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Part Number: PMP11254
This is a snubber that reduces the peak voltage and ringing across D10 at turn off. The current that was flowing prior to D10 turning off builds up energy in the leakage inductance of the transformer. When D10 turns off, the energy resonates with stray circuit capacitance (diode, transformer, PCB) if the snubber is not present. Since there is little stray resistance to damp this LC circuit the voltage can get quite high, possibly exceeding the rating of D10. The ringing can also result in EMI issues. C16 is sized so that it is larger than the stray capacitance in order to direct the energy into R18. The value of R18 is selected to be close to the critical damping resistance SQRT(L/C) where L is the stray inductance and C is the stray capacitance plus snubber capacitance. It is usually necessary to make measurements on an actual board to obtain the correct values.
One of my colleagues wrote an article on calculating snubber circuit values. I have attached it for your reference.
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