Re: 170VDC to 185VDC boost circuit

Jamie Morken wrote...

I simulated a boost circuit for 170VDC to 185VDC, but the efficiency
is very low, it draws about 25watts when outputting about 9watts
(50mA at 185V).

This is at a 200kHz PWM and 4% dutycycle (not sure I can drive the
fet with the required 0.2uS pulse). The dutycycle is so low because
the boost voltage ratio is small I think. The inductor is running
in discontinous mode. I picked a high frequency to keep the inductor
size small.

Any ideas on how to make this work better?

Hmm, I wonder if you're properly driving the MOSFET? Or did you
select too large a FET with too much capacitance? You'll want to
use small, low-capacitance MOSFETs like those offered by Supertex,
e.g., the 250V 7-ohm TN5325, in stock at Mouser for 40 cents each.
http://www.supertex.com/pdf/datasheets/TN5325.pdf

For even lower capacitance you could use the TN2130, with Coss less
than 15pF (compare to 60pF) at 25V. It's a 44-cent sot-23 surface-
mount part. http://www.supertex.com/pdf/datasheets/TN2130.pdf

But this type of application, which is akin to 185V in and 15V out,
cries out for using a transformer rather than an inductor. That is
to say, consider the inductor having a secondary winding: Instead of
operating at 50mA it can operate at 50mA * 15/170 = 4.4mA, isn't that
attractive? The FET will switch at much lower currents. Furthermore,
you can likely lower the switching frequency, and hence the switching
losses, and still keep the transformer small.

Another thought, if you can use a fixed 15V step-up, you can use
a fixed duty cycle on the MOSFET switch and avoid a dc-dc regulator.
For example, if the transformer winding is 15/170, or say 34:3 turns
(with some turns-ratio adjustment for losses, etc), you can simply
run it at fixed a 50% duty cycle in a voltage-delivery mode.

BTW, using a transformer instead of an inductor means the FET could
see 340V, so http://www.supertex.com/products/selector_guides/101/100
helps us choose a small 500V FET like the VN0550, 99 cents at Mouser.
http://www.supertex.com/pdf/datasheets/VN0550.pdf


--
Thanks,
- Win
.

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