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Measuring Drain Current on LNK625

Posted by: adicarlo on Mon, 04/26/2021

Hello,

in your technical documentation you advise not trying to measure IDrain via the voltage drop of a resistor placed in series to the Source, as this would modulate the Source voltage and the device may malfunction. You recommend to use an adequate scope current probe clamped to a small wire loop in series to the Drain terminal, instead.

Now, would it be possible to place a small resistor in series to the Drain terminal instead than to the Source, and measure the current via the voltage drop on it? An isolated scope voltage probe would of course be used for performing such measure.

In case this is feasible, what kind (value and technology) of resistor would you suggest?

 

Best Regards

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Submitted by PI-Tarmo on Sat, 05/01/2021

Hi adicarlo,

Good day!

In theory, yes you can measure the current via the voltage drop using a series resistor to the drain pin. Maybe pick a value on the same order of magnitude as the Rds(on). Any resistor that works for current sensing will work here. Make sure the inductance is low enough and the power dissipation/peak power/peak current are high enough. In reality, that'll have some inaccuracies in the measurement. We have to factor the uncertainties in the resistance into your measurements. Also the limitations of the differential probe used since those typically have rather poor CMRR at higher frequencies.

Hope I answered your question!

Best Regards.

PI-Tarmo