Power BJT: Bipolar Junction Transistor
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Questions and Answers

What is the primary difference between a Power BJT and a signal BJT?

  • The Power BJT has a higher current gain compared to signal amplifiers
  • The operating characteristics of a Power BJT differ significantly from the signal BJT due to voltage and current ratings requirements (correct)
  • The construction of the Power BJT is simpler than the signal BJT
  • The Power BJT is used for signal amplification

    • What is the purpose of the base current in the on-state?

  • To block negative collector-emitter voltages
  • To decrease the collector-emitter voltage
  • To ensure the operating point lies on the hard saturation line associated with the lowest voltage drop across the device (correct)
  • To increase the collector current

    • What is the maximum voltage between collector and emitter in the cutoff region denoted by?

  • VBE
  • VCC
  • VCEO (correct)
  • VCE

    • What is the reason for the second breakdown in Power BJTs?

    <p>When both the collector-emitter voltage and the collector current are high</p> Signup and view all the answers

    What is the consequence of the second breakdown in Power BJTs?

    <p>The appearance of local hot spots in the semiconductor</p> Signup and view all the answers

    What is the characteristic of Power BJTs in terms of current gain?

    <p>Low current gain compared to signal amplifiers</p> Signup and view all the answers

    What is the fundamental output frequency of an AC voltage controller?

    <p>Equal to the input frequency</p> Signup and view all the answers

    What is the primary component of a single-phase AC controller?

    <p>Triac</p> Signup and view all the answers

    What is the characteristic of power switches used in AC voltage controllers?

    <p>Bidirectional conduction</p> Signup and view all the answers

    What is the relationship between input current and output current in an AC voltage controller?

    <p>Input current is equal to output current</p> Signup and view all the answers

    What is the purpose of connecting two antiparallel switches in an AC voltage controller?

    <p>To achieve bidirectional conduction</p> Signup and view all the answers

    What is the type of load assumed in the analysis of single-phase AC controllers?

    <p>RL load</p> Signup and view all the answers

    What is the primary factor that determines the extinction angle?

    <p>Both firing angle and load angle</p> Signup and view all the answers

    What happens when one of the SCRs is conducting in a single-phase AC controller?

    <p>The other SCR is reverse biased</p> Signup and view all the answers

    For a resistive load, what is the relationship between power and voltage?

    <p>Power is proportional to the squared rms value of voltage</p> Signup and view all the answers

    What is the relationship between input voltage and output voltage when either SCR is conducting in a single-phase AC controller?

    <p>Input voltage is equal to output voltage</p> Signup and view all the answers

    What is the magnitude control ratio for a 50% reduction in power?

    <p>1/2</p> Signup and view all the answers

    What is the load angle for a resistive load?

    <p>0°</p> Signup and view all the answers

    What is the purpose of multipulse gate signals in the controller?

    <p>To activate the controller's triacs</p> Signup and view all the answers

    What is the percentage reduction in power for a firing angle of 60°?

    <p>19.5%</p> Signup and view all the answers

    What is the value of the magnitude control ratio for a firing angle of 60°?

    <p>0.897</p> Signup and view all the answers

    What is the type of load in Example 2?

    <p>RL load</p> Signup and view all the answers

    When T1 is forward biased and fired at 𝛼f, what happens to the conducted current?

    <p>It initially increases then drops to zero</p> Signup and view all the answers

    What happens to the conduction angle (𝛽) when the firing angle (𝛼f) increases?

    <p>It decreases</p> Signup and view all the answers

    What is the condition for the minimum feasible firing angle?

    <p>It is equal to the load angle</p> Signup and view all the answers

    What happens when the AC controller is fired with a single triggering pulse?

    <p>It acts as a half-wave rectifier</p> Signup and view all the answers

    What is the purpose of using multipulse gate signals in phase-controlled ac voltage controllers?

    <p>To avoid the AC controller acting as a half-wave rectifier</p> Signup and view all the answers

    What is the condition for no output control to occur in phase-controlled ac voltage controllers?

    <p>When 𝛼f &lt; ∅</p> Signup and view all the answers

    What is the formula for the rms value of the output voltage (𝑉𝑜)?

    <p>𝑉𝑜 = 𝑉𝑖 (𝛼 - 𝛼𝑓 - [sin 2𝛼𝑒 - sin 2𝛼𝑓])</p> Signup and view all the answers

    What is the significance of the extinction angle (𝛼e) in the formula for the rms value of the output voltage?

    <p>It is used to find the rms value of the output voltage</p> Signup and view all the answers

    Study Notes

    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.

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    Description

    Learn about the construction and operating characteristics of Power BJT, its differences from signal BJT, and its applications.

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