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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
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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!
You mean the "reset winding" of the forward converter? Although textbooks usually show a reset winding, commercial designs often use an RCD circuit to clamp the reset voltage as this results in a cheaper transformer build. (There are 3 basic ways of achieving core reset in forward converters: 1. Reset winding+diode, 2. RCD clamp, 3. Assymmetric half-bridge.)
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?
Unlike a flyback transformer the forward converter transformer doesn't usually have an air gap. This means magnetising inductance is high, and stored energy in the transformer is low. (The complete opposite of a flyback transformer) so dissipation in the RCD clamp circuit is comparatively low.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
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Marko wrote ...
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.
Since the TL431 can't withstand more than 36V I decided to regulate the 12V line, while leaving the 45V somewhat unregulated. A load on the 45V line should weigh down the 12V line, so I think the regulation of the 45V line would work ok.
I made an attempt at a forward converter but it tripped the breaker as soon as I turned it on, so something was terribly wrong with it. I lost my only 600V mosfet and a good bridge rectifier in the process. I've given this some thought and decided to leave the forward converter for another project. I'll build a buck converter instead to drop the 45V down to regulated 12V for the logic supply, and just run the coil from my bench supply when needed. The time I've put into the power supply is already more than I spent on the coil!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
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Unlike a flyback transformer the forward converter transformer doesn't usually have an air gap. This means magnetising inductance is high, and stored energy in the transformer is low. (The complete opposite of a flyback transformer) so dissipation in the RCD clamp circuit is comparatively low.
Hmm.. so, not to make the core 'walk' into saturation in any case I need <50% duty cycle, right? I was probably overestimating the energy stored in the transformer, but it might still be god idea to re-use it.
UC3842/43 go to 100% if I don't limit them by adjusting the timing network, so could this have been your problem Uzzors?
From what it appears now you could indeed make a forward converter with actually changing very little in the circuit, one additional diode and buck inductor.. IRFP450 should work well for switch, I hope you still have lots of them around.
Big transformer from PC power supply will probably work well as it is, for 2x voltage on the primary you should probably double the frequency though... your max output voltage is going to be some 45..50 volts with fullwave rectifier.
I guess you know you should wind the buck inductor with turns ratio = voltages ratio, and etc.
If this fails, Uzzors, have you considered ''electronic transformer'' power supply, since you don't really need any specially stable voltage regulation for a SSTC?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
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jmartis wrote ...
Marko, what do you mean by "electronic transformer"?
It is a kind of unregulated SMPS commonly used to drive 12V halogen lamps. As lamp driver it has no rectifiers or filter but it doesn't mind you adding one, and increasing the input filter caps to 100-200uF to stop the diac circuit from retriggering.
I tried it up to some 100 watts and works very well, transistors hardly needed heatsinking especially after I mastered the CT design.
Only drawback is that it is easily killed by an output short, so it is good idea to have some sort of overcurrent protection, preferably and SCR which would lock the circuit until power is cycled.
I had some threads on it around the forum, not sure if I'm hijacking this one too much -
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
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Marko wrote ...
UC3842/43 go to 100% if I don't limit them by adjusting the timing network, so could this have been your problem Uzzors?
I think the current sense limited the duty cycle to 40-50%. When viewing the gate waveform it would limit itself even when the transformer was struggling to charge the filter cap. The problem I was getting was a lack of overall power. The smps was running full bore with no load, which naturally heats the drive mosfet. It was probably avalanching a large spike too since that energy had to go somewhere, and it wasn't going into the load! Using one of those auxiliary standby transformers from an ATX supply with the exact same circuit corrected the problem. Increasing the air gap to an obscene size fixed the problem slightly, and therefor I think the flyback transformer I made wasn't able to store/couple energy correctly. I would just use the flyback transformer from a monitor if size wasn't an issue. This power supply was meant for my small class E coil, and the way things were headed it was going to be larger, more expensive, and more complicated than the coil itself.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
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I was talking about forward, not flyback converter about the duty cycle thing.
Flyback converter should turn off/go into hiccup mode once output cap is charged adn there is no load, something is surely very wrong if it runs all the time!
Not to repeat all the story, forward is the way to go, not flyback.
This power supply was meant for my small class E coil, and the way things were headed it was going to be larger, more expensive, and more complicated than the coil itself.
And you ignored the electronic transformer idea? :p
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
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The time I've put into the power supply is already more than I spent on the coil!... This power supply was meant for my small class E coil, and the way things were headed it was going to be larger, more expensive, and more complicated than the coil itself.
Nobody ever claimed switched-mode power supply design was easy! It is one of the most demanding areas of electronics because it requires knowledge of magnetics design, power electronics, thermal management, control theory and good layout practices. (What you have done is actually quite good, and you probably learned loads along the way.)
If in doubt build a linear supply - It's much easier and there are about a hundred less ways for a linear supply to blow up or damage whatever it is driving if you get something wrong! On the other hand if the PSU really does need to be lightweight and high performance and dirt cheap in volume, then it's worth taking 4 months to develop a reliable SMPSU.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
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I had stripping down my breadboard and was ready to start on the buck converter when it struck me. I had used a 200V diode on the reset winding! Unable to let it go I rebuilt the circuit, harvested a high voltage mosfet and bridge and tested it. Woe and behold it worked! At least with a flyback transformer modified for forward mode. I'll rewind it and see how it works with the correct voltages + aux winding.
One question first: If I keep the primary current/ duty cycle below a certain threshold can the reset winding be removed? If so, I assume I'll need to ensure the primary current never goes so high as to not return to zero again. This can be solved with the current/inductance formula but which inductance will I need, the open circuit primary inductance?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
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Yay uzzors, glad to see you getting it to work! (you might want to change thread title after it).
One question first: If I keep the primary current/ duty cycle below a certain threshold can the reset winding be removed? If so, I assume I'll need to ensure the primary current never goes so high as to not return to zero again. This can be solved with the current/inductance formula but which inductance will I need, the open circuit primary inductance?
well, if you just pinged the transformer without means of resetting the core I think you would get in trouble, as mosfet would start avalanching, this alone might demagnetize the core but I don't think that's a good idea to do.
RCD clamp just moves this power away from the mosfet, turning it into heat. I'm not sure how to properly design a clamp but I think the obvious objective you need to do is to have no current flowing in the transformer before mosfet turns on again, or it will walk into saturation over a number of cycles.
I think your maximum duty cycle in any case is 50% and you should limit it there.
Reset winding actually recycles this power (which I don't know how relevant it really is, a fraction of watt at your power level?), and if you are winding your own transformer I don't think it's too hard to implement.
Good thing about RCD clamp is that it would allow you to use existing transformers without reset winding like PC PS transformers.
What are you using for your output inductor? If you want multiple output voltages make sure to wind it on a single powdered iron core, with winding ratios equal to your output voltage ratios.
One thing I'm wondering about is:
Ideal buck converter has it's output voltage equal to duty cycle*Uin, but it is obvious in practice, that if I just leave the forward converter running without feedback, the output caps will charge to peak transformer secondary voltage regardless of anything;
I noticed though that if I add some minimum load to output the voltage will drop and start following the duty cycle rule pretty well.
What is that determines this 'minimum load' in practice? I think that's especially important in multi-output converters where feedback is taken from just one output.
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Flyback smps problems (Now forward converter)
Unable to let it go I rebuilt the circuit, harvested a high voltage mosfet and bridge and tested it. Woe and behold it worked! At least with a flyback transformer modified for forward mode. I'll rewind it and see how it works with the correct voltages + aux winding.
(you might want to change thread title after it).