In a single-excited switching power supply, whether it is a forward or a flyback switching power supply, it is required to apply overvoltage protection to the power switch tube to prevent the flyback pulse generated by the switch transformer primary coil when the switch tube is suddenly turned off. After the peak voltage and the input voltage are superimposed, they are applied to the D and S poles of the power switch tube to break down the power switch tube.

In order to prevent the breakdown of the power switch tube, FIG. 7 is an RCD spike absorbing circuit for suppressing the spike voltage of the flyback and having an overvoltage protection function for the power switch tube. It is called the RCD spike absorbing circuit because the main components in the figure are composed of R, C, and D.

For the convenience of analysis, we equivalent the switching transformer to an ideal (leakage is equal to 0) switching transformer T in series with a leakage inductance Ls, the inductance Lu of the primary transformer N1 of the switching transformer is called excitation inductance; distribution The capacitance Cs is the total equivalent capacitance after the distributed capacitance of the initial coil of the switching transformer and the distributed capacitance of the secondary coil are equivalent to the initial coil.

In Figure 7, when the power switch Q1 is turned off, the flyback voltage pulse generated by the primary winding of the switching transformer (including the flyback voltage pulse generated by the leakage inductance) will be superimposed with the input voltage U and applied to the power switch tube. Both ends of D and S of Q1.

At this time, the rectifier diode D will be turned on and charge C; the role of C is to absorb the spike voltage applied to the D and S poles of the switch tube to prevent the switch tube from being broken by the spike voltage; The function is to discharge the accumulated charge generated by the C-absorption spike voltage to prepare for the absorption of the next spike.

Otherwise, after a plurality of spike voltages are absorbed, the accumulated charge of the capacitor will be more and more, and the voltage across the capacitor will be higher and higher, and finally the absorption spike will be lost.

It is worth noting that when the power switch tube Q1 is turned off, due to the access of the transformer secondary coil rectifying and filtering circuits D2 and C2, the equivalent distributed capacitance Cs of the primary and secondary coils of the switching transformer is relative to the filter capacitor C2. The effect will become negligible; at this time, because the output voltage Uo is coupled and reflected by the primary and secondary coils of the transformer, the back electromotive voltage generated by the primary coil of the transformer is completely clamped to a proportional to the secondary output voltage Uo. Numerically, ie:

In addition, since the equivalent distributed capacitance Cds across the switch Q1 is not a pure capacitor, but actually an impedance is small to large, the impedance change process is similar to a capacitively charged variable resistor, which only absorbs energy, and does not Will release energy.

After the voltage Uds across it is higher than the voltage across the capacitor C, that is, after the rectifier diode D is turned on, the effect of the distributed capacitor Cds becomes a shunt resistor Rds. At this time, the larger the current flowing through the resistor Rds, the larger the loss of the switching tube. The capacity of the capacitor C and the resistance of the resistor R in FIG. 7 are appropriately selected, and the flow resistance Rds can be reduced (the switching tube is turned off). The equivalent resistance of the current, which can reduce the loss of the switching tube.

In other words, the RCD spike absorbing circuit over-voltage protection of the switch tube is achieved by shunting the current flowing through the power switch tube (Rds) through the capacitor C and the resistor R; the RCD spike absorbing circuit can not only reduce the switch The peak voltage across the drain and source of the tube also reduces the losses in the switching transistor.