Essential Concepts in Power Electronics Devices and Circuits

Thyristors and SCR Fundamentals

Working Principle of a Thyristor

A thyristor is a four-layer, three-junction semiconductor device that functions as a switch. It consists of two p-n junctions and operates in four different regions:

• Forward Blocking
• Forward Conducting
• Reverse Blocking
• Reverse Conducting

Thyristor Working Mechanism

When a small trigger pulse is applied to the gate, it switches the thyristor from the forward blocking state to the forward conducting state, allowing current to flow between the anode and cathode. Once turned on, the thyristor remains conducting even after the trigger pulse is removed. Current flow continues until the voltage polarity across the device reverses, or the current drops below the holding current, at which point it turns off.

Turn-On and Turn-Off Characteristics

Turn-On Characteristics

When a small gate current is applied, the thyristor enters the forward conducting state. The device starts conducting once the forward voltage exceeds the threshold, and the current increases exponentially. The thyristor remains in the "on" state as long as the current is above the holding current, even if the gate current is removed.

Turn-Off Characteristics (Commutation)

The thyristor turns off when the anode current falls below the holding current. It can also be forced to turn off by reversing the voltage across the device (i.e., by applying a negative anode voltage). Once turned off, it cannot conduct again unless triggered with a gate pulse.

Silicon Controlled Rectifier (SCR) Basics

The SCR (Silicon Controlled Rectifier) is a four-layer, three-terminal device used for power control.

Forward Conduction Mode

In this mode, the anode is positive, the cathode is negative, and a gate pulse is applied. The device turns ON and conducts until the current drops below the holding current.

SCR Commutation Techniques

Commutation refers to the process of turning off an SCR.

• Natural Commutation: Occurs when the AC current naturally goes to zero (used in AC circuits).
• Forced Commutation: An external circuit forces the current to zero (used primarily in DC circuits).

SCR Connections for Power Handling

To handle higher power requirements, SCRs can be connected:

• Series Connection: Used to increase the overall voltage rating.
• Parallel Connection: Used to increase the overall current rating.

Note: Both series and parallel connections require equalizing circuits to ensure uniform voltage and current sharing among the devices.

Controlled Rectifiers and Converters

Single-Phase Half-Controlled Rectifier (R-Load)

In a single-phase half-controlled rectifier, only one of the two primary switching devices is controlled (e.g., a thyristor), while the other is uncontrolled (a diode).

Working Operation

During the positive half-cycle of the AC input, the controlled device is triggered and conducts, providing a positive output voltage. During the negative half-cycle, the uncontrolled diode conducts, allowing current to flow and providing an output voltage that is less than the peak input voltage (due to the rectification process). The average output voltage depends critically on the triggering angle of the controlled device.

The Role of the Freewheeling Diode

A freewheeling diode (also known as a flyback diode) is connected across the load in a controlled rectifier circuit.

Effect on Output

During the off period of the controlled rectifier, the freewheeling diode allows the current to circulate through the load, preventing sudden voltage spikes, especially with inductive loads. This helps to reduce the ripple in the output and improves the waveform, making the system more stable by providing a continuous path for the current when the main rectifier switches are not conducting.

Three-Phase Full Converter Analysis

A three-phase full converter uses six thyristors for full control over the output voltage.

Output Voltage Calculation

The average DC output voltage (V_dc) of a three-phase full converter is given by:

V_dc = (3 √3 / π) · V_m · cos(α)

Where:

• V_m = Maximum phase voltage
• α = Firing angle (control angle)

Output Current Calculation

The output current (I_dc) is related to the load resistance (R_load) and the output voltage, calculated using Ohm's law:

I_dc = V_dc / R_load

Types and Applications of Converters

Half-Controlled vs. Fully Controlled Converters

• Half-Controlled: Uses a mix of diodes and SCRs, resulting in unidirectional current flow.
• Fully Controlled: Uses only SCRs, allowing for bidirectional control (motoring and regeneration) under certain conditions.

Four-Quadrant Converter Design

A four-quadrant converter uses two full-bridge converters or specialized chopper circuits to control both the polarity of the voltage and the current. This allows operation in all four quadrants (forward motoring, forward braking, reverse motoring, and reverse braking).

Converter Applications in Power Electronics

Converters are essential for:

• AC-DC conversion (Rectifiers)
• DC-AC conversion (Inverters)
• Voltage regulation in motor drives
• Integration in renewable energy systems

DC Choppers: Principles and Performance

Definition and Types of DC Choppers

A DC chopper is a static device that converts a fixed DC input voltage into a variable DC output voltage. It effectively acts like a DC transformer.

Choppers are widely used in electric traction, electric vehicles, and battery-powered systems.

Principle of Operation

The average output voltage (V_o) is controlled by rapidly switching the input voltage ON and OFF. The relationship is defined by the duty cycle (D):

V_o = D · V_in

Where D is the duty cycle, D = T_on / (T_on + T_off).

Classification of Choppers

• Type A (Step-Down Chopper): Output voltage is less than the input (V_o < V_in).
• Type B (Step-Up Chopper): Output voltage is greater than the input (V_o > V_in).
• Type C (Two-Quadrant Chopper): Can operate in both motoring and regenerative braking modes.
• Type D (Four-Quadrant Chopper): Allows control in all four quadrants.
• Type E (Class E): Also known as a dual converter configuration.

Transistor-Based Choppers: These use BJT or MOSFET switches, offering faster response times and are typically used in low to medium power systems.

Performance Analysis of a Step-Up Chopper

For a step-up chopper, also called a boost converter, the output voltage is always greater than the input voltage.

Key Performance Parameters

1. Output Voltage (V_o):

   V_o = V_in / (1 - D)

   Where D is the duty cycle.

2. Duty Cycle (D):

   D = T_on / (T_on + T_off)

3. Ripple Factor: Determines the variation (ripple) in the output voltage or current.
4. Efficiency (η): The ratio of output power to input power.

Applications of Choppers

Choppers are utilized across various power electronic systems:

• Electric Traction: Controlling the speed of DC motors in trains.
• Battery-Powered Vehicles: Efficient control of energy flow and motor drives.
• Solar Power Systems: Used in Maximum Power Point Tracking (MPPT) circuits for optimizing power extraction.
• SMPS (Switched Mode Power Supplies): For efficient DC voltage regulation.
• Electric Welding: Providing variable DC voltage for arc control.
• Trolley Buses and Forklifts: Efficient DC motor drive control.

Inverters: DC to AC Conversion

Inverter Definition and Applications

An inverter is a power electronic circuit that converts DC power into AC power.

General Applications

• UPS (Uninterruptible Power Supplies)
• Solar power systems (PV grid integration)
• Motor drives (Variable Frequency Drives)
• HVDC transmission systems

Single-Phase Half-Bridge Inverter Operation

The single-phase half-bridge inverter uses two switches (typically MOSFETs or IGBTs) and requires either two equal DC sources or a center-tapped transformer.

Operation Cycle

1. During the first half cycle, the first switch conducts, and the output voltage is +V_dc/2.
2. During the second half cycle, the second switch conducts, and the output voltage is -V_dc/2.

The resulting output is an alternating square wave. Filtering is often used to obtain a sinusoidal waveform.

Specialized Inverter Types

McMurry-Bedford Half-Bridge Inverter

This type of inverter utilizes a resonant LC circuit for commutation. It is designed to provide zero current turn-off for thyristors, making it suitable for high-frequency applications.

Inverter Applications in Renewable Energy

Inverters are critical components in renewable energy systems:

• Conversion: Converting DC power generated by solar panels or stored in batteries into usable AC power for the grid or household use.
• Grid-Tied Systems: Synchronizing the generated AC power precisely with the grid frequency and voltage.
• Off-Grid Systems: Providing reliable, standalone AC power.
• Wind Turbines: Converting the variable frequency AC output from the turbine generator first to DC, and then inverting it back to fixed frequency AC suitable for the grid.
• Hybrid Systems: Managing and directing energy flow efficiently from multiple sources (solar, battery, grid).

AC Voltage Controllers and Phase Control

Single-Phase AC Voltage Controller (R-Load)

An AC voltage controller uses thyristors or TRIACs to regulate the RMS value of the voltage supplied to an AC load.

Operation with Resistive Load (R-Load)

Thyristors are fired at a specific angle (α), delaying the conduction part of each half-cycle. The output voltage is directly dependent on this firing angle. Since the load is purely resistive, no current flows during the blocked period (as no energy is stored).

Concept of Phase Control

Phase control is the fundamental method used in AC voltage controllers. It involves delaying the turn-on angle (α) of the thyristors within each half-cycle of the AC waveform. By controlling α, the effective RMS voltage across the load is precisely regulated.

Phase Control Applications

• Light dimming systems
• Speed control of fans and small motors
• Industrial heating control

Three-Phase AC Voltage Regulator Analysis

A three-phase AC regulator uses thyristors to control the voltage delivered to 3-phase loads by delaying the firing angle of each phase. This is commonly used in industrial heaters, lighting, and motor speed control.

RMS Output Voltage Calculation

The RMS output voltage (V_o(rms)) for a phase-controlled controller is calculated as:

V_o(rms) = V_in · √ [ (1/π) · ∫_α^π sin²(θ) dθ ]

Where α is the firing angle.

The voltage equation for an AC regulator is also given by:

V_rms = V_m · √ [ (1/π) · ∫_α^π sin²(θ) dθ ]

Output Current Calculation

For a purely resistive load (R), the RMS output current (I_o(rms)) is:

I_o(rms) = V_o(rms) / R

Note: For complex or practical loads, simulation or numerical calculation methods are often required to find exact values.

Power Semiconductor Devices

Power MOSFET Characteristics

The Power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is crucial for high-speed switching applications.

Transfer Characteristic

This characteristic shows the relationship between the gate-to-source voltage (V_gs) and the drain current (I_d).

Output Characteristic

This characteristic shows the relationship between the drain-to-source voltage (V_ds) and the drain current (I_d) for different levels of V_gs.

Operating Regions

The Power MOSFET operates in three main regions:

• Cutoff Region: V_gs < V_th (Threshold voltage).
• Triode Region: Linear operation.
• Saturation Region: Switching mode operation.

Advantages of MOSFET-Based Converters

• High-speed switching capability
• High efficiency
• Low gate drive power requirement
• Compact design

Insulated Gate Bipolar Transistor (IGBT)

The IGBT (Insulated Gate Bipolar Transistor) is a hybrid power device combining the input characteristics of a MOSFET (voltage control) and the output characteristics of a BJT (high current capability).

Basic Structure

The IGBT has a four-layer structure (P⁺N^-P N⁺) and is controlled by a MOS gate structure.

Applications

IGBTs are ideal for high voltage and high current applications, such as large motor drives and Uninterruptible Power Supplies (UPS).

Motor Control and Frequency Conversion

Electric Drives Systems

An electric drive is a system used to control the speed, torque, and position of an electric motor. Key components include:

• Power Modulator (Converter/Chopper)
• Controller
• Electric Motor
• Feedback System

Rotor Resistance Control

This method is specifically used in slip ring induction motors to control speed by inserting variable resistance into the rotor circuit.

Frequency Conversion

The principle of frequency conversion involves changing the AC frequency using a two-stage process: AC input is rectified to DC, and then the DC is inverted back to AC at the desired new frequency. This technique is essential for motor speed control and grid interfacing.

Cycloconverters

Cycloconverters convert AC power at one frequency directly to AC power at a different, usually lower, frequency without an intermediate DC link.

• Step-Down Cycloconverter: Output frequency is less than the input frequency.
• Step-Up Cycloconverter: Output frequency is greater than the input frequency (rare).
• Available in Single-Phase and Three-Phase configurations.