Power electronics is one of the branches of electrical engineering, it concerns devices (converters) allowing the form of electrical energy to be changed.
It includes the study, the realization, the maintenance:
- electronic components used at high power
- converters structures
- ordering these converters
- industrial applications of these converters
Power electronics, which we should also call “energy conversion electronics” is less than 50 years old. It has experienced such a boom that today nearly 15% of the electrical energy produced is converted in one form or another. During these years the size, weight and cost of converters has only diminished, in large part due to the progress made in the field of electronic switches.
Let us recall that a power converter with unit efficiency (without losses) can only be made up of ideal switches and purely reactive dipoles, therefore without the least parasitic resistance: capacitors and inductors. Reactive dipoles are energy storage elements whose size (and therefore cost) is inversely proportional to the operating frequency.
In addition to traditional power electronics applications such as electric traction and industrial drives, new areas of application have emerged:
- Management of the distribution network :
- FACTS: Flexible Transmission Systems in Alternating Current.
- Active filtering and improved power factor
- HVDC: THT direct current transmission
- Appliances :
- various variators,
- switching power supplies,
- induction hobs.
- Portable devices (camcorders, computers, etc.) :
- smart battery chargers
- DC / DC conversion TBT
- The automobile: very strong increase in the use of electric energy in current and future automobiles: there will be a very large market at the time of the planned (but delayed?) Switch to 42 V, hybrid vehicles, …
It was in the field of high power rectification that the first static converters were developed to replace electromechanical converters. In the 1950s, for electric traction, we turned towards the solution – alternative transport + continuous motorization. The necessary static converters are made using mercury vapor rectifiers (ignitrons) with the same functionality as thyristors.
- The first silicon power diodes appeared in 1956 and thyristors in 1961. In the 1970s, thyristors and diodes were used in self-switched devices such as choppers and inverters, the following years saw the development of bipolar power transistors which promotes the development of low and medium power conversion electronics.
- At the start of the 1980s, transistor devices pushed thyristor devices to increased power: around 90, GTOs were no longer used except at very high power (> 1 MW) or for voltages greater than 2kV.
- The IGBT appeared in 1985, first in the field of medium powers (a few tens of kW), it supplanted Darlington transistors. Within 10 years it becomes a component that can be used at high power.
- The advent of the IGCT (Integrated Gate Commutated Thyristor) thyristor around 1997 in the field of voltages above 6 kV risks causing the disappearance of the GTO thyristor in the medium term.
- In the field of low powers, because of its speed and the simplicity of its control, the power MOSFET transistor supplants the bipolar transistor. Thanks to planar integration techniques and the boom in the portable market (telephone, computer, CD player, etc.) requiring efficient and miniaturized conversion electronics, it even supplants diodes in applications such as rectifier (synchronous rectifier).
- Silicon carbide (SiC) -based components appear in 2002. Diamond-based components are still under study in 2004. Their high ionization energies allow higher voltage blocking and / or high operation. temperature.
They are equivalent to a valve in a hydraulic installation.
The two important parameters to take into account are:
- the maximum blocking voltage of the component, that is to say the voltage beyond which the breakdown occurs and therefore the destruction of the diode.
- The maximum intensity of the current which can pass through it.
The three main faults of the component are:
- The threshold voltage VS
- Dynamic resistance RD
- The parasitic capacitance C.
Currently diodes are available in several categories:
- Low dynamic resistance RD power silicon diodes
They are used in the field of high power converters such as traction inverters. They are made in an encapsulated case. The junction which constitutes them is of type PiN (P – Intrinsic – N), or PN-N +. The introduction of a very lightly doped zone makes it possible to obtain a high blocking voltage.
- Fast diodes of low stray capacitance C.
They have recovery times of the order of a few tens of nanoseconds.
- Schottky diodes: low threshold voltage VS and low C.
They consist of a metal – semiconductor junction. Compared to PiN diodes, the threshold voltage is lower, but the resistance is higher (hence a voltage drop which depends more strongly on the current flowing through it). They can operate at very high frequencies but the maximum permissible reverse voltage is lower. For all these reasons, they are mainly used in converters operating at low voltage and at high frequency: switching power supplies.
- Silicon carbide (SiC) Schottky diodes.
They combine very low C and a higher blocking voltage than conventional Schottky diodes but these improvements come at the expense of the increase in VS
These are electronic switches whose blocking or starting are controlled by a voltage (they behave like doors that can be opened or closed at will). They are the most used in the field of low and medium powers ( a few kW).
Their range of use is limited to a few hundred volts, except for the high frequency range where the MOSFET outperforms all other components.
Their main drawback is that in the on state they behave like resistors (RDSon) of a few tens of mΩ. This resistance is responsible for the conduction losses. The MOSFET can also exhibit switching losses when used as a switch in switching power supplies. In fact, at each switching, the parasitic capacitances present at its terminals must be charged or discharged leading to losses in CV².
Bipolar Power Transistors
Compared to power MOS transistors, they require more complicated control and have poorer dynamic performance. However, they are thermally more stable and above all, due to current control, they are less sensitive to electromagnetic disturbances.
The MOS transistor is fast and easy to control, but bipolar transistors have better voltage withstand and exhibit lower on-state voltage drop at high currents. The desire to combine these two advantages has given birth to hybrid components called IGBTs.
Since the 1990s, these have been the most widely used components to make converters operating with voltages from a few hundred volts to a few kV and with currents from a few tens of amperes to a few kA.
Component functioning roughly like a valve controlled by a “tire-Suisse”:
- In order for it to turn on, it must be primed: the trigger current must be maintained until the main current reaches the sticking current
- When blocking, it is necessary to wait for a certain duration for turn-off so that the thyristor can effectively block the reverse voltage.
For these reasons the thyristor is reserved for applications concerning very high voltages (> kV) and high currents, where its lower cost compensates for its technical limitations. For example, long distance or submarine direct current – high voltage (HVDC) links are almost always carried out with thyristors.
Example of values: Thyristor 16 kV – 2 kA, frequency 300 Hz.
Hard switching and soft switching
The rise in frequency of static converters leads to an increase in switching losses in the switches. These losses can be reduced, but above all relocated by adding a switching assistance circuit (CALC) without modifying the operating principle of the converter.
Another possibility is to modify the nature of the switches so that they perform spontaneous switching, also known as soft switching because the losses are zero, but also that of the converters which must then create the switching conditions. These converters are called (quasi) resonant converters.
Two types of switches can be used, leading to two types of soft switching:
- Switch with controlled start and spontaneous blocking, like the thyristor. The blocking is then carried out at the zero crossing of the current, called ZCS (Zero Current Switching) in English.
- Switch with controlled locking and spontaneous priming. The blocking is then carried out at the zero crossing of the voltage or ZVS (Zero Voltage Switching) in English
To achieve the zero crossing of one of the quantities it is necessary to add an oscillating circuit in the assembly, hence their names of quasi-resonant converters.
There are generally four main functions of power electronics converters:
Conversion direct – direct, alternating – direct, direct – alternating and alternating – alternating.
But in addition to its purely functional names, special names have been given to certain converters.
- Continuous – continuous conversion
– Charge pump converters
- Alternating – continuous conversion
– Switching power supplies
- DC – AC conversion
- Alternative – alternative conversion
-Uninterruptible power supplies (UPS)