mosfet model with shunted zener, is it dangerous to turn it on

[kanmaedexandzelbladex]

Hello,

I have a boost converter which I am still fabricating and I want to test its openloop characteristics first. I am afraid of this because my specs is: Input (11V-19V) and Output is 45V and the MOSFET I am using has a Vdss of 55V (IRFZ34N). I am afraid of possible voltage spikes across the Vds of my mosfet during switching transitions. My question is, will such spikes destroy my mosfet? or will it only clamp the voltage spike to 55V? The model of the mosfet in the picture in the datasheet is a mosfet in parallel with a zener diode so I'm thinking maybe the 55V is the zener voltage and it's not too dangerous if there is a large spike since the zener would probably only clamp it and the spike only occurs for a short time.

Thanks

 

[emontllo]

The model of the mosfet in the picture in the datasheet is a mosfet in parallel with a zener diode

This "Zener" represent the avalanche behaviour of MOSFETS that start conducting current when a certain drain to source voltage is applied. Properly designing a MOSFET to operate in avalanche region is not easy and if not properly designed it would cause the MOSFET to fail See: http://www.irf.com/technical-info/appnotes/an-1005.pdf,.

I think 55 V is too low for your application, you should work with a higher safety margin (like > 70 VDS) especially if you have observed spikes during switching transitions.

 

[kanmaedexandzelbladex]

Awww. Well I still have a lot of time and I plan to test my openloop boost first starting with a low input voltage and low duty cycle (signal generator + pushupull/totempole as gate driver) to see if there are any spikes and slowly rise the input and/or duty cycle and check for spikes. That way, I can already see if there are spikes even without reaching my 45V output first. Is this a good idea? If there are spikes which are really large, then I will just change my mosfet. Let's say my spikes reach up to 50V only, is this safe? XD. I really want to use this mosfet since from my choices it has the smallest capacitances.

 

[FvM]

The essential point is "spike" energy. If it's suffciently below avalanche handling capability of the device, there should be no problem. in other words, you are asking about leakage inductances and currents that energize it.

Some switcher designs can't avoid to drive MOSFETs repeatedly into avalanche clamping, with respective low energy.