Flyback converter "snubber" circuit

　　The breakdown stress of the secondary side is often solved by a "snubber circuit". The figure shows a typical circuit. The design of the buffer network is described in detail in the front power adapter manufacturer Jiuqi Industry.

　　In order to reduce the breakdown stress of the secondary side in the offline flyback converter, a snubber network needs to be connected across both ends of the switching transistor and a snubber rectifier diode is often required to reduce the breakdown stress and RF radiation problems.

　　In the figure, the buffer elements Ds, Cs, and Rs of a typical flyback converter are connected across Q1. Their function is to provide a bypass for the primary-side induced drive current when Q1 is turned off and to reduce the rate of change of Q1's collector voltage. .

　　The working principle is as follows: When Q1 starts to turn off, the voltage on its collector will increase, and the primary current will flow through diode D. Transfer to capacitor Cs. The transistor Q1 turns off very quickly, and the dv/dt on its collector will be determined by the original current of the collector and the value of Cs when it is turned off.

　　The voltage of the collector will suddenly rise up to the limit value (2V.). After a short time, the voltage on the output secondary winding will reach V due to leakage inductance. (Equivalent to the output voltage plus the diode voltage drop), the flyback current will be exchanged from the primary side to the secondary side, and the current rate established by D1 is determined by the leakage inductance of the secondary side.

　　In fact, Q1 will not turn off immediately. If you want to avoid the secondary side breakdown voltage, the buffer component should be selected so that the voltage on the collector of Q1 does not exceed Vceo before the current drops to zero, as shown in the figure.

　　Unless the turn-off time of Q2 is known, the optimal selection of these components is empirical, based on the measurement of the turn-off voltage and current on the collector. The figure shows the typical turn-off waveform and switching stress with and without the snubber network.

　　When the current reaches zero, a safe voltage margin should be provided for the collector voltage. Since the operating temperature, load, dispersion of transistor parameters, and drive design have considerable influence on these parameters, it is considered that this value should be at least lower than V. Figure 2.3.3 shows the restriction conditions of 30%. In this example, when the collector voltage reaches V=, the collector current just drops to zero.

　　On the one hand, C should be avoided. The value is too large, because at the end of the flyback period, the energy stored in the capacitor must be consumed on R2 during the first part of the on period.

　　The value of Rs should be a compromise choice. If the resistance is too small, the current on Q at the turn-on moment will be too large, which will increase the conduction loss. On the other hand, the resistance is too large, and Cs cannot be fully discharged during the minimum on period.

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