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Synchronizing DC-DC converters to reduce emissions?

P

Pat Lawler

Jan 1, 1970
0
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?

2) If the supplies are synchronized, would a phase shift between
converters (avoiding simultaneous switching of all converters) help?

Thanks,
---
 
K

Ken Smith

Jan 1, 1970
0
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?

If you have more than one PWM circuit in the system, it is a good idea to
make sure that they run at exactly the same frequency to reduce the
likelihood of beats between them. I have done this using a circuit that
made one PWM chip tend to run at 180 degrees to the other. The results
were good.

In another case, I fed some of the transistor turn off edge into the
oscillator of the other supplies PWM. My theory was to make one
transistor turn on just as the other turned off. Although it did work, I
found that the regulation of the second supply suffered.
 
N

Nico Coesel

Jan 1, 1970
0
Pat Lawler said:
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?

I think it's actually a bad idea, the emission will be concentrated at
one single frequency. It's even better to use spread spectrum PWM
chips. These will vary the operation frequency deliberately.
 
K

Ken Smith

Jan 1, 1970
0
Nico Coesel said:
I think it's actually a bad idea, the emission will be concentrated at
one single frequency. It's even better to use spread spectrum PWM
chips. These will vary the operation frequency deliberately.

Any day now, I expect the (US)-FCC and the EU to change the rules to make
the limit be the total RF power and not the strongest peak. Until then,
spreading the noise out does help to pass the standards.

Often you don't need a special chip for the spreading. Just don't filter
the "DC" going to the chip too well. The ripple from the rectifiers will
make the frequency wobble around.
 
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