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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
Jan, did you use current feedback when running at 300W? I would expect heavy loading and especially short circuit of the output to result in dead switches or very poor efficiency in all cases without some means of current control and preventing saturation.
Yes I used current feedback, but it will work fine without it if you don't mindlessly short circuit the output for long time. I think in the case of short-circuit, most of the leftover energy in the LC is returned back to the supply rails through the MOSFET body diode.
I used two parallel transistors just because I had them and I liked that the little heatsink got just lightly warm. Of course it will work with one with sensibly sized heatsink.
One problem I ran into with current feedback is the feedback oscillating in the audio range. I have no fix for that and I tried various filters in the fb path.
To the "bare" 555 driver, you are right the transistor will get very hot with more load on the secondary. Because of this, it is good for powering lifters etc. which require high voltage at low current, but for arcing fun it is no good.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Dr. Kilovolt did you use primary current feedback, or feedback of the HV secondary current? If the latter, you could have the dreaded right-half-plane zero.
If you take your feedback from the switch current, it should be easier to avoid instability. (just use Antonio's circuit, or a UC384x controller chip since these have peak current control with a built-in sense amp)
If you were taking it from the switch current already, then maybe there's a mistake in the circuit, or maybe it's an interaction between the internal capacitance of a DC flyback and the negative resistance of the arc.
Or, just leave it the way it is... squealing is annoying, but blown MOSFETs are even more annoying.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Dr. Conner wrote ...
Dr. Kilovolt did you use primary current feedback, or feedback of the HV secondary current? If the latter, you could have the dreaded right-half-plane zero.
I got it from the secondary ground return pin, after all I wanted to build a constant current-limited HV PSU. What do you mean by "right-half-plane zero"?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
What do you mean by "right-half-plane zero"?
I think it's that bizarre and unexplained instability that happens in negative feedback control loops.
I was always troubled by it, and especially if using opto-couplers in feedback path.
As conner says, implementing current mode control would probably be just easier than messing with negative feedback.
And no IC's like UC3842's are needed (which is still considered 'exotic' by some people); CM control can be easily implemented by just few logic IC's rather easily, and actually getting us rid of all the unnecessary parts of that IC.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The long answer:
In a flyback converter, the current goes to the load during the periods when the switch is off. So the immediate effect of an increase in duty cycle (assuming continuous mode) is a *decrease* in average output current, until the increase in duty cycle has made the inductor current ramp up. This initial setting off in the wrong direction destabilizes the control loop. And, an unstable control loop has a zero in the right half of the s-plane when you model the closed-loop dynamics with Laplace transforms: hence the name. See e.g. and the articles that it references. Any unstable system has a RHP zero, but when we're talking about boost and flyback converters, we specifically mean the instability caused by the effect described above.
The short answer: Mix the switch current into your feedback loop. In order not to spoil the DC accuracy, put it through a high-pass filter so you use only the AC component.
Shorter answer: Slug hell out of it using an integrator with a long time constant, and it might sort of work.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
In a flyback converter, the current goes to the load during the periods when the switch is off. So the immediate effect of an increase in duty cycle (assuming continuous mode) is a *decrease* in average output current, until the increase in duty cycle has made the inductor current ramp up. This initial setting off in the wrong direction destabilizes the control loop.
OK, to get it straight, *why would secondary current ever drop with increase of primary current?* I guess that's the major basic thing I have been missing all the time.
In any case, is this instantaneous drop in current the source of the mysterious 180 degree phase shift, which leads to oscillation?
Does something similar happen with voltage feedback with forward converters?
The short answer: Mix the switch current into your feedback loop. In order not to spoil the DC accuracy, put it through a high-pass filter so you use only the AC component.
How exactly is this 'mixing' done? , you mean current mode control (in which case he doesn't need output current feedback at all), or something else?
Shorter answer: Slug hell out of it using an integrator with a long time constant, and it might sort of work.
I think we would all greatly appreciate if you could explain us how to implement an integrator in the feedback loop, and what does it do.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Detailed answers to all of these questions would be outside the scope of this thread. They're meant as brain teasers for Jan or anyone else wanting to experiment with negative feedback in their HV projects. (Yes, even Google can't replace the need for thinking.)
I'll just say that when a flyback converter operates in continuous mode (and I'd be very surprised if these high-powered drivers didn't) there is a steady "flow" of DC amp-turns with a small AC ripple, just the same as in the output filter choke of a buck converter.
I say amp-turns, not current, because we all know that in the flyback circuit the current in any given winding is discontinuous. But overall, though the current commutates between the primary and secondary windings in proportion to their turns ratio, it is a constant current and the magnetizing inductance of the transformer smooths it and resists changes in it.
So, when the duty cycle is increased, this current takes a while to catch up, and until it does, the mean output current decreases, and a control loop sensing average output current would see a destabilizing error signal of the wrong polarity.
That's all I'm prepared to say, at the risk of dragging us off-topic. As for the integrator thing: The compensation capacitor on a TL494, SG3525, UC384x, whatever, makes the on-chip error amp function as an integrator in the feedback loop. You often see a resistor in series with the capacitor, this is because the power supply designer decided that a plain integrator wasn't good enough.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Marko: Current flows on the output side of a flyback converter when the primary side switch is OFF. Therefore increasing the duty ratio of the primary side switch initially leaves LESS time remaining for current to flow on the secondary side. Hence the AVERAGE secondary side current falls initially when the duty ratio is increased. As Mr Conner said, it starts out going in the wrong direction for some time before it turns round and starts increasing instead.
When negative feedback is used the "starting out in the wrong direction" effectively turns negative feedback around the control loop into positive feedback around the control loop and the whole system oscillates.
Some advice: Most flyback supplies will transition into the continuous mode with output s/c because there is no output voltage being developed across the sec winding other than the diode drop to remove energy from the transformer. Cycle-skipping is the way to combat this, and is the way that most flyback converters stop their output current integrating up out of control.
EDIT: In general up to about 40 watts is reasonable for discontinuous current mode flyback, then you want to be operating in continuous mode above that power. At about 80 watts you would want to be into heavy continuous mode and above that power level the flyback is no longer the topology of choice.
Re Instability of Forward derived converters with feedback: Forward converters like buck, half-bridge, full-bridge etc don't have a right-half-plane zero in the open loop transfer function because they transfer power to the output when the primary switch is ON. It is only the boost and flyback operating in continuous current mode that exhibit the moving RHP zero in the open-loop transfer function. There are however many ways to wrongly compensate a forward derived converter that can make it go unstable without the RHP zero problem. If you're interested in this stuff I highly recommend the papers by Lloyd Dixon on SMPSU control theory that I have mentioned previously.
-Richie, <---- who also doesn't want to get into a control theory lecture situation. :)
Registered Member #1749
Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497
i agree with Dr.SSTC i don't think you wanna take the pin 3 resister off go any lowe than 40ohms and your 555 is gone in a very short time use a bd139/bd140 gate driver combo
i never burned the mosfet i only fried 555 timers because i never used a gate resistor until a few weeks ago when i learned a little more about mosfets
Registered Member #579
Joined: Mon Mar 12 2007, 09:45AM
Location: Burntwood, Staffs, England
Posts: 46
I have built most flyback drivers on the web and got all of them to work Excluding Dr Killovolts one - I will try it in the next couple of days The most reliable is the one with one 2n3055 and the true flyback mode (2) windings The Mazilla circuit with the two IRF250 - a ZVS - circuit works very well but as you have found the fet blow easily. I have a demonstration on YOU TUBE - sparkfishes of how to use a Compact fluorescent Lamp _ Energy Saving Lamp to drive a flly back transformer using the original pins
I hoped to have a display of the different types of Flyback Transformers and the various drivers for the Cambridge Teslathon but could not get there. I will display the drivers/ flyback transformers etc soon on YOU TUBE
Big sparks can be obtained from Car Ignition coils also - if you can get hold of the old round ones
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Problems with 555 Flyback Driver and Flyback Driver Discussion
