Question about a mosfet driver gate resistor

Question

I'm considering the ZXGD3003E6 for driving an N-channel MOSFET - PSMN4R5-40PS,127. There is a typical application schematic in the MOSFET driver datasheet (as shown below). My question is how do I calculate the value of R1 and R2?

Edit: Will be driving the gate driver with a TLC272 opamp as a buffer which is connected to a 12V supply. Also the current through the MOSFET will be no more than 10A DC.

Answer 1

From the driver's point of view, a power MOSFET looks like a capacitor from the gate to the source. This is why the notation says that varying the resistors changes the turn on and turn off times; those resistors work with the FET capacitance to form an R-C delay network. If you want to slow down the turn-on and turn-off voltage ramps htat the load sees at the FET drain, then R1 and R2 can do that for you. If you are just switching a relatively static load on and off, they are not needed.

Another reason for R1 is to reduce ringing caused by a resonant tank formed by the FET capacitance and lead inductance. This is an issue only in repetitive high speed circuits like switching power supplies, PWM motor drivers, etc. Again, if that is not what you are doing then R1 is not needed.

Answer 2

The N-Channel MOSFET that you have selected is listed as a "standard level MOSFET" in the title of the datasheet. This means that the gate drive voltage needed to turn on the device will be higher than a more specialized "logic level MOSFET". You should check out the transfer function charts in the data sheet to become familiar with the device characteristics. You will require a gate drive voltage (gate to source) of at least 5 Volts to achieve a decent level of drain current flow. In fact to achieve the advertised on resistance value of 4.6 mΩ you would require gate drive voltage of 10 Volts.

The gate driver IC that you have selected is not a particularly elegant part. Due to the emitter follower nature of the device the high (low) level output levels are a V_BE voltage drop less than (more than) the input pin voltages. This means that if your microcontroller GPIO drive signals are a high drive of 5 Volts you would not see the output get to 5 Volts. In fact in the data sheet they state a typical value of 0.4V V_BE when the current flow is 1uA. So even if you get the gate capacitance of the MOSFET fully charged the drive voltage can only get to around 4.6 Volts.

This is hardly a good match for part selection even if your MCU has 5V swing GPIOs. If the MCU you are using has 3.3V GPIOs (which is much more common) you have a non-starter solution here.

Further consider that if you do choose to use the R1 and R2 resistors as shown in your diagram they act as a voltage divider which will further reduce the gate drive voltage.