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[moved] Boost Power Supply EFI Help Requested

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Newbie level 4
Sep 6, 2015
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I am a ham radio operator and am seeing EMI noise from a small Chinese 12VDC to 20VDC boost power supply in the 3Mhz to 30Mz range. The noise is on harmonics of approx 100Khz. The boost device is powered from a 12V battery (no input noise).

The EMI only appears when the boost circuit is wired to the radio (no emissions are visible when the boost is simply turned on). The small boost power supply only drives a 500ma load (but the load is a very sensitive preamplifier).

I am hoping that this is a simple problem for someone with the right knowledge. There is no input noise to the boost circuit, and I want to filter all noise on the output above 3Mhz. I have tried multiple loops on ferrite beads (type 31, 43, and F), without much success.

I would appreciate any advice on a simple LC circuit (with values), voltage regulators or any other simple solutions for a novice with minimal electronics theory background.

Thanks, Ron

Break open the unit and put snubbers on the mosfet(s) and diodes..

Easy Peasy, the unit is sealed in a small metal casing. The small size (approx 1 1/2 inch) was part of the advantage for a portable radio box. Is there anything you could recommend for the output?

Thanks, Ron

The only thing you can do is add LC filters on the output then. Maybe 1uH inductors on both the output leads, followed by a ceramic 1uF capacitor on the output.

This may be a stupid question, but can I use a small axial leaded inductor or does it need to be a wire wound coil?


Any inductor can be described as a "wire wound coil," so I'm not sure what you mean. So long as the inductor is rated for your desired current, and doesn't have a self resonant frequency below your bandwidth of interest, it should work. For a high SRF, using an air core inductor would be wise.

That helps, thanks. One final question, I cannot find any ceramic disk capacitors above .47uf. There are SMD caps above this value. Am I looking at the right type of capacitor?

Thanks again.

assuming its mostly common mode interference, find a ferrite toroid (high u) say 30-50mm OD, and wrap the two o/p wires thru the toroid say 5 -10 times and see if there is an improvement, if this has no effect, then the noise is differential and you will have to put some inductance in series with each line and then a cap to filter the noise

Just for a blind test, had you already tried to ground some parts of the circuit ( housing, battery negative, etc ... ) at a reliable electric Earth ?

Yes, I have tried grounding the boost chassis to the load negative. It shifts the EMI frequency slightly but does not reduce it. Since the circuit is driven by a battery it really is independent of earth or the mains ground.

Any further recommendation on the 1uf cap type? If I go with a ceramic disk capacitor they only seem to go up to .47uf.


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