A multiphase chopper is a chopper which consists of two or more choppers connected in parallel. A multiphase chopper may be operated in two modes.
In Phase Operation Mode:
In this mode of operation, all parallel connected choppers are on and off at the same time.
Phase Shifted Operation Mode:
In this mode, different choppers are ON and OFF at different instants of time. The advantage of multiphase choppers are as follows :
- Due to increase in phases, total output current is sum of currents of all choppers.
- Frequency of ripple in output becomes higher, so filtering is easy to filter OFF ripple currents.
- Peak-to-peak amplitude of ripple is reduced.
Following Fig. 1 shows the power circuit of multiphase chopper and L1 and C as the input source filter.
Fig. 1: Multiphase chopper
Here, in this circuit, three choppers S1, S2 and S3 are connected in parallel. Each chopper has a separate but identical output smoothing inductance L2. An additional common inductance L1 is present in the load circuit. Each chopper will have the same switching frequency and duty cycle. However, starts of the switching period of chopper S2 will be delayed with respect to start of switching period of chopper S1 by an interval equal to T/3 seconds will correspond to a phase of 120° in angular measure. Similarlly, chopper 3 i.e. S3 will have a phase shift of 120° with respect to chopper S2.
The effect of this phase shift on the currents of the individual choppers and on the load current is shown in Fig. 2. Voltage V1, V2 and V3 are voltages at output terminals of choppers S1, S2 and S3 respectively. I1, I2 and I3 are currents supplied by three choppers S1, S2 and S3 respectively. From output current waveform it is observed that, output current of a multiphase chopper is sum of all currents supplied by individual chopper. Also it is calculated that ripple frequency of resultant outputs current is multiple of the chopper frequency. Therefore, output load elements require low values of L and R. Since load is inductive, diodes D1, D2 and D3 perform free-wheeling action.
Fig. 2: Output voltage and Current waveforms
Advantages of Multiphase chopper
- The main advantage of this chopper over single chopper that its input current has reduced ripple amplitude and increased ripple frequency.
- Size of filter for a multiphase chopper is reduced.
Disadvantages of Multiphase chopper
- Extra commutation circuits.
- Additional external inductors.