Electrical – 24V H Bridge Gate driver – FOD3184 & FQP47N06


enter image description here

I am trying to design a H-bridge to drive 24V motors. No issue on the low side but I would like to use the FOD3184 Opto/Gate driver on the high side of the H-bridge to drive a P MOSFET and I am somewhat worried about the P-channel MOSFET being pulled to GND by the gate driver.

2 Questions for the community:

Question 1:
On the high side I would like to used a 12V Zener diode to protect the P MOSFET and limit the voltage drop to -12V. I am also thinking about adding a resistor to limit the current pulled through the diode to GND via the gate driver. I am not sure where to place the resistor (which value?) or even if this is the right approach. I already have the gate driver and the MOSFET so i would like to make use of them instead of buying new parts.

Question 2:
I also read threads on adding a capacitor across the MOSFET to control the slew rate but the thread all talked about this in the context of driving capacitive loads. In my case the load will be inductive in nature (Motors). Should i worry about this here?

I am using the Fairchild FQP47P06 and FQP50N06L on each side of the H-bridge with the FOD3184 to drive them. Supply voltage is 24V DC. PWM frequency 10-15 KHz. the motors are small… RC car type of things. I have 2 steeper and 2 BLDC running at less than 1Amp current.

Proposed circuit change based on the answer from Rioraxe:

-12V Negative power supply: (is the polarity of the capacitors correct?). In this circuit the LM7912 makes me think of a normal voltage regulator connected upside down Power to GND Pin and GND to the Input pin… i'll have to test this on a breadboard.

Let me know.

enter image description here

Best Answer

The simple zener across gate-source would waste a lot of power. You would need to add a current limit resistor from driver to gate and you want to size the resistor for significant driver current, that significant driver current would persist through the zener and the resistor when driven to ground by the gate driver.

I would create an intermediate supply for the Vss of FOD3184s. The average current requirement would not be high, perhaps less than 10mA per driver-MOSFET. If a 7912 or similar is handy, that may be what I would use. Connect the regulator common/reference to PWR. Put a few uF of decoupling cap next to each optocoupler-driver.

I don't see how adding capacitors across the MOSFET outputs would be beneficial.