1. AC-DC Converters (Rectifiers)

Also known as phase-controlled converters, these devices convert fixed-voltage, fixed-frequency AC power into a variable DC voltage output. This is achieved by using phase-controlled thyristors and altering the firing angle (α) at which they are triggered. The commutation (switching off) is natural or line-commutated.

• Pulse Configurations: The smoothness of the DC output and converter utilization improve with the number of pulses.
  • 2-Pulse Converter: Used for single-phase systems (half-bridge or full-bridge).
  • 3-Pulse Converter: Used for three-phase systems, with each thyristor conducting for one-third of the supply cycle.
  • 6-Pulse Converter: Standard for three-phase bridge converters, where two devices conduct at any given time.

1. DC-DC Converters (Choppers)

Choppers convert a fixed DC input voltage to a variable DC output voltage using a high-speed switching element.

• Step-Down (Buck) Converter: The average output voltage (V_o) is less than the input voltage (V_s). The output is controlled by the duty cycle (D), which is the ratio of ON-time to the total period. V_o = D * V_s.
• Step-Up (Boost) Converter: The average output voltage is greater than the input voltage. V_o = V_s / (1 – D).
• Step-Up/Down (Buck-Boost) Converter: Can produce an output voltage that is either higher or lower than the input. V_o = (D * V_s) / (1 – D). It provides buck operation for D < 0.5 and boost operation for D > 0.5.

1. DC-AC Converters (Inverters)

Inverters convert DC power into AC power at a required output voltage and frequency.

• Types:
  • Voltage Source Inverter (VSI): Fed by a stiff DC voltage source with low impedance.
  • Current Source Inverter (CSI): Fed by a variable DC current source with high impedance.
• Configurations:
  • Single-Phase: Includes half-bridge and full-bridge inverter designs.
  • Three-Phase: Consists of three arms, typically delayed by 120°, to generate a three-phase AC output.
• Conduction Modes for Three-Phase Inverters:
  • 180° Mode: Each device conducts for 180°, and three devices are ON at any time.
  • 120° Mode: Each device conducts for 120°, and only two devices are ON at any instant. This mode is suitable for delta-connected loads.

1. AC-AC Converters

These converters change AC power from one voltage or frequency to another directly.

• AC Voltage Controllers: Vary the RMS value of an AC voltage by introducing phase control using thyristors to chop the AC waveform.
• Cycloconverters: Convert AC power at one frequency to AC power at another (usually lower) frequency in a single stage using line commutation.
• Integral Cycle Control: A method of direct AC-AC conversion that switches full or half cycles of the input to the output, enabling lossless switching at zero voltage crossings.
• Matrix Converters: A single-stage converter that uses an array of bidirectional switches to directly convert AC to AC, offering a controllable input power factor and a compact design.

1. Converter Performance and Control

• Effect of Source Inductance: In practical systems, source inductance prevents instantaneous commutation, creating an “overlap interval” where multiple switches conduct simultaneously. This reduces the average DC output voltage.
• Performance Parameters:
  • Form Factor (FF): V_L / V_DC (Ratio of RMS load voltage to average DC load voltage).
  • Ripple Factor (RF): A measure of the pulsating AC component in the rectified DC output. sqrt(FF^2 – 1).
  • Efficiency (η): Ratio of DC output power to AC input power. (1 / FF)^2 for ideal devices.
  • Transformer Utilization Factor (TUF): Ratio of DC power delivered to the VA rating of the transformer.
• Control Methods:
  • Time Ratio Control: Varies the output voltage by controlling the ON/OFF time ratio (duty cycle). Can be achieved with constant frequency (Pulse Width Modulation) or variable frequency.
  • Current Limit Control: The switch is turned ON and OFF to maintain the load current between a predefined maximum and minimum level.
• Pulse Width Modulation (PWM): A key technique to control inverter output and reduce total harmonic distortion (THD). It involves comparing a reference waveform (e.g., sinusoidal) with a high-frequency carrier wave (e.g., triangular) to generate the switching pulses.
• Voltage and Harmonic Control: Harmonics are unwanted frequencies that are integer multiples of the fundamental frequency. THD is the measure of the total contribution of all harmonics. Filters and specific control techniques are used to mitigate them.
• Power Factor Control (PFC): The process of reducing reactive power to improve the power factor (the ratio of real power to apparent power). PFCs and specialized harmonic filters are used to achieve this.
• Space Vector Modulation (SVM): An advanced PWM algorithm used to control AC waveforms, particularly for driving three-phase AC motors.