Oh dear - should have gone resonant !

Quite often, for power supply designers - the last phase is EMC testing & remediation.

This is somewhat logical as you have to have something stable to test - before you can do long EMC scans at full power - usually worst case for EMC.

Note the scan above - it is pretty awful - peaking at about 108dBuV ( 250mV @ 3MHz ), this is really not what you would hope to see on 1st scan.

Note the * in the top right corner - this tells you the spec ann is being over-driven on its input - and you should add a 20dB pad at least ! - as the results will be in error - also severe over-driving can damage the spec ann ! ( N.B. repairs and calibration are costly ).

You can make your life easier by choosing a switching topology that makes the least RFI ! - i.e. a resonant one.

This means the output rectifiers are soft switched ( a big source of noise to 300MHz ),

the main mosfets are soft switched, the dV/dt and di/dt are limited in the transformer,

and the heat-sinking is well thought out to limit RF propagation.

If the above is true then designing a matched EMC filter and passing EMC becomes much easier.

It is an unfortunate fact of life that hard switched converters make a lot of noise, - and the higher the Vin or Vout ( or Iin or Iout ) - the more noise they make !

this can only be cured by snubbers, lots of heatsinking - lowering switching rates - and finally - lots and lots of filtering - L's and C's . . .

And all this has to be done near the end of the project - usually requiring modifying the case, filter pcb's - increasing airflow - re-wiring connections - the list goes on.

Some times we are asked - why go resonant when you have to add resonant chokes and caps ? - these are expensive and take up space - and need a share of the airflow.

The answer can be divined from the above - would you rather add chokes and caps to the input and output filters ? these certainly add cost - often needing a steel case themselves - and take up their fill of room . . . and then there is the time to add these remedies . . .and likely the need to re-design the case.

So - starting with a " quiet " converter really does save time and cost and gets you to an EMC pass without having to carve up your enclosure to fit larger filters, heat-sinks and fans.

for more of the good stuff . . . .

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