MOSFET Switches and Zener References

An N-channel enhancement MOSFET has source, drain and gate terminals. An insulating oxide separates the gate from the semiconductor. A sufficiently positive gate–source voltage creates a conducting channel between drain and source. The very high input resistance means negligible steady gate current is required.

The threshold voltage Vth marks the onset of conduction, not a guarantee of a very low on-resistance at a large load current. Read the supplied transfer curve of drain current against VGS and output curves of drain current against VDS for different gate voltages. IDSS denotes the small drain leakage with zero gate–source voltage in this enhancement-device context.

Connect a load between the positive supply and drain, with source at 0 V. When the MOSFET is off, little current flows, so almost no voltage is dropped across a resistive load and VDS is near the supply voltage. When on with low channel resistance, VDS is small and almost all the supply appears across the load.

An inductive load can produce a large voltage on switch-off. A correctly oriented flyback diode provides a path for the decaying current. A gate pull-down resistor avoids an undefined floating input. Keep the distinction between high input resistance and low on-state drain–source resistance.

A Zener diode has anode and cathode and is designed to operate in controlled reverse breakdown. Beyond its knee and minimum current, the voltage changes only slightly as current varies. Connect the cathode to the positive reference node, anode to 0 V, and a series resistor from the supply to limit current.

The resistor carries the sum of Zener and reference-load currents. Check that the Zener current exceeds its specified minimum and that power is acceptable. This option requires a reference source, rather than detailed regulator design.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.