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Registered Member #537
Joined: Sun Feb 18 2007, 07:22PM
Location:
Posts: 10
Shaun: At the science fairs that I've been to almost everyone says that I need to shorten the wires. When I get around to making a portable version 2.0 that'll be a priority. Thanks.
Sofi: I thought about that solution, but I was worried that it would both rob me of efficiency, and possibly blow the resistor because of the larger current draw during firing. I actually was originally using an ir2110 to drive the gates, except they kept overheating and exploding. I guess I was hooking them up wrong or something. It didn't just overheat when I was firing, as soon as I applied power it started heating, then a few minutes later it would explode, releasing the magic blue smoke
I'll will definitely be getting some fast or ultrafast diodes.
I was initially hesitant to use a breadboard, but I've seen a couple other high voltage boost converters built on them. I haven't had any problems with arcing over, which was the only thing that I was worried about. I did end up melting a bit of the plastic when an IGBT that I was using blew when one of my diodes went bad, but that was the extent of my problems. The high current sections of the circuit are all done in 10 gauge wire. If I were working with voltages on the order of maybe 1Kv, I wouldn't use a breadboard.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Great work. I was lucky enough to get to see your project in person and I was very impressed. Im glad you decided to post your work for all to see. If your interested, my ISEF project log is here.
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
Yeah, I've destroyed a lot of IGBTs. I had 3 smaller IGBTs in parallel to get a higher current carrying capacity, but they kept blowing, I think because they had a negative thermal coefficient, so one of them would get thermal runaway, then the others would blow because they couldn't handle the current. I got tired of trying to get the smaller IGBTs to work, so I bought the beefiest ones that I could find.
Some IGBTs have a positive temp coefficient and are designed for parallel operation, most have a negative temp coefficient and do not want to share. This can be helped by puting a very low value resistor in series with each IGBT to even out the currents.
What speed diode do you recommend that I use? I wasn't aware that the difference between standard and high speed diodes would be significant enough to make a difference. The IGBTs that I'm currently using seem to be tolerant of these voltage spikes, though. They're rated for 1200 volts.
In theory with the half bridge design the voltage accross the coil is clamped by the diodes to the capacitor voltage. In practice there is a delay between the IGBT switching off and the diode switching on, during this time the voltage will shoot up very rapidly. The rate of rise of voltage is limited only by the output capacitance of the IGBT. Standard rectifier diodes are very slow (1000nS) in this time the voltage could reach 1000s of volts.
I didn't think that I needed a gate drive resistor. Could you please explain what purpose they serve? I just thought that you wanted to turn the IGBTs on and off as fast a possible, thus no gate resistor.
The IGBT gate has some capacitance, the wires to the IGBT have some inductance, together they form an LC circuit. When the driver IC goes from 0V to 12V the gate will actually shoot up to 24V and then oscillate between 24V and 0V, the resistance of the circuit is usually low (10 gauge wire) so the oscillation will continue for many cycles until the voltage settles at 12V. During this time the IGBT is trying to switch on and off very rapidly as the gate voltage swings up and down, this leads to huge switching losses. At turn off the same thing will happen but the gate will oscillate between -12V and +12V until it settles at 0V. A gate drive resistor (10R or 20R) will provide damping and actually help the IGBT to switch faster. If using parallel IGBTs you need a resistor for each gate.
I was hoping that using the half bridge design would mean that I didn't need snubbers. The modules have an anti-parallel diode, so that may help them to avoid blowing from reverse voltage, but as far as I know that's all they have.
The Toshiba modules also contain a zenner clamp between the gate and emitter to protect against overvoltage on the gate.
I was worried about the long wires, but I didn't think that they would matter that much considering that I'm driving a coil that has a much higher inductance. I guess that the next revision will have either shorter or flat wires to limit the stray inductance.
It`s not the wires between the coil and the bridge that matter, its the wires between the bridge and the caps that will cause the problems. Also try to keep the gate wires short for the same reason.
All of this is somewhat academic as you have got the coilgun to work successfully already. I hope this discusion will help you see why some people avoid using IGBTs and why some people who do use IGBTs see a lot of magic blue smoke
Registered Member #1544
Joined: Sun Jun 15 2008, 02:18PM
Location:
Posts: 2
OZZY
wrote ...
In theory with the half bridge design the voltage accross the coil is clamped by the diodes to the capacitor voltage. In practice there is a delay between the IGBT switching off and the diode switching on, during this time the voltage will shoot up very rapidly. The rate of rise of voltage is limited only by the output capacitance of the IGBT. Standard rectifier diodes are very slow (1000nS) in this time the voltage could reach 1000s of volts.
Standard rectifier diodes and ultrafast/fast/whatever diodes turn ON at the approximately same speed, in practice instantly. So there is no voltage peak acccros diode at turn-ON
Fast recovery diodes turn OFF faster than standard diodes but it is a lesser(or non-existent) problem in a coilgun use. When standard recovery diode is conducting and voltage is reversed so that diode is in blocking state it takes some time to sweep off charge carriers from the pn-junction. This can be a problem in switch-mode power supples etc. because of high peak current during reverse recovery causing losses and heating but hardly in a coil-gun half-bridge
Registered Member #511
Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55
mzzj wrote
Standard rectifier diodes and ultrafast/fast/whatever diodes turn ON at the approximately same speed, in practice instantly. So there is no voltage peak acccros diode at turn-ON
My mistake I misunderstood the term "reverse recovery time". Your comment has prompted me to try and find out how diodes really behave. The best explanation I can find is here Diode turn on time. His tests show that a standard rectifier diode has a forward recovery time of a few nanoseconds but a reverse recovery time of a few microseconds.
However this does not explain the overshoot common when switching inductive loads. More searching turned up this Application note. This suggests that the overshoot is caused by stray inductance in the circuit not clamped by the diode or by stray inductance in series with the diode. If the diode is capable of switching on in 3nS and conducting 100s of Amps then the dI/dt is huge so any inductance in series with the diode could develop a significant voltage.
Registered Member #537
Joined: Sun Feb 18 2007, 07:22PM
Location:
Posts: 10
Well, I just got my hands on a bunch of UF4007 diodes, so I'll probably put a bunch of them in parallel. Anyway, I just got my research paper together and I thought I'd post it here. The equation referenced in the figures and appendix 1 is not the correct equation, but the equation in the discussion and introduction is correct. The major difference between the two equations is that the one in my original post here accounts for the changing inductance of the coil as the projectiles moves through it. Other than that the equation in the research paper is as correct as I know.
Registered Member #1544
Joined: Sun Jun 15 2008, 02:18PM
Location:
Posts: 2
OZZY wrote ...
mzzj wrote
Standard rectifier diodes and ultrafast/fast/whatever diodes turn ON at the approximately same speed, in practice instantly. So there is no voltage peak acccros diode at turn-ON
My mistake I misunderstood the term "reverse recovery time". Your comment has prompted me to try and find out how diodes really behave. The best explanation I can find is here Diode turn on time. His tests show that a standard rectifier diode has a forward recovery time of a few nanoseconds but a reverse recovery time of a few microseconds.
However this does not explain the overshoot common when switching inductive loads. More searching turned up this Application note. This suggests that the overshoot is caused by stray inductance in the circuit not clamped by the diode or by stray inductance in series with the diode. If the diode is capable of switching on in 3nS and conducting 100s of Amps then the dI/dt is huge so any inductance in series with the diode could develop a significant voltage.
Ozzy
Yes, stray inductance is the main evil causing voltage spikes. As a rule of thumb 1cm of wire has 10nH (25nH/inch) of inductance and in a good layout there could be approx 5cm of wiring, component legs and internal current pathways on components. This would give us rough estimate of inductance as 50nH. Switching speed could be for example 100A in 50nS, causing voltage spike of E=L*di/dt E=50nH*100A/50nS= 100V. If you use for example 500v igbt's with 350V supply voltage you have only 50 volts of voltage margin left in this imaginary example. There is two things to do to keep your mosfets/igbt's happy, keep all wires short with compact layout and control turn-off speed so that its enough SLOW. Proper gate drive is important, not too fast or not too slow.
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180J Half-Bridge Coilgun Complete
