Finding a I/O expander that supports 18 PWM outputs or another solution for MOSFET control

Question

I am a student working on a university project. I need help figuring out how I can supply 18 PWM signals to 18 gate drivers, in a 3-phase inverter.

The inverter is built up of 3 half bridges, with 6 transistors per half bridge, 3 on the high side and 3 on the low side.

The gate driver takes a simple HIGH/LOW signal, and has a built in signal inverter for low side. The switching frequency is at 24kHz. I want a I/O expander or a slave microcontroller to supply the PWM, as the main controller sits quite a bit away from the 3-phase bridge. My wish is to turn off some of the transistors when the inverter is running at a low load to improve efficiency.

As far as I am aware, using GPIO pins is out of the question, so I need dedicated PWM pins, my issue is that I don’t have enough PWM outputs.

I have been looking at I/O expanders, but the vast majority I found only gave GPIO. I have also been looking at LED drivers, but I am not sure if they are the best choice.

Another option might be using AND gate logic, supplying the PWM to one input, and the ON/OFF command to the other input. This way I might be able to reduce the PWM to 6 outputs.

Thanks.

Answer 1

The inverter is built up of 3 half bridges, with 6 transistors per half bridge, 3 on the high side and 3 on the low side.

OK so you basically have a requirement for 6 control lines that need to use PWM. Each PWM control line activates/deactivates 3 parallel MOSFETs yes?

My wish is to turn off some of the transistors when the inverter is running at a low load to improve efficiency.

Then instead of using 18 PWM lines to control all 18 MOSFETs, use extra IO lines to disable some MOSFETs from activating; you'll still use 6 PWM controllable lines but now, instead of an extra 12 PWM controllable lines, you use logic IO lines to disable the 12 MOSFETs that no longer need to play a role.

The number of logic lines could be one i.e. if line=high then the extra 12 MOSFETs still receive PWM drive signals and, if line=low then those MOSFETs are disabled via their respective drivers.

You might want to do this in stages so you might want three extra control lines so that 1 bank, 2 banks or all 3 banks of MOSFETs can be disabled.

Answer 2

In a typical setup an ESP32 has 22 PWM-capable free GPIO pins left plus some input-only pins. It can be used as a slave IC or a slave board.

Answer 3

Motion control MCU can have enough PWM embedded. Look for Texas Instruments or ST products, if should be easy to find.

If, however, you are binded to a CPU that has nothing like that embedded, consider using an FPGA. There you can implement almost any number of PWM, flexible and efficient. Even with variable dead time.