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Driving a Igbt

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Kolas

Mon Jan 12 2015, 06:10PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

haha i never thought of that but yes! that will work!! very cool. remember that isolated power will be needed for all of these gates.

Andy

Sun Jan 18 2015, 06:21AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Kolas, with the isolation is that optocoulpes for driving ics?
Something along the same lines but different part of the circuit, im trying to workout or understand pnp mosfets, I dont really understand the below picture with recards the to resistors, the divided voltage is 1.812 volts on the gate?

Cheers

1421562091 4266 FT167928 Img 20150118 191744

With the mosfet instead of switchs.

mister_rf

Sun Jan 18 2015, 09:15AM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

By the way, thatâ€™s a bipolar transistor H-bridge.
And in practice thereâ€™s a difference for using BJT transistors vs MOSFET transistors.

For BJT normal polarization the base emitter voltage Vbe = around 0.6 volts up to 0.7 volts

Using MOSFET the excessive voltage will punch through the gate-source oxide layer and result in permanent damage. Notice that we need to use a voltage clamp (a conventional zener diode is suitable for this purpose) to prevent the gate-source voltage rating from being exceeded.
The gate â€“ source voltage will not reach more than the zener voltage.

Kolas

Mon Jan 19 2015, 06:05AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

I see lines blurring here somewhat. BJT's do "switch" much differently then igbt's and mosfets to a lesser extent.
For my own purposes I treat both mosfets and igbt's in much the same way; in that they switch from 0 to 1 with very little time. One way to achive this is by what is called a GDT or gate driver transformer.

This is because typically two or more of these switches need to be driven at once, and in that case it is even more often that they need to be switched exactly opposite of each other. Hence the transformer. To get exactly inverted signals, all one must do is flip the polarity of one of the windings. GDT's also provide decent isolation from the gate drive circuitry.
Generally in this case, both igbt's and mosfets should be provided as square a wave as possible so that rise and fall times are short.
It is worth noting that SCR's can also be driven by GDT's, however they trigger, instead of switch.

Optocoupling can also be used, and are certainly getting popular for SMPS design. However it is important to note that they are just non linear signal transistors and can not: A) switch much power B) switch anything without a secondary power source. so at the end of the day, optocouping is probably going to end up being an unnecessary complication.

Your goal was at one point to discharge a bank of capacitors. If that is still your goal, I would like to reiterate my suggestion that you use SCR's over any other type of solid state switch. They can handle much larger impulse currents and have very little in the way of losses. They are also fairly simple to work with and are very robust and forgiving. For an avalanche switch, there is no better suited a device that i know of.

Andy

Mon Jan 19 2015, 01:51PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thanks mister rf, I just don,t get the negtive voltage on the gate compared to source, and how to acheive it. I understand you picture were the zener makes a voltage drop going up, but down the resistors make the drop, is that correct?.
Kolas I apprecate your help, and plan on using a Scr for the discharge part throught water, and the can crusher part for magnetic feild, but ive run into a barrier, I would like to get more thrust than one kg per two kilowatts per hour, so trying to make a barrier discharge system to lower the ohms of the discharge part of the circuit throught water.
The goal is 15kv @ 25mA @ 50khz, which if the circuit above works and I can understand p enchered mosfets should do.

Cheers all

Kolas

Mon Jan 19 2015, 04:57PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

To drive a p and n mosfet is the same. Reasons I know for using p channel mosfets are inversion of a signal, and creating a bridge without the need for isolation in a mid level inverter stage. The resistors are for biasing of the gate so that you can operate said devices in their linear region. It is not particularly necessary to drive the mosfet's gate below 0V. It is done so often because over driving causes the switch to switch more reliably and insures faster gate charge dissipation.(I think) It should not be needed at low frequencies with small mosfets that have small gate charge(s.)

Signification

Sat Feb 21 2015, 07:05AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Andy,
have you ever looked at this guy called SUBCOOLEDHEATPUMP on youtube? You should...all kinds of very clever things from start to finish about how IGBT's work and how to drive them.

And I also prefer SCR's mainly because of their absolutely huge pulse rating for a firing cycle of ~8-10ms. I have a puck type with a pulse rating of 56,000 Amps at 8ms. I plan to use it with a coil gun.

...cool idea about the capacitor-charging / SCR hold-off scheme!

Newton Brawn

Sat Feb 21 2015, 11:16PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Andy,
Do you mind especify the load characteristics ?

Series Resistance

Capacitance, inductance.

Schematic

Cheers

Andy

Sun Feb 22 2015, 12:05AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi Newton
Still fine turning the designs, I was going to use a capacitor discharge but for power you need volts, but I only need amps, so will try inductors as some type of buck or trasfomer type.

I dont think a silcon switch would be the best, but will try have a 15 amp fuse to complete the circuit for the inductor, and use silcon switch to discharge a capactor throught the fuse to break the main circuit.

It will only be a one shoot obviously, so still working on it.

Looked on Mouser and there is a 2volt 60amp mains voltage dc to dc converter, but that dreaming getting that at this stage.

fred

Mon Feb 23 2015, 02:11AM

Registered Member #54441 Joined: Mon Feb 09 2015, 11:33AM
Location:
Posts: 9

discharging a capacitor quickly
reduce ESR.
the following pic shows one of my experimental fast cap discharge methods. large copper strips reduce resistance.
lots of smaller capacitors reduce resistance and provide for faster discharge/charge times.

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