Hi,
I am working on a switcher with ADC sampling at the same frequency as
the main switcher PWM frequency, they are already synchronized to reduce
noise when sampling the ADC. There are also several small flybacks
(10watts at 12Volts output) on the board, I am thinking of syncing their
PWM as well, but I am wondering if syncing a flyback is the same
as syncing a forward converter for timing? Is it worthwhile to do this
or is the noise reduction going to effectively be minimal since there is
noise all the time in SMPS due to the fast current changes in the
inductors, or is switching noise really something that is worth taking
into account even for small switchers?
cheers,
Jamie
Others have mentioned the desirability of not having switchers
operating on different but nearly the same frequency. I can attest to
that being a very good idea. I just got bit by it. ;-) Two supplies
running from the same input bus, physically separated by a few inches
but "talking" to each other on the input bus were the problem.
Apparently the control loop in them is noisy (marginally stable?) at
around 4-5kHz; there's a broad peak in the spectral noise out at their
outputs around that freq. It's low enough frequency to be impractical
to L-C filter. But when we got a board with those two switchers
running at frequencies different by about 5kHz, the 3.3V-output one
(which supplied the ADCs through some filtering that was quite good at
the switching freq) had a couple millivolts of 5kHz on its output, and
that caused sideband spurs at 5kHz on the digitized samples.
Something not mentioned by other posters, at least as far as I could
see, is that you can make things much better if you not only
synchronize them but run them at phase offsets to interleave the
pulses drawn from the input side. This is a really big advantage.
Some switcher controller manufacturers such as Linear Technology offer
parts that do this for you.
If the current drawn from each supply is essentially constant so that
the switcher runs with each cycle like every other cycle, then if the
ADC sampling is synced to the switching frequency, effects from the
switchers alias to DC, and to the degree they are constant, they can
be calibrated out (subtracted out--one nice thing about aliasing to DC
is that you have no phase to worry about!). Another trick is to put
the (aliased) spur signal in a frequency band you aren't interested
in.
But I can also offer you some hope: the board I've been working on
has several switchers on it, to supply 1.0V, 1.2V, 1.8V and 3.3V, for
some fairly heavy-duty digital processing. There are two analog -->
ADC channels that run at about 100Ms/s. I can see spurs on one
channel at the switching frequencies and harmonics, with max amplitude
around -120dBm for one of the fundamentals, rapidly trailing down to
much lower levels. Almost all that is picked up by amplifiers and
filters in front of the ADC. On the other channel, I see three power
supply fundamental spurs at -135dBm or less, and practically
everything else is lost in the noise floor at -155dBm (this with a
10Hz resolution bandwidth). I believe almost all of what I see is
being coupled in magnetically at this point. We did kill the 3.3V
switcher noise by going to a linear regulator down from 5V for the
critical analog parts; the very slight overall lowering of efficiency
was well worth the performance gain.
Cheers,
Tom