High voltage limiter with discrete components

Question

I'm designing high-voltage enclosure circuitry that engages a lock when either of two HV supplies reach 50V or greater. Both are capable of 6kV output. Below is what I have so far, where R7 represents the lock:

The two similar blocks form comparators against a 5V reference, and Q3 and Q6 form an AND gate. Q9 drives the output.

I've had issues with op-amps and logic ICs burning up with previous designs, so all components are discrete.

This design works well in simulation so far, but I have only yet cranked up the sources to 60V. In reality, a 100:1 resistive divider (10M:100k) will be used to yield a 0-60V range, or thereabouts (200:1 for 0-30V, perhaps).

The issue is that, at 50V, the voltage at the comparator input is too low to overcome Q1's or Q4's \$V_{be}\$. I thought about adding a common-emitter amp with a gain of 10 at the inputs that just saturate a bit above this 50V mark, but that would also require another inverting amp following it.

Is there a simple way to go about this that I'm not thinking of? Or is a change in configuration needed?

Answer 1

You can make it work by providing a negative supply for R1/R2, something like -5V will do.

But there are probably easier ways, especially if you know why exactly you had part failures. If your failures are due to the divider flashing over, then discrete parts are not going to be all that much more robust.

I would suggest something more along these lines (concept only at this point), where U1 is an open-collector or drain output dual comparator with low input bias current:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Another possibility would be to build the comparators out of PNP transistors, so they can sense voltages near ground. A simplified schematic would look like this:

This also removes the need for the transistor AND before the output. A diode is needed to clamp the HV input when it gets much above the common emitter node. Also, I have not included any base or collector resistors but they might be needed, depending on the source impedances of the various signals.

Note that a similar diode could be needed to bypass the BE junction of Q2 and Q5 in your original schematic to prevent the BE junctions to get reverse-biased beyond breakdown.