WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Geeng-Kwei Chang, Shu-Yuan Fan, Sheng-Yu Tseng Multi-output Auxiliary Power Supply with Lossless Snubber Geeng-Kwei Chang Shu-Yuan Fan Department of Electrical Engineering Wufeng University Ming-Hsiung, Chia-Yi, Taiwan, R.O.C e-mail: [email protected] Sheng-Yu Tseng Green Power Evolution Applied Research Lab (G-PEARL) Department of Electrical Engineering Chang Gung University Kwei-Shan, Tao-Yuan, Taiwan, R.O.C e-mail: [email protected] Abstract: - A multi-output auxiliary power supply realized by a flyback converter is presented for providing the essential low power supplies for control circuits and driving circuits in power processing systems. In the pro- posed circuit topology, lossless snubbers are introduced into the flyback converter to recover energy trapped in leakage inductor of transformer, and as a result, smoothes out voltage surge across drain-source of the switch, and alleviates oscillation caused by parasitic capacitance of the switch and leakage inductance of the transfor- mer, hence reduces switching losses and improves conversion efficiency. Based on these concepts, an auxiliary power supply with five sets of output voltage has been designed and implemented to verify the feasibility of the proposed multi-output auxiliary power supply. Experimental results show that the conversion efficiency has been increased about 5% and reached 82.5% under full load as compared with flyback converter with hard- switching. Key-Words: - flyback converter; lossless snubber; auxiliary power supply. 1 Introduction SMPS, such as half-bridge converter, full-bridge Power processing systems play a major role in to- converter, push-pull converter, flyback converter or day's electric utilities, for instance, in solar power forward converter [3], and so on. generation systems, in power unit of electric vehicles, Among various selecting criteria for an appropri- in battery chargers, in space satellites, in uninterrupt- ate converter, galvanic insulation is an decisive one ible power supplies (UPS), in active power filters, etc. when insulation from power line is recommended. In the power processing systems, the auxiliary power Since the auxiliary power supply takes input voltage supply is needed to provide different voltages for the from power line and has to provide several regulated control circuits and driving circuits. The needed vol- and isolated dc output voltages with different levels, tages are ranged from 3.3V to 24V. Two different the galvanic insulation becomes the most important design concepts are available, one is low-frequency L D ac transformer, and the other is high-frequency K 2 Tr1 N1 N2 switch-mode power supply (SMPS) [1, 2]. Although C1 the ac transformer can provide stable and indepen- Lm CO RO dent power source, it is bulky and uneasy to install on R 1 D1 1:N circuit board. On the other hand, SMPS, been com- Vi pact in weight and size and more efficiency, is find- M ing increased attention in industry and academia as 1 one of the preferred choices for low-power applica- tion. SMPS has successfully made its way into the industry and is now a mature and proven technology. Fig. 1. Schematic diagram of flyback converter with Many kinds of converters can be used to realize the conventional RCD snubber. E-ISSN: 2224-266X 191 Issue 6, Volume 12, June 2013 WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Geeng-Kwei Chang, Shu-Yuan Fan, Sheng-Yu Tseng LK D2 LK D2 Tr Tr1 N 1 N2 N1 N2 D2 Lm CO RL Lm CO RO 1:N RCD snubber 1:N Vi V i C1 D L1 R1 1 M1 M1 D1 C1 Fig. 2. Schematic diagram of flyback converter with Fig. 4. Schematic diagram of flyback converter with lossless LCD snubber. conventional RCD snubber in parallel with the LK D2 conversion efficiency. Therefore, as shown in Fig. 3, an active clamp circuit composed of an active switch Tr1 N1 N 2 C1 is introduced into the flyback converter. Besides of Lm CO RO resetting the transformer and suppressing the surge, M2 the circuit can store and release the surge energy at 1:N V i adequate timing through the active switch to raise efficiency and achieve soft-switching [12-14]. Al- M1 though the active clamp circuits can increase conver- sion efficiency of converter, it needs extra driving circuit and active switch, resulting in a higher cost. Hence, a lossless snubber shown in Fig. 4 is proposed Fig. 3. Schematic diagram of flyback converter with to recover energy trapped in leakage inductor and conventional active clamp circuit. reduce switching losses. As shown in Fig. 4, the pro- posed lossless snubber is composed of passive com- factor to be taken into consideration. Hence, flyback ponents only and can achieve a lower cost. type or forward type converter with multiple outputs A variant of the RCD type snubber is adopted for coupled magnetically through transformer are more the multi-output flyback converter. The RCD snubber suitable for auxiliary power applications [4]. in Fig. 1 can be placed in parallel with the switch, as In reality, no matter how one arranges the winding shown in Fig. 4. In order to recover the energy structure inside the transformer, it brings about lea- trapped in leakage inductor of transformer, the resis- kage inductance. As a result, a flyback converter generally suffers from low efficiency and voltage tor R1 in the RCD snubber can be replaced by a surge across power transistor switches. To reduce transformer coupled to the output, resulting in the voltage surge, an RC clamp circuit is usually inserted circuit in Fig. 5. It can be seen that the proposed to absorb the energy stored in the leakage inductance snubber can recover the energy trapped in leakage and suppress the spike. This absorbed energy is dissi- inductor to output load and thus increases the overall pated in the snubber resistor during transistor turn-on. L T D To increase the efficiency, a non-dissipative LC K1 r1 O clamp circuit was proposed. In the circuit, the ab- D3 L O sorbed energy is returned through the snubber induc- Tr2 C R tor to the power source [5-7]. Both of the aforemen- O O VO Lm1 tioned circuit do not consider the diode reverse re- covery effects which may cause the oscillation in vol- 1:N 1:N V tage surge during switching, RCD as well as RC- i RCD clamp circuits are proposed [8-11]. A typical Tr2 RCD snubber is shown in Fig. 1. With the RCD D snubber, spike voltage across switch is reduced, but 1 Lm21 D 2 LK21 M1 additional power loss is generated. In Fig. 2, a loss- CM1 1:N less LCD snubber is used to replace RCD snubber to DM1 C1 increase the conversion efficiency. Since RCD and LCD snubber can only improve voltage surge across Fig. 5. Schematic diagram of flyback converter switch to reduce switching loss, it does not increase with the proposed lossless snubber. E-ISSN: 2224-266X 192 Issue 6, Volume 12, June 2013 WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Geeng-Kwei Chang, Shu-Yuan Fan, Sheng-Yu Tseng V1 LK11 Tr1 D O1 diode Do keeps forward biased, which makes trans- T r2 N4 former T continue transferring its energy to the N1 DL3O 1r L m1 1 CO1 R1 VO 1:N load. Additionally, before t0 , voltage across capacitor CO2 R2 DO2 C reaches the level (/)VVNi+ o , higher than Vi 1 1:N1:N2:N3:N4:N5 V2 V3 VNo / , which, when switch M 1 is ON, been applied Tr2 DO3 C O3 R3 across the primary winding of transformer Tr2 , will D 1 Lm21 M1 D2 L K2 1 N3 1:N make diodes D2 and D3 forward biased. Thus, C1 V4 DO4 energy stored in capacitor C1 can be transferred to CO4 R4 the output through diode D3 . Meanwhile, the circuit V5 including diodes D2 and D3 is equivalent to an LC DO5 CO5 R5 circuit (suppose that Vo is constant), and current iD3 GND1 will resonant from zero to a positive value, then back to zero while cut off diode D . Fig. 6. Schematic diagram of the proposed 3 Mode 2 [Fig. 7(b); t≤ t< t ] auxiliary power supply with five sets of output 1 2 voltages. At t1 , I LK11 equals I Lm11 and I N1 equals zero, which makes the diode D reversely biased. The efficiency. Fig. 6 shows the complete circuitry with o five sets of output. input side circuit behavior is dominated by diLK11 VLLi=( K11 + m11 ) (3) dt 2 Operational Principles of the Pro- That is, within this time interval, IILm11= LK11 posed Auxiliary Power Supply increases linearly and transformer T 1r is storing In order to simplify the analysis of the proposed energy. The capacitor C1 keeps releasing its energy snubber, the one output converter as shown in Fig. 5 to the load through diode D2 、 transformer Tr2 and is considered in the analysis. Operation of this circuit diode D3 . This mode ends when diode D3 is OFF. can be divided into seven operation modes, and sup- Mode 3 [Fig. 7(c); t2≤ t< t 3 ] pression of transistor surge voltage is different in This mode begins at t when diode D is OFF. In each mode. In Fig. 7, (a)~(g), the operational prin- 2 3 ciples corresponding to each mode are illustrated. this mode, both transformers T 1r and Tr2 are storing From the key waveforms shown in Fig. 8, the detail energy, T 1r from the source Vi and Tr2 from the operation of this converter can be clarified. capacitor C1 .