Power Electronics -PE-EC505C- Module4 ( MAKAUT-Syllabus)

 

Today, Our Communication Elements Are: - Principle of Operation of Full Bridge Square Wave Inverter, Quasi-Square Wave, PWM Inverters, and Comparison of Their Performance, Driver Circuits for Inverters and Fourier Analysis of Output Voltage with Harmonic Control, Filters at the Output of Inverters, Single-Phase Current Source Inverter (CSI).

Principle of Operation of Full Bridge Square Wave Inverter: -

Power electronics plays a vital role in converting and controlling electrical power for different applications. One of the most important circuits in this area is the full-bridge square-wave inverter. It is widely used for applications where a simple and cost-effective conversion from DC (Direct Current) to AC (Alternating Current) is required. In this blog, we will understand the principle of operation of a full-bridge square wave inverter, its working stages, advantages, limitations, and real-world uses.

What is a full bridge inverter

A full bridge inverter is an electronic circuit that converts DC power into AC power using four controlled switches, usually power transistors, IGBTs, or MOSFETs. The output waveform obtained is a square wave, hence the name square wave inverter.

Unlike a half-bridge inverter, which uses only two switches, the full-bridge configuration offers double the output voltage, making it more efficient for higher power applications.

Principle of Operation

The working of the full bridge square wave inverter is based on the controlled switching of four devices arranged in an H-shaped bridge. Let us go step by step.

1. Circuit Configuration

• Four switches (S1, S2, S3, S4) are connected in a bridge across the DC supply.
• The load is connected between the middle points of the two bridge arms.
• To avoid short circuits, the switches must be turned on in a specific sequence.

2. Positive Half Cycle

• When switches S1 and S4 are turned ON, current flows from the positive terminal of the DC source, through S1, through the load, then through S4, and finally back to the negative terminal.
• This makes the load experience a positive polarity of voltage.

3. Negative Half Cycle

• When switches S2 and S3 are turned ON, current flows from the positive terminal of the DC source, through S3, through the load in the opposite direction, then through S2, and finally back to the negative terminal.
• The load now experiences a negative polarity of voltage.

4. Resulting Waveform

• By alternating between these two states, the inverter produces a square wave AC output across the load.
• The frequency of the output depends on the switching speed of the devices.

Key Features of Square Wave Inverter

• Output waveform: square wave (contains fundamental frequency plus harmonics).
• Output voltage: Equal to the DC input voltage (in the ideal case).
• Switching devices: Usually MOSFETs, IGBTs, or thyristors with proper driver circuits.
• Control: Simple switching sequence compared to sinusoidal inverters.

Advantages

• Simple and cost-effective design.
• Higher output voltage compared to half-bridge inverters.
• Easy to implement with fewer control complexities.
• Useful for low-cost power conversion needs.

Limitations

• The square wave contains harmonics, which can cause heating and noise in sensitive loads.
• Not suitable for running inductive loads like induction motors (without filtering).
• Output quality is poor compared to sine wave inverters.

Applications

• Used in simple power supply units.
• Inverters for low-cost appliances such as fans, lights, and heaters.
• DC to AC conversion in emergency backup power systems.
• Testing equipment where a pure sine wave is not required.

Conclusion

The full bridge square wave inverter is a fundamental building block in power electronics. Its principle of operation is simple: alternating pairs of switches conduct in sequence to reverse the polarity across the load, thus producing an AC square wave. While its waveform quality is not suitable for sensitive devices, its simplicity and low cost make it an ideal choice in many basic applications.

Quasi-Square Wave: -

When we talk about inverters in power electronics, one of the most important waveform types that comes up is the quasi-square wave. It sits somewhere between the basic square wave and the pure sine wave. The quasi-square wave inverter is one of the simplest ways to convert DC into AC while reducing harmonics compared to a pure square wave. Let’s break it down in simple words.

What is a quasi-square wave

A square wave inverter switches the output between positive and negative DC voltage levels directly. While it is simple, it has a major drawback: it produces too many harmonics, which makes it unsuitable for sensitive loads like computers or audio equipment.

A quasi-square wave is an improved version of the square wave. Instead of switching instantly from +Vdc to –Vdc, it introduces a zero-voltage step in between. This makes the waveform look like a flattened version of a square wave, hence the name 'quasi'.

Working Principle

The inverter uses power semiconductor switches (like MOSFETs, IGBTs, or thyristors) to chop the DC supply into a stepped waveform. The control circuit is designed to hold the output at zero for a small duration before switching to the opposite polarity. This zero interval significantly reduces the third harmonic and other lower-order harmonics, making the waveform smoother and closer to sine wave behaviour.

For example, if a square wave directly jumps between +230V and –230V, a quasi-square wave may follow this sequence: +230V → 0V → –230V → 0V → +230V.

Advantages of Quasi-Square Wave

• Reduced Harmonics: By inserting the zero-voltage step, the distortion decreases compared to a square wave.
• Simplicity: The circuit design is straightforward and cheaper than producing a sine wave.
• Better Load Compatibility: Inductive and resistive loads can handle quasi-square waveforms more efficiently than square waves.
• Good for Low-Cost Applications: Useful in devices like small UPS systems, fans, lights, and simple household electronics.

Limitations

However, quasi-square wave inverters are not perfect. They still generate harmonics (though less than a square wave) and are not suitable for highly sensitive electronic gadgets or medical instruments. Today, PWM (Pulse Width Modulated) sine wave inverters are replacing them in most critical applications.

Applications

• Low-cost UPS systems
• Emergency power supplies for fans, lights, and pumps
• Some motor drive applications
• Small household inverters

Conclusion

The quasi-square wave inverter represents a smart compromise between simplicity and performance. While it cannot achieve the smoothness of a pure sine wave, it offers a practical and cost-effective solution for many everyday applications. By inserting zero-voltage steps, engineers reduce harmful harmonics and improve overall efficiency.

PWM Inverters and Comparison of Their Performance: -

In modern power electronics, Pulse Width Modulation (PWM) inverters play a vital role in efficiently converting DC power into high-quality AC power. These inverters are widely used in motor drives, renewable energy systems, UPS, and industrial applications because they provide better voltage control, reduced harmonic distortion, and higher efficiency compared to traditional inverters.

What is a PWM Inverter

A PWM inverter generates alternating voltage from a DC source by controlling the width of switching pulses. Instead of directly switching on and off at fixed intervals, the inverter modulates the width of pulses within a switching period to match the desired sinusoidal reference. This technique ensures that the output waveform is very close to a pure sine wave, making it suitable for sensitive loads.

The major PWM techniques are:

• Sinusoidal PWM (SPWM): Uses a sinusoidal reference compared with a triangular carrier wave.
• Space Vector PWM (SVPWM): More advanced, improves DC bus utilisation, and reduces harmonics.
• Hysteresis Band PWM: Commonly used in motor drives with current control.

Advantages of PWM Inverters

• Improved Output Quality: PWM produces nearly sinusoidal voltages with lower Total Harmonic Distortion (THD).
• Better Efficiency: Reduced switching losses in advanced PWM methods.
• Voltage Control: Output voltage can be easily controlled by adjusting the modulation index.
• Compatibility with Motors: Smooth torque control and reduced heating in induction and synchronous motors.
• Flexibility: Suitable for both low- and high-power applications.

Comparison of PWM Inverters

To understand the performance of PWM inverters, let us compare them with square-wave and modified sine-wave inverters:

Feature	Square Wave Inverter	Modified Sine Wave Inverter	PWM Inverter (SPWM/SVPWM)
Waveform Quality	Poor, high harmonics	Moderate, stepped waveform	Nearly sinusoidal, low THD
Efficiency	High at switching, but poor with loads	Better than square	Very high with controlled switching
Harmonics	Very high	Moderate	Very low (reduced THD)
Motor Drive Compatibility	Causes heating and noise	Acceptable for small loads	Excellent, smooth torque
Cost	Low	Medium	Higher but justified
Applications	Simple low-cost needs	Household appliances	Motors, renewable, UPS, industry

Applications of PWM Inverters

• Motor Drives: Smooth control of induction and synchronous motors.
• Renewable Energy Systems: Used in solar and wind power converters.
• UPS Systems: Provides clean power for sensitive electronics.
• Electric Vehicles: Essential for efficient motor operation.

Conclusion

PWM inverters represent a major advancement in power electronics by combining efficiency with superior output quality. While square and modified sine wave inverters are still used for cost-sensitive applications, PWM inverters dominate wherever performance, reliability, and power quality matter. With innovations like SVPWM, even better DC bus utilisation and reduced harmonic distortion are achieved, making PWM inverters the standard in modern power conversion.

 

 

Driver Circuits for Inverters and Fourier Analysis of Output Voltage with Harmonic Control: -

Inverters play a vital role in modern power electronics. They convert direct current (DC) into alternating current (AC), making them essential for applications ranging from renewable energy systems to uninterruptible power supplies (UPS) and motor drives. However, an inverter by itself cannot function unless it is supported by driver circuits, which control the switching devices. Its performance must be evaluated using mathematical analysis of the output voltage, especially through Fourier series expansion, to understand harmonics.

In this document, we will explore driver circuits for inverters, the Fourier analysis of their output voltage, and the methods of harmonic control used in practice.

1. Driver Circuits for Inverters

The heart of an inverter is its power semiconductor switches, such as IGBTs, MOSFETs, or thyristors. These switches cannot be directly connected to control logic due to differences in voltage and current levels. Hence, driver circuits act as an interface between low-voltage control signals (from microcontrollers, DSPs, or PWM generators) and high-power inverter switches.

Functions of Driver Circuits

• Level Shifting: Control signals are usually 3.3 V or 5 V, but power switches require 10–20 V gate drive.
• Isolation: Galvanic isolation (via optocouplers or pulse transformers) ensures safety between control electronics and high-voltage inverter circuits.
• Amplification: Drivers provide sufficient current to rapidly charge/discharge the gate capacitance of MOSFETs or IGBTs.
• Dead-Time Insertion: Prevents simultaneous conduction of upper and lower switches in the same inverter leg.
• Protection: Drivers often include short-circuit, over-temperature, and under-voltage lockout features.

Types of Driver Circuits

• Optocoupler-based Drivers: Provide optical isolation between logic and power sides.
• Pulse Transformer Drivers: Common in high-frequency applications due to efficiency and isolation.
• Dedicated Gate Driver ICs: Modern integrated circuits simplify the design with built-in protections.

2. Output Voltage of an Inverter

The output of an ideal inverter should be a pure sinusoidal waveform. In practice, however, since inverters switch between positive and negative DC levels, the actual output is a square wave or pulse-width modulated (PWM) waveform. This non-sinusoidal waveform can be expressed as a Fourier series – a mathematical tool that breaks down periodic signals into a fundamental sinusoidal component plus harmonics.

3. Fourier Series Analysis of Inverter Output

Consider a single-phase full-bridge inverter with a square wave output of amplitude Vdc. The output voltage waveform is an odd function, symmetrical about the origin. Therefore, the Fourier series contains only odd harmonics of sine terms.

vₒ(t) = (4Vdc/π) [ sin(ωt) + (1/3)sin(3ωt) + (1/5)sin(5ωt) + … ]

Here:

• ω = 2πf is the angular frequency,
• The first term (sin ωt) is the fundamental component,
• Higher terms (sin 3ωt, sin 5ωt, …) are harmonics.

Fundamental RMS value of output voltage:

V₁,rms = (2√2 Vdc) / π

Only odd harmonics are present, and total harmonic distortion (THD) is high for a square wave inverter.

4. Harmonics in Inverter Output

Harmonics are undesirable because they cause:

• Heating in machines,
• Additional copper and iron losses,
• Torque pulsations in motors,
• Interference in communication systems,
• Reduced efficiency of the overall system.

Thus, harmonic control becomes an essential task in inverter design.

5. Harmonic Control Methods in Inverters

(a) Pulse Width Modulation (PWM) Techniques

PWM is the most popular method for harmonic control. Instead of a fixed square wave, the inverter output is generated using switching pulses of variable width. By controlling the duty ratio, the fundamental voltage can be adjusted, and harmonics can be minimized.

• Sinusoidal PWM (SPWM): A sinusoidal reference signal is compared with a triangular carrier wave. This reduces lower-order harmonics significantly.
• Space Vector PWM (SVPWM): A more advanced digital technique that improves DC bus utilization and reduces harmonics.
• Selective Harmonic Elimination (SHE): Switching angles are calculated to eliminate particular harmonics.

(b) Filtering Techniques

Even with PWM, high-frequency harmonics remain. These are filtered using:

• LC Filters: Placed at the inverter output to smooth the waveform.
• Active Filters: Use power electronics to inject compensating currents that cancel harmonics.

6. Fourier Analysis of PWM Inverter Output

When PWM is used, the output waveform is more complex, but still periodic. Using Fourier analysis:

• The fundamental component is controlled by the modulation index (M).
• Harmonics are shifted to higher frequencies (around the carrier frequency and its sidebands).
• Since most loads (like motors) have inductance, these high-frequency harmonics are naturally filtered out.

Mathematically, for sinusoidal PWM:

vₒ(t) = V₁ sin(ωt) + Σ Vn sin(nωt)

where V₁ is the fundamental amplitude and Vn represents harmonic components located around multiples of the carrier frequency.

7. Practical Importance

Renewable Energy Systems: Solar inverters rely heavily on PWM techniques to supply grid-quality sinusoidal voltage.

• Motor Drives: Harmonic control ensures smooth torque and higher efficiency.
• UPS Systems: Require nearly pure sinusoidal outputs to prevent damage to sensitive loads.

Conclusion

Driver circuits are the backbone of inverter operation, ensuring safe and reliable switching of semiconductor devices. However, the quality of the inverter’s output is equally important. By applying Fourier series analysis, we can mathematically understand the harmonic content in the inverter’s output. While simple square wave inverters have high harmonic distortion, advanced PWM methods and filtering techniques help achieve a sinusoidal output with minimal harmonics.

In short, driver circuits enable the inverter to function, while Fourier analysis and harmonic control ensure it functions efficiently and with power quality suitable for modern applications.

Filters at the Output of Inverters: -

The main purpose of filters is to smooth the inverter’s stepped or pulse-width modulated (PWM) output into a waveform that closely resembles a pure sinusoid. By reducing unwanted harmonic components, filters improve power quality and ensure the connected load receives stable AC voltage and current.

Types of Filters

1. Low-Pass Filters (LC filters): The most common type, consisting of an inductor and a capacitor. They allow the fundamental frequency (50/60 Hz) to pass through while attenuating higher-order harmonics.

2. LCL Filters: A more advanced form of an LC filter with two inductors and one capacitor. They provide superior harmonic suppression, especially for grid-connected applications like solar inverters.

3. Trap Filters: Designed to eliminate specific harmonic frequencies by using resonant circuits tuned to those harmonics.

4. Active Filters: Instead of just passive elements, active filters use power electronic devices to dynamically cancel out harmonics. These are more expensive but highly effective in sensitive industrial applications.

Benefits of Output Filters

• Reduction of Total Harmonic Distortion (THD).
• Improved efficiency and reliability of electrical loads.
• Compliance with international power quality standards.
• Protection of connected equipment from overheating or malfunction.

Single-Phase Current Source Inverter (CSI): -

Unlike a voltage source inverter (VSI), which is driven by a stiff DC voltage, a current source inverter is supplied by a stiff DC current, usually ensured by a large series inductor at the input. The CSI is less common than VSI but offers certain advantages in specific applications.

Working Principle

The single-phase CSI converts a constant DC current into an alternating current waveform using controlled switching devices (such as thyristors or IGBTs). Since the input current is constant, the output is also controlled in terms of current, making this topology highly suitable for current-driven loads like induction heating or synchronous motors.

Characteristics of CSI

• Input Side: A large inductor maintains a constant input current.
• Output Side: The output current is nearly sinusoidal, but the output voltage depends on the load.
• Switching Devices: Require reverse-blocking capability since the current direction is enforced by the input source.
• Natural Commutation: Thyristor-based CSIs often rely on load characteristics for commutation, which restricts their application.

Advantages of CSI

• Better performance for motor drive applications that require current control.
• Inherent protection against short circuits, unlike VSIs.
• Simple structure for supplying constant current loads.

Disadvantages of CSI

• A bulky input inductor is required.
• Voltage control is more complex compared to VSI.
• Limited applications due to load dependency.

Applications

• Induction Heating Systems: where constant current is required.
• High-Power Motor Drives: especially for synchronous motors.
• Renewable Energy Systems: in niche cases where current-fed topologies are preferred.

Conclusion

Filters at the output of inverters are essential for improving waveform quality and reducing harmonics, thereby enhancing efficiency and protecting connected loads. On the other hand, the single-phase current source inverter represents a specialized topology where constant current output is the priority. While less common than voltage source inverters, CSIs serve important roles in industrial and heating applications. Together, these concepts highlight the versatility of inverter technology in shaping the future of power electronics.

-------------------------------------------MODULE-5 NEXT PAGE-------------------------------------------