[time-nuts] Thunderbolt - any negatives ?
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Jun 1 17:17:25 UTC 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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