Power electronics:

Introduction:

The use of electronics in the conversion and control of electric power is known as power electronics.

 

Power electronics

Mercury-arc valves were used to create the first high-power electronic gadgets. In contemporary systems, the conversion is carried out by power transistors like the power MOSFET and IGBT and other semiconductor switching devices such diodes, thyristors, and diodes. In contrast to electronic systems that deal with the transmission and processing of signals and data, power electronics processes a significant amount of electrical energy.The most prevalent power electronics component present in many consumer electronics products, such as television sets, personal computers, battery chargers, etc., is an AC/DC converter (rectifier). Tens of watts to several hundred watts is the usual power range. The variable speed drive (VSD), which is used to regulate an induction motor, is a popular use in the industrial sector. VSDs have a power range that ranges from a few hundred watts to tens of megawatts.

 

The kind of input and output power can be used to categorize power conversion systems:

a) Ac to Dc (rectifier)

b) Dc to Ac  (inverter)

c) Dc to Dc (DC-to-DC converter)

d) Between AC (AC-to-AC converter)

 

Power electronics

History:

The mercury arc rectifier was the first power electronic device. It was created in 1902 by Peter Cooper Hewitt to change alternating current (AC) into direct current (DC). Thyratrons and grid-controlled mercury arc valves for power transmission have been studied since the 1920s. Mercury valves with grading electrodes were created by Uno Lamm to be used for high voltage direct current power transmission. Selenium rectifiers were created in 1933.

 

Field-effect transistors were first conceptualized by Julius Edgar Lilienfeld in 1926, but no practical devices could be built at that time. Walter H. Brattain and John Bardeen created the bipolar point-contact transistor in 1947 at Bell Labs with the help of William Shockley. The bipolar junction transistor (BJT), developed by Shockley in 1948, increased the stability and performance of transistors while lowering their price. Higher power semiconductor diodes became accessible in the 1950s and began to take the place of vacuum tubes. The field of power electronics applications was significantly expanded in 1956 when General Electric released the silicon controlled rectifier (SCR). High frequency DC/DC converters were made possible by the improved switching speed of bipolar junction transistors by the 1960s.

 

Power electronics benefited greatly from R. D. Middlebrook's innovations. He started the Power Electronics Group at Caltech in 1970. Along with other techniques essential to contemporary power electronics design, he created the state-space averaging method of analysis.

 

Power MOSFET:

The development of the MOSFET (metal-oxide-semiconductor field-effect transistor) by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959 marked a significant advance in the field of power electronics. Power designers were able to achieve performance and density levels with MOSFET transistor generations that were not feasible with bipolar transistors. The power MOSFET was first made accessible in the 1970s as a result of advancements in MOSFET technology, which was initially utilized to create integrated circuits.

 

The first vertical power MOSFET, subsequently known as the VMOS, was introduced by Hitachi in 1969. (V-groove MOSFET). Yamaha, JVC, Pioneer, Sony, and Toshiba all started making audio amplifiers utilizing power MOSFETs in 1974. A 25 A, 400 V power MOSFET was first offered by International Rectifier in 1978. Compared to a bipolar transistor, this device can operate at greater frequencies, however it can only be used in low voltage applications.

 

Due to its low gate drive power, quick switching speed, simple advanced paralleling capability, wide bandwidth, robustness, easy drive, simple biasing, ease of application, and ease of maintenance, the power MOSFET is the most widely used power device in the world. It has a wide range of power electronic applications, including mobile phones, laptop computers, portable information appliances, power integrated circuits, and the network that makes the Internet possible.

 

Insulated-gate bipolar transistors (IGBTs) were first introduced in 1982. In the 1990s, it was readily accessible. This component combines the benefits of the isolated gate drive of the power MOSFET with the power handling capacity of the bipolar transistor.

 

Power electronics

Devices:

The available active devices dictate the capabilities and efficiency of a power electronics system. A crucial component in the design of power electronics systems is their characteristics and constraints. Power electronics previously made extensive use of the mercury arc valve, high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices like the thyratron and ignitron. Vacuum devices have been mostly supplanted with solid-state devices as their ratings in terms of voltage and current-handling capabilities have increased.

 

Power electronic components can be utilized as amplifiers or switches. An ideal switch is either open or closed, dissipating no power; it can either pass no current while being applied with a voltage, or it may flow any current with no voltage drop. The majority of power electronic applications rely on switching devices on and off because semiconductor devices employed as switches may approximate this perfect quality. This results in extremely efficient systems because very little power is lost in the switch. In contrast, the current flowing through an amplifier varies continually in response to a controlled input.The power dissipation inside the device is significant relative to the power given to the load, and the voltage and current at the device terminals follow a load line.

 

How gadgets are used is determined by a number of factors. Devices like diodes start conducting when a forward voltage is provided; there is no external control over this process. Power devices like mercury valves and thyratrons, as well as silicon-controlled rectifiers and thyristors, enable control of the onset of conduction but depend on periodic reversal of current flow to turn them off.Devices that offer full switching control and may be turned on or off regardless of the current flow through them include gate turn-off thyristors, BJTs, and MOSFET transistors. Although proportional amplification is also possible with transistor devices, it is rarely employed for systems with a power rating higher than a few hundred watts. The design of a device is also greatly influenced by its control input characteristics; occasionally, the control input is at a very high voltage relative to ground and needs to be powered by an isolated source.

 

The losses produced by a power electronic device should be as small as feasible because efficiency is crucial in a power electronic converter.

 

Device switching rates vary. Diodes and thyristors can be used for power frequency switching and control at relatively low speeds; certain thyristors work well at a few kilohertz. In power applications, devices like MOSFETS and BJTs can switch at frequencies ranging from tens of kilohertz to a few megahertz, but with diminishing power levels. Applications requiring high power (hundreds of kW) at extremely high frequency (hundreds or thousands of megahertz) are dominated by vacuum tube devices. Faster switching mechanisms reduce the energy lost during the switchovers from on to off and back, but they may cause issues with radiated electromagnetic interference.To obtain the maximum switching speed a device is capable of, gate drive (or comparable) circuits must be built to generate enough drive current. Excessive heating could ruin a device that lacks the drive to switch quickly.

 

Practical devices require some time to travel through an active zone before they reach the "on" or "off" state, have a non-zero voltage drop, and dissipate power when turned on. These losses account for a sizeable portion of the overall power lost in a converter.

 

 

 

Device power handling and dissipation are important design considerations as well. Power electrical equipment may need to dissipate tens or even hundreds of watts of waste heat while switching between conducting and non-conducting states as effectively as feasible.The power controlled in the switching state is significantly greater than the power lost in the switch. Heat must be released as a result of the forward voltage drop in the conducting condition. Exotic semiconductors like silicon carbide have an advantage over straight silicon in this regard, and germanium, once the mainstay of solid-state electronics, is now little used because of its unfavorable high-temperature properties. High power semiconductors need specialized heat sinks or active cooling systems to manage their junction temperature.

 

There are semiconductor devices with single-device ratings of up to a few kilovolts. Multiple devices must be connected in series to regulate very high voltage, and networks must be employed to maintain an even voltage across every device.Once more, switching speed is important since the device with the slowest switching will need to withstand a disproportionate amount of the total voltage. Mercury valves were originally offered with 100 kV ratings in a single unit, making it easier to use them in HVDC systems.

 

Heat created in the dies and heat developed in the resistance of the interconnecting leads limit the current rating of a semiconductor device. The internal junctions (or channels) of semiconductor devices must be equally spread with current; otherwise, breakdown processes can quickly kill the device once a "hot spot" forms. There are certain SCRs that can handle currents of up to 3000 amps in a single unit.

 

Converters for DC/AC (inverters):

DC to AC converters turn a DC source into an AC output waveform. Uninterruptible power supplies (UPS), flexible AC transmission systems (FACTS), voltage compensators, and solar inverters are a few examples of applications. Voltage source inverters and current source inverters are two unique groups of topologies for these converters. The independently regulated output of voltage source inverters, often known as VSIs, is a voltage waveform. The controlled AC output of current source inverters (CSIs) is unique in that it has a current waveform.

Power switching devices, which are frequently completely programmable semiconductor power switches, are responsible for converting DC to AC power. Due to the discrete values used in the output waveforms, quick transitions rather than smooth ones are produced. Even a crude approximation of the sinusoidal waveform of AC power suffices for some purposes. The switching devices are run much faster than the desired output frequency when a nearly sinusoidal waveform is required, and the amount of time they spend in each state is adjusted to provide an output that is nearly sinusoidal on average. The carrier-based technique, often known as pulse-width modulation, the space-vector technique, and the selective-harmonic technique are all common modulation methods.

 

Both single-phase and three-phase applications can effectively use voltage source inverters. Power supplies, single-phase UPSs, and complex high-power topologies when utilized in multicell arrangements are all common uses for single-phase VSIs, which employ half-bridge and full-bridge configurations. Applications requiring sinusoidal voltage waveforms, such ASDs, UPSs, and some FACTS devices like the STATCOM, use three-phase VSIs. Additionally, they are utilized in situations where arbitrary voltages are necessary, such as voltage compensators and active power filters.

 

Current source inverters are employed to convert a DC current source into an AC output current. When three-phase applications need for high-quality voltage waveforms, this kind of inverter can be useful.

 

Multilevel inverters, a comparatively new class of inverters, have drawn a lot of attention. Due to the fact that power switches connect to either the positive or the negative DC bus, CSIs and VSIs operate normally as two-level inverters. The AC output would more closely resemble a sine wave if there were more than two voltage levels available at the inverter output terminals. Because of this, multilevel inverters perform better despite being more complicated and expensive.

 

Different inverter types use different DC connections and have different requirements for freewheeling diodes. Depending on the application, either can be configured to run in square-wave or pulse-width modulation (PWM) mode. Square-wave mode is more straightforward, whereas PWM may be used in a variety of ways and generates waveforms of superior quality.

 

Voltage Source Inverters (VSI) provide a roughly constant voltage source for the output inverter portion.

 

 

 

For a particular application, the appropriate modulation technique is determined by the desired quality of the current output waveform. A VSI's output is made up of discrete values. The loads must be inductive at the chosen harmonic frequencies to provide a smooth current waveform.A capacitive load will result in a choppy current waveform with significant and frequent current spikes if there is no inductive filtering between the source and load.

 

Three basic categories of VSIs exist:

l a half-bridge inverter for one phase.

l full-bridge single-phase inverter

l voltage source inverter for three phases

 

A half-bridge inverter for one phase:

The single-phase voltage source half-bridge inverters are frequently used in power supply and are designed for lower voltage applications.

The inverter's functioning causes low-order current harmonics to be injected back into the source voltage. This indicates that two substantial capacitors are required in this design for filtering, and that only one switch can be activated simultaneously in each leg of the inverter. The DC source would be shorted out if both switches in a leg were on at once.

 

Several modulation methods can be used by inverters to regulate their switching protocols. A carrier voltage signal, v, and an AC output waveform, vc, are compared using the carrier-based PWM approach. S+ is activated when vc exceeds v, and S- is activated when vc is less than v. The PWM transforms into a specific sinusoidal case of the carrier based PWM when the triangle carrier signal is at frequency f with its amplitude at v and the AC output is at frequency fc with its amplitude at vc. The term for this situation is sinusoidal pulse-width modulation (SPWM). The modulation index, also known as the amplitude-modulation ratio.

 

A larger basic AC output voltage will be seen, albeit at the cost of saturation, if the over-modulation region, ma, surpasses one. The output waveform's harmonics for SPWM have well-defined frequencies and amplitudes. This makes it easier to design the filtering parts required for the low-order current harmonic injection resulting from the inverter's operation. In this mode of operation, the maximum output amplitude is equal to half the source voltage. When the maximum output amplitude, ma, is greater than 3.24, the inverter's output waveform changes to a square wave.

 

Both switches in a leg of square wave modulation cannot be switched on simultaneously since doing so would result in a short across the voltage source, as was the case with pulse-width modulation (PWM). S+ and S- must both be on for a half cycle of the AC output period in order for the switching method to function.

 

As a result, the inverter does not control the AC output voltage; rather, the inverter's DC input voltage magnitude does.

 

Selective harmonic elimination (SHE) is a modulation method that enables the inverter to be switched while removing specific inherent harmonics. Additionally, the basic element of the AC output voltage can be changed within a desired range. Even harmonics do not occur since the AC output voltage created using this modulation approach has odd half-wave and odd quarter-wave symmetry.  It is possible to remove any unwelcome odd (N-1) intrinsic harmonics from the output waveform.

 

Full-bridge single-phase inverter:

Similar to the half bridge inverter, the full bridge inverter contains an extra leg that connects the neutral point to the load.

 

S1+ and S1-, as well as S2+ and S2-, cannot be turned on at the same time in order to prevent shorting out the voltage source. When using a modulation method for a full-bridge configuration, only one of the switches on each leg should be active at any given time—either the top switch or the bottom switch. The additional leg causes the output waveform's maximum amplitude, Vi, to be twice as large as the half-bridge configuration's maximum practicable output amplitude.

 

Bipolar SPWM is utilized to produce States 1 and 2 from Table 2 for the AC output voltage. There are only two possible options for the AC output voltage: Vi or -Vi. Using a half-bridge design, a carrier-based method can be employed to obtain the same states. S1+ and S2 are on for the full-bridge when S+ is on for the half-bridge. Similar to how S- is on for the full bridge, S1- and S2+ are on for the half bridge. This modulation method produces an output voltage that is roughly sinusoidal, with a fundamental component whose amplitude in the linear area is less than or equal to one.

 

The unipolar approach generates its AC output voltage using states 1, 2, 3, and 4 from Table 2 as opposed to the bipolar PWM method. Consequently, the AC output voltage can be Vi, 0 or -Vi. As shown in Figure 4, two sinusoidal modulating signals, Vc and -Vc, are required to produce these states.

 

VaN is produced by Vc, whereas VbN is produced by -Vc. The relationship described below is known as unipolar carrier-based SPWM.

 

VaN and VbN's phase voltages are identical but 180 degrees out of phase with one another. There are no even harmonics in the output voltage, which is equal to the difference between the two-phase voltages. As a result, if mf is measured, even the harmonics of the AC output voltage will manifest at normalized odd frequencies, fh. The center of these frequencies is two times the normalized carrier frequency. When attempting to produce an output waveform of higher quality, this particular feature enables smaller filtering components.

 

Due to its odd half-wave and odd quarter-wave symmetry, the AC output voltage does not contain any even harmonics, as was the case for the half-bridge SHE.

 

 

 

 

Voltage source inverter for three phases:

While three-phase VSIs span both the medium and high power ranges, single-phase VSIs are typically employed for low power applications.

 

This prevents switches from being turned off simultaneously in any of the three legs of the inverter, which causes the voltages in each leg to rely on the polarity of the corresponding line current. Zero AC line voltages are produced by states 7 and 8, which cause AC line currents to freely flow through either the upper or lower components. The line voltages for states 1 through 6, however, result in an AC line voltage that is made up of the discrete values Vi, 0 or -Vi.

 

In order to create out-of-phase load voltages for three-phase SPWM, three modulating signals that are 120 degrees out of phase with one another are used. The normalized carrier frequency, mf, must be a power of three greater than the single carrier signal in order to maintain the PWM features. As a result, the phase voltages remain the same size but remain 120 degrees out of phase with one another. The linear region's highest phase voltage amplitude, ma less than or equal to one.

 

Changing the input DC voltage is the sole technique to regulate the load voltage.