Power electronics digital notes b. Tech III year

Comparison between BJT and MOSFET:
Sl 
No
BJT
MOSFET
1 
It is a Bipolar Device 
It is majority carrier Device 
2 
Current control Device 
Voltage control Device.
3 
Output is controlled by controlling base current 
Output is controlled by controlling gate voltage 
4 
Negative temperature coefficient 
Positive temperature coefficient 
5 
So paralleling of BJT is difficult. 
So paralleling of this device is easy. 
6 
Dive circuit is complex. It should provide 
constant current(Base current) 
Dive circuit is simple. It should provide 
constant voltage(gate voltage) 
7 
Losses are low. 
Losses are higher than BJTs. 
8 
So used in high power applications. 
Used in low power applications. 
9 
BJTs have high voltage and current ratings. 
They have less voltage and current ratings. 
10 
Switching frequency is lower than MOSFET. 
Switching frequency is high. 

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53 | 
P a g e
UNIT 
–
 II 
Single phase and three phase controlled rectifiers 
Phase control technique 
–
 Single phase Line commutated converters 
Unlike diode rectifiers, PCRs or phase controlled rectifiers has an advantage of regulating the output 
voltage. The diode rectifiers are termed as uncontrolled rectifiers. When these 
diodes are switched 
with 
Thyristors, then it becomes phase control rectifier. The o/p voltage can be regulated by changing the 
firing angle of the Thyristors. The main application of these rectifiers is involved in 
speed control of DC 
motor
. 
What is a Phase Controlled Rectifier? 
The term PCR or Phase controlled rectifier is a one type of rectifier circuit in which the diodes are 
switched by 
Thyristors or SCRs (Silicon Controlled Rectifiers).
Whereas the diodes offer no control over 
the o/p voltage, the Thyristors can be used to differ the output voltage by adjusting the firing angle or 
delay. A phase control 
Thyristor is activated by applying a
short pulse to its gate terminal and it is 
deactivated due to line communication or natural. In case of heavy inductive load, it is deactivated by 
firing another Thyristor of the rectifier during the negative half cycle of i/p voltage.