Output of IGBT not returning to 0V - Page 1
Forum section: electronics / beginners

author: sonnytiger
author_rank: Frequent Contributor
timestamp: December 02, 2012, 06:29:55 pm
content: I have the following circuit:The IGBT is a IXGR40N60C2D1, and basically i am measuring the voltage across a 300 ohm load and am getting a square wave as i should going to 60V but only down to 20V. It is not returning to 0 and this is the problem, reading the article on Wikipedia for IGBT's it seems i need a negative voltage in order to close off the channel formed from the applied voltage, can anyone confirm this and perhaps give me some solutions? or if it is not the case tell me what the problem appears to be? If you need anymore information let me know of course.Thanks!

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: December 02, 2012, 09:20:21 pm
content: Hi, got a part number for the igbt,I know nothing about IGBTs but what would happen if you switched low side?Seems like the gate voltage needs to be referenced to the emitter.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: December 02, 2012, 10:00:59 pm
content: ok i will try that shortly thanks!

author: Kremmen
author_rank: Super Contributor
timestamp: December 02, 2012, 10:44:20 pm
content: Definitely move the IGBT to the low side if you drive it like this.You don't indicate the load current, but since you have selected an IGBT one assumes it is not insignificant. In that case take care to drive the IGBT properly. Usually the driver circuits have guard circuits against loss of saturation and control of dv/dt and di/dt. On the other hand the main circuit voltage in your schematic is pretty low for a typical IGBT.But changing to low side switching will solve your primary issue.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: December 03, 2012, 12:50:38 am
content: the load current will be quite low in fact. less than 2A at the absolute most. i simply had it lying around and it was convenient as i would not have to change the output voltage on my 555 to drive another transistor. As for reversing the source and drain leads or emitter and collector leads (the datasheet uses both terms for some reason, another thing i would like explained) all that did was old the output on all the time, not what i wanted and obviously not the proper wayt to connect it. now, i tried it in the above configuration, but at a lower voltage (11VRMS) so about 15 peak. I also connected the load intended which is a large air core inductor (~2200 turns of 30AWG wire) so to be expected i go spikes of voltage in the negative, just like id expect to get measuring the voltage across the inductor. Also notice the fact that it is returning to 0V or very close, zooming on the waveform with my scope reveled it is a few hundred millivolts above 0V. I am not willing to try it on the full 60Volts peak due to my fear that the spikes might ruin my scope but perhaps the negative spikes are turning the transistor fully off whereas with the resistive load there are no spikes so it remains floating? any ideas as to why it remains floating at 20V with the resistive load and the full 60Vpeak are welcome. Please help! hahasorry i also moved the transistor to the low side in all my tests, i just put it in the wrong place on my schematic.haha another edit I assume when you say move it to low side switching you mean put the transistor on the other side of the load? IE connect the load to ground with the transistor?

author: Kremmen
author_rank: Super Contributor
timestamp: December 03, 2012, 08:37:33 am
content: Yes, that is what low side switching meansTo avoid the inductor from spiking everything around itself to ruin, you should include a freewheeling diode in the circuit to manage the inductive kickback. See attached pic what i mean.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: December 03, 2012, 10:56:03 am
content: yes i have done this and it helped of course, i initially didn't do so because the datasheet indicated that the IGBT already had one built in but obviously id didn't work. so yeah, i need to try it out on the higher voltage when i get home from school but so fa it works perfectly on 12VHave to see when I get home.

author: sorin
author_rank: Frequent Contributor
timestamp: December 03, 2012, 04:13:05 pm
content: Use NPN or mofet

author: sonnytiger
author_rank: Frequent Contributor
timestamp: December 03, 2012, 06:04:20 pm
content: I want to get to the bottom of this not just give up and use something else. I want to know why its doing this s I can learn from it.

author: SeanB
author_rank: Super Contributor
timestamp: December 03, 2012, 06:24:36 pm
content: The diode in the IGBT does not allow the current in the inductor to circulate, you need one across the load, either direct or with a resistor in series to absorb the energy ( longer time to decay to zero, if directly across then the load resistance does this). Consider that when you switch off the current in the coil is tending to keep flowing, and then pulls the junction to a high positive voltage, biasing the diode in the IGBT and the IGBT transistor firmly off until either the energy is dissipated or the IGBT breaks down and dissipates the energy.

author: sorin
author_rank: Frequent Contributor
timestamp: December 04, 2012, 09:31:51 am
content: http://www.amplimos.it/images/IXTH16N20D2.pdfThe IGBT are for hight power hight voltage not fot 421VA