RE: OLTC

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Dan,

You may want to pass this on to your friend.  He is pretty sharp :-))

>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 "think" the theoretical limit is 677V.

240V*1.41*2=677V

>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.

Voltage spikes like this are generated across the inductor when the IGBTs fire.

http://hot-streamer-dot-com/temp/OLTC08-16-01.gif

I don't think a diode will work since the voltage spike is in the 'same
direction' as the normal voltage on the inductor in this case.  The IGBTs
are protected by the 47uF cap so it never gets to them, the danger is to
the other side of the inductor.  The voltage spikes are not too terrible
considering the bridge rectifier is 1000PIV.  there are MOVs there at 700V
but best not to depend on them for other than 'emergency' duty.  But the
solution should be simple and it is on the detailed diagram:

http://hot-streamer-dot-com/temp/OLTC08-16-03.gif

Just two little caps keep a current path for the inductors and reduce and
control the high voltage spikes:

http://hot-streamer-dot-com/temp/OLTC08-16-02.gif

http://hot-streamer-dot-com/temp/OLTC08-16-04.gif

There are still big spikes, but they are well within the system's ability
to handle.  It is hard to get rid of them without making something hot.

So I 'think' I have it figured out ;-)  If the primary voltage really
should be 1350V, I would like to know about it :-))

Thanks for the insight ;-))

Cheers,

	Terry


>
>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
>