A ZVT-PWM single stage PFC converter with an active snubber1 Wei Gu, Jaber Abu-Qahouq, Shiguo Luo, Issa Batarseh Electrical and Computer Engineering Department University of Central Florida Email:
[email protected] Abstract-A single-stage power factor correction converter with soft-switching is proposed in this paper. Besides the ZVT turn-on performance of the main switch, the converter has ZCS for the auxiliary switch with an active snubber. High frequency operation of the proposed converter makes the ac-dc power supply possible to be minimized in size and weight. A 50-W 500kHz prototype has been built in the laboratory to experimentally verify the analysis.
II. PROPOSED CONVERTER AND ITS OPERATION The proposed soft-switching converter with an active snubber shown in Fig 1 is based on the hard-switching single-stage PFC converter with two bulk capacitor proposed first in [3]. Besides achieving zero-voltage transition (ZVT), the circuit also using a transformer as a snubber to lower voltage and current stresses of switches. The main waveforms and the equivalent circuits for the modes of operation are shown in Fig. 2 and Fig 3.
I. INTRODUCTION As the use of power supplies continues to increase, more distorted mains current is drawn from the line, resulting in lower power factor and high total harmonic distortion. Power factor correction (PFC) is becoming more and more common in single-phase off-line switching-mode power supplies, not only because low power factor limits the maximum available power drawn from mains, but also agency regulation requires that the harmonic current of the line current of mains-connected equipment remains below certain limits. For years a great deal of effort has been made to development efficient and cost-effective power factor correction schemes. As a branch of active PFC techniques, the single-stage technique receives particular attention because of its low cost implementation [1] [2]. Moreover, with the residential industry and defense industry continuously demanding for even higher power density, switching mode power supply operating at highfrequency is required because at high switching frequency, the size and weight of circuit components can be remarkably reduced. But with the increasing of switch frequency, the switching loss becomes intolerable, resulting in very low conversion efficiency. Furthermore, the presence of leakage inductance in the high-frequency transformer and junction capacitance in the semiconductor devices causes the power devices to turn-off and turn-on with more energy loss and noise. Because of this reason, switch frequency of the traditional SMPS is limited within 100kHz. To boost the switching frequency, the soft-switching technique [4], [5], was introduced to alleviate the switching loss.
Li
Di
Dao
S
Lp1
Lap
Ds
Las
Cs
VAC
Cp2
Dp Lak Da
Sa
Lp2 Ca
Cp1
Fig 1 Basic circuit
S
Sa
Vcs
I Lak
I Di
ILp1
t0
t1 t 2
t 3t 4
t5
t6
Fig 2 Circuit waveforms
1
This work is supported by NSF (contract number NSF 99 608 03)
Ls
Lk1
Lk2
Do Co
Ro
Li
Di
Li
Dao
Lap
Ds S
Las Dp
Lak
Dao
Cp2
Lp1
Cs
VAC
Di
Do
S
Ls Co
Lk1
Da
Ro
Ca
Cp1
Lk2
Ca
Cp1
Lk2
Di
Dao
Do
Las Dp
Co
S
Ro
Cp1
Ca
Dp Da
Lk2
Sa
Ls
Lk1
Lak
Lp2
Da
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Cs
VAC
Cp2
Lp1
Lap
Ds
Ls
Lk1
Lak
Li
Cp2
Lp1
Cs
Sa
Do Co
Lp2 Ca
Cp1
Lk2
(f) Mode 6
(b) Mode 2
Fig 3 Modes of operation Li
Di
Dao
Lap
Ds S
Cp2
Lp1
Las
Cs
VAC
Dp Lak
Cp1 Lk2
Di
S
Mode 2: t1 < t < t2
Dao
Las Dp
Cs
VAC
Cp2
Lp1
Lap
Ds
Lak
Sa
Ls
Lk1
Da
Lp2 Ca
Cp1
(d) Mode 4
At t = t0 , the auxiliary switch S a is turned on.
conduct when auxiliary switch is on because the energy is transferred from the primary winding to the secondary winding. After that, S is ready to be turned on at ZVS.
(c) Mode 3 Li
Ro
C s , C a , and Lrak form a resonant tank as can been in Fig. 2a. At the end of this mode (t = t0 ), the capacitor voltage of the main switch S hits zero. Dao starts to
Lp2 Ca
Mode 1: t0 < t < t1
Do
Ls Co
Lk1
Da Sa
Ro
(e) Mode 5
Lap
VAC
Co
Lp2
Sa
Dao
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Dp Lak
(a) Mode 1 Li
Las
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Lp2
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Lp1
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Cp2
Lk2
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Co
Ro
After turning S on at t = t1 , the diode Di conducts and the source voltage is applied to the input choke inductor L, causing the current through the inductor increasing linearly. During this period, energy is transferring from the source to the choke inductor. The period ends when the leakage inductor current I Lak reaches zero and Dao is turned off. The choke current continues to increase linearly. Mode 3: t2 < t < t3 The equivalent circuit for this freewheeling mode is shown , The auxiliary switch is turn off in this period.
Ro
Mode 4: t3 < t < t4 At t = t3 , S is turned off. The main switch output
Using the component values from table1, the schematic (Fig 4) produces the result shown in Fig 5, which verifies the theoretical analysis.
capacitor C s is quickly charged up to 2V p1 by the current
iL1 . Under the constraint of KCL, both the storage capacitors, C p1 and C p 2 are being charged by current iL1 + iL 2 during this operation period. With the inductor current i Li decreasing linearly, magnetic energy stored in the choke is being converted into electric energy and being stored into the storage capacitors. Thus the energy loss of the storage capacitors during Mode 2 is being recovered. Mode 5: t4 < t < t5 The choke inductor current, I L
continues to
decrease linearly. Owing to the existence of diode Do , the primaries of the transformer present very high impedance with the currents through the windings can be negligible. This period ends when the choke inductor current reaches zero.
Fig 4 Simulation schematic
Mode 6: t5 < t < t6 This is a free-wheeling stage for regulation purpose.
III Soft-switching condition Soft-switching is maintained for wide line and load range, which is a unique feature for ZVT. In mode 1, the differential voltage across the main switch is
VCs = V0 cos(
1 t) Lak C s
V0 is the Vcs at t = 0 . For ZVS, Vcs drops to zero before the mode 2 begins. So we get
t1 − t0 >
π Lak C s 2
The right side of the above inequality is a constant, which means for this converter, soft-switching operation will be ensured for the whole load and line range as long as we choose proper values for the time delay between the gate signals of main and auxiliary switches and the leakage inductance of the snubber transformer.
IV. COMPUTER SIMULATION
Fig 5 Simulation result
As can be noted, the simulated waveforms are the same as theoretical waveforms. Also, the ZVS and ZCS are achieved for the main switch and auxiliary switch, respectively.
V. EXPERIMENT RESULTS A 50-W 500-kHz prototype has been built in the laboratory to experimentally verify the analysis. It agrees well with the computer simulation. In fig 6, the upper waveform is the drive signal of the power MOSFET and the lower waveform is the voltage across the MOSFET. It’s shown that drive signal starts rising after the voltage across the MOSFET drops down to zero, which is zero voltage switching. However, from fig 6, we can see the noise, which
should be reduced by using a transformer with a smaller parasitic capacitance and inductance.
Fig 6 Experiment result
6 Conclusions Due to high frequency operation, the proposed converter makes the ac-dc power supply possible to be minimized in size and weight. At the same time, the active single-stage PFC techniques help to reduce the component count and cost. Therefore, for the low power application, this proposed converter could be considered as a strong competitor. ACKNOWLEDGMENT The authors of this paper would like to thank Dr. Peter Kornetzky and Dr. Huai Wei, for providing the advice. REFERENCES 1. R. Redl, “Power-factor correction in single-phase switching-mode power supplies-an overview”, Int. J. Electronics, Vol. 77, No. 5, 555-582, 1994. 2. B. Sharifipour, J. Huang, and P. Liao, “Manufacturing and Cost Analysis of Power-Factor-Correction Circuits”, IEEE Applied Power Electronics Conf. (APEC) Proc., pp 490-494, 1998. 3. P. Kornetzky, H. Wei, G. Zhu and I. Bartarseh, “A Single-Switch AC/DC Converter with Power Factor Correction,” Proceedings of PESC97, pp. 527525,1997. 4. G. Hua, C.S. Leu, Y. Jiang, and Fred C. Lee, “Novel Zero-Voltage-Transition PWM Converters,” Proceedings of PESC92, pp. 55-61, 1992. 5. J.P. Gegner and C. Q. Lee, “Zero-voltage-Transition Converters Using an Inductor Feedback Technique”, Proceedings of PESC94, pp. 590-596, 1994.
