Zener diode with high source impedance

Question

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

Answer 1

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".

Answer 2

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.