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Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Referring to my Flyback Driver project: I was thinking that driving a flyback with a perfectly symetrical squarewave may not be a good method. Here's why..... In a TV or computer monitor that uses a CRT, the high voltage waveform that drives the flyback is a linear sawtooth which is generated in the horizontal sweep circuit. Flyback transformers produce the HV pulse during the fast-falling trailing edge of the sawtooth (which is during the time period when the CRT beam is retracing (or "flying back") to the left side of the screen). During the time that the flyback produces the HV pulse, it rings at its resonant frequency just before the driving voltage begins to rise again to strart another cycle (sweep).
Okay, now here's the problem as I see it. If you drive a flyback with a square-wave, the transformer will produce a HV pulse each time the driving voltage falls to zero volts, but unlike a sawtooth waveform, the leading edge of the following driving pulse rises to max voltage very quickly (but with opposite polarity), essentially putting the brakes on the ringing flyback pulse that was created during the trailing edge of the previous square-wave driving signal. So, it seems to me, that it would be better to allow the flyback to ring before the next driving pulse starts to rise above zero volts. This can be done either with a sawtooth, or with a pulse train that has a short duty-cycle (say, 10% for example).
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
bwang wrote ...
A half/full bridge driver for LOPTs indeed generates less voltage, but allows for *much* more power throughput.
Ahhh... Good point! But it's really not the driving bridge that determines whether you get more voltage or more power, but it's the driving waveform. Right?
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
While the voltage may quickly reverse polarity, the current waveform will not. I'm not sure if this actually matters, but electromagnetism is governed by amp per turn, so it would make sense if the quick voltage transition doesn't matter.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Arcstarter wrote ...
While the voltage may quickly reverse polarity, the current waveform will not. I'm not sure if this actually matters, but electromagnetism is governed by amp per turn, so it would make sense if the quick voltage transition doesn't matter.
I believe it DOES matter (but of course I could be wrong). I'm thinking of the old automobile ignition systems that used "breaker points". They were only effective in producing high voltage up to a critical engine RPM because if the driving pulses from the points exceeds the time at which the magnetic field of the coil (impulse transformer) can collapse, then the system can no longer produce efficient sparks. That's because the coil tries to generate a hv pulse during the magnetic field collapse, but gets stopped when the breaker points close, energizing the magnetic field again. Isn't it a similar situation with a flyback transformer?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Xray wrote ...
.I'm thinking of the old automobile ignition systems that used "breaker points". They were only effective in producing high voltage up to a critical engine RPM because if the driving pulses from the points exceeds the time at which the magnetic field of the coil (impulse transformer) can collapse, then the system can no longer produce efficient sparks. That's because the coil tries to generate a hv pulse during the magnetic field collapse, but gets stopped when the breaker points close, energizing the magnetic field again. Isn't it a similar situation with a flyback transformer?
From what I remember about points ignition systems, the governing factor is the 'dwell angle', which relates to the time that the points are 'closed'. It takes time for the current to build up, if the current doesn't build up to a high enough level, you get a weaker spark.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Ash Small wrote ...
From what I remember about points ignition systems, the governing factor is the 'dwell angle', which relates to the time that the points are 'closed'. It takes time for the current to build up, if the current doesn't build up to a high enough level, you get a weaker spark.
Exactly! The dwell angle is just another way of saying "duty cycle".
So, what this all boils down to (I think) is, if I want a "powerful" arc (and not necessarily a LONG arc) then driving my flyback with a symetrical square-wave from a half-bridge made up of two MOSFETS is the right way to go.
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Flyback Transformer Waveform question.
