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I'm using MC34063 in step-up configuration to charge 4S lithium-ion battery pack. Input is 12 V output is set to 16.9 V. Module consists of a 0.33 Ω current sensing resistor, a shielded 2 A 100 uH inductor, SS34 Schottky diode and 100 uF 1206 SMD ceramic capacitor. While battery is charging there is no noise but as soon as batteries are full and charge controller disconnects the charger from battery pack MC34063 module starts emission of an annoying noise. I cannot find which component is the source. The inductor, Schottky diode, capacitor or the IC itself? Can you help me please find a solution for this annoying issue? enter image description here enter image description here

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  • \$\begingroup\$ Is there a particular reason you are using MC34063 in 2023? It lacks any skip mode at low or no load so it will tend to oscillate around any minimum duty cycle. Any modern boost converter would not have this issue. \$\endgroup\$
    – winny
    Commented May 25, 2023 at 11:07
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    \$\begingroup\$ Source of the noise is most probably in the inductor and ceramic capacitors due to Lorentz forces inside them. \$\endgroup\$
    – winny
    Commented May 25, 2023 at 11:14
  • \$\begingroup\$ Thanks @winny, I'm open to suggestions... What is the commonly used alternative which hopefully has the circuit as simple as MC34063? \$\endgroup\$
    – AKTanara
    Commented May 25, 2023 at 11:15
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    \$\begingroup\$ Out of stock, all of them at all distributors? The pitch would be due to how MC34063 handles low duty cycle. Changing to components with less “microphony” would just attenuate it. Nail polish/conformal coating/hot melt glue is also good for attenuating the sound. \$\endgroup\$
    – winny
    Commented May 25, 2023 at 18:24
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    \$\begingroup\$ You might try this: remove the battery and replace it with a load resistor to see how little load you can get away with to keep the noise from occurring. If the load is not too high, you can leave it there so the converter will see a load after the battery is charged. You probably want to add a diode between the load and the battery to keep the battery from discharging through the load resistor the charger is unplugged, and keep your feedback point at the battery. This will hurt the efficiency, but at least you won't have the noise. \$\endgroup\$ Commented May 26, 2023 at 12:45

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I've tried to calculate the conditions with the values in your schematic and the online calculator: http://www.nomad.ee/micros/mc34063a/
I adjusted the frequency until the calculated capacitance showed as 680pF. enter image description here When charging, the frequency would be little above 19kHz. The MC34063 does not PWM but will miss pulses when the output voltage is reached. 19kHz is too close to what you can hear.

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