Electronics > Projects, Designs, and Technical Stuff
Shoot-through 3 phase bridge? gate drive waveform
jmelson:
--- Quote from: duak on June 09, 2020, 05:42:06 pm ---Questions for JMelson:
1.) any thoughts on why the IGBT's diode is so slow to turn on? I note that the data sheet does not show the forward recovery time and the diode's Vf graph shows a higher than expected voltage.
2.) have you observed this effect in other IGBTs?
--- End quote ---
I can't say for IGBT's. Since my servo amps are relatively low voltage, I only use big MOSFETs in them. The "body diode" is, in most cases, a side effect of the MOS structure, and not a designed-for-purpose diode. You CAN buy MOSFETs that have a separate diode that has much better charateristics.
But, i THINK that many IGBTs have a similar body diode that is an artifact of the transistor structure, and may have similar (poor) characteristics.
I have seen in the literature that experienced designers have said "never let the body diode conduct."
So, my observations are only certain for FETs, not IGBTs. And, the measurement I reported required a fairly large inductive load sinking current from the half-bridge.
The problem with the added resistor on VS1 is that it goes in series with the gate charging circuit of the high-side transistor. To be large enough to limit current, it probably will be too large for your desired gate charge/discharge time.
Jon
Jon
Yansi:
I've yet to see a diode with any significant forward recovery. As far as I know, it does not exist. Turn on time is dominated by parasitic inductance. Or not? I stay being corrected.
Yansi:
--- Quote from: jmelson on June 10, 2020, 06:07:01 pm ---But, i THINK that many IGBTs have a similar body diode that is an artifact of the transistor structure, and may have similar (poor) characteristics.
I have seen in the literature that experienced designers have said "never let the body diode conduct."
--- End quote ---
That thinking is incorrect. Substrate diode in a mosfet is an unintentional structure, whose parameters such as the Qrr/trr are a result of a compromise in the design of the mosfet. You can't simply change parameters of one, without affecting the other.
IGBTs do not have any intrinsic diode in its structure. Hence why you can get IGBTs with and without integrated diode. The integrated diode is usually a separate die, that can be made using a different process with significantly better Qrr/trr than a similar mosfet substrate diode would achieve.
That kind of rule of thumb of "never let the body diode conduct." sure has some truth in it, but mostly with old nasty mosfets. There is plenty of choice these days with reasonably fast substrate diodes. Also, in a motor control application, Qrr/trr of mosfet substrate diode is mostly non-issue. Switching frequencies are rather low.
duak:
Ghoetic, have you considered the change that Yansi and I have suggested? (page 23 of the IRS2334 data sheet I referred to above.)
Infineon/IRF suggests Rvs not be greater than 5 ohms, I suppose to not affect gate drive.
Also, when I was looking for something else I found this on parasitic turning on: https://www.controleng.com/articles/tutorial-mitigating-parasitic-turn-on-effect-in-igbt-output-drives-to-improve-drive-performance/
Jon, I also worked with power FETs, but it was years ago. I developed a 3 phase MOSFET servo amp in the mid to late 80s with a switching rate of 100 kHz. MOSFETs at that time had bog slow body diodes so I had to isolate it with a Schottky rectifier and then add a fast recovery diode to handle the freewheeling current. Fortunately Unitrode made a 50 ns, 50 A part but it was only available in a stud mount package. I recall that the forward recovery time was doggone fast (<< 100 ns) and more of a function of the loop area and inductance. If I remember my device physics correctly, an increased forward voltage simply causes the diode's depletion zone to close and conduct quicker. Within a few years, similar parts were available in TO-247 style packages and a good bit of the series inductance went away.
I haven't designed with IGBTs. I understand that they do not have body diodes as do MOSFETs, however they have other parasitic devices like a thyristor. I wonder if the IGBTs that have anti-parallel diodes add another die to the package or do some sort of trick with a diode connected transistor hence the higher forward voltage drop? Maybe it's just a smaller die that is optimized for speed and has a higher VF.
BTW, I tore down a 20 year old Mitsubishi VFD that had popped its IPM. I wanted to see if I could convert it to generate a variable frequency PWM pulse train to drive an external H bridge for a single phase lab test source for the lab. The potting compound was clear and I could clearly see the damaged IGBTs and that it had separate freewheeling diode dice.
Cheers,
nuno:
--- Quote from: Yansi on June 10, 2020, 07:54:23 pm ---I've yet to see a diode with any significant forward recovery. As far as I know, it does not exist. Turn on time is dominated by parasitic inductance. Or not? I stay being corrected.
--- End quote ---
Not entirely sure if we can draw conclusions about the forward recovery, but I remember seeing almost 2us turn off response latency difference in using a 1N4148 versus a BAT85/54 as an anti-saturation diode for a small signal BJT. Of course, a us may or may not be significant depending on your application...
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