AbstractâThis paper proposes a lossless snubber cell for all single-stage PFC with a boost type input current shaper and an isolated dc/dc converter. The single ...
A snubber cell for single-stage PFC with a boost type input current shaper and isolated dc/dc converter Qian Zhang, Habing Hu, Abdel-Rahman Osama, Shen John, Issa Batarseh dept. of electrical engineering and computer science University of Central Florida Orlando, USA Abstract—This paper proposes a lossless snubber cell for all single-stage PFC with a boost type input current shaper and an isolated dc/dc converter. The single auxiliary switch of the snubber cell shares the same switching signal as the PFC main switch; both switches operate under soft switching way. Various stages of the switching cycle have been comprehensively analyzed to explain the operation principle of the soft switching transition. This soft switching technique reduces switching power loss and improves efficiency. The simulation results verify the practicable of the new snubber cell. 100W prototypes are built on the bi-flyback PFC converter and the BIFRED PFC converter; experimental waveforms match with simulation results.
I.
In recent years, many soft switching techniques and snubber techniques have been proposed for PFC converter [6-8]. However, there are few topologies that include both functions in one cell. In this paper, a new snubber cell combined soft switching technique with snubber technique together is proposed, moreover this snubber cell can be implemented generally to all isolated single-stage PFC with a boost type input shaper. Besides these topology advantages, the auxiliary switch and the main switch share the same PWM signal which also simplify the control scheme. Operating statuses of the soft switching technique are analyzed in detail, and parameters design is discussed theoretically.
INTRODUCTION
Single stage PFC converters with advantages in efficiency, size, reliability and quality, are commonly applied in low power area to satisfy harmonic standards [1-3]. In a single-stage PFC converter, the functions of input current shaping, isolation, and output voltage regulation are all achieved by one switch [4]. Due to the simplicity in both circuit design and controller design, a boost converter is generally selected as an input current shaper type. In a boost converter topology, only a partial power passes through the switching device whereas the rest of the power passes directly to the output side, and this property increases the efficiency of the converter [4-5]. Moreover, a boost inductor helps reducing the current stress on power components. In the design of a boost type converter, to reduce the size of an inductor, high switching frequency is a preferred developing trend. However, high switching frequency of the PFC converter increases not only power losses but also EMI emissions. For the requirement of isolation, a forward or flyback dc/dc converter with transformer is usually cascaded with the boost converter. Meanwhile the leakage inductor of the transformer causes high voltage spike on the switching device. This voltage spike increases voltage stress on the switching device extensively. To reduce the voltage spike, a snubber circuit design is very necessary.
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II.
OPERATION ANALYSIS AND PARAMETERS DESIGN
A. Operation principle Figure 1 presents the new active snubber cell and its applications in both bi-flyback and BIFRED topologies as
examples. The main switch S and the auxiliary switch Sr share the same PWM signal. To simplify the operation analysis, assume that the capacitor Cs is large enough, therefore the voltage across this capacitor can be represented
by a constant value VCs . Take the bi-flyback topology as an example to analyze the operation principles of the snubber cell. Assume that the whole PFC converter system has achieved a steady status. Operation of the snubber cell can be divided into three stages as illustrated in Figure 2. The detailed analysis follows. Stage 1[t0-t1]: During this stage both switches S and Sr are off. Since the bi-flyback PFC converter has two operation modes: boost mode and flyback mode [9]; if the converter operates under boost mode, leakage current of the transformer T 1 flows through the diode Dr1 to charge the capacitor Cr , otherwise the converter operates under flyback mode, leakage currents of both T 1 and T 2 flow through Dr1 to charge Cr , shown as Figure. 2. The diodes Dr 2 and Dr 3 are both off as no currents flow through them.
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Stage 2[t1-t2]:Both switches S and Sr are turned on at
Figure. 2. On the other side the leakage currents
(a) (b) (c) Figure 1. (a)proposed snubber cell, (b)proposed snubber circuit for bi-flyback PFC converter, (c)proposed snubber circuit for BIFRED PFC converter
Figure 2. various operation stages during one switching cycle
(a) Figure 3. key waveforms of the snubber cell during one switching cycle
(b)
the same time. Whereas the diodes Dr1 and Dr 3 are turned off by the reverse voltage across them individually. Energy stored in the capacitor Cr discharges through the inductor Lr and the diode Dr 2 , shown as Figure. 2. The current iLr increases when the voltage VCr is larger than VCs , and decreases when the voltage VCr drops below VCs . During this discharging period, since both diodes Dr1 and Dr 3 can prevent the voltage across Cr from going negative, the voltage across the capacitor Cr continuously decreases until the voltage reaches zero. Stage 3[t2-t3]: Stage 3 only exists when the current iLr has not reached zero at the end point of the stage 2. The next turn-off signal turns off both switches S and Sr ; the current iLr flows through Dr 2 , Cs and Dr 3 to keep decreasing until iLr reaches zero, then Dr 2 and Dr 3 are turned off by the reverse voltage across them individually, shown as
are charging Cr as analyzed in stage 1. Key waveforms of the snubber cell during one switching cycle are shown as Figure 3. It is obvious to observe from this figure that, when the inductor current iLr reaches zero during the turn-on period, the switching cycle has two operation stages, shown as Figure 3(a), otherwise it has three stages, shown as Figure 3(b). Regardless of the number of operation stages, the main switch S realizes ZVS in all turnoff transitions, while the auxiliary switch Sr realizes ZCS in all turn-on transitions. B. Parameters design To ensure of the soft switching conditions, observing from Figure 3, two requirements have to be pre-satisfied: VCr has to achieve zero during the turn-on period, and iLr has to achieve zero before the end of turn-off period. The second requirement is easy to meet because of the free wheeling diode Dr 3 . To satisfy the first requirement,
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parameters design for the snubber circuit should be systematically analyzed.
VinDT ⎧ ⎪ I 01 = L1 ⎨ VCsDT ⎪ I 02 = L2 ⎩
Observe from Figure 2, it is obviously to get equation (1) during the voltage VCr discharging period
diLr ⎧ ⎪VCr − Lr dt = VCs ⎨ dVCr ⎪ Cr = −iLr dt ⎩
(1)
Assume that at the beginning of the turn-on period,
⎧V Cr = V Cs + V con ( t = 0 , Vcon > 0 ) ⎨ i Lr = 0 ⎩
(2)
and at the end of the discharging period, the voltage VCr should decrease to zero VCr = 0 ( t = tx ≤ ton ) (3) Take (2-3) as boundary restriction for equation (1), it is directly to derive equation (4)
⎧V Cr = V con cos ω t + V Cs ⎪ ⎨ i Lr = V con C r sin ω t ⎪⎩ Lr
(ω =
1 ) LrCr
(4)
Since the voltage VCr has to achieve zero during the turn-on period, among (4), the amplitude Vcon should be greater than VCs , and Vcon is larger, the voltage VCr is decreasing faster to zero. However for the current iLr , the amplitude is preferred to be smaller in order to reduce the current stress on auxiliary diodes Dr1 and Dr 2 . To get further restriction for parameters selection, consider the charge balance. The electric charge stored on the capacitor Cr is charged by the leakage current
Cr (Vcon + VCs ) = ∫ ileakdt
where L1 stands for the primary side inductance of T 1 . For the flyback mode, the charge should be calculated separately for each transformer and added up together. Consider practical operation conditions, the leakage inductance are usually about 5% of the transformer inductance. Combine all these restrictions above, values of Lr and Cr could be appropriately selected. III.
I0 2π LleakCr
SIMULATION RESULTS
A simulation bi-flyback prototype is built up using Matlab Simulink to test the proposed snubber cell. Parameters of the simulation system are set as below: Input: 110Vac; Output: 20Vdc @100W; T 1 : primary inductance L1 = 90uH , turns ratio n1 = 5 ; T 2 : primary inductance L 2 = 600uH , turns ratio n 2 = 3 ; leakage inductors are 5% of primary inductors for individual transformers; Switching frequency: 100kHz. The auxiliary components: Cr = 4.4nF , Lr = 35uH . Figure 3.(a) shows the waveform of the voltage VCr , while Figure 3.(b) shows the waveform of the current iLr .
(5)
When every time the main switch turns off, the leakage current decreases to zero to charge the capacitor Cr . Assume the initial leakage current is I 0 , and the leakage inductor is Lleak ,
Cr (Vcon + VCs ) =
(10)
(a)
(8)
For the bi-flyback PFC converter system, when the system works under boost mode, the leakage current is mainly supplied by the second transformer T 2 ,
I0 =
VCsDT L2
(9)
(b)
where L 2 stands for the primary side inductance of T 2 ; D stands for the duty cycle, and T stands for the switching cycle. When the system works under the flyback mode, the leakage currents are supplied by both transformers,
Figure 3.(a)voltage waveform across through
Cr , (b)current waveform
Lr
The prototype of BIFRED PFC converter is also built up with simulink, simulation waveforms are similar with Figure 3. Waveforms of Figure 3. illustrate the same shape as
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induced before in the expression (4). When applied to both bi-flyback PFC converter and BIFRED PFC converter in simulation, this snubber cell operates the same way as analyzed in parameters design. The simulation results have proved that the proposed snubber cell is practicable. IV.
EXPERIMENTS RESULTS
A 100W bi-flyback PFC converter prototype with the proposed snubber cell is built. The same controller implemented by microchip dspic30f2023 as in paper [9] is applied here. Parameters of the circuit system are set as below: Input: 110Vac; Output: 20Vdc @100W; T 1 : primary inductance L1 = 86uH , turns ratio n1 = 5 ; T 2 : primary inductance L 2 = 640uH , turns ratio n 2 = 3.5 ; leakage inductors are 5% of primary inductors for individual transformers; Switching frequency: 100kHz. The auxiliary components: Cr = 4.4 nF , Lr = 35uH .
VCr
PWM Vo
iLr
auxiliary switch operates under ZCS on both turn-on and turn-off transitions. By disconnect the secondary side of the transformer T 1 , a BIFRED PFC converter is formed from the bi-flyback PFC converter, and experiments are also carried out on the converter. Key waveforms are shown as Figure 5. Figure 5. shows the key waveforms of the snubber cell measured when the input voltage is 80Vac, and the output power is 25Vdc @50W. It also confirms that the practicability of the proposed snubber cell. V.
CONCLUSION
This paper proposed a new snubber circuit for the isolated single-stage PFC converters with any boost type input current shapers. This snubber cell not only realizes snubber affection on the main switch, but also achieves soft switching on both the main switch and the auxiliary switch. The prototypes of the snubber cell are built on the biflyback PFC converter and the BIFRED PFC converter individually. Simulation waveforms prove its practicability, while experiment results confirm it. However, experiments haven’t been carried under full voltage and full load at designed. It is mainly because the PCB board design hasn’t include any isolation between the DSP circuit and the main power circuit, and the disturbance current (when big enough) caused by the snubber circuit makes the DSP works unstable. APPENDIX For the derivation of expression of the voltage VCr and the current iLr .
Figure 4. snubber cell of bi-flyback converter: inductor current (light blue), capacitor voltage (deep blue), driving signal (purple), output voltage (green)
diLr ⎧ ⎪VCr − Lr dt = VCs ⎨ dVCr ⎪ Cr = −iLr dt ⎩
Vo
VCr
iLr
(1)
Solution of (1) is,
⎧ VCr = V 1 sin(ωt + θ ) + V 2 cos(ωt + θ ) + VCs ⎪ Cr Cr ⎨ cos( + ) + sin(ωt + θ ) i ω t θ V 2 Lr = −V 1 ⎪⎩ Lr Lr
PWM
Figure 5. snubber cell of BIFRED PFC converter: inductor current (light blue), capacitor voltage (deep blue), driving signal (purple), output voltage (green)
1 ) (2) LrCr Take iLr = 0 ( t = 0 ) into the expression of iLr , get V 1 cos θ = V 2 sin θ (3) Take VCr = VCs + Vcon ( t = 0 ) into the expression of VCr , (ω =
Figure 4. shows the key waveforms of the snubber cell measured when the input voltage is 80Vac, and the output power is 20Vdc @50W. Obviously the snubber cell has two operation stages as analyzed before: the inductor current iLr achieves zero before the switches turn off. Moreover, the waveforms of iLr and VCr match with simulation waveforms. Figure 4. illustrates that the main switch operates under ZVS during turn-off transitions, whereas the
get
V 1 sin θ + V 2 cos θ = Vcon
(4)
Combine (3) and (4) together, solve out
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⎧ V 1 = Vcon sin θ ⎨ ⎩V 2 = Vcon cos θ
(5)
[4]
Take (5) back into (2),
⎧V Cr = V con cos ω t + V Cs ⎪ 1 ⎨ i Lr = V con C r sin ω t ( ω = LrCr ⎪⎩ Lr
) (6)
[6]
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