Power electronics involves using electronics to control and change electric power. This helps manage how electricity is used in many devices.

The first high-power electronic devices were made using mercury-arc valves. Today, modern systems use special types of electronic components, such as diodes, thyristors, and power transistors like the power MOSFET and IGBT, to change electric power. Unlike systems that handle signals and data, power electronics deals with large amounts of electrical energy. An AC/DC converter, also called a rectifier, is a common power electronics device found in many consumer products, such as televisions, computers, and battery chargers. The power range usually goes from tens of watts up to several hundred watts. In industry, a common use is the variable-speed drive (VSD), which controls the speed of an induction motor. VSDs can handle power starting from a few hundred watts up to tens of megawatts.

Power conversion systems can be grouped based on the type of input and output power:

History

Power electronics began with the invention of the mercury-arc rectifier by Peter Cooper Hewitt in 1902. This device changed alternating current (AC) into direct current (DC). In the 1920s, scientists studied thyratrons and grid-controlled mercury-arc valves for power transmission. Uno Lamm created a mercury valve with special parts that made it suitable for high-voltage DC power transmission. In 1933, selenium rectifiers were developed.

Julius Edgar Lilienfeld proposed the idea of a field-effect transistor in 1926, but it could not be built at that time. In 1947, the bipolar point-contact transistor was invented by Walter H. Brattain and John Bardeen, working under William Shockley at Bell Labs. In 1948, Shockley developed the bipolar junction transistor (BJT), which improved transistor performance and reduced costs. By the 1950s, higher power semiconductor diodes became available and replaced vacuum tubes. In 1956, General Electric introduced the silicon controlled rectifier (SCR), which expanded the use of power electronics. By the 1960s, faster bipolar junction transistors enabled high-frequency DC/DC converters.

R. D. Middlebrook made important contributions to power electronics. In 1970, he started the Power Electronics Group at Caltech. He created the state-space averaging method and other tools essential for modern power electronics design.

In 1957, Frosch and Derick built the first silicon dioxide field-effect transistors at Bell Labs. These transistors had drain and source areas next to each other on the surface. In 1960, Dawon Kahng and his team at Bell Labs demonstrated a working MOSFET. Other team members helped create the device, develop processes, and test its performance.

In 1969, Hitachi made the first vertical power MOSFET, later called the VMOS. From 1974, companies like Yamaha, JVC, Pioneer, Sony, and Toshiba used power MOSFETs in audio amplifiers. In 1978, International Rectifier produced a 25 A, 400 V power MOSFET. This device could operate at higher frequencies than bipolar transistors but was limited to low-voltage uses.

The power MOSFET is the most common power device because it uses little power to control, switches quickly, and is easy to use and repair. It is used in many applications, such as portable devices, power circuits, cell phones, laptops, and internet infrastructure.

In 1982, the insulated-gate bipolar transistor (IGBT) was introduced. It became widely used in the 1990s. This component combines the power handling of bipolar transistors with the easy control of power MOSFETs.

Devices

The performance and cost of power electronics systems depend on the types of active devices used. These devices' features and limits are important when designing power electronics systems. In the past, devices like mercury-arc valves, vacuum and gas-filled diodes, and triggered devices such as thyratrons and ignitrons were commonly used. As solid-state devices improved in their ability to handle higher voltages and currents, vacuum devices were mostly replaced by solid-state devices.

Power electronic devices can function as switches or amplifiers. An ideal switch is either fully open or fully closed, which means it uses no power. It can withstand voltage without allowing current to flow or allow current without voltage drop. Semiconductor switches can closely match this ideal behavior, making them widely used in power electronics because they reduce power loss. In contrast, amplifiers allow current to change continuously based on an input signal. The voltage and current in amplifiers follow a load line, and the power used inside the device is much greater than the power delivered to the load.

Several factors determine how devices are used. Diodes allow current to flow when a forward voltage is applied but cannot be controlled externally. Devices like silicon-controlled rectifiers and thyristors (as well as mercury valves and thyratrons) can be controlled to start conducting but require current reversal to turn off. Devices such as gate turn-off thyristors, BJTs, and MOSFET transistors can be turned on or off independently of current flow. Transistors also allow proportional amplification, but this is rarely used in systems requiring more than a few hundred watts. The control input requirements of a device also influence design; sometimes, the control input operates at very high voltages and needs an isolated power source.

Efficiency is important in power electronic systems, so power loss in devices should be minimized.

Devices differ in how quickly they can switch on and off. Some diodes and thyristors work well at slower speeds and are used for power frequency applications. Other thyristors operate at a few kilohertz. Devices like MOSFETs and BJTs can switch at tens of kilohertz up to a few megahertz, but their power handling decreases at higher frequencies. Vacuum tubes are used for very high power applications at extremely high frequencies. Faster switching reduces energy loss during transitions but may cause electromagnetic interference. Gate drive circuits must provide enough current to enable a device to switch quickly. If a device cannot switch rapidly, it may overheat and be damaged.

Practical devices have some voltage drop when active and use power even when on. They also take time to transition between states. These losses contribute significantly to total power loss in a system.

Managing heat is a key design factor. Power electronic devices may need to remove tens or hundreds of watts of heat, even when switching efficiently. The power controlled is much greater than the power lost in the switch. The voltage drop in the conducting state creates heat that must be removed. High-power semiconductors often use special heat sinks or cooling systems to manage heat. Materials like silicon carbide handle heat better than regular silicon. Germanium, once widely used, is now rarely used because it performs poorly at high temperatures.

Semiconductor devices can handle voltages up to several kilovolts in a single unit. When very high voltages are needed, multiple devices must be connected in series with systems to balance voltage across them. Switching speed is important because the slowest device will handle more voltage. Mercury valves once reached 100 kV in a single unit, making them useful for high-voltage direct current systems.

The current capacity of a semiconductor device is limited by heat generated in the device and the resistance of its connections. Devices must distribute current evenly across their internal parts to avoid hot spots, which can damage the device. Some SCRs can handle up to 3,000 amperes in a single unit.

DC/AC converters (inverters)

DC-to-AC converters change direct current (DC) into alternating current (AC). These devices are used in adjustable-speed drives, uninterruptible power supplies, flexible AC transmission systems, voltage compensators, and photovoltaic inverters. There are two main types of these converters: voltage source inverters (VSIs) and current source inverters (CSIs). VSIs control the output as a voltage, while CSIs control the output as a current.

DC-to-AC conversion uses power switches, which are semiconductor devices that can be fully controlled. These switches create output waveforms with sudden changes instead of smooth curves. For some uses, a rough version of a sine wave is enough. When a nearly perfect sine wave is needed, the switches operate very quickly, and their on/off times are carefully adjusted to make the output wave look like a sine wave. Common methods to control this process include carrier-based pulse-width modulation, space-vector modulation, and selective-harmonic elimination.

Voltage source inverters are used in both single-phase and three-phase systems. Single-phase VSIs use half-bridge or full-bridge designs and are often found in power supplies and uninterruptible power systems. Three-phase VSIs are used in applications that need smooth voltage waves, such as adjustable-speed drives, uninterruptible power supplies, and some flexible AC transmission systems like STATCOMs. They are also used in systems requiring custom voltages, like active power filters and voltage compensators.

Current source inverters create an AC current from a DC current source. These inverters are useful in three-phase systems where high-quality voltage waves are needed.

A newer type of inverter, called a multilevel inverter, has become popular. Traditional VSIs and CSIs are two-level inverters because their switches connect to either the positive or negative side of the DC power source. If more voltage levels are available, the AC output can more closely resemble a sine wave. Although multilevel inverters are more complex and expensive, they provide better performance.

Each inverter type uses different DC connections and may or may not require freewheeling diodes. They can operate in square-wave mode or pulse-width modulation (PWM) mode, depending on their purpose. Square-wave mode is simpler, while PWM offers better waveform quality.

Voltage source inverters (VSIs) take power from a nearly constant-voltage source. The quality of the current output determines which modulation method is used. The output of a VSI has discrete values. To create a smooth current wave, the load must be inductive at certain harmonic frequencies. Without inductive filtering, a capacitive load can cause sudden, large current spikes.

There are three main types of VSIs:
1. Single-phase half-bridge inverter
2. Single-phase full-bridge inverter
3. Three-phase voltage source inverter

Single-phase half-bridge inverters are used for lower-voltage applications, such as power supplies. A diagram (Figure 9) shows the circuit design of this inverter. These inverters require two large capacitors to filter out unwanted current harmonics. In each leg of the inverter, only one switch can be on at a time. If both switches in a leg are on, the DC power source would short out.

Inverters use different methods to control their switching. Carrier-based PWM compares the AC output voltage to a triangular carrier signal. When the AC voltage is higher than the carrier signal, one switch turns on; when it is lower, another switch turns on. If the AC frequency and carrier frequency are set properly, this becomes sinusoidal pulse-width modulation (SPWM). The modulation index (ma) is calculated as the ratio of the AC voltage to the carrier voltage. The frequency-modulation ratio (mf) is the ratio of the carrier frequency to the AC frequency.

If the modulation index exceeds 1, the AC output voltage increases, but this can cause saturation. In SPWM, the harmonics in the output have predictable frequencies and amplitudes, making it easier to design filters. The maximum output voltage in this mode is half the DC source voltage. If the modulation index is higher than 3.24, the output becomes a square wave.

In square-wave mode, both switches in a leg cannot be on at the same time, as this would short the DC source. The switching pattern requires both switches to be on for half a cycle of the AC output. The fundamental AC voltage is calculated as 2 times the DC voltage divided by π. The harmonics in the output have amplitudes that decrease with increasing frequency.

The AC output voltage is determined by the DC input voltage, not by the inverter itself.

Selective harmonic elimination (SHE) is a method that removes unwanted harmonics from the AC output. This technique allows the fundamental voltage to be adjusted within a desired range. The output has odd half and odd quarter-wave symmetry, so even harmonics are absent. Unwanted odd harmonics can be eliminated using this method.

A full-bridge inverter is similar to a half-bridge inverter but has an extra leg to connect the neutral point to the load. A diagram (Figure 3) shows the circuit design of a single-phase full-bridge inverter.

To prevent shorting the DC source, switches in the same leg cannot be on at the same time. In the full-bridge configuration, the maximum output voltage is twice that of a half-bridge inverter. States 1 and 2 from Table 2 are used to generate the AC output voltage with bipolar SPWM. The output can only be at two voltage levels: +Vi or -Vi. In a half-bridge setup, the same states can be achieved using a carrier-based method. The output voltage is mostly sinusoidal, with a fundamental component that depends on the modulation settings.

AC/AC converters

Converting AC power to AC power allows control of the voltage, frequency, and phase of the electricity sent to a device from an AC power source. The two main types of converters are divided based on whether the frequency of the electricity can be changed. Converters that do not change the frequency are called AC Voltage Controllers or AC Regulators. Converters that allow frequency changes are called frequency converters. Under frequency converters, there are three common types: cycloconverters, matrix converters, and DC-link converters (also called AC/DC/AC converters).

AC Voltage Controller: The purpose of an AC Voltage Controller or AC Regulator is to adjust the average voltage across a device while keeping the frequency constant. Three common control methods are ON/OFF Control, Phase-Angle Control, and Pulse-Width Modulation (PWM) AC Chopper Control. These methods can be used in both single-phase and three-phase circuits.

• ON/OFF Control: This method is often used for heating devices or motor speed control. It turns a switch on for a set number of full cycles and then turns it off for another set of full cycles. To reduce electrical distortion, switches are turned on and off only when the voltage and current are at zero (zero-crossing).
• Phase-Angle Control: This method uses components like diodes, SCRs, and Triacs to control electricity flow. It delays the start of a wave, allowing only part of the wave to be sent to the device.
• PWM AC Chopper Control: This method improves electrical quality compared to other methods. It uses switches that turn on and off rapidly during each half-cycle of the input voltage.

Matrix Converters and Cycloconverters: Cycloconverters are widely used in industry for AC-to-AC conversion because they work well in high-power applications. They directly change the frequency of electricity and are connected to the power supply. Their output has complex distortions, but these are reduced by the inductance of the device they power. Cycloconverters do not use storage devices like inductors or capacitors, so the power input and output are always equal.

• Single-Phase to Single-Phase Cycloconverters: These have become more popular recently due to smaller and cheaper power switches. They can produce either a smooth or a trapezoidal-shaped voltage wave. Some designs use zero-voltage intervals for control.
• Three-Phase to Single-Phase Cycloconverters: These include two types: 3φ to 1φ half-wave and 3φ to 1φ bridge cycloconverters. Both can produce voltage in either positive or negative directions, allowing current to flow in only one direction at a time.

Newer versions of cycloconverters, called matrix converters, use switches that can handle electricity in both directions. A single-phase matrix converter connects three input phases to three output phases using nine switches. These switches must not be connected in a way that causes a short circuit. Matrix converters are lighter, more compact, and more flexible than other types. They can operate at higher temperatures, support a wide range of output frequencies, and allow electricity to flow back to the power source.

Matrix converters are divided into two types: direct and indirect. Direct matrix converters use switches that can handle electricity in both directions. This allows for higher output voltage and reduces electrical waste. However, their complex control and switching issues limit their use in industry. Indirect matrix converters use separate input and output sections connected through a DC link without storage elements. They include a four-quadrant rectifier and an inverter. Their control method is simpler and more reliable than direct matrix converters.

DC Link Converters: DC Link Converters, also called AC/DC/AC converters, change AC power to DC using a rectifier and then back to AC using an inverter. This process allows for lower voltage and adjustable frequency output. These converters are widely used because they work well under heavy or no load conditions and can disconnect from a device without damage.

Hybrid Matrix Converters: Hybrid matrix converters combine features of AC/DC/AC converters and matrix converters. Some designs use one-way switches and two stages without a DC link, reducing size and weight. These converters are divided into two types: hybrid direct matrix converters (HDMC) and hybrid indirect matrix converters (HIMC). HDMC changes voltage and current in one step, while HIMC uses separate steps like AC/DC/AC converters but without storage elements.

Applications:
– AC Voltage Controller: Used for lighting control, heating devices, motor speed control, and starting motors smoothly.
– Cycloconverter: Used in high-power motor drives, variable frequency power supplies, power factor correction, and connecting separate power systems.
– Matrix Converter: Currently limited in use but being developed for advanced applications.

Simulations of power electronic systems

Power electronic circuits are tested using computer programs such as SIMBA, PLECS, PSIM, SPICE, MATLAB/Simulink, and OpenModelica. These programs create models of circuits before they are built to see how they work in different situations. Using a simulation is less expensive and quicker than building a real model for testing.

Applications

Power electronics are used in many different devices and systems, from small ones like battery chargers and phone chargers to large systems that move electricity across countries. These systems are found in almost every electronic device. For example:

• DC/DC converters help keep the voltage in mobile devices, like phones and tablets, at a steady level even when the battery’s voltage changes. These converters are also used to isolate electronics and fix power issues. A special type of DC/DC converter, called a power optimizer, helps get the most energy from solar panels or wind turbines.
• AC/DC converters (also called rectifiers) are used whenever a device, like a computer or TV, is plugged into a wall outlet. These converters change alternating current (AC) from the outlet into direct current (DC) for the device. Some also change the voltage level.
• AC/AC converters change the voltage or frequency of electricity. They are used in international power adapters and light dimmers. In power grids, they help transfer electricity between systems that use 50 Hz and 60 Hz frequencies.
• DC/AC converters (inverters) are used in systems like backup power supplies (UPS), solar energy systems, and emergency lighting. When the main power fails, inverters take energy from batteries and turn it into AC electricity to keep devices running. Solar inverters, which come in different sizes, are used in solar power systems to connect solar panels to the electrical grid.

Motor drives are used in machines like pumps, fans, and large equipment in factories. These drives help control power and movement. For AC motors, systems like variable-frequency drives and motor soft starters are used.

In hybrid electric vehicles (HEVs), power electronics help manage energy between the battery and the engine. These vehicles use either a series or parallel system, depending on how the engine and motor work together. Most electric vehicles use DC/DC converters for charging batteries and DC/AC converters to power the motor. Electric trains use power electronics to get energy from power lines and control movement using special systems like pulse-width modulation (PWM) rectifiers. Power electronics are also used in elevator systems with devices like thyristors, inverters, and permanent magnet motors.

Inverters are used to change electricity from DC to AC or to change AC to AC indirectly. This is important for systems like power conditioning, motor control, and connecting renewable energy to the grid.

In power systems, active power filters (using voltage source inverters) help remove unwanted electrical signals called harmonics. These filters use measurements of current and voltage to create signals that cancel out the harmonics. This process does not use extra power because the system is powered directly from the grid.

Uninterruptible power supply (UPS) systems are used in places like hospitals and airports where electricity is always needed. In a standby system, an inverter starts working when the main power fails, using battery energy to keep devices running. In an online system, a combination of a rectifier, battery, and inverter ensures power is always stable and free from electrical issues.

AC motor drives control the speed, torque, and movement of AC motors. These drives are either low-performance (using simple controls) or high-performance (using advanced controls). Low-performance drives are used in devices like fans, while high-performance drives are used in elevators and electric cars.

Inverters are also used in renewable energy systems. In solar power, inverters (often using PWM technology) change DC energy from solar panels into AC electricity for use in homes or the grid. In wind turbines, inverters help stabilize the electricity produced when the turbine speed changes.

A smart grid is an updated electrical system that uses technology to collect and use information about energy use automatically. This helps make electricity production and distribution more efficient and reliable.

Power from wind and hydroelectric turbines can cause changes in the frequency of electricity. Power electronics convert this AC electricity into high-voltage direct current (HVDC), which is easier to adjust for the grid. This makes the electricity cleaner and improves its efficiency. Wind power systems use either gearboxes or direct drive technology to reduce the size of power electronics.

Solar power is generated using photovoltaic cells and power electronics. Solar inverters change the DC energy from solar panels into AC electricity. Inverters are divided into three types: central inverters (used in large solar farms), module-integrated inverters (used in individual solar panels), and string inverters (used in groups of panels).