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Power rating for a current sense resistor for SMPS application

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david.cano90

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Hello everyone,

If I am about to use a shunt resistor for measuring the current of a transistor to make current mode control in a push-pull topology, the power rating of that resistor (defined as: P_r=(I_r)^2*R_s) should be calculated with the RMS value of the current or the average value?

Thank you guys!

David.
 

RMS. By definition, RMS power is the equivalent DC heating power. But, the difference between average(mean) power and RMS power is not that much; you're not giving yourself too much margin if you're even asking that question.
 

For a current sense resistor calculation we should know sense voltage (Voltage applied to current sense pins of controller when maximum rated current flow )
Resistance Rsc = V sense / Ipk
For push pull topology Ipk = 2.8*Pout/Vin

For selecting resistor power ratting
You select W = Ipk^2*Rsc always select more than this power rating for resistor
 

Hi,

With typical SMPS switching frequencies there is no need to take care about peak currents.
RMS is the value to use.

Klaus
 

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