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I have two questions about the RCD clamp and the RC snubber question in flyback. Below the figure is a CCM flyback converter with primary and secondary clamps and snubbers.

Here are my questions:

  1. What's the difference between RC snubber and RCD clamp? As I know, the RCD clamp could dampen the voltage spike, RC snubber also could do that. So I am confused about two.
  2. I do not understand why need to use both of them at the same time.
  3. I know how to design the RCD clamp value, but I have no idea about how to design RC snubber, could you give me some advice?

enter image description here

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  • \$\begingroup\$ RC snubber limits dV/dt. RCD clamp limits peak voltage. \$\endgroup\$ Commented Oct 29, 2021 at 10:58

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There is a need for using the right terminology here. In your circuit, you have a RCD clamping network or RCD clamp also sometimes found as peak clamp in the literature. The other \$RC\$ network is a damping network or snubber not to be mixed with the RCD snubber intended to delay the voltage waveform at the switch opening and reduce turn-off losses.

The RCD clamp operating principle is equivalent to building a low-impedance dc voltage as represented below:

enter image description here

The energy stored in the leakage inductance with a bit of magnetizing current contribution is absorbed by the resistance while the capacitor is scaled to limit the acceptable ripple. If the voltage on the \$RCD\$ clamp is equal to \$V_{clp}\$ then the voltage on the drain at the switch opening is equal to \$V_{HV}+V_{clp}+V_{os}\$ in which \$V_{os}\$ relates to the voltage overshoot brought by the diode forward transit time.

The calculation of this \$RCD\$ clamp network is not that complicated but requires the determination of several parameters such as the maximum peak current in the converter in the worst-case situation. Usually, this is with the maximum input voltage from the specifications and a true short circuit at the output terminals (at the board level). Under these conditions, the voltage on the MOSFET shall never exceed its \$BV_{DSS}\$ derated by a 10-20% ratio. For instance, should you choose a 600-V MOSFET and apply a 15% derating factor, the maximum drain voltage shall never exceed 600 x 0.85 = 510 V. I detailed all the calculations linked to the determination of this peak clamp in an APEC seminar that I taught in 2011: The Dark Side of the Flyback Converter.

Now the damper. This guy is here to dissipate power, that it the way you damp a system. However, you want ac power and not dc power, hence the need to cut the dc component via the series capacitor. Below is the way to size this damper and it uses a simple method proposed by Dr. Ridley many years ago:

enter image description here

In the vast majority of designs I have seen, a snubber takes place across the secondary-side diode for reducing the ringing and its radiating contribution. I haven't seen many across the primary side of the transformer.

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  • \$\begingroup\$ Hi Kint, Could you kindly tell me where I can find the RC snubber information in your answer? How do you know how much you could damp in the RC snubber, I am curious about these 5 step is based on what theory? \$\endgroup\$ Commented Oct 29, 2021 at 15:44
  • \$\begingroup\$ There is nothing specific here: the idea is to derive the equation for the quality factor of the parallel resonant network involving the leakage inductance and the parasitic capacitance. Then express \$Q\$ and make it equal 1. This is what these equations are all about : ) \$\endgroup\$ Commented Oct 29, 2021 at 17:03
  • \$\begingroup\$ Thank you. I know it. \$\endgroup\$ Commented Nov 3, 2021 at 15:16
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You can't learn how to design a snubber until you discover why they exist. The main idea is to help protect the FET from nonlinear switching and to compensate for parasitic capacitance when the FET turns off without adding significant load when they are ON.

This helps reduce VI=P(f,t) resonant power losses in the FET by adjusting the impedance to dampen the response under varying load conditions. Switching is not ideal like a sine transformer. In some cases the energy is stored like a flyback transfomer as in this case. Others are less demanding for snubbing ,such as in a forward transformer converter .

Until you know what goes on inside the operating mode in time and frequency domains, I can't do much to generalize yet.

This is where analytic simulations help with some understanding of conjugate impedance matching and know the RLC values of the transformer and FET is necessary.

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  • \$\begingroup\$ Thanks, I will try to use simulation to know it. \$\endgroup\$ Commented Oct 29, 2021 at 15:46
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RCD CLAMP is the most used clamp network for flyback power supply topologies like the one you posted.

The diode Dclamp1 allows the capacitor Cclamp1 to discharge into resistor Rclamp1 during Ton, that is, during the fraction of the period T where the MOSFET is on.

Without that diode, the capacitor Cclamp1 would discharge also into the MOSFET during Ton.

I've never used snubber networks on the primary side of flyback transformers. That doesn't mean that it may work well.

I remember that in a circuit I designed, I had to put 2 diodes Dclamp1 in parallel because one was too hot and that I had to put 2 resistors Rclamp1 in series in a 1206 package for the same reason. I designed the transformer in order to have the minimal leakage inductance but still I had to dissipate 0.6 Watt of power into the RCD network.

Around you may find a 2-stage RCD network that is supposed to be more efficient and effective but back then I didn't have the time to test it.

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  • \$\begingroup\$ What's the circuit look like in your design? \$\endgroup\$ Commented Oct 29, 2021 at 15:45
  • \$\begingroup\$ I'll post it here next week on Thursday or Wednesday, hold up. \$\endgroup\$ Commented Oct 29, 2021 at 17:00
  • \$\begingroup\$ Hi Enrico, Thanks, I will wait for your answer. \$\endgroup\$ Commented Nov 3, 2021 at 15:17
  • \$\begingroup\$ Hi, I'm out of office. Be back on friday. \$\endgroup\$ Commented Nov 3, 2021 at 15:22

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