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Registered Member #1211
Joined: Fri Jan 04 2008, 11:20AM
Location: England - UK
Posts: 19
After receiving in the post, the components needed to build a Half-Bridge Flyback driver, I got straight down to putting it together yesterday, but am now just a bit stuck and going nowhere fast..
The single-ended 555 / IRF740 driver worked great as a plasma string machine, but under advice, I need to make a proper Half-Bridge for a plasma tweeter/speaker, to get around the problems of audio distortion & excessive drive requirements.
Using a TL494 & TC4422 with a 30V/3A supply for the FETS (IRF540's), I get 5mm 'arclets' - very spindly things indeed. So, I tried the 555 with the TC4422 as well, but just the same - tiny squeaky arcs.
There are lovely clean square waves at both gates of both FETs, so the drive is just fine - they run stone cold, but I guess they would be, drawing only a couple of hundred mA at best. The individual FETs drive the primary of a Flyback (12 turns), to +30V OK, without the capacitors in place.
Caps are 250V Mylar Film - have a dozen 2.2uf's - seems to make no difference how many I parrallel for the divider - can't even get a small, 12V/5W bulb to light up at full duty cycle.
Have tried different operating frequencies, 5Khz upwards - 'best' output is at about 18Khz...
Thanks for any advice you can give me on this one.
Registered Member #1211
Joined: Fri Jan 04 2008, 11:20AM
Location: England - UK
Posts: 19
Thanks for the reply.
Not using a GDT, as one isn't needed apparently, but with you on 3-4 turns for the primary.
The circuit I'm using is shown below - I added a 4422 gate drive IC on the output of the 555. Also got some inverting 4429's at the same time, for a future full bridge.
Just not happy driving MOSFET's with less than knife-edged waveforms - lost too many in the past.
It was either a 4422 or a standard totem-pole driver. Bad idea for this circuit ?
One thing that did notice as strange, was the large size of the rail-rail divider caps at '3-100uf'. I would have thought much lower values would be needed, matching the resonate frequency of the Flyback ?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Richard wrote ...
I would have thought much lower values would be needed, matching the resonate frequency of the Flyback ?
The last thing you want in a halfbridge is when the decoupling caps start resonate with your primary winding, this would lead to destruction of the bridge. The caps must be big enough to ensure no resonance can happen, the bigger the better of course.
If you want better efficiency you can go up to cca. 60V with IRF540s, at this voltage I would use ca. 7 turn on the primary.
Registered Member #1211
Joined: Fri Jan 04 2008, 11:20AM
Location: England - UK
Posts: 19
Nice work Andrew.
Finally got the H-bridge working - it was all down to fine setting the resonate frequency & precise pulse-width in the end. Unfortunately, no use to me, as I need to drive a Flyback at a much higher frequency, than the low 15.5Khz this runs at, when it is providing any useable output.
I get much better output over a wider frequency range, from a single driver using the same supply. The IRF540 also runs barely warm without a heatsink and when correctly snubbered, seems very efficient. The trace below shows the waveform on the Drain of the IRF540, powering the flyback primary, at 5us/cm.
Using the H-bridge circuit , the Drain trace is awash with dozen on harmonics of differing amplitudes. Both FETs run very hot as expected - and consequently, power consumption is high.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Richard wrote ...
Nice work Andrew.
Finally got the H-bridge working - it was all down to fine setting the resonate frequency & precise pulse-width in the end. Unfortunately, no use to me, as I need to drive a Flyback at a much higher frequency, than the low 15.5Khz this runs at, when it is providing any useable output.
This circuit is really not meant to run at 15khz. I use halfbridge flyback drivers with ~30-50kHz. Try removing some turns or powering it with higher voltage, if you haven't done already. The halfbridge should really have advantage over single transistor
Registered Member #558
Joined: Thu Mar 01 2007, 10:28AM
Location:
Posts: 9
Hi,
This is a bit out of date but asking anyway. I have major problems get this half bridge working. I'm driving it with TL494, but each time I test run it - the R6 47-100ohm resistor starts smoking. The reason is that FET get shorted, what's wrong with my circuit. I have proper heatsink. Diodes I use are MUR1620 and FETs IRF540. Caps are around 3uF Any ideas?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Mark wrote ...
Hi,
This is a bit out of date but asking anyway. I have major problems get this half bridge working. I'm driving it with TL494, but each time I test run it - the R6 47-100ohm resistor starts smoking.
It must be rated at a few watts (3W is fine). Make sure your "12V" is really 12V and not more, so you are no overvolting the gate driver.
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Half-Bridge Flyback Blues
