POWER ELECTRONICS

INTRODUCTION     

Power electronic devices: Power electronic devices are the electronic devices that can be directly used in the power  processing circuits to convert or control electric power.  

The control of electric motor drives requires control of electric power. Power electronics have eased the concept of power control. Power electronics signifies the word power electronics and control or we can say the electronic that deal with power equipment for power control.

So, WHAT IS POWER ELECTRONICS??

Power Electronics is a branch of Electrical Engineering which deals with power conversion from one from to another form using Inductors, Capacitors, Semiconductor devices (Diode, Thyristor, MOSFET, IGBT etc.). The power may be from mW(point on load applications) to MW(Power Systems).     [mW- milliWatt; MW- MegaWatt]

Power Electronic

The basic functions of importance for power electronics are- 

(1) power conversion, ac to dc, dc to ac, ac to ac, 

(2) power conditioning to remove distortion, harmonics, voltage dips and over-voltages,

(3) high speed and/or frequent control of electrical parameters such as currents, voltage impedance, and phase angle, and 

(4) high speed and/or frequent circuit interruption transfer, and current limiting functions.

CLASSIFICATION OF POWER ELECTRONIC DEVICES     

Power electronic devices are classified into three types-

• Uncontrolled Device (such as Diode)
• Half-Controlled Device (such as Thyristor)
• Fully Controlled Device (such as MOSFET, IGBT & GTO)

Power Devices are : 

1. Power Diodes  

2. Power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)  

3. Power Bipolar -Junction Transistor (BJT)  

4. Insulated-Gate Bipolar Transistor (IGBT)  

5. Thyristors (SCR, GTO, MCT, IGCT) 

6. DIAC 

7. TRIAC

Types of Diodes: 

P-N Junction Diode: 

A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only

This is the basic diode formed with the interaction of p-type and n-type materials. It deals with the concept of biasing. This biasing make it classify into various operating modes. 

This diode conducts only during forwarding bias. In reverse bias, there is no evident flow of the current. It indicates that current gets blocked during reverse bias. 

There are three biasing conditions for p-n junction diode and this is based on the voltage applied: 

1. Zero Bias – No external voltage potential is applied to the PN junction diode.
2. Reverse Bias – The voltage potential is connected negative, (-ᵛᵉ) to the P-type material and positive, (+ᵛᵉ) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
3. Forward Bias – The voltage potential is connected positive, (+ᵛᵉ) to the P-type material and negative, (-ᵛᵉ) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.

V-I characteristic of Diode

Zener Diode:

Are specifically designed to operate under reverse breakdown  conditions. These diodes have a very accurate and specific reverse breakdown voltage. 

Zener diode is basically like an ordinary PN junction diode but normally operated in reverse biased condition. But ordinary PN junction diode connected in reverse biased condition is not used as Zener diode practically. A Zener diode is a specially designed, highly doped PN junction diode.

 Hence Zener Diode has a very thin depletion region. 

Applications of Zener Diode:

There are extensive applications of a Zener diode and few of those are:

• It is used as a voltage limiter to regulate voltage levels across the minimal value of loads
• Employed in the applications those need over-voltage safeguarding
• Used in clipping circuits

Power Diode:

A power diode has a P-I-N structure as compared to the signal diode having a P-N structure. Here, I (in P-I-N) stands for intrinsic semiconductor layer to bear the high-level reverse voltage as compared to the signal diode. However, the drawback of this intrinsic layer is that it adds noticeable resistance during forward-biased condition. Thus, power diode requires a proper cooling arrangement for handling large power dissipation. 

Power diodes are used in numerous applications including rectifier, voltage clamper, voltage multiplier and etc. Power diode symbol is the same as of the signal diode

DIAC:

The DIAC is a diode that  conducts electrical current only  after its break-over voltage, VBO,  has been reached momentarily.

Constructions:-

– Three alternately doped layers. 

– The doping concentration around both  junctions is equal. 

– Leads are only attached to the outer layers.  

– Packaged like a PN junction diode. 

– A DIAC has the same effect on current  regardless of the direction of flow. 

– One junction is forward biased.  

– The other is reverse biased.  

– It is a bidirectional  device that can pass the current in both  forward and reverse  biased conditions and  hence it is an AC  control device.  

– Performs as if it contained two PN junction  diodes connected in series back-to-back. 

• Useful for controlling 

 – lamps 

 – heaters 

 – speeds of small motors  

V-I Characteristic of DIAC

1. The diac performs like an open-circuit until its switching is exceeded. 

2. At that position the diac performs until its current decreases toward  zero. 

3. Because of its abnormal construction, doesn’t switch sharply into a low  voltage condition at a low current level like the triac or SCR, once it goes  into transmission, the diac preserves an almost continuous –Ve  resistance characteristic, that means, voltage reduces with the enlarge in  current. 

4. This means that, unlike the triac and the SCR, the diac cannot be  estimated to maintain a low voltage drop until its current falls below the  level of holding current. 

TRIAC:

 

Construction And Operation: 

The triac is another three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal triggers it into conduction. Thus the triac is a three terminal, four layer bidirectional semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes. Triacs of 16 kW rating are readily available in the market.   

• TRIACs 

– An acronym for triode AC semiconductor.  

– Conduct both directions of AC current flow. 

– Have the same switching characteristics as  SCRs. 

– Equivalent to two SCRs connected in parallel,  back to back.

  

TRIACs are widely used to control  application of power to various types of  loads.  

• TRIAC construction  

– A four-layer NPNP device in parallel with a  PNPN device.  

– Designed to respond to a gating current through  a single gate.  

– Not equally sensitive to the gate current  flowing in opposite directions.                  

TRIACs from 50 to 500 hertz.                

TRIACs have difficulty switching power to  inductive loads.

V-I Characteristic of TRIAC

1. The V-I characteristics of  triac in the first and third  quadrants are basically equal  to those of an SCR in the first  quadrant.  

2. It can be functioned with  either +Ve or –Ve gate control  voltage but in typical  operation generally the gate  voltage is +Ve in first quadrant  and -Ve in third quadrant.