 W (3.3 times growth). Similarly, 8.8 (from 4.1 W to 36 W) and 12.9 (from 2.1 W to 27 W) values as the growth rate of power loss are calculated about the HF-transformer and the rectifier with the output filter, respectively. Figure 1 shows loss ratio of elements which compose the converter. The primary-side MOSFET transistors which construct the full-bridge circuit in the soft-switching DC-DC converter account for 8% of a whole consumption in the range of 1 kw or less and shows about 6% of that at 1.6 kw 1118

or more. This ratio still shows 56.3% at a full-load. These results indicate that most of the total converter loss is obviously due to the full-bridge circuit which consists of MOSFET switches. The loss ratio of HF-transformer decreases from about 3% to 25% with increasing output power, in the range of 1.6 kw or more. On the contrary, the loss ratio of the rectifier with the filter grows from 7.5% to 18.8% due to increasing conduction loss. Comparing Fig. 7 and Fig. 1, these results indicate that the effect of increasing loss of the rectifier with the filter on efficiency degradation of the converter becomes more significant as output power increases. V. CONCLUSION In this paper, the latest high-efficiency phase-shift softswitching transition pulse modulation DC-DC power converter with for PV generation systems has been presented, which is based on the secondaryside LC series resonant circuit principle. The operation principle of this new DC-DC converter in a steady state has been schematically described with the aid of simulation analysis and confirmed in experiment. This new conceptual high-frequency link is achieved by the highefficiency power conversion processing based on the secondary-side LC resonant principle. The comparative key operating waveforms between simulation and experimental results have been discussed and evaluated in details. The actual efficiency vs. output power characteristics have been demonstrated and discussed in this paper. Further considerations for this new conceptual softswitching DC-DC converter proposed have should be discussed as the following topics; (1) The soft-switching operating ranges for some circuit parameters, lossless snubbing capacitors dead time setting leakage and magnetizing inductance of highfrequency transformer, and the secondary-side LC resonant constants. (2) SiC power MOSFETs and SiC-SBD Integration. 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