Power electronics

Power electronics refers to the branch of electrical engineering, which has the conversion of electric energy with electronic components to the task. Typical applications include inverter or inverter in the field of electric drive technology.

The conversion of electric energy converters, which are rotating machine sets consisting of an electric motor and generator, or power transformers are not expected to power electronics. Also audio amplifier or transmitter output stages do not count on power electronics, although the electric power can be significant here.

History

With the invention of the first rectifier 1902, the power electronics began, but later got the name. The rectifier was a gas discharge vessel with liquid mercury cathode. This mercury arc rectifying alternating currents could rectify up to several kilo amperes and voltages up to 10 kV tolerated. From 1930 it has mercury current judges were provided with a grid control similar to tube technology, and as a controllable DC was producing ( Ignitron, thyratron ). Due to the relatively high forward voltage of about 20 V, which multiplied by the forward current a fairly high loss of electrical power means and the elaborate construction and the resulting high acquisition and operating costs, these converters were used not to the extent as today's power electronics. The first semiconductor to the current direction were selenium and cuprous oxide rectifier.

In 1957 at General Electric, the first controllable power semiconductors was developed and later referred to as a thyristor. The following development has produced a number of other controllable power semiconductors and passive, which are used in many parts of the drive technology today.

Types and applications

The power electronics provides especially the conversion of electrical energy with respect to the voltage waveform, the amount of voltage and current and frequency. The arrangements for this transformation are called converters. They are differentiated according to their function in DC, AC and inverter.

Furthermore, there are power electronic devices and assemblies which are for the connection and disconnection of electrical loads. These include not only the switching function is often more protective and monitoring functions. They differ from relays and contactors in that they operate without moving parts.

• Umrichtung AC voltage into DC voltage by rectifier
• Umrichtung DC voltage into AC voltage supplied by inverter
• Umrichtung DC voltage into a higher or lower DC voltage by DC-DC converter (DC / DC ) converter
• Umrichtung AC voltage into AC voltage with another frequency or amplitude, for example, by AC controller or inverter

Advances in microelectronics resulted in the field of power electronic devices to further improved control and regulation possibilities and have the power electronics can continue to win important.

In drive technology, the electric motors can be described by the control possibilities of the power electronics, the operating points of electrical machinery set very flexible. Today, even large machines drives and electric locomotives are equipped with power electronic controllers.

Is also used in areas of power generation and transmission power electronics ever larger areas of application. In systems with lower power plants or with conditions in which the classical synchronous generator as a power generator can not be used, frequency converters are used to feed the electrical energy generated into the grid. In power transmission, the power electronics are used in so-called HVDC short couplers for frequency decoupling between interconnected systems. The same technique is used for the high voltage direct current transmission (HVDC, also called cross- coupling ) for use. Substations for traction power and overhead tram power line work with power electronics.

In the field of targeted control in high voltage systems with three-phase alternating current power electronics comes in the context of Flexible AC Transmission System ( FACTS) to the application. Using Unified Power Flow Controller ( UPFC ) can selectively power flows are adjusted to individual lines in a meshed interconnections and thus transmission lines are optimally utilized in its transport performance.

Power electronics is becoming increasingly important in the automotive industry. Here, a plurality of electrical loads is connected in series with power electronic components, and controlled. One of the first applications in the car were the alternator regulator, which allowed, among other things, instead of DC generators use the more efficient, smaller and poorer maintenance three-phase alternators. Other applications include electronic ignition ( thyristor ) and the electronic injection.

In hybrid vehicles (eg Toyota Prius ) is a part of, electric vehicles ( forklift, " E-Car " ) is generated the entire driving power to an electric motor. The electrical energy must be converted to the appropriate for the electric drive motor voltage and frequency by means of power electronics. For high-performance DC choppers and inverters are used, which prepare for hybrid vehicles, the energy in generator mode, the motor also for intermediate storage in accumulators or double-layer capacitors ( " Super Caps ").

In the high -frequency technology, power electronics solves the slow obsolescent tube technology from gradually. At very high frequencies and power but electron tubes ( klystrons, magnetrons ) are still used. In the induction heating exist thanks to the power electronics nowadays small, effective, maintenance-free and long-lasting devices.

Other examples are welding inverter for arc welding and medium frequency inverter resistance welding.

Devices and Circuits

Rectifiers are used to generate direct current from alternating current. They consist of several non-controlled diodes or active controlled components such as thyristors or IGBTs, which are by their interconnection to other rectifiers. In power electronics, in particular three-phase rectifier as the six-pulse circuit or the twelve-pulse circuit are applied.

Controlled converters are called rectifier circuits operating with thyristors, GTO thyristors or IGBT and allow you to adjust the output voltage continuously. This shifts a phase control switch on the electronic switch within the period by an adjustable angle. They are often able to supply the power from the DC side and back into the grid ( four-quadrant operation ).

When AC power controller circuits are denoted by thyristors or triacs, which controls brightness of lamps ( dimmers ), temperature control of heating resistors or activations of solenoids or motors can be realized. Two counter- parallel connected thyristors or triac switch here also with a phase control both half cycles of the alternating current in a certain phase angle by.

SSR (English solid state relay ) are electronic AC voltage switch and also work with thyristors or triac, they have a built-in potential separation between supply and signal circuit and turn either immediately or always at the zero crossing of the AC voltage, so as to prevent spurious emissions.

DC choppers are (engl boost converters. Step up converter, boost converter ) for DC supply with up- converters and down-converters ( buck converters, buck- regulator to step down converter ). The so-called " active PFC " ( Power Factor Correction) works with a the mains rectifier downstream upconverter. This DC-DC converter operate with bipolar transistors, MOSFETs or IGBTs. Often in DC choppers instead of synchronous rectifier diodes can be realized, so that in principle it is also possible to feed back current.

Switching power supplies convert AC voltage to electrically isolated, regulated DC voltages and work with a controlled inverter, the first of an alternating voltage high frequency generated from the rectified AC line voltage, which is then transformed and rectified. Switching power supplies operate with bipolar transistors, MOSFETs or IGBTs.

The following components are characteristic of the power electronics:

• Diac ( to control thyristor)
• Bipolar power transistor (switching power supplies and DC / DC converter)
• Power MOSFET (switching power supplies and DC / DC converter)
• GTO thyristor ( integrated gate - commutated thyristors, IGCT ) ( inverter high power )
• (IGBT switching power supplies, motor controls, inverters )
• Thyristor ( power converters, solid state relays, pulse power sources )
• Triac (Dimmer, SSR )
• Diodes for rectification and freewheeling diode ( Schottky diodes for small voltages up to 200 V, silicon diodes for voltages up to several kilovolts )
• Power capacitors

Power and efficiency

The efficiencies of power electronic systems are often very high ( about 70 to about 95%) and exceed, for example, those of small power supplies with power transformer. Nevertheless, lead to spurious emissions power electronic assemblies and often reactive power and distortion ( harmonics ) in the power grid. The non- sinusoidal shape of the current consumption causes a reactive power, it is called distortion power.

Therefore, also in power electronics reactive and apparent power of particular importance - the apparent current record or the recorded RMS current of a rectifier or switching power supply, despite high efficiency often much higher than the ratio of power output and supply voltage ( Pnutz / U1 ), leading to higher thermal stress ( heat flow ) of the network or dining transformers leads.

Uncontrolled rectifiers

Rectifiers are often required for the operation of the power electronics to the power supply, in order to first generate a DC voltage. A charging capacitor connected downstream flows only during brief periods in the region of the apex of the input AC voltage power. This current flow angle is smaller, the greater the capacitor and the lower the internal resistance of the supply system. Low conduction angle of the rectifier circuits lead to a high crest factor ( ratio of rms to peak current). They result in a higher load on the network, the leads and the rectifier.

When you turn on the charge of the capacitor causes an inrush current, which must be often reduced with an inrush current limiter.

The problems caused by the small current flow angles network impurities ( harmonics ) can be reduced with a power factor correction (PFC ).

The efficiency of a power rectifier to the power supply 230 V is about 99%. Be used silicon diodes.

Voltages below about 100 volts may be rectified by Schottky diodes. They have a lower forward voltage (approximately 0.3 to 0.6 V versus about 0.7 to 1 V for silicon diodes) and work faster. They are therefore often used at the output of switching power supplies.

Controlled rectifiers

Phase control

The DC side power at a controlled bridge rectifier with a control angle is obtained with resistive load without considering the losses to:

This includes the DC voltage of the DC, the rms value of the supply, sinusoidal alternating voltage, the rms value of the fundamental current and the phase angle of the current fundamental. For uncontrolled rectification Two pulse.

A larger steering angle results in a larger phase shift between current drawn and line voltage. It causes at> 0 reactive power in the network and is undesirable.

The efficiency of controlled power rectifier is only slightly less than that of uncontrolled power rectifier.

Synchronous rectifier

Synchronous rectification is used mainly in the production of low voltages at high power. In this case, the MOSFET switching transistors can be used instead of diodes, which are respectively at the right time and off. The higher circuit complexity is taken into consideration in order to increase the efficiency of such power sources significantly. This is achieved by that the forward voltage of the components is much lower than that of the diodes.

Synchronous rectifiers ( for example for the supply of the CPU of a computer with 1.8 V and 60 A ) have an efficiency of over 95% even at voltages of a few volts.

Power factor correction

Power electronic modules with input rectifier and storage capacitor cause reactive and distortion power. You can by implementing power factor correction modules ( engl. power factor correction, PFC) are eliminated.

Measures to PFC are necessary to meet the " contamination " of the network by more and more powerful electronic devices and are working to specific performance classes and consumers required by law ( EN 61000-3-2 ).