 Gate resistor value tradeoff
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Steve Ward
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Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
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I believe too small a gate resistor can cause reliability issues due to excessive gate voltage overshoot and collector voltage spikes caused by turning the IGBT off faster than it was designed to go.
I didnt think turn-off too fast was ever a problem with IGBTs because of the tail currents being somewhat independent of the gate voltage. I use GDTs almost exclusively, and always use a diode to bypass the gate R for fast turn off.
Ive had good results using fairly large gate resistance (with reverse schottkey diode) on my IGBTs with the primary goal of adding dead-band to allow for zero volt switching (at turn on) of my tesla bridges. This requires a phase-lead type driver so that you can command the IGBT to turn off before the current zero crossings, this way the load current causes the companion IGBT VCE to transition to "0V" BEFORE the IGBT is commanded to be ON. I find this type of tuning gives the cleanest waveforms and minimal overshoot.
You can verify whether or not you have enough dead-time by checking an IGBT's Vce and Vgs simultaneously on an oscilloscope**. You want to see the Vce fall to 0 before Vgs hits the turn on voltage. You can observe the turn on voltage in the VGS waveform: it shows up as a little hesitation in voltage rise at 5-10V typically. If the IGBT turn on is too early, you will see large voltage transients (undershoot) in Vce (and Vgs for that matter) as the turn on is very rapid. If the phase lead is not great enough, then no amount of deadtime will work because this switching method relies on there being enough primary current to charge the IGBT junctions while in the deadtime portion of the switch cycle. I reduce Rgs to avoid excessive dead-time period, so this is more or less how to pick an Rg in my opinion.
For CM300s i found 4.7 ohms to work well with 24V drive through a GDT.
For FGH60N60SMD im using 15 ohms with 2:1 stepdown GDTS for 12V drive.
**Be careful about connecting your oscilloscope ground probe to your power electronics.
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