Leaky thyristors, part 2

24.10.2010 12:14

A while ago I wrote how thyristors in my power supply seem to be conducting a small, but significant current in the reverse direction.

Yesterday I took a handful of thyristors and some equipment and made a few measurements in an attempt to get to the bottom of this effect.

This is the circuit I used:

Measurement circuit for reverse anode currnt in SCR.

I measured Bourns SCR type TIC106D and TIC106N. Both have a specified peak reverse current of 1 mA and gate trigger current of 200 μA.

Note that this circuit operates the SCRs in the 4th quadrant - Gate-cathode voltage is positive while anode-cathode voltage is negative. While this is a valid mode of operation for triacs, I haven't seen any discussion of behavior of SCRs under these conditions. The reverse voltage is significantly smaller that allowed peak reverse voltage (400 V for TIC106D and 800 V for TIC106N)

Here are the results. Red graph shows measurements of two different TIC106N elements while the blue graph shows measurements for one TIC106D:

Reverse anode current versus gate current in SCR.

It appears that approximately half of the gate current flows into the anode instead of the cathode under these conditions. Also, lower the rated voltage of the SCR larger the reverse current.

Gate current used here is much larger than the minimum trigger current required. However, I've often seen advice that thyristors should be triggered by current pulses with an amplitude much larger than the required trigger current.

I would welcome any explanations of these measurements from the perspective of the two-transistor model.

Posted by Tomaž | Categories: Analog

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