App note: Power MOSFET Basics: Understanding the Turn-On Process

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More to know about MOSFET gate threshold voltages, an application note from Vishay. Link here (PDF)

The question of how to turn on a MOSFET might sound trivial, since ease of switching is a major advantage of field-effect transistors. Unlike bipolar junction transistors, these are majority carrier devices. One does not have to worry about current gain, tailoring the base current to match the extremes of hfe and variable collector currents, or providing negative drives. Since MOSFETs are voltage driven, many users assume that they will turn on when a voltage, equal to or greater than the threshold, is applied to the gate. However, the question of how to turn on a MOSFET or, at a more basic level, what is the minimum voltage that should be applied to the gate, needs reappraisal with more and more converters being controlled digitally. While digital control offers the next level of flexibility and functionality, the DSPs, FPGAs, and other programmable devices with which it is implemented are designed to operate with low supply voltages. It is necessary to boost the final PWM signal to the level required by the MOSFET gate. This is where things begin to go wrong, because of the misconceptions about what really turns on a MOSFET. Many digital designers look at the gate threshold voltage and jump to the conclusion that, just as with their digital logic, the MOSFET will change state as soon as the threshold is crossed.

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2 Comments

  1. I know some people found this app-note post useless but this one came with a perfect timing for me :)
    I was looking for some Mosfet you can turn-on efficiently at 3.3V and was struggling with what to look for in all these datasheets.
    Thanks for the knowledge sharing :-D

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