[time-nuts] Switching regulator replacement for 7805
scott.j.stobbe at gmail.com
Mon Dec 5 13:33:11 EST 2016
FWIW, you can snub the switch node to dampen parasitic ringing in exchange
of a loss in efficiency of a couple percent. I'm pretty sure I have seen
this outlined in a few app notes.
On Mon, Dec 5, 2016 at 1:22 PM, jimlux <jimlux at earthlink.net> wrote:
> On 12/5/16 9:13 AM, Dan Kemppainen wrote:
>>> Depending on the application, another possible application is to
>>> sync up the DC/DC converter to the "main" clock source. This makes
>>> the switching noise then coherent to the system, which either makes
>>> it average out completely, or possible to filter it out in the digital
>>> domain using a deep notch-filter in receiver applications.
>> You are assuming the switching noise is directly caused by the
>> fundamental switching frequency, this may not always be the case. Often
>> the fast edges can excite parasitic reactances in the board and
>> components, which in turn oscillate and radiate at their own frequency.
>> This is not necessarily related to the switching frequency.
> On big science spacecraft this is pretty common (which have all sorts of
> sensitive science instruments, some of which may not have the best EMI/EMC
> characteristics from both a source and susceptability standpoint). On the
> other hand, as switching rates have gone up, it's not clear that
> distributing a 20kHz "PWM reference clock" around is a good idea.
> On a radio I was recently working with, it used to radiate quite nicely at
> about 15-19 MHz, even though the switching rate was in the hundreds of kHz
> range - we figured that this is where the "antenna efficiency" (better with
> higher freq) of the traces matched with the "harmonic power" (lower with
> higher freq)
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