Split voltage mystery

Question

As I was designing a split voltage supply for an opamp supply, I thought that cutting one rail if the other rail wasn't at it's nominal voltage would be a good idea if I wanted a "good" output signal. Turns out it isn't that useful (at least in my particular case). BUT I'm still wondering how to do that. So for my curiosity:

How would one do that?

I have a split voltage (in my case it was +/-30 V and a load of about 25 mA. Although this post is hypothetical so not sure it's useful).

My idea was to use some comparator and the output would activate or deactivate a switch. Maybe a BJT or a MOSFET with a MOSFET driver. If I use a comparator like the LT1017, in it's datasheet the absolute maximum rating for the input voltage is -0.3 V to 40 V. So how can I monitor the negative rail with that? In one application example, V+ and V- are "switched". But can I use a negative Vref on INA- or INA+? And if it's the case, can the output of the negative rail comparator drive a switch (MOSFET or BJT)? Or MOSFET driver?

LT1017 datasheet

Answer 1

How about something like this?

When the voltage difference between the rails is lower then the zener voltage, no current can flow. This means no voltage drop across R1/R2, keeping both transistors off. Otherwise current can flow trough the zener, when the drop across R1/R2 is larger then 0.7V both transistors turn on. R3 limits base current for both transistors.

^{simulate this circuit – Schematic created using CircuitLab}

If the zener knee isn't sharp enough for your purposes one could replace it with a TLV431 configured for the desired voltage.

Answer 2

Linear Technology (now a part of Analog Devices) has a ton of voltage monitors and hot-swap controllers that can handle positive and negative voltage combinations, have on-board MOSFET drivers, etc.

There is a catch-22 (metastability) issue with the task, that is solved by delays. Since at power-up no two supplies come up at exactly the same moment, there needs to be a power-on delay before the comparator outputs are taken seriously.

Next, if you want the system to restart automatically when both supplies are back within range, there needs to be a way to power the monitor circuit during the unbalanced period.

Last, there probably should be a short delay when power is back to make sure it is there to stay. My home generator waits 1 minute, before switching back to grid power. That probably is a bit long for a small electronic system, but only you can say.