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Topic: TVS diode next to zener diode- does it make sense? (Read 545 times) previous topic - next topic

ArduRobotBuilder

Apr 05, 2018, 07:25 pm Last Edit: Apr 05, 2018, 07:27 pm by ArduRobotBuilder
Hello,

my question is pretty straight forward:
Does it make sense to put a TVS diode parallel to the zener diode to protect the gate of the mosfet? Or is the zener diode alone protection enough? The powerline is very unsteady and reaches "working" voltages between 20V and 40V but transients will happen since I have a lot of switching and inconsistent loads (flyback diodes are already implemented). The "load" in this schematic is exemplary the LED.

I know how and according to what parameters a TVS must be chosen, I'd just like to know if the additional TVS at the gate would make sense.
Thanks!

Edit: Please ignore the "sense if S1 is closed" text


larryd

For large short duration spikes.
TVS is faster than a zener, that would be one argument to have one.
The zener might be a better clamp clamp.

Remember there is a need to keep the input capacitance to a minimum.

However, for a LED and a series resistor of 330Ω looks like over kill.



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jremington

#2
Apr 05, 2018, 08:46 pm Last Edit: Apr 05, 2018, 08:46 pm by jremington
If this is for a project to be used in an automotive electrical environment (or similar with very large voltage spikes), then the TVS diode should be used to protect the entire circuit, not just the gate of the transistor.

MorganS

Think of the circuit protection as creating a safe perimeter. Everything inside the perimeter can assume that there's no bad spikes coming down the line. You just have to apply protection strategies to every wire that crosses that perimeter.

Usually that's pretty clear from the layout of your box. All wires coming into your box from outside are suspect. Some idiot (usually you the designer) is going to hook this up backwards or drag a live wire across the pins of the connector at some stage in the future.

Every wire entering or leaving the box needs to have protection near where it enters the box. Don't collect all of the protection diodes into a group on the far side of the circuit board. Those traces passing under sensitive components can zap them before the protection circuit can clamp the spike. Put the protection circuits between the socket and the rest of your circuit, to make a protected perimeter.

TVS diodes are very high-level magic. They can withstand big voltage zaps. You don't always need to have one TVS per wire. Look at the internal circuitry of the TPD2E001. That protects two data lines and a power wire with only one TVS diode.

But a TVS cannot protect against everything. If you hook up 12V to the TPD2E001 then it will explode and release the magic smoke. The sustained power dissipation of that kind of hard fault will overheat the TVS. So you need to add more components, such as a fuse. You will end up with layers of protection to cover each different type of fault.
"The problem is in the code you didn't post."


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