[Prev][Next][Index][Thread]

Re: DC power supply again


From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Thursday, August 28, 1997 2:45 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: DC power supply again

Gudday Harri,
              Nice to hear from you again.....
 
> From:   Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
> Sent:   Thursday, August 28, 1997 6:31 AM
> To:     Tesla List
> Subject:    Re: DC power supply again
> 
> On Sun, 24 Aug 1997, Tesla List wrote:
> > > > loser in a number of systems, I have decided on a completely 
> > > > different approach. I am currently designing an off-line flyback 
> > > > switcher to run at the 2kW+ level. What I am looking for:
> > >  
> > > > - efficiency in the supply of around 80 - 85%
> > > > - being able to use any size cap without having to deal with primary 
> > > >   charging resonances
> > > > - good gap quenching. I am looking at using optical feedback from the 
> > > >   gap to shut the supply down if necessary although that is 
> > > >   peripheral to getting the supply up and running. Experiments 
> > > >   suggest that with this type of supply, quenching is solely dependent
> > > >   on output discharges anyway.
> > > > - continuously variable power setting. Break rate is obviously 
> > > >   dependent on gap and power setting.
> > > > - Max output of 20kVDC
> > > > - No smoothing cap required. The supply charges Cp raw.
> > > > - Incorporation of power factor correction by using the input 
> > > >   haversine to modulate the internal reference (not yet tested).
> 
> Humm.. I'll be very interested to hear how practical prototype will
> perform out. I've hade the same idea for quite a long time ago already.
> However, I've thought there may be some serious problems with flyback
> topology.
> 
> Assume the gap fires, primary resonates and transfers only partially the
> energy during 1st half cycle to the secundary. Then, the primary current
> after resonance to 0 volts will start going partially to the cap (charging
> it negative!) and partially through the flyback secundary and diode.
> All the energy left in the cap (now negative) will have to be dissipated
> somewhere. Possibility of distruction? Have you thought about this?

Indeed yes. I have already successfully run coils from such a supply. 
The supply behaves like a current source of sorts and the gap shunts
the output while conducting so the rectifiers need not hold off more
than that required by the flyback design alone.

> Therefore I had the conclusion that the topology must be something else
> to be safe. Almost every topology will still be in trouble when the
> partially charged cap is reverse biasing the system. I can think of no
> other solution but preventing that like with a dc cap and large inductor
> after that. Unfortunately that means a large, expencive, bulky inductor.
> There are some other rather exotic solutions but I'm not for them either.

In fact using inductive storage is _exactly_ the right topology for 
the job. You will instantly blow a forward or transformer type 
converter if it has no current limiting and if it does, you are 
likely to suffer serious gap quench problems. I have total confidence 
in my choice based on experiments I have already conducted.

> I'd still love the SMPS approach becouse of the same benifits you
> described. Variable power & voltage power source would be quite usefull
> for other purposes as well.

I am going to wind a second transformer for using the supply for an 
arc welder.
 
> For optical feedback I'd also suggest using that to determine need of
> eg. large magnetic fields in the gap to quench it. Rather large magnetic
> fields are quite quickly generated with suitable coils driven by say a
> FET from a 400V source.. I bet that kind of device would be actually
> rather simple to build and it would allow tailoring quenching times just
> what you like. I guess that could also be used for preventing gap firing
> before the suitable moment.
> 
> > > >and hopefully switch losses of the same order. Please don't bombard 
> > > > me with "how's it going". I am currently suffering financially and
> > > > time to completion is indeterminate at this stage. I see this as a 
> 
> The same old problem. I usually get the same problem as well. :(

My timetable has accelerated now I have a job. It is crucial I get 
this thing going now. I will be winding the primaries today. 

> > > > mature this technology and have tested a number of designs in order 
> > > > to arrive at a well behaved one that is scalable to the kW range.
> 
> Those kind of devices for kW use are mentions even in literature. Usually
> they are nowadays resonant mode devices directly charging a cap. Those
> tend to be rather cheap.
> 
> The best transformer solution may be something like primarys in parallel,
> secundarys in series (for non flyback). Usually many small separate
> transformers is better than a huge one made from E-cores. Large ones
> do not dissipate heas as well. If it is made of small units anyway, I'd
> certainly go for multiple transmormer system. It can be prototyped with
> low output voltage and expanded later as needed.

I see simplicity as the best approach. I looked at multiple windings, 
multiple transformers and it didn't look good. With good design, 
switchers can be made pretty efficient with regard to heat 
generation.

Malcolm