Power Tip 57: Design a flyback primary switch snubber

March 15, 2013 //By Robert Kollman
Power Tip 57: Design a flyback primary switch snubber
Robert Kollman of Texas Instrumentsexamines how to design a flyback primary switch snubber.

How to best control the voltage stress on the primary switch in a single-ended flyback converter (shown in Figure 1 ) is a multi-faceted problem. You have to solve a combination of technical issues while still keeping an eye on the overall cost. You have to:

  • Limit the MOSFET voltage stress to an acceptable level
  • Discharge the leakage inductance very quickly to maintain good efficiency (see Power Tip 17 )
  • Minimize circuit losses due to adding the snubber
  • Avoid impacting the power supply dynamics

The lowest cost approach to solve these issues is shown in Figure 1 of Power Tip 17 and consists of a standard recovery diode, capacitor and loading resistor. The circuit works by transferring excessive transformer leakage energy onto the snubber capacitor and dissipating it over the switching period. Unfortunately in this approach there is always energy dissipated in the snubber resistor, regardless of output power. In each switching cycle, the voltage on the capacitor always will be recharged to at least the reflected output voltage. This degrades the efficiency, particularly, at light loads.

Figure 1 of this power tip presents an alternative circuit approach, which replaces the resistor/capacitor with a resistor (R1) and zener diode (D1). When the FET turns off, the drain voltage rises to the point that the diodes conduct to discharge the leakage inductance of the transformer. The rate at which the current discharges is set by the difference between the reflected output voltage and the clamp voltage. Note that for best efficiency, as Power Tip 17 points out, it is critical to discharge the leakage inductance energy as quickly as possible. In choosing values, first consider the MOSFET voltage rating and derating criterion to determine a suitable maximum voltage stress on the MOSFET. First choose the zener voltage to be above the reflected output voltage so that it does not continue to conduct after the leakage inductance has been reset. Next size

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