Hello. I've a question about transistor IRF-740. This component has a

[Ravaner]

Hello. I've a question about transistor IRF-740. This component has a Zener diode connected between drain and source. Has this diode a function of limiting over voltage ?

 

[yungman]

Other MOSFET has the same, this is not unique for the 740. It is the body diode in the MOSFET structure that breaks down beyond the specified voltage. Just look at it as the limit of the MOSFET. It is not as if they put a zener diode in, even a regular diode starts to break down beyond the specified reverse voltage.

 

[Ravaner]

Many thanks for your reply

 

[Ravaner]

I want you use this MOSFET in the primary of a trigger coil. To avoid overvoltage which can destroy this transistor, what do you think is better : Zener diodes or Varistor ?

 

[yungman]

I worked with a lot of high voltage pulsing circuits, only the transorbs are fast enough

http://www.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=p6ke

http://www.digikey.com/product-search/en/circuit-protection/tvs-diodes/655429?k=1.5ke

Important is don't let the MOSFET breakdown at any time. If the transistor rated 1000V, at no time, you should allow the voltage to go beyond 1000V. When you use the TVS diode, make sure you read the guaranty UPPER limit. They always spec a range of break down voltage, eg. for P6KE5.1, they spec the conduction voltage of say between 4.7 to 5.5V. You need to honor the 5.5V as some will only break down at 5.5. So if your transistor is 1000V, and if a P6KE200 is rated between 170 and 230, You can only use 4 in series to guaranty the highest break down voltage is 4X230=920V ( below 1000V).

 

[Ravaner]

Many thanks for this complete answer.