1. POWER ELECTRONIC DEVICES

 For 4th Semester Polytechnic EE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic

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Course Code EE 4001
Course Title Fundamentals of Power Electronics

1. Power Electronic Devices

Power electronics is the study of electronic devices and circuits that control and convert electrical power efficiently. Power devices play a significant role in the conversion and regulation of electrical energy in applications such as motor drives, power supplies, renewable energy systems, and more. Two important power electronic devices are Power Transistors and Insulated Gate Bipolar Transistors (IGBTs). Let’s break down each topic.

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1.1 Power Transistors

1.1.1 Construction of Power Transistors:

• A power transistor is a type of transistor used for switching and amplification of electrical signals in power electronic circuits.
• It consists of three layers of semiconductor material: Emitter (E), Base (B), and Collector (C).
  • The transistor can be NPN (negative-positive-negative) or PNP (positive-negative-positive) based on the arrangement of materials.
  • Power transistors are designed with large physical size and high voltage/current handling capacity.

1.1.2 Working Principle:

• Power transistors operate based on the principle of current amplification. When a small current is applied to the base terminal, it controls a larger current flowing between the collector and the emitter.
• In the on-state, the transistor acts like a switch and allows the current to flow between the collector and emitter, leading to low resistance (saturation region).
• In the off-state, the transistor does not allow current to flow, leading to high resistance (cut-off region).

1.1.3 V-I Characteristics:

• V-I Characteristics (Voltage-Current) show how the transistor behaves when the voltage across the collector-emitter is varied, with respect to the current.

  • In the cut-off region, the transistor is off and no current flows (acts as an open switch).
  • In the saturation region, the transistor is fully on, and the current flow is determined by the external circuit (acts as a closed switch).
  • In the active region, the transistor works as an amplifier and controls the output current.

  Graph:

  • X-axis: Collector-Emitter Voltage (Vce)
  • Y-axis: Collector Current (Ic)

  Key Points:

  • When Vce = 0, Ic = 0 (cut-off).
  • As Vce increases, Ic increases linearly (active region).
  • At a certain Vce, the Ic reaches a saturation point (saturation region).

1.1.4 Uses of Power Transistors:

• Power Amplification: Used in audio and RF amplifiers.
• Switching Applications: Used in power supplies and motor drives.
• Pulse Circuits: Used in pulse-width modulation (PWM) for motor control.

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1.2 Insulated Gate Bipolar Transistor (IGBT)

1.2.1 Construction of IGBT:

• An IGBT is a combination of BJT (Bipolar Junction Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).
• It has four layers of semiconductor material: Emitter (E), Base (B), Collector (C), and Gate (G).
  • The Gate terminal is insulated from the rest of the transistor, making it behave like a MOSFET for switching.
  • The BJT section (base, collector, and emitter) provides the current-carrying capability.
• The combination of both gives the IGBT better performance, combining high switching speed (MOSFET) with high current handling capacity (BJT).

1.2.2 Working Principle:

• Gate Control: The IGBT is controlled by the gate voltage. A small voltage applied to the gate terminal turns the transistor on and allows current to flow from the collector to the emitter.
• On State: When the gate voltage is positive, the device behaves like a closed switch, and current flows.
• Off State: When the gate voltage is 0V, the device behaves like an open switch, and no current flows.
• The IGBT combines the voltage control of MOSFET and high current capability of BJT, making it suitable for high power applications.

1.2.3 V-I Characteristics:

• The V-I characteristics of an IGBT are similar to a BJT in terms of its voltage-current behavior:

  • In the off-state, the IGBT behaves like an open switch.
  • In the on-state, it acts like a closed switch, with a small voltage drop across it.
  • It has a lower saturation voltage compared to power transistors.

  Graph:

  • X-axis: Collector-Emitter Voltage (Vce)
  • Y-axis: Collector Current (Ic)

1.2.4 Uses of IGBT:

• Motor Drives: Widely used in motor control circuits, particularly in variable speed drives.
• Power Inverters: Used in converting DC to AC, such as in solar inverters and UPS systems.
• Switching Applications: Used in high-voltage power switching, such as in rail traction and electric vehicle drive systems.

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Summary of Key Differences:

Feature	Power Transistor	IGBT
Construction	BJT	Combination of BJT & MOSFET
Switching Speed	Moderate	High
Voltage Control	Current-driven	Voltage-driven
Current Handling	Moderate	High
Efficiency	Moderate	High
Applications	Amplifiers, switching	High power inverters, motor drives

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Important Questions to Practice:

1. What are the key differences between Power Transistors and IGBTs?
2. Explain the construction, working principle, and V-I characteristics of a power transistor.
3. What is the significance of the gate in an IGBT?
4. Describe the working principle of an IGBT with its V-I characteristics.
5. What are the applications of Power Transistors and IGBTs in power electronics?
6. Why is an IGBT preferred over a BJT in high power applications?
7. Draw the V-I characteristics of a power transistor and explain its regions.
8. How does the combination of BJT and MOSFET in an IGBT benefit its operation?

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Conclusion:

Both Power Transistors and IGBTs are essential devices in power electronics used for controlling electrical power. While power transistors are more suitable for lower power applications, IGBTs are ideal for high-power, high-efficiency, and fast-switching applications. Understanding their construction, working principles, and characteristics will help in their effective application in various engineering fields.

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