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Registered Member #2655
Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18
Hello again, I've tried to use a GDT to drive a high capacity gates (30nF each) at approx. 80kHz and I've ran into this issue with any ferrite toroidal core I have here. I'm driving the core with discrete IRF530/IRF9530 H-bridge, +-15Vpk. As a DC blocking cap I'm using a 1uF pulse rated (Wima) and two 10uF electrolytics (antiparallel). Bottom signal is the signal input to the driver, 5V/div. Top signal is the voltage across 33nF capacitor - aslo 5V/div. This shot was taken at unknown ferrite core with Ae approx 80mm^2, 25 turns, but I can't get better shape with other core/different number of turns anyway. Am I correct this is probably caused by a low permeability? Thanks a lot!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, what is the issue? No transformer is perfect, the output will never be identical to the input. You have to decide whether that gate waveform is good enough for your application.
Is the output voltage really only 8V peak-peak? That's certainly not good enough for many applications.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
I always recommend simulting your gate drive circuits using PSPICE or similar program. Modeling gate drivers, transformer, gate loads is relatively simple and you can get a very good idea of how the circuit will behave prior to building it.
Adjusting turns, core type, drive type, etc... is much easier in PSPICE than trying to do it on the bench.
Registered Member #2655
Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18
Sorry, I haven't said it clearly - the voltage is really 8V peak to peak (and that's the issue) - insufficent to drive almost any gate. The MOSFETs are connected as a "H-Bridge" - simillar to Steve Ward's driver and without GDT connected, the voltage is 30V peak to peak on the output terminals. Of course I can "solve" it by increasing input voltage - but I really think there's a design flaw that need to be found and removed (winding is made of twisted pair of #25 AWG ECW so I don't think the leakage inductance is here a big issue).
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
A few things that come to mind;
Have you checked the amplitude of the waveform on the gdt primary when the secondary load is connected?
If it's low then measure your gate driver dc supply voltage (30Vdc), is it low too? If the dc supply is ok then the driver fets aren't turning on 'hard' enough. If the primary voltage is correct and the secondary voltage is low then ... the leakage inductance is too high due to low core permeability.
If you are driving two transistors from one GDT at 80 kHz then 160k times a second one gate-emitter is charged and one discharged. On the datasheet for your transistor find Qg for Vg=30 V. multiply by 2x 160k = 320k this gives minimum gate driver current required. e.g. Qg= 800nC @ Vge = 30V, Idrive >= 256 mA, for a 30V psu that's 7.68W minimum/ideal
Most toroids that have an overall black coating seem to be high flux, Ur c90 To calm your mind, wind 10 to 30 turns of wire and measure the inductance (use series or parallel resonance with a known capacitor) knowing the core size and Al (inductance/turns-squared) find a similar core to get an estimate of Ur. For gdt Ur >1000 is best for c80kHz.
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[SOLVED] Driving high capacity gates at low freq with GDT?
