Semiconductor Devices and Power Electronics
Semiconductor Devices and Power Electronics
This domain tests PN junctions, diodes, transistors and the switching devices used in charge controllers and inverters: SCR, TRIAC, MOSFET and IGBT.
Core rules
- A PN junction conducts when forward biased; silicon drops about 0.7 V, germanium about 0.3 V.
- A Zener diode works in reverse breakdown to regulate voltage.
- BJT is current-controlled; MOSFET is voltage-controlled and suited to high-frequency switching; IGBT combines MOS input with low ON-state loss.
- SCR latches ON when gate-triggered and turns OFF only when current falls below holding current; TRIAC is bidirectional.
- Rectifier converts AC to DC; chopper converts fixed DC to variable DC; inverter converts DC to AC; PWM controls output voltage and reduces harmonics.
Worked example
A half-wave rectifier is fed with 12 V RMS AC. Peak voltage Vm = 12 × 1.414 = 16.97 V. Ideal average DC output = 0.318 × Vm = 5.4 V. The diode must withstand a peak inverse voltage of about 17 V, so a 50 V rated diode gives margin. With a capacitor filter the DC voltage rises toward Vm but ripple depends on load and capacitance.
Traps that cost marks
Using RMS instead of peak for PIV; expecting an SCR to turn off by removing the gate signal; confusing TRIAC and SCR; ignoring switching losses; assuming a real diode has zero forward drop.