semiconductor physics and Devices

Week 13: Power Electronics – Semiconductor Devices in Power Systems

Lecture Topics:

1. Introduction to Power Electronics

   • Power electronics deals with the conversion, control, and conditioning of electrical power using semiconductor devices.

   • Importance in modern technology:

     • Used in energy conversion systems (AC-DC, DC-DC, DC-AC, AC-AC).

     • Applications include renewable energy systems, electric vehicles, motor drives, and power supplies.

   • Overview of power semiconductor devices:

     • Diodes, thyristors, MOSFETs, IGBTs, and SCRs (Silicon-Controlled Rectifiers).

2. Power Diodes

   • Power Diodes: Handle high voltage and current, used in rectification and freewheeling applications.

   • Characteristics of Power Diodes:

     • High current handling capacity.

     • Fast switching times, especially in Schottky diodes (used in high-frequency applications).

   • Applications:

     • Used in rectifier circuits to convert AC to DC.

     • Freewheeling diodes used in DC motor drives and inductive loads to provide a path for current when the switch is off, preventing voltage spikes.

3. Thyristors and SCRs (Silicon-Controlled Rectifiers)

   • Thyristors: Four-layer semiconductor devices used to control large power flows.

   • Silicon-Controlled Rectifiers (SCRs):

     • A type of thyristor used in applications requiring controlled rectification or switching.

     • SCRs can be turned on by applying a gate pulse but remain on until the current falls below a threshold (latching).

     • Used in phase control, motor drives, and controlled rectifiers.

   • Triacs: A type of thyristor that can control current in both directions, often used in AC power control (e.g., dimmers, motor speed control).

4. Power MOSFETs

   • Power MOSFETs: Designed to handle high voltages and currents, with fast switching speeds.

   • Advantages:

     • High efficiency, fast switching times, and good thermal stability.

     • Used in low- to medium-power applications like switching power supplies, DC-DC converters, and motor drives.

   • Switching characteristics:

     • Power MOSFETs operate with a high input impedance and are controlled by gate voltage.

     • They exhibit low on-resistance, reducing power loss when in the on-state.

   • Applications:

     • Widely used in DC-DC converters, inverters, motor controllers, and power supplies.

5. Insulated Gate Bipolar Transistor (IGBT)

   • IGBT: Combines the high input impedance of a MOSFET with the high-current capability of a BJT.

   • Advantages:

     • High power-handling capability with relatively fast switching speed.

     • Lower conduction loss compared to MOSFETs in high-power applications.

   • Applications:

     • Used in high-voltage, high-power applications such as electric vehicles, renewable energy systems (e.g., solar inverters), motor drives, and uninterruptible power supplies (UPS).

   • Switching characteristics:

     • Slower than MOSFETs, making IGBTs more suitable for lower-frequency switching in high-power systems.

6. DC-DC Converters

   • Step-down (Buck) converters:

     • Convert a higher input voltage to a lower output voltage.

     • Use a power MOSFET or IGBT as a switch, and an inductor to store energy.

   • Step-up (Boost) converters:

     • Convert a lower input voltage to a higher output voltage.

     • Use a power diode and inductor to step up voltage when switching.

   • Buck-boost converters:

     • Combine buck and boost functions to either increase or decrease voltage.

     • Widely used in battery-powered systems where the input voltage may vary.

   • Applications:

     • Power supplies for mobile devices, laptops, renewable energy systems, and electric vehicles.

7. DC-AC Inverters

   • Inverters: Convert DC power to AC power, used in applications like motor drives, uninterruptible power supplies (UPS), and renewable energy systems (e.g., solar power inverters).

   • H-Bridge Inverter:

     • A common topology that uses power switches (MOSFETs or IGBTs) to control the output AC waveform.

     • Can generate a square wave or use pulse-width modulation (PWM) to approximate a sine wave.

   • Three-phase inverters:

     • Used in industrial applications to drive three-phase motors and in renewable energy systems to connect to the power grid.

     • Use six switches (e.g., IGBTs or MOSFETs) to generate three-phase AC power.

8. AC-DC Rectification

   • Diode Rectifiers: Convert AC power to DC power.

     • Half-wave rectifier: Uses a single diode to allow one half of the AC waveform through.

     • Full-wave rectifier: Uses four diodes in a bridge configuration to allow both halves of the AC waveform, providing a smoother DC output.

   • Controlled Rectifiers:

     • Use thyristors or SCRs to control the output voltage by varying the phase at which the devices are turned on.

     • Used in motor control, industrial power supplies, and heating systems.

9. Thermal Management in Power Electronics

   • Heat dissipation:

     • Power semiconductors generate heat due to conduction losses (resistance) and switching losses.

     • Efficient heat dissipation is essential to ensure reliable operation of power devices.

   • Cooling techniques:

     • Heat sinks: Passive devices that increase the surface area for heat dissipation.

     • Fans: Active cooling to increase airflow over heat sinks.

     • Thermal interface materials: Used to improve thermal conductivity between the semiconductor and the heat sink.

   • Thermal shutdown protection: Many power devices include built-in protection to turn off the device if it overheats, preventing permanent damage.

10. Renewable Energy Applications

    • Solar inverters:

      • Convert DC from solar panels to AC for use in homes or feeding into the power grid.

      • Use IGBTs or MOSFETs to perform DC-AC conversion.

    • Wind power systems:

      • Use power electronics to convert the variable-frequency AC generated by wind turbines into grid-compatible AC.

    • Electric vehicles:

      • Power electronics play a critical role in converting stored battery energy (DC) to power the vehicle’s AC motors.

Examples:

• Design of a basic DC-DC buck converter using a power MOSFET, including calculations for inductor size and switching frequency.

• Explanation of the operation of an H-Bridge inverter and the generation of a square-wave AC output.

• Calculation of power loss in an IGBT and the required heat sink size for thermal management.

• Design of a full-wave rectifier circuit using diodes and the calculation of ripple voltage.

Homework/Exercises:

1. Design a buck converter circuit that steps down a 12V DC input to 5V, and calculate the inductor value for a switching frequency of 50 kHz.

2. Compare the characteristics and applications of SCRs and IGBTs regarding switching speed, power handling, and efficiency.

3. Explain the operation of a three-phase inverter and its use in driving an industrial motor.

4. Calculate the thermal resistance required for a heat sink to maintain a power MOSFET below its maximum junction temperature in a DC-DC converter.

Suggested Reading:

• Charles Kittel, Introduction to Solid State Physics, Chapter 8: Semiconductors (continued).

• Research papers on power electronics and renewable energy systems.

Key Takeaways:

• Power semiconductor devices such as diodes, thyristors, MOSFETs, and IGBTs play a critical role in controlling and converting electrical power in various applications.

• Devices like power MOSFETs and IGBTs are widely used in converters, inverters, motor drives, and renewable energy systems, offering high efficiency and fast switching.

• Proper thermal management is crucial in power electronics to ensure device reliability and longevity, requiring efficient heat dissipation techniques.

• Power electronics are essential in the growth of renewable energy technologies and electric vehicles, making them key to the future of energy conversion and management.

This week focuses on the use of semiconductor devices in power electronics, covering their roles in DC-DC converters, inverters, and rectifiers and their applications in renewable energy and motor control systems. Understanding these devices and their thermal management is essential for designing efficient power systems.