Hello Guys,
Good day.
Our customer is having trouble calculating the gate resistance for a n-mosfet driver. He is driving an n-mosfet with a gate voltage of 14v and he need to know the external gate resistor he should use.
He is thinking of using cc28740 mosfet with STF18N65M2.Are they a fit?
Hi, Art:
UCC28740 is okay to drive STF18N65M2.. The STF18N65M2's Ciss is 770pF, and the driver current of UCC28740 is 25mA and sink current is 50mA. Based on the information, You may refer the gate resistor selection guild as below link to calculate an initial RDRV and RSINK first. These value is used to avoid gate oscillation during MOSFET ON/OFF.
External Gate Resistor Selection Guide (Rev. A)
After put the calculated RDRV and RSINK, please check the gate waveform to make sure there is no gate oscillation because the parasitic L and C might be different from calculation results. As the figure as below, the blue RG is too small to make gate oscillated. The red RG is too big and it makes higher switching loss. To adjust the RG value to make the gate waveform is closed to the black one.
Please note it is usually to choose smaller RSINK. It is because the IDS is max when flyback MOSFET turns off. A smaller RSINK is help to speed up MOSFET off to reduce turn-off switching loss.
Please let me know if you need any assistance. Thanks.
Wesley.
