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I have designed a boost converter with a 400V output which uses a PFC IC For regulation, the IC measures the output via a potential divider which is fed into a pin on the IC

The IC is looking for approx 2.5V so with 400V output I use a divider with 3M and 19k

All is good in the world, very basic stuff

Now I am looking to protect the IC, I had a failure where I lost a ground and a voltage exceeding the 5V max was applied to the IC pin and the smoke was let out

To avoid this possibility I would like to clamp the maximum voltage to the pin and I thought lets drop a zener in there, in parallel with the 19k resistor so any misbehaviour will be clamped

When I actually sat down and thought about it I realised that 3M resistor is going to seriously limit the current and the zener wont be able to get past the knee point which typing this made me think it might not matter the voltage would still be limited. I dont want any regulation just protection

I plan on adding a TVS diode as well but thats just for spikes I dont think it can be expected to work with constant DC applied

Am I still ok to use a zener in this application?

Is there a better way?

Edit

Added schematic showing the zener in place, as you can see R2 and R8 come to approx 3M

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    \$\begingroup\$ You might want to add schematic to help visualizing the important part \$\endgroup\$ – PlasmaHH May 15 '17 at 10:35
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    \$\begingroup\$ Better way: spend time and effort into not losing your ground connections. Use a proper circuit board etc.. \$\endgroup\$ – Andy aka May 15 '17 at 10:36
  • \$\begingroup\$ I can add a schematic no problem \$\endgroup\$ – Jamie Lamb May 15 '17 at 10:38
  • \$\begingroup\$ The ground was lost due to overcurrent blowing the current sense resistor, mitigation is already in place (diode in parallel) but I have always been a belt and braces man! I want to protect it against this fault even if its not going to happen \$\endgroup\$ – Jamie Lamb May 15 '17 at 10:39
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    \$\begingroup\$ Your schematic is not complete - I cannot see the current sense resistor? \$\endgroup\$ – Andy aka May 15 '17 at 10:52
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Taking the example of a BZX84C5V1 zener (because that sprang into my head), it regulates at 5.1 volts with a cathode current of between 4.8 and 5.4 mA. With a cathode current of 1 mA, the regulation point might be down at 4.1 volts.

With a much lower cathode current it might start to regulate at 3 volts (maybe 100 uA) and at 10 uA it might start trying to regulate at some voltage well-below 2.5 volts. The data sheet states that at 2 uA it will be naturally sitting at 2 volts so be aware of any zener diode's limitations - they are not perfect on-off devices and will consume hundreds of uA below their zenering voltage.

This then becomes problematic because your reference voltage is 2.5 volts and the zener may still be taking several tens of micro amps at this voltage and ruining the regulator's stability. It might not be a big deal to you but, then again, it might be enough to put you off the idea.

This is why precision zeners are sometimes used but, are they usable as a protection device? No, not really - they can be just as susceptible to problems as any other chip. Maybe use a 15 volt zener or a reverse diode to the Vcc rail. Maybe try a 10k in series with the pin as a form of protection. On this particular occasion, a zener would be well-down the list of options.

Stick with my original advice of spending "time and effort into not losing your ground connections".

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  • \$\begingroup\$ Thanks for your answer Andy, it certainly is enough to put me off and its totally negated the idea, it will break the circuit as the uA the zener will take at 2.5V is going to be sourced through that 3Meg so every uA is 3V drop, its just like a high value resistor in parallel so it will make the converter chop more to sustain 2.5V on the input pin hence the output will rise, 400VDC output is plenty enough for most! Trying to stop it happening was the first action a diode in parallel is probably the easiest way but I am still 'thinking about it'. \$\endgroup\$ – Jamie Lamb May 15 '17 at 12:19
  • \$\begingroup\$ A TVS diode wouldnt work either and if I was really needing to protect this pin then I would have to look at the VDC input which serves the divider, a TVS diode would ensure then 400VDC was clamped to whatever so the pin cant be stressed \$\endgroup\$ – Jamie Lamb May 15 '17 at 12:21
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What you are suggesting is a silly over-reaction.

You had a connection failure and a part blew up. That can happen when you have bad connections. The right way to deal with this is to examine the ground connection of the IC and see what happened to make it fail. That really shouldn't fail. If everything is remotely competently designed, then this will be a very rare failure. Going out of your way to protect against it doesn't make much sense.

Also, what exactly is the cost of the chip blowing up versus the board failing for other reasons? Most likely, you're going to replace the board either way. Think about it. If you do board replacement, there is no difference in cost between any two failures. Either way the board is dead and you replace it.

Note that your proposed "fix" also relies on a ground connection. What's the chance of that ground connection opening? Also, now you have additional failure modes you didn't have before. For example, the zener or TVS could fail shorted for some reason. That's far fetched, but probably less so than a ground connection between a chip and the ground net on a circuit board opening.

Think about it.

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  • \$\begingroup\$ I understand what you are saying I just feel that the cost of a zener is negligible and if it protects the IC once in a thousand times it will be worth it for me \$\endgroup\$ – Jamie Lamb May 15 '17 at 10:46
  • \$\begingroup\$ Oops pressed enter for a new line!, I am redesigning the PCB to add a H Bridge to run from the 400V output so its not like I am doing this just for the zener it was an extra. The main reason I posted the question is because I am learning Olin, I love learning and its something I dont know the behaviour of so here I am. This isnt a commercial product its all just a bit of fun the board blowing up doesnt matter to anyone on earth I just want to make electronics as reliable as I possibly can at a reasonable cost \$\endgroup\$ – Jamie Lamb May 15 '17 at 10:48
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    \$\begingroup\$ @JamieLamb so now you've had the two highest reputation site members offer the same advice. What ya gonna do? \$\endgroup\$ – Andy aka May 15 '17 at 10:55
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    \$\begingroup\$ And it is much appreciated and well received, I am probably going to leave it and take the advice however I would love an answer to my question. Would the zener still do the job? ie will it still limit the voltage despite being well below the knee and what is the alternative with high source impedance sure it might not be necessary here but I think its a decent enough question how would we protect with such high source impedance. Your comment seems a bit condescending if I am honest its not the most ridiculous question in the world is it? or is it? \$\endgroup\$ – Jamie Lamb May 15 '17 at 11:03

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