Electron wrote:
> Hi,
> can anyone suggest a good, cheap, IC to make a 8..15V->300V DC-DC convert=
er?
> I wish to have about 50W output, I can go as low as 20W if necessary. It =
is
> for a CDI.
>
> The most important thing is not to have fancy components, transformers ar=
e
> OK as long as they're easy to wind by hand, or commercially available at =
a
> reasonable price.
>
> So far the best I came with is a UC3845 + MOSFET, in flyback topology, us=
ing
> a transformer which doesn't seem too easy to wind nor commercially availa=
ble..
>
> You have alot of experience with this kind of converters.. maybe You have
> something really good to advice.
>
> Thank You!
>
> Kind regards,
> Mario
>
>  =20

LT1073 + external 900V MOSFET. You may need to have two LT1073. The 1st=20
to make a 12V supply so the 2nd one has 12V gate drive.
I just designed one for 450V 20W.  You need one inductor for each=20
converter. some diodes and FET

This may help in coil selection.
http://www.ladyada.net/library/diyboostcalc.html

The LT1073 is self contained needing  no external  switch, just coil, =20
capacitor, diodes etc for Boost , buck, sepic, Cuk etc. as  the =20
internal switching transistor has uncommitted collector & emitter. It=20
works directly to 45V from as low as 1V.

It's  only 19KHz,  but  this  means  if you boost the 8V to 16V with a=20
suitable inductor and internal switch, this powers 2nd LT1073. The gate=20
capacitance of cheap high power 950V mosfets is 1000pF to 2200pF, so=20
19kHz better than 40, 100, 500KHz etc. The 2nd LT1073 switch collector=20
goes to your +16V boost rail and its emitter drives the Big MosFet with=20
a 1K to 2K   resistor to  ground to  ensure  turn off  quick enough. =20
The  2nd coil  is fed from the 8V to 15V original supply to drain of=20
FET. Diodes and capacitors etc as per normal.

The 19kHz also means a suitable 400V to 600V rated Fast rectifier will=20
work. You won't easily get rectifiers for 500KHz and 350V!
The UF 120 or URM120 is OK for 90V & 19kHz (tested and recommended by=20
Linear Technology)
I have some BYV27-600 which seem OK for 19KHz switcher (up to 600V & 2A,=20
40ns reverse, 250ns forward, ). 10% of 19kHz is 5uS? 1% of 19kHz is 500ns



Note there is fixed 5V and 12V versions of LT1073, you want the=20
uncommited version with 2 resistors to set voltage and one to set=20
current limit. The resistor from FB pin to HT 350V line must be 500V=20
rated. Or 2 x 2M2 250V rated parts in series.

--=20
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