TEL423 — Power Electronics: Lecture Note

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Course code: TEL 423 Course title: Power Electronics Course credit load: 2 Units Series 1: Power Semiconductor Devices and Switched Mode Power Control • Part 1.1: Principles of Power Control by Switching o Sub Part 1.1.1: Linear Versus Switched Mode Power Control o Sub Part 1.1.2: Benefits of Switched Mode Circuits o Sub Part 1.1.3: Switching Losses and Efficiency Considerations • Part 1.2: Power Semiconductor Switching Devices o Sub Part 1.2.1: Power Diodes o Sub Part 1.2.2: Power BJTs and Power MOSFETs o Sub Part 1.2.3: Thyristors and IGBTs • Part 1.3: Switching Device Characteristics and Ratings o Sub Part 1.3.1: Voltage and Current Ratings o Sub Part 1.3.2: Switching Characteristics and Dynamic Behaviour o Sub Part 1.3.3: Thermal Considerations and Heat Sinking • Part 1.4: Circuit Analysis Techniques for Switched Mode Circuits o Sub Part 1.4.1: Idealised Switch Models o Sub Part 1.4.2: Analysis of Switching Waveforms o Sub Part 1.4.3: Practical Considerations in Switch Selection Introduction Every electrical load, from a mobile phone charger to an electric vehicle drive, needs power delivered in a form quite different from the form in which it is generated or stored, and the circuits that perform this conversion sit at the heart of modern electrical engineering. Rather than

dissipating unwanted energy as heat across a resistive element, as older linear regulation methods do, power electronic circuits achieve conversion by rapidly switching semiconductor devices between fully on and fully off states, a technique that underlies almost every efficient power supply, motor drive, and renewable energy interface in use today. This Series opens the course by asking why switching, rather than continuous control, has become the dominant approach to power conversion, and by introducing the semiconductor devices that make switched mode operation possible. The aim of this Series is to equip you with the principles of power control by switching, an understanding of the commonly used power semiconductor switching devices, familiarity with their key characteristics and ratings, and the basic circuit analysis techniques applicable to switched mode circuits, laying the foundation for the converter topologies examined later in this course. We shall commence this Series by examining the principles of power control by switching. Next, we will consider the power semiconductor switching devices used to implement it. Following that, we will examine switching device characteristics and ratings. Finally, we will conclude by examining circuit analysis techniques for switched mode circuits. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: distinguish switched mode power control from linear power control, • SLO 1.2: explain the benefits of switched mode circuits, • SLO 1.3: identify the commonly used power semiconductor switching devices, • SLO 1.4: compare the operating principles of power diodes, BJTs, MOSFETs, thyristors, and IGBTs, • SLO 1.5: interpret voltage and current ratings for power switching devices, • SLO 1.6: describe the switching characteristics and thermal behaviour of power devices, • SLO 1.7: apply idealised switch models to analyse switched mode circuits, • SLO 1.8: evaluate practical considerations in selecting a switching device. 1.1 Principles of Power Control by Switching 1.1.1 Linear Versus Switched Mode Power Control

A linear power regulator controls the output voltage by continuously varying the resistance of a pass transistor operating in its active region, dropping the excess supply voltage across that transistor as heat, so that a regulator supplying 5 V from a 12 V source must dissipate the difference across every unit of current it delivers. This continuous dissipation makes linear regulation simple and quiet in operation, producing an output free of switching noise, but it also makes efficiency fall sharply whenever the input-output voltage difference is large, since the wasted power grows in direct proportion to that difference and to the load current drawn. A switched mode power supply instead drives its main transistor fully on or fully off in rapid succession, so the device is either conducting current with very little voltage across it, or blocking voltage with very little current through it, and in both states the instantaneous power dissipated in the switch itself is small. Because a switch spends almost no time in the lossy intermediate region that a linear regulator occupies continuously, switched mode conversion can achieve efficiencies well above ninety percent even across a wide input-output voltage range, at the cost of a more complex circuit, an inductor or transformer for energy storage, and switching noise that must be filtered from the output. Worked example: A linear regulator converting 12 V to 5 V at 2 A dissipates (12−5)×2 = 14 W in the pass transistor alone, whereas a well-designed switched mode converter performing the same conversion at ninety percent efficiency dissipates only around 1.1 W in total losses. Application: Battery-powered equipment, where every watt wasted as heat shortens operating time between charges, relies almost exclusively on switched mode conversion rather than linear regulation. Fill in the blank: A linear regulator dissipates excess supply voltage as ________ across its pass transistor. 1.1.2 Benefits of Switched Mode Circuits Beyond the efficiency advantage already introduced, switched mode circuits offer several further benefits that explain their dominance in modern power conversion. Because losses are low, the semiconductor devices and any associated heat sinks can be made physically much smaller than the equivalent linear components handling the same power, and because switching frequencies typically range from tens of kilohertz into the megahertz region, the energy storage elements, inductors and capacitors, can likewise be made smaller than the low-frequency transformers required in older linear or ferro-resonant supplies. A switched mode circuit can also readily step voltage up as well as down, and can invert polarity, options that are difficult or impossible for a simple linear regulator, which can only reduce voltage. This flexibility allows a single switching topology, selected appropriately, to serve applications ranging from battery chargers to grid-tied inverters, and it is precisely this versatility, combined with high efficiency and compact size, that has made switched mode conversion the standard approach examined throughout the remainder of this course.

Worked example: A switched mode supply operating at 200 kHz requires an inductor many times smaller than the 50 Hz transformer a linear equivalent would need, allowing the switching supply to fit into a compact wall-mounted charger. Application: The compact size and light weight made possible by high-frequency switching are essential in portable and aerospace applications, where every gram of mass and every cubic centimetre of volume carries a direct cost. Fill in the blank: Higher switching frequencies allow the energy storage components in a converter to be made ________. 1.1.3 Switching Losses and Efficiency Considerations Although an ideal switch dissipates no power, a real semiconductor switch cannot change state instantaneously, so during every turn-on and turn-off transition it briefly carries significant current while simultaneously supporting significant voltage, producing a short pulse of power dissipation known as a switching loss. Unlike conduction loss, which depends only on the on-state resistance and the current level, switching loss grows directly with switching frequency, since a switch operating faster passes through its lossy transition region more often per second. Designing an efficient switched mode circuit therefore involves balancing switching frequency against switching loss: raising the frequency shrinks the passive components, as noted in the previous sub-Part, but also raises switching loss, so practical designs select a frequency, and a device technology suited to that frequency, that minimises total loss for the application at hand. Conduction loss, the loss occurring while the device is fully on, together with switching loss and any gate or base drive losses, make up the total losses that determine a converter’s overall efficiency. Worked example: A MOSFET switching at 500 kHz incurs proportionally higher switching losses than the same device switching at 50 kHz, even though its conduction loss at a given current remains unchanged. Application: Selecting a switching frequency is one of the first design decisions in any switched mode converter, balancing component size against total efficiency for the target application. Fill in the blank: Switching loss grows directly with the converter's switching ________.

Figure 1.1: Voltage, current, and instantaneous power during a switch turn-on transition. Pilot questions 1.1 1. Distinguish between linear and switched mode power control. (Linked to SLO 1.1) 2. State why linear regulation becomes inefficient at large input-output voltage differences. (Linked to SLO 1.1) 3. List two benefits of switched mode circuits over linear regulation. (Linked to SLO 1.2) 4. Explain why higher switching frequencies allow smaller passive components. (Linked to SLO 1.2) 5. Define switching loss and state how it varies with switching frequency. (Linked to SLO 1.2) 1.2 Power Semiconductor Switching Devices 1.2.1 Power Diodes A power diode is a two-terminal device that conducts current in one direction only, from anode to cathode, once forward biased beyond a small threshold voltage, and blocks current in the reverse direction up to its rated reverse breakdown voltage. In power electronic circuits, diodes are rarely used to control the flow of power directly, since they cannot be turned on or off on

command, but they perform essential supporting

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