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Registered Member #2725
Joined: Tue Mar 09 2010, 02:31AM
Location:
Posts: 6
I've recently finished a 555 flyback driver similar to this one and it works fairly well. The flyback begins arcing at about 1.5 cm and will draw out to about 4 cm. However, the circuit seems to draw a lot more current than I expected. When idle the flyback circuit will sink about 2A, this value goes up to around 12A when pulling an arc. My power supply will often trigger short circuit protection while I'm drawing an arc, and I've melted several breadboard contacts just from the joint overheating. The temperature on my mosfet rises to 230F in just under a minute.
I am using a 555 timer tuned to 23Hkz and 68% duty cycle. This in turn pulses a mosfet though a 22 ohm gate resistor. The circuit is powered with 19v, although my power supply will sag to 15v when pulling an arc. This amounts to around 180W of power going into the flyback. The schematic calls for a 35V, 5A supply. Yet my circuit is drawing over 12A at only 15v. What could I be doing wrong?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, that circuit is tuned for 24..35V operation with ca. 10-15 primary turns, if you use lower voltage you must use a bigger resonant cap (needs experimenting) and less primary turns [though your turns seem ok], also experiment with core air gap.
The gate driver shouldn't run much over 50% duty cycle, try changing the "upper" 555 resistor to 1k, the "lower" to >10k and size the timing cap accordingly.
I would try a higher voltage mosfet, such as the IRFP260, I think your 100V FET is cutting it a bit and maybe is heating from avalanching.
Registered Member #2725
Joined: Tue Mar 09 2010, 02:31AM
Location:
Posts: 6
Thanks for the response Dr. Kilovolt. You were right about my 100V mosfet cutting it too close. It turns out that the voltage across my mosfet is spiking up to 144v and causing excessive heating. I've replaced the mosfet with a 250v FQA 40N25 which now runs a lot cooler. Additionally, idle current draw is now down to 800mA.
However, when drawing an arc this current still rises to around 11 amps. I tried changing the resonant cap without much success. Lowering the capacitance decreases the arcing current, but increases idle current. Sparks get longer, but weaker (blue and thin). Increasing capacitance decreases idle current but increases arcing current. The sparks do get more powerful (purple, thick), but arcs aren't as long.
Also, what exactly is a "core air gap" and how can I modify it? Additionally, would removing the metal clip that holds the two ferrite core halves together would help any?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Core airg gap is, well the gap between the core halves. When you remove the core from any flyback, you'll see it has plastic spacers between the core halves. Try fiddling with the air gap vs. primary turns.
Oh I almost forgot the most important thing, do you use a diode split (rectified) transformer? If so, have you tried swapping the primary connections?
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Help: Flyback drawing too much current
