IGBT alternative? - Page 1
Forum section: electronics / beginners

author: ACDCAC
author_rank: Newbie
timestamp: January 19, 2015, 05:35:30 pm
content: Hi,I am trying to replicate a 2 self-commutating VSCs (Voltage Source Converter (rectifier/inverter)), as used in HVDC systems, on a much smaller scale using as little power as possible. To do this I am looking for an IGBT that requires a low gate current. From what I've researched so far, and by looking at datasheets, most IGBTs have a very low gate resistance (Rg ~10 Ohms) and typical gate voltages of +/-15 V which leads to a currents of over an amp. Ideally I would like to use only a fraction of that current for safety reasons. By increasing the gate resistance I would radically increase Turn-Off time, giving far less control of the commutation process. I'm looking at rectifying 240V or possibly less.So my question is- What alternatives to IGBTs are there for low current applications? Or is there such a thing as low current IGBTs?Thanks in advance for your help.

author: dannyf
author_rank: Super Contributor
timestamp: January 19, 2015, 06:28:25 pm
content: QuoteBJT, mosfets, scr, triac, or low-current IGBT?

author: klr5205
author_rank: Regular Contributor
timestamp: January 19, 2015, 06:52:00 pm
content: Alternatively, you could consider using a gate driver IC which is specifically designed safely to handle current pulses on the order of a couple amps. They usually have some handy protection circuits built in as well.

author: owiecc
author_rank: Frequent Contributor
timestamp: January 19, 2015, 07:31:53 pm
content: IGBTs have insulated gate (IGBT) therefore it only needs power for charging the gate capacitance. It needs amps in very short pulses. Gate driver ICs will help you doing just that. There is no safety hazards in charging and discharging gate capacitances with amps as gate drivers operate with low voltages.If your voltage levels are below 300V you should consider using power MOSFETs.

author: ACDCAC
author_rank: Newbie
timestamp: January 19, 2015, 09:46:18 pm
content: Thanks for your help guys!dannyf: Triacs and SCRs are line-commutated I think? I didn't want to use BJTs as the gate isn't insulated. Is there such a thing as a low-current IGBT because that would be great! My limited experience with reading datasheets could be holding me back in finding 1- every IGBT I've looked at seems to require a gate current >1A.klr5205 and owiecc: I should have said in my original post that I am using a 6 pulse bridge for each converter, so that's 6 gates per converter to be supplied which is increasing the power needed. I really would prefer to use lower currents but if there is no other way using a gate driver chip will be the way I will go.MOSFETS seem to be the theme of your responses so I'm off to research them now. I know they are similar to IGBTs but my knowledge of their use is limited, why might they be a better option to IGBTs?

author: owiecc
author_rank: Frequent Contributor
timestamp: January 19, 2015, 09:56:53 pm
content: You need a 1A pulse for few microseconds. There is very little power loss involved in the gate circuit. Really, don't worry about the amps at this stage.MOSFETs have better conduction characteristics at small currents (they behave like resistors). IGBTs are better at higher currents. MOSFETs are generally faster so switching loss is lower then IGBTs.Powering gate circuits will be your next problem. Bottom gate drivers are easy. Top ones require special attention (isolated power source/bootstraping).

author: dannyf
author_rank: Super Contributor
timestamp: January 19, 2015, 10:08:58 pm
content: QuoteThat's because of the way you asked the question - very little context provided.IGBTs are really BJTs driven by a mosfet. As such, they are naturally suited for switching at (very) high current levels (where ohmic losses from a mosfet dominate) - we are talking about 100amp or over.So if all you need is to south of that, consider a mosfet or a bjt.QuoteNot sure why that's important to you but if you really want the "gate" to be insulated, put a mosfet in front of your low-current bjt now you have made yourself a small current IGBT.It makes no sense but does precisely what you want.

author: Leadfootin
author_rank: Contributor
timestamp: January 20, 2015, 01:01:32 am
content: IGBT's can be very useful down to as low as 3 amp ratings. The big caveats, same as mosfet are turn on, turn off and dv/dt considerations. I recently blew thru the gate junction of a 30 amp IGBT with only 50 ma being switched. Not enough load on a 15mh inductor regulated boost converter for a geiger counter.They are great for switching inductive loads as long as voltage rise rates are controlled during turn off and cross conduction is avoided - thus the advice to use the recommended driver chips.

author: bugs
author_rank: Contributor
timestamp: January 21, 2015, 03:14:14 am
content: http://en.wikipedia.org/wiki/High-leg_deltaIs this the net you want to connect to?Does it have to be bidirectional?Remember that MOSFETs and IGBTs are not self commutating nor reverse blocking, so you probably have to put a diode in series.

author: ACDCAC
author_rank: Newbie
timestamp: January 22, 2015, 07:32:13 pm
content: Hi guys,I've attached a picture of the LTspice circuit I'm trying to make work. I tested a single IGBT circuit and got it to work but sticking 6 of them together is proving a problem.The primary side of each driver chip represents the control signals from my micro-controller, each on for 120° and delayed until their conduction period is due to start. The secondary side isbasedon the second picture I attached from the driver chip datasheet.For this simulation, I'm using this IGBT:[url=http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00003695.pdf]http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00003695.pdfAnd this Driver Chip:http://www.avagotech.com/docs/AV02-0161ENThanks for your patience, any advice is much appreciated, I'm sure there is more than one problem with it.

author: bugs
author_rank: Contributor
timestamp: January 24, 2015, 08:30:45 am
content: I suspect that the body diode is causing unwanted effects. Try putting an extra diode in series with each IGBT.