low side mosfet drive circuits and 23

Posted on Feb 7, 2014

MOSFETs cannot always just be connected to the drive signal and be expected to work properly. Due to the construction of the MOSFET, driving it is not the simplest of tasks, especially for beginners. There are many users who regularly ask for help on MOSFET drive related issues or problems on different blogs, websites and forums. So, here I will show some MOSFET drive

low side mosfet drive circuits and 23
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techniques/methods for MOSFETs configured as low-side switches. Before I head on to MOSFET drive, let me just tell you what a low-side switch is, in case you don`t know. When the MOSFET (that you`re using as a switch) sinks current, it is a low-side switch. The load will be between the drain and +V supply. The source will be connected to ground. Gate will be driven with respect to ground. Let me start with N-channel Power MOSFETs. One thing to keep in mind is that these MOSFETs are fully turned on when the voltage at the gate with respect to the source (VGS) is > 8V. So while at voltages less than 8V, the MOSFET will be on (as long as VGS > VGS(threshold) ), the MOSFET won`t be fully turned on. It will operate in the linear region and there will be very high losses. Now let`s go to Logic Level MOSFETs. These MOSFETs tend to have much lower VGS (threshold) and thus are usually fully turned on with VGS of 4V to 5V. One downside to Logic Level MOSFETs is that they usually have higher on-state resistances compared to similarly rated Power MOSFETs. So, this would mean higher conduction losses for the same load. Another thing to remember is the gate capacitance. So, when the MOSFET is to be turned on or off, the gate must be driven high or low with sufficient current quickly enough so as to charge or discharge the gate capacitance quickly enough so that the MOSFET spends minimal time in the linear region and is quickly turned...

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