Question

MOSFETs are very useful as a voltage controlled switch. For instance, if an input voltage of VGS = 10V

was applied to the 2N7000, a large amount of current could flow through the drain. What limits the
current flow is the internal resistance between the source and the drain, RDS(ON). If the resistance is
high, the power dissipated would generate a large amount of heat that would destroy the device.
Determining this resistance is important. Develop a circuit in the space below and then describe a
procedure that would determine the RDS(ON) when VGS = 5V.

Answer 1

To find the RDS(ON) for a MOSFET at VGS=5V, create a test circuit with a known resistor in series, apply 5V to the gate, measure voltage across the resistor and current through it, then calculate RDS(ON) using Ohm's Law. Temperature can affect RDS(ON), so maintain a stable temperature during measurements.

Step-by-step explanation:

To determine the RDS(ON), or the on-state drain-source resistance of a MOSFET such as the 2N7000 at a gate-source voltage (VGS) of 5V, we need to design a simple test circuit and perform measurements. The circuit should include the MOSFET, a known resistor (R), and a voltage source for the gate voltage to turn on the MOSFET.

First, connect the R in series with the drain and source of the MOSFET. Then, apply a VGS of 5V to turn the MOSFET on. Measure the voltage across the known resistor (VR) and the current through it (I) using a multimeter or a current probe. As VGS=5V is a fixed value, we are primarily concerned with VDS, the voltage drop across the MOSFET when it is on.

Next, using Ohm's Law (V=IR), calculate the resistance of the MOSFET (RDS) by taking the voltage drop across the MOSFET (VMOSFET) and dividing it by the current through the known resistor. The equation for finding RDS(ON) will be RDS = VMOSFET / I. Keep in mind, RDS(ON) may vary with temperature, so make sure to record your measurements at a stable temperature.

Remember that RDS(ON) greatly affects efficiency and thermal performance. A lower RDS(ON) means less power loss and less heat generation, which is crucial for the reliability of the MOSFET in high-current applications.