Overvoltage Protection Circuit : Damaging buck converter - Page 1
Forum section: electronics / projects

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 07:05:50 am
content: 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,

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 27, 2021, 07:49:39 am
content: can u post something bigger?

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2021, 08:15:53 am
content: +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.

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 08:16:26 am
content: Hi MasterTech,I have added a larger image.My apologies for the size of the initial post.Kind Regards,

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 08:23:05 am
content: 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,

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 27, 2021, 08:26:42 am
content: All I can say is that L3 seems fishy, why would you put an inductor like that in a path that can be cut at any time, it should be with C37+ a diode, if necessary at all.That's why maybe you don't see any overvoltages when you isolate the converter, there a no currents to replicate the inductor behaviour, which could be generating a spike going thru the transistor as avalanche.L3 should not be there, whether it is the culprit or not.

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 08:54:43 am
content: Hi MasterTech,Thanks for find,will remove.To replicate the inductor behaviour,can I use a dummy load instead of hooking the converter back up?,just to avoid me blowing up regulators unnecessarily.Kind Regards,

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2021, 08:56:19 am
content: 24V > Vmax/2I suggested 20V for a reason . . .

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 27, 2021, 08:57:11 am
content: Quote from: Saboteur94 on November 27, 2021, 08:54:43 amYes you can do that.Also I'm curious the ferrite filter BLM21 is for small currents, but the converter is a 4A one, what is your average output current? It is customary to add a resistor is parallel to reduce frequency ringing in these types of filters:look at figures 23 and 25https://www.analog.com/media/en/technical-documentation/application-notes/an-1368.pdf

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 09:10:55 am
content: Noted,thanks

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 09:20:37 am
content: Quote from: MasterTech on November 27, 2021, 08:57:11 amAverage output current from the converter is 3A at 5V. However,the input voltage to this the system will be 11V at a minimum so the current through BLM21 will be less than 2A majority of the time owing the "buck" effect.

author: magic
author_rank: Super Contributor
timestamp: November 27, 2021, 09:27:07 am
content: Quote from: MasterTech on November 27, 2021, 08:26:42 amYes, you have made yourself a nice boost converter. When the protection PMOS turns on, capacitors short the inductor to ground and current ramps up. Then voltage ramps up too and when the capacitors are fully charged, the energy stored in the inductor's magnetic field keeps pushing current forward and overchares the capacitors.BTW, is there any problem with sensing the output voltage (drain side) of the protection circuit rather than its input?editNevermind, the protection would probably turn into a crude linear regulator under such arrangement - not good for the MOSFET.

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2021, 09:54:46 am
content: Yes, Magic has described the problem. Its going to ring to over twice Vpwr_in as you've chosen a non-polar, so presumably ceramic, Vin decoupling capacitor (C37: 4.7uF 50V). If its value was stable with respect to voltage, it would ring to a bit under 72V (or whatever the LM61460 breaks down at), but as a high-K capacitor will typically loose two thirds of its value by the time it reaches its rated voltage, if you test with a HV current sink load in place of the LM61460, I'd expect either it will peak at over 150V or (far more likely) the capacitor will fail.Note that even if you delete L3, you may get some ringing due to loop inductance, so adding an electrolytic in parallel to C37 to provide damping due to its ESR may be advisable.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2021, 10:02:23 am
content: The capacitance at the buck converter input is relatively small, so less energy is needed to cause spikes in the voltage. This also cause the current through the P-MOSFET to still have quite some ripple which is not so good for the efficiency.Besides too high a voltage, though could worst case also be the voltage going negative from the buck converter trying to suck out the last bit of charge, before it turns off.L3 could be a problem, especially if the OVP circuit part start oscillating in an unintended mode. There are some circuit parts to act against this, but it may not be enough.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 27, 2021, 10:18:40 am
content: Quote from: Saboteur94 on November 27, 2021, 09:20:37 amOk, but at 1.5A or more passing thru that BLM21 its inductance is going to drop quite a lot nulling its filtering effect. You should find a 3 or 4A rated ferrite if you want to maintain a its HF filtering specs. This is described also in the PDF I linked.

author: Saboteur94
author_rank: Contributor
timestamp: November 27, 2021, 10:49:22 am
content: Quote from: MasterTech on November 27, 2021, 10:18:40 amNoted,will definitely ammend accordingly.Thanks

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 27, 2021, 11:21:56 am
content: Another point, don't know if your input diode S3M is put on purpose for some reason, but it will dissipate more than 1W which seems unnecessary. You could use a schotky diode to reduce it to 1/2 or a PMOS to even reduce it further.