In chopper circuits, unidirectional power semiconductors are used. If these semiconductor devices are arranged appropriately, a chopper can work in any of the four quadrants. we can classify chopper circuits according to their working in any of these four quadrants as type A, type B, type C, type D and type E. Let us now take a look of these classifications and the characteristics of various classifications.
Type A Chopper or First–Quadrant Chopper
This type of chopper is shown in the figure. It is known as first-quadrant chopper or type A chopper. When the chopper is on, v0 = VS as a result and the current flows in the direction of the load. But when the chopper is off v0 is zero but I0 continues to flow in the same direction through the freewheeling diode FD, thus average value of voltage and current say V0 and I0 will be always positive as shown in the graph.
In type A chopper the power flow will be always from source to the load. As the average voltage V0 is less than the dc input voltage Vs
Type B Chopper or Second-Quadrant Chopper
In type B or second quadrant chopper the load must always contain a dc source E . When the chopper is on, v0 is zero but the load voltage E drives the current through the inductor L and the chopper, L stores the energy during the time Ton of the chopper . When the chopper is off , v0 =( E+ L . di/dt ) will be more than the source voltage Vs . Because of this the diode D2 will be forward biased and begins conducting and hence the power starts flowing to the source. No matter the chopper is on or off the current I0 will be flowing out of the load and is treated negative . Since VO is positive and the current I0 is negative , the direction of power flow will be from load to source. The load voltage V0 = (E+L .di/dt ) will be more than the voltage Vs so the type B chopper is also known as a step up chopper .
Type -C chopper or Two-quadrant type-A Chopper
Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to Vs . The direction of the load current i0 will be reversed. The current i0 will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.
The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.
Type D Chopper or Two-Quadrant Type –B Chopper
The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage v0 will be equal to Vs . When v0 = – Vs the two choppers will be off but both the diodes D1 and D2 will start conducting. V0 the average output voltage will be positive when the choppers turn-on the time Ton will be more than the turn off time Toff its shown in the wave form below. As the diodes and choppers conduct current only in one direction the direction of load current will be always positive.
The power flows from source to load as the average values of both v0 and i0 is positive. From the wave form it is seen that the average value of V0 is positive thus the forth quadrant operation of type D chopper is obtained.
From the wave forms the Average value of output voltage is given by
V0= (Vs Ton-VsToff)/T = Vs.(Ton-Toff)/T
Type –E chopper or the Fourth-Quadrant Chopper
Type E or the fourth quadrant chopper consists of four semiconductor switches and four diodes arranged in antiparallel. The 4 choppers are numbered according to which quadrant they belong. Their operation will be in each quadrant and the corresponding chopper only be active in its quadrant.
- First Quadrant
During the first quadrant operation the chopper CH4 will be on . Chopper CH3 will be off and CH1 will be operated. AS the CH1 and CH4 is on the load voltage v0 will be equal to the source voltage Vs and the load current i0 will begin to flow . v0 and i0 will be positive as the first quadrant operation is taking place. As soon as the chopper CH1 is turned off, the positive current freewheels through CH4 and the diode D2 . The type E chopper acts as a step- down chopper in the first quadrant.
- Second Quadrant
In this case the chopper CH2 will be operational and the other three are kept off. As CH2 is on negative current will starts flowing through the inductor L . CH2 ,E and D4. Energy is stored in the inductor L as the chopper CH2 is on. When CH2 is off the current will be fed back to the source through the diodes D1 and D4. Here (E+L.di/dt) will be more than the source voltage Vs . In second quadrant the chopper will act as a step-up chopper as the power is fed back from load to source
- Third Quadrant
In third quadrant operation CH1 will be kept off , CH2 will be on and CH3 is operated. For this quadrant working the polarity of the load should be reversed. As the chopper CH3 is on, the load gets connected to the source Vs and v0 and i0 will be negative and the third quadrant operation will takes place. This chopper acts as a step-down chopper
- Fourth Quadrant
CH4 will be operated and CH1, CH2 and CH3 will be off. When the chopper CH4 is turned on positive current starts to flow through CH4, D2 ,E and the inductor L will store energy. As the CH4 is turned off the current is feedback to the source through the diodes D2 and D3 , the operation will be in fourth quadrant as the load voltage is negative but the load current is positive. The chopper acts as a step up chopper as the power is fed back from load to source.