Hello all,

I have been working on a project for a while and I am trying to build a MOS gated thyristor to protect my circuit. I have added schematics of my circuit. The main idea is when the gate reaches a certain value of voltage, thyristor latches on to ground and shuts the MOSFET connected in parallel. I have looked up principles for a MOS gated thyristor and it should work on theory. On the other hand I have obtained different results in both simulation and real. First of all when I try to simulate it on LTSpice it does not work. It behaves like it is always latched. No matter what I tried (added pull down resistors to base, added a series resistor between MOSFET's drain and BJT's base etc.) I was not able to make it work on LTSpice. Then I decided to make it on board. On board, I have obtained a different result. For example when I apply an 80% PWM through gate of the MOSFET, the circuit latches on for 80% of time but during the 20% it latches off again. I want it to latch on completely so that I can shut the MOSFET. Do you have any suggestions or ideas?

Your help is much appreciated.

Figure 1: Circuit I assembled on board. I have not used T2507 yet and worked on thyristor equivalent circuit only.


Figure 2: Latch on - Latch off behavior on oscilloscope

 

There is probably always enough leakage current in SPICE to latch the circuit without a resistor across base and emitter of the PNP device.
Why do you want to build a thyristor out of a MOSFET? One built out of a thyristor will be more robust. You could always put a MOSFET on the gate if you need a high resistance.
By the way, IXYS makes MOS- gated Thyristors.

 

Thanks for your response. I just wanted to try some equivalent circuit model since I had necessary components. First I built the thyristor with PNP and NPN but as you mentioned because of leakage it was not working. It was leaking current and it was kept latched until capacitors have been discharged. So I decided to use MOSFET for high resistance but right now I went back even further. I guess I will try it with a proper thyristor.

 

It was leaking current and it was kept latched

Try adding a 1kΩ resistor between the base a emitter of the PNP transistor.

 

The 2N7002 is shunting all the current form the lower BC848. Put a resistor ( 1k to 10k) between base of upper transistor and the 2n7000 drain and you will find it works better.

 

Hello all, thanks for all your responses. Putting a resistor and capacitor at base-emitter of PNP made it much stable. No luck for the simulation though. On the other hand there are still some instability about the MOSFET trigger but I will work on it. As it seems a proper thyristor would be much better. I will try it.

Thank you for all of our response.