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high current spikes in SMD sense resistors of flyback

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eem2am

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The current sense resistors in a ~20W offline flyback suffer spikes of several amps due to the capacitance at the drain node being discharged through the fet every time it switches on.

......despite being high current, the duration is so short that the power rating of the resistor is not broached.

...but do you feel that special pulse rated resistors should still be used for this purpose, due to the height of the current peaks?
 

Did you see the peak current in a measurement, and are you sure, that you actually measure current instead of inductive voltage peaks? If it's real current, I would mainly fear EMC issues.

But there are cases, where resistor pulse current rating should be considered, e.g. gate resistors in high power circuits. Without knowing the circuit parameters and resistor type, I can't determine if your circuit belongs to this class.
 

I have often measured this current in the past....

For example , in a flyback, when the fet turns on, the voltage across the capacitance of the output schottky is suddenly increased.....and this must be accompanied by a large current spike , since as you know , i = cdv/dt.

..this current spike is transferred through the transformer, and goes through the fet as it turns on....

also, the drain-source capacitance is suddenly discharged by fet turn on , and that adds to the current spike.


so i am certain that these current spikes exist.
 

The drain-source capacitance discharge current surely doesn't flow through the sense resistor.

The diode capacitance charge does, but dV/dt at the primary is limited by transformer leak inductance. Several A at the sense resistor sounds unlikely for a 20 W converter.
 
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    eem2am

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