Misplaced Pages

#275724

112-470: The first high-power electronic devices were made using mercury-arc valves . In modern systems, the conversion is performed with semiconductor switching devices such as diodes , thyristors , and power transistors such as the power MOSFET and IGBT . In contrast to electronic systems concerned with the transmission and processing of signals and data, substantial amounts of electrical energy are processed in power electronics. An AC/DC converter ( rectifier )

224-511: A . Unlike the bipolar PWM technique, the unipolar approach uses states 1, 2, 3, and 4 from Table 2 to generate its AC output voltage. Therefore, the AC output voltage can take on the values Vi, 0 or −V [1]i. To generate these states, two sinusoidal modulating signals, Vc and −Vc, are needed, as seen in Figure 4. Vc is used to generate VaN, while –Vc is used to generate VbN. The following relationship

336-754: A SCR , a GTO, a MCT , etc.) is still often used. This device can be turned on by a pulse provided by a driving circuit, but cannot be turned off by removing the pulse. A thyristor turns off as soon as no more current flows through it; this happens automatically in an alternating current system on each cycle, or requires a circuit with the means to divert current around the device. Both MCTs and GTOs have been developed to overcome this limitation, and are widely used in power distribution applications. A few applications of power semiconductors in switch mode include lamp dimmers , switch mode power supplies , induction cookers , automotive ignition systems , and AC and DC electric motor drives of all sizes. Amplifiers operate in

448-440: A 'depleted region' that supports the high voltage during the off-state. On the other hand, during the on-state, the higher doping of the drift region allows for the easy flow of carriers, thereby reducing on-resistance. Commercial devices, based on this super junction principle, have been developed by companies like Infineon (CoolMOS products) and International Rectifier (IR). The major breakthrough in power semiconductor devices

560-433: A 25 A, 400 V power MOSFET in 1978. This device allows operation at higher frequencies than a bipolar transistor, but is limited to low voltage applications. The power MOSFET is the most common power device in the world, due to its low gate drive power, fast switching speed, easy advanced paralleling capability, wide bandwidth , ruggedness, easy drive, simple biasing, ease of application, and ease of repair. It has

672-579: A Mercury arc valve to provide power to the tram which carries visitors between its two sites. Special types of single-phase mercury-arc rectifiers are the Ignitron and the Excitron . The Excitron is similar to other types of valve described above but depends critically on the existence of an excitation anode to maintain an arc discharge during the half-cycle when the valve is not conducting current. The Ignitron dispenses with excitation anodes by igniting

784-536: A bipolar transistor, but is limited to low voltage applications. The Insulated-gate bipolar transistor (IGBT) was developed in the 1980s, and became widely available in the 1990s. This component has the power handling capability of the bipolar transistor and the advantages of the isolated gate drive of the power MOSFET. Some common power devices are the power MOSFET , power diode , thyristor , and IGBT . The power diode and power MOSFET operate on similar principles to their low-power counterparts, but are able to carry

896-474: A disproportionate share of the overall voltage. Mercury valves were once available with ratings to 100 kV in a single unit, simplifying their application in HVDC systems. The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads. Semiconductor devices must be designed so that current is evenly distributed within

1008-500: A few hundred microseconds. Nominal voltages for MOSFET switching devices range from a few volts to a little over 1000 V, with currents up to about 100 A or so, though MOSFETs can be paralleled to increase switching current. MOSFET devices are not bi-directional, nor are they reverse voltage blocking. An example of this new device from ABB shows how this device improves on GTO technology for switching high voltage and high current in power electronics applications. According to ABB,

1120-765: A few tens of milliwatts for a headphone amplifier, up to around a gigawatt in a high voltage direct current transmission line. The first electronic device used in power circuits was the electrolytic rectifier - an early version was described by a French experimenter, A. Nodon, in 1904. These were briefly popular with early radio experimenters as they could be improvised from aluminum sheets, and household chemicals. They had low withstand voltages and limited efficiency. The first solid-state power semiconductor devices were copper oxide rectifiers, used in early battery chargers and power supplies for radio equipment, announced in 1927 by L.O. Grundahl and P. H. Geiger. The first germanium power semiconductor device appeared in 1952 with

1232-458: A forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron ) allow control of the start of conduction but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to

SECTION 10

#1732791510276

1344-481: A half-bridge configuration, a carrier based technique can be used. S+ being on for the half-bridge corresponds to S1+ and S2− being on for the full-bridge. Similarly, S− being on for the half-bridge corresponds to S1− and S2+ being on for the full bridge. The output voltage for this modulation technique is more or less sinusoidal, with a fundamental component that has an amplitude in the linear region of less than or equal to one v o1 =v ab1 = v i  • m

1456-434: A larger amount of current and are typically able to withstand a larger reverse-bias voltage in the off-state . Structural changes are often made in a power device in order to accommodate the higher current density, higher power dissipation, and/or higher reverse breakdown voltage. The vast majority of the discrete (i.e., non-integrated) power devices are built using a vertical structure, whereas small-signal devices employ

1568-461: A lateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved in the height of the die. With this structure, one of the connections of the device is located on the bottom of the semiconductor die . The power MOSFET is the most common power device in the world, due to its low gate drive power, fast switching speed, and advanced paralleling capability. It has

1680-414: A mercury arc valve takes one of two basic forms — the glass-bulb type and the steel-tank type. Steel-tank valves were used for higher current ratings above approximately 500 A. The earliest type of mercury vapor electric rectifier consists of an evacuated glass bulb with a pool of liquid mercury sitting in the bottom as the cathode . Over it curves the glass bulb, which condenses the mercury that

1792-472: A power device are either related to excessive temperature or fatigue due to thermal cycling. Research is currently carried out on the following topics: Research is also ongoing on electrical issues such as reducing the parasitic inductance of packaging; this inductance limits the operating frequency, because it generates losses during commutation. A low-voltage MOSFET is also limited by the parasitic resistance of its package, as its intrinsic on-state resistance

1904-425: A rough approximation of the sinusoidal waveform of AC power is adequate. Where a near sinusoidal waveform is required, the switching devices are operated much faster than the desired output frequency, and the time they spend in either state is controlled so the averaged output is nearly sinusoidal. Common modulation techniques include the carrier-based technique, or Pulse-width modulation , space-vector technique , and

2016-599: A single tank. As solid-state metal rectifiers became available for low-voltage rectification in the 1920s, mercury arc tubes became limited to higher voltage and especially high-power applications. Mercury-arc valves were widely used until the 1960s for the conversion of alternating current into direct current for large industrial uses. Applications included power supply for streetcars, electric railways, and variable-voltage power supplies for large radio transmitters. Mercury-arc stations were used to provide DC power to legacy Edison -style DC power grids in urban centers until

2128-407: A single unit. A six-phase rectifier rated 150 amperes has a glass envelope approximately 600 mm (24 inches) high by 300 mm (12 inches) outside diameter. These rectifiers will contain several kilograms of liquid mercury. The large size of the envelope is required due to the low thermal conductivity of glass. Mercury vapor in the upper part of the envelope must dissipate heat through

2240-439: A smoother direct current. Three phase operation can improve the efficiency of the transformer as well as providing smoother DC current by enabling two anodes to conduct simultaneously. During operation, the arc transfers to the anodes at the highest positive potential (with respect to the cathode). In HVDC applications, a full-wave three-phase bridge rectifier or Graetz-bridge circuit was usually used, each valve accommodated in

2352-423: A starting electrode. The starting electrode is brought into contact with the pool and allowed to pass current through an inductive circuit. The contact with the pool is then broken, resulting in a high emf and an arc discharge. The momentary contact between the starting electrode and the pool may be achieved by a number of methods, including: Since momentary interruptions or reductions of output current may cause

SECTION 20

#1732791510276

2464-401: A steel tank with ceramic insulators for the electrodes is used, with a vacuum pump system to counteract slight leakage of air into the tank around imperfect seals. Steel-tank valves, with water cooling for the tank, were developed with current ratings of several thousand amps. Like glass-bulb valves, steel-tank mercury arc valves were built with only a single anode per tank (a type also known as

2576-630: A wide range of power electronic applications, such as portable information appliances , power integrated circuits, cell phones , notebook computers , and the communications infrastructure that enables the Internet . As of 2010, the power MOSFET accounts for the majority (53%) of the power transistor market, followed by the IGBT (27%), then the RF amplifier (11%), and then the bipolar junction transistor (9%). Switching times range from tens of nanoseconds to

2688-416: A wide range of power electronic applications, such as portable information appliances , power integrated circuits, cell phones , notebook computers , and the communications infrastructure that enables the Internet . In 1982, the insulated-gate bipolar transistor (IGBT) was introduced. It became widely available in the 1990s. This component has the power handling capability of the bipolar transistor and

2800-626: A working MOSFET with their Bell Labs team in 1960. Their team included E. E. LaBate and E. I. Povilonis who fabricated the device; M. O. Thurston, L. A. D’Asaro, and J. R. Ligenza who developed the diffusion processes, and H. K. Gummel and R. Lindner who characterized the device. In 1969, Hitachi introduced the first vertical power MOSFET, which would later be known as the VMOS (V-groove MOSFET). From 1974, Yamaha , JVC , Pioneer Corporation , Sony and Toshiba began manufacturing audio amplifiers with power MOSFETs. International Rectifier introduced

2912-616: Is a semiconductor device used as a switch or rectifier in power electronics (for example in a switch-mode power supply ). Such a device is also called a power device or, when used in an integrated circuit , a power IC . A power semiconductor device is usually used in "commutation mode" (i.e., it is either on or off), and therefore has a design optimized for such usage; it should usually not be used in linear operation. Linear power circuits are widespread as voltage regulators, audio amplifiers, and radio frequency amplifiers. Power semiconductors are found in systems delivering as little as

3024-412: Is a physical limit, no improvement is expected in the design of a silicon MOSFET concerning its maximum voltage ratings. However, its excellent performance in low voltage applications make it the device of choice (actually the only choice, currently) for applications with voltages below 200 V. By placing several devices in parallel, it is possible to increase the current rating of a switch. The MOSFET

3136-878: Is a type of electrical rectifier used for converting high- voltage or high- current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube , but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50- ampere currents, for decades. Invented in 1902 by Peter Cooper Hewitt , mercury-arc rectifiers were used to provide power for industrial motors, electric railways , streetcars , and electric locomotives , as well as for radio transmitters and for high-voltage direct current (HVDC) power transmission. They were

3248-508: Is able to withstand very high reverse breakdown voltage and is also capable of carrying high current. However, one disadvantage of the thyristor in switching circuits is that once it becomes 'latched-on' in the conducting state; it cannot be turned off by external control, as the thyristor turn-off is passive, i.e., the power must be disconnected from the device. Thyristors which could be turned off, called gate turn-off thyristors (GTO), were introduced in 1960. These overcome some limitations of

3360-401: Is an efficient rectifier. Hot-cathode, gas discharge tubes such as the thyratron may also achieve similar levels of efficiency but heated cathode filaments are delicate and have a short operating life when used at high current. The temperature of the envelope must be carefully controlled, since the behaviour of the arc is determined largely by the vapor pressure of the mercury, which in turn

3472-547: Is as low as one or two milliohms. Some of the most common type of power semiconductor packages include the TO-220, TO-247, TO-262, TO-3, D Pak, etc. The IGBT design is still under development and can be expected to provide increases in operating voltages. At the high-power end of the range, the MOS-controlled thyristor is a promising device. Achieving a major improvement over the conventional MOSFET structure by employing

Power electronics - Misplaced Pages Continue

3584-469: Is called unipolar carrier-based SPWM v o1 =2 • v aN1 = v i  • m a . The phase voltages VaN and VbN are identical, but 180 degrees out of phase with each other. The output voltage is equal to the difference of the two-phase voltages, and do not contain any even harmonics. Therefore, if mf is taken, even the AC output voltage harmonics will appear at normalized odd frequencies, fh. These frequencies are centered on double

3696-489: Is evaporated as the device operates. The glass envelope has one or more arms with graphite rods as anodes . Their number depends on the application, with one anode usually provided per phase. The shape of the anode arms ensures that any mercury that condenses on the glass walls drains back into the main pool quickly to avoid providing a conductive path between the cathode and respective anode. Glass envelope rectifiers can handle hundreds of kilowatts of direct-current power in

3808-471: Is expected from the replacement of silicon by a wide band-gap semiconductor. At the moment, silicon carbide (SiC) is considered to be the most promising. A SiC Schottky diode with a breakdown voltage of 1200 V is commercially available, as is a 1200 V JFET . As both are majority carrier devices, they can operate at high speed. A bipolar device is being developed for higher voltages (up to 20 kV). Among its advantages, silicon carbide can operate at

3920-403: Is in linear regulated power supplies, when an amplifier device is used as a voltage regulator to maintain load voltage at a desired setting. While such a power supply may be less energy efficient than a switched mode power supply , the simplicity of application makes them popular, especially in current ranges up to about one amp. The role of packaging is to: Many of the reliability issues of

4032-400: Is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require specialized heat sinks or active cooling systems to manage their junction temperature ; exotic semiconductors such as silicon carbide have an advantage over straight silicon in this respect, and germanium, once

4144-448: Is necessarily associated with a lower performance in the on-state. The trade-offs between voltage, current, and frequency ratings also exist for a switch. In fact, any power semiconductor relies on a PIN diode structure in order to sustain voltage; this can be seen in figure 2. The power MOSFET has the advantages of a majority carrier device, so it can achieve a very high operating frequency, but it cannot be used with high voltages; as it

4256-523: Is particularly suited to this configuration, because its positive thermal coefficient of resistance tends to result in a balance of current between the individual devices. The IGBT is a recent component, so its performance improves regularly as technology evolves. It has already completely replaced the bipolar transistor in power applications; a power module is available in which several IGBT devices are connected in parallel, making it attractive for power levels up to several megawatts, which pushes further

4368-442: Is set by the coolest spot on the enclosure wall. A typical design maintains temperature at 40 °C (104 °F) and a mercury vapor pressure of 7 millipascals . The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapor. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light. The construction of

4480-468: Is the high voltage drop it exhibits in the on-state (2-to-4 V). Compared to the MOSFET, the operating frequency of the IGBT is relatively low (usually not higher than 50 kHz), mainly because of a problem during turn-off known as current-tail : The slow decay of the conduction current during turn-off results from a slow recombination of a large number of carriers that flood the thick 'drift' region of

4592-501: Is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers , battery chargers , etc. The power range is typically from tens of watts to several hundred watts. In industry, a common application is the variable speed drive (VSD) that is used to control an induction motor . The power range of VSDs starts from a few hundred watts and ends at tens of megawatts . The power conversion systems can be classified according to

Power electronics - Misplaced Pages Continue

4704-410: Is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line , and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when

4816-620: The Nelson River DC Transmission System high-voltage DC-power-transmission project. The valves for the Inter-Island and Kingsnorth projects used four anode columns in parallel, while those of the Nelson River project used six anode columns in parallel in order to obtain the necessary current rating. The Inter-Island link was the last HVDC transmission scheme in operation using mercury arc valves. It

4928-487: The Power Electronics Group at Caltech . He developed the state-space averaging method of analysis and other tools crucial to modern power electronics design. In 1957, Frosch and Derick were able to manufacture the first silicon dioxide field effect transistors at Bell Labs, the first transistors in which drain and source were adjacent at the surface. Subsequently, Dawon Kahng led a paper demonstrating

5040-606: The STATCOM . They are also used in applications where arbitrary voltages are required, as in the case of active power filters and voltage compensators. Current source inverters are used to produce an AC output current from a DC current supply. This type of inverter is practical for three-phase applications in which high-quality voltage waveforms are required. A relatively new class of inverters, called multilevel inverters, has gained widespread interest. The normal operation of CSIs and VSIs can be classified as two-level inverters, due to

5152-464: The excitron ) or with multiple anodes per tank. Multiple-anode valves were usually used for multi-phase rectifier circuits (with 2, 3, 6 or 12 anodes per tank) but in HVDC applications, multiple anodes were often simply connected in parallel in order to increase the current rating. A conventional mercury-arc rectifier is started by a brief high-voltage arc within the rectifier, between the cathode pool and

5264-698: The thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices. Power electronic devices may be used as switches, or as amplifiers. An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power

5376-399: The "on" or "off" state. These losses are a significant part of the total lost power in a converter. Power handling and dissipation of devices is also critical factor in design. Power electronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled

5488-485: The 1930s and 1940s, leading to the first truly practical mercury-arc valve for HVDC transmission, which was put into service on the 20 MW, 100 kV HVDC link from mainland Sweden to the island of Gotland in 1954. Uno Lamm's work on high voltage mercury-arc valves led him to be known as the "Father of HVDC" power transmission and inspired the IEEE to dedicate an award named after him, for outstanding contributions in

5600-470: The 1950s, higher power semiconductor diodes became available and started replacing vacuum tubes . In 1956, the silicon controlled rectifier (SCR) was introduced by General Electric , greatly increasing the range of power electronics applications. By the 1960s, the improved switching speed of bipolar junction transistors had allowed for high frequency DC/DC converters. R. D. Middlebrook made important contributions to power electronics. In 1970, he founded

5712-595: The 1950s. In the 1960s, solid-state silicon devices, first diodes and then thyristors , replaced all lower-power and lower voltage rectifier applications of mercury arc tubes. Several electric locomotives, including the New Haven EP5 and the Virginian EL-C , carried ignitrons on board to rectify incoming AC to traction motor DC. One of the last major uses of mercury arc valves was in HVDC power transmission, where they were used in many projects until

SECTION 50

#1732791510276

5824-409: The 1970s. In 1969, Hitachi introduced the first vertical power MOSFET, which would later be known as the VMOS (V-groove MOSFET). From 1974, Yamaha , JVC , Pioneer Corporation , Sony and Toshiba began manufacturing audio amplifiers with power MOSFETs. International Rectifier introduced a 25 A, 400 V power MOSFET in 1978. This device allows operation at higher frequencies than

5936-504: The AC output waveform, v c , to a carrier voltage signal, v Δ . When v c is greater than v Δ , S+ is on, and when v c is less than v Δ , S− is on. When the AC output is at frequency fc with its amplitude at v c , and the triangular carrier signal is at frequency f Δ with its amplitude at v Δ , the PWM becomes a special sinusoidal case of the carrier based PWM. This case is dubbed sinusoidal pulse-width modulation (SPWM).For this,

6048-636: The AC side, while inductors are commonly employed on the DC side. Due to the absence of freewheeling diodes, the power circuit is reduced in size and weight, and tends to be more reliable than VSIs. Although single-phase topologies are possible, three-phase CSIs are more practical. In its most generalized form, a three-phase CSI employs the same conduction sequence as a six-pulse rectifier. At any time, only one common-cathode switch and one common-anode switch are on. Mercury-arc valve A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier

6160-465: The AC waveform are utilised. The cathode is connected to the + side of the DC load, the other side being connected to the center tap of the transformer secondary winding, which always remains at zero potential with respect to ground or earth. For each AC phase, a wire from each end of that phase winding is connected to a separate anode "arm" on the mercury-arc rectifier. When the voltage at each anode becomes positive, it will begin to conduct through

6272-481: The DC input voltage of the inverter. Using selective harmonic elimination (SHE) as a modulation technique allows the switching of the inverter to selectively eliminate intrinsic harmonics. The fundamental component of the AC output voltage can also be adjusted within a desirable range. Since the AC output voltage obtained from this modulation technique has odd half and odd quarter-wave symmetry, even harmonics do not exist. Any undesirable odd (N-1) intrinsic harmonics from

6384-505: The DC traction motors for trolleybuses , trams, and subways, and electroplating equipment. The mercury rectifier was used well into the 1970s, when it was finally replaced by semiconductor rectifiers . Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor at very low pressure. A pool of liquid mercury acts as a self-renewing cathode that does not deteriorate with time. The mercury emits electrons freely, whereas

6496-432: The IGBT during conduction. The net result is that the turn-off switching loss  [ de ] of an IGBT is considerably higher than its turn-on loss. Generally, in datasheets, turn-off energy is mentioned as a measured parameter; that number has to be multiplied with the switching frequency of the intended application in order to estimate the turn-off loss. At very high power levels, a thyristor -based device (e.g.,

6608-567: The IGCT devices are capable of switching in excess of 5000 VAC and 5000 A at very high frequencies, something not possible to do efficiently with GTO devices. A power device may be classified as one of the following main categories (see figure 1): Another classification is less obvious, but has a strong influence on device performance: A majority carrier device is faster, but the charge injection of minority carrier devices allows for better on-state performance. An ideal diode should have

6720-406: The active region, where both device current and voltage are non-zero. Consequently power is continually dissipated and its design is dominated by the need to remove excess heat from the semiconductor device. Power amplifier devices can often be recognized by the heat sink used to mount the devices. Multiple types of power semiconductor amplifier device exist, such as the bipolar junction transistor,

6832-403: The advantages of the isolated gate drive of the power MOSFET. The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve , the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as

SECTION 60

#1732791510276

6944-406: The anodes and cathode. Development of high-current rectifiers required leadwire materials and glass with very similar coefficients of thermal expansion in order to prevent leakage of air into the envelope. Current ratings of up to 500 A had been achieved by the mid-1930s, but most rectifiers for current ratings above this were realised using the more robust steel-tank design. For larger valves,

7056-402: The arc each time conduction is required to start. In this way, ignitrons also avoid the need for control grids. In 1919, the book "Cyclopedia of Telephony & Telegraphy Vol. 1" described an amplifier for telephone signals that used a magnetic field to modulate an arc in a mercury rectifier tube. This was never commercially important. Mercury compounds are toxic, highly persistent in

7168-525: The arc has been established, it cannot be stopped by grid action, because the positive mercury ions produced by ionisation are attracted to the negatively charged grid and effectively neutralise it. The only way of stopping conduction is to make the external circuit force the current to drop below a (low) critical current. Although grid-controlled mercury-arc valves bear a superficial resemblance to triode valves, mercury-arc valves cannot be used as amplifiers except at extremely low values of current, well below

7280-406: The carbon anodes emit very few electrons even when heated, so the current of electrons can only pass through the tube in one direction, from cathode to anode, which allows the tube to rectify alternating current. When an arc is formed, electrons are emitted from the surface of the pool, causing ionization of mercury vapor along the path towards the anodes. The mercury ions are attracted towards

7392-425: The cathode spot to extinguish, many rectifiers incorporate an additional electrode to maintain an arc whenever the plant is in use. Typically, a two or three phase supply of a few amperes passes through small excitation anodes . A magnetically shunted transformer of a few hundred VA rating is commonly used to provide this supply. This excitation or keep-alive circuit was necessary for single-phase rectifiers such as

7504-411: The cathode tanks either water-cooled or air-cooled. Single-phase mercury-arc rectifiers were rarely used because the current dropped and the arc could be extinguished when the AC voltage changed polarity. The direct current produced by a single-phase rectifier thus contained a varying component (ripple) at twice the power supply frequency , which was undesirable in many applications for DC. The solution

7616-477: The cathode, and the resulting ionic bombardment of the pool maintains the temperature of the emission spot , so long as a current of a few amperes continues. While the current is carried by electrons, the positive ions returning to the cathode allow the conduction path to be largely unaffected by the space charge effects which limit the performance of vacuum tubes . Consequently, the valve can carry high currents at low arc voltages (typically 20–30 V) and so

7728-464: The circuit schematic for a three-phase VSI. Switches in any of the three legs of the inverter cannot be switched off simultaneously due to this resulting in the voltages being dependent on the respective line current's polarity. States 7 and 8 produce zero AC line voltages, which result in AC line currents freewheeling through either the upper or the lower components. However, the line voltages for states 1 through 6 produce an AC line voltage consisting of

7840-441: The concept of a field-effect transistor in 1926, but it was not possible to actually construct a working device at that time. In 1947, the bipolar point-contact transistor was invented by Walter H. Brattain and John Bardeen under the direction of William Shockley at Bell Labs . In 1948 Shockley's invention of the bipolar junction transistor (BJT) improved the stability and performance of transistors , and reduced costs. By

7952-407: The critical current needed to maintain the arc. Mercury-arc valves are prone to an effect called arc-back (or backfire ), whereby the valve conducts in the reverse direction when the voltage across it is negative. Arc-backs can be damaging or destructive to the valve, as well as creating high short-circuit currents in the external circuit, and are more prevalent at higher voltages. One example of

8064-426: The current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also significantly affect design; sometimes, the control input is at a very high voltage with respect to ground and must be driven by an isolated source. As efficiency is at a premium in a power electronic converter,

8176-447: The design of the filtering components needed for the low-order current harmonic injection from the operation of the inverter. The maximum output amplitude in this mode of operation is half of the source voltage. If the maximum output amplitude, m a , exceeds 3.24, the output waveform of the inverter becomes a square wave. As was true for Pulse-Width Modulation (PWM), both switches in a leg for square wave modulation cannot be turned on at

8288-715: The device across its internal junctions (or channels); once a "hot spot" develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 amperes in a single unit. DC to AC converters produce an AC output waveform from a DC source. Applications include adjustable speed drives (ASD), uninterruptible power supplies (UPS), Flexible AC transmission systems (FACTS), voltage compensators, and photovoltaic inverters . Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters. Voltage source inverters (VSIs) are named so because

8400-489: The discrete values of Vi, 0 or −Vi. For three-phase SPWM, three modulating signals that are 120 degrees out of phase with one another are used in order to produce out-of-phase load voltages. In order to preserve the PWM features with a single carrier signal, the normalized carrier frequency, mf, needs to be a multiple of three. This keeps the magnitude of the phase voltages identical, but out of phase with each other by 120 degrees. The maximum achievable phase voltage amplitude in

8512-525: The disused deep-level air-raid shelter at Belsize Park . After they were no longer needed as shelters, Belsize Park and several other deep shelters were used as secure storage, particularly for music and television archives. This led to the mercury-arc rectifier at the Goodge Street shelter featuring in an early episode of Doctor Who as an alien brain, cast for its "eerie glow". Auckland's Museum Of Transport And Technology (MOTAT) still employs

8624-631: The early 1970s, including the HVDC Inter-Island link between the North and South Islands of New Zealand and the HVDC Kingsnorth link from Kingsnorth power station to London . However, starting about 1975, silicon devices have made mercury-arc rectifiers largely obsolete, even in HVDC applications. The largest ever mercury-arc rectifiers, built by English Electric , were rated at 150  kV , 1800 A and were used until 2004 at

8736-546: The environment, and present a danger to humans and the environment. The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC converter stations have required extensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps, which continually emitted small amounts of mercury vapor. Power device A power semiconductor device

8848-459: The excitron and for mercury-arc rectifiers used in the high-voltage supply of radiotelegraphy transmitters, as current flow was regularly interrupted every time the Morse key was released. Both glass and metal envelope rectifiers may have control grids inserted between the anode and cathode. Installation of a control grid between the anode and the pool cathode allows control of the conduction of

8960-721: The fact that power switches connect to either the positive or to the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave. It is for this reason that multilevel inverters, although more complex and costly, offer higher performance. Each inverter type differs in the DC links used, and in whether or not they require freewheeling diodes . Either can be made to operate in square-wave or pulse-width modulation (PWM) mode, depending on its intended usage. Square-wave mode offers simplicity, while PWM can be implemented in several different ways and produces higher quality waveforms. Voltage Source Inverters (VSI) feed

9072-509: The field of HVDC. Mercury arc valves with grading electrodes of this type were developed up to voltage ratings of 150 kV. However, the tall porcelain column required to house the grading electrodes was more difficult to cool than the steel tank at cathode potential, so the usable current rating was limited to about 200–300 A per anode. Therefore, Mercury arc valves for HVDC were often constructed with four or six anode columns in parallel. The anode columns were always air-cooled, with

9184-437: The following characteristics: In reality, the design of a diode is a trade-off between performance in on-state, off-state, and commutation. Indeed, the same area of the device must sustain the blocking voltage in the off-state and allow current flow in the on-state; as the requirements for the two states are completely opposite, a diode has to be either optimised for one of them, or time must be allowed to switch from one state to

9296-482: The full-bridge configuration should have either the top or the bottom switch of each leg on at any given time. Due to the extra leg, the maximum amplitude of the output waveform is Vi, and is twice as large as the maximum achievable output amplitude for the half-bridge configuration. States 1 and 2 from Table 2 are used to generate the AC output voltage with bipolar SPWM. The AC output voltage can take on only two values, either Vi or −Vi. To generate these same states using

9408-399: The glass envelope in order to condense and return to the cathode pool. Some glass tubes were immersed in an oil bath to better control the temperature. The current-carrying capacity of a glass-bulb rectifier is limited partly by the fragility of the glass envelope (the size of which increases with rated power) and partly by the size of the wires fused into the glass envelope for connection of

9520-451: The independently controlled output is a voltage waveform. Similarly, current source inverters (CSIs) are distinct in that the controlled AC output is a current waveform. DC to AC power conversion is the result of power switching devices, which are commonly fully controllable semiconductor power switches. The output waveforms are therefore made up of discrete values, producing fast transitions rather than smooth ones. For some applications, even

9632-845: The introduction of the power diode by R.N. Hall . It had a reverse voltage blocking capability of 200 V and a current rating of 35 A . Germanium bipolar transistors with substantial power handling capabilities (100 mA collector current) were introduced around 1952; with essentially the same construction as signal devices, but better heat sinking. Power handling capability evolved rapidly, and by 1954 germanium alloy junction transistors with 100 watt dissipation were available. These were all relatively low-frequency devices, used up to around 100 kHz, and up to 85 degrees Celsius junction temperature. Silicon power transistors were not made until 1957, but when available had better frequency response than germanium devices, and could operate up to 150 C junction temperature. The thyristor appeared in 1957. It

9744-425: The limit at which thyristors and GTOs become the only option. Basically, an IGBT is a bipolar transistor driven by a power MOSFET; it has the advantages of being a minority carrier device (good performance in the on-state, even for high voltage devices), with the high input impedance of a MOSFET (it can be driven on or off with a very low amount of power). The major limitation of the IGBT for low voltage applications

9856-553: The linear region, ma less than or equal to one, is v phase = v i  / 2 . The maximum achievable line voltage amplitude is V ab1 = v ab  • √ 3  / 2 The only way to control the load voltage is by changing the input DC voltage. Current source inverters convert DC current into an AC current waveform. In applications requiring sinusoidal AC waveforms, magnitude, frequency, and phase should all be controlled. CSIs have high changes in current over time, so capacitors are commonly employed on

9968-619: The losses generated by a power electronic device should be as low as possible. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in

10080-437: The main-stay of solid-state electronics is now little used due to its unfavorable high-temperature properties. Semiconductor devices exist with ratings up to a few kilovolts in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switching device will have to withstand

10192-543: The mercury vapor from the cathode. As the anodes of each AC phase are fed from opposite ends of the centre tapped transformer winding, one will always be positive with respect to the center tap and both halves of the AC Waveform will cause current to flow in one direction only through the load. This rectification of the whole AC waveform is thus called full-wave rectification . With three-phase alternating current and full-wave rectification, six anodes were used to provide

10304-473: The modulation index, or amplitude-modulation ratio, is defined as m a = v c /v ∆ . The normalized carrier frequency, or frequency-modulation ratio, is calculated using the equation m f = f ∆ /f c . If the over-modulation region, ma, exceeds one, a higher fundamental AC output voltage will be observed, but at the cost of saturation. For SPWM, the harmonics of the output waveform are at well-defined frequencies and amplitudes. This simplifies

10416-422: The ordinary thyristor, because they can be turned on or off with an applied signal. The MOSFET was invented at Bell Labs between 1955 and 1960 Generations of MOSFET transistors enabled power designers to achieve performance and density levels not possible with bipolar transistors. Due to improvements in MOSFET technology (initially used to produce integrated circuits ), the power MOSFET became available in

10528-424: The other (i.e., the commutation speed must be reduced). These trade-offs are the same for all power devices; for instance, a Schottky diode has excellent switching speed and on-state performance, but a high level of leakage current in the off-state. On the other hand, a PIN diode is commercially available in different commutation speeds (what are called "fast" and "ultrafast" rectifiers), but any increase in speed

10640-425: The output inverter section from an approximately constant-voltage source. The desired quality of the current output waveform determines which modulation technique needs to be selected for a given application. The output of a VSI is composed of discrete values. In order to obtain a smooth current waveform, the loads need to be inductive at the select harmonic frequencies. Without some sort of inductive filtering between

10752-444: The output waveform can be eliminated. The full-bridge inverter is similar to the half bridge-inverter, but it has an additional leg to connect the neutral point to the load. Figure 3 shows the circuit schematic of the single-phase voltage source full-bridge inverter. To avoid shorting out the voltage source, S1+, and S1− cannot be on at the same time, and S2+ and S2− also cannot be on at the same time. Any modulating technique used for

10864-407: The primary method of high power rectification before the advent of semiconductor rectifiers, such as diodes , thyristors and gate turn-off thyristors (GTOs) in the 1970s. These solid state rectifiers have almost completely replaced mercury-arc rectifiers thanks to their higher reliability, lower cost and maintenance and lower environmental risk. In 1882 Jules Jamin and G. Maneuvrier observed

10976-689: The problems caused by backfire occurred in 1960 subsequent to the electrification of the Glasgow North Suburban Railway where steam services had to be re-introduced after several mishaps. For many years this effect limited the practical operating voltage of mercury-arc valves to a few kilovolts. The solution was found to be to include grading electrodes between the anode and control grid, connected to an external resistor - capacitor divider circuit. Dr. Uno Lamm conducted pioneering work at ASEA in Sweden on this problem throughout

11088-514: The rectifying properties of a mercury arc. The mercury arc rectifier was invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920s and 1930s by researchers in both Europe and North America. Before its invention, the only way to convert AC current provided by utilities to DC was by using expensive, inefficient, and high-maintenance rotary converters or motor–generator sets. Mercury-arc rectifiers or "converters" were used for charging storage batteries, arc lighting systems,

11200-414: The same time, as this would cause a short across the voltage source. The switching scheme requires that both S+ and S− be on for a half cycle of the AC output period. The fundamental AC output amplitude is equal to v o1 = v aN = 2v i /π . Its harmonics have an amplitude of v oh = v o1 /h . Therefore, the AC output voltage is not controlled by the inverter, but rather by the magnitude of

11312-480: The selective-harmonic technique. Voltage source inverters have practical uses in both single-phase and three-phase applications. Single-phase VSIs utilize half-bridge and full-bridge configurations, and are widely used for power supplies, single-phase UPSs, and elaborate high-power topologies when used in multicell configurations. Three-phase VSIs are used in applications that require sinusoidal voltage waveforms, such as ASDs, UPSs, and some types of FACTS devices such as

11424-418: The source and load, a capacitive load will cause the load to receive a choppy current waveform, with large and frequent current spikes. There are three main types of VSIs: The single-phase voltage source half-bridge inverters are meant for lower voltage applications and are commonly used in power supplies. Figure 9 shows the circuit schematic of this inverter. Low-order current harmonics get injected back to

11536-444: The source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design. As Figure 9 illustrates, only one switch can be on at a time in each leg of the inverter. If both switches in a leg were on at the same time, the DC source would be shorted out. Inverters can use several modulation techniques to control their switching schemes. The carrier-based PWM technique compares

11648-439: The super junction charge-balance principle: essentially, it allows the thick drift region of a power MOSFET to be heavily doped, thereby reducing the electrical resistance to electron flow without compromising the breakdown voltage. This is juxtaposed with a region that is similarly doped with the opposite carrier polarity ( holes ); these two similar, but oppositely doped regions effectively cancel out their mobile charge and develop

11760-490: The transitions from on to off and back but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating. Practical devices have a non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach

11872-576: The type of the input and output power: Power electronics started with the development of the mercury arc rectifier. Invented by Peter Cooper Hewitt in 1902, it was used to convert alternating current (AC) into direct current (DC). From the 1920s on, research continued on applying thyratrons and grid-controlled mercury arc valves to power transmission. Uno Lamm developed a mercury valve with grading electrodes making them suitable for high voltage direct current power transmission. In 1933 selenium rectifiers were invented. Julius Edgar Lilienfeld proposed

11984-473: The value of the normalized carrier frequency. This particular feature allows for smaller filtering components when trying to obtain a higher quality output waveform. As was the case for the half-bridge SHE, the AC output voltage contains no even harmonics due to its odd half and odd quarter-wave symmetry. Single-phase VSIs are used primarily for low power range applications, while three-phase VSIs cover both medium and high power range applications. Figure 5 shows

12096-444: The valve in the non-conducting state, a negative bias of a few volts or tens of volts is applied to the grid. As a result, electrons emitted from the cathode are repelled away from the grid, back towards the cathode, and so are prevented from reaching the anode. With a small positive bias applied to the grid, electrons pass through the grid, towards the anode, and the process of establishing an arc discharge can commence. However, once

12208-452: The valve, thereby giving control of the mean output voltage produced by the rectifier. Start of the current flow can be delayed past the point at which the arc would form in an uncontrolled valve. This allows the output voltage of a valve group to be adjusted by delaying the firing point, and allows controlled mercury-arc valves to form the active switching elements in an inverter converting direct current into alternating current. To maintain

12320-464: The vertical MOS field effect transistor, and others. Power levels for individual amplifier devices range up to hundreds of watts, and frequency limits range up to the lower microwave bands. A complete audio power amplifier, with two channels and a power rating on the order of tens of watts, can be put into a small integrated circuit package, needing only a few external passive components to function. Another important application for active-mode amplifiers

12432-695: Was formally decommissioned on 1 August 2012. The mercury arc valve converter stations of the New Zealand scheme were replaced by new thyristor converter stations. A similar mercury arc valve scheme, the HVDC Vancouver Island link was replaced by a three-phase AC link. Mercury arc valves remain in use in some South African mines and Kenya (at Mombasa Polytechnic - Electrical & Electronic department). Mercury arc valves were used extensively in DC power systems on London Underground , and two were still observed to be in operation in 2000 at

12544-448: Was to use two-, three-, or even six-phase AC power supplies so that the rectified current would maintain a more constant voltage level. Polyphase rectifiers also balanced the load on the supply system, which is desirable for reasons of system performance and economy. Most applications of mercury-arc valves for rectifiers used full-wave rectification with separate pairs of anodes for each phase. In full-wave rectification both halves of

#275724