Causes of inductor whistling in DC/DC circuits and related solutions

What is DC/DC circuit inductor whistling? How to solve it? The frequency of the switching current output by the regulated power supply circuit, or the periodic frequency of the periodic pulse group, or the periodic frequency of the glitch falls into the audio range of 20~20kHz, and is periodic The changing current passes through the inductor coil and generates an alternating magnetic field, causing the inductor coil to produce mechanical vibrations at an almost fixed frequency like a "horn" under the action of the alternating magnetic field and howl.

The size of the whistling sound has a certain relationship with the quality of the inductor winding. The looser the winding, the bigger the whistling sound will be.

Factors causing inductor whistling in DC/DC circuits

1. The load current is too large. There is a current-limiting protection circuit inside the DC/DC chip. When the load exceeds the maximum current of the switching MOS tube inside the IC, the current-limiting circuit detection circuit will adjust the duty cycle inside the chip or stop working until the load current is detected. When it is within the standard range, restart the normal working switch.

If the time period from the stop switch to the restart switch happens to be within the frequency range of 20~20kHz, the vibration frequency of the inductor will be heard.

2. The load voltage is too high. If the load is empty or lightly loaded, the overvoltage protection will be triggered and the switch will stop. If the voltage drops, the operation will be restarted. The time from stop to restart will easily fall within the audio range.

3. The circuit generates self-excitation. When the power circuit is designed, the stability is not enough. When the load changes, it is prone to self-excitation. When the frequency of self-excitation falls within the audio range, it will cause howling.

4. The crossover frequency falls within the frequency range of 20~20kHz. The crossover frequency is usually 1/5 to 1/20 of the switching frequency. The audio frequency range needs to be ignored during design, otherwise the howling problem will not be solved.

5. The switch switching overshoot is large. During the switching process between the DC/DC high-side tube and the low-side tube, due to the existence of parasitic capacitance and parasitic inductance, an overshoot will occur at the moment when the top tube is turned on, and the amplitude will be large, resulting in a periodic overshoot voltage being applied to the inductor. This causes the howling sound to become louder.

Howling solutions

1. Increase the switching frequency. Make the crossover frequency deviate from the audio range to avoid howling;

2. Ensure that the circuit works stably. To avoid self-excitation, the parameters involved are many and complex, such as:

a) Adjust the compensation loop. It is usually difficult to adjust, but the effect is the best;

b) Adjust the output inductor value, but the effect is not very obvious;

c) Adjust the output capacitor value, the effect is obvious;

d) Adjust the ESR of the output capacitor, the effect is obvious.

3. Reduce overshoot voltage. Adding a buffer circuit between the upper and lower tubes of the switch, such as the R4 and C2 circuit in the left picture above, or the D1 and C2 circuit in the right picture above, can reduce the whistling, but it cannot eliminate it.

4. Reduce the impact of parasitic parameters on the circuit. Lay out the wiring reasonably to reduce the parasitic inductance and capacitance caused by the power supply and ground loops, and keep the feedback loop as far away from the switching circuit as possible;

5. Reduce the mechanical vibration of the inductor. Choose good quality inductors;

6. When selecting the power supply IC, do not exceed OCP, OLP, OTP and OVP. If there is a exceedance, the gap frequency should also avoid falling in the audio area, and margin design needs to be considered. The above is the inductor whistling in DC/DC circuits and the corresponding solutions. I hope it can help you.