High-side N-channel MOSFETS without a zener diode?

Thread Starter

Chris_Swift

Joined Oct 30, 2019
8
Hello!
Hypethically speaking, if one made a circuit board without a Zener diode located here:
1579215115792.png

Would you end up destroying mosfet Q3?

Because let's just say, the circuit seems to ALMOST work... but Q3 just won't budge, nor does it seem that its body diode is doing anything either..?
 
Last edited:

Thread Starter

Chris_Swift

Joined Oct 30, 2019
8
A bq40z80 chip (Texas Instruments battery management chip)
The chip raises the gate voltage 11.5 volts above the source (I see ~35 volts against the source: ~24v). HOWEVER, only for Q2 right now.. The Vgs of Q3 is just resting at 22v. I posted a message on TI's forum as it may be related to the chip actually.
Thank you for taking a look though!
-Chris
 

Alec_t

Joined Sep 17, 2013
10,901
The Vgs of Q3 is just resting at 22v
If Vgs is really 22V then that may exceed the rated maximum for that FET (I haven't checked) but should have been prevented by the 16V zener. Or do you mean Vg? What is the Vs voltage?
Is the bq40z80 correctly programmed to handle a 7S pack?
 
Last edited:

Thread Starter

Chris_Swift

Joined Oct 30, 2019
8
If Vgs is really 22V then that may exceed the rated maximum for that FET (I haven't checked) but should have been prevented by the 16V zener. Or do you mean Vg? What is the Vs voltage?
Is the bq40z80 correctly programmed to handle a 7S pack?
My apologies, I meant Vg -> bat-
My circuit gets confusing because I also have a low-side mosfet to act as a on/off for the end device that gets connected to "Pack+ & Pack-"
So, Vgs of Q3 is actually stuck at 0v.
And you bring up a good point, the bq40z80 is not configured to handle the battery yet.

..sometime later..

I issued a "reset" command to the battery chip and it does indeed drive the pin to 35v.. My apologies, that sort of makes this thread useless, but thank you for the assistance regardless. I'll order a new EV2400 board to properly program the parameters of the chip (I also discovered why I fried my other EV2400 board).

Engineering is hard..
 
Top