Russell McMahon <apptech@paradise.net.nz> wrote:
> I see buck boost as a clever combination of just what its name 
> suggests. Someone decided they could make a buck boost as well by 
> switching both ends at once.

Is that really what they do? I've never looked closely, but it seemed
to me that you'd switch one end or the other, but not both at the same
time.

> Note that a pure boost converter is fundamentally MORE efficient than 
> a buck boost in boost mode.
> This is because the boost converter output "stands on" the input rail 
> and that part of the input is 100% efficient*.

Not really. You're always wasting power in the Vf of the diode. Saying
that "part of" the input is 100% efficient seems like pointless hair-
splitting anyway. I sort of understand what you're saying, but in terms
of real efficiency, you need to look at the real power dissipated in
the switches, diodes, coil and capacitors. Not to mention the energy
pumped through the gate-drive circuit(s), which can be rather significant
if you're going for fast switching.

Assuming you do indeed switch only one end at a time, the only additional
loss in a buck-boost relative to a pure boost is the on-resistance of the
input-side switch. The only additional loss relative to a pure buck is
the extra series diode on the output. Synchronous rectification helps
a lot with diode losses (but then there are more gate drivers).

-- Dave Tweed
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