Electronic – Surprising Inrush Current on a MOSFET Switch

inrush-currentmosfetswitches

I have the following P-ch MOSFET switch used to supply a 12V, 6-20A load:

P-CH MOSFET Switch

When I enabled the switch, I measured the following voltage drops across +12VDC & +12VDC_GR. The yellow line is +12VDC (my source voltage) and the blue line is +12V_GR (my 0.5ohm load). As you can see, the +12VC has a sudden drop followed by a ripple.

0.5ohm load on +12VDC_GR

I assumed this was caused by inrush current on my load, but when I removed the load and tried it again, I got an almost identical result:

No load on +12VDC_GR

I'm hoping someone in the community can give me some insight into what is going on/fix the issue:

  1. Is this actually inrush current causing the issue or something else to do with the MOSFET?
  2. Would adding a soft start between P-Channel gate and drain do anything to mitigate this? I'd like to understand the actual issues going on here.

Thanks!

Best Answer

It looks like your power source is on the verge of instability. Could start oscillating anytime, and you will have a bigger problem.

Keep in mind that the SQM40031EL has about 30 nF input capacitance, which can provide a good impulse kick (~30 ns) when the control transistor U28 turns on. As result the input has the same drop of ~1 V in both cases, with load, and without it. The rest is the reaction of PSU to the step load.

As a matter of fact, this kind of step load test is a must for checking stability margins of power supplies.

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