Practical sharing: Forward active clamp soft switching power supply design

In recent years, the power supply industry has been committed to the research and development of 80PLUS products. How to achieve the requirements of 85PLUS? This is not a problem for general adapters and is easy to achieve. However, for PC power supplies or server power supplies with multiple outputs of medium and low DC voltages, it is not so easy to achieve 85PLUS. To deal with such problems, it is more appropriate to use a dual-transistor forward power supply. The range from 300W to 1200W is widely used by manufacturers, and some technical means can meet the requirements of 80PLUS. This article introduces a design method for realizing soft switching on a dual-transistor forward using active clamping technology, and gives actual cases and experimental results.

Working Principle of Dual Transistor Forward Active Clamp Soft Switching

The main circuit of the dual-transistor forward active clamp soft switch is shown in Figure 1.

Referring to Figures 2 to 7, the working process of the dual crystal forward active clamp switching power supply is described in detail as follows:

(1) Power transmission stage (t0-t1), as shown in Figure 2, in this stage, the first main switch VT1 and the second main switch VT2 are turned on at the same time, while the clamp switch VTR1 is in the off state. The input voltage applied to the transformer causes the excitation current to rise linearly, and the primary transfers energy to the secondary through the transformer. The secondary VD1 is turned on, VD2 is turned off, and the current on L1 rises linearly, and is supplied to the load RL after rectification and filtering. Under this condition, VD1 and VD2 are just turned on under ZVS because their body diodes have been in the on state before (as shown in Figure 6) (2) Resonance stage (t1-t2), as shown in Figure 3, under the control of the duty cycle, the first main switch VT1 and the second main switch VT2 are turned off at the same time at t1, and the polarity of the transformer core is reversed. Due to the input power supply and the excitation inductance of the transformer, the parasitic capacitors COSS1 and COSS2 of VT1 and VT2 are charged. Since the capacitor voltage cannot change suddenly, the first main switch VT1 and the second main switch VT2 are turned off under ZVS. At the same time, the transformer's excitation current begins to discharge the parasitic capacitor COSS of the clamp switch tube VTR1, and charges the clamp capacitor CR1 through the body diode of VTR1 . The secondary VD1 is cut off, VD2 is turned on, and L1 continues to supply power to the load RL through the freewheeling current of VD2.

(3) Active clamping stage (t2-t3), as shown in Figures 4 and 5, at time t2, the clamping switch tube VTR is turned on in the ZVS state. Since the body diode of VTR1 has been turned on before, the UDS voltage of VTR1 is very low. The clamping switch tube VTR1 is in the on state throughout the stage, and the transformer excitation current continues to charge the clamping capacitor CR1 through the clamping switch tube VTR1. After the clamping capacitor CR1 is fully charged, it is discharged through the transformer excitation inductance. The secondary is powered by L1 through VD2 to supply power to the load during the entire stage, and VD1 is cut off.

The dual-transistor forward active clamp switching power supply introduced in this article has the advantages of both single-crystal forward active clamp and dual-crystal forward. It is suitable for high-voltage and medium-power applications, and the magnetic core is effectively reset, the magnetic core utilization is improved, and the duty cycle can exceed 0.5, or even reach 0.7. If the input voltage is 380V, when the duty cycle is 0.7, the reverse voltage of the main switch tube is only about 634V, which has great benefits in high-voltage applications, achieving zero-voltage switching, and greatly improving efficiency compared to dual-crystal forwards, while also reducing EMI interference. The secondary waveform has no dead time, and is suitable for self-driven synchronous rectification , which is of great benefit to low-voltage and high-power output. The frequency can also be increased accordingly, which can save magnetic core materials, reduce volume, and reduce the voltage stress of the primary and secondary switch tubes accordingly.

There is another structure of the dual crystal forward active clamp soft switch power supply , as shown in Figure 8. Its structure is basically similar to the dual crystal forward active clamp soft switch power supply shown in Figure 1, only the clamp switch tube VTR3 and the clamp capacitor CR3 are set on the secondary side, one end of the clamp capacitor CR3 is connected to the same-name end of the transformer, and the other end is connected to the D pole of the clamp switch tube VTR3, and the S pole of the clamp switch tube VTR3 is connected to the opposite-name end of the transformer, please refer to Figure 8. Its working principle is similar to that of the primary clamp. Actual waveform results

We actually use a general dual-transistor forward product and improve it to the above-mentioned active clamping method. The actual dual-transistor operating waveforms are shown in Figures 9 to 12.

From the actual waveforms above, the UDS voltage of the two transistors is much lower than the original hard switch, which is conducive to the selection of MOSFET switch tubes in the design. At the same time, the voltage margin of the same specification material is greatly improved, which increases the reliability of the product. In addition, from the figure, we can clearly see that the conduction and shutdown of the MOSFET are basically zero voltage conduction and shutdown, which reduces the switching loss. At the same time, it is also very beneficial to electromagnetic compatibility.

From Figure 13, we can see that the forward voltage is 39V, the negative voltage is 26V, and the duty cycle is 0.42. Therefore, the withstand voltage of the components in the secondary rectifier part can be much lower than the original specifications, which is of great benefit to improving efficiency.

Summarize

The circuit introduced in this article has been verified in actual use, giving full play to its advantages, and is of great help in improving the margin of material selection and product efficiency. Using this circuit to make a 1000W high-power server power supply not only meets the 80PLUS silver standard, but also can smoothly reach the gold standard by slightly improving the secondary output rectification and material selection. Therefore, this circuit can open up the thinking of power supply designers and provide them with an additional choice in circuit selection.