I am planning to use some power MOSFETS as simple on/off switches for a high voltage battery discharge circuit I'm designing. The circuit will consist of several parallel resistors of relatively high resistance. I want to be able to switch off individual rungs of the ladder in order to add or remove parallel resistance paths to manipulate the overall resistance of the circuit. It will look sort of like this.
simulate this circuit – Schematic created using CircuitLab
I know I haven't shown how I'll drive the gates of the MOSFETs, I left that out of this schematic for simplicity's sake because I have that part of the equation well figured out. R1, R2, and R3 will actually be comprised of multiple 100w resistors in series. Heat dissipation has been considered! Once again, they're represented this way schematically for simplicity.
M4 is supposed to be a master on/off for the circuit. It's sort of redundant given the circuit breaker, but it's easier to automate voltage to a gate than physically throwing a breaker- and the whole purpose of this design is automation.
Also, I'm not really sure if the 300k ohm resistor is necessary, but in my head the idea is to allow a very small current through the circuit when the MOSFETS are turned off, which should mean that if I throw the circuit breaker switch to OFF that the circuit will bleed off any residual charge into the negative terminal of the battery. Correct me if that is wrong or unnecessary or I should go about it in a different way.
I believe I have properly sized the MOSFETS I want to use, but I've run across the following graph in a data sheet that I somehow overlooked before. At first glance I thought it disqualified the MOSFET I'm looking at, but I think I was misinterpreting. Here is the digikey page for the actual FET in question and here is the graph I'm a bit puzzled by:
Now here is how I think I can interpret this:
If, for instance, M1 and M4 are both ON given R1's resistance the current through each FET should be about 240 mA. And if I look at the graph above, it would seem that at 240v that 240mA is beyond the Safe Operating Limit for continuous DC.
Except the Vds isn't actually 240 when the M1 is ON, it is actually basically zero since R1 drops almost all of the voltage. So both M1 and M4 are going to be seeing decent current but at very low Vds. In other words, they'll be operating in the safe region.
When M1, M2, and/or M3 are ON but M4 is OFF, R1, R2, and/or R3 won't drop the voltage, so the Vds of M1, M2 and M3 will indeed be 240v, however the current will be near zero. In other words, they'll be operating in the safe region.
Is this correct? Do I have it right?
If I do have it right, it would seem that the only time this would be an issue is if I were to, say, swap the positions of M4 and the Circuit Breaker. If I did that, there would be no resistive load to drop the voltage before the FET, so Vds would actually be 240v and current would be the same, moving things outside of the Safe Operating Area.
Perhaps this is exactly what the term "Forward-Bias" means in this context. I admit to not fully understanding the term as applied here.