Author Topic: Why is this MOSFET here?  (Read 1459 times)

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Offline Gelf54Topic starter

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Why is this MOSFET here?
« on: October 04, 2019, 09:30:56 pm »
I've just fixed my old Dell C600 laptop (after our cat decided to mark it as his own, which corroded away some IC pins) and noticed in the circuit diagrams this MOSFET circuit. I've seen similar before and haven't figured out why it's used.
It can't be providing reverse voltage protection, since the body diode only has a Vsd of around 0.7V, and it can't be providing under-voltage lockout, since the Vgs threshold is only around 1.7V.
Thoughts?

[Edit] Never mind, I think I just figured it out.
The 33K & 100K resistors do provide an under-voltage lockout of around 7V I'd estimate.
Not sure why this is necessary. The battery is around 14.8VDC. Guess it prevents spurious logic signals before the +5V & +3.3V rails have stabilised, or during power down.
« Last Edit: October 05, 2019, 04:27:52 am by Gelf54 »
 

Offline Gelf54Topic starter

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Re: Why is this MOSFET here?
« Reply #1 on: October 04, 2019, 09:41:20 pm »
Because of the MOSFET body diode.
 

Online Zero999

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Re: Why is this MOSFET here?
« Reply #2 on: October 04, 2019, 10:31:57 pm »
Are you sure it's drawn correctly?
 

Offline Gelf54Topic starter

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Re: Why is this MOSFET here?
« Reply #3 on: October 05, 2019, 04:26:33 am »
I have no reason to think otherwise atm. Is there a slightly different circuit that you have in mind?
 

Offline JackJones

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Re: Why is this MOSFET here?
« Reply #4 on: October 05, 2019, 05:20:01 am »
It looks like a soft start circuit rather than polarity protection.
 
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Offline Gelf54Topic starter

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Re: Why is this MOSFET here?
« Reply #5 on: October 05, 2019, 06:27:53 am »
That's about a 15mS time constant, isn't it, 0.47uF & 33K ?
Maybe it's to delay past any initial power-on ripple from the external power brick?
 

Offline Gelf54Topic starter

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Re: Why is this MOSFET here?
« Reply #6 on: October 05, 2019, 08:35:02 am »
Good to know! Thanks.
 

Offline sanwal209

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Re: Why is this MOSFET here?
« Reply #7 on: October 15, 2019, 08:58:09 am »
It is an In rush control circuit.
 

Offline MagicSmoker

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Re: Why is this MOSFET here?
« Reply #8 on: October 15, 2019, 10:19:43 am »
Why doesn't it provide reverse input protection?

 :palm:

On a second thought, I have no idea what I've been drinking.

I feel your pain - I made the exact same mistake about a week ago...  :-DD
 


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