In my last post I wrote about using an N-channel MOSFET as a low side switch element. In many H-bridge applications, the low side switch is PWM’d allowing for control over the average current / voltage applied to the load. This is typical of robotics systems and other motion control designs. In a motor drive application the PWM of a motor allows you to control the motor’s speed and torque. The PWM signal is often generated by a microcontroller pin, which creates certain issues. One substantial issue is that a microcontroller pin is not an appropriate current source for high current MOSFETs.
N-channel MOSFETs typically have a low on resistance (Rds(on)) which makes them efficient at passing higher currents with minimal power dissipation. However, this low on resistance is not present when a MOSFET is being turned on or off, which is happening all the time during PWM. Also, the lower the Rds(on) specification the higher the capacitance of the part’s gate-source ( “gate capacitance” Cgs). This means that the better your MOSFET is at handling heavy loads (high currents), the worse it is at being driven directly by a microcontroller.
You can see the turn-on, turn-off procedure in the images below, which originally come this Texas Instruments Design And Application Guide For High Speed MOSFET Gate Drive Circuits, by Laszlo Balogh.
During turn on the voltage applied to the gate exceeds the threshold voltage Vth, the current required to charge the gate capacitance is initially high, and as the MOSFET turns on the load current increases and the voltage across the MOSFET decreases. You can see that in sections 2 and 3 above that you have both voltage and current applied to the MOSFET (Vds, Id). So during this turn-on period the MOSFET has to dissipate power.
During turn-off the MOSFET also has to dissipate power (sections 2 and 3).
Rgate in the schematics above can reduce instantaneous current source requirements by the device driving the MOSFET. So you can use a large resistor (say 1K ohm) between a microcontroller pin and a MOSFET gate to protect the microcontroller. This increases the rise and fall times of the MOSFET’s gate voltage when turned on/off by the controller, increasing the amount of time in sections 2 and 3 of the timing diagrams above, thereby increasing the power dissipated in the MOSFET. Extended rise and fall times can seriously degrade your design’s ability to deal with high currents.
There are a number of manufacturers of low side gate drive ICs designed to sit between a microcontroller pin and the gate of a MOSFET. Two part families sold by Microchip are the MCP1401/02 and the TC4426, TC4427, and TC4428. These gate driver ICs are pretty standard, commodity parts, so its always a good idea to select something that is both inexpensive and crosses with a similar part by another manufacturer that has an identical footprint. The MCP1401 below is designed to provide 500mA to a MOSFET with a 470pF gate capacitance. The TC4426 can provide 1.5A to drive a 1000pF MOSFET load. Using these chips in your circuit protect your controller and allow the most efficient power dissipation of your H-bridge circuit.