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# UCC27517A: mosfet gate resistor

Part Number: UCC27517A

hi, using 4 of UCC27517A Single-Channel High-Speed Low-Side Gate Driver and 4 150v Infineon BSC110N15NS5 mosfets to drive unipolar 4 coil stepper motor.  9 amps/winding, 0-10khz switching speed.

Mosfet Ciss=2080pF, turn ON=13.3ns, Qgs=12nC, 10v drive, RDson=8.8mohm.

I want to limit gate drive current to protect UCC27517A; maximize switching time while reducing 9mhz ringing somewhat.

1.  what gate resistor can you recommend?  from studying many app notes, I am thinking 5 ohms.
2.   what is total ma & power required by UCC27517A at 10v drive?  I am guessing about 5ma average needed, & 10uF & .1uF located as close as possible to UCC27517A .
• Hello Vern,

what gate resistor can you recommend?  from studying many app notes, I am thinking 5 ohms.

I also did some calculations and I think 4-6Ω should be a good value. Often, it is good to try a few different values of gate resistor and see how they perform. Another option to prevent high-frequency ringing, is to use a series ferrite bead.

what is total ma & power required by UCC27517A at 10v drive?  I am guessing about 5ma average needed, & 10uF & .1uF located as close as possible to UCC27517A .

The total power dissipated in the gate drive loop will be:

P = Fsw * Vdd * Qg

If you divide out Vdd, you will have the current. You can also add the quiescent current as an offset:

your load is close to 1.8nF (bigger) but vdd is lower (10V vs. 12V). These are canceling out somewhat, so your final current should be somewhere close to the red line. At 10kHz, the current required should be below 2mA. Multiplied by 4 for all drivers, and somewhere around 5-6mA total seems reasonable. Also, the 10uF and 1uF bypass capacitance is good also.

Best of luck with your design!

Thanks,

Alex M.