Self-oscillating SMPS with saturable drive transformer

[N-Channel]

QEX is published by the American Radio Relay League (ARRL), along with QST. The article is at thr ARRL's website (www.arrl.org). I have the article in PDF format. Send me your e0mail and I will send it to you.

[richwalters]

Originally posted by Dem
Rich:
Billing for poor cos factor ... just nightmare... Did You buy PFC power supply Your PC?
Could You decribe more detailed about "squared" B-H ferrites in this application? I used simple ferrites (like 3C85, 3F3) for drive transformer without problem, I paid on this only by not very optimal design of output transformer (lower than need B - less than 0.15T) to be sure that drive tr-r will saturated before even during asymmetric start-up.

Hi there.....my pc runs off a few VA mains (don't bother)....my tube power amp is more greedy....250VA min quies (8x KT88's (pout=2x250W) but used pfc with ZVT as my mains fluctation is so wild.....For real simplicity at the time of design I had thought of PFC flyback for min 200 ACV operation but from previous experience with another commercial design, interference problems mainfested. Perhaps I should revisit.

A few years back Allied signal did some exotic amorphous core materials (metglas)....extremely low loss and mighty square...were perfect tempter for smps designs....The bog standard ferrites as you mentioned are more suitable...

Somehow I was thinking of the Royer design.....that was short circuit proof......however the smps mode isn't so forgiving. It's turning the page back to Baker clamps and when trannies were in smps before mos took over the show......

richj

[Kenshin]

I'm also planning to do a SMPS of 50~100W power level use some scheme like this / electronic ballast / halogen lamp electronic transformer. I'm keeping wondering if somebody has done it before. In 1990s, many people modify electronic ballast for SMPS use, but seldom any now. Also, commercial SMPS seldom use these "electronic ballast" schemes.

Some of these schemes can ZVS, and some are said to have a efficiency up to 95%.

The circuit I planning is more like the electronic ballast circuit -- positive feedback is taken from a current transformer in main circuit. The satuation magnetics device will be taken from a electronic ballast . (I have collected many dead CFL lamps.) And current limiting will be done by a inductor series with the transformer.

Electronic ballast circuit has series resonance, electronic transformer not. Whether to have a (diode clamped) series resonance or not, is undecided.

[Dem]

Kenshin:
50~100W is not a problem for such SMPS, but typical electronic ballast / halogen electronic transformers use pure current-feedback with proportional-drive, and it has 2 disadvantages:
1. It stops to work at no-load or very light load;
2. It continues to work under shorted load.

It doesn't matter when load is more or less stable (electronic ballast / halogen ) but for audio (It's DIYaudio forum, isn't it? ) 50~100W I suggest You voltage-feedback, up to 200W it should be OK to my opinion.

[Eva]

Dem :

Proportional drive requires a proportional negative base current of about Ic/2 for optimum turn-off. This is achieved by inserting some components between the drive transformer and the base pin, and as a side effect, the volts*second product seen by the pulse transformer is no longer constant but proportional to Ic

[Kenshin]

Originally posted by Dem
Kenshin:
50~100W is not a problem for such SMPS, but typical electronic ballast / halogen electronic transformers use pure current-feedback with proportional-drive, and it has 2 disadvantages:
1. It stops to work at no-load or very light load;
2. It continues to work under shorted load.

It doesn't matter when load is more or less stable (electronic ballast / halogen ) but for audio (It's DIYaudio forum, isn't it? ) 50~100W I suggest You voltage-feedback, up to 200W it should be OK to my opinion.

Can it keep running if the transformer's primary turns have a relatively low inductance?

[Dem]

Kenshin: Can it keep running if the transformer's primary turns have a relatively low inductance?

I don't have deep knowlege or experience with royer/Jensen oscillators , that's why I started this thread - to look for people who knows more...
But I know that the problem of current-feedback driving self-oscillator is when magnetizing current is greater then load-reflected current, in other words - at light load.
If You reduce the primary inductance, You increase magnetizing current, so as I understand it, it will even worse. In addition, lower primary inductance increases max flux density towards saturation point, but You want be sure that driver tr-r will saturated before the output tr-r, so I always kept high inductance and low B at primary.
But for Your power level, voltage FB is good enough to my opinion.

[Kenshin]

Since voltage feedback is good, I want to build one, just as yours.

What's the actual size/turns/parameters of the transformers? And how did you debugged the circuit ?

Another problem: how to simulate the satuable magnetics behavior?

[Dem]

Originally posted by Kenshin
Since voltage feedback is good, I want to build one, just as yours...
Another problem: how to simulate the satuable magnetics behavior?

I am working now on 200W prototype, I'll update You about the progress. My main goal is to make something as simple as possible, without hard to find parts, I'm attempting to use parts only from old PC power supplies, dimmers, halogen tr-rs etc.

About simulation - I do it with a piece of paper and a pencil , sometimes Excell for recursive calculations. I don't use p-spice, because I'm sure that simple simulation can be done manually with the same accuracy... I built a small jig to measure saturation point of drive tr-rs.
I am building this not only for myself, so I'm thinking about building and testing methods, I'll update You too...

[Kenshin]

Originally posted by Dem
About simulation - I do it with a piece of paper and a pencil , sometimes Excell for recursive calculations.

The main parameters could be calculated on a piece of paper.
But how to get the switching waveform and calculate the switching loss?

I have done some experiment on a 220V AC electronic ballast and got more than 400V peak output using 30V DC input. It's bigger than my former calculation. Maybe because the inductance of driving pulse transformer falls gradually during the satuation, so the effective satuation current is much bigger.

Apart from the difficulty of control, it's great! Low voltage debugging, very smooth switching waveform, and almost no heat from the transistor. (Though the inductor and capacitors are heated hard by the large resonance current.)

Maybe switching waveform measurements using low voltage supply and transformer output voltage / waveform measurements can replace waveform measurements powered directly by mains voltage. The latter one require either a mains isolation transformer or a isolation probe.

After get control of the resonance amplitude (with 30V DC to 220V AC input), I will connect a isolation transformer at the output and use it as a PSU.