Hello,

I am thinking about a battery management system that has both high voltage cutout and low voltage cutout circuits. planning on using a FET or IGBT for these, which I think is pretty standard. When the battery is just siting idle (charged up, no charge current in and no discharge current out) I would want the LVC and HVC to be OFF, meaning the FETS would be ON. I don't want to drain the battery, so I wonder about having them just sitting there in the on state. The gate capacitor should be charged and hardly leaking any current, and with no current across the drain/source pins, I'm thinking this should be OK, but is this a common/acceptable thing to do? Will I "wear out" the FET buy keeping it in this state for extended periods? Am I forgetting about some power drain that will consume current and discharge my battery?

Thanks!

 

I don't want to drain the battery, so I wonder about having them just sitting there in the on state.

Post a schematic so we know what you're talking about.

Will I "wear out" the FET buy keeping it in this state for extended periods?

Your usage of FET is imprecise. There are many types of FETs (enhancement mode MOSFET, depletion mode MOSFET, JFET, IGFET, MESFET, ...).

 

@DubP
The major reason FETS are used in semiconductors is for that very reason. Use voltage and little current to charge the gate capacitor and then leave it alone. BJTs on the other hand require constant current on the base.

Read and understand your datasheets for any FETs you plan to use- FETS are in huge varieties now- choose carefully.

 

It's perfectly fine to leave a FET "on" without any current on the drain.

But, nobody here can say if you have any battery discharge paths while the circuit is "idle" without seeing the circuit.

EDITED: must learn not to use personal terminology.

 

It's perfectly fine to leave a FET's gate open without any current on the drain.

The way I read the OP, the gates are connected and powered. ?
Drain to source non-powered.
Max.

 

Yea, ok...by "open" I meant "on".

Poor choice of words.

It's called a "gate" and that's how I envision them IE: open or closed.

 

The major reason FETS are used in semiconductors is for that very reason. Use voltage and little current to charge the gate capacitor and then leave it alone. BJTs on the other hand require constant current on the base.

It's perfectly fine to leave a FET "on" without any current on the drain.

Thanks for such fast responses. Glad to know that I can use them this way.

Read and understand your datasheets for any FETs you plan to use- FETS are in huge varieties now- choose carefully.

But, nobody here can say if you have any battery discharge paths while the circuit is "idle" without seeing the circuit.

Yes, There are a lot of MOSFETs available for this (BMS) application. I haven't selected one yet, but will check all of the specs to make sure the one I get that works in my application.

 

It's perfectly fine to leave a FET "on" without any current on the drain.

Then why even use a mosfet? There has to be something else shutting down the power before the drain in that case.

 

Are you asking me? I would have to assume the current stops on the drain because the battery is now charged...

 

That's what I was intending. Basically battery is charged, LVC function is disabled, meaning the MOSFET has been turned ON and a load COULD draw current, but most of the time the battery is just sitting idle with no current demand, thus no current on the drain.

 

The main loss with an ON MOSFET is current through the drain-source on-resistance so, if there's no current, the MOSFET will be dissipating no power (as if it weren't there).

This is different than a BJT, which will will be dissipating base-emitter-current power when ON, even if there's no collector current.