Analysis of Spike Hazard and Absorption Circuit in Switching Power Modules
2022-08-16

In switching power supplies, inductance and capacitance are common, parasitic capacitance is generally connected in parallel with the switching element or diode, and parasitic inductance is connected in series with it. Due to the presence of inductance and capacitance, the switching power supply module will generate large voltage and current surges when it is working on and off. The hazard of spikes in the switching power supply module will cause component damage or abnormal circuit operation. The use of a absorbing circuit can suppress the surge current.

Voltage spikes are caused by inductance freewheeling. The inductances that cause voltage spikes include line distributed inductance, transformer leakage inductance, and inductive components in the equivalent model of the device. The current caused by voltage spikes includes topology current, diode reverse recovery current, and inappropriate the resonant current, etc.

The role of absorption and buffer circuits:

1. It can prevent damage to components, the absorption circuit is used to prevent voltage breakdown, and the buffer circuit is used to prevent current breakdown.

2. It can reduce the loss of switching devices, or achieve a certain degree of soft opening.

3. Reduce di/di and dv/dt, reduce ringing and improve EMI.

Common absorption circuits include LC absorption circuit, RC absorption circuit, and RCD absorption circuit. The working principle is to provide a bypass for the switch when the switch is turned off, absorb the energy accumulated in the parasitic inductance, and make the switch voltage clamped.

The LC absorption circuit is composed of capacitors, inductors, resistors and other components and electronic devices. It is a circuit that can generate oscillating current or filter.

As shown in the figure above: when the switch is turned off, the energy accumulated in the inductance such as leakage or excitation can be discharged through the capacitor C through VD, so that the voltage of the absorption capacitor C is reversed, and then the transformer is demagnetized by the capacitor voltage.

The RC absorption circuit is shown in the figure above: it is a circuit composed of a resistor and a capacitor in series, and is connected in parallel with the switch. When the switch is disconnected, the energy accumulated in the parasitic inductance charges the parasitic capacitance of the switch, and will absorb The resistor charges the snubber capacitor. Due to the function of the absorption resistor, its impedance will become larger, and the absorption capacitor will increase the parallel capacitance of the switch quite equivalently, and suppress the voltage surge when the switch is turned off. When the switch is turned on, the snubber capacitor discharges again through the switch.

The RCD absorption circuit is shown in the figure above: it is composed of a resistor, a capacitor and a diode, and the resistor can also be connected in parallel with the diode. When the switch is turned off, the energy stored in the parasitic inductance is charged through the parasitic capacitance of the switch, and the switch voltage rises. When its voltage rises to the voltage of the absorption capacitor, the absorption diode is turned on so that the switching voltage is clamped by the absorption diode, and the energy accumulated in the parasitic inductance charges the absorption capacitor. During the on-time of the switch, the snubber capacitor is discharged through the resistor.