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**

**8.2**

**PRINCIPLE OF RECTIFIER**

**8.3 SEMI-CONTROLLED RECTIFIER**

i.
Diode rectifiers

ii.
AC - DC converters (controlled rectifiers)

**8.4 HALF-WAVE RECTIFICATION**

**8.5 FULL-WAVE RECTIFICATION**

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.

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.**

**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**

*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**

*a classical chopper.*At the beginning of operation, thyristor

*T*is fired and this causes the commutation capacitor

_{2}*C*to charge through the voltage

*V*, which should be supply voltage

_{c}*V*in the fist cycle. The plate

_{s }*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

*I*during the commutation process. We shall also redefine the time origin,

_{m }*t = 0*, at the beginning of each mode. Mode 1 begins with

*T*is fired. The load is connected to the supply. The commutation capacitor

_{1 }*C*reverses also its charge through the resonant reversing circuit formed by

*T*, and

_{1}, D_{1}*L*

_{m}.

**8.8.2.**

**IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS**

*D*

_{1}_{ }with thyristor

*T*, as shown in Fig. 8.8(c). In good chopper, the commutation time,

_{3}*t*, should ideally be independent of the load current

_{c}*. t*

_{c}_{ }could be made less dependent on the load current by adding an antiparallel diode

*D*

_{f}_{ }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

*T*by firing

_{1}*T*. There are four possible modes and their equivalent circuits are shown in Fig. 8.8(e).

_{3}**8.8.2.**

**RESONANT PULSE CHOPPERS**

*V*

_{c}_{ }through

*L*, and load. The circuit operation can be divided into six modes and the equivalent circuits are shown in Fig. 8.8(g).

_{m}, D_{1}**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

*L*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_{m}C*L*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:_{m}C
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

*L*that would satisfy the devices._{m}CA chopper with a highly inductive load is shown in Fig. 8.9(a). The load current ripple is negligible (

**=0). If the average load current is I**

*DI*_{a}, the peak load current is

*I*The input current, which is of pulsed shape as shown in Fig 8.9(b).

_{m}=I_{a}+**DI= I**_{a}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

*T*is fired and the supply is connected to the load. This mode is valid_{1}*for t = kT.**T*is fired. The commutation capacitor reverses its charge through

_{2}*C, L*, and

_{m}*T*

_{2}.Mode 3 begins when

*T*is self-commutated and the capacitor discharges due to resonant oscillation

_{2}*through diode D*an

_{1}*d T*. Assuming that the capacitor current rises linearly from

_{1}*0*to

*I*and the current of thyristor

_{m}*T*falls from

_{1}*I*to 0 in time

_{m }*t*

_{x}.Mode 4 begins when current through

*T*falls to zero. The capacitor continues to discharge through the load at a rate determined by the peak load current.

_{1}Mode 5 begins when the freewheeling diode

*D*starts conducting and the load current decays through

_{m}*D*. the energy stored in commutation inductance

_{m}*L*and source inductance

_{m}*L*is transferred to capacitor

_{s}*C*.

Mode 6 begins when the overcharging is complete and diode D

_{1}turns off. The load current continues to decay until the main thyristor is refired in the next cycle. In the stedy-state condition

*V*.

_{c}= V_{x}**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.

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

**8.11.1**

**PRINCIPLE OF STEP-UP OPERATION**

*t*, the inductor current rises and energy is stored in the indicator

_{1}*, L*. if switch is opened for time

*t*, the energy stored in the inductor is transferred to load through diode

_{2}*D*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

_{1}*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

*t*, the output voltage V

_{1}_{s}appears across the load. If the switch remains off for a time t

_{2}, 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.