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thyristors in series


thyristors in series  

  1. 1. thyristors in series

    • turn on time
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    • gate command
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    • increasing the rating
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Please help me to solve a problem.
I work to a project with power thyristors in serial connection. The problem is linked to the simultaneous turn of all the thyristors.
If some of the thyristors are turned on before the other, then a very high voltage is applied to the terminals of one or more thyristor for a certain time interval, and this can provoke the destruction of the thyristor.
The turn on of the thyristors is commanded by a gate current.
All of the thyristors are of the same tyme but, of course, they are not perfectly identical: this is the problem because, the time interval of the application of the gate current in order to turn on the different thyristors is a little bit different from one thyristor to other. This little difference can provoke the destruction of certain thyristors by application of an overvoltage.
Can anybody help me with this problem?
Thank you in advance.
Alex

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Hi alex,

You can’t use thyristors in series to increse the voltage rating. It is not possible to get accurate enough trigging for a reliable circuit. It might work if the two thyristors where made on the same chip (substrate) with a common gate and shearing the same temperature but it’s simpler to use a correctly rated thyristor instead.     

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In fact I am not sure you have understood me well.
I will try  to explain better the matter.
I use the thyristors in series in order to have a greater rating, to have a greater voltage at the 2 terminals of my set of thyristors.
For exemple, let us use 2 thyristors in series.
If one of them turns on before the other, then a too great rating will be applied to the terminals of this thyristor, and it will be destroyed.
This difference of time can be due to the different physical characteristics of the 2 thyristors.
Let me explain.
We have to apply a current on the gate of each of the thyristors before they turn on. And this operation must take a certain amount of time. This amount of time is, of course, a little different from a thyristor to another, even if they are of the same type.
Well, my problem is to know whether this little time difference is big enough to provoke the destruction of the turn-off thyristor.
Thank you very much in advance.
Alex

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A thyristor doesn't get distroyed when the terminal voltage exceeds the rating on the datasheet, it just acts like a diac and triggers with no input pesent, you could connect them in series and if one turns on before the other the ther would just turn on anyway.

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Hi alex,

Yes, I thought you meant just that. I still say the explanation is valid for such a scenario.
Bottom line: It won’t work properly. So, why not just use one single thyristor with high enough voltage rating or maybe the voltage is extremely high and no such thyristor exists? 8)

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Hi Alun,

So you consider this an acceptable circuit solution instead of using a single thyristor with the correct ratings then?


Of course using a componant with the correct rating is better but I was making the point that thyristors aren't normally damaged by forward overvoltage if the current is limited to a suitable value by the load.
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