Analysis and Design of Active Clamp Flyback Converter

1. Introduction

Flyback converters are widely used in small and medium power conversion applications due to their advantages such as simple circuit topology, electrical isolation of input and output, wide voltage step-up/step-down range, and easy multi-channel output. However, the voltage and current stress of the power switch of the flyback converter is large, and the voltage spike of the power switch caused by leakage inductance must be limited by a clamping circuit. In the literature [1], the author conducted a comparative study on RCD clamping, LCD clamping, and active clamping flyback converters, and concluded that active clamping technology enables the flyback converter to achieve the best comprehensive performance.

Figure 1 Active clamp flyback converter circuit topology

Figure 2 Active clamp flyback converter principle waveform

2. Analysis of the Steady-State Principle of Active Clamp Flyback Converter

The circuit topology and principle waveform of the active clamp flyback converter are shown in Figure 1 and Figure 2 respectively [2]. The transformer is represented by the magnetizing inductance Lm, the resonant inductance Lr (including the transformer leakage inductance and the external small inductance) and the ideal transformer T with only the ratio relationship. Cr is the equivalent capacitance , including the output capacitance of the two switches S and SC. In steady-state operation, each switching cycle is divided into seven switching state stages, and the equivalent circuits of each switching state are shown in Figure 3. The seven switching states are:

① t=t0～t1: At t0, the power switch S is turned on, the clamp switch SC and its parasitic diode Dc and the rectifier diode D are all turned off, and Lm and Lr are linearly charged;

② t=t1～t2: At t1, S is turned off, the magnetizing inductance current, i.e. the resonant inductance current, charges Cr in a resonant manner, and the drain-source voltage uDS of the switch tube S rises approximately linearly;

③ t=t2～t3: At t2, uDS rises to Ui+uC, DC is turned on, Lr and Lm are connected in series The branch voltage is clamped at uC≈Uo(N1/N2), the magnetizing current charges CC through the clamping branch (CC>Cr), and the u1 decreases as u1=-uCLm/(Lr+Lm);

④ t=t3～t4: At t3, u1 has dropped to make D forward-biased, and then u1 is clamped at -Uo(N1/N2), Lr and CC begin to resonate, the voltage on Lr is uC-Uo(N1/N2), and the iC drop rate is [uC-Uo(N1/N2)]/Lr. SC is turned on before iC starts to reverse, and SC obtains zero voltage turn-on (ZVS);

⑤ t=t4～t5: At t4, SC is turned off, Lr resonates with Cr, and u1 is still clamped at -Uo(N1/N2) value during Cr discharge;

⑥ t=t5～t6: At t5, uDS=0. Assuming that the energy storage of Lr is greater than that of Cr, which is enough to turn on the parasitic diode Ds in S, and the voltage on Lr is clamped at the value of Ui+Uo(N1/N2), the current i2 in the secondary rectifier diode D decreases at a rate of

(Lm>>Lr) (1)

⑦ t6～t7: At t6, S zero voltage ZVS is turned on. As iLr rises, i2 gradually decreases. At t7, iLr has risen to the magnetizing current iLm value, i2=0, D is reverse biased, u1 changes from -Uo(N1/N2) to Ui, and then Lm and Lr are linearly charged again, and a new PWM switching cycle begins again.

To achieve ZVS opening of power switch S, the following conditions must be met: ① The driving signal should be added during t5-t6, otherwise after iLr crosses zero and becomes positive, Lr will charge Cr again, and power switch S will lose the ZVS condition. Strict requirements should be imposed on the interval between S opening and SC closing, and its value should not exceed one-fourth of the resonant period of Lr and Cr, that is,

(2)

② When SC is turned off, the energy storage of Lr should not be less than that of Cr, so that the charge on Cr can be drained out.

(3)

From the above analysis, it can be seen that the active clamp flyback converter has the following advantages: ① The clamping capacitor Cc absorbs the energy in the transformer leakage inductance and feeds it back to the grid side, eliminating the turn-off voltage spike caused by the leakage inductance, and the power switch is subjected to minimum voltage stress; ② The clamping capacitor Cc and the resonant capacitor Cr resonate with the resonant inductor Lr, so that both the main and auxiliary switches obtain ZVS switching; ③ The resonant inductor Lr reduces the turn-off current change rate of the rectifier diode D, reducing the turn-off loss and switching noise caused by the reverse recovery of D.

3. Key circuit parameter design

3.1 Magnetizing inductance Lm

The magnetizing inductance Lm determines the boundary conditions of the CCM/DCM operating mode. If the system operates in CCM mode,

(4)

Where, Pomin—output power when the inductor current is critically continuous, Fs—switching frequency

η—conversion efficiency, D—switch S duty cycle

3.2 Resonant inductor Lr and power switch S

The voltage stress of the power switch S and the clamp switch SC is

(5)

The last term in the formula is the increase in power switch voltage stress caused by the introduction of the resonant inductor Lr.

With the introduction of the resonant inductor Lr, the actual effective duty cycle is slightly smaller than the switch S drive signal duty cycle D, and the lost duty cycle △D is

From formula (3), we can get

(7)

Where Isp—peak current of power switch

The resonant capacitor voltage is

(8)

UCr is a function of the resonant inductance Lr, and it is difficult to accurately solve equation (8). In fact, the voltage ULr is small compared to Ui+(N1/N2)Uo, so the Lr value for the power switch S to obtain ZVS can be approximately expressed as

(9)

The resonant inductor current iLr (i.e. the primary inductor current of the transformer) is the sum of the power switch current iS and the clamp capacitor current iC, and its effective value is

3.3 Clamping Capacitor Cc

The principle for selecting the Cc value is: the half resonant period of Cc and Lr should be greater than the cut-off time of the power switch S, that is,

(11)

The clamping capacitor voltage is the sum of the primary winding voltage and the Lr terminal voltage, that is,

(12)

The effective value of the clamping capacitor current is

3.4 Clamp switch Sc

The clamp switch voltage stress is determined by equation (5). From equation (11), we have

The current through the clamp switch Sc (same as iC) is approximately a sawtooth wave, and the peak current is equal to the peak current through S. The conduction time of the clamp switch Sc and its internal diode Dc is approximately (1-D)TS/2, so the average current in Dc and the effective value of the current in Sc are respectively

3.5 Rectifier diode D

The voltage stress of the rectifier diode D in the active clamp flyback converter is the same as that of the traditional flyback converter, which is Ui (N2/N1) + UO, but the current stress is very different. Due to the introduction of the active clamp branch, although the magnetizing inductance works in CCM mode, D works in DCM mode, and the current peak IDp increases, that is,

(16)

The effective value of the current in D is the effective value of the secondary current of the transformer, that is,

3.6 Output filter capacitor Cf

The effective current of the output filter capacitor Cf is

4. Test results

Based on the current control active clamp flyback converter internal voltage regulation Power supply Design example: Ui=18～32VDC, three output groups Uo/Io=+15V/1.0A, -15V/0.2A, +5V/0.4A, rated output power 20W, FS=300KHz, Dmax=0.6, η=78.5%, critical continuous power Po, min=1/6Pomax, Lm=52.3μH, Lr=2μH, Cc=0.47μF, Cf=100μF, power switch S and clamp switch Sc are both IR F530. +15V, -15V, +5V three output rectifier diodes are SR506, 1N5819, 1N5819 respectively, and the control circuit uses UC3843 current mode PWM controller. When the input voltage Ui=27V, the principle test waveform of the active clamp flyback converter is shown in Figure 4. As shown in Figure 4(a), the turn-off voltage spike caused by the transformer leakage inductance is eliminated. As shown in Figures 4(a) and (b), both the main switch and the clamp switch achieve ZVS switching. As shown in Figure 4(f), the di/dt is small when the rectifier diode is turned off. The test waveform is completely consistent with the theoretical analysis waveform shown in Figure 2.

5. Conclusion

Applying active clamping technology to flyback converters can overcome the shortcomings of traditional flyback converters and achieve ZVS switching of the power switch; suppress the turn-off voltage spike of the power switch; reduce the turn-off loss and switching noise of the secondary rectifier diode, thereby achieving high conversion efficiency and high power density of the flyback converter.