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Re : DC power supply again


From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Sunday, September 07, 1997 3:50 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re : DC power supply again

Hi Harri,

> Better core utilization in other topologies helps a lot. You can push
> something like 1kW through a supprisingly small core in bridge topologies.

Agreed. The major difference in running a core up to Bpk with bridge 
vs flyback is that power is being delivered 100% of the time vs < 50% 

> >      Any choke _must_ be in the secondary side or you throw a huge 
> > voltage burden on the secondary windings.
> 
> Not nessessarily. Resonant topologies often have some LC-circuit in
> the primary side. Sometimes it is a series LC network, sometimes
> a series L with C in parallel with the transformer. Those do not
> put huge voltage stress like a normal bridge with just a choke in
> series with the transformer would.

Don't disagree. However, the primary choke in a resonant topology is 
not there to boost output voltage.

> I would be very tempted to try some multiplier in the secundary side.
> Perhaps some nise doubler to keep transformer smaller. I've began to
> put a lot of thought on that kind of device. It has surely lots of
> disadvantages as well but it is always something that can be given
> a thought.
> 
> > The transformer design I've come up with is about 6" diameter by 
> > around 3 - 4" high. A lot of core gapping is needed. I am going to 
> 
> Wow! That's quite a huge transformer for 700W level! Voltage level
> does have the nasty effect of making things big.. :(

As I said, it designed for 2.3kW throughput in flyback mode. It would 
doubtless be capable of a lot more in an H-bridge.

> > use IRF840's. That allows a lot of lattitude for changing to an H-
> > bridge if I wish because I chose the flyback voltage = rectified peak 
> > mains (about 330V). The compromise in choosing that value is that the 
> > output rectifiers have to hold off around 40kV (I'm building a 50kV 
> > stack - or was going to until the wire problem appeared). This design
> > is fine for about 2.3kW including losses.
> 
> 50kV diode stack ain't too fun to have around.. That's one reason I'd
> go for bridge design with eg. a half wave doubler.. Diodes would have
> to take only the full output voltage (plus some marigin).
> 
> BTW, what's the diode stack about? The simples and most reliable thing
> for fast hv diode stack could well be a lot of avalance diodes (like
> BYV96E) in series. However, that is certainly not the cheapest one.
> Designing a fast stack is not fun at all with normal diodes!

_No way_ can you use ordinary diodes in this application, no way! The 
BYV96E is also a bit of a slug (250nS rev recovery). I have chose
UF5408 - 3A I(fwd), 50nS rev recovery time, 1kV holdoff. I was 
careful to choose not only appropriate rectifiers for the original 
application but some that I could use in a forward converter as well.

    NB- you _must_ also use diodes with this kind of speed to rectify 
the mains as well if you use the PFC configuration.

 > > I'll post. I am going to mount the core halves on paxolin this 
> > weekend. If I choose to go to H-bridge, I simply remove the core 
> > gapping and operate at a frequency that gives B(pk) of 250mT. I am 
> 
> That's quite a high B. I'd assume core losses at B peak-to-peak of
> 500mT would he huge even at a very modest frequency! What's the
> frequency range, 20-30kHz perhaps? That would certainly explain the
> need for a huge core as well..

I carefully checked the Hanna curves and bulk loss curves before 
deciding on that figure, then built and ran a smaller one at the same 
level. There is not a huge amount of difference between 200 and 250mT 
but I wouldn't go beyond that. This seems fine for the core material I 
am using. The core barely gets warm after running for several minutes.
This is for _the particular material_ I am using. One could easily be 
deceived by this because most people typically run saturating core 
inverters and then wonder why the core gets hot (we are talking 
typically 400 - 450mT pk in this app for the N27 type material).

Malcolm