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2.1 INTRODUCTION TO BASIC RECTIFIER CIRCUITS, Several types of rectifier circuits are available: Single-phase and threephase, half-wave and full-wave, controlled and uncontrolled, etc. For a, given application, the type used is determined by the requirements of that, application. In general the types of rectifiers are:, , 1. Uncontrolled Rectifiers : Provide a fixed d.c. output voltage for a, given a.c. supply where diodes are used only., Controlled Rectifiers : Provide an adjustable d.c. output voltage by controlling the phase at, which the devices are turned on, where thyristors and diodes are used., , Three Phase Rectification, 3-phase rectification is the process of converting a balanced 3-phase power supply into a, fixed DC supply using solid state diodes or thyristor

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We saw in the previous tutorial that the process of converting an AC input supply into a fixed, DC supply is called Rectification with the most popular circuits used to perform this, rectification process is one that is based on solid-state semiconductor diodes., In fact, rectification of alternating voltages is one of the most popular applications of diodes,, as diodes are inexpensive, small and robust allowing us to create numerous types of rectifier, circuits using either individually connected diodes or with just a single integrated bridge, rectifier module., Single phase supplies such as those in houses and offices are generally 120 Vrms or 240, Vrms phase-to-neutral, also called line-to-neutral (L-N), and nominally of a fixed voltage and, frequency producing an alternating voltage or current in the form of a sinusoidal waveform, being given the abbreviation of “AC”., Three-phase rectification, also known as poly-phase rectification circuits are similar to the, previous single-phase rectifiers, the difference this time is that we are using three, singlephase supplies connected together that have been produced by one single three-phase, generator., The advantage here is that 3-phase rectification circuits can be used to power many, industrial applications such as motor control or battery charging which require higher power, requirements than a single-phase rectifier circuit is able to supply., 3-phase supplies take this idea one step further by combining together three AC voltages of, identical frequency and amplitude with each AC voltage being called a “phase”. These three, phases are 120 electrical degrees out-of-phase from each other producing a phase sequence,, or phase rotation of: 360o ÷ 3 = 120o as shown.

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Three-phase Waveform, , The advantage here is that a three-phase alternating current (AC) supply can be used to, provide electrical power directly to balanced loads and rectifiers. Since a 3-phase supply has, a fixed voltage and frequency it can be used by a rectification circuit to produce a fixed, voltage DC power which can then be filtered resulting in an output DC voltage with less, ripple compared to a single-phase rectifying circuit., , Working of Three Phase Half Wave Uncontrolled Rectifier, , Figure A shows a three phase half wave uncontrolled rectifier, circuit using delta – star transformer., •, The anode of the diode D1, D2 and D3 are connected to the, secondary winding of the transformer with R phase, Y phase and B, phase respectively., •, The cathode of the three diodes is connected to the neutral, point of the secondary winding., •, The resistive load is connected between neutral point and, common cathode point of the diodes., •

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Working, •, , The three phase supply voltage waveform is shown in the figure, , B., The diode can block negative voltage and it conducts when, phase voltage is maximum of the three phases., •, The effect of the transformer winding reactance is neglected., The working of three phase uncontrolled rectifier is given below., point a, •, , The voltage of phase R becomes zero ., •, The voltage of phase B becomes positive with respect to phase, Y resulting diode D3 conducts., •, The load current flows through phase B – diode D3 – load, resistance RL – n., •, , Point b, The voltage of phase Y becomes negative ., •, The voltage of phase R becomes positive with respect to phase, B resulting diode D1 conducts., •, The load current flows through phase R – diode D1 – load, resistance RL – n., •, , Point c, The voltage of phase B becomes negative ., •, The voltage of phase Y becomes positive with respect to phase, R resulting diode D2 conducts., •, The load current flows through phase Y – diode D2 – load, resistance RL – n., •, There are three diodes and each diode conducts in a proper, sequence of D3 – D2 – D1 – D3 and so on., •, The conduction angle for each diode is 120 degree., •

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Three Phase Uncontrolled Full Wave Rectifier, The connection diagram for three phase full wave uncontrolled, rectifier using Delta – star transformer is shown in the figure A., •, There are two diodes used for each phase and load is connected, between common anode terminal and common cathode terminal of, diodes., •, There are two three phase half wave rectifiers which are made by, diode D1, D2 and D3 and diodes D4, D5 and D6 respectively., •

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Working, The three phase supply voltage waveform is shown in the figure B., •, The diode can conduct when phase voltage ( either positive or, negative ) is maximum out of three phases., Point a, •, The voltage of phase R becomes zero., •, The voltage of phase B becomes nearly maximum positive, whereas the voltage of phase Y becomes negative therefore diode D3, from phase B positive half leg and diode D5 from phase Y negative, half leg conducts., •, The current flows from phase B – diode D3 – load resistance RL –, diode D5 – phase Y., Point b, •, The voltage of the phase R increases towards positive maximum, whereas the voltage of phase B decreasing towards positive to, negative., •, The voltage of the phase Y already negative as earlier. Therefore, the diode D1 from phase R leg conducts whereas the diode D5, continues its conduction., •

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Therefore the current flows from phase R – diode D1 – load, resistance RL – diode D5 – phase Y., Point c, •, The voltage of the phase R becomes positive maximum., •, The voltage of the phase B decreases towards negative, maximum whereas the voltage of phase Y increases from negative to, positive., •, Therefore the diode D1 from phase R positive leg continues its, conduction whereas the diode D6 from phase B negative leg, conducts., •, The load current flows from phase R – diode D1 – load resistance, RL – diode D6 – phase B., •, , This will result in one diode from positive group D1, D2 or D3 and one, diode from negative group D4, D5, or D6 conducts at a same time e.g. D3, and D5, D5 and D1, D1 and D6, D6 and D2, D2 and D4.

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Each diode conducts for 120o. It should be noted that two diodes, of a same phase never conduct otherwise the supply should be short, circuited., •, , Three-Phase Full-Wave Uncontrolled Bridge Rectifier, Fig.2.22 shows a three-phase full-wave uncontrolled bridge rectifier, with resistive load. The rectifier is fed from an ideal three-phase supply, through delta-star three-phase transformer. The principle of operation of, this convertor can be explained as follows:, Each three-phase line connects between pair of diodes, one to route power, to positive (+) side of load, and other to route power to negative (-) side of, load., • Diode 1, 3 and 5, whichever has a more positive voltage at its, anode conducts., • Similarly, diode 2, 4 and 6, whichever has more negative voltage at, its cathode return the load current., • The conduction pattern is: 16-36-34-54-52-12., • Each diode conducts for 120˚ in each supply cycle as shown in, Fig.2.23., , Fig.2.22 The three-phase full-wave, uncontrolled rectifier., The output voltage is the instantaneous difference between two appropriate phases at each instant as depicted in Fig.2.23, and the resultant d.c., output voltage wave is shown in Fig.2.24., To find the average voltage Vdc on the load, assume that the line to line, voltages are represented by the following equations,, vab = van – vbn = Vm sin(wt)-Vm sin( w t + 2 π/ 3 ) ,

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Power electronics and drives, , Fig.2.23 Each diode conducts for 120˚., , Difference Between Half Wave And Full Wave Rectifier In, Tabular Form, BASIS OF, COMPARISON, , HALF WAVE, , FULL WAVE, , Description, , Half wave rectifier, current only during, positive half cycle, of the applied, input, therefore, it, shows, unidirectional, characteristics., , Full wave rectifier,, both the halves of the, input signal is utilized, at the same time of, operation, therefore it, shows bidirectional, characteristics., , Fundamental, Ripple, Frequency, , Output frequency, (fundamental, ripple frequency) of, half wave rectifier, is equal to the, , Full wave rectifier, output frequency, (fundamental ripple, frequency) is twice, that of the applied, input i.e 100Hz.

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frequency of input, i.e 50Hz., , Ripple Factor, , Half wave rectifier, has less ripple, factor when, compared to full, wave rectifier. For, half wave rectifier, it is about 1.21., , Full wave rectifier has, more ripple factor, when compared to, half wave rectifier. For, full wave rectifier, it is, about 0.482., , Peak Inverse, Voltage, , The peak inverse, voltage for half, wave rectifier is, equivalent to the, maximum value of, applied input, voltage., , Peak inverse voltage, for full wave rectifier, is twice the maximum, value of applied input, voltage., , Voltage, Regulation, , Half wave rectifier, has a fair good, voltage regulation, mechanism., , Full wave rectifier has, a better voltage, regulation mechanism, when compared to, half wave rectifier., , Number Of, Diodes, , Half wave rectifier, circuit requires only, one diode., , In full wave rectifier, circuit, two or even 4, diodes are used in the, circuit., , Efficiency, , Half wave rectifier, has an efficiency of, 40.6%., , Efficiency of full wave, rectifier is 81.2%., , Center, Tapping, , Half wave rectifier, does not require, center tapping of, the secondary, winding of, transformer., , Full wave rectifier, requires center, tapping of the, secondary winding of, the transformer., , DC Saturation, , DC saturation of, the transformer, core is a common, problem in the half, waver rectifier, circuit., , Full wave rectifier, circuit does not have, DC saturation of, transformer core, because the current in, the secondary winding, flows in two halves of, the secondary winding

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of the transformer, and in opposite, direction., , Cost, , Half wave rectifier, is less costly since it, requires only one, diode., , Full wave rectifier is, more costly since it, requires more than, 1diodes., , Difference Between FWR and FWBR, Bridge, rectifier, , Definition, , Center tapped, rectifier as the, name suggests, requires a center, tapped, transformer, (secondary, winding)., , No center tapped, transformer is, required in a, bridged rectifier., , Number of, diodes, , Center tapped, rectifier uses, only two diodes, in its circuit., , Bridge rectifier, uses four diodes, in its circuit., , Peak inverse, voltage, , The peak inverse, voltage (PIV) of, the diode in the, center tapped, full wave rectifier, is twice the, transformer, secondary, terminal voltage., , Peak inverse, voltage PIV of the, diode is equal to, the transformer, secondary, voltage. Thus this, type of rectifier, can be used for, high voltage, application., , Transformer, Utilization, Factor (TUF), , The transformer, utilization factor, , The transformer, utilization factor, (TUF) is equal to

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(TUF) is equal to, 0.672, Voltage drop, across the two, diodes of the, Voltage drop, center tapped, across, rectifier is less, diodes, when compared, to bridge, rectifier., , 0.810 for the, bridge rectifier., The voltage drop, across the 4, diodes of the, bridge rectifier is, more than the, voltage drop, across the center, tapped rectifier., , Size Of, Transformer, , The transformer, required in the, bridge rectifier is, smaller than that, required in the, center tapped, rectifier in terms, of kVA rating., , The transformer, required in the, bridge rectifier is, smaller than that, required in the, center tapped, rectifier in terms, of kVA rating., , Economic, efficiency, , Center tapped, transformer is, economically, efficient since it, uses only two, diodes in its, circuit., , Bridge rectifier is, economically, inefficient since it, uses four diodes, in its circuit., , Control rectifier, Concept of firing angle, Firing Angle of SCR is defined as the angle between the instant SCR would conduct if it, were a diode and the instant it is triggered., , We know that, there are two conditions which must be satisfied for turn on of an, SCR. They are:, •, •, , SCR must be forward biased i.e. its anode voltage must be positive with, respect to cathode voltage., It must be gated i.e. a gate signal must be applied across the Gate and, Cathode Terminals., , This means, even though the SCR is forward biased, it is not going to conduct until a, gate signal is applied. This is not the case with a diode. In a diode, as soon as it gets, forward biased, it starts conducting. No gate signal is required to be applied for, turning on a diode. In fact, there is no such Gate terminal in diode.

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Let us understand the concept of firing angle in detail. For the sake of better, understanding, consider the figure below., , In the above figure, a thyristor T is connected to AC source v s and load resistance R. Just, think, if the thyristor is replaced with a diode, it will start conducting for positive half cycle of, the supply voltage as it is forward biased for this period. But will it be the same case, thyristor?, , Obviously, No. It’s true that SCR is forward biased for positive half cycle of the supply, voltage but unfortunately gate signal is not applied. Hence, it will not conduct or turn on. Let, us now apply gate signal at some angle α on the source voltage curve as shown in figure, below., Now, SCR is forward biased and gate signal is also applied at wt = α. Hence, SCR will turn, on and will start conducting. This angle, at which gate signal is applied to SCR Gate and, Cathode terminal is called Firing Angle. Application of gate signal is also called Firing of, SCR. Once SCR is forward biased and fired, it becomes ON. The load voltage and current, will have a wave shape similar to supply voltage as the load is resistive, Firing Angle may also be defined as the angle measured from the instant SCR gets, forward biased to the instant it is triggered. From this definition, the firing angle for, our example is α., Half wave converter, Controlled rectifiers, or converters, as they are generally called, are broadly classified into, fullcontrolled and half-controlled types. The full controlled or two quadrant type uses SCRs as the, rectifying devices. The DC current is unidirectional, but the DC voltage may have either polarity. With

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one polarity, the flow of power is from the AC source to the DC load this is called rectification. With, reversal of the DC voltage by the load, the flow of power is from the DC source to the AC supply ; this, process is called inversion. In this article we will discuss half-wave controlled rectifiers., Half Wave Controlled Rectifiers, With A Resistive Load, Figure 1[a] shows a half-wave controlled rectifier circuit with a resistive load. During the positive halfcycle of the supply voltage, the SCR is forward-biased and will conduct if a trigger is applied to the, gate. If the SCR turns on at t ω load current flows and the output voltage V0 will be the same as the, voltage At time t= π, the current falls natural to zero, since the SCR is reverse-biased. During the, negative half- cycle, the SCR blocks the flow of current, and no voltage is applied to the load The SCR, stays off until the gate signal is applied again at (to +2 π). The period from 0 to t0 in figure 1[b], represents the time in the positive half- cycle when the SCR is off This angle [measured in degrees] is, called the firing angle or delay angle α The SCR conducts from to to π this angle is called the, conduction angle θ, The average or DC value of the load voltage is given by, , V0(avg)=(Vm(1+cos α ))/2π, , Vm= maximum value of the AC source voltage=

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Freewheeling (Flyback) Diode, Definition: Freewheeling diode is used to protect the circuit from unusual damage caused due to, abrupt reduction in the current flowing through the circuit. It is also known as Flyback diode and, forms connection across the inductor to remove Flyback voltage generated across it., Freewheeling diodes are also known as kickback diode, clamp diodes, commutating diodes,, suppression diodes, or snubber diode etc., Here in this article, we will discuss the factors responsible for the need of such diodes in switching, circuits. But first, we must have the basic idea of diodes., , What is a Diode?, A diode is a semiconductor device composed of P and N-type semiconductor material. It, conducts under forward biased condition when the applied potential exceeds the barrier potential., Thus acts as a closed switch., While under reverse biased condition, the diode stops conducting and functions as an open switch., So a Freewheeling (Flyback) diode operates in the same way that it conducts in forward biased, condition but do not conducts in reverse biased condition., • What is Flyback?, , Flyback is basically defined as an abrupt increase in voltage across the inductive load when the, current through the circuit shows a reduction., , Need for Freewheeling (Flyback) Diode Consider, the circuit shown below:, , As we can see that the circuit shown above is composed of a diode, a switch and RL load. Also a, supply voltage V is provided to it.

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Once the switch gets closed so due to applied external potential, the diode in the circuit gets, forward biased and current starts flowing through the load RL., We know that an inductor is basically a conductive loop of wire that produces a magnetic field when, current flows through it. The inductor holds the energy in the form of an electromagnetic field., So, in closed switch condition, the flow of current through the inductor leads to the generation of, the magnetic field, causing it to get fully charged., But as the switch in the circuit gets opened as shown in the figure below:, , Then this will lead to an interruption in the flow of current through the circuit. Resultantly this will, cause the collapsing of the earlier generated field., And according to Lenz law, this field sets up a current in the circuit in the opposite direction, thereby, leading to the production of negative potential across the inductor. This potential is known as, Flyback voltage., And this Flyback voltage across the inductor has significantly greater value than actually applied, potential by the external source., This leads to a flow of high current through the circuit. Resultantly causing a high reverse voltage to, set up across the switch as well as the diode, that may lead to damaging of the devices in the circuit., The voltage spike across the inductor is given as:, V = L di/dt, : di/dt is the rate of change of current across the inductor and, L denotes the inductance of the coil., Thus it can be said that voltage across the inductor and current flowing through the circuit holds the, relation of direct proportionality.

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So, due to this reason, a free-wheeling diode is connected across the inductor to avoid the damage, in the circuit., Working of Freewheeling (Flyback) Diode, The figure below represents a circuit with a freewheeling diode:, , It is clear from the figure that the freewheeling diode is connected directly across the inductor. The, presence of Flyback diode gives an alternate path to the current, produced due to Flyback voltage at, the inductor., Under normal operating conditions when the switch is closed, the external potential reverse biases, the freewheeling diode present in the circuit. And so the freewheeling diode plays no such crucial, role under normal or steady-state condition., But in the presence of FD when the switch is opened, the voltage across the inductor forward biases, the freewheeling diode., , Due to small resistivity offered by FD, current in open switch condition now flows through the part of, a circuit comprising of the freewheeling diode, R and L. This resultantly leads to the protection of, switching device present in the circuit., , Applications of Freewheeling Diode, As we have already discussed that these diodes are used for the protection of switching devices., Thus majorly finds applications in full-wave rectifiers, relay drivers and H-bridge motor drivers etc., , Single Phase Full Wave Controlled Rectifier, Single Phase Full Wave Controlled Rectifier with 'R' load:

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Figure below shows the Single phase Full Wave Controlled Rectifiers with, R load, , • The single phase fully controlled rectifier allows conversion of single, phase AC into DC. Normally this is used in various applications such as, battery charging, speed control of DC motors and front end of UPS, (Uninterruptible Power Supply) and SMPS (Switched Mode Power Supply).

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• All four devices used are thyristors. The turn-on instants of these, devices are dependent on the firing signals that are given. Turn-off, happens when the current through the device reaches zero and it is, reverse biased at least for duration equal to the turn-off time of the, device specified in the data sheet. • In positive half cycle, thyristors T1 & T2 are fired at an angle α ., • When T1 & T2 conducts, Vo=Vs, IO=is=Vo/R=Vs/R, • In negative half cycle of input voltage, SCR's T3 &T4 are triggered at, an angle of (π+α), • Here output current & supply current are in opposite direction ∴, is=-io, T3 & T4 becomes off at 2π.