Pat Lawler wrote:
> We are designing a switching power supply for a customer that has
> multiple outputs. Due to the tight regulation requirements, all
> outputs have their own PWM modulators and control loops.
>
> The customer feels the RF emissions (as measured by CISPR 11) will be
> reduced by synchronizing the frequencies of all the converters. I
> think synchronizing the PWM sections will make the total power supply
> emissions as high as possible (emissions are coherent?).
>
> 1) What has been your experience with controlling noise from multiple
> switching power supplies? Is synchronizing a good idea?
>
Synchronizing is a two edged sword as I think you have correctly
surmised. It is a good technique at reducing EMI susceptibility where
there are clocked processes such as logic and A to D conversion.
The biggest problem is the PWMs will have a spectrum of emissions that
varies as a function of load current. Synchronizing will make it easier
measure emissions by eliminating the variable beat frequencies. This
usually results in higher apparent emissions.
>
> 2) If the supplies are synchronized, would a phase shift between
> converters (avoiding simultaneous switching of all converters) help?
>
Yes. Using a multi-phase clock will help. The biggest effect will be
seen at light loads. As loading increases the PWM pulse widths will
start to overlap and the emissions will go up.
Overall I think synchronizing is a good idea. There is a way to test the
hypothesis. Find a way to run each of the converters by itself. (I do
this by replacing all the converters but the UUT converter, with linear
lab supplies.) If any of the emissions of each individual supply is say
greater than 6db above the others, synchronizing will have minimal
effect. If all emissions are approximately equal, synchronizing will keep
the levels close to the single converter values. Don't forget to test at
all load levels.
Fred Townsend
>
> Thanks,
> ---
>
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