Power Electronics

power semiconductor devices are as follows:

1. p-n junction diode - conventional semiconductor-semiconductor  junction diode
   
2. schottky diode - faster switching, low forward voltage drop diode
3. bipolar junction transistor (BJT) - conventional where both polarity charges operate (electrons & holes)
   
4. field effect transistor (FET) - a unipolar transistor
   
5. metal oxide semiconductor FET (MOSFET)
6. Insulated gate bipolar transistor (IGBT)
7. silicon controlled rectifier (SCR)
8. triode for AC (TRIAC) - a bidirectional thyristor
   

Junction Diodes:
A p-n junction is the basic building block of a semiconductor. p indicates positive charges and n indicates negative charges.
Zener diode is a special diode which is utilised in the reverse-biased mode in voltage regulating apllications.
Schottky diode has very low forward voltage drop but at the same time weak reverse bias withstand capability.
High Amp power diodes are type of p-n jucntion diodes designed to handle high currnet with proper heat dissipation and easy mounting arrangements.
Rectificaion of AC to DC wave can be performed by diodes. A single diode can rectifiy half of an AC waveform, but a full bridge rectifier which contains 4 diodes in a special arrangement can result in fully-rectified DC waveform.

A simple junction diode

  

Transistors:
Bipolar transistors can be either pnp transistor or npn transistor.
In  operation they employ both polarities of charges (i.e. electrons and holes) so the name bi-polar transistors. These are the conventional transistors.
A transistor can be used in a circuit as a switch which turns ON or OFF in specific conditions, or, as an amplifier which amplifies the signal fed into the transistor. Other evolutions of bipolar transistors are IGBTs, FETs, MOSFETs which have certain advantages depending on the application for which it is used for. For e.g. some of the above are fast-switching.

Symbol of transistor with 3 legs & model of a BJT

   
Thyristors :
Firing angle is the main aspect in the operation of a thyristor. A thyristor will start to conduct when a certain threshold current flows through its secondary circuit. These are mostly applied in high current switching.

Thyristors: with mounting arrangement and heat sink

 HVDC -
Not forgetting the historical fact that there was an AC/DC war between Westinghouse & Edison , AC became predominant in transmitting power to long distances due to the inherent features of AC. Saying that, nowadays for ultra-long distance power transmission DC has been found more beneficial than AC. HVDC refers to high voltage direct current (dc) system. As the power generation is easy in AC form, in this HVDC system, it is then inverted into DC, transmitted over very long distance, and then converted back to conventional AC.

AC power--->Converter--->DC transmission line--->Inverter---->AC power

Here comes the ultimate importance of REAL high current low loss Power Electronic Converters and Inverters.




A 2000A 250 kV high voltage direct current (HVDC) thyristor valve rated 2000 A,250 kV dc at Manitoba Hydro's Henday converter station: Source unknown  

















FACTS -
Flexible alternating current (ac) transmission system is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability."


To summarise the confusing AC/DC below provided is an application example.

1. AC to DC: rectifiers e.g. full bridge rectifier
2. DC to AC: inverters e.g. UPS in battery mode
3. AC to AC: transformers e.g. a 33/11kV power transformer
4. DC to DC: switching/chopping e.g. switch mode power supply