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Flyback smps problems (Now forward converter)

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GeordieBoy

Tue Apr 29 2008, 12:56PM

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

say my on time is 1us, this would equate 300uVs, which would with 100V reset voltage need at least 3us off time, right?
This would equate max duty cycle of 25%.
This is what I meant, hope I'm not failing to see fallacies in there?

Yeah, looks right to me. The mosfet would see 300V plus 100V plus the leakage inductance spike if a seperate reset winding is used. As long as there is sufficient time allowed for core reset at all DC bus voltages and the MOSFET Vds(max) is never exceeded, all is well. The beauty of the RCD clamp is that the reset voltage can stay high even when the DC bus voltage is at a low, so the time required for core reset doesn't eat into the available duty ratio. This can be important at low line voltage where you need max duty ratio.

I wanted a small simple SMPS that would not use any specially critical components.

I'm not sure what you mean by specially critical components. The only part you will have to design and wind yourself will be the flyback or forward transformer as that is usually designed for the specific line and load specification.

I need no more than like 15W of power, and half bridge converter would be an overkill.

Discontinuous current-mode flyback converter is the topology of choice in this power range. If DC steady-state regulation and load transient response are not critical you can use primary side feedback with something called dominant pole compensation. The pole of the power circuit itself dictates the closed-loop crossover frequency and the control loop is unconditionally stable.

If DC and transient response are critical then buy a ready built unit by Lambda or Astec etc, and tweak the output voltage if necessary to meet your spec.

-Richie,

Marko

Tue Apr 29 2008, 03:06PM

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

Thanks guys..

Feedback really is the part that ruins all the fun in these things. :(

Steve Conner wrote ...

About disabling the error amplifier:

One technique I've seen is to tie the + and - inputs to some fixed voltages. So the - to ground and the + to Vref or whatever. If you get it the right way round, the error amp will actually turn itself off. They usually only have a single-ended output, so if it tries to pull its output all the way high, it ends up disabling itself. This is what people do to use the TL494 as a SSTC driver.

You can now feed the signal from your optocoupler into the compensation pin, since this is the error amp's output and also the input to the comparator.

I forgot that diodes which I thought can be used to disable the error amp are after, and not before the comp pin, so my first idea is not possible.

I measured the output of error amplifier and it definitely both sources and sinks significant current... only sink rating is given as absolute maximum in datasheet though.

Since there is a clamp zener on the output anyway it may be OK to force the output down, but I don't know.

IIRC, TL494 has 'or' circuit of diodes at error-amp outputs, which allow them to be 'disabled', and also a free + input which makes things pretty easy.

Steve Conner wrote ...

Another way to "disable" the error amp would be to use heavy negative feedback with a resistor from the compensation pin to the - input. In this case, you're just lowering its gain. As Richie mentioned, a little gain in the error amp can be helpful, in case the optoisolator isn't powerful enough to drive the compensation pin directly.

The Fairchild datasheet even shows how to do that, see Figure 1:

Well steve that is exactly what I'm trying to do now, but I can't stop output from oscillating.
I'm using a simple zener, resistor and optocoupler at output for feedback.

I tried 100ohm-100kiloohm range of err-amp negative feedback resistors, gain of the error amp just doesn't seem to be important much.

The output is oscillating at several Hz, just going on and off. I'm using external power supply in place of auxiliary winding supply to remove it as a variable.

I'm not sure how much you guys can help there.

I'm not sure what you mean by specially critical components. The only part you will have to design and wind yourself will be the flyback or forward transformer as that is usually designed for the specific line and load specification.

Discontinuous current-mode flyback converter is the topology of choice in this power range. If DC steady-state regulation and load transient response are not critical you can use primary side feedback with something called dominant pole compensation. The pole of the power circuit itself dictates the closed-loop crossover frequency and the control loop is unconditionally stable.

If DC and transient response are critical then buy a ready built unit by Lambda or Aztec etc, and tweak the output voltage if necessary to meet your spec.

-Richie,

Well, for a professional, yes, a series of 10 000 flyback converters is a cost save compared to forward at that power level, but for amateur purposes the tiny cost saving of flyback converter just doesn't mean anything.

I've actually built a small flyback converter with UC3842 some time ago, with a completely random transformer from junk - so it was a miracle it actually worked - I simply used primary, secondary and aux winding for what they were before, and actually made something to work.
It is an ugly and crammed up design though, which I'm not too proud off.

The primary side feedback you're talking about never worked with it.

It's not powerful enough though for what I need here I fear, and feedback sometimes seems to oscillate under too light load.

Getting specifically gapped transformer cores and bobbins is very hard for a Croatian teenager, and I just didn't want do depend on scrapped transformers while having lots of toroidal cores around.

Flyback converter design is very challenging even if I have right transformer, so I thought forward converter would be far easier.

All I wanted from this circuit:

electronics inside my SSTC grew to size and power level where I just couldn't easily fit an iron transformer of required size.
At that point I started thinking if it would be cool to have a small CPS to power everything, and learn something in process, it's just it.

Otherwise I'm probably just going to drag a power supply around with it. :P

I'll report back if I find anything on this feedback issue. thanks guys,

Marko

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