Assuming your wanting to use the mosfet as a switch. A voltage potential greater than the source by the amount defined for the particular mosfet (P/N) for the switched current. To derive the minimum look at the "transfer characteristics". For example, IRL540, source at 0V, and you want to switch 5A. Curves show that 4.5V at the drain as a minimum (depending on temperature), but for reliability a higher value should be used. If the same part has the source at 40V, then to turn it on it would have to 44.5V at the gate minimum. Realistically off the shelf mosfet drivers would use a voltage difference maybe 8 to 12V at the gate. For the mosfet to be off, the gate should be less than 2V of the same value of the source. In the middle of that region the mosfet becomes a linear device.
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
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