# Thyristor Forced Commutation

#### HotFurnace

Joined Mar 31, 2018
26
Hi Everybody,
As you know, thyristor are a bistable switch and are unable to switch off once on, unless the external circuit forced the ANODE current to go down to zero.
I really wonder, what if, for example a thyristor is conducting 10A from anode to cathode, then my external gate circuit forced the cathode current to zero (by making the gate voltage go negative related to cathode, all the 10A flowing into the anode now flow out of the GATE, and none flow out of cathode) for some time higher than the commutated turn off time, then could the thyristor turn off? In short, is there any possible commutation mode of the thyristor, except making the anode current to go to zero?
If we consider the following equivalent circuit, then we can see it is possible If all the current is taken from the NPN transistor by the gate circuit, then after some time it will recover and then we stop the gate current and the anode current too.
However, if we consider the real physical structure of the thyristor, then? Any master please help me out.
I added a picture to demonstrate what I mean Simulation link: http://www.falstad.com/circuit/circuitjs.html?cct=\$+1+3.4722222222222224e-7+0.20306040966347483+50+5+43 g+-160+240+-160+288+0 r+-144+112+-192+112+0+1000 177+320+144+304+192+0+311.1270049804743+311.127004980474+1+1+1e-9 v+256+304+256+272+4+5+20000+12+0+0+0.159154943092 r+240+256+240+224+0+10 w+240+256+256+272+0 g+256+304+256+320+0 v+320+32+320+96+0+0+40+311.1269837220809+0+0+0.5 g+320+32+384+32+0 g+320+192+320+224+0 c+272+240+256+272+0+0.000003+3.076680941197582e-13 r+272+240+288+192+0+0.1 w+240+224+272+240+0 l+320+96+320+144+0+0.00009999999999999999+0.0003111699402228361 r+384+144+384+96+0+50 w+384+144+320+144+0 w+384+96+320+96+0 o+11+1+0+4353+10+102.4+0+2+11+3 o+14+1+0+4099+1280+25.6+1+2+14+3

#### HotFurnace

Joined Mar 31, 2018
26
Yes, I know about that already, but will ordinary thyristor turn off just like a GTO do, provide that I had enough current handling capability on the gate side of the thyristor to conduct all the anode current and maybe even more?

#### ian field

Joined Oct 27, 2012
6,543
Yes, I know about that already, but will ordinary thyristor turn off just like a GTO do, provide that I had enough current handling capability on the gate side of the thyristor to conduct all the anode current and maybe even more?
If that was good to go - a lot less people would've bothered developing elaborate commutating circuits.

#### Janis59

Joined Aug 21, 2017
767
Firstly, for amperage until 50...100 A and voltage up to 1,7 kV exist a plenty of cheap igbt what are thousand times faster as SCR, have no need for witchcraft about switch-off capacitor etc etc.
However if the aim is more than 2 kV and Amperes are at least 2000 or more, then sorry, the scr indeed is still the best choice however painful. Normally people try to not use them in DC/DC, but are using at AC/DC or AC/AC, anjd then Your problem never arise.

#### ebp

Joined Feb 8, 2018
2,332
I recommend seeking some info on the actual physical structure of an SCR die, noting the physical arrangement of the gate region and how connection is made to the gate. Look also at the structure of an "amplifying gate" which is used in some high-power SCRs to aid turn-on.

A big impediment to trying to pull sufficient current from the gate to turn off the anode-cathode current is that the gate region is very thin and spread out over the whole die area. The connection to the gate is, of necessity, quite localized. To get the device to turn off you would have to mange to get all of the carriers that are blasting through the gate region to find their way to the gate terminal. The thinness of the gate makes its resistance comparatively high so while you might deplete the carriers near the metalization connecting to the gate you would have a very hard time getting out the carriers farther way. The way the gate is arranged even makes it somewhat of an issue to get the SCR turned on properly in the first place. To get enough carriers into the gate so that turn-on is reasonably uniform over the whole die area, thereby avoiding "hot spotting" where part of the die turns on and conducts too much current for the area, it is common to drive the gate with a current pulse far higher in magnitude than the minimum required to start turn-on.

#### crutschow

Joined Mar 14, 2008
23,359
but will ordinary thyristor turn off just like a GTO do
To sum up the above, no.

#### HotFurnace

Joined Mar 31, 2018
26
Hi everyone,
I'm sorry I should have mentioned at the beginning of the thread that I just need a theory explaining why I can't turn off the thyristor without forcing anode current to zero. In practical yes like what you say using commutation circuit or IGBT is much cheaper& more effective, but I want to know about the theoretical aspect. I'm not an engineer, but a high school student.

#### Janis59

Joined Aug 21, 2017
767
1) Then You ought go to the forum section "help in homeworks"
2) But, to understand the reasons WHY is happen what is happen there may be enough with analyzing the VoltAmpere characteristic of thyristor - there are more than obvious that is only one way to get out of "clinch" toward graph beginning.

#### HotFurnace

Joined Mar 31, 2018
26
It had nothing to do with homeworks(this stuff is only taught at university) , it just my curiosity. So not sure, anyway i will contact a moderator so he could move this thread to the right section.
What voltamp plot of the thyristor you are talking about? Thyristors are three terminals device. If you say it is anode cathode plot then one terminal (gate) is not considered?
I would rather to test with a thyristor... Hope it will not be destroyed in the process...

#### ebp

Joined Feb 8, 2018
2,332
There is nothing whatever wrong with putting the question in this section. The "Homework Help" section is appropriate for students asking for assistance with assigned problems, mostly so people who answer can do it in ways that provide guidance and foster learning instead of answers with all the details. (It does have the advantage that you will not likely get "why would you want to ... you should ..." non-answers.)
It is quite beyond me to see what a the "VoltAmpere characteristic" plot is supposed to reveal with regard to your original question.

If you were to use two discrete transistors, as shown in the diagram at #1, then you most definitely could turn it off by connecting the "gate" to the "cathode", but that circuit only approximates an SCR.

If you are going to experiment, I suggest using a small thyristor and a setup that limits the current to quite a low level (say 50 mA or even less depending on the SRC's hold-current spec - it could be as little as a milliamp with a sensitive gate device like the EC103 series). Of course you also want to use a power supply voltage that is safe - maybe 12 volts.

#### Ludens

Joined Nov 12, 2014
3
I will give you a simple answer, based on the two-transistor model:

If you pull the gate low, you turn off the NPN transistor, but not the PNP one. The current continues to flow through the PNP and the gate. As soon as you release the gate, the PNP transistor, being on, turns back on the NPN, and the SCR continues conducting!

For an SCR to be off, both transistors need to be off. If any of the two is turned on, for example by applying a gate pulse, it will turn on the other. To turn off a common SCR, you need to turn both transistors off, not just one. And that can't be done via the gate.

#### crutschow

Joined Mar 14, 2008
23,359
If you pull the gate low, you turn off the NPN transistor, but not the PNP one.
If the NPN is off, then no PNP base current can flow, so how can the PNP stay on?

#### ebp

Joined Feb 8, 2018
2,332
You can also turn off the PNP by shorting its base to its emitter, stopping the base current to the NPN and turning off the whole assembly.

#### HotFurnace

Joined Mar 31, 2018
26
Sorry for reviving the thread, but I must add information for who wonder why thyristor can't be turned off:
Please take a look at the first diagram (the actual construction of the thyristor), we can see there is 4 layer: pnpn.
When this stack is turned on (or the middle n-p junctions switch from blocking to conduct) , regardless of the gate current (the gate might sink or source current), if there is anode current larger than the holding current, then the aforementioned anode current always go through the middle n-p junction, and therefore it must remain conducting.
If we want to turn off thyristor, the n-p junction current must be exhausted for charge recombination to take place, ensuring the transition of the n-p junction from conducting to blocking. Or in other words, the anode current must be switch off.
This is the reason, in theory and in practice, why a thyristor can never be turned off without first switching the anode current off.

#### atferrari

Joined Jan 6, 2004
3,456
I recall using a thyristor as a cutoff in a PSU in case of a short circuit. To restore it to normal operation there was a button you pressed to deviate the current to common making current through the thyristor =0.
Current was, IIRC, around 50 mA.

Not practical for high currents, I know.