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From: "Frithiof Andreas Jensen"
Subject: Re: Switching regulator noise
Date: Tue, 7 Jan 2003 10:44:43 +0100
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"James Fitzsimons" wrote in message
> Any suggestions will be much appreciated,
Ok - In order of priority:
The PCB layout is critical - it is useful to ensure that the current loops
terminate exactly at the filter capacitor(s). A local ground plane on top of
the PCB connected only at the "ground point" (filter capacitor) is good
too - one can use it as a heat-sink as well. The output filter ground point
can be connected to it too, but it should be "isolated" from the rest of the
circuit. We want to have a "deserted island" for the noise ;-)
The free-wheeling diode can oscillate badly if it is a fast-recovery type, a
soft-recovery type may help, or in pathological cases an RC network in
parallel with the diode is one solution. The capacitor should as small as
possible - a good first guess is five times the diode junction capacitance;
the resistor is determined experimentally but in the tens of Ohm's usually
is right. (The objective is to force the parasitic network to oscillate with
the capacitor and then kill the q with the resistor).
Murata does some neat integrated filters with the proper layout for killing
the noise. One on the output is good.
An oscilloscope is very useful in finding out where the ringing is.
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