Zener in power stage

[Denniz]

Hi, evry!

I'm relatively new to class-D amps, to begin with.

In many designs one can see a 15V zener in gate-source net of the power stage FETs.

1. What is the goal of putting them there?
2. While bootstrap nets are sometimes 12V, why using 15V zener?

Thanks.

[Eva]

The high side gate drive supply rail is usually produced from a resistive divider at startup and then from bootstrapping when the amplifier is switching. The resistive divider needs a zener for voltage limiting (although it may be included in the driver IC sometimes).

Switching speed (di/dt and dv/dt) is strongly dependent on gate drive supply voltage, another reason to ensure it does not get too high. Additionally, some degree of regulation in gate drive voltage can improve switching timing accuracy and reduce open-loop THD.

Sometimes MOSFET fail with a gate to drain short. In these circumstances a zener (with the help of schottkys to clamp driver output to the rails) can reduce the amount of damage to the drive circuit (zener should fail shorted due to overcurrent, then forcing a properly sized gate resistor to fail open before PCB tracks start to blow).

And sometimes the zeners are just cosmetic... (circuit designers playing vodoo games, without deep functionality considerations)

[Denniz]

Originally Posted by Eva

The high side gate drive supply rail is usually produced from a resistive divider at startup and then from bootstrapping when the amplifier is switching. The resistive divider needs a zener for voltage limiting (although it may be included in the driver IC sometimes).

Like that in the picture attached?
Thank You.