Electronics of power

General information

The electronic of power , which one should name besides “electronic energy transformation” has less 50 years. She made such great strides that today close to 15 % of produced electrical energy is converted in a form or another. During these years the size, the weight and the cost of the converters made only decrease, mainly thanks to the progress made in the field of the interruptory electronic.

Let us recall that a converter of unit power of output (without losses) can be made up only of ideal switches and reactive dipoles purely thus without least the resistance parasite: condensing and Inductance S. the reactive dipoles are elements of storage of energy whose size (and thus cost) are inversely proportional to the frequency of operation.

In addition to the traditional applications of the electronics of power like the industrial electric traction and drives, it appeared new scopes of application:

the management of the distribution network :

FACTS : Systems of Flexible Transmission in Alternative course,
active Filtering and improvement of the power-factor,
HVDC: Transmission in D.C. current HTA.

the electric household appliances :

various variators,
food with cutting,
hotplates to induction.

the portable units (Video camera S, Computer S, etc):

intelligent chargers of accumulating S,
conversion DC/DC TBT.

the Automobile : very strong increase in the electric energy utilization in the current cars and in prospect: there will be a very large market at the time of the passage envisaged, (but delayed?) in 42 V, hybrid vehicles,…

the switches

Historical

It is in the field of the rectification of strong power that the first static inverters intended develop to replace the electromechanical converters. In the years 1950, for the electric traction, one directs oneself towards the solution - transport into alternate + motorization uninterrupted. The static inverters necessary are produced using rectifiers with mercury vapor (ignitrons) having the same functionality as the thyristors.

the first Diode S of power to the Silicium appear in 1956 and the Thyristor S in 1961. In the years 1970, thyristors and diodes are used in devices autocommutés like the choppers and the inverters, the years which follow see the development of bipolar transistors of power which supports the development of an electronics of conversion of low and average power.

With the beginning of the year 1980, the devices with transistors push the devices with Thyristor S towards increased powers: about 1990, GTO are used more only in very strong powers (> 1 MW) or for tensions higher than 2 Kv.

IGBT appears in 1985, initially in the field of the average powers (a few tens of kilowatts), it supplants the transistors Darlington. It becomes in the 10 years which follow a component usable in strong power.

the advent of the thyristor IGCT (Integrated Spoils Commutated Thyristor) about 1997 in the field of the tensions higher than 6 Kv is likely to involve in the medium term the disappearance of the thyristor GTO.

In the field of the low powers, because of its speed and of the simplicity of its order, the transistor MOSFET of power supplants the bipolar transistor. Thanks to the techniques of integration double diffusion and the rise of the market of portable (telephone, computer, reader CD, etc) requiring an effective and miniaturized electronics of conversion, it supplants even the diodes in applications like the rectification (rectifying synchronous).

the components containing Carbure of silicon (Sic) appear in 2002. Those containing Diamant are still being studied in 2004. Their strong energies of ionization allow a blocking of higher tension and/or operations at high temperature.

the diodes

They are equivalent to a valve in a hydraulic installation.

The two parameters important to be taken into account are:

the maximum tension of blocking of the component, i.e. the tension beyond which occurs the breakdown and thus the destruction of the diode.
maximum intensity of the current which can cross it.

The three principal defects of the component are:

the tension of threshold V_S
dynamic resistance R_D
stray capacity C .

Currently the diodes are declined in several categories:

the diodes silicon of dynamic power of resistance weak R_D .

They are used in the field of the converters of strong power like the inverters of traction. They are made out of encapsulated case. The junction which constitutes them is of PiN type (P - Intrinsic - NR), or PN-N+. The introduction of a zone very slightly doped makes it possible to obtain a high tension of blocking.

fast diodes of stray capacity C weak.

They have times of covering about a few tens of nanoseconds.

the Schottky diodes: tension of weak threshold V_S and C weak.

They consist of a junction metal - semiconductor. Compared to the diodes Pine, the tension of threshold is weaker, but resistance is higher (from where a voltage drop which more strongly depends on the current which crosses it). They can function at very high frequencies but the authorized maximum opposite tension is weaker. For all these reasons, they are mainly used in the converters functioning in TBT and at high frequency: food with cutting.

the Schottky diodes in Carbide of silicon (Sic).

They combine C very weak and a tension of blocking higher than the traditional Schottky diodes but these improvements are made with the detriment increase in V_S .

MOSFET of power

These are electronic switches whose blocking or starting is ordered by a tension (They behave as doors which one can open or close at will). It is used in the field of the weak ones and average powers (a few kilowatts).

Their field of application is limited to a few hundreds of volts, except the field of the raised frequencies for which MOSFET outclasses all the others composants.

Their principal defect is that with the state passing they behave like resistances ( R_DSon ) of a few tens of mΩ. This resistance is responsible for the losses in conduction. MOSFET can also present losses of commutation when it is used as switch in the food with cutting. Indeed, with each commutation, the stray capacities present at its terminals must be charged or discharged involving losses in CV ² .

Bipolar Transistors of power

Compared to transistors MOS of power, they require a more complicated order and have poorer dynamic performances. However they are thermically more stable and especially, because of an order while running, they are less sensitive to the electromagnetic disturbances.

IGBT

Transistor MOS is fast and easy to order, but the bipolar transistors have the best held in tension and present a voltage drop at weaker state passing for high currents. The will to cumulate these two advantages gave rise to named hybrid components IGBT.

Since the years 1990, they are the components most used to produce converters functioning with tensions of a few hundreds of volts to a few kilovolts and with currents of a few tens of amps to a few kiloamperes.

Thyristors

Component functioning coarsely like a valve ordered by “car-Switzerland”:

So that it becomes busy it should be started : it is necessary to maintain the current one of trigger until the principal current reaches the latching current.
With blocking it is necessary to await a certain duration the draining (turn-off) so that the thyristor can block the opposite tension indeed.

For these reasons the Thyristor is reserved for applications concerning the very strong tensions (> kilovolts) and the strong currents, where its lower cost compensates for its technical limitations. For example the connections long distances or underwater by D.C. current - high voltage (HVDC) are almost always carried out with thyristors.

Example of values: Thyristor 16 Kv - 2 kA, frequency 300 Hz.

hard Commutation and soft commutation

The rise in frequency of the static inverters involves an increase in the losses by commutation in the switches. These losses can be reduced, but especially delocalized by the addition of circuit of assistance to commutation (CALC) without modifying the principle of operation of the convertisseur.
Another possibility consists in modifying the nature of the switches so that they carry out a spontaneous commutation, known as also soft commutation because the losses are null, but also that of the converters which must then create the conditions of commutations. These converters are known as converters (quasi) résonnants.

Two types of switches can be used, leading to two types of soft commutations:

Interruptory with ordered starting and spontaneous blocking, like the thyristor. Blocking is then carried out in the passing to zero of the current, named ZCS (Zero Current Switching) in English.
Interruptory with ordered blocking and spontaneous starting. Blocking is then carried out in the passing to zero of the tension or ZVS (Zero Switching Voltage) in English.

To arrive at the passage to zero of the one of the sizes it is necessary to add an oscillating circuit in the assembly, from where them name of converters quasi resounding.

Some devices

One generally distinguishes four great functions from converters in the electronics of power:
Conversion continuous - continuous, alternate - continuous, continuous - alternate and alternate - alternate.

But in addition to these purely functional denominations, of the particular names were given to certain converters.

Conversion continuous - continuous

Chopper S
Converters with pump of load

Conversion alternate - continuous

Rectifying S
Food with cutting

Conversion continuous - alternate

Inverter S

Conversion alternate - alternative

Gradateur S
Food without interruption (ASI)
Cycloconvertisseur

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