Design of spike absorption circuit based on switching power supply

Abstract: In order to prevent the distributed inductance and capacitance in the high-speed switching circuit of the switching power supply system from generating surge voltage and noise under the influence of the accumulated charge of the diode. In this paper, the surge voltage generated by the accumulated charge of the diode is magnetically absorbed by amorphous cores and rectangular cores by using RC or LC absorption circuits, thereby solving the generation and suppression of surge current in the switching power supply.
Keywords: switching power supply; surge current; absorption circuit

0 Introduction
Most of the main components of the switching power supply have parasitic inductance and capacitance. The parasitic capacitance Cp is generally connected in parallel with the switching element or diode, and the parasitic inductance L is usually connected in series with it. Due to the effects of these parasitic capacitances and inductances, the switching element often generates large voltage surges and current surges when it is turned on and off.
The on and off of the switch and the reverse recovery of the diode both generate large current surges and voltage surges. The most effective way to suppress the current surge when the switch is turned on is to use a zero voltage switching circuit. On the other hand, the voltage surge when the switch is turned off and the voltage surge when the diode is reversed may damage the semiconductor components and is also the cause of noise. For this reason, an absorption circuit is required when the switch is turned off. When the diode reverse recovers, the voltage surge generation mechanism is the same as when the switch is disconnected. Therefore, this absorption circuit is also applicable to the diode circuit. This article introduces absorption circuits such as RC, RC D, and LC. The basic working principle of these absorption circuits is to provide a bypass for the switch when the switch is disconnected to absorb the energy accumulated in the parasitic inductance and clamp the switch voltage, thereby suppressing the surge current.

1 RC absorption circuit
Figure 1 shows a circuit diagram of an RC absorption network. It is a circuit in which the resistor Rs and the capacitor Cs are connected in series and connected in parallel with the switch. If the switch is disconnected, the energy accumulated in the parasitic inductance will charge the parasitic capacitance of the switch while also charging the absorption capacitor through the absorption resistor. In this way, due to the effect of the absorption resistor, its impedance will become larger, so the absorption capacitor will equivalently increase the capacity of the parallel capacitance of the switch, thereby suppressing the voltage surge when the switch is disconnected. When the switch is turned on, the absorption capacitor discharges through the switch, and at this time, its discharge current will be limited by the absorption resistor.

2 RCD absorption circuit
The RCD absorption circuit given in this article is shown in Figure 2. It consists of a resistor Rs, a capacitor Cs and a diode VDs, where the resistor Rs can also be connected in parallel with the diode VDs. If the switch is disconnected, the energy accumulated in the parasitic inductance will be charged through the parasitic capacitance of the switch, and the switch voltage will rise. When its voltage rises to the voltage of the absorption capacitor, the absorption diode is turned on, so that the switch voltage is clamped by the absorption diode (about 1 V), and the energy accumulated in the parasitic inductance also charges the absorption capacitor. During the switch on period, the absorption capacitor is discharged through the resistor.

The transformer can also be demagnetized by using RC and RCD absorption circuits without setting up a demagnetization circuit composed of transformer windings and diodes. The excitation energy of the transformer will be consumed in the absorption resistor. RC and RCD absorption circuits can not only consume the energy accumulated in the transformer leakage inductance, but also consume the transformer excitation energy. Therefore, this method also reduces the conversion efficiency of the converter. Since the RCD absorption circuit clamps the switch voltage through a diode, the effect is better than RC. At the same time, it can also use a larger resistor, but the energy loss is also smaller than RC.

3 LC absorption circuit
The LC absorption circuit is shown in Figure 3, which consists of Ls, Cs, VDs1 and VDs2. If the switch is disconnected, the energy accumulated in the leakage or excitation inductance can be discharged through VDs1 through the capacitor Cs, so that the voltage of the absorption capacitor Cs is reversed, so that the transformer is demagnetized by the capacitor voltage. During this period, the voltage polarity of the input voltage and the voltage of the absorption capacitor added to the switch is reversed again. Under normal circumstances, the LC absorption circuit does not consume energy.

4 Conclusion
To increase the switching frequency and improve the quality of switching power supply products, voltage surge and current surge issues must be considered. This article proposes this effective suppression measure based on the analysis of the interference generation mechanism and a lot of practice. Therefore, to solve the surge problem, it is also necessary to analyze the surge generation mechanism in combination with the actual design, and design the surge absorption circuit in combination with the actual situation to minimize the surge interference of the switching power supply.