Simulating an avalanche transistor for an impulse generator

[A-ron]

Hi, I'm currently trying to simulate an impulse generator that makes use of the avalanche transistor FMMT415. I have found SPICE code for this transistor and have used it in model maker to try and simulate the transistor. However, I want to know if this is enough to simulate a transistor operating in the avalanche region. Can multisim simulate avalanche transistor or will further circitry be required. Any advise on the topic would be much appreciated. Regards.

[OStep]

A-ron,

You shouldn't need to use a model maker if you have found a SPICE model. The model maker generates SPICE models. So I am a little confused by what you have done. But let me try to help anyway.

The primitive BJT device (npn and pnp) in Multisim is based on the gummel-poon model which doesn't model avalanche breakdown. In your circuit, the FMMT415 component uses a single primitive BJT device for its model so its impossible to model avalanche breakdown.

However,I have found a SPICE macro model for the FMMT415 on diodes.com. It contains a primitive BJT device in its core but is supported by many other devices, presumably to characterize avalanche breakdown characteristics.

Its @ http://www.diodes.com/products/catalog/detail.php?item-id=824#0

But I have created the component with the model for you. See attached.

[A-ron]

Hi max, thanks a lot for your reply. I used the transistor which you made, unfortunately the output was not as expected. I was expecting a much higher voltage from the transistor as itwent into the avalanche region. I believe that it is because my circuit doesn't provide the transistor with a high enough voltage to push it into the avalanche region. I have tried the samecircuit but with the FMMT413 transistor instead, as it has a lower collector-base breakdown voltage. However, still no luck. I am trying to follow a design that I found in a book, however, a few of the key components were omitted, such as the avalanche transistor used and the inductor and resistor values conected to it. Would you have any suggestions for this simulation. Many thanks Aaron.

[OStep]

Aaron,

It seems that the avalanche transistor is the key component in your design. I would isolate that transistor and run some unit tests on it to determine whether it behaves as you expect.