[eagle] Re: CAN-Do Power Supply Instability

Robert McGwier rwmcgwier at gmail.com
Mon Jun 18 21:40:16 PDT 2007


Juan Rivera wrote:
> All,
> 
>  
> 
> I dug into the CAN-Do circuitry today.  The CAN-Do module uses a Maxim 
> max1836 switching down-converter.  I found the following quote in the 
> spec sheet, "Commonly, instability is caused by excessive noise on the 
> feedback signal or ground due to poor layout or improper component 
> selection.  When seen, instability typically manifests itself as 
> ‘motorboating,’ which is characterized by grouped switching pulses with 
> large gaps and excessive low-frequency output ripple during no-load or 
> light-load conditions."
> 
>  

I do believe you have found the culprit and nailed it down completely.
> 
> It reminded me of the waveform I captured earlier from CAN-Do inductor, 
> L3. (See the waterfall display below.)  Those are definitely grouped 
> pulses with large gaps.  Notice also that there is tremendous splatter 
> over a large bandwidth that can just be seen as faint horizontal lines.
> 
>  
> 
> I’d like to propose an alternative – Move the switching down-converters 
> out to the power distribution and management area and feed DC to the 
> payloads at an appropriate voltage so switching power supplies can be 
> removed from all analog payloads.  This would go a long way towards 
> cleaning up what is shaping up to be a nasty situation.  It’s just not 
> good design practice to bundle switching supplies with low-level analog 
> signals.


It is understood why you would want this. But this will not be done.  We 
distribute main bus power, the best that can be conditioned from the 
solar arrays through storage batteries and charge regulation for the 
batteries.  We do not build power supplies.  We build battery charging 
systems with highly variable sources (rotating spacecraft with varying 
degrees of illumination on them) and all we attempt to do is maintain 
something like a stable voltage without wrecking the batteries.  The 
analog payloads must provide conditioning from this nasty source to 
derive for themselves what they need.  What cannot be allowed is 
radiation sources that are the cause of interference to this and other 
systems, such as you have found with the Can Do. You message there is 
received loud and clear.


> 
>  
> 
> 73,
> 
>  
> 
> Juan
> 
>  


I want us to do what is necessary to get clean power onto the receiver 
to complete testing of the proposed RX circuity assuming clean power, so 
we can make modifications to the RX design outside of power.  In other 
words, I want RX testing to commence with the assumption we will deal 
with the Can-Do radiation problem.  We will not provide more than  the 
usual spacecraft DC bus.  It must be assumed it will be somewhat nasty, 
and the analog payloads must deal with it to provide the voltages it 
needs for proper operation.


Thank you for all of your fantastic work.  It is really appreciated.

Bob


-- 
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"If you're going to be crazy, you have to get paid for it or
else you're going to be locked up." Hunter S. Thompson


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