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Recently I was going through the data sheet of the TOP264 TOP Switch IC. In a typical application circuit diagram, at the secondary of the transformer they have used a configuration for the diode as follows instead of using a simple diode for a half wave rectifier. enter image description here

What difference does it actually make in the output? Can anybody explain the advantages of this configuration?

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  • \$\begingroup\$ I think it's to keep a small load on the output of the switcher's isolation transformer when it reverses polarity. I guess it will prevent ringing and may also aid reverse recovery time of the schottky diodes. \$\endgroup\$ – Andy aka Jun 4 '13 at 10:24
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It is a snubber network.

Its purpose is to suppress high frequency ringing at the turn-off of the rectifier diode, caused by resonance between the diode capacitance and the flyback transformer leakage inductance. This is necessary not only for EMI purposes but to protect the device from excessive voltage stress.

The snubber won't have a meaningful impact on the output, since it's only 'working' on the edge of the pulse train coming in.

The values can be estimated by calculation, but the ringing driving sources are parasitic in nature so by and large they're empirically determined (measure ringing frequency, set values, measure, adjust values, measure again, etc.)

Here are some app notes that touch on the topic:

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Protection of the Schottky diode

It is a snubber circuit and it is to protect the schottky diode from reverse voltage breakdown. Here is an interesting article from ST and the relevant points I've copied below. Simply, the article tells you why a snubber is needed even if you are using a normal PN diode. Note figure 9 for schottky and figure 11 for PN.: -

enter image description here

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I would say it was an RC snubber circuit.enter image description here

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    \$\begingroup\$ Can you please explain something more on your answer ? \$\endgroup\$ – noufal Jun 4 '13 at 11:10
  • \$\begingroup\$ I think usually a snubber like this across an SCR or triac is to make it less likely the device will get stuck on when the load is inductive. What's the point of a snubber on a Schottky diode? \$\endgroup\$ – Phil Frost Jun 4 '13 at 13:12
  • \$\begingroup\$ That's not why you place a snubber across an SCR. They are meant to carry on conducting, even with inductive loads. Scr's are however prone to premature gating due to high dv/dt across the device \$\endgroup\$ – JonRB Oct 25 '15 at 19:35
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This is a RC snubber to protect the diode. RC snubber's acts like a damper and dampens the resonance created because of the parasitic capacitance and inductance.

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It appears to be a precharge circuit to prevent a high inrush current to the output capacitors when the circuit is initially turned on.

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    \$\begingroup\$ As it is in parallel with the diode it also does nothing to limit the inrush current. \$\endgroup\$ – Brian Drummond Jun 4 '13 at 10:27

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