A New Active Soft Switching Technique for Pulse Width Modulated Full Bridge DC-DC Converters Naga Brahmendra Yadav Gorla and N. Lakshmi Narasamma auxiliary switches are not soft switched. A new active soft switching circuit for non-isolated and isolated converters is proposed in [6]-[9]. The novelty of these circuits lies in achieving soft switching for both main, auxiliary switches and gating to auxiliary switches are ground referred. Proposed active soft switched ZVS PWM full bridge converter has two identical auxiliary cells connected to conventional full bridge as shown in Fig. 2. S1 to S4 are main switches and D1 to D4 are output rectifier diodes of the converter.c1 to C4 are capacitors and DB1 to DB4 are anti parallel diodes across the main switches S1 to S4 respectively. Auxiliary cell-1 consists of an active switch S4a, a diode D4a, resonant inductor L4r, resonant capacitor C4r, a winding (L4T) coupled to the primary of power transformer. Auxiliary cell-1 when operated properly achieves ZVS to main switch S4. Gating sequence to main (S1 to S4) and auxiliary switches (S2a, S4a) are as shown in Fig. 3. Auxiliary switch S2a should be gated immediately when main switch S3 is turned off and before S2 is gated. Auxiliary switch S4a is also gated in the similar way as that of S2a. Turns ratio between primary and secondary of power transformer is n. Turns ratio between primary and coupled windings (L2T, L4T) is kt. Abstract A new active soft switching circuit for Zero Voltage Switched Pulse Width Modulated (ZVS-PWM) full bridge converter is presented in this paper. The proposed circuit has two auxiliary circuit cells (Auxiliary circuit cell-1, Auxiliary circuit cell-2), one for each ground referred active switch. Auxiliary circuit cell consists of an active switch, a diode, a resonant inductor and a capacitor, and a coupled winding derived from main power transformer. Auxiliary circuit when gated properly creates zero voltage across the main switch during its turn-on. Winding coupled to the power transformer helps in resetting auxiliary inductor current to zero and hence turn-off of auxiliary switch is lossless. Steady state operation of proposed circuit with necessary analytical expressions is presented. Circuit simulation results of the proposed active soft switched ZVS-PWM full bridge converter are presented. Index Terms Zero voltage switching, active soft switching, ZVS PWM full bridge. I. INTRODUCTION The constant demand in Power Processing Systems (PPS) for applications like water pumps, air conditioner fed from Photo Voltaic (PV) systems (as shown in Fig. 1) is towards higher efficiency and power density with low EMI. PPS for example in applications like air conditioners has to convert 48 VDC input from PV to 230 VAC. This paper focuses on the front end DC-DC converter required to meet such load demands. Fig. 3. Gating pulses (G1-G4) to main switches (S1 -S4) and gating signals (G2a, G4a) to auxiliary switches (S2a, S4a) respectively. Fig. 1. PV system feeding solar pumps, air conditions systems etc. Full bridge DC-DC converters are conventional choice for medium and high power applications. Isolation transformer provides high voltage gains apart from providing isolation. Switching at high frequency provides better power densities but overall system efficiency reduces because of increased switching losses. Soft switched full bridge converters such as ZVS-PWM full bridge converters addresses this issue [1]-[5]. Salient features of the circuits proposed in these papers are wide range of ZVS, complexity in implementation; higher conduction losses during freewheeling intervals, some of the II. STEADY STATE ANALYSIS Steady state analysis of the proposed active soft switched ZVS PWM full bridge converter is presented in this section. To reduce the complexity of analysis, following assumptions are made. y All the devices are assumed ideal (no Rdson for switches, no forward voltage drop for diodes). y Output filter inductor is large enough to treat it as a constant current source. y Parasitics of transformer such as inter winding capacitance, leakage inductance are neglected. Manuscript received June 10, 2013; revised August 27, 2013. The authors are with Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai 600036, India (e-mail: gorla.indra@gmial.com, lakshmin@ee.iitm.ac.in). DOI: 10.7763/IJESD.2014.V5.444 20
Fig. 2. Proposed active soft switching circuit for pulse width modulated full bridge DC-DC converterssteady state operation of the circuit is divided into twelve intervals (I1 -I12). Circuit conditions during this interval are as shown in the Fig. 6. All rectifier diodes in the secondary D1 -D4 are in conduction sharing full load current equally. Interval1 (I1) (t0 < t < t1): This is a power transfer interval and circuit conditions during this interval are as shown in Fig. 4. Devices S1, S2, D1 and D2 are in conduction during this interval. dim (t ) 0;i p (t ) I m dt I id1 (t ) id 3 (t ) 2 Lm Fig. 4. Positive power transfer interval. Lm dim Vdc dt im (t ) im (t0 ) Vdc (t t0 ) Lm Fig. 6. Freewheeling interval. This interval ends when S1 is turned off. Interval 4 (I4) (t3 < t < t4): This interval starts when auxiliary switch S4a is gated immediately when S1 is turned off. Circuit conditions during this interval are depicted in Fig. 7. Resonant inductor current raises linearly during this interval until it reaches reflected load current in the primary. All the rectifier diodes (D1 -D4) are in conduction during this interval. Equations governing this interval are as follows. i p (t ) i' +im (t) = ni +im (t) This interval ends when gating to the main switch S2 is turned off. Interval 2 (I2 ): (t1 < t < t2 ) Next switch to turn-on is S3, by the end of this transition interval capacitor C3 across S3 has to discharge completely and body diode DB3 should be in conduction. Gating S3 while body diode DB3 in conduction ensures ZVS for main switch S3. Circuit conditions during this interval are as shown in Fig. 5. il4 r (t ) Vdc (t t3 ) L4 r i' (t ) n(id1 (t ) id 3 (t )) Fig. 5. Transition from positive power transfer to freewheeling interval. dvc 3 (t ) i p (t ) ;i p (t ) ni I m dt 2C ni I m ni I m vc 2 (t ) (t t1 );vc 3 (t ) Vdc (t t1 ) 2C 2C Fig. 7. Linear charging interval in auxiliary cell-1. This interval ends when resonant inductor current reaches reflected load current. Interval 5 (I5) (t4 < t < t5): Resonant inductor (L4r) resonates with resonant capacitor (C4r ) during this interval. Voltage across the main switch S4 decreases from Vdc to zero Interval 3 (I3) (t2 < t < t3): This is a freewheeling interval. 21
during resonance. Circuit conditions during this interval are as shown in Fig. 8. Equations governing this interval are as follows. By the end of this interval, body diode of main switch S4 will be in conduction. Gating S4 while DB4 in conduction ensures ZVS during turn-on. This interval ends when main switch S4 is turned off. Interval 8 (I8): Next switch to conduct is S1, hence capacitor C1 has to discharge completely and anti parallel diode DB1 should conduct to ensure ZVS turn-on for S1. Circuit conditions during this interval are shown in Fig. 11. Fig. 8. Resonant interval in auxiliary cell-1. Fig. 11. Transition from negative power transfer to freewheeling interval. il4 r ( t ) = ni + Vdc Interval 9 (I9): This is a freewheeling interval. Main switch S3 and anti parallel diode DB1 are in conduction during this interval. All the rectifier diodes D1-D4 are in conduction since voltage applied across the primary of transformer is zero during this interval. C4 r Sin (ω ( t t4 ) ) L4 r vc4 r (t ) = Vdc cos (ω ( t - t4 ) ), where ω = 1 L4 r C4 r Interval 6 (I6) (t5 < t < t6): Circuit conditions during this interval are as shown in Fig. 9. Voltage across resonant capacitor is Vdc/kT. This capacitor is clamped by the body diode of S4 to resets the resonant inductor current to zero by the end of this interval. Turning off S4a after complete reset of auxiliary inductor current ensures ZCS during its turn-off. Equations governing this interval are as follows. Fig. 12. Freewheeling interval. Circuit conditions during I9 are as shown in Fig. 12. This interval ends when gating to active switch S3 is removed. Interval 10 (I10): Auxiliary switch S2a is gated immediately when S3 is turned off as shown in Fig. 3. The circuit condition during this interval is as shown in Fig. 13. Current through resonant inductor (L2r) raises linearly. All the rectifier diodes D1-D4 continue to conduct during this interval. Fig. 9. Resetting interval in auxiliary cell-1. il4 r ( t ) = il4 r ( t5 ) Vdc ( t t5 ) KT L4 r 2 V C4 r KT 1 dc ( t t5 ) = ni + Vdc L4 r KT KT L4 r Interval 7 (I7): This is a negative power transfer interval. Main switches S3 and S4, output rectifier diodes D3, D4 are in conduction during this interval. Circuit conditions during I7 are as shown in Fig. 10. Fig. 13. Linear charging interval in auxiliary cell-2. This interval ends when resonant inductor current reaches reflected load current. Interval 11 (I11): Resonant inductor L2r and resonant capacitor C2r resonates during this interval. Output rectifier diodes D3 and D4 gets reverse biased and D1 and D2 conducts full load current. By the end of this interval, capacitor C2 across the main switch S2 discharges completely and body diode DB2 starts conducting. Gating S2, while DB2 in conduction ensures ZVS during turn-on. Circuit conditions during this interval are depicted in Fig. 14. Fig. 10. Negative power transfer interval. 22
Fig. 14. Resonant interval in auxiliary cell-2. Interval 12 (I12): The circuit condition during this interval is as shown in Fig. 15. The resonant capacitor voltage is clamped to Vdc/kT since DB2 is in conduction during this interval. This resets resonant inductor current linearly to zero. Turning off S2a when resonant inductor current is zero ensures ZCS during its turn-off. Fig. 15. Resetting interval of auxiliary cell-2. Next interval is positive power transfer interval i.e., I1. Equations governing interval 1 to interval 6 also holds good for interval 7 to interval 12. Modes of operation along with interval description are shown in Table I. Theoretical waveforms for proposed converter is shown in Fig. 16 & Fig. 17. Auxiliary circuit intervals i.e., linear charging interval (t3 < t < t4), Resonant interval (t4 < t < t5) and resetting interval (t5 < t < t6) are indicated in Fig. 17. As mentioned earlier, voltage across the coupled winding helps in resetting the resonant inductor current to zero. Turning off the auxiliary switch after resetting the resonant inductor current reduces the switching losses in the auxiliary switches as shown. Fig. 16. Theoretical waveforms of proposed active soft switched ZVS PWM full bridge DC-DC converter. TABLE I: MODES OF OPERATION OF PROPOSED ACTIVE SOFT SWITCHED ZVT PWM FULL BRIDGE DC-DC CONVERTER Fig. 17. Theoretical waveforms showing gating to auxiliary switch, corresponding auxiliary inductor current, gating and drain to source voltage of main switch. III. RESULTS AND DISCUSSIONS Circuit simulations results of proposed active soft switched ZVS-PWM full bridge DC-DC converter is presented in this section. Gating to the main switches are 23
shown in Fig. 18. It can be seen from the figure that gating to the ground referred switches S2, S4 are PWM controlled. Main switches S1, S3 are gated complementary with proper dead time between switches of same legs i.e., between S1 and S4 or S2 and S3. Gating to auxiliary switch S4a of auxiliary cell-1 and main switches S1, S4 are shown in Fig. 19. It can be seen from this figure that auxiliary switch S4a is gated immediately after S1is turned off and before S4 is turned on. Gating to auxiliary switch S4a, current through auxiliary inductor L4r, gating to main switch S4 and drain to source voltage of main switch S4 are shown in Fig. 20. Similarly gating to auxiliary switch S2a, current through auxiliary inductor L2r, gating to main switch S2 and drain to source. Voltage of main switch S2 is shown in Fig. 21. The following inferences are drawn from these figures. Drain to source voltage of main switches S2 and S4 is zero during their turn-on because of auxiliary cell-1 and auxiliary cell-2. Hence main switches are turn-on with ZVS. Auxiliary switch current which is same as auxiliary inductor current is made zero before its turn-off. This ensures ZCS turn-off for auxiliary switches. Auxiliary switches conduct for smaller duration of time (apx 7-10%) of total switching period. Hence conduction losses due to additional circuit will be less. Fig. 18. (from top) Gating to main switches S1-S4. Fig. 22. (from top) Gating to the main switches S1 and Drain to source voltage of S 1. Fig. 19. (from top) Gating to main switches S1, Gating to auxiliary switch S4a, Gating to main switch S4. Fig. 23. (from top) Gating to the main switches S3 and Drain to source voltage of S3. Gating and drain to source voltage of main switches S1and S3 are shown in Fig. 22 and Fig. 23 respectively. It can be seen from these figures that drain to source voltage has come down to zero before gating to main switches are given. It can be concluded that additional auxiliary cells did not disturbed the ZVS mechanism for main switches S1and S3. Fig. 20. (from top) Gating to auxiliary switches S4a, Resonant inductor current IL4r, Gating to main switch S4, Drain to source voltage of S4. IV. CONCLUSION A new active soft switched ZVS-PWM full bridge DC-DC converter is proposed in this paper. The proposed auxiliary circuit achieves zero voltage during turn-on for main switches without affecting the zero voltage turn-on conditions for other switches. Proposed auxiliary circuit operates for small duration of time (10% of Ts) and hence additional conduction losses would be less. Current through the auxiliary switch is made zero during turn-off which reduces the turn-off losses in the auxiliary switches. Fig. 21. (from top) Gating to auxiliary switches S2a, Resonant inductor current IL2r, Gating to main switch S2, Drain to source voltage of S2. 24
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] J. G. Cho, J. A. Sabate, G. Hua, and F. C. Lee, Zero voltage and Zero current switching full bridge PWM converter for high power applications, in Proc. IEEE Power Electronics Specialists Conference, June 1994, pp. 102-108,. K. Yoshida, N. Nagagata, T. IsHii, and H. Handa, ZVS-PWM full-bridge converter using active current clamping with synchronous rectifiers, in Proc. 30th Annual IEEE Power Electronics Specialists Conference, August 1999, vol. 1, pp. 257-262. G. Moschopoulos and P. Jain, ZVS PWM full-bridge converters with dual auxiliary circuits, in Proc. 22nd International Telecommunications Energy Conference, 2000, pp. 574-581. M. Borage, S. Tiwari, S. Bhardwaj, and S. Kotaiah, A Full-Bridge DC-DC Converter With Zero Voltage Switching Over the Entire Conversion Range, IEEE Transactions on Power Electronics, vol. 23, no. 4, pp. 1743-1750, July 2008. M. Ordonez and J. E. Quaicoe, Soft-Switching Techniques for Efficiency Gains in Full-Bridge Fuel Cell Power Conversion, IEEE Transactions on Power Electronics, vol. 26, no. 2, pp. 482-492, February 2011. N. Lakshminarasamma and V. Ramanarayanan, A family of auxiliary switch ZVS-PWM DC-DC converters with coupled inductor, IEEE Transactions on Power Electronics, vol. 22, no. 5, pp. 2008-2017, September 2007. G. N. B. Yadav and N. Lakshminarasamma, Novel soft transition pushpull converter: Analysis, modeling, design and implementation, in Proc. IECON 2011-37th Annual Conference on IEEE Industrial Electronics Society, 7-10 November 2011, pp. 1486-1491. [9] G. N. B. Yadav, and N. Lakshminarasamma, A new active soft switching circuit for full bridge converters, in Proc. IEEE international conference on power electronics, drives and energy systems, 16-19 December 2012, pp. 1-6. G. N. B. Yadav and N. Lakshminarasamma, Analysis and Design of a Novel Active Soft Switched Phase-Shifted Full Bridge Converter, World Academy of Science, Engineering and Technology 73, 2013. Naga Brahmendra Yadav Gorla received B.Tech degree in Electrical and electronics Engineering from Acharya nagarjuna university, India in 2010. He is currently working towards MS (Research) in Power Electronics at Indian Institute of Technology Madras, Chennai, India. His areas of interest include DC-DC conversion, inverters, parasitic effects at higher switching frequencies etc. N. Lakshmi Narasamma obtained her Ph.D. degree in Electrical Engineering from the Indian Institute of Science and joined the faculty of Electrical Engineering at the Indian Institute of Technology, Madras as an Assistant Professor in the year 2009. She has coauthored four journal papers in peer-reviewed journals, including the IEEE Transactions on Power Electronics and several premier conferences. Her research interests are in the areas of Power Electronics and drives. 25