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| Current pulse generator project |
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| PartialDischarge:
--- Quote from: daqq on December 17, 2018, 12:02:13 pm --- --- Quote ---This is one of the pending improvements, I have a couple of 10V zeners from G-E, but there is ringing in the gate, and for short time it crosses the 20V barrier, will add a resistor to damp it also. --- End quote --- This could be a measurement issue. When measuring this kind of thing, it's better to do it differentially (hook one probe directly onto the GATE input, a second one on the EMITTER (the terminal close to the gate input) and do a substraction of the two), since when dealing with 100s of amps, grounds tend to bounce about. --- End quote --- Yes, could be, these fast signals are hard to measure accurately, although I'd rather use my 7A13 plugin since digital scopes are bad for this kind of diff measurement. Anyway the current trace also shows some ringing so I believe its coming from the GE. |
| duak:
When I was debugging fast power FET circuits, I found that the 'scope would show a damped ringing during the circuit's transitions even with the probe connected but grounded to itself. This was due to HF common mode currents from the circuit passing thru the probe and scope. I could eliminate them by passing the probe cable thru a high mu common mode choke a few times. Another use for a high mu choke is to use it as a pulse compressor or intensifier. The theory is to apply a voltage pulse to a high mu choke in series with the load. The choke's inductance will initially limit the di/dt to allow the switch to fully close. At some point the current saturates the core of the choke and it essentially shorts out and not longer limits di/dt giving a sharper, higher current pulse. If another parallel C and series L is added, the pulse can be further compressed. I found this effect when performing ESD susceptibility tests on a product when a common mode choke saturated and passed an intensified pulse to the power supply which then glitched. I then found that this effect was used to generate narrow pulses for lasers and radar transmitters, far sharper and narrower than you'd think the switching devices would be able to generate. I found a few papers on drivers using this effect, but no tutorials on how to roll your own - an exercise for the student, I suppose. I've never tried to use this effect, so I can't give much direct advice. If memory serves, a 10 A pulse became a 30 A pulse with about 1/4 the duration. Intuitively, it might help here because it could allow the IGBT to operate within its safe operating area by reducing Vce more rapidly during turn on, perhaps compensating for the Miller effect. Cheers, |
| David Hess:
--- Quote from: MasterTech on December 17, 2018, 08:04:17 am --- --- Quote ---What about grounding the gate of the IGBT and driving the emitter with the drain of a power MOSFET which then only has to sustain low voltages? --- End quote --- Not a bad idea, and wouldn't mind having a negative rail, but I had already this drivers which are overkill here since I don't use the isolation or the protections but.. --- End quote --- The driver drives the MOSFET instead. The IGBT gate is then tied to a low impedance positive bias supply high enough to fully turn it on. Bipolar designs that work this way are more difficult because the positive base bias has to supply high current but this is often done with a separate low voltage high current winding off of a transformer. They do not literally ground the base. Higher performance designs yet use special packaging for the transistor which does allow the base/gate to be directly grounded. In the past, you could get TO-39 transistors with the base instead of the collector tied to the metal case. Big stripeline RF transistors are still available with the emitter and base leads swapped. |
| PartialDischarge:
--- Quote from: David Hess on December 17, 2018, 08:25:46 pm ---The driver drives the MOSFET instead. The IGBT gate is then tied to a low impedance positive bias supply high enough to fully turn it on. --- End quote --- Got it, will try it in a future version. --- Quote ---I found this effect when performing ESD susceptibility tests on a product when a common mode choke saturated and passed an intensified pulse to the power supply which then glitched. I then found that this effect was used to generate narrow pulses for lasers and radar transmitters, far sharper and narrower than you'd think the switching devices would be able to generate. I found a few papers on drivers using this effect, but no tutorials on how to roll your own - an exercise for the student, I suppose. I've never tried to use this effect, so I can't give much direct advice. If memory serves, a 10 A pulse became a 30 A pulse with about 1/4 the duration. --- End quote --- Interesting, will try to look up something on this |
| PartialDischarge:
--- Quote from: daqq on December 16, 2018, 09:55:12 pm ---A steeper edge could possibly be achieved by using a higher resistance (thus lowering the L/R constant) with higher voltage. At these speeds you could be dealing with a limitation of the transistor. --- End quote --- Eventually this worked very well, I added another 3.3 ohms in series (3x10ohm paralleled) with the 2.2ohm and the rise time nearly halved. This limits the current pulse of this setup to about 200A, but with this I can test current probes of >2MHz bandwidth. I shortened the pulse to 5us for the sake of the resistors well-being. Here are the 50A and 100A screenshots 50A and VCE 50A and VCE 100A pulse rise time (545Volts VCE) 100A pulse (545Volts VCE) |
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