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schematic

simulate this circuit – Schematic created using CircuitLab

The circuit above is used to control a 240W ceramic heater by means of a PWM signal, generated by a microcontroller, with 5V logic. The input is a standard 280W AC-DC external power adapter, the Meanwell GST280A24.The C1 is a large electrolytic capacitor, 2200 uF, with 35V maximum voltage, placed close to the load. This capacitor has a maximum ripple current of 2,06A @ 100 KHz. We can noticed a low but clear humming sound when the circuit is switched on. The sound is coming from both the C1 and the snubber capacitor. We measured the frequency of the acoustic noise and it is around 1.200 Hz. We do not clearly understand what it is happening. The load should be drawing pulsed current at 10A, and there is no inductor in series with the 24V line to help with the current requirements. Could this be the problem? The voltage signal measured in the MEAS node is very clean, there are no spikes or other strange behaviour. Could this be the sign of the C1 suffering because of the exceeding limits of the ripple current? Or would that not be a problem at 30 Hz? Should we skip the C1 completely if reliability over time is a requirement? We do not have physical space on the control card for inserting large inductor coils and several large capacitors, thus the filter on the power input line should be kept at a bare minimum.

Francesco
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  • If the snubber capacitor is class 2 MLCC (X7R, Y5V...) it will act like a piezo speaker. Does removing the cap fix the issue? – bobflux Oct 15 '21 at 08:04
  • Unfortunately not, we tried to remove it and the issue stays there. – Francesco Oct 15 '21 at 08:13
  • Any other class 2 MLCCs decoupling the big power supply cap? Does the whine come from the power supply itself? – bobflux Oct 15 '21 at 10:14
  • At a closer look, the whine seems to come from the DC connection to the control card. There is a large circular connector and the sound is coming from there. Yes, there are some small ceramic capacitors to decople the big power suppy cap. My question is, am I going to fry anything if I leave as is over time? Temperatures of all the components (including the large cap) are at RT, thus not heating is occurring apparently. – Francesco Oct 15 '21 at 10:22
  • Did you measure the audible noise to 1.2 kHz, or 1.2 Hz? The latter wouldn't be audible, but I the former seems too high frequency for your description of a "humming" sound. Are you positive that this measurement was done correctly? You could try changing the 30Hz input frequency, and listen if the audible noise also changes freqency. This would tell you if the noise is caused by some harmonic of the 30Hz, or if it's caused by ripple from the AC-DC converter. – sondre99v Oct 15 '21 at 12:32
  • We did try changing the PWM frequency with no results, the audible sound frequency seems to be 1,2 Khz, thus 1200 Hz. It could be some armonics of the 30Hz? – Francesco Oct 15 '21 at 13:03
  • We located the sound and it seems to come from the magnetic contact that is used to power the circuit. Thus, it is not a problem of the control card itself but instead it is coming from the DC connection to the power supply. I would feel pretty safe now. What do you think? Do you see any flaws in this circuit? – Francesco Oct 18 '21 at 15:23

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