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I am trying hard to remember the role of the Diode labeled "D1" and marked with a red circle in the following schematic of the flyback converter below:

Sample Flyback converter with full bridge rectifier

Why do we need a diode there?

I know the RLC circuit left to that is for damping the oscillations.

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This is a snubber circuit. Together with R3 and C3 the diode catches some or most of the ringing on the primary side and burns the excess energy in R3. D2, D3 and D9 are part of it as well and clamp the maximum voltage. More about it here: Wikipedia snubber

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  • \$\begingroup\$ Oh I think I remembered it now. When we don't use the snubber the transistor is gonna breakdown (burn) from a huge voltage spike right? I think I had that once happen to me when I used an LED in series with the 1N4007 (D1) diode for an LED "power ON indicator" . For some reason my LED stopped working and this caused the transistor to burn. It was a sloppy design choice, I had to resolder the thing which was quite a mess. \$\endgroup\$ – NerdyNerdie Feb 14 at 4:34
  • \$\begingroup\$ An LED has a rather high junction capacitance. Depending on the switching frequency of your converter it might not even conduct until the operation time of the switch S1 is already over. As you put it in series, the slowest one dominates, so D1 is effectively eliminated. \$\endgroup\$ – letsfetz Feb 14 at 7:47
  • \$\begingroup\$ In that case, the function of the snubber circuit is not provided, the main power switch S1 tales All the energy of the overshoot and eventually can get destroyed. \$\endgroup\$ – letsfetz Feb 14 at 7:48
  • \$\begingroup\$ Other design philosophies try to eliminate the overshoot to start with by minimizing the leakage inductance of the transformer and/or using this energy for soft-switching of the power semiconductors. This also depends on various other design specifications. \$\endgroup\$ – letsfetz Feb 14 at 7:51
  • \$\begingroup\$ That explains why the LED was a bad choice. And how about the 2x 33V + 100V = 166V zener diodes (D3 D2 and D9)? What do they provide us in this snubber circuit? \$\endgroup\$ – NerdyNerdie Feb 14 at 20:01
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Why do we need a diode there?

You need a snubbed to catch excess stored energy that isn't delivered to the secondary circuit. You need a diode to prevent the snubbed taking current when switch S1 activates and ruining efficiency. The snubbed should only take current when the back emf from the primary rises above the bus voltage, Vin.

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