Can someone please help me find why the LM2596 is blowing up on my PCB ?

the Id max could be too small for the inrush current and you turned it into a resistor - there are many things to consider for a properly reliable power electronics design .... as you are finding out - slowly ....
 

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nishantnidaria

Joined Sep 27, 2021
34
perhaps you exceeded the Vgs, or the Vds of the mosfet, perhaps you static damaged it ? there are a myriad ways you could have ruined it - over zealous with a soldering iron ... the list goes on ...
The Vgs limit is +-20V and Vds is -30V. The datasheet of the PMOS is in the attachment, i hope it helps get closer to a solution.
 

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nishantnidaria

Joined Sep 27, 2021
34
the Id max could be too small for the inrush current and you turned it into a resistor - there are many things to consider for a properly reliable power electronics design .... as you are finding out - slowly ....
Drain Current-Continuous is -4.3A, Yes i agree with you on the reliable power electronics design and learning part.
 

Thread Starter

nishantnidaria

Joined Sep 27, 2021
34
hi nishant,
The internal diode is under rated for your application.
You could consider a parallel shottky diode.
E
Yes, i could use a parallel but then i would have to change the fuse right ?
Could i not do it using a pmos ? maybe one which is more suitable for my application ?
 

ericgibbs

Joined Jan 29, 2010
16,808
hi,
You have to decide under what conditions do you want the fuse to blow.?
By the time it has blown , some other part of the circuit that is causing the high current will be damaged.
So what is the fuse protecting.? usually the power source , not the project circuit.

You could use a higher power P MOS.
E


Update:
Do you have a 10Amp general purpose Silicon Diode in stock.?
For a circuit test ONLY, use that diode in place of the PMOS, fit a 5Amp fuse.

Try powering up, if all appears OK, check the project components for signs of overheating.
 
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Thread Starter

nishantnidaria

Joined Sep 27, 2021
34
hi,
You have to decide under what conditions do you want the fuse to blow.?
By the time it has blown , some other part of the circuit that is causing the high current will be damaged.
So what is the fuse protecting.? usually the power source , not the project circuit.

You could use a higher power P MOS.
E
Okay i see!! fuse is just for battery protection and series diode or parallel diode or P MOS is for the project circuit ? If i understand correctly?

Would you have a reference for a higher P MOS for this purpose (12V input goes to two DC-DC converters) ?

Also do you think putting a resistor at the gate and zener diode between source and gate would help ?
 

ericgibbs

Joined Jan 29, 2010
16,808
hi,

Okay i see!! fuse is just for battery protection and series diode or parallel diode or P MOS is for the project circuit ? If i understand correctly?

The PMOS is for power supply reverse polarity protection only, if incorrect polarity then the PMOS is not conducting, no current to the project.

The PMOS internal diode is forward biassed, if the polarity is correct, so current will flow into the project circuitry, via the diode.

Added sim.
 

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Thread Starter

nishantnidaria

Joined Sep 27, 2021
34
hi,

Okay i see!! fuse is just for battery protection and series diode or parallel diode or P MOS is for the project circuit ? If i understand correctly?

The PMOS is for power supply reverse polarity protection only, if incorrect polarity then the PMOS is not conducting, no current to the project.

The PMOS internal diode is forward biassed, if the polarity is correct, so current will flow into the project circuitry, via the diode.
Oh but if there's no current to the project in case of reverse polarity then why are my ICs getting blown?
 

ericgibbs

Joined Jan 29, 2010
16,808
hi
Most likely cause is that the internal diode had blown short circuit due to excess high current when the polarity was correct.! The internal diode is unrated for the project current.

added: being a short circuit it would not give reverse protection

You posted:
In the dead one, the measured resistance between gate and source is 12.1Ω, whereas in the new one (unused) it's 9.1 MΩ
 

Thread Starter

nishantnidaria

Joined Sep 27, 2021
34
hi
Most likely cause is that the internal diode had blown short circuit due to excess high current when the polarity was correct.! The internal diode is unrated for the project current.

added: being a short circuit it would not give reverse protection

You posted:
In the dead one, the measured resistance between gate and source is 12.1Ω, whereas in the new one (unused) it's 9.1 MΩ

This is quite interesting!! do you think i could check if the internal diode has been shorted or not with a multimeter on my used PCB which has never experienced reverse polarity ? How ? :)
 

ericgibbs

Joined Jan 29, 2010
16,808
hi,
Try measuring resistance from Drain to Source one way then the other way around,
One way should give high resistance and the other way low, lets know what you measure.
 

Irving

Joined Jan 30, 2016
3,190
The PMOS internal diode is forward biassed, if the polarity is correct, so current will flow into the project circuitry, via the diode.
And then the PMOS conducts as gate more -ve than source, although its 'upside-down' it still conducts well enough to reduce the voltage drop to a few mV and the losses to a few tens of mW (here approx 120mW) compared to a diode alone (approx 2W). When I first saw a PMOS used backwards like this I couldn't see the point - why just not use a diode? But then I found an article from Infineon explaining why/how and it all made sense. But you do need a device big enough to cope... most TO220 devices with no heatsink will handle 2W easily, so 120mW is a non-issue. But SMD parts need a little more thought.

1632835534678.png
 
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ericgibbs

Joined Jan 29, 2010
16,808
OK,
I suggest you take the MOS off the PCB.

Set up a little test jig, so that you have say a 100R load on the Source as the the project load, connect the Gate to 0V and the apply 12V to the Drain one polarity then the other.

Measure the current thru the 100R or the volts across it.. what do you measure.?
 

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