Power Semiconductor Devices
Power diodes, SCR characteristics, turn-on and turn-off methods, latching and holding current, di/dt and dv/dt protection, snubbers, GTO, power BJT, MOSFET and IGBT comparison.
Power electronics switches large currents and voltages with semiconductor devices. Choosing a device means trading voltage and current rating against switching speed and ease of control.
Power diodes
General-purpose (slow), fast-recovery and Schottky (very fast, low drop, low voltage) diodes. Reverse recovery time t_rr and stored charge Q_rr matter at high frequency.
Thyristor (SCR)
A four-layer PNPN device with anode, cathode and gate.
- Forward blocking until triggered; then it latches on and the gate loses control.
- Turn-on methods: gate triggering (normal), forward breakover voltage, dv/dt triggering (unwanted), temperature, light (LASCR).
- Latching current I_L: minimum anode current just after triggering for the SCR to stay on. Holding current I_H: minimum current to stay on once conducting. I_L > I_H (typically 2–3 times).
- Turn-off (commutation): reduce the anode current below I_H and apply reverse voltage for longer than the turn-off time t_q. Natural (line) commutation in AC circuits; forced commutation in DC circuits.
- Protection: a series inductor limits di/dt; an RC snubber across the device limits dv/dt; fuses for overcurrent.
- Two-transistor model: α₁ + α₂ → 1 at turn-on.
Other devices
| Device | Control | Turn-off by gate | Speed | Notes |
|---|---|---|---|---|
| SCR | Current pulse | No | Slow | Highest ratings, rectifiers |
| TRIAC | Current pulse | No | Slow | Bidirectional, AC regulators |
| GTO | Current pulse | Yes (large negative gate current) | Medium | High power |
| Power BJT | Base current | Yes | Medium | Needs continuous base drive |
| Power MOSFET | Gate voltage | Yes | Fastest | Low-voltage, high-frequency; positive temperature coefficient (easy paralleling) |
| IGBT | Gate voltage | Yes | Fast | MOSFET input + BJT conduction; medium to high power |
Firing circuits
R, RC and UJT relaxation oscillator triggering; pulse transformers or opto-isolators give isolation. For a UJT oscillator, f ≈ 1/(RC ln(1/(1 − η))).
Worked example
An SCR has latching current 40 mA and is fed from a 100 V DC source through a 0.5 H inductor. What minimum gate pulse width keeps it on?
- i = (V/L) t, so t = 0.04 × 0.5/100 = 200 µs.
Practice
An SCR in series with L = 0.2 H across a 200 V DC source has a latching current of 50 mA. What is the minimum gate pulse width in µs?
Show answer and explanation
Answer: 50i = (V/L)t, so t = I_L L/V = 0.05 × 0.2/200 = 50 µs.
Which is generally true for an SCR?
Show answer and explanation
Answer: AMore current is needed to establish conduction (latching) than to maintain it (holding).
A snubber circuit across an SCR protects it mainly against:
Show answer and explanation
Answer: BThe RC snubber slows the rate of rise of voltage across the SCR; a series inductor limits di/dt.
Which devices can be turned off by their gate or base signal? (Select all that apply.)
Show answer and explanation
Answer: A, B, CA conventional SCR cannot be turned off from its gate; it needs the current to fall below the holding value.