[time-nuts] Switching regulator replacement for 7805

jimlux jimlux at earthlink.net
Mon Dec 5 13:22:59 EST 2016


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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