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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, here's another one in my series of solutionless problems
I'm trying to make a gate driver for half-bridge pwm control. As I found this is not possible with a GDT because it cannot transfer DC offset. I can do dead time power control with a GDT, but I don't want any higher dead time because I had problems with that before.
The driver is KA3525 pwm going to IR2111 half bridge driver, which outputs are boosted with TC4422 gate drivers. These drive a half-bridge of 6 IRF740's (3 per "leg") which drives two 25 turn parallel primaries of ferrite transformers (pri/sec are opposite each other for maximum leakage inductance, so the xfmrs can work into short circuit).
First tests are with a small 25W bulb in series with mains and the transformers' secondaries shorted. As I turn it on, I am greeted with a repeating "chirping" noise coming of the xfmrs. On closer inspection, the output of the KA3525 goes mad when I apply power to the bridge. There must be some weird form of noise/feedback getting to the pwm chip. I have decoupling caps everywhere, I don't get it.
The strange thing is that this chirping stops when I lower the duty cycle a bit, or if I leave the xfmrs secondaries open circuit.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Oh well, please ignore this thread! I was debugging the circuit and I actually forgot to connect the IR2111 to supply voltage! It's funny that it kind of worked even without the supply voltage
The most complex problems usually have the simplest solution.
Problem fixed, feel free to delete/lock this thread
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I'm trying to make a gate driver for half-bridge pwm control. As I found this is not possible with a GDT because it cannot transfer DC offset. I can do dead time power control with a GDT, but I don't want any higher dead time because I had problems with that before.
The driver is KA3525 pwm going to IR2111 half bridge driver, which outputs are boosted with TC4422 gate drivers. These drive a half-bridge of 6 IRF740's (3 per "leg") which drives two 25 turn parallel primaries of ferrite transformers (pri/sec are opposite each other for maximum leakage inductance, so the xfmrs can work into short circuit).
What do you mean when you say you need ''DC offset'' on your gates?
You clearly don't want DC on your power transformers (flybacks?) and there is no reason why you wouldn't be able to use deadtime on your GDT.
You seem to be really frustrated with GDT's for some reason and venture for rather expensive IC's...
GDT's work for entire world and use any amount of deadtime happily.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko, I need only minimal dead time on the output of the bridge, otherwise when using GDT and dead time control, things starts getting funny (the primary noise, which I described in some other thread, gets into the gate waveform through reverse transfer capacitance) and the transistors get hot. So instead of varying dead time, I change the "on/off" ratio of the gate drive. This creates a DC offset which a GDT cannot transfer.
The level shift has an added benefit of near-perfect waveform on the gates, only minimal gate resistors requied.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
GDTs only work for symmetric wave forms. There are *many* cases where you need a high side driver to convey the "DC" component of the signal. Ive tried half-bridge drivers from TI and intersil so far, and they are far less robust than they might claim... in the long run i prefer ultrafast opto-isolators and using good old UCC3732X drivers with boot-strapping diodes to supply them.
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Level shifted gate driver problems
It's funny that it kind of worked even without the supply voltage 
