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Flyback smps problems (Now forward converter)

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uzzors2k

Sat Feb 09 2008, 06:35PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I like to make projects completely self-sustained, so to wrap up my mini class E coil I need a 80W 40V supply. Since I've made a 15W 12V offline flyback supply before without problems I though I could do it again.

Circuit:

1202581509 95 FT0 Uc3842 Flyback Converter

80W is a fair amount of power so I decided to wind my own core, and after going through some datasheets I designed the transformer. I used 88 primary turns, 7 aux turns and 7 + 4 secondary turns. The core cross-sectional area is 0.7cm2, air gap about 2mm. Now for the problem: the IRFP450 dissipates power with no load! With the transformer from an ATX supply the IRFP450 doesn't heat even with a load. With my homemade transformer the smps is running max duty with no load! I tried increasing the air gap which helped a little. It seems to me that it can't couple enough energy from primary to secondary, what should I do?

Inductances:
Total Secondary inductance = 24ÂµH
Primary Inductance = 1.4 mH
Leakage Inductance with secondary and aux shorted = 50ÂµH
Leakage Inductance with primary and secondary shorted = 0.5ÂµH

These numbers don't deviate much from the commercial ATX flyback transformer I have, so I'm stumped.

Dr. Dark Current

Sat Feb 09 2008, 07:34PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Maybe you have the wrong material for the core? I have cores from some SMPS's that will take a lot of flux easily but others of similar size draw significant no-load currents.

uzzors2k

Sat Feb 09 2008, 08:16PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

You might be right. I have tried winding a new transformer with the same results, but both cores were previously used in inductors. Wouldn't inductor cores be ideal though? One was an EI and the other two Es if it matters, both with an air gap.

Marko

Sat Feb 09 2008, 08:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi uzzors,

80W is a fair amount of power so I decided to wind my own core, and after going through some datasheets I designed the transformer. I used 88 primary turns, 7 aux turns and 7 + 4 secondary turns. The core cross-sectional area is 0.7cm2, air gap about 2mm. Now for the problem: the IRFP450 dissipates power with no load! With the transformer from an ATX supply the IRFP450 doesn't heat even with a load. With my homemade transformer the smps is running max duty with no load! I tried increasing the air gap which helped a little. It seems to me that it can't couple enough energy from primary to secondary, what should I do?

2mm airgap looks huge from first look, 'd reduce it to 0.2..0.3mm or so and slowly increase if needed; I'd maybe also try some kind of snubber on the mosfet.
Is the transformer you are talking about from a 80W flyback ATX supply?

I can't help you much more, maybe if you give some waveforms of mosfet voltages and currents?

I think all I can say now, is, that I don't believe anymore that these flyback converters are of too much use for amateur projects. I once ran into similar situation as you, by using recycled transformer from a flyback converter mosfet was just grossly heating and I couldn't figure out what was wrong.

For your hobby you will build few SMPS's, not a series of 10 000 of them, and I really don't think you care so much if your design saves you $2 or so.

To keep OT short, if I needed really small (few W) cheap and robust, I'd use a BJT halfbridge with saturable CT (ala ''electronic transformer'') and a linear regulator on the output. To power some electronics in a SSTC you don't need anything more!
The circuit could actually work for few hundred W if you don't need regulated output!

If I wanted more, I'd just take a step to SG3525 controlled halfbridge. Very robust and forgiving, lots of power, feedback isn't anymore life-supporting, but actually, forward converter has pretty good load regulation without feedback at all!

And if done right it probably wouldn't take much more board space than the UC3842 SMPS!

Cheers,

Marko

Dr. Dark Current

Sat Feb 09 2008, 09:41PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You can actually make a half bridge by using a very few parts: 2x MOSFET and 1x half-bridge drive chip (plus filter caps, PSU etc.). This is how would I do it as the drive chips are easily obtainable, relatively cheap and you can build really simple and powerful PSUs.

GeordieBoy

Sat Feb 09 2008, 09:41PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

80W is a lot of power for a flyback power supply. It is no surprise that the transformer designed for a lower power level is unsuitable for this high power. The transformer design is the most critical part of a flyback converter as it determines the peak component stresses. In summary you want to go for a design where the current is more heavily continuous (less discontinuous) to get decent efficiency at this power level. At a guess i'd say the primary inductance was about 3 times too much. Flyback transformer design is a complex iterative procedure and you might need to go many times round the loop.

EDIT: http://www.edn.com/archives/1995/041395/08df3.htm

80W is getting into the region where a forward converter would make a better choice, because losses on the primary side clamp are quite high even for a well manufactured flyback transformer. You can still use the same controller IC, but you need to add an output filter choke. The design optimisations are actually easier for the forward converter than the flyback, and this is definitely the route I would take.

Sulaiman

Sun Feb 10 2008, 06:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A few quick calculations reveal your problem(s);

Since I = N.B.Ae/L , at 100 kHz 0.1T is about maximum so Ipk = 0.44 Amp.
Using E = 0.5 x L x I^2 , Epk = 136 uJ ,
so Pmax = 100E3 x 136E-6 = 13.55 Watts maximum for this core/winding combination at 100kHz.

Marko

Mon Feb 11 2008, 11:01PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Done anything on this, Uzzors?

Building 80W flyback supply isn't really impossible (I had an 80W PC power supply around here, used UC3842 a single IRFP450 too), but as already said, the small cost save is something really meaningless to you.

One thing that interests me, do you want to regulate the input voltage to the coil? In that case you won't be able to provide supply voltage for your logic from the same supply.

(then you need a small transformer or auxiliary SMPS, that's how it needs to be done)

If you don't need regulation, for this power level you may try to use a ''electronic transformer'' with choke, rectifier and filter on output, which is simplest you can get and still pretty bulletproof, especially if you add overcurrent protection with small SCR. It is a good use for bipolar transistors you may have laying around...

Just want to see what have you come up to...

Marko

GeordieBoy

Tue Feb 12 2008, 09:56AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Building 80W flyback supply isn't really impossible (I had an 80W PC power supply around here, used UC3842 a single IRFP450 too),

It isn't impossible, it's just a tricky design excercise to make it efficient, and the forward converter design is easier. Are you sure that it was an 80W *flyback* power supply Marko? If it had a big toroidal output choke on the LV side with multiple windings on it, then it was a forward converter. Flyback converters don't need output chokes, and at most only employ stick inductors for post filtering.

Marko

Tue Feb 12 2008, 12:12PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Are you sure that it was an 80W *flyback* power supply Marko? If it had a big toroidal output choke on the LV side with multiple windings on it, then it was a forward converter. Flyback converters don't need output chokes, and at most only employ stick inductors for post filtering.

Yes, my words are usually faster than my thinking, I remembered that something wasn't alright, I checked and it definitely *has* the buck choke on it!

I also always thought that cross regulation of flyback converter can't be good enough for PC power supply, but this was while ago and at that time I thought that choke is just a ripple filter or something.

But what I still find very odd is that it has *no* freewheeling winding and diode on it! Primary side is just like a flyback converter!

I would expect the snubber network and the mosfet to get blown up by magnetizing energy dumped into it with each cycle, how is it recycled, I really don't see any means?

I assume transformer is not gapped if it's forward converter?

]fonte_atx.pdf[/file]

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