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Re: hvguy-dot-com new flyback driver

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

> Hi!

Hey Jan ; )

> you can use a single CD4049 (hex inverter, costs maybe 10 cents??) 
> to replace the four transistors in the gate drive.
> http://student.fortlewis.edu/~gwdurland/4049.html

Sweet. I have seen that before in class-D audio amplifier designs for
car audio, but have never thought about using it here. Maybe I'll
build it tonight. Actually, I know I'll build it tonight.

Yah, I know the Z20's and 9540's are overkill --- I knew they worked
though, so they went on the schematic. SO LISTEN TO JAN and save some

> One question about the protection diodes around the mosfet: what 
> are they for? I may be mistaken but I thought that the voltage 
> spike on the mosfet is always positive (ok I never measured...), so

> the mosfet intrinsic diode/bipolar transistor never conducts, thus,

> doesn't need to be disabled?  ;o) You could save some bucks by 
> leaving out the FR and schottky diodes? Or?

> reverse diode accross the flyback primary helps hugely against 
> voltage spikes, but, it will get a bit hot so a heat sink is 
> necessary. Some TO-220 packaged ultrafast diode would be best for
> this. It's called a "flyback diode" in this flyback transformer 
> setup.

Yep. The latter paragraph is true I'm sure.

By the way, I do have the part # for that diode now. I used an IXYS
30-06A in TO-247 package. You could probably get away with something
less expensive...maybe...I killed an NTE577 (70nS, 1kV, 5A) with it,
but I think this was due to overcurrent (HOT).

The series shottky (D1 - three 1N5822's in my schem) just helps to
make sure that the MOSFET diode is never forward biased. The fast
recovery diode (D2 - IXYS 30-06A) is the one that does the work. It
shorts the reverse current to ground, it should be beefy. Like maybe
faster than 100nS Trr (reverse recovery time), a good surge current
rating, and lots of voltage...say more than 600V. The series shottky
is probably overkill but I haven't tried the circuit without it.

The MOSFET body diode could probably handle the current, but wow does
this circuit work better with the external diodes. That's a major
change from the first driver I posted. Jan, if I'm not mistaken,
don't you have a schematic with these two diodes in it also? I want
to say it is hand-drawn.

> Umm yeah and a final note: arcs from flybacks with internal 
> rectifier + cap are by far the longest. 6-7cm. It's sure worth 
> trying out! :)
> cheers,
> - Jan

Oh yeah. I'm making a variable HV current supply for a laser with
it...I figured this one was worth it. 

Justin Hays
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com