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4hv.org :: Forums :: High Voltage
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Problems with 555 Flyback Driver and Flyback Driver Discussion

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Firefox
Fri Jul 04 2008, 02:54AM Print
Firefox Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I just finished Uzzor's simple 555 flyback driver, pictured here, and tried testing it, only to have one cooked 200V 10A MOSFET. The driver seemed to be working fine (I'm going to have to find a speaker to hook it to) but the FET didn't like being where it was, and was hot enough to burn my finger after only a couple seconds of power. My power source was the 12V 15A rail of a ATX PSU. Could it be that the impedance of my circuit is too low, and I am drawing more amperage than the FET can handle?

I have also made a few small changes to the circuit. I used a 100k potentiometer and a 10 ohm gate resistor. I also added a 2uF film cap across the power supply.

[Edit] I'm expanding the scope of this thread to include all discussion on flyback driver, due to the wealth of information posted as alternatives and upgrades to the 555 driver. Feel free to add your thoughts and experiences as you see fit.
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Dr. Dark Current
Fri Jul 04 2008, 06:39AM
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have this circuit on my pc and post it every time someone asks about 555 flyback driver tongue
1215153281 152 FT49027 555flyback

Some notes:
-the fet shoult be rated approx. 6-10 times the voltage of your PSU, but not more as it will have too high Rds(on)
-your 68ohm gate resistor is too big, you can remove it completely or use something like 10 ohms
-your FET has probably too high Rds. Something aroud 50-100mOhm max. should be okay (e.g. IRF540)
-it will get quite hot anyway, there is a "quasi-resonant" mod for this circuit which involves few extra components but makes the fet run often much cooler.
-these drivers work best at around 70V supply voltage (555 has its own supply) and 500V FETs (IRFP460)



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cjk2
Fri Jul 04 2008, 02:06PM
cjk2 Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
A snubber is probably needed to stop the fet from overheating. Try a TVS or chose a capacitor of the correct value.
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Marko
Fri Jul 04 2008, 03:45PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
It is normal for mosfets to die in a circuit like this when we attempt to draw arcs, or charge large capacitances which will both 'clamp' the output voltage to some value.

I did not understand it myself until recently, but I now only see a great amount of misconceptions being spread over and over, which I would like to stop.


It needs to be understood that any transformer or inductor can work only with AC, and will represent a dead short to any DC component. Volt-seconds in positive halfwave need to always be same as volt-seconds in negative halfwave; although their shape is irrelevant.


A flyback is just a transformer with relatively low magnetizing inductance due to large air gap introduced in it. It works by putting an amount of energy into this magnetizing inductance while switch is on and releasing it into secondary while it is off.


If we drive the switch by constant duty cycle from NE555, we have predetermined the volt-seconds we put into positive halfwave with each cycle.

The expected problem is, if we loaded the output of the transformer with an arc - there is absolutely nothing assuring that negative half-wave volt-seconds will be the same as what we predetermined.

Due to low impedance of an arc the negative voltage is actually going to be clamped to some very low value. If we attempt to charge a large HV capacitor, like to run a tesla coil, the same will happen as capacitor basically represents a short to DC output.


In any case, when positive and negative Vs aren't trimmed, the current will integrate up for their difference. This is how the DC component appears in the primary -not instantly, but over a period of several cycles. At some point the flux (which follows the integrating current) will saturate the core, decreasing the primary inductance heavily and overheating the switch.

I'm pretty sure this is what happens in all those ''NE555 flyback drivers''.


Snubber like a TVS or RCD clamp will do nothing for this. It will absorb energy put into the transformer in case output is unloaded (which will be a great deal of heat on the snubber).

In case clamp is not present this heat will simply be dissipated in the switch avalanching.


The quasi-resonant scheme is what is used in TV's, IIRC, and it is only that can actually restore energy to supply bus and increase efficiency a bit. Still it does absolutely nothing about the core saturation problem.


Sometimes the circuit can work, but relatively poorly. I think it is because, as the core approaches saturation, the output diminishes, and voltage drop across the arc increases until it gets high enough to balance the volt-seconds and stop the flux walking.

But still it usually results in hot transistor and hot ferrite core.


The industry standard way of solving this problem is called Current Mode Control, which actively prevents the current integration by cutting the cycle off when current reaches predetermined value. Look for datasheets and application notes on UC384* series of SMPS controllers.


Only with current mode control you could safely charge capacitors and draw arcs without problems.
But, still there would be the problem of what happens to the converter when unloaded - this may be solved by quasi-resonant drive, or voltage feedback. Voltage feedback would again, along with all control loop problems, probably make impossible drawing of stable arcs from the output - and who wants a flyback driver he can't draw arcs from!

And after all you have done you would just have a flyback driver, and I guess not a too powerful one.


To cut it short, building an efficient and powerful flyback converter that can perform all feats you guys want is probably one of hardest and most pointless things in electronics I can think off.


So, my advice is:

- Ignore Jan Martis, his site, and all the schematics of ''NE555 flyback drivers'', never build them. And I'm saying that solely because I'm evil. ( :D )

- For driving flybacks at low power levels (up to 500W or so), with all feats you need (drawing arcs, capacitor charging..), I can't think of anything better then ''Mazzili'' flyback driver, or a simple halfbridge of mosfets driven by SG3525, TL494 or some other controller. They each have their + and - but it's mostly about what input voltage do you want to use.

- For higher power levels, it is just that flyback transformers won't stand them for long. But if you own a dedicated transformer, my probably sole recommendation would be an SLR inverter.


Marko









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Firefox
Fri Jul 04 2008, 04:37PM
Firefox Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Marko, if I'm understanding what you are typing, are you saying its only luck that people have been able to get 555 drivers to work at all?
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Marko
Fri Jul 04 2008, 08:54PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Firefox wrote ...

Marko, if I'm understanding what you are typing, are you saying its only luck that people have been able to get 555 drivers to work at all?

Well, you could get it to work, and I mentioned a reason why, although efficiency will always be poor. I don't see a point as equally simple or only slightly more complex circuits can easily achieve what you guys want. There is no point pursuing the wrong way.
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GeordieBoy
Fri Jul 04 2008, 09:35PM
GeordieBoy Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Marko wrote:

Volt-seconds in positive halfwave need to always be same as volt-seconds in negative halfwave; although their shape is irrelevant.

A flyback is just a transformer with relatively low magnetizing inductance due to large air gap introduced in it. It works by putting an amount of energy into this magnetizing inductance while switch is on and releasing it into secondary while it is off.

If we drive the switch by constant duty cycle from NE555, we have predetermined the volt-seconds we put into positive halfwave with each cycle.

...if we loaded the output of the transformer with an arc - there is absolutely nothing assuring that negative half-wave volt-seconds will be the same as what we predetermined.

Due to low impedance of an arc the negative voltage is actually going to be clamped to some very low value. If we attempt to charge a large HV capacitor, like to run a tesla coil, the same will happen as capacitor basically represents a short to DC output.

In any case, when positive and negative Vs aren't trimmed, the current will integrate up for their difference. This is how the DC component appears in the primary -not instantly, but over a period of several cycles. At some point the flux (which follows the integrating current) will saturate the core, decreasing the primary inductance heavily and overheating the switch.

I'm pretty sure this is what happens in all those ''NE555 flyback drivers''.

Totally agree with everything Marko wrote here. This is *exactly* what happens when you short-circuit the output of a flyback converter that has fixed programmed duty ratio. The fixed on-time keeps pushing more energy packets into the core but the shorted output doesn't allow any energy to escape during the off-time. Marko's explanation is spot on.

Snubber like a TVS or RCD clamp will do nothing for this. It will absorb energy put into the transformer in case output is unloaded (which will be a great deal of heat on the snubber).

In case clamp is not present this heat will simply be dissipated in the switch avalanching.

Agreed again. The way that circuit is drawn, with no load on the output the peak voltage developed across the switch immediately after the on-time will be the MOSFET avalanche rating. All of the energy put into the flyback transformer during each on-time will be dissipated in avalanching the MOSFET during the early part of the off-time.

The flyback circuits in TV receivers actually use a quasi-resonant arrangement where a resonant capacitor connected across the switch makes the energy stored in the primary winding re-circulate resonantly if there is no load on the secondary of the flyback transformer. It also fixes the peak voltage across the switch by design, so that it can not go any higher even when the HV output is open circuit. In a TV the on-time is typically something like 52us and the half-sine shaped flyback pulse is produced in the 12us of off-time. Peak switch voltage on the primary side is usually somewhere in the region of 1200V for a 150V supply, but the turns ratio of the flyback transformer steps this up to maybe 20kV on the secondary.

I've never seen a good diagram of the waveforms of a TV's flyback (line output stage) on the net but discussions about this frequently come up here, ...so i've posted my own sketch here to go along with this thread:

Link2

-Richie,
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Antonio
Sat Jul 05 2008, 01:48AM
Antonio Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The drawing is correct. The idea that a low-impedance load will make the primary current grow indefinitely, limited only by the current that the transistor can handle and resistances in the circuit, is correct too. So, if you want to make arcs, always put some significant resistance in series with the arc, or in series with the primary coil, or use a more powerful transistor at the input. A regular flyback transformer will not last much making arcs without current limitation. A regular transistor will not last much too if used without a snubber, or that diode and capacitor in parallel with it (better than a snubber, as they recycle unused energy).
In working with these 555 flyback drivers, always guarantee first that the oscillator is working before connecting the power supply to the transformer. Any element that is too hot to touch is really too hot, and if this happens something is wrong.
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Dr. SSTC
Sat Jul 05 2008, 07:04AM
Dr. SSTC Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222
Dr. Kilovolt wrote ...

I have this circuit on my pc and post it every time someone asks about 555 flyback driver tongue
1215153281 152 FT49027 555flyback

Some notes:
-the fet shoult be rated approx. 6-10 times the voltage of your PSU, but not more as it will have too high Rds(on)
-your 68ohm gate resistor is too big, you can remove it completely or use something like 10 ohms
-your FET has probably too high Rds. Something aroud 50-100mOhm max. should be okay (e.g. IRF540)
-it will get quite hot anyway, there is a "quasi-resonant" mod for this circuit which involves few extra components but makes the fet run often much cooler.
-these drivers work best at around 70V supply voltage (555 has its own supply) and 500V FETs (IRFP460)






i dont 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
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Steve Conner
Sat Jul 05 2008, 12:26PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I agree with everything Marko and Richie said, flyback drivers are for kids :P

Although, I thought of one thing. Antonio once mentioned that the single transistor blocking oscillator circuit can have a form of peak current control, because the cycle can be ended by the transistor coming out of saturation, as well as by the core going into saturation. In the light of what was said here, I think this makes it a good candidate for a kiddie friendly flyback driver.

It also explains why it's used to charge camera flash capacitors, and why you hear the rising pitch whine as the capacitor charges. When the capacitor voltage is low, the circuit must run at a low duty cycle to give the core time to reset. The on-time is constant, so the off-time and hence the frequency varies with capacitor voltage.
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