7.0 AC–DC CONVERTER (RECTIFIER) AND DC–DC CONVERTER (CHOPPER)

Uncontrolled rectifier circuits; half-wave and full wave. Halfcontrolled
rectifier circuits. Controlled rectifier circuits; half-wave
and full wave. Function of choppers; commutator principles,
operation of chopper circuits, mark space ratio or time ratio
control, step-up and the step-down chopper.

RECTIFIER AND CHOPPER

General objective  :  To understand the concept of a rectifier.
Specific objectives :   At the end of the unit you should be able to:
  • Identify the power of an uncontrolled rectifier, semi-controlled rectifier, controlled rectifier and chopper.
  • Identify  the uncontrolled rectifier  and chopper circuit.

8.1       INTRODUCTION OF RECTIFIER
           
            The process of converting  alternating  current (or  alternating voltage) into pulsating direct current (or  pulsating direct voltage) is known as rectification. Rectification is accomplished with the help of diodes. Circuits  which provide rectification are called rectifier circuits. Rectifier circuits can provide either half-wave rectification or full-wave rectification.
           
8.2              PRINCIPLE OF RECTIFIER

            Assume a  half-wave rectifier output is to be used to supply current to a load. The output of  the rectifier gives the expected half-cycle of sinusoidal output once every cycle except that conduction of the rectifier diode is not allowed to begin at the start of the cycle but after an angular  measure of θ radians has occurred. The resulting current waveform is shown in Fig. 8.2(a).

            If the angle θ can be  varied form 0 to P /2 radiants (or even from 0 to P radians) then the  mean value  of current taken by the load can be varied as can the rms current to be derived.



8.3       SEMI-CONTROLLED RECTIFIER

            For control of electric power or semi control power conditioning, the conversion of electric power from one form to another is necessary and the switching characteristic of the power device permit these conversions. The static power converter may be considered as a switching matrix. The power electronics semi-control rectifier circuits can classified into two types:

                                                              i.      Diode rectifiers
                                                            ii.      AC - DC converters (controlled rectifiers)


8.4       HALF-WAVE RECTIFICATION
           
            The result of half-wave rectification is illustrated in Fig 8.4 (a), and the circuit which performs the rectification is drawn in Fig 8.4 (b).  The ground symbol in 8.4 (c) is the reference point for voltages referred to in the discussion which follows.


8.5       FULL-WAVE RECTIFICATION

            Full wave rectification can be provided with two diodes and a center-tapped transformer as shown in  Fig. 8.5 (a) , or it can be accomplished with four diodes  and a nontapped transformer (see Fig. 8.5 (b) ).

            Figure 8.5.1(a)  shows  the direction and path of current flow for the ½ cycle when the polarity of the transformer is as marked. Notice that only D1  is conducting and that only the top half of the transformer is providing power. This is because D2 is reverse-biased.

            During  the second ½  cycle (see 8.5.1 (b) ), the polarities of the  transformer windings are  reversed. Therefore, D1 is now reverse-biased and D2 allows the current to flow in the indicated direction and path. Notice that current through R1 is in  the same direction for each ½ cycle.


  


             Figure 8.5(b)  shows a full-wave, bridge rectifier circuit. Notice  that this  circuit provides twice as much dc voltage as does the previous full-wave circuit when both circuits use the same transformer. The bridge rectifier circuit does not use the center tap of the transformer and  it requires four diodes.

             During ½ cycle, two of the diodes in Fig. 8.5(c ) conduct and allow the full secondary voltage to force current through load resistor R1. the remaining two diodes are reverse-biased  and thus  prevent the diode bridge from short-circuiting the transformer secondary.


 

8.6       INTRODUCTION  OF CHOPPER.
           
            A dc chopper is the equipment that can be used as a dc transformer to step up or step down a fixed dc voltage.  The chopper can also be used for switching- mode voltage regulators and for transferring  energy between two dc resources.  However, harmonics are generated at the input and load side of the chopper, and these harmonics can be reduced by input and output filters.
           

8.7        PRINCIPLE OF CHOPPER.

            A chopper can operate on either fixed frequency chopper or variable frequency. A variable-frequency chopper generates harmonics of variable frequencies and a filter design. A fixed – frequency chopper is normally used. A chopper  circuit uses a fast turn off as a switch and requires commutation circuitry to turn it off.  The circuits are the outcome of meeting  certain  criteria: (1) reduction of minimum on-time limit, (2) high frequency of operation, and (3) reliable operation.

8.8       TYPE AND BASIC OPERATION OF CHOPPER FUNCTION CIRCUIT

            The development of alternative switching (e.g., power transistors, GTO s),  the applications for type and circuit of choppers are limited to high power levels and especially, to traction motor control. Some of chopper type and circuit used by traction equipment manufactures are discussed in this section.


8.8.1        IMPLUSE-COMMUTATED CHOPPERS 


            The impulse-commutated chopper is a very common circuit with two thyristors as shown in figure   8.8(a) and is also known as  a classical chopper.  At the beginning of operation, thyristor  T2 is fired and this causes the commutation capacitor C to charge through the voltage Vc , which should be supply voltage  Vs  in the fist cycle. The plate A becomes positive with respect to plate B.  The  circuit  operation can be divided into five modes, and the equivalent circuits under steady-state conditions are shown in Fig. 8.8(b).  We shall assume that the load current remains constant at a peak value  Im during the commutation process.  We shall also redefine the time origin, t = 0, at the beginning of each mode.  Mode 1 begins with T1  is fired. The load is connected to the supply.  The commutation capacitor C reverses also its charge through the resonant reversing circuit formed by T1, D1, and Lm.



 


 8.8.2.      IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS

             The problem of undercharging can be remedied by replacing diode D1  with thyristor T3, as shown in Fig. 8.8(c).  In good chopper, the commutation time, tc, should ideally be independent of the load current. tc  could be made less dependent on the load current by adding an antiparallel diode Df  across the main thyristor as shown in Fig. 8.8(c) by dashed lines.  A modified version of the circuit is shown in Fig. 8.8(d)., where the charge reversal of the capacitor is done independently of main thyristor  T1 by firing T3 .  There are four possible modes and their equivalent circuits are shown in Fig. 8.8(e).










 

 8.8.2.       RESONANT PULSE CHOPPERS

              A resonant pulse chopper is shown in Fig. 8.8(f).  As soon as the supply is switched on, the capacitor is charged to a voltage Vc  through Lm, D1, and load.  The circuit operation can be divided into six modes and the equivalent circuits are shown in Fig. 8.8(g).







8.9        SKETCHING CURRENT AND VOLTAGE WAVE

             There are no fixed rules for designing or sketching of copper circuit and the design  varies with the types of circuit used. The designer has a wide range of choice and values of LmC components are influenced by the designer’s choice of peak resonant reversal current, and peak allowable voltage of the circuit. The voltage and current  ratings LmC components and devices is left to the designer based on the considerations of price, availability, and safety margin. In general, the following steps are involved in the design:

a.       Identify the modes of operations for the copper circuit.
b.      Determine the equivalent circuits for the various modes.
c.       Determine the currents and voltages for modes and their waveforms.
d.      Evaluate the values of commutation components LmC that would satisfy the devices.

            A chopper with a highly inductive load is shown in Fig. 8.9(a).  The load current ripple is negligible (DI=0). If the average load current is Ia, the peak load current is Im=Ia + DI= Ia The input current, which is of pulsed shape as shown in Fig 8.9(b).



            The wave forms for currents and voltages are shown in figure 8.9(b). In the following analysis, we shall redefine the time origin t=0 at the beginning of each mode.





 Fig 8.9(c): Equivalent circuit for modes.
             Mode 1 begins when main thyristor T1 is fired and the supply is connected to the load. This  mode is valid for t = kT.

             Mode 2 begins when commutation thyristor T2 is fired. The commutation capacitor reverses its charge through C, Lm, and T2.

             Mode 3 begins when T2 is self-commutated and the capacitor discharges due to resonant oscillation through diode D1 and T1. Assuming that the capacitor current rises linearly from 0 to Im and the current of thyristor T1 falls from Im  to 0 in time tx.

             Mode 4 begins when current through T1 falls to zero. The capacitor continues to discharge through the load at a rate determined by the peak load current.

             Mode 5 begins when the freewheeling diode Dm starts conducting and the load current decays through Dm. the energy stored in commutation inductance Lm and source inductance Ls is transferred to capacitor C.

             Mode 6 begins when the overcharging is complete and diode D1 turns off. The load current continues to decay until the main thyristor is refired in the next cycle. In the stedy-state condition Vc = Vx. 


8.10     DEFINITION OF MARK SPACE RATIO (TIME RATIO CONTROL)

            The sequence of events within the frequency counter is controlled by the time ratio base, which must provide the timing for the following events: resetting the counter , opening the count gate, closing the count gate, and storing the counted frequency in the latch. The resetting of the counter and storing the count are not critical events as long as they occur before and after the gate period, respectively. The opening and closing of the count gate, on the other hand, determine the accuracy of the frequency counter and are very critical in its timing.

             Since the accuracy of the frequency counter depends directly on the accuracy of the time ratio base signal, the time base is driven from a accurate crystal controlled (e.g; oscillator). This element of the time base is typically a temperature compensated crystal oscillator operating at several megahertz. A crystal oven could be used to supply a similar accuracy, except that the oven require the application of power to provide the correct frequency and is available for use immediately after power-on. Fig. 8.10(a)  shows a simplified diagram of temperature-compensated crystal oscillator.




8.11     COMPARING STEP-UP AND STEP-DOWN CHOPPER DOWN.

            A chopper can be considered as dc equivalent to an ac transformer with a continuously variable turns ratio. Like a transformer, it can be used to step-down or step-up a dc voltage source.


8.11.1    PRINCIPLE OF STEP-UP OPERATION

            A chopper can be used to step-up a dc voltage and an arrangement for step-up operation is shown in Fig. 8.11(a).  When switch SW is closed for time t1, the inductor current rises and energy is stored in the indicator, L. if switch is opened for time t2, the energy stored in the inductor is transferred to load through diode D1 and the inductor current falls. Assuming a continuous current flow, the waveform for the inductor current is shown in Fig. 8.11(b).  For values of  k tending to unity, the output voltage becomes very large and is very sensitive to changes in k, as shown in Fig. 8.11(c).

 



8.11.1    PRINCIPLE OF STEP-DOWN OPERATION  
            
            The principle of operation can be explained by Fig. 8.11(d).  When switch SW is closed for time t1 , the output voltage Vs appears across the load. If the switch remains off for a time t2 , the voltage across the load is zero. The waveforms for the output voltage and load current are also shown in Fig. 8.11(e). The chopper switch can be implemented by using a (1) power BJT, (2) power MOSFET, (3) GTO, or (4) forced-commutated thyristor. The practical devices have a finite voltage drop ranging from 0.5 to 5 V, and for the sake of simplicity we shall neglect the voltage drops of these power semiconductor devices.

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