How to improve switching time for SCR - Page 1
Forum section: electronics / beginners

author: IIS
author_rank: Newbie
timestamp: August 23, 2020, 12:30:46 pm
content: This is my 1st post so pls be patientIs it possible to force SCR to switch on faster ?I build some education test jig that drop out ~146J of energy by SCR.Below img of this pulse taken on 100A 75mV shunt .But curve that i have form scope don't look greateIt's rather slow. So i have 1 SCR 300A cont. current - its old polish tech device but can stand lot of ennergy in pulse.Question is - how to do it better ?

author: janoc
author_rank: Super Contributor
timestamp: August 23, 2020, 12:48:22 pm
content: You can try to overdrive the gate of the SCR, that will make it turn on faster a bit.Otherwise you have two options:Find a faster SCRUse an IGBT which switch much faster (order of magnitude)

author: IIS
author_rank: Newbie
timestamp: August 23, 2020, 02:36:57 pm
content: HI,As for IGBT - parts that I can afford are in TO247 case and it is 200A max current for single part. Now I have from 9kA to 23kA of current so that why i search some "voodoo" that i can apply to gate my SCR ( It M18 mount type ) Also what i see in example ofIGW75N60Isc is only 670A I think it will be hard to drive 20ish parts than single one.

author: janoc
author_rank: Super Contributor
timestamp: August 23, 2020, 04:17:18 pm
content: 23kA of peak current? What are you driving? A tram car??OK this is beyond my field of expertise, sorry. Someone smarter will need to answer.The problem with SCRs in general is that they are slow. And especially if you are using a communist-era recycled junker - those were almost certainly not designed for high speed switching. Such high power SCRs were likely meant for things like trams or trolleybuses and those inverters operated with frequencies from around 100Hz to 10kHz or so (that's where the characteristic high pitched noise of these vehicles came from too).Other thing that comes to mind is that parasitic inductance of the wiring will play a significant role too when you are trying to do an instant energy dump like that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2020, 07:53:44 pm
content: How much loop inductance? What supply voltage?Tim

author: bdunham7
author_rank: Super Contributor
timestamp: August 23, 2020, 08:54:44 pm
content: Can you provide the specific circuit you are using, including part numbers? And the vertical scales of your graph so we don't have to guess?I assume you are using a capacitor and charging it, then discharging it through the shunt. It's not obvious to me that your current pulse shape is caused only by the SCR rise time.Also, try putting the scope across the SCR if you can do it safely (mind the grounds). You might need to reconfigure the circuit to do this.

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2020, 12:52:13 pm
content: It appears you have a significant amount of inductance in the circuit.What rise time do you require?Are you're using a high enough voltage?v = Ldi∕dt

author: IIS
author_rank: Newbie
timestamp: August 24, 2020, 03:59:42 pm
content: HiMy PSU give 62V ( Capacitors are on 63V - generic type cap.)SCR is old Polish part PN T00-300-22-504All wires are short in box (max 10cm from cap to main bus )

author: janoc
author_rank: Super Contributor
timestamp: August 24, 2020, 07:50:10 pm
content: I wonder what sort of answer are you looking for that is going to be different than what you have got on this Polish forum already?https://www.elektroda.pl/rtvforum/topic3722967.htmlRitterX explained to you that thyristors are not any speed demons, that the one you have there was intended for a low frequency converter (likely for a tram or something like that), so its switching speed is not going to be great. Also the reason why SCRs are slow (due to their construction) has been explained to you by another person.And that you will have better results either with MOSFETs or IGBTs. And also how to choose those is explained here. The need for low inductance of the wiring is explained there as well.I have found your post when I was looking for some data on that SCR of yours. And the only two places that have turned up were some Polish auction site selling these and your post on that forum.

author: filssavi
author_rank: Frequent Contributor
timestamp: August 24, 2020, 08:34:45 pm
content: Even assuming you have an ideal switch (which most definitely you have not) 63V is way too low a voltage to be able to push that much current, capacitor ESR and ESL give you a hard limit of how fast energy can be pushed to the rest of the circuit, moreover the rest of the circuit will also have parasitic inductance and by judging at the pictures you have far too much wire and not enough busbars to push 100s of amps (let alone 23 kA)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2020, 09:02:36 pm
content: The plots are not from the same condition.This is about as close as I can get the voltage waveform, assuming the measurement is some mixture of voltage drops; this is a reasonable assumption as the resistance and inductance are distributed over the circuit, and can't be measured pure in any one location.The inductance is reasonable, corresponding to about a meter of wire. (Maybe not reasonable for whatever the intended purpose is, but that's another matter.) The resistance is, as well. The initial step-in of the shunt waveform cannot be so easily explained*, but the horizontal scale is completely incommensurate.*Probably, if I modify the simulation to include SCR rise-time, its steplike dI/dt, dropped across some fraction of loop inductance, will account for the difference. This is apparent in the initial slope of the voltage plot: the simulation assumes the pulse is instant-on, which is clearly not the real case.Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2020, 01:13:22 pm
content: I wouldn't normally use an SCR at such a low voltage. A MOSFET will switch much faster and have a lower voltage drop.

author: IIS
author_rank: Newbie
timestamp: August 27, 2020, 10:35:32 am
content: Thank you all for response. As for topic on other forum - i write also on this to check if someone have different approach to problem .I will try to make it on some mosfets - use 20 or 30 pcs and divide each to its own smaller cap. bank.