Introduction of Type C Chopper
The type C chopper is parallel combination of type A and type B chopper. Here, only power circuit is shown, control circuit is not given. The output voltage is always positive but output current either positive or negative in this type of chopper therefore it works in first as well as second quadrant.
When output voltage and current are positive, chopper operates
in first quadrant. Similarly, when output voltage positive but output current
negative, chopper operates in the second quadrant. In the type C chopper,
Chopper CH1 and CH2 never conduct simultaneously otherwise supply source is
short circuited. Similarly, diode D1 and D2 never conduct simultaneously,
otherwise load is short circuited.
Type C chopper power circuit diagram
The power circuit diagram of type c chopper is shown
in the figure.
Working of type C Chopper
The working of type C chopper is explained here.
CH1 and CH2 OFF
The diode D1 and D2 does not forward
biased because of CH1 and CH2 remain off at starting condition. The load is
completely isolated from the supply system.
CH1 conducts
When chopper CH1 conducts, the load current flows through Vdc – CH1 – L – Load – Vdc. The output voltage and current both are positive therefore the chopper operates in the first quadrant.
Diode D1 conducts
When chopper CH1 is switched off, the voltage
across inductor changes its sign and current flows from L – Load – D1 – L path.
The output voltage becomes zero but load current continues to flow in the
positive direction. The voltage across
inductor is VL (di/dt).
CH2 conducts
When voltage across inductor is less than back emf
of load, the current continues to flow in the positive direction. When voltage
across inductor is equal to back emf of load, the current flows through load is
zero. When the voltage across inductor is more than back emf of load, the
current continues to flow in the negative direction and at that time pulse to
the chopper CH2 is given. When chopper CH2 is switched on, the voltage across
load becomes zero.
|
Voltage condition |
Direction of current |
|
Voltage across inductor VL
(di/dt) > Back emf of load |
Current flow in positive direction |
|
Voltage across inductor VL
(di/dt) = Back emf of load |
Current = 0 |
|
Voltage across inductor VL
(di/dt) < Back emf of load |
Current flow in negative direction |
D2 conducts
When chopper CH2 is switched off, the direction of
inductor voltage change, the current flows through Load – L – D2 – Vdc – Load. In
this condition, voltage across load is positive but current is negative
therefore the chopper operates in the 2nd quadrant.
Output voltage
The output voltage becomes zero when diode D1 or
CH2 conducts. The output voltage becomes VDC When diode D2 or
chopper CH1 conducts.
|
Conduction |
Output voltage |
|
Diode D1 / chopper CH2 |
Zero |
|
Diode D2 / chopper CH1 |
VDC |
Type C chopper waveform
- The waveforms for gate signal G1, gate signal G2, output current and output voltage is shown in the figure C. The chopper CH1 is switched on at point A and load current flows from supply to load side.
- The inductor current increases exponentially. The chopper CH1 is switched off at point B and load current flows through path L – LOAD – D1 – L. When the value of L ( di/dt ) is equal to back emf Eb, the load current becomes zero ( Point C ). The diode D1 conducts during point B to point C.
- The chopper CH2 is switched on at point C due to back emf of the DC motor. The load current flows in the negative direction through path LOAD – L – CH2 – LOAD. The negative current flows continue until chopper CH2 is switched and chopper CH1 is switched on ( Point D ).
- When the chopper CH2 is switched off, the stored energy of inductor dissipated its energy through path L – D2 – VDC – LOAD – L. The chopper CH1 remains off in spite of it receives gate signal. The chopper CH1 switched on at point E where the negative current becomes zero.
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