Rectifier

Rectifiers are used in electrical engineering and electronics to convert AC voltage to DC voltage. They form, in addition to inverters and converters, a subset of the current judges. To attenuate alternating components, a rectified voltage is usually smoothed.

A rectifier is used, for example,

• To supply DC -powered electrical load from the AC mains;
• For connection to remote power grids or the coupling of non-synchronous power systems via high- voltage direct -current transmission;
• For measurement purposes or
• For applications in telecommunications, for example to envelope detection.

The rectification is usually uncontrolled by semiconductor diodes with an efficiency up to 95 %. Active electronic components, such as thyristors, phase control by allowing a controlled rectification. Field effect transistors ( MOSFETs ) may be used for synchronous rectifiers - particularly in the case of equality of the direction of lower voltages and large currents - and allow due to the lower forward voltage higher efficiency than is possible with semiconductor diode.

• 3.1 Uncontrolled rectifiers 3.1.1 Half-wave rectifier (E1 ) ( M1U )
• 3.1.2 bridge rectifier ( B2 ) ( B2U )
• 3.1.3 midpoint rectifier ( M2)
• 3.1.4 for rectifying three-phase AC
• 3.1.5 rectifier circuits for voltage multiplication

History

In 1873, Frederick Guthrie discovered that a positively charged electroscope is discharged when they brought a grounded, glowing piece of metal in the vicinity. When a negatively charged electroscope nothing happens, resulting followed that the electric current could flow in only one direction.

1874 Karl Ferdinand Braun discovered the direction -dependent conduction in certain crystals. He settled the crystal rectifier patented 1899.

The Indian scientist Jagadish Chandra Bose used 1894 as the first crystals to detect electromagnetic waves. The first practically usable crystal silicon detector was developed in 1903 for harmful applications by Greenleaf Whittier Pickard, who this was patented in 1906 .. In the follow-up period, however, lead sulfide was more frequently used because it was cheaper and easier to use.

Thomas Edison rediscovered Guthrie's observation in 1880 in experiments with light bulbs and had patented the effect in 1884 without knowing a possible application. Since then, Edison Richardson effect it is called. Some twenty years later, John Ambrose Fleming realized that was the first employee of Edison and later scientific adviser to the Marconi Wireless Telegraph Company, the Edison - Richardson - effect could be used to detect weak radio signals. He had his first practical application, the " Fleming valve" patenting in 1904.

All effects described so far could only be used for very low currents. However, the increasing use of electrical energy needed powerful rectifier, because those is preferably produced by alternators. As the enigmatic electrical processes have been elucidated in crystals only after about 1950 ( after the invention of the transistor ), we devised the meantime diverse principles for rectification. With very few exceptions after 1980 all exotic methods were quickly displaced by the almost universal silicon diodes. Everything else can be found only in museums.

Physical principles

Mechanical rectifier

At the beginning of the 20th century there were only electromechanical rectifier to convert AC voltage to DC voltage:

• With appropriate converters sit an AC motor and a DC generator on a common shaft. These converters often served simultaneously for voltage transformation and galvanic isolation from the mains.
• So-called chopper were used as an inverter from DC to the subsequent transformation by means of a transformer. They combine a self-oscillating, working with switch contacts inverter and coupled thereto a second set of contacts for synchronous rectification in itself. These were due to the contact - wear interchangeable in ( socket ).
• The rotating mechanical rectifier have a wheel with electric contacts, which enables the exact switching time of the alternating voltage. The wheel is driven by a synchronous motor, which ensured the synchronization between the rotational movement and the change in polarity of the alternating voltage. The design has been supplied, for example, a high voltage transformer with alternating current to generate the high DC voltage for the electrostatic precipitator. A further development of this principle embodied the Kontaktumformer.
• Until about 1970, the charging current to the batteries through DC alternator with mechanical rectifier ( commutator ) was created in cars. After the invention of high-performance semiconductor diodes, these were quickly replaced by much more powerful alternators.

A disadvantage of the contact erosion, especially at higher currents, synchronization problems and the limitation to relatively low frequencies below about 500 Hz are The great advantage of vanishing forward voltage and thus very low power dissipation has been achieved only recently by controlled MOSFET rectifier again.

Electrolytic rectifier

One way of rectifying opened the principle of anodic oxidation, which played a role in the early days of electric telegraphy and telephony. Two submerged in an electrolyte electrodes may have rectifiers similar properties. An electrode needs to from a noble metal, for example platinum, are made, and the other made ​​of a metal, forming a thick oxide layer by anodic oxidation, such as niobium. With this arrangement, current can only flow when the anodically oxidizable metal acts as a cathode. These rectifiers are also referred to as a wet electrolytic rectifier or rectifier and could be used to voltages up to 300V. The main disadvantages of location sensitivity and toxic or corrosive vapors limited the number of applications.

Mercury arc rectifiers

Another development was the mercury vapor rectifier, which could also be used for higher power ratings. It consists of a glass bulb, at its lower end is a cathode with a mercury pool ( pool cathode). In addition to warp the glass flask fitted with condensing the mercury again. Side arms are fused with graphite electrodes as anodes. Electrons can flow only from the pond to the cathode graphite electrodes.

Dry rectifier

Some decades later, the first semiconductor rectifiers such as selenium rectifier and cuprous oxide rectifiers were invented. They are, as that no liquid were used, referred to as dry rectifier. They consist of a metal plate on which a layer of selenium or copper oxide is applied. The reverse voltage is only about 15 ... 50 V, the forward voltage is relatively high. To rectify large voltages and dissipate the waste heat, the selenium plates were stacked. The plate surface determines the current, the plate number of the maximum reverse voltage. So-called selenium rods contained a large number of small selenium rectifier slices and served until the 1970s, inter alia, for rectifying the anode voltage of cathode ray tubes in black and white televisions. They had off-state voltages up to 20 kV.

Because of the typical horseradish or garlic -like odor to be described in overload of a selenium rectifier spoke the word " rectifier " also known as " direct it smells " off.

At the beginning of the 20th century detector crystals were used from galena or pyrite, especially in detector receivers: A semiconductor-metal transition, which consisted of a semiconductor crystal and a tentative metal tip, were sought with the appropriate points for demodulation of AM radio broadcasts. This mechanically very sensitive and not very effective experimental design was quickly displaced by the rapidly progressing development of the electron tube circuits enabled, which amplify and simultaneously could rectify ( Audion ).

Disadvantages are the low reverse voltages of about 15 V and above all the low ratio barrier resistance / transmission resistance, which is just over one and today's standards is no longer enough.

Semiconductor diodes

The major breakthrough in the development of the rectifier was achieved only after a thorough exploration of the pn junction, following the all revolutionary invention of the bipolar transistor in 1947. Was used over a longer period of time sensitive germanium diodes, which later by the temperature-resistant silicon diodes were replaced. Their excellent properties displaced all other rectifier effects in Meaningless:

• The comparatively very low threshold voltage of about one volt ensures low power dissipation and simplifies cooling.
• Switching power supplies were reliable and maintenance -free only after the invention of the semiconductor diodes. Since then they replace heavy toroid transformers and they are probably completely displace in the foreseeable future. DC-DC converter with semiconductor diodes, as an essential components have become standard components in electronic devices.
• Only after the availability of powerful rectifier from semiconductors economically operating frequency could be built even for very large benefits. This revolutionized the electric drive technology because they provide stepless speed change of the highly reliable three-phase asynchronous machines.
• High voltage direct current transmission was only made possible by the invention of the semiconductor diodes. Maybe with early invention, the current war would have been different.

Exemplary widespread components in the medium power range for blocking voltages up to 1,000 volts, the rectifier diodes 1N4001 ... 1N4007, for currents up to 1 ampere and the rectifier diodes 1N5400 1N5408 ... for currents up to 3 amps.

Disadvantage in comparison to all other rectifier types is the very low thermal overload. Causes are the low mass of the crystal in semiconductor diodes and the poor heat dissipation via the connecting wires. Therefore (a few milliseconds), the crystal temperature rises against overcurrent after a very short time so far that the PN layer is irreversibly destroyed.

Tube diodes

A tube or diode vacuum diode is an electron tube with a heated cathode, and one or two ( more often ) the anode (s ). There is no sense applicable tax possibility of the transmitted stream. Tube diodes were in the early days of broadcasting technology in tube receivers, the standard component for rectifying the supply voltage and the ( amplitude modulated ) signal. Disadvantages are the bulky and fragile glass housing, the necessary heat to the cathode and the high forward voltage of about 40 V at currents to 100 mA. Unmatched contrast, the extreme maximum reverse voltage (> 100 kV) and the infinite high reverse resistance.

Glimmgleichrichter

End of the 1930s was also experimenting with rectifiers on the basis of glow discharges and specific types of neon lamp. The method is similar to the diode of the tube, the tube but it is filled with a gas and the cathode is not heated. The rectifying effect is based on an asymmetrical shaping of the two discharge electrodes or electrode coatings on the reduction of the cathode case. The Glimmgleichrichter could not prevail because of its very poor ratio of forward -to reverse current (< 100:1), the low maximum current and the incomparably high forward voltage of about 70 V; the use of capacitors for voltage smoothing increased the reverse current even further.

Rectifier circuits

Uncontrolled rectifiers

In these circuits, diodes are only used, the conductivity is dependent on the polarity of the applied voltage:

• When the cathode is more negative than the anode and the necessary threshold voltage (for Silicon: 0.6 V ) is exceeded, the diode.
• Upside Down in sign, the diode, as long as the breakdown voltage is not exceeded.

In both cases, the current must be limited, otherwise, the diode will be destroyed. The following are some typical rectifier circuits are outlined, in parentheses, the technical abbreviation is indicated.

Half-wave rectifier (E1 ) ( M1U )

At a half-wave rectifier (also Einzweigschaltung ) only one half cycle of the AC voltage is rectified, and the other is not used. During the half period in which the diode is operated in the forward direction, is at the output voltage in the second half period of the diode. Disadvantages are the relatively large ripple on the DC side, the poor efficiency and the unbalanced load of the AC voltage source. Characterized the upstream transformer is magnetized, since it is traversed in one direction by the current. For such a rectifier requires only a single diode. The pulsating DC voltage needs to be smoothed as a rule. The ripple, the frequency of the input voltage.

Half-wave rectification is outdated and comes from a time when the rectifier were still very expensive. They are found today only in flyback converters. Black and white televisions had a one-way rectifier for generating the high voltage of the picture tube anode voltage from the line -return pulses of the line transformer. " Allstrom " tube radios and televisions had a half-wave rectifier selenium, and later of silicon for mains rectification and for obtaining the anode voltage. A Netzpol was used as a ground potential in order to build a series circuit for directly operated at the power tube heaters can.

Rectifier bridge (B2 ) ( B2U )

Standard for single-phase rectifier is called the bridge rectifier, also Graetz circuit or two-pulse bridge circuit. It is named after the German physicist Leo Graetz. The circuit is formed by four diodes: The left alternating current voltage that comes directly from a transformer, for example, is converted into a pulsating DC voltage ( shown at right ). Since this is a full-wave rectification, the negative half cycle of the AC voltage in a DC circuit on the load R appears exclusively positive. Unlike other rectifier types, the reverse voltage of the rectifier diodes must be only as large as the peak voltage of the AC voltage at the rectifier circuit. It is chosen for safety reasons, however, slightly higher ( in case of power rectifiers on the 230 -volt AC, for example, about 400 volts).

The ripple is twice the frequency of the input voltage, which in the subsequent filter costs are reduced.

Bridge rectifier for AC and AC are often considered together already interconnected diodes in a common housing offered. At higher currents, they contain a cooling surface and a bore for mounting on a heat sink.

Midpoint rectifier ( M2)

At the midpoint rectifier both half cycles of the AC voltage is also rectified. However, a transformer with a Mittelpunktanzapfung, this requires that forms one pole of the rectified output voltage simultaneously.

The advantage of this circuit lies in center to manage with only two diodes D1 and D2 and the voltage is only reduced by a diode forward voltage. The disadvantage is that it requires a special transformer, which is more powerful unit, as always contributes only half of the copper to current flow. For a given volume of copper each half of the secondary winding because of the thinner wire ( double turns, must be found ) in approximately twice the internal resistance, which enters into the power loss (P = R · I2).

It is further noted that the blocking voltage of the diode must be at least twice the output voltage. When a blocking diode and the other passes, located on the closed to the full voltage transformer.

The ripple is twice the frequency of the input voltage, which reduces the subsequent filter complexity compared to the half-wave rectification. The circuit is mainly used at low voltages (below 10 V ) as well as switching power supplies, since the benefits outweigh the disadvantages. The center tap is often used in earlier times, tube devices for the generation of the anode voltage, since you only needed an expensive rectifier unit with her using Duodioden, rectifier tube with two anodes and a common cathode or mehranodigen mercury vapor rectifiers.

Today one often uses double diodes ( silicon or Schottky diodes ) with a common cathode. They consist of a chip including two diodes and the rear side thereof is soldered as a common cathode, a cooling fin.

Rectifier for three-phase AC

→ Main article: Three -phase rectifier

For medium power of a few kilowatts up the three-phase AC voltage is rectified from the power grid, because then the ripple voltage on the DC side is small and only little effort to smooth the DC voltage is produced.

Application, see three-phase rectifier in electrical power engineering such as trams, which are usually operated with DC voltage of 500 V ... 750. Even with today's conventional three-phase alternator motor vehicles it is used. In both cases, no smoothing is necessary.

Before the time of the semiconductor rectifier is also produced multi-phase mercury vapor rectifier, which were characterized by a common pool cathode and a plurality of upwardly anodes.

Rectifier circuits for voltage multiplication

Special rectifier circuits are used for voltage multiplication. In this case, combinations of diodes and capacitors are connected so that an alternating voltage produces a multiplied voltage. Typical circuits are voltage doubler, the high voltage cascade and the Greinacher circuit. These circuits find application, inter alia, in television receivers having cathode ray tubes for generating the anode voltage in the range of 18 to 24 kV.

Controllable rectifier

All rectifier described so far are uncontrolled. The switching is made without an additional control electronics, only because of the sign of the applied electrical voltage to the diodes. Controllable rectifier is used in the field of energy and drive technology. They do not just rectification is possible, but by phase angle control and power control, because you can set the time, these components from which the rectifier is electrically conductive - before he isolated.

The picture to the ignition pulse of the thyristors is shown below as a blue rectangle. Fading of the control pulse, the current flow remains ( shown in red ) to the zero crossing exist. By sliding the switch-on, the energy (gray area) can be changed, which flows to the consumer. Areas of application are, for example, the speed control of DC and universal motors in industrial facilities and small appliances such as drills, in modern electric locomotives for loading the DC voltage intermediate circuit and equipment of high-voltage direct -current transmission. Controlled rectifiers in converters enable very effective frequency and power control of three-phase motors in rolling mills, electric locomotives and washing machines.

Previously, grid-controlled mercury arc rectifiers such as thyratron and ignitrons were used for this purpose, which are large, expensive and lossy. Today, thyristors, insulated gate bipolar transistors (IGBT ) and partly power MOSFETs are used, which require much lower cooling requirements.

There are also so-called GTO thyristors, which allow the locking of a valve by means of a pulse. However, these values ​​indicate poorer electrical and are increasingly being replaced by IGBTs.

If a bridge rectifier fully controllable branches, with it's four- quadrant operation is possible, that is, it can provide with appropriate control of both energy from the ac side to the dc side and vice versa. The equivalent circuit in the form of an H-bridge is known as a four -quadrant controller, and used, inter alia, in inverters for generating an alternating voltage from a direct voltage.

The classification of the controllable rectifier is carried out in a plurality of circuit topologies, these include B2HZ, B2HK, B2C and B6C rectifier. A controllable rectifier circuit with very low harmonic represents the Vienna rectifier

In addition, a controlled rectifier has importance in metrology, for example in the AC bridge.

Synchronous rectifier

The rectification very low voltages is problematic because there is no uncontrolled diode with threshold voltages below 0.2 V ( Schottky diode). Small stresses can be so do not rectify, for larger, there are unpleasant side effects:

• In the envelope detectors demodulated voltage is distorted.
• For large currents, the efficiency drops significantly, the rectifier is hot.

Therefore, high-performance synchronous rectifiers are used, among other things, in switching power supplies with low output voltage.

In this type of rectifiers no components are used, the permit on his own initiative a current flow in one direction only; Instead, one uses transistors ( MOSFETs often ), which are controlled by a control electronics so that they act as a semiconductor diode with a very low forward voltage.

For example, a modern Pentium processor requires a direct current of 75 A at 1.3 V. This power is only generated in the laptop 18 V by means of miniaturized switching converter using MOSFET rectifier. Because of these components, the loss of tension in the range of only a few are 10 mV, this rectifier having an efficiency of about 99% and can be achieved in a few square centimeters, without cooling. When using a dry rectifier more heat would be produced in this than in the processor itself

The disadvantage of synchronous rectifiers is the higher component costs, there must be operated for synchronous drive MOSFETs additional circuitry. Most of these additional circuit parts for the time -precise control are summarized finished in integrated circuits.

In the circuit diagram shown on the right part of a switching regulator, the AC voltage is transformed to the left via a transformer to the required voltage level, in the same direction on the two MOSFETs and the output voltage V0 is smoothed by means of coil L and capacitor C. Due to lack of clarity, the drive circuit for the two MOSFETs.

Synchronous rectifier for low currents are found in the chopper amplifier, auto-zero amplifiers and lock -in amplifiers.

Smoothing

Each rectifier makes the original waveform of the voltage, which is often sinusoidally periodically fluctuating voltage. However, most consumers as computers require very constant DC voltage, which is why the troughs must be balanced. This process is called smoothing, which is normally done by a parallel with the load switched capacitor with sufficient capacity. This capacitor is charged by short current pulses, which can assume considerable values ​​. Alternatively, this can be done by smoothing inductor in series to the consumer, whereby the current flow through the rectifier is more uniform. The remaining after smoothing alternating component, also called ripple voltage or ripple can be reduced by subsequent screening members called on.

The smoothing simplifies with increasing frequency of the AC voltage, which is why more and more frequently switching power supplies are used with frequencies above 40 kHz. Very efficient rectifiers can be always operated with three -phase alternating current, since the direct current without smoothing often has sufficiently low ripple voltage with use of a twelve-pulse rectifier.

Small power supplies for electron tubes are pi filter (two shunt capacitors, between a series inductance ), in principle, but that also works with any other low-pass. At very high demands on the quality of the DC voltage stabilization by an electronic voltage regulator as in laboratory power supplies used. This can eliminate the built-in amplifier output voltage ripple, almost entirely because the circuit acts as a negative feedback.

Rectifier in metrology

In the electrical metrology precision rectifier for low voltage and small Strömstärken are necessary. They are not the power supply of electronic components, but for processing measurement signals.

In contrast to the simple uncontrolled rectification may be for the AC voltage measurement technique for rectification, which is synchronous to an external process, reverse the sign of the output voltage.

Rectifier in communications technology

Rectifiers are used in the communication equipment for detecting or demodulating RF signals. A simple example is the amplitude modulated envelope detection signals at the detector receivers. It is shown in the adjacent circuit diagram.

The coil and the variable capacitor in this case represent a resonant circuit that is tuned to the desired radio frequency carrier frequency. The received voltage, the amplitude of which varies in dependence on the useful signal is rectified through the diode, whereby the voltage waveform applied to headphones as the envelope of the desired signal - higher frequency components of the transmit frequency can be suppressed by the inductance of the headphones.

This form of a receiver is not particularly sensitive and suitable only for receiving timely and strong transmitter. The materials for the diode previously existed among other things, galena or pyrite and were contacted through a fine metal tip ( which is rather to be regarded as a Schottky diode ), whereby a low-capacitance rectifying effect was achieved. Germanium diodes were later used for this purpose.

The underlying principle is also used in current radio receivers when receiving amplitude-modulated signals.

Non-linear circuit as a rectifier

All rectifiers are non-linear circuits, which cause a non-sinusoidal current waveform on the DC side for sinusoidal voltage profile in power supplies. The non-linearity is due to the non-linear function and the amount due to the smoothing effect on the direct voltage side.

The current on the AC side is composed of several frequency components, called harmonics, which can cause interference in AC systems. In order to keep these small harmonics, power supplies with rectifiers of certain benefits must have a power factor correction to minimize the distortion power. This is a special form of filtering, which tracks the desired sinusoidal current waveform on the AC side.

Moreover occurs in rectifiers, as with all non-linear circuits, a special form of reactive power, which is referred to in the literature as non-uniform distortion power or distortion performance and affects similar to the reactive power. This is in contrast to the reactive power, which is also called a shift of reactive power and is characterized by a certain phase shift between voltage and current of the fundamental component to a form of reactive power, which is caused by strong harmonics, caused by the usually small conduction angle. This distortion power factor load as well as the displacement of reactive power lines and is undesirable since it does not work is done by the consumer.

On 1 January 2001 an EMC standard went into effect, lays down rules on the permissible low- noise spectrum ( harmonics ) for electronic consumer from 75 watts.

Others

• The largest ever used -down mercury arc rectifiers were in the high-voltage direct current transmission system Nelson River Bipole 1 had a blocking voltage of 150 kV and a maximum forward current of 1800 A. mid-1990s, they were replaced by controlled rectifier thyristor-based.

• Immersed to a voltage and applies this in dilute sulfuric acid, a platinum electrode, and niobium, so only current flow when the niobium electrode is the cathode. This is called an electrolytic rectifier. Such rectifiers can also be implemented with other electrolytes and metals. It is important that an electrode made ​​of metal with a high propensity to passivate such as niobium, tantalum, or aluminum.

• With Schottky diode rectifier can be built with lower forward voltage than conventional diodes. The forward voltage or forward voltage describe the voltage drop across the diode in the conductive state and is undesirable in rectifiers. Schottky diodes are mainly used in switching power supplies are used.

• High- voltage rectifier, as used in television receivers, high-voltage laboratories for coating the toner in laser printers or high voltage direct current transmissions, consisting of a series circuit of conventional diodes. This is necessary because silicon diodes have a maximum reverse voltage of a few kilovolts and the breakthrough comes when exceeding this voltage. The manufacture of semiconductor devices having reverse blocking voltage of greater than several kV is not possible.