Lets imagine a Forward/Flyback converter with a zener snubber. How to analyse how this DZ snubber works?

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My problem is that how can I know what is the voltage of the common node of the two diodes? What is the right method to approach it? An example with values would be great.


Vz + Vf must be << Vds max at a current starting at V+/DCR of coil. The energy absorbed with no secondary load must be absorbed by the Pd rating of the Zener for the duration of T=L/Zzt terminal resistance of the Zener.

The diode must be rated at well with the same current pulse but has a much lower Pd requirement.

If you are using a large power pulse and cannot find an adequate Zener then other methods of clamping must be used to protect the NFET such as devices with built in Avalanche diode and higher V rating and in a different config.


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  • \$\begingroup\$ Vz + Vf can be any value you like it does not need to be <<Vds. The diode blocking voltage MUST be > Vds, but the Zener voltage can be anything you want. The larger the Zener voltage the quicker you dissipate the stored inductive energy. Vz must obviously be < Vds(max). \$\endgroup\$ – Jack Creasey Jun 26 '18 at 3:04
  • \$\begingroup\$ I said Vds max , you’re thinking Vds \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 26 '18 at 3:07
  • \$\begingroup\$ Ah ...you are oh so right ….apologies, my mistake. \$\endgroup\$ – Jack Creasey Jun 26 '18 at 4:55
  • \$\begingroup\$ Okay, I got it, that this is how to select those components, but how to approach how it works? What is the voltage between the two diodes? If input is 24V, the voltage on the drain is 150V and the zener is 75V for example. How to get the voltage of the common point of two diodes? How do I know which diode conducts when? \$\endgroup\$ – U.L. Jun 26 '18 at 7:09
  • \$\begingroup\$ It must conduct the same current therefore voltage is obvious in forward condition in reverse same idea except current is 1uA or so \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 26 '18 at 14:10

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