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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am trying to make an isolated gate drive for an IGBT. I was hoping to do this without using a bootstrap chip. I have a variable duty signal input to a TC4420 and TC4429 gate drive chip. The output goes through a 1uf capacitor to a GDT. What I find is if the duty is 50% half of the output signal is above 0v and half is below, which is good for turning the gate on and off. For a 15v input, I get +/-15v. If the duty goes below or above 50% the signal shifts up or down, causing almost the entire waveform to be above zero or below. I wind up with the signal going from 0 to -15v, or 0 to 15v. I guess the average voltages above and below zero must stay equal.
How do I fix this so the maximum and minimum voltages are the same? Is it possible or should I just get the bootstrap chip?
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635
If the output of a transformer were uneven, that would imply that the core is heading for saturation (I think this is called flux walking?). I've used GDT with slight adjustment to duty cycle, but too much and like you said, it causes uneven voltages. GDTs, like any ideal transformer, only pass AC without saturating. You'll hear people say GDTs don't like uneven duty cycle, and this is why. I don't know what your application is but I chose to stick with GDTs and switch to frequency modulation rather than pulse width modulation.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Why not go for a "homemade" approach? Optoisolators and transistors are very cheap. You can even make an optoisolator with an led, phototransistor and shrink tube if you don't have one.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
IamSmooth wrote ...
I guess the average voltages above and below zero must stay equal.
Right, the average value of the GDT output must be zero (DC component does not pass the transformer), so if you reduce the waveform duty-cycle you will obtain uneven positive and negative gate values.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have optoisolators. How would I go about getting an isolated +/-15v drive signal to the gate without using a gdt or bootstrap? Is there a link I can look up?
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help with gate drive transformer
