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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Dark Current wrote ...
The gate threshold voltage is when the FET or IGBT starts turning on. It is insufficient if the gate voltage is just above this threshold, it must be high enough to fully saturate the transistor. This usually happens with >10V for a MOSFET or >12V for an IGBT.
yes if this wasnt clear, that is what i meant, if his droop never falls below some critcal value, then it still stays on, i think he needs to know if the droop in gate drive cuases a droop in proportional conduction, which it wont if it (gate voltage) stays high enough.
if he would list a part number for his transistors and post an actual waveform we could tell him if hes done well or poor.
EDIT: he means the IRF740... i missed that, let me see... ]irf740.pdf[/file]
Given what i have highligthed in green i htink its clear... if he stays above 5-6 volts or so then the transistor is just about on, and definately on at 10 volts or more.
EDIT: (Given the graph on transfer charcteristics, above 8 volts Vgs, hell still be able to conduct high current at nearly 0.5 ohms on state R.)
Registered Member #2578
Joined: Sun Dec 27 2009, 09:21PM
Location: USA Utah
Posts: 31
well I figured it out, it was the capacitors they where too low a value for below 48khz. mostly the gate side capacitor. now they are almost perfectly square but still a little slope @24khz. I still have a little playing around with it but now I have an idea of how to get it working, the way I want it to. @48khz it is square.
the idea of adding free wheel diodes across the mosfets in the totem pole is good and I will likely use it.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Even if your gate signal isnt perfectly square, it will still produce a square wave if it has high rate of rise and fall edges. remember these MOSFETs are switches, not amplifiers like the NPN/PNP BJTs....
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GDT driver
