RE: OLTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dan Kline by way of Terry Fritz <twftesla-at-qwest-dot-net>" <daniel_kline-at-med.unc.edu>

Hi Terry,
I forwarded the OLTC info to an engineer-friend of mine who talked about
building this sort of thing a lot over the last few years. He had a comment
about the design and asked me to forward his email
on to the list.

Dan Kline

-----

Hey,

I looked at the OLTC. Really cool idea! If he gets things working, He should
be able to deliver about 1300V to the primary coil! I spotted something
important that Terry appears to have overlooked, however. He should place a
FAST recovery diode (1kV) reversed biased from the 150mH coil to the minus
return of the DC input (Cathode would go to the 150mH coil side opposite the
47uF cap and the anode goes to the minus DC return).

WHY? Let's say that current to the 150mH coil is switched on and continues
to flow until the 47uF cap is fully charged to the DC input level
(240V*1.41*2=677V). The coil current will be max at this point. You can
still leave the switch on and the 47uF cap will continue to charge to a
voltage greater than the DC input because the coil field is collapsing and
will source extra voltage to charge the 47uF cap. This will continue until
the coil current is zero, corresponding to the max voltage on the 47uF cap
(theoretically 2*Vin or 1350V) This is resonant rise. You can open the
switch at this point and take the 1350V on 47uF, or you can leave the switch
closed and the 47uF cap will ring with the 150mH coil until eventually the
47uF cap reaches the level of the DC input (677V). With the current monitor
Terry has integrated into his design, my guess is that he wants the 1350V
level on the 47uF cap. This requires that he open the charging switch at
precisely the moment when the 150mH coil current is zero. In the real world
he can get close, but there will likely still be a little bit of current
flowing when he opens the switch to take the 1350V charge on the 47uF cap.
HERE IS WHY YOU NEED THE FAST DIODE: When the switch is opened while current
is flowing in the 150mH coil, the collapsing field will generated as much
voltage as it can to try to continue the current flow. This will
over-voltage the IGBT switch and cause its destruction! PREVENT IT> Add the
FAST recovery diode (like an International Rectifier brand; irf-dot-com) as
previously described to provide a current path for the 150mH coil when the
switch is opened. I have learned this the hard way in high power inductive
switchers.

It's late and I'm harping. I just don't want him to get discouraged when his
IGBT explodes. He has a very good idea and it is not too complex to build. I
hope he gets 4 foot white hot bolts!

Perhaps you could forward my comments to the list?

-Brian