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


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.


  • 1
    \$\begingroup\$ I think this answers your question (page 3 on specifically): onsemi.com/pub/Collateral/AND9093-D.PDF \$\endgroup\$
    – Big6
    Apr 18 '18 at 23:15
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    \$\begingroup\$ When you removed the load was the 0.1uF capacitor still connected? \$\endgroup\$
    – EE_socal
    Apr 18 '18 at 23:25
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    \$\begingroup\$ If you don't use a low impedance probe ( 50 Ohm AC coupled ) or very short ground leads < 1cm, you can capture probe ground wire resonance. Pls confirm your results are valid. \$\endgroup\$ Apr 18 '18 at 23:30
  • \$\begingroup\$ That's why engineers invented specialized integrated circuits, so-called "Hi-Side Switches", with controlled inrush currents, undervoltage, overvoltage, thermal protection, etc. etc. \$\endgroup\$ Apr 19 '18 at 2:06

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.

  • \$\begingroup\$ Ok, Awesome! Reading up on AMD9093-D was a big help. I selected values that increased my switching time from under 10nS to about 10uS and the voltage drop/oscillation is gone. Ali Chen, I am concerned about the instability issue. Looking at the oscillation, it's 400ns between peaks, so I'm assuming I would simply build a high amperage filter that was capable of filtering out 2.5Mhz, correct? \$\endgroup\$
    – Peter
    Apr 20 '18 at 19:19
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    \$\begingroup\$ @Peter, no, a filter is not a solution. You need to examine your DC supply, and fix its feedback transfer function somehow, so the regulator will have more stable phase margins. Maybe you will need to use a better (or opposite, worse) output cap to make the DC supply more stable. You might need to decrease the feedback gain, or use some RC snubber to correct its transfer function, I don't know. \$\endgroup\$ Apr 20 '18 at 19:32

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