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TitleEffect of Source Inductance
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Page 1

TYPES OF CONVERTERS

AC to DC Converters (RECTIFIER)

Introduction

One of the first and most widely used application of power electronic devices have been in

rectification. Rectification refers to the process of converting an ac voltage or current source to

dc voltage and current. Rectifiers specially refer to power electronic converters where the

electrical power flows from the ac side to the dc side. In many situations the same converter

circuit may carry electrical power from the dc side to the ac side where upon they are referred to

as inverters. In this lesson and subsequent ones the working principle and analysis of several

commonly used rectifier circuits supplying different types of loads (resistive, inductive,

capacitive, back emf type) will be presented. Points of interest in the analysis will be.

• Waveforms and characteristic values (average, RMS etc) of the rectified voltage and

current.

• Influence of the load type on the rectified voltage and current.

• Harmonic content in the output.

• Voltage and current ratings of the power electronic devices used in the rectifier circuit.

• Reaction of the rectifier circuit upon the ac network, reactive power requirement, power

factor, harmonics etc.

• Rectifier control aspects (for controlled rectifiers only)

In the analysis, following simplifying assumptions will be made.

• The internal impedance of the ac source is zero.

• Power electronic devices used in the rectifier are ideal switches.

The first assumption will be relaxed in a latter module. However, unless specified otherwise, the

second assumption will remain in force.

Rectifiers are used in a large variety of configurations and a method of classifying them into

certain categories (based on common characteristics) will certainly help one to gain significant

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insight into their operation. Unfortunately, no consensus exists among experts regarding the

criteria to be used for such classification. For the purpose of this lesson (and subsequent lessons)

the classification shown in Fig 9.1 will be followed.

DC – DC CONVERTER

There are three basic types of dc-dc converter circuits, termed as buck, boost and buck-boost. In all

of these circuits, a power device is used as a switch. This device earlier used was a thyristor, which is

turned on by a pulse fed at its gate. In all these circuits, the thyristor is connected in series with load

to a dc supply, or a positive (forward) voltage is applied between anode and cathode terminals. The

thyristor turns off, when the current decreases below the holding current, or a reverse (negative)

voltage is applied between anode and cathode terminals. So, a thyristor is to be force-commutated,

for which additional circuit is to be used, where another thyristor is often used. Later, GTO’s came

into the market, which can also be turned off by a negative current fed at its gate, unlike thyristors,

requiring proper control circuit. The turn-on and turn-off times of GTOs are lower than those of

thyristors. So, the frequency used in GTO-based choppers can be increased, thus reducing the size of

filters. Earlier, dc-dc converters were called ‘choppers’, where thyristors or GTOs are used. It may be

noted here that buck converter (dc-dc) is called as ‘step-down chopper’, whereas boost converter (dc-

dc) is a ‘step-up chopper’. In the case of chopper, no buck-boost type was used.

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For any current to flow in the load at least one device from the top group (T1, T3, T5) and one

from the bottom group (T2, T4, T6) must conduct. It can be argued as in the case of an

uncontrolled converter only one device from these two groups will conduct.

Then from symmetry consideration it can be argued that each thyristor conducts for 120° of the

input cycle. Now the thyristors are fired in the sequence T1 → T2 → T3 → T4 → T5 → T6 → T1

with 60° interval between each firing. Therefore thyristors on the same phase leg are fired at an

interval of 180° and hence can not conduct simultaneously. This leaves only six possible

conduction mode for the converter in the continuous conduction mode of operation. These are

T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each conduction mode is of 60° duration and appears in the

sequence mentioned. The conduction table of Fig. 13.1 (b) shows voltage across different

devices and the dc output voltage for each conduction interval. The phasor diagram of the line

voltages appear in Fig. 13.1 (c). Each of these line voltages can be associated with the firing of a

thyristor with the help of the conduction table-1. For example the thyristor T1 is fired at the end

of T5T6 conduction interval. During this period the voltage across T1 was vac. Therefore T1 is fired

α angle after the positive going zero crossing of vac. Similar observation can be made about other

thyristors. The phasor diagram of Fig. 13.1 (c) also confirms that all the thyristors are fired in the

correct sequence with 60° interval between each firing.

Fig. 13.2 shows the waveforms of different variables (shown in Fig. 13.1 (a)). To arrive at the

waveforms it is necessary to draw the conduction diagram which shows the interval of

conduction for each thyristor and can be drawn with the help of the phasor diagram of fig. 13.1

(c). If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero

crossing of the line voltage with which it’s firing is associated. Once the conduction diagram is

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