Field Effect Devices and Power Control
5. Field Effect Devices and Power Control
This domain tests voltage-controlled devices - JFET and MOSFET - and the thyristor family used for switching AC power.
FETs
- A JFET is voltage controlled: the gate-source voltage controls the channel, and the gate draws almost no current, giving very high input impedance.
- Key parameters: I_DSS is the saturation current at V_GS = 0; V_GS(off) (pinch-off) is the voltage that stops conduction; the transfer characteristic follows Shockley's equation I_D = I_DSS(1 − V_GS/V_GS(off))².
- MOSFETs have an insulated gate; the depletion type is normally on, the enhancement type is normally off and needs V_GS above a threshold.
Power control
- SCR: anode, cathode and gate; conducts in one direction only after a gate pulse and latches until current falls below the holding current; used in controlled rectifiers, dimmers and motor drives.
- DIAC: bidirectional trigger diode that breaks over at a fixed voltage and fires a TRIAC.
- TRIAC: bidirectional, equivalent to two SCRs back to back; controls AC power in lamp dimmers and fan regulators.
- UJT: used as a relaxation oscillator to generate trigger pulses.
- Phase control varies the firing angle to set the average load voltage.
Worked example
A JFET has I_DSS = 8 mA and V_GS(off) = −4 V. At V_GS = −2 V: I_D = 8(1 − (−2/−4))² = 8(1 − 0.5)² = 8 × 0.25 = 2 mA.
Traps
- Confusing pinch-off voltage with breakdown voltage.
- Expecting a MOSFET gate to draw steady current.
- Assuming an SCR turns off when the gate signal is removed - it does not.
- Handling MOSFETs without grounding; the gate oxide is easily destroyed by static.