MOSFET Gate-Source protection question

Question

I have seen different schemes in the three phase motor inverter design when people try to protect gate-source from high voltage as shown in the picture.

I understand the usage of zener diode to limit Vgs, and the usage of R to prevent unwanted switch-on when gate is floating (Am I correct in the understanding?). Here are my questions.

1. why are zener and R used together as the 3rd picture shows? Are those different config related with inverter power ratings?
2. what's the difference between using zener diode and TVS diode as in 4th picture?
3. How to choose zener diode in term of breakdown voltage Vc? Vc higher than Vth but lower than Vgsmax?

Edited

One more question, is it common to use zener and R in parallel between gate and source to both limit Vgs and avoid unwanted switch-on?

Answer 1

Your interpretation for the reason why Z-diodes or resistors are used is corrent, in my opinion.

When the driver leaves the MOSFET's gate floating, you want at least something to give you a defined voltage, and a single resistor is an often-used solution. You choose the resistor's value as a trade off between a "good" low with a floating driver output (i.e. a small value for the resistor) and low losses while the driver has a high signal while turning on the MOSFET (i.e. a large value for the resistor).

1. A series combination of a diode and a resistor is just another trade-off, and one I have hardly ever seen. It prevents excessive currents should the diode ever start to conduct, while it "just somewhat" limits the gate-to-sorce voltage in such events. Also, a resistor is always a dampening component in any circuit prone to oscillations; it may have been designed in to prevent ringing once the diode conducts, but I'm really just guessing here and have never used such a snubber myself; usually, you want the snubber in the drain-to-source path, because this is where oscillations are usually caused. Putting it in the gate path will likely only limit effects that originated somwhere around the drain.

2. A (bidirectionial, like in your picture) TVS is just two zeners. Some diodes marketed as TVS have higher peak power handling capabilities or are designed for really fast breakdown. The anti-serial-connection of two diodes goes along with the specification of V_{GS, max} = ± (some voltage). In words: Negative gate voltages are OK for the MOSFET, and sometimes, a gate driver will provide a negative voltage to turn off the MOSFET, either for a "better" off-state, or because V_{GS, th} is quite low, or because your FET is a normally-on type (although that's not mainstream for the application of motor drives).

3. Yes, that's basically it. Just for completeness, one should as well add the obvious V_Z > V_{driver, ON}, because otherwise, the driver will just dissipate power into the protection diode.

4. (One more answer to one more question ;-) Sure, this sounds like a good idea and I believe I've seen it many times.