In general, the MOSFET should be chosen with V DS rating that is at least 125% of the voltage to be tested. Most discrete power MOSFETs are rated for 12 V, 25 V, 30 V, or even higher, so selecting a MOSFET with V DS suitable for testing low- to mid-voltage supplies isn’t difficult. The drain-to-source voltage (V DS) must be high enough to withstand the voltage of the supply under test, including any transient surges or overshoot. For many MOSFETs, the total gate charge Q G is a good metric, and comparisons between MOSFETs of similar on-resistance R DS(ON) can be made quickly, using just this parameter. Therefore, it’s important to select a MOSFET with low “figure of merit” 1 and correspondingly low parasitic capacitances. A MOSFET with high gate-to-source and gate-to-drain capacitances (C GS and C GD, respectively) will require higher drive current to reach a desired slew-rate. To modulate the current flowing through the MOSFET with good stability and high bandwidth, the operational amplifier must be able to rapidly change the gate-to-source voltage.
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