Power BJT: Bipolar Junction Transistor

Power BJT: Bipolar Junction Transistor

• First SCSD to allow full control, later replaced by other devices with higher performance.
• Construction and operating characteristics differ from signal BJT due to voltage and current ratings requirements.

Operating Characteristics

• In cutoff region, collector current is almost zero and maximum voltage between collector and emitter is denoted by VCEO (rated voltage).
• Power transistors have very small reverse voltage withstanding capability.

Ideal Switch Operation

• Base current in on-state must be high enough for operating point to lie on hard saturation line, associated with lowest voltage drop across the device.
• BJTs cannot block negative collector-emitter voltages, requiring a diode connected in series with the collector.

Limitations and Failure

• Second breakdown occurs when both collector-emitter voltage and collector current are high, leading to high power loss and local hot spots in the semiconductor.
• Hot spots cause current density increase, leading to damage.

Characteristics

• Current-Controlled Device with low current gain compared to signal amplifiers, typically around 10.

AC-AC Voltage Controllers

• AC controllers are power electronic converters that produce an adjustable output voltage supplied from a fixed AC source.
• The fundamental output frequency is equal to the input frequency.

Single-Phase AC Controllers

• Supplied from a single-phase AC voltage source and have a single-phase output.
• Use triacs or two antiparallel connected SCRs (Silicon-Controlled Rectifiers) for bidirectional conduction.
• The input current (ii) equals the output current (io), and the output voltage (vo) equals the input voltage (vi) when either SCR is conducting.

Output Voltage and Current

• The output voltage and current waveforms have half-wave symmetry and no DC component.
• The firing angle (αf) determines the conduction angle (β), which affects the output current and voltage.
• When the firing angle increases, the conduction angle decreases, and the current pulses become smaller and shorter.

Operation with αf < φ

• When fired with a single triggering pulse, the AC controller acts as a half-wave rectifier, which is not desirable.
• Multipulse gate signals are used to avoid this and ensure output control.

AC Controller Analysis

• The rms value of the output voltage (Vo) can be found using the equation: Vo = Vi * (α - αf - [sin(2αe) - sin(2αf)]) / π
• The extinction angle (αe) depends on the firing angle (αf) and the load angle (φ).

Example 1

• A single-phase AC voltage controller is used to control lighting, and the firing angle is 60°.
• The reduction in power consumed by the lighting is 19.5%.

Example 2

• A single-phase AC voltage controller supplies a load consisting of a resistance and an inductance.
• The rms output voltage can be calculated for different firing angles using the AC controller analysis equation.