Power electronics digital notes b. Tech III year

Power transistors
Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices 
are used a switching devices and are operated in the saturation region resulting in low on-state voltage 
drop. They are turned on when a current signal is given to base or control terminal. The transistor 
remains on so long as the control signal is present. The switching speed of modern transistors is much 
higher than that of thyristors and is used extensively in dc-dc and dc-ac converters. However their 

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voltage and current ratings are lower than those of thyristors and are therefore used in low to medium 
power applications. Power transistors are classified as follows o Bipolar junction transistors(BJTs) o 
Metal-oxide semiconductor filed-effect transistors(MOSFETs) o Static Induction transistors(SITs) o 
Insulated-gate bipolar transistors(IGBTs) 
Advantages of 
BJT’S
 
i.
BJT’s have high switching frequencies since their turn
-on and turn-off time are low.
ii.
The turn-on losses of a BJT are small.
iii.
BJT has controlled turn-on and turn-off characteristics since base drive control is possible.
iv.
BJT does not require commutation circuits 
Demerits of BJT
i.
Drive circuit of BJT is complex.
ii.
It has the problem of charge storage which sets a limit on switching frequencies. 
iii.
It cannot be used in parallel operation due to problems of negative temperature coefficient. 
Thyristors 
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Silicon Controlled Rectifiers (SCR’s)
 
A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate current-
controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of 
thyristor. 
SCRs are mainly used in electronic devices that require control of high voltage and power. This makes 
them applicable in medium and high AC power operations such as motor control function. 
An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of 
semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three junctions 
labeled as J1, J2 and J3 and three terminals(anode, cathode and a gate). An SCR is diagramatically 
represented as shown below.