EVALSTDRIVE101 - Questions about FET's gate circuits

[mual]

Hello,

I use ST's STDRIVE101 in a design to drive a three-phase BLDC motor and used the reference design EVALSTDRIVE101 as a design guideline. My design has a MOSFET switching frequency of 20 kHz and a MOSFET drain-source voltage between 36 and 42 V.

I have some questions about the design’s power stage (see screenshot from the design’s documentation below) and would be very grateful for an answer.  All designators mentioned are taken from the screenshot.

My questions are:

• Gate resistor R11: Is there a strong reason for choosing 33 Ohm? My thinking is that when using a smaller resistor, the MOSFET’s switching on process could be sped up as my design uses a MOSFET capable of turning on faster. Is this possible with STDRIVE101 and to what extent could the gate resistor be reduced?
• Gate driver diode D1: This is used for faster switching off the gate by bypassing R11 if I get the design correctly?
• Resistor R14: I assume this is a pull-down resistor to drain the gate from unwanted charge when the MOSFET is turned off?

Thanks in advance for taking the time for an answer!

Regards,

mual

[AScha.3 · Accepted solution]

Hi,

> R11: Is there a strong reason for choosing 33 Ohm?

No. Choose any value...but be aware : the faster you switch, the more EMI you generate, up to (maybe) self destruction of the mosfet (oscillation or shoot might happen, depends on layout, load, current, capacities and the ruggedness of the mosfet).

The higher you choose the R11 value, the more "smooth" it will switch on, but switching loss will increase also.

So its a compromise, to get good balance EMI/spike vs. switching loss.

If you think, you can do it better (with other mosfet, it might be needed to adapt), try it.

And in a half-bridge, as its here, you change the timing - maybe dead-time needs adjusted also.

Avoid any shoot - this might kill all mosfets and driver...so if you want to experiment, have some extra parts at hand. 🙂

>D1: This is used for faster switching off the gate

Right.

>Resistor R14: I assume this is a pull-down

Is more for safety, if driver not active (maybe at power on or disable driver), to have a "safe" voltage at the gate.

[mual]

Thank you!