How Safe this TVS Diode?

[Botvink]

Hello friends,

I am designing one brushless motor driver. It is targeted for 6S (25.2V Max) power motors. And max current planned is 100A. I selected this TVS diode:

SMC5K28A-M3/H

https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/SMC5K28A-M3-H/7427140

IT has breakdown voltage between 31 to 34V and max clamping voltage is 45V.

My mosfets are 40V rated mosfets, planned working voltage of board is 25,2V. What do you think about this option?I am not sure if I have chosen the right model.

 

[Jrbe]

I would think you'd want a bidirectional TVS but it depends on how you're implementing this.

TVS are good for quick overvoltage (transient) events like a static shock or quick inductive spikes. They don't do so well dealing with long term overvoltage current but are fast.

You could look at using a TVS that's under the max mosfet voltage but paired with a MOV that's a bit lower voltage. MOVs are typically used for lightning protection. A TVS reacts quicker than a MOV but the MOV can handle more current. So the TVS kicks in quickly, then the MOV takes over once it's able to react. But this can get pricey and challenging at lower voltages with wide windows of part tolerance and are especially difficult to deal with long term events.

These both have limited lifetimes when they're doing their jobs. Both of these protection devices can fail shorted. They could possibly take out the mosfet / controller. Depending on what your expected protection needs are / what overvoltage and overcurrent events look like it might be better to look at higher voltage fets both for cost and simplicity.

 

[Botvink]

Yes, actually I made exactly same thing.. Here my design board uses 6 x Mov (Red marked components) and 4 TVS. I
designed this board for combat robots and afraid of all motor inductive kickbacks.. That TVS and Mov paralleled, but I need to control voltage levels of which is lower... Usually they cover very broad voltage..

TVSs are 28V rated..
Big 3 Mov paralelled to TVS motor output lines are 26V DC rated: https://eu.mouser.com/ProductDetail/Bourns/AV20K1812801NIR1HT?qs=OlC7AqGiEDnE1PLpXDnjBg%3D%3D

So I am afraid I made opposite

 

[Jrbe]

If it's just the inductive spike you're worried about from say a motor disconnection mid battle, a good sized TVS should be fine handling that by itself. If the motor is AC / bldc the diodes in the mosfets likely would handle the flyback from a disconnect as long as they're decent.

If you're bringing this thought from a stepper motor protection circuit design, the diodes built into stepper driver chips aren't very strong and typically need backups or don't exist.

I know a 12v system can make about an 80v inductive spike without protection. I'd expect you could see about double that with 26v but it depends on your components.

You could add a fuse in line if you're worried about the possibility of a failed shorted protection item. You'd probably have to do some destructive testing to verify that it works as expected. Of course this means no protection and a likely blown mosfet next event if the fuse blows.