Using zener diode as a switching diode?

[treez]

Hello,

I wish to add a diode to the gate of a thyristor in a trigger circuit for a xenon flash lamp.

This is because the BT168G thyristor gate jumps below 5V negative when the thyristor is triggered, and i want to stop this.

However, i do not want to add another new part to the BOM......so....i already have a 30V zener diode (BZX84 C30) in the circuit......can i just use it as a switching diode for my use at the thyristor gate?

Surely it will switch on quick enough?
(of course, it wont be seeing use as a zener diode)

 

[ishailesh]

**broken link removed**

Well BT168G is a Gate Assisted Turn Off Thyristor. The GATT is used in circumstances where a fast turn-off is needed. To assist in this process a negative gate voltage is applied. In addition to reducing the anode cathode voltage ,This reverse gate voltage helps in draining the minority carriers stored on the n-type base region and it ensures that the gate-cathode junction is not forward biased.The structure of the GATT is similar to that of the standard thyristor, except that the narrow cathode strips are often used to enable the gate to have more control because it is closer to the centre of the cathode.

 

[FvM]

This is because the BT168G thyristor gate jumps below 5V negative when the thyristor is triggered, and i want to stop this.

Not understandable without circuit information where the negative voltage comes from. Are you talking about a real circuit measurement or a simulation result (possibly an artefact)?

Anyhow, if you're worried about a negative gate voltage, you would want to place an antiparallel 1N4148 between gate and cathode.

Well BT168G is a Gate Assisted Turn Off Thyristor.

Sure? Where did you get the information? Not from the BT168 datasheet, I suppose.

 

[Raza]

I would also wish to know the information source and as said earlier surely not from the datasheet.

 

[treez]

Ishailesh

The GATT is used in circumstances where a fast turn-off is needed.

Likewise i do not understand how you can know it is a GATT.

...It made my heart jump into my mouth because i most definetely do not want fast run off.......its the thyristor in a xenon flash lamp trigger circuit.

FvM:

Not understandable without circuit information where the negative voltage comes from

It is due to the fact that the microcontroller which turns on the thyristor is distant from it............when the thyristor turns on, the trigger capacitor suddenly discharges and a huge, brief oscillating surge of current flows along the ground track, causing volt drop across its stray inductance........

the microcontroller drives the thyristor gate with a voltage thats 3.3V above the microcontroller ground, but this ground is at times during the surge, well below the voltage at the ground local to the thyristor, and so the thyristor gate goes well negative of its cathode.

This is what i have seen on the oscilloscope when connected from cathode to gate of thyristor

 

[FvM]

I never heard about a SCR being damaged by a current limited negative gate voltage, but may be there's a risk.

A BZX84C30 with its cathode connected to the gate can work of course. The capacitance is sligthly higher than a 1N4148, but this shouldn't be a problem.