Selection of RCD snubber circuit based on MOSFET in series with the primary side of the transformer

For MOSFET, it is connected in series with the primary side of the transformer during operation. Due to the influence of the transformer leakage inductance and the large turn-on capacitance of MOSFET, it will be subjected to a very high voltage spike when it is turned off.

In order to reduce this peak, we use an RCD snubber circuit. As shown in the figure:

There are many types of RCD circuits: pure RCD, RCD with feedback inductor, and RCD with feedback transformer. The latter two are to convert the energy consumed on R into electromagnetic energy and feed it back to the grid to reduce losses.

When the RCD is working, the voltage across the switch tube is limited to 2Vdc due to the position relationship, and C slows down the rise of the collector or drain voltage of the switch tube. Then C discharges through the resistor R, or transfers the energy to the inductor or transformer and finally feeds back to the grid.

The value of C needs to be large enough to slow down the voltage rise of the switch tube and ensure that the switch tube is not impacted. C cannot be too large due to loss, and there is no special requirement for the size of R. The smaller R is, the faster C discharges. It only needs to ensure that C discharges all its charge when the switch tube is turned off next time within the time of Ton.