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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
hey again. today i just put together a crude gate drive section on my breadboard. it runs off of 5v from an arduino, and is controlled by the arduino as well. my gdt calculations worked out well: 202khz gives a cycle time of 5uS(so on time is 2.5uS). the arduino is incapable of being accurate a these speeds though... it still works with my gdt(no saturation) with an on time of ~10uS.
for those who want to know, it is 9 turns around a 105mm^2 core of epcos n30 material with an al of 4620. time is 10uS as stated.
my problem is with the waveform. it will work, it doesn't ring, or overshoot excessively, nor is it over or under dampened. it looks like it *may* have too low inductance. but its strange.
here are the waveforms: first unloaded secondary then loaded with igbts(no resistor)
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm guessing it's not saturating, at 9 turns that would only be 0.2A magnetization current at 200khz. A core with 1cm^2 cross section should easily handle that on 9 turns. You can confirm for sure if you know the saturation flux of the core. Do you have plenty of buscap on the gate driver output?
Also I assume you're testing at 200khz? It's hard to tell from your description. If you're accidentally testing it at some low frequency then saturation would be expected
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Sorry for the lack of clarity. Above are some tests i ran. The number in the top left is the number of turns on my core. Using the N V T B Ae formula I get a minimum turns of 1.58. This matches well with the above data correct? As for the circuit, it is USB powered, with a 10uF ceramic cap on each ucc. If you want to re calculate it, the time is ~10uS, voltage is 5v, Ae is 105mm^2, and I think n30 material has peak flux density of .4, but use .3 to be safe. And for good measure, the al is 4620nH/t^2.
Edit: the gdt in the above test is not the same one with problems. It is 1:1 with 1 primary and 1 secondary. It uses thicker wire, and it's loaded with 1 igbt. Why it looks the same loaded as the first one does unloaded? I don't know. It may be that cat5 has too thin wire and is causing resistance. As it stands, the data from that one holds no value and was just to test the effects of the number of windings.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Looks fine. The funny appearance of the peaks is probably because you're running the UCCs off 5V. The peaks probably always look like that, but most people use 12 or 15V so the funkiness is relatively 3 times smaller.
It's caused by the GDT's magnetising current. During the first half of the peak, it's returning to the supply through the UCC's on-chip diodes, pushing the output 0.7v outside the rail. During the second half, it flows in the forward direction and the voltage drop changes sign. This can be seen very clearly in the scope shots with few turns.
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help with my gdt waves
