Peter Mcalpine wrote:
> Hi All,
> Trying to get my head around DC:DC buck regulators..
> Have a circuit that generals runs from 12V input
> and has 3.5V output at 150ma.
> At present am using an ON Semi MC33063 chip. Although
> cheap it uses transistor switches inside, and would like
> to be able to have a bit more head room with a FET style
> unit..
>
> Anyway, at present with 12Vin, 3.5Vout@ 150ma I am
> getting 69% efficiency.
>
> What components most effect this efficiency number?

Switching frequency: every time the transistor goes into a transition
period, you lose more energy than if it is just on or off.

Switch: a MOSFET is more efficient than a bipolar transistor most of the
time

Diode: The 'free-wheel' diode conducts when the switch is turned off, so
duty cycle can have some effect on losses.

Hope that helps. A low frequency super-low Rds(on) MOSFET and high speed
schottky diode would have the highest efficiency, but as the frequency
goes down, inductors and capacitors tend to get huge.


>
> Also, it seems that the DC:DC solution likes a bit of
> current going through it, as when I switch off things
> my efficiency drops to 48% (13ma output current).
> Should I do things differently?

Since the MC33063 is a fixed frequency device, it may force the switch
on every cycle, but since no current is being drawn, the duty cycle is
very short. Bipolar devices can be "slow", it may not have enough time
to fully turn on before the control is telling it to shut down. That's
my guess.

--
--
Martin Klingensmith
http://infoarchive.net/
http://nnytech.net/


>
> Thanks!
>
> Regards,
> Peter Mcalpine

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