Topologies of Power Electronic Circuits in PE Power Exam

author Wasim Asghar
author July 31, 2023

Welcome to the second guide of the series Devices and Power Electronic Circuits in the PE Power exam. As you prepare for the NCEES® PE Power exam, developing a solid foundation of power electronic circuits is essential. 

With the NCEES® exam guidelines and course roadmap in mind, we will further help you explore the critical components and working principles of power electronic topologies, providing a clear understanding of how different frameworks work.

If you have not read the first part of this study guide, read to learn the fundamentals of devices and power electronic circuits in the PE Power exam.

The second part of our study guide will help you explore different power electronic circuit topologies instrumental in the interconversion of AC and DC. Let’s dive deep into the details.

Power Electronic Circuit Topologies

Various power electronic circuit topologies use different types of diodes and other electronic components. These topologies or methodologies are intended for the interconversion of AC and DC. Let’s discuss the 4 power electronic circuit topologies with different devices and power electronic circuits in PE Power Exam.

AC to DC Converters – Rectifiers

rectifiers

Introduction to Rectification

Rectification converts alternating current (AC) to direct current (DC). In power electronics, rectifiers are essential to convert the sinusoidal AC voltage from the power supply into a pulsating DC voltage, which can be further smoothed to obtain a nearly constant DC voltage. Rectifiers are widely used in various applications, including power supplies, battery chargers, and motor drives.

1. Half-Wave Rectifier

The half-wave rectifier is the simplest form of a rectifier that uses a single diode to convert the positive half-cycle of the AC input waveform to DC. During the negative half-cycle, the diode blocks current flow.

Working of Half-Wave Rectifier
  • During the positive half-cycle of the AC input voltage, the diode becomes forward-biased, allowing current to flow through the load resistor in the same direction as the input voltage.
  • During the negative half-cycle of the AC input voltage, the diode becomes reverse-biased, blocking the current flow through the load resistor.
Output Voltage of Half-Wave Rectifier

The output voltage of the half-wave rectifier is the positive half-cycle of the input voltage waveform. The output voltage can be represented as:

Vout = Vm * (1 – e^(-t / RC))

Where:

  • Vout is the output voltage,
  • Vm is the peak value of the input voltage,
  • t is time,
  • RC is the product of the load resistor (R) and the output capacitor (C) time constant.
2. Full-Wave Rectifier

The full-wave rectifier is a more efficient type of rectifier that utilizes both the positive and negative half-cycles of the AC input waveform. It employs a bridge configuration of diodes, typically four, to rectify the entire AC waveform.

Working of Full-Wave Rectifier
  • During the positive half-cycle of the AC input voltage, the diodes D1 and D3 become forward-biased, allowing current to flow through the load resistor in the same direction as the input voltage.
  • During the negative half-cycle of the AC input voltage, the diodes D2 and D4 become forward-biased, and the current flows in the opposite direction through the load resistor.
Output Voltage of Full-Wave Rectifier

The output voltage of the full-wave rectifier is the combination of the positive and negative half-cycles of the input voltage waveform, resulting in a pulsating DC voltage. The output voltage can be represented as:

Vout = |Vm| – 2 * Vd – Vr

Where

  • Vout is the output voltage,
  • |Vm| is the absolute peak value of the input voltage,
  • Vd is the forward voltage drop of the diodes, and
  • Vr is the voltage drop across the load resistor.

Why Do We Use Capacitor Filter in Rectification?

capacitor filter in rectification

A capacitor filter is employed to convert the pulsating DC voltage from rectification into a nearly constant DC voltage. The capacitor acts as a reservoir, charging during the peak portions of the output waveform and discharging during the valleys.

Working of Capacitor Filter
  • During the positive half-cycle, the capacitor charges to the peak value of the output voltage.
  • During the negative half-cycle, the capacitor discharges, supplying current to the load and maintaining the output voltage.

Why is Zener Diode Used as a Voltage Regulator in Rectification?

voltage regulator in rectification

While the capacitor filter helps in smoothing the DC voltage, it may not provide a precise constant DC voltage. To achieve a stable and regulated DC voltage, a Zener diode voltage regulator can be added.

Working of Zener Diode Voltage Regulator

The Zener diode is reverse-biased and connected in parallel with the output. It has a breakdown voltage known as the Zener voltage (Vz).

When the output voltage exceeds the Zener voltage, the Zener diode begins to conduct in reverse breakdown, providing a constant voltage drop across its terminals.

This ensures that the output voltage remains close to the Zener voltage, providing a stable DC voltage output.

A power electronic circuit can efficiently convert AC voltage to a regulated DC voltage, vital for various electronic devices and systems, by combining half-wave or full-wave rectification, capacitor smoothing, and Zener diode voltage regulation.

Zener Diodes – Everything You Need to Know

zener diodes

Zener diodes are special semiconductor devices designed to operate in the reverse-biased breakdown region, known as the Zener breakdown region. They are commonly used as voltage regulators in power electronics circuits to provide a stable and constant DC voltage output. In this guide section, we will explore the working of a Zener diode voltage regulator in detail.

Zener Diode Characteristics

A Zener diode is similar to a regular diode but is specifically designed to have a sharp breakdown voltage (Zener voltage, Vz) in the reverse-biased condition. When the reverse bias voltage across the Zener diode reaches or exceeds its breakdown voltage, it starts conducting in the reverse direction.

Zener Breakdown

In a Zener diode, breakdown occurs due to the Zener effect. The Zener effect is a quantum mechanical phenomenon where, at a certain level of electric field strength, electrons in the valence band acquire enough energy to cross the forbidden energy gap and enter the conduction band. This results in the generation of electron-hole pairs, leading to a sharp increase in reverse current.

Operating in Zener Breakdown Region

When a Zener diode is reverse-biased, and the applied reverse voltage exceeds the Zener voltage (Vz), the diode enters the Zener breakdown region. In this region, the Zener diode behaves as a voltage regulator, maintaining a relatively constant voltage drop across its terminals.

Zener Diode Voltage Regulation

The Zener diode voltage regulation is achieved by connecting the diode in parallel with the load in a power electronic circuit. Here’s how it works:

Below Zener Voltage (Vz)

When the output voltage (Vout) of the circuit is below the Zener voltage (Vz), the Zener diode remains in the off state, and only a small reverse saturation current (Is) flows through it. The voltage across the load remains almost equal to the input voltage, and the Zener diode does not play a significant role.

Above Zener Voltage (Vz)

The Zener diode enters the breakdown region when the output voltage (Vout) exceeds the Zener voltage (Vz). The diode starts conducting heavily in the reverse direction, providing a nearly constant voltage drop equal to its Zener voltage (Vz) across its terminals.

Why Choose Zener Diode as Voltage Regulator?

Zener diodes significantly contribute to some significant benefits of power electronics, such as:

  • Simple and Cost-Effective – Zener diode voltage regulators are simple to implement and cost-effective compared to other voltage regulation techniques.
  • Fast Response – Zener diodes respond quickly to changes in the load current and maintain a stable output voltage.

Limitations of Zener Diode as Voltage Regulator

Simulation of power electronics systems using zener diodes comes amid some limitations. Let’s have a quick look.

  • Power Dissipation – Zener diodes have limited power handling capabilities, and excessive current through the diode can lead to overheating and failure.
  • Voltage Variation – The output voltage of a Zener diode regulator is not precisely constant but can vary slightly with changes in load current and temperature.

Load Regulation and Line Regulation with Zener Diode

The performance of a Zener diode voltage regulator is characterized by load regulation and line regulation:

  • Load Regulation – Load regulation refers to the ability of the voltage regulator to maintain a constant output voltage as the load current changes. It is expressed as a percentage change in output voltage for a specified change in load current.
  • Line Regulation – Line regulation refers to the ability of the voltage regulator to maintain a constant output voltage when the input voltage varies. It is expressed as a percentage change in output voltage for a specified change in input voltage.

Capacitor Filter – Everything You Need to Know

A capacitor filter is often added in parallel with the output to reduce voltage ripples caused by the Zener diode’s inherent limitations. The capacitor smooths the voltage variations and provides a more stable output.

Zener diode voltage regulators are valuable components in power electronics circuits, providing stable and regulated DC voltage outputs. By understanding the Zener effect and the operating characteristics of Zener diodes, students can comprehend the role and significance of Zener diodes as voltage regulators in various electronic applications.

DC-to-DC Converters

DC-to-DC converters, also known as voltage regulators or voltage converters, are essential power electronic circuits that efficiently convert one DC voltage level to another.

They are widely used in various applications, including battery-powered devices, power supplies, and renewable energy systems. In this guide section, we will explore the working of three common types of power electronics circuits as DC-to-DC converters.

1. Buck Converter

The Buck converter, or the step-down converter, converts a higher DC voltage to a lower DC voltage. A switching converter uses a switching element (usually a MOSFET) and an inductor to achieve the voltage conversion.

Working of Buck Converter
  • During the ON period of the switching element, the inductor current increases, storing energy in the inductor.
  • During the OFF period of the switching element, the inductor current decreases, releasing energy to the output load through the diode.

*The output voltage is controlled by varying the switching element’s duty cycle (ON time vs. OFF time).

Components and Purpose
  • Switching Element – A MOSFET or a transistor is used as the switching element to control the current flow through the inductor.
  • Inductor – The inductor stores and releases energy during the ON and OFF periods.
  • Diode – The diode allows the inductor current to flow to the output during the OFF period of the switching element.
Operation of Components
  • Switching Element – The switching element is controlled by a pulse-width modulation (PWM) signal. When it is ON, it connects the input voltage to the inductor. When it is OFF, it disconnects the inductor from the input voltage.
  • Inductor – During the ON period of the switching element, the inductor current ramps up, storing energy as a magnetic field. During the OFF period, the inductor releases energy to the output load, maintaining a continuous current flow.
  • Diode – The diode acts as a freewheeling path for the inductor current during the OFF period of the switching element, preventing voltage spikes.
2. Boost Converter

The Boost converter, or the step-up converter, converts a lower DC voltage to a higher DC voltage. It is another switching converter that uses an inductor and a diode to achieve voltage conversion.

Working of Boost Converter
  • During the ON period of the switching element, the inductor current increases, storing energy.
  • During the OFF period of the switching element, the inductor releases energy to the output load through the diode and capacitor.

*The output voltage is controlled by varying the switching element’s duty cycle (ON time vs. OFF time).

Components and Purpose
  • Switching Element – Similar to the Buck converter, a MOSFET or a transistor is used as the switching element to control the current flow through the inductor.
  • Inductor – The inductor stores and releases energy during the ON and OFF periods.
  • Diode – The diode allows the inductor to discharge its stored energy to the output during the OFF period of the switching element.
  • Capacitor – The capacitor smooths the output voltage and reduces voltage ripples.
Operation of Components
  • Switching Element – The switching element is controlled by a PWM signal. When it is ON, it connects the input voltage to the inductor. When it is OFF, it disconnects the inductor from the input voltage.
  • Inductor – During the ON period of the switching element, the inductor current ramps up, storing energy as a magnetic field. During the OFF period, the inductor releases energy to the output through the diode and charges the output capacitor.
  • Diode – The diode acts as a freewheeling path for the inductor current during the OFF period of the switching element, preventing voltage spikes.
3. Buck-Boost Converter

The Buck-Boost converter is a versatile switching converter that can step up or down the input voltage, making it suitable for higher-to-lower and lower-to-higher voltage conversion.

Working of Buck-Boost Converter
  • During the ON period of the switching element, the inductor current increases, storing energy.
  • During the OFF period of the switching element, the inductor releases energy to the output through the diode and capacitor.

*The output voltage is controlled by varying the switching element’s duty cycle (ON time vs. OFF time).

Components and Purpose
  • Switching Element – Similar to the Buck and Boost converters, a MOSFET or a transistor is used as the switching element to control the current flow through the inductor.
  • Inductor – The inductor stores and releases energy during the ON and OFF periods, respectively.
  • Diode – The diode allows the inductor to discharge its stored energy to the output during the OFF period of the switching element.
  • Capacitor – The capacitor smooths the output voltage and reduces voltage ripples.
Operation of Components
  • Switching Element – The switching element is controlled by a PWM signal. When it is ON, it connects the input voltage to the inductor. When it is OFF, it disconnects the inductor from the input voltage.
  • Inductor – During the ON period of the switching element, the inductor current ramps up, storing energy as a magnetic field. During the OFF period, the inductor releases energy to the output through the diode and charges or discharges the output capacitor, depending on the input and output voltage relationship.
  • Diode – The diode acts as a freewheeling path for the inductor current during the OFF period of the switching element, preventing voltage spikes.

DC-to-DC converters are crucial components in power electronics, allowing efficient and precise voltage conversion for various applications. The Buck, Boost, and Buck-Boost converters offer different voltage conversion capabilities and can be combined to create more complex power electronic systems, catering to diverse power requirements. 

Understanding these converters’ working principles and components is essential for PE Power exam aspirants studying devices and power electronic circuits in the PE Power Exam.

DC-to-AC Converters

DC-to-AC converters, or inverters, are essential power electronic circuits that convert direct current (DC) into alternating current (AC). They find widespread use in various applications, such as solar power systems, uninterruptible power supplies (UPS), motor drives, and grid-tie systems. 

In this study note, we will explore the working of inverters in all technical detail, covering their components, purpose, and operation.

1. Basic Inverter

The basic inverter converts a fixed DC input voltage to a variable AC output voltage, typically with a fixed frequency. It is also called a square wave inverter or a single-phase inverter.

Working of Basic Inverter
  • The inverter circuit consists of a DC input source connected to a switching element (such as a MOSFET or an IGBT) and an output transformer.
  • During the ON state of the switching element, the DC input voltage is applied to the primary winding of the transformer.
  • During the OFF state of the switching element, the primary current is interrupted, causing the transformer core to demagnetize.
  • The changing magnetic field induces an AC voltage in the secondary winding of the transformer, generating the AC output.
Components and Purpose

DC Input Source: The DC input source provides the fixed input voltage to the inverter.

  • Switching Element – The switching element controls the connection and disconnection of the DC input to the transformer’s primary winding.
  • Output Transformer – The output transformer converts the DC input to AC output, isolating the input and output.
Operation of Components
  • Switching Element – The switching element is controlled by a PWM signal, which regulates the ON and OFF times to generate the desired AC waveform.
  • Output Transformer – The transformer steps up or down the voltage to achieve the desired output voltage level and provides isolation for safety and protection.
2. Modified Sine Wave Inverter

The modified sine wave inverter, also known as a quasi-square wave inverter, aims to approximate a sine wave by employing a stepped waveform with various levels.

Working of Modified Sine Wave Inverter
  • The modified sine wave inverter uses multiple switching states to create a stepped waveform.
  • The stepped waveform consists of alternating positive and negative voltage levels that approximate the shape of a sine wave.
Components and Purpose

The components and purpose of a modified sine wave inverter are similar to those of the basic inverter.

3. Pure Sine Wave Inverter

The pure sine wave inverter generates an output waveform that closely resembles a sinusoidal waveform, similar to the AC power supplied by the utility grid.

Working of Pure Sine Wave Inverter
  • The pure sine wave inverter uses more complex circuitry and control algorithms to generate a smooth sinusoidal output.
  • It employs pulse-width modulation (PWM) techniques to produce precise voltage levels at specific intervals, producing a high-quality sine wave.
Components and Purpose

A pure sine wave inverter’s components and purpose are more sophisticated than the fundamental and modified sine wave inverters. It includes additional filtering and control circuitry to achieve high-quality sinusoidal output.

Operation of Components

PWM Controller – The PWM controller generates precise control signals for the switching element to achieve the desired sine wave output.

Filtering Components – Additional filtering components, such as inductors and capacitors, are used to smooth out the stepped waveform, resulting in a near-perfect sine wave.

DC-to-AC converters, or inverters, are essential in power electronics for converting DC power to AC power in various applications. Understanding the working principles and components of inverters is vital for students studying devices and power electronic circuits in the PE Power Exam

AC-to-AC Converters

AC-to-AC converters, also known as cyclo converters, are power electronic devices used to convert one AC voltage and frequency to another AC voltage and frequency. Unlike traditional AC-to-DC and DC-to-DC converters, cyclo converters provide AC-to-AC conversion without needing intermediate DC stages. 

They are mainly used in applications that often require variable-frequency and variable-voltage AC power. Let’s discuss the working of different cyclo converters crucial for PE Power exam aspirants studying devices and power electronic circuits in the PE Power Exam.

1. Single-Phase to Single-Phase Cyclo Converter

The single-phase to single-phase cyclo converter converts the input single-phase AC voltage at one frequency to a single-phase output AC voltage at a different frequency. It is widely used in applications like speed control of single-phase induction motors and variable voltage/frequency drives.

Working

A single-phase to single-phase cyclo converter consists of pairs of thyristors (SCRs – Silicon-Controlled Rectifiers) or GTOs (Gate Turn-Off Thyristors). These devices are arranged in various switching configurations to control the output voltage and frequency.

Components and Purpose
  • Thyristors (SCRs/GTOs) – The thyristors are the switching devices responsible for controlling the output voltage and frequency.
  • Output Filter – An output filter smokes the output waveform and reduces harmonics.
Operation of Components
  • Thyristors (SCRs/GTOs) – The thyristors are controlled by firing pulses at specific angles to determine when they should conduct during each half-cycle of the input waveform.
  • Output Filter – The output filter removes higher-order harmonics from the output waveform, resulting in a smoother sinusoidal waveform.
2. Three-Phase to Single-Phase Cycloconverter

The three-phase to single-phase cycloconverter converts a three-phase AC voltage at one frequency to a single-phase output AC voltage at a different frequency. It is used in applications requiring variable-frequency and variable-voltage single-phase power.

Working

A three-phase to single-phase cyclo converter combines the input three-phase AC voltage into a single-phase output. It uses switching devices, such as thyristors or GTOs, to control the power flow during each half-cycle of the input waveform.

Components and Purpose

Same as single-phase to single-phase converter

3. Three-Phase to Three-Phase Cycloconverter

The three-phase to three-phase cyclo converter converts a three-phase AC voltage at one frequency to another three-phase AC voltage at a different frequency. It is commonly used in applications requiring variable-speed drives for three-phase motors.

Working

A three-phase to three-phase cyclo converter uses multiple switching devices, such as thyristors or GTOs, to control the power flow between the input and output three-phase systems.

Components and Purpose

Same as single-phase to single-phase converter

Purpose and Applications of Cyclo Converters

Cycloconverters find applications in variable-speed motor drives, induction heating systems, and grid interconnections.

They offer flexibility in generating different output frequencies and variable voltage magnitudes.

AC-to-AC converters, or cyclo converters, are essential in power electronics for converting AC power to different frequencies and voltage levels without needing intermediate DC stages. By controlling the switching devices at specific angles, cyclo converters can generate the desired output waveform with the desired frequency. 

Understanding cyclo converters’ working principles, components, and applications is crucial for Power exam aspirants studying devices and power electronic circuits in the PE Power exam.

Conclusion

Understanding power electronic topologies is a crucial part of devices and power electronic circuits in the PE Power exam. According to the NCEES® PE Power exam guidelines and course roadmap, a solid grasp of power electronic components, such as power circuit topologies, diodes, transistors, thyristors, and inverters, is vital for successfully tackling exam questions and practical scenarios.

For those looking to take their preparation to the next level, Study for FE provides a recognized platform offering comprehensive PE Power exam preparation courses. Explore Study for FE today to achieve your career aspirations and make a lasting impact in the world of power engineering.

wasim-smal
Wasim Asghar – P.E, P.ENG, M.ENG

Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.