Build and test a general-purpose high-side PWM controller
This circuit cobines a variable-duty cycle PWM generator with a high-side p-channel MOSFET driver circuit, making it suitable for controlling loads up to 15V/23A.
The PWM signal is generated with a modified 555 Timer astable oscillator.
Steering diodes are used to isolate the 100kΩ pot so that it primarily adjusts duty cycle with little change to the frequency.
A switch allows selection of 1nF & 10nF capacitors, selecting a base PWM frequency of:
- 10nF - 1.25kHz
- 1nF - 12.6kHz
The PWM output switches an n-channel MOSFET which in turn controls the high-side p-channel Power MOSFET.
The shunt resistor R4 limits the bypass current when the n-channel MOSFET is on. At 1kΩ, it wastes up to 15mA (at 15V) but in return allows fast switching of the Power FET. A higher value of R4 (10kΩ, 100kΩ) saves power, but switching time is affected.
A pair of capacitors (220µF, 100nF) provide some smoothing of the power supply to the Power FET and decouple the PWM and FET loads.
At 50% duty cycle:
At minimum duty cycle (~1.25%):
At maximum duty cycle, the output approaches 99.9% fully on.
At 50% duty cycle:
At minimum duty cycle (~1.25%):
At maximum duty cycle, the output approaches 100% fully on.
Protoboard/boxed construction:
