# Electrical – Driving a 35V PMOS circuit from a Grounded/Floating input

mosfetpcbpmosswitchingzener

I have an application where I need to switch a contactor on and off using a spare output of an off the shelf Battery Management System (BMS)

The input is the signal coming from the BMS, while the output is the signal going to the contactor

## Input/Output

• When the input is FLOATING, the output should be OFF
• When the input is GROUNDED, the output should be ON

The manufacturer of the BMS provided the following circuit diagram to follow: ## Constraints

• BMS output must not sink more than 175mA
• BMS output must not have more than 24V across it

The main supply of this circuit is a battery which will have a maximum operating voltage of 35V. Looking through the P MOSFETS available in my country, I can only find a max Vgs of -20V. I have looked at gate voltage protection circuits and tried to adapt the following circuit to work: The circuit I have derived is the following:

Zener diode: BZX85C22V
PMOS: Vishay IRF9540PB My logic is that the 10K resistor acts as the pull-up when the BMS input is floating. When the input is grounded, then the Zener starts conducting and clamps the gate to 22V. Therefore Vgs is -13V (well under the -20V maximum). Iz looks to be about 22mA for the zener to conduct, so I assume let 25mA flow. so then

R2 = (35-22)/0.025 = 520R. Choose 470R, which gives 28mA. Resistor power dissipation is P = I^2 * R = 0.36W (Choosing 0.6W resistors)

Zener dissipation = V * I = 22 * 0.025 = 0.55W (Under the maximum of 1.3W).

My question is that will this circuit work reliably? I am not experienced with circuit design, so your feedback would be very appreciated. 