Answer Key
- The fundamental goal of power electronics is to control the flow of current and voltage, converting it into a form suitable for user loads. The key objectives are to achieve maximum efficiency, reliability, and availability, while also ensuring minimum cost, least weight, and a small size.
- The three modes of operation for an SCR are the OFF state (forward blocking), where junctions J1 and J3 are forward biased but J2 is reverse biased; the ON state (conducting), which is triggered by a gate pulse or by exceeding the avalanche voltage; and Reverse blocking, which compensates for forward voltage drop.
- A TRIAC is a bi-directional semiconductor device, whereas an SCR is unidirectional. This bi-directional capability allows the TRIAC to control current flow for both halves of an AC cycle, making it ideal for AC power switching applications like residential light dimmers.
- A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that can act as a switch, amplifier, or oscillator. It is called “bipolar” because its operation depends on two types of charge carriers: holes, which are the dominant carriers in P-type semiconductors, and electrons, which are the main carriers in N-type semiconductors.
- An IGBT combines the characteristics of a MOSFET and a BJT. It integrates an isolated gate using FET technology, similar to a MOSFET, for control input, while achieving the high current and low saturation voltage capacity characteristic of a BJT.
- A chopper is a power electronic device that uses high-speed switching to connect and disconnect a source from a load, converting a fixed DC voltage to a variable DC voltage output. The three main classifications are the Step-Up chopper (boost converter), the Step-Down chopper (buck converter), and the Step-Up/Down chopper (buck-boost converter).
- Pulse Width Modulation (PWM) is a technique used to reduce the total harmonic distortion (THD) in a load current. A sinusoidal PWM waveform is generated by comparing the desired modulated waveform with a high-frequency triangular carrier waveform; the output voltage is switched between positive and negative based on whether the signal voltage is larger or smaller than the carrier voltage.
- A cycloconverter is a frequency changer that converts AC power from one frequency to another in a single stage. Its primary limitation is that its operations are only smooth at frequencies that are not equal to or greater than half the input frequency value, meaning its output frequency is typically lower than the input frequency.
- The two basic methods for controlling the output voltage in a converter are Time Ratio Control and Current Limit Control. Time Ratio Control varies the duty ratio (TON/T) using either constant frequency (PWM) or variable frequency, while Current Limit Control switches the chopper on and off to maintain the current between set maximum and minimum levels.
- A dual converter consists of two converters linked together back-to-back. They are primarily found in applications requiring four-quadrant operation, such as variable speed drives (VFDs).