In my design, I have a +3.3 V SMPS that provides power to digital circuits (microcontroller, EEPROM, etc.). The microcontroller allows applying a 100 Hz PWM signal with variable duty cycle and with an amplitude of +24 V to a resistive load. This is done with the following circuit:
- HEATER_PWM is a 100 Hz PWM signal with an amplitude of +3.3 V coming from the microcontroller.
- HEATER_A and HEATER_B are the connections to the resistive load.
- HEATER_ISENSE+ is the voltage signal used for current sensing.
+3.3 V power supply
+24 V power supply
This is the plot of the +24 V power supply when the PWM is activated.
When the 100 Hz PWM is enabled (and only when it is enabled), the +3.3 V power supply produces an audible noise which seems to be at the PWM frequency. I suspect that this is due to the power inductor "vibrating" at 100 Hz.
I tried to increase the value of R116 to 100 kOhm without any effect.
How could I get rid of this acoustic noise?