[time-nuts] Thunderbolt - any negatives ?

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Jun 1 13:17:25 EDT 2009


Hi there,
 
A switcher has much more stresses on the components, since it  usually 
switches the primary side rectified 110/220V high-voltage across a  transformer. 
Thus the switching FET has to be very high voltage capable (about  ~170V DC 
in the US), and the second  component under stress is the primary  high 
voltage capacitor, because it sees a very fast AC switching current on it  
(current draw is on when the FET is on, and off when the Fet is off).  Also 
there has to be a fast snubber network to prevent the back-emf from  destroying 
the primary Fet with over-voltage.
 
A linear supply has none of these fast current/voltage transients on it,  
only a couple of diodes switching the 60Hz secondary onto a capacitor at low  
voltage.
 
A secondary concern is thermally induced stress, switchers will usually be  
packed into a very small enclosure with very high power capability/density. 
This  is not possible for linear supplies, since the transformer size will 
usually  determine overall sizing. Compare a Laptop power supply size 
(usually these have  between 40W and 90W rating!) to a similar rated linear supply.
 
bye,
Said
 
 
In a message dated 6/1/2009 09:48:29 Pacific Daylight Time,  
hmurray at megapathdsl.net writes:

Is there  something I don't understand in this area?  What makes a linear  
supply more reliable than a switcher?

My first guess would be a  switcher would be more reliable because it would 
run cooler.

That's  probably assuming the same amount of design effort which is 
probably 
not a  valid assumption if I'm comparing a brand-X linear with a brand-Z  
switcher.  A quick glance at the general construction might give a  better 
answer.





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