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Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
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Posts: 179
Hello. I want to build another flyback driver circuit. I built one before but it wasn't so great.
I am thinking about using 120Volts from the electric socket, rectifying this voltage using a bridge and smoothing it out with a large capacitor filter. It makes no sense to convert 120 volts AC to 24 or 48 volts DC and than step it up to kilovolts. More current = more losses...
I want to make something like this:
This is what I used to have before I burned it in an experiment: but IRFP 250 Mosfets limit my input voltage to 40 Volts. Why not use 120 volts in my next project?
This way I won't have to worry about the current rating of my 12 to 48 volt power supply that I would probably have to get from an LED lighting supplier...
The top two links show some Flyback driver circuits that use mosfets rated up to 500 volts drain source voltage.
IRFP 250 can handle 200 volts maximum...
What determines my input voltage in the circuits mentioned above is a voltage divider that consists of the resistors in series with the Gate pins of the MOSFETS and those resistors that connect them to the ground rail. Am I correct? Voltage on the gate must remain slightly below Gate opening Vgs and Zener diodes have to purge the gate once the voltage on the gate exceeds 12 V. Is this pair of Zener Diodes a safety measure to protect the gate or is this to make this circuit oscillate?
I am a novice with MOSFETS so I am not sure why Vgs threshold for IRFP250 is 2 to 4 volts even though all circuits with this mosfet seem to use 10 to 12 volts to open the gate... I don't see 10 to 12 volts anywhere in the datasheet... But this is just case study for IRFP250. I am not sure if I should pick a different MOSFET to run off ~170 volts (120VAC rectified -- 120*sqrt(2) = 169.705627485 -- peak DC voltage).
So should I increase resistance of the resistor that is in series with the gate, get a 0.5A or more powerful zeners or a combination of both? What should be my MOSFET choice and how do I determine the optimal Gate voltage for it to fully open?
Registered Member #3282
Joined: Wed Oct 06 2010, 05:01PM
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Posts: 224
your never going to get a zvs to run at over 100 volts. which is why you don't see them used in computers. use this circuit http://danyk.cz/igbt_2_en.html
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
This circuit can, in principle, run with 100 V or more, but: The inductances from the primary coil and of the series coil must be much larger than the ones required for low-voltage power supplies, and if their cores saturate the circuit may easily be destroyed. The gate drive through resistive dividers is already inefficient al low voltages, and much less efficient at higher voltages. In any case this circuit is dangerous because nothing impedes the symultaneous conduction of both mosfets. The question about 12 V drive is because the threshold vgs voltage of 2-4 V is the minimum for the mosfet conduct, but the maximum current that it can conduct and the conductance when conducting are higher with higher vgs.
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
Antonio wrote ...
This circuit can, in principle, run with 100 V or more, but: The inductances from the primary coil and of the series coil must be much larger than the ones required for low-voltage power supplies, and if their cores saturate the circuit may easily be destroyed. The gate drive through resistive dividers is already inefficient al low voltages, and much less efficient at higher voltages. In any case this circuit is dangerous because nothing impedes the symultaneous conduction of both mosfets. The question about 12 V drive is because the threshold vgs voltage of 2-4 V is the minimum for the mosfet conduct, but the maximum current that it can conduct and the conductance when conducting are higher with higher vgs.
Interesting. I am still unsure how optimal Vgs is determined. It is not on the datasheet... Higher inductance? I need to wind 10X the amount of turns to get the same inductance? Is there a way to increase the frequency along with voltage to compensate for that?
Were there any circuits where 120AC was rectified and used directly to drive the primary coil of the flyback transformer? It makes sense for someone to develop such circuit, even if it doesn't use voltage dividers or uses TRIACs instead of MOSFETs. It would make sense to use inductive or capacitive feedback instead of resistive feedback in such oscillator. Maybe even a small transformer to trigger the gates (or bases) of semiconductors of the oscillator.
Maybe this 47 - 100 uH choking coil in Mazzilli circuit can be replaced with some sort of a transformer that can be used for feedback purposes - to open gates.
It would make sense to use semiconductors with higher Vgs all together or to use thyristors. Did anybody ever build something like this? I am too inexperienced to develop my own circuit.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874
Hi I've been wondering the same thing, is it that the fly-back isn't designed for that input, flux density/cross section?
Mains voltage with current behind it, at high frequency wouldn't be to safe for me, have noticed that high frequency seems to propagate through insulators over a longer distance.
EDIT, I miss understood what you said, didn't release you are limiting current, just assumed with the large filter cap....that it was on the ac side, maybe not the dc side
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
Andy wrote ...
Hi I've been wondering the same thing, is it that the fly-back isn't designed for that input, flux density/cross section?
Mains voltage with current behind it, at high frequency wouldn't be to safe, have noticed that high frequency seems to propagate through insulators over a longer distance.
12 volts 120 volts - difference of one order is not that great. If you are afraid of the mains voltage an high currents - put a fuse in the circuit or limit current with a ballast.
A chopper is a more modern way of controlling current... I would reduce the pulse duration on primary to limit current consumption and prevent saturation from happening.
More voltage would saturate the core as much as less voltage... Compensate by reducing currents or pulse duration. It would make sense to make this circuit work faster with higher voltage.
If this circuit used feedback that was wound onto the primary of the flyback, than saturation could be avoided because if the core is saturated, the feedback would not work.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I have succesfuly run a zvs/royer on full wave rectified 110 Vac but I used a separate 12 Vdc supply for the gate drive. I did not use it to draw arcs as I'm sure that would have blown something.
Using rectified 240 Vac I had one instant failure and one delayed failure and gave up.
I would not reccomend rectified mains for a zvs/royer.
On the bright side, the original primary in most flybacks is designed for around 110 Vac rectified supply so a conventional flyback circuit using a 555 or smpsu ic driver works excellently.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
tarakan2 wrote ...
I am still unsure how optimal Vgs is determined. It is not on the datasheet... Higher inductance? I need to wind 10X the amount of turns to get the same inductance? Is there a way to increase the frequency along with voltage to compensate for that? Were there any circuits where 120AC was rectified and used directly to drive the primary coil of the flyback transformer?
Look at the curves showing current x voltage for different vgs. The minimum vgs depends on the maximum drain current. It's safe to use 12V, as the maximum is usually 20V. Inductance is proportional to the square of the number or turns, for the same core. So, if you increase the voltages 10 times and want the currents 10 times smaller (same power), you need inductances 100 times larger, or 10 times more turns. Usual flyback transformers in TV/monitor applications operate with around 100V primary voltage. Just copy the circuits from the schematics of the boards. The same boards usually have also an useful multi-output switching power supply that can be copied too and used to power the flyback circuit.
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
Sulaiman wrote ...
I have succesfuly run a zvs/royer on full wave rectified 110 Vac but I used a separate 12 Vdc supply for the gate drive. I did not use it to draw arcs as I'm sure that would have blown something.
Using rectified 240 Vac I had one instant failure and one delayed failure and gave up.
I would not reccomend rectified mains for a zvs/royer.
On the bright side, the original primary in most flybacks is designed for around 110 Vac rectified supply so a conventional flyback circuit using a 555 or smpsu ic driver works excellently.
I am glad that you had successfully done it. I live in United States so our mains are 120VAC.
It makes sense to use a small transformer to power the gates or to make a capacitor voltage divider. Some way of controlling current by limiting the amount of time gates remain on can be applied to draw arks and not to burn anything.
Maybe some native flyback transformer windings can be used with rectified mains... 555 may be used. I can even make it with a microcontroller to adjust the chopper and other characteristics inside a program.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I have used 555 to drive an igbt with c150 Vdc for the flyback primary, I did not use any voltage feedback for that circuit so output voltage varied as the load current changed. I used lots of 1.5KE350CP TVS diodes (because I bought a lot cheap from eBay) to protect the transistor from over-voltage spikes with one eht diode in series to reduce capacitive charge/discharge losses. I think I used 3x series by 5x parallel to absorb full power input with no output load. (not enough, thay got quite hot at full power setting with no load) I should have built in voltage regulation So I have since then bought a few UC2825 smpsu IC (for an improved version with output voltage regulation and current limit) and 10x 35kV flyback transformers ... as usual, another project waiting .......
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Another Flyback Driver circuit... Why use 24 volt power supply? I want to use 169.70 VDC
