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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
This seems to resemble an amplifier, but using the HV secondary of the coil as a resonant feedback with the resistors setting the DC bias level to slightly turn on the gate.
Interesting circuit though.
Can't say I've run into this one before, usually they need feedback windings as well.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
It could be relying on the leakage inductance of the secondary and the capacitance of the MOSFET gate to set the frequency of oscillation. I'd also be somewhat skittish about connecting the HV return directly to the gate of the MOSFET, I like to keep low voltage as separate as possible from high voltage.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
When I first looked at the circuit I thought 'That's clever'. Then I looked again and thought 'Hang on, the base of the secondary won't go negative until the MOSFET is switched off'.
I've read a bit on leakage inductance (Wikipedia, etc.) but that doesn't really explain what exactly is happening here.
Are you saying that when current begins to flow in the primary, the base of the secondary goes negative, pulling the gate of the MOSFET negative? if so, how exactly?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I'm prepared to believe that under certain circumstances the output might be fed back to the input 180° out of phase, the basic condition required for oscillation.
The author writes of "some kind of delay between each pulse which is sent to the primary coil" which might suggest squegging or other disruptive oscillatory form.
The statement that the circuit will oscillate at 30kHz, I take with a pinch of salt. What are the frequency determining elements? It might well work with the components and physical layout the author has used, but not with others.
As for frequency stability, it looks like a bit of a wanderer to me.
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Interesting flyback circuit
