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Overvoltage Protection Circuit : Damaging buck converter

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Saboteur94:
Hi Guys,

I have an issue whereby an overvoltage protection circuit is destroying an buck converter when it "disengages".

I have attached the schematic of the protection circuit and the regulator(LM61460) it feeds,herewith.

The exact sequence of steps followed to produce the issue is:
(1) Power up the device/PCB with a voltage that is less than 36v.
(2) Gradually increase the voltage until it is greater than 36v causing the OVP circuit to engage protecting the buck converter and upstream circuitry.I confirmed that the circuit functioned as expected at this point(0V present at test point 5 in schematic below).
(3) I then decrease the voltage to below 36v,the OVP then disengages and immediately following this the buck converter gets shorted to ground.

I suspected that when the OVP disengages there might have been some transient generated that is higher than the maximum input voltage(42V) of the buck converter so I isolated the converter from the circuit,hooked up my oscilloscope and repeated steps (1)-(3) above to see if I could catch any potential transients to no avail.

Any assistance in helping debug this issue would be greatly appreciated.

Kind Regards,

PartialDischarge:
can u post something bigger?

Ian.M:
+1.  The text on your schematic is essentially unreadable.  Its difficult to extract a hi-rez bitmap from most PDF readers, so maybe export direct from your schematic editor?   If you reduce the color depth to 4 bits (16 colors)  even rather large bitmaps have reasonably small file sizes in GIF or PNG format.

Also, if you temporarily mod the OVP circuit to act at 20V (less than half the LM61460 abs.max. input voltage rating), you can probably observe the OVP engage and disengage transients under load without killing the LM61460. If it dies with 20V OVP you've also learned something!  Scope traces would be useful to  let us see if there's any unexpected interaction between the LM61460 and your OVP circuit.

Saboteur94:
Hi MasterTech,
I have added a larger image.

My apologies for the size of the initial post.

Kind Regards,

Saboteur94:
Hi Ian,

Noted,will change diode D9 to allow a the circuit to shutdown a lower voltage(24V) to see if that kills the chip.

I have uploaded a higher quality image.

Kind Regards,

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