I have a circuit circuit fed by a 48V nominal lithium battery source. I'm using a few simple resistors to divide the voltage down to a suitable range to drive a p-channel mosfet gate. When the battery goes into BMS cutoff, I expected 0V and open circuit would be the conditions, and for that set of conditions, my circuit works perfect in bench tests.
But it turns out the BMS manufacturer delivers 6V during BMS cutoff, and they don't provide any info about the exact nature of that source. They even called the 6V "fake volts", whatever that means. So the end result is that I want to block those low voltages so my circuit behaves like an open circuit below some threshold voltage, say 39V. A zener diode seems like the answer, but they have a minimum current of about 1-3mA needed to go into breakdown, and my circuit requires low power consumption, so it normally only uses 350 uA. I really don't want to waste more power to solve this problem.
Is there any other simple way to block low voltages (< 10V), or only allow high voltages (> 39V) to pass? Key word is simple, and with 250-500 uA power consumption.
If it just won't work this way, I'll redesign the whole thing using a voltage comparator or op-amp, etc, but I'd much rather use my existing circuit. It's very stable, has very low current consumption, has a test function, an LED indicator, and allows for a separate power source to deliver the output signal of + 12V which I use to shut down alternator charging.
Any help is greatly appreciated. The only thing I need to know is how to block low voltages or create an open circuit condition below some set threshold voltage without wasting a lot of power.
Here's a schematic of just the core circuit without the LED indicator on Vout and the pushbutton tester that I use to short the output of R1 to ground. Please note Vstarter and Vhouse share a common ground.
The p-FET is rated for 30V so that it can handle a gate voltage of 16.3V to 24.5V from the divider depending on the output of a 48V nominal battery (40V to 60V). The 12V nominal source can be as high as 15V, so I wanted the gate voltage to be higher when the battery is operational. I was told by another helpful person on this forum that was important. I think he said the FET could be damaged otherwise, but maybe it was only to keep the gate switched off.
I was able to briefly test during a BMS cutoff and the 6V output will power an LED directly, so it does deliver some amount of mA. My circuit also appears to work under that condition. I get the expected 12V at Vout. But is the p-FET okay with 2.4V on the gate input and 12V (can be up to 15V) on the source? I thought @nanofarad explained that having the source voltage higher than the gate was a problem. Is that only for normal operation, but for my case a good thing since I actually want the p-FET to be conducting when the voltage is significantly lower, like the 2.4V it gets during this odd 6V BMS cutoff output?