Re: boost converter

Hello Michael,


I was reading up on the theory behind the boost converter:

http://en.wikipedia.org/wiki/Boost_converter

and I thought, you know, there is a short circuit for a certain amount
of time. Actually, for a fairly long time, since Figure 5 of the above
link seems to show best performance for a duty cycle of 80% (please
correct me if I'm wrong).


Figure 5 shows just a graph. Duty cycle versus performance depends on
the output load (voltage and current), it cannot be generalized. 80%
typically happens when the output voltage needs to be 4-5 times higher
than the input.



Would having a resistor (even just one ohm) in the path of the
switching transistor improve efficiency at all...?


Nope, that would increase the losses because it would be like degrading
the Rdson by 1ohm. When we design switchers there is always that hunt
for the least in Rdson that a nickel or two can buy. No free lunches there.

There really isn't a short. The inductor current begins to ramp up. At
some point the switch opens and the inductor dumps its current into the
load side capacitor. "Some point" means before the inductor core
saturates. Else there will be an impressive and loud pyrotechnical show
followed by a plume of smoke and wailing fire alarms. Same if it goes
into continous current mode (CCM) and the inductor current "ratchets up"
for some reason.


Ok, thanks for the reply.

Loud pyrotechnical show, smoke, wailing fire alarms... just *not* what
I'd want in a boost converter...

On that thought... put a fuse in the same path where I'd previously
recommended a resistor?


You could but by the time it blows the transistor is most likely already gone. A fuse is typically placed before the whole converter, in an attempt to prevent a major explosive event.

--
Regards, Joerg

http://www.analogconsultants.com
.

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