- The most fundamental as well as important control of any machine or a process is the ON-OFF control.
- When the power supply is provided to the device, the device turns in ON state and when the power supply is removed, the device turns in OFF state. The duration for which the device remains in ON and OFF states are called the ON time and the OFF time.
- The voltage across the device can be controlled by using chopper circuits.
- Smaller size.
- Smooth control.
- Power saving.
- Fast response.
- High efficiency.
APPLICATIONS OF CHOPPER:
- Trolley cars.
- Mine haulers.
- D.C. drive.
- SMPS (Switch Mode Power Supply)
- Battery charging circuit.
- Time ratio controller.
- D.C. current source.
CONCEPT OF A CHOPPER:
A chopper is a D.C. to D.C. converter. It can be of D.C. type and A.C. type and can be used for motor control, temperature control and illumination control purposes. The chopper switch can be implemented by using power BJT, power MOSFET, GTO or forced commuted thyristors as the voltage drop across these devices are very less and can be neglected.
DEFINITION OF A CHOPPER:
A chopper is a device which chops off the supply voltage in ON and OFF period respectively. A chopper is also known as D.C to D.C. converter, as it converts directly from D.C. to D.C. Choppers are also called as TRC (Time Ratio Controller).
BASIC BLOCK DIAGRAM OF A CHOPPER:
CLASSIFICATION OF CHOPPERS:
- Choppers are classified on various basis. According to output voltage level compared with input voltage level, choppers are classified as:
- Step-up choppers: In step-up chopper, chopping element is connected in parallel with load and source. So it is also called as parallel chopper.
- Step-down choppers: In step-down chopper, chopping element is in series, therefore, it is called as series chopper.
- Choppers are also classified according to the type of commutation used for SCR turn-OFF. Three types of choppers are:
- Voltage commutation choppers
- Current commutation choppers
- Load commutation choppers
STEP-DOWN CHOPPER (CLASS A):
In this chopper the load voltage can be controlled between 0 to Vin volts i.e. the output voltage can at the most be equal to the input voltage.
The working operation can be divided into two modes:
- Mode 1 : When chopper switch is ON, the current flows from supply voltage Vs to the load R. Therefore there is output voltage across R.
- Mode 2 : When chopper switch is OFF the current does not flow from supply to load R.
STEP-DOWN CHOPPER WITH RL LOAD:
As the load is inductive the freewheeling diode is connected in shunt with load as shown in above circuit diagram. The N-channel enhancement type MOSFET is used as a switch.
WORKING: When the chopper is switched ON, the current flows from supply load. Therefore, the coil is energised. When the chopper is switched OFF and the load current continues to flow through freewheeling diode as it is forward bias. The freewheeling diode provides discharging path for indicator coil.
In this chopper, load voltage can be higher than the input D.C. voltage. A MOSFET switch chopper can be used to step-up a D.C. voltage. An inductor L is connected in series
WORKING: When switch is closed i.e. ON for time t1, the inductor L current rises and energy is stored in the inductor L. The voltage across the inductor L is equal to the input D.C. voltage V with the polarity +ve and -ve as shown in the above circuit. If the switch S is open circuited for time t2, the energy stored in the inductor is transferred to the load through diode and inductor current falls.
Half wave rectifier