Re: gEDA-user: Switching regulator question

[Dan McMahill <dan@xxxxxxxxxxxx>]

Bob Paddock wrote:
> On Saturday 16 June 2007 19:43, Randall Nortman wrote:
> 
>> It seems that the lower the average input
>> voltage, the higher the average efficiency of the regulator is going
>> to be, so I would ideally aim to have the largest input ripple
>> possible,
> 
> You have to consider things like loop filter lag etc.  A more
> stable input will give a more stable output.
> 
>> which coincidentally allows me to choose a smaller, cheaper 
>> input capacitor.
> 
> Do not over look the "ripple current" rating of the capacitor, when used
> in a switching regulator.  Each topology of regulator places different
> demands on the input and output capacitors.  Using a capacitor
> that does not have a good ripple current rating (we are not talking
> about your 120 Hz here, but the 500 kHz that is reflected back
> to the input) generally lets the magic smoke out of them.
> Once the magic smoke escapes you have to get a new one.


I just want to emphasize what Bob said here.  Ripple current is a Big 
Deal.  Personally I avoid aluminum electrolytics like the plague.  If 
you have to use them be sure to look at the ripple current rating and 
also figure out what temperature the cap will operate at.  Aluminum 
electrolytics have a temp rating and a lifetime at that temp.  Some are 
85 deg C, 2000 hour.  Think about it, 2000 hours is not much.  You get a 
factor of 2 more for every 10 degrees you back off.  Still for an 85 
degree cap, you have to back off a lot.  If it were me I'd want 105 deg 
C caps, 5000 hour so you have a better lifetime.  Or better yet, use 
tantalums or cermaics if you can get enough C.  For tantalums in 
switchers, it used to be the case that you should derate the voltage by 
2x since you can get uneven voltage drops in the dielectric under 
transient conditions.  The last switcher I designed for a product had 
caps which were chosen completely based on temp rating, lifetime, and 
ripple current rating.  The actual capacitance was basically a secondary 
consideration.  I just hoped the bean counters never tried to find a 
cheaper "replacement part".

Your comment about lower input voltage giving higher efficieny is more 
for a linear regulator.  For a switcher it will depend on many things.

-Dan


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