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Frankenstein's Fix: Power Supply Parables

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Caleb Kraft
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surprises about
Caleb Kraft   10/18/2013 11:50:10 AM
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it is always interesting how wrong the designs can be for a specific purpose product. it is as though they never researched their target use.

rick merritt
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Re: surprises about
rick merritt   10/18/2013 7:07:58 PM
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Cool!

salbayeng
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Re: surprises about
salbayeng   10/24/2013 4:36:44 AM
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Decades ago we had (switch mode) power supply problems on the new fangled Apple II computers.

Turns out there was an  electrolytic coupling capacitor that provided PWM feedback to the power stage through a transformer (rather than an optocoupler). 

The capacitor was forward biassed under light load, but we were a research lab and we filled all the available slots with cards. But under this heavy load condition, the coupling capacitor was reverse biassed and would eventually depolarise. 

Swapped the cap for a bipolar type and it fixed the problem.

 

_hm
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Good to know fix
_hm   10/19/2013 5:31:09 AM
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@Geln: It is good to know the fix. However, all experienced designers make some small or not so small mistakes. May be it is in hurry or sudden change ofspecs etc.

Similary I have observed, people using 5V TVS for 12V lines too. And many standard MOSFETs do not get proper VGS(TH) to drive it.

Caleb Kraft
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Re: Good to know fix
Caleb Kraft   10/24/2013 5:36:47 PM
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yeah, often times if you get a chance to talk to the designers you'll find they probably knew about this and it was in fact a last minute change, or possibly an oversight discovered after production.

elctrnx_lyf
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Satisfying
elctrnx_lyf   10/19/2013 9:16:18 AM
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It would be definitely a satisfying experience to actually dig deep into the unknown design and fix the issue.

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