Electronic – Audible noise in DC-DC buck converter

acoustic-noisebuckpwmswitch-mode-power-supply

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:

PWM driver circuit

Where:

  • 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

+3.3V power supply circuit

+24 V power supply

This is the plot of the +24 V power supply when the PWM is activated.

+24V power supply output plot

Issue

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?

Best Answer

The first question is, what is switching 5A or 7A at 100Hz doing to your +24V supply? It will be adding some ripple to it, can you measure how much on a scope? I'm guessing the ripple looks a bit like a square wave.

The second is, how much current do you need from that 3.3V supply? 0.1A, 1A, 10A?

Assuming less than 1A or less than 3W total, so the current drawn from the 24V supply is 0.1 or 0.2A...

Then you can feed the 3.3V supply from 24V via a low value resistor : 1 to 5 ohms, dropping less than an volt across the resistor - max 0.1 or 0.2W wasted power.

And replace C11 with a much larger capacitor - 100 to 1000 uf. Then R-C provide a low pass filter to reduce the PWM ripple, or at least slow its edges to a ramp, so that the regulator can respond fast enough to maintain control of the 3.3V output without "singing".