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Simple inrush limiter has problems

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cupoftea

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Hi,
We all know the attached inrush limiting circuit, its well known, cheap-as-you-like, 2-penny-bit one that
uses the switching FET as a charge pump so as to provide the hi side supply for the Inrush-resistor-NFET.
(LTspice and PNG attached, also inrush power waveform in the inrush NFET)

Crude, because the SMPS power current inevitably gets drawn through the just-turning-on inrush NFET.
Also, since the Vin range is from 18-33Vin, then you have to set the UVLO to around 16V, and then of course, if Vin
is 33V, then the inrush fet gets turned on prematurely and suffers an enormous inrush power spike which is
outside of its SOA. (as shown)

Adding a delay cap of some 10uF from D6 cathode to ground (as well as a diode across D15 to discharge it later)
appears to solve the issue, but has other problems.

Would you agree that the attached inrush power pulse, would kill the PSMN1R8-40 NFET after about a year or so?
The VDS x IDS is within the SOA but after a year or so of this happening once per day?

PSMN1R8-40
 

Attachments

  • Inrush power pulse.png
    Inrush power pulse.png
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  • Schem_inrush limiter.png
    Schem_inrush limiter.png
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  • SingleBuck_INRUSH_cheapo.zip
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  • Inrush IDS and VDS.png
    Inrush IDS and VDS.png
    31.9 KB · Views: 28
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Which facts form the basis of your opinion?

sine Inrush power Ppk*PW50 = 1.6kW * 8 us = 13 mJ impulse

your design? or all inrush designs using this FET? or Nexperia MTBF results? or the assumption that the events accumulate daily wear & tear? why?

Thus if I am correct you are saying ~50A*32V for 8 us vs SOA of 120 us is fatal. Maybe but why?

1715538009661.png
 

Attachments

  • 1715536933728.png
    1715536933728.png
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Thanks vert much for your reply.., it was the fear that even though SOA is the "one off survive" thing...maybe after a year of it it goes kapputt.
1600W just seems crazy for a 5x6mm SMD FET, even if only for 100us or so.
 
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D.A.(Tony)Stewart said:
You have a few unanswered questions to respond to. Did I change your assumptions?
Click to expand...

You stated "the enormous inrush power spike which is outside of its SOA. "

I showed it was well inside its SOA.

Still waiting for answers.
 
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