Buck regulator frequency

buckemcground

I am trying to use a buck regulator. Usually I make it use at its Max frequency and then just use it. But now, we want to go for Emi and emc certification. I am hence told that this switching (a few hundreds of kHz) can generate noise in the system. So what is the optimal frequency to prevent rfi or noise ?

Best Answer

Hundreds of khz is not your biggest problem. You should worry about hundreds of mhz that are involved in switch rise time, boost capacitor charging, etc. It all, by the way, starts with good layout, which if you don't have, is the reason for EMI problems.

There are several simple rules to follow, i hope not to miss something.

  1. Keep magnetic flux as constant as you can: try making inductor current flow almost same way when the switch is on and off.
  2. Where you have pulsed current, make sure return parh is right near the line. Of course, the both paths must be very short. Consider ferrite for boost cap to limit bandwidth.
  3. Prevent capacitive coupling with switching node by removing plane underneath.

2 is most tricky, so pay close attention to whe pulsed currents flow. Use capacitors to provide the pulses, where needed, mostly on switch input.

By the way, you should also use a nice common mode filter on your input to prevent conductive emission.

Good luck!

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