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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I was bored today, so I though revisiting the 555 flyback driver would be fun. I came up with the "foolproof" 555 driver, which can run from nearly any voltage and with a totally isolated 555. I've only run it from 24 volts so far, but the results are promising, a good 30kV. Once I make a low voltage MOT power supply I'll start pushing things. More info here. (scroll down to 555 driver MKII)
I've got two questions though, what should the breakdown voltage of the mosfets be? (Like the ratio between input and voltage spikes.) And why can't GDTs have an air gap?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
And why can't GDTs have an air gap?
Well obivously because you want coupling to be as high as possible for sharpest drive waveform.
Regarding it, 1:4 ratio looks like a lot of burned power and stress on zeners and driver.
IIRC, a 1:2 GDT is used in such configurations in order to keep mosfet's gate voltages equal to your +- supply voltage. A DC blocking cap may be used, and resistor is used on secondary side, rather than primary to prevent ringing. Similar to this, but with TC4422 instead of transistors:
After all, GDT isn't actually needed for such circuit to work, and you may even be better without it once you debug the circuit.
Breakdown voltage of your mosfet is dictated by your primary inductance (lower inductance, lower amplitude of spike mosfet is going to see for same secondary voltage).
I would actually recommend using higher voltage mosfets or IGBT's, 500 or 600 volt like regardless of higher ON resistance. This way it runs on smaller currents, reducing switching losses and leaves more space for twidling.
TV HO drivers after all use very high voltage transistors with often puny current ratings, and are pretty efficient in their job.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
There is no reason why GDTs can't have an air gap, but there is no reason why they need one either. I've seen one exception to this: a base drive circuit for TV HOTs that uses the GDT in the flyback mode. Ie: you turn the drive transistor OFF to turn the HOT ON. This won't work without an air gap, because it uses the stored magnetic energy to drive the HOT. A core with no air gap can store practically no energy.
The voltage rating of the MOSFETs is a function of the transformer magnetizing inductance and the stray capacitance (both primary and secondary reflected) It's usual to place a small capacitor between drain and source of your MOSFETs to reduce the amplitude of the spikes. But remember, the bigger you allow the spikes to get, the higher the output voltage will be! The spike is just the peak output voltage, transformed by the turns ratio of the flyback. So high voltage transistors (like recommended by firkragg) are a good thing.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I used the "standard" (without GDT) 555 flyback driver with great success, I was getting like 70kV from a standard DC flyback, with next to none mosfet's heating. The trick is to use a proper value "damper" capaitor (across D-S of the fet) which reduces mosfet heating and even increases output voltage (I know this is strange, but it usually works for me this way).
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The Foolproof 555 driver
