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RE: DC power supply question

Posted by VoltSecond (M) on November 7, 2024 at 19:23:47

In Reply to: DC power supply question posted by MylesJ on October 17, 2024 at 18:33:04:

Switching supplies can be very quiet with some effort (I've made the output noise and ripple better than 110dB down). Often the most bothersome noise problems from switchers is "Common Mode" (the hot and cold bouncing up and down in phase.) Just adding a common mode choke can help at RF frequencies, BUT at the LC resonant point, the choke makes the noise issues much worse.

What is the LC? The "C" is the "Y" capacitance in the switcher added to the switcher's transformer's primary to secondary capacitance. The "L" is the inductance of the choke. The way around this resonance is to use a damped common mode choke with the damping loosely coupled to the choke.

Standard off the shelf common mode chokes with 1/2 the turns on one side of the core and the other 1/2 on the other side of the core can make things worse. I've seen it at work more than once. The split winding chokes need to be used with ultra careful layout and/or shielding.

Many switchers do not "like" to run into less than 10-20% of full load. Into light loads, the switchers hop switching cycles and do other things to make noise. If your switcher is running with less than 10% load, add a resistor to get it above 10 to 20% of rated output current (on all outputs).
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Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.

. . .Oh!. . .Remember: Modifying things voids their warranty.

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