Chopper is a power electronic converter which converts fixed DC voltage to variable DC voltage.
Chopper uses thyristors for power conversion. As thyristors can carry current in only one direction, by properly arranging the thyristors and diodes, four quadrant operation of chopper can be achieved.
Types of chopper based on the quadrant of operation
- Type A or I quadrant chopper
- Type B or II quadrant chopper
- Type C chopper (I and II quadrant)
- Type D chopper(I and IV quadrant)
- Type E or Four quadrant chopper
In the first quadrant, the average voltage and the current are both positive.
We have already seen about stepdown chopper which gives a positive voltage and current. Hence a stepdown chopper can be used as a first quadrant chopper.
When the switch is on, the inductance stores the energy and the load voltage is equal to the source voltage.
When the switch is off, the load current flows through the freewheeling diode and load voltage is equal to zero.
As α varies between 0 and 1, Vo < Vs.
Power flow is from source to load.
For detailed explanation read step down chopper
In the second quadrant, the voltage is positive while the current is negative. For the current to be negative, the load must have an energy source like battery or back emf E.
When CH2 is ON, L stores energy
Vo=0
When CH2 is OFF,
Vo=(E+VL)
Vo > Vs
D2 conducts
Power flows to source
As current always flows out of the load, the current is said to be negative.
Net power flow is from load to source.
As Vo > Vs, this chopper is called step up chopper.
In the first quadrant,voltage is positive, current is positive
In the second quadrant, voltage is positive, current is negative.
To get two quadrant operation, connect both Type A and Type B chopper in parallel.
Load must contain a dc source to achieve second quadrant operation.
The output voltage is always positive because of the freewheeling diode connected across the load.
For first quadrant operation,
CH1 is operated ( ON/OFF)
When CH1 is ON,
Current flows through CH1 and load.
Vo =Vs , io is positive
When CH1 is OFF
Current through L cannot change immediately
FD conducts
Vo =0 , io is positive
In first quadrant operation, both Vo and Io are always positive.
For the second quadrant operation,
CH2 is operated ( ON/OFF)
When CH2 is ON,
Current flows through CH2 and load.
Vo =0 , io is negative
When CH2 is OFF
Current through L cannot change immediately
D2 conducts
Vo =Vs , io is positive
In the second quadrant operation, Vo is positive and Io is negative.
In the first quadrant,voltage is positive, current is positive
In the fourth quadrant,
voltage is negative, current is positive.
When CH1 and CH2 are turned ON, the current flows through the load.
Vo =Vs , io is positive
When diodes D1 and D2 conduct,
Vo =-Vs , io is positive
When Ton > Toff , average output voltage is positive.
When Ton < Toff , average output voltage is negative.
Current is always positive.
Four quadrant chopper consists of 4 switches and 4 diodes. A diode is connected in antiparallel to each switch.
By operating the switching ON/OFF, different quadrant operations can be achieved.
CH4 is always kept ON while CH1 is operated.
When CH4-ON, and CH1 is ON,
Vo =Vs , io is positive
When CH4- ON, and CH1 is OFF,
The current freewheels through D2.
Vo =0 , io is positive
Both Voltage and current are positive in first quadrant.
CH2 is operated.
When CH2 ON
Current flows through CH2, D4, load
Vo =0 , io is negative
When CH2 off,
Current flows through D1 and D4.
Vo =Vs , io is negative
In third quadrant both voltage and current should be negative.
E should be reversed in this mode of operation.
CH2 is always kept ON while CH3 is operated.
When CH2-ON, and CH3 is ON,
Vo =-Vs , io is negative
When CH2- ON, and CH3 is OFF,
The current freewheels through D4.
Vo =0 , io is negative
Both Voltage and current are negative in this quadrant.
CH4 is operated.
When CH4 ON
Current flows through CH4, D2, load
Vo =0 , io is positive
When CH2 off,
Current flows through D2 and D3.
Vo =-Vs , io is positive
Thus by appropriately operating the switches, four quadrant operation can be achieved.
References
P.S.Bimbra Power Electronics, Khanna Publishers, third Edition, 2003.
Read Electric Vehicle 