Types of Single Phase Semi Converter

Generally, there are two types of configurations of semi-converter.

1. Symmetrical Semi Converter.
2. Asymmetrical Semi Converter.

Single Phase Symmetrical Semi Converter with R Load

(a) Circuit Diagram.

(b) Input, output voltage and current waveforms

Fig. 1: Single Phase Symmetrical Semi Converter with R Load

In Fig. 1 (a), shows the circuit diagram of single-phase half-controlled bridge rectifier with R load. In this rectifier two SCRs and two diodes are connected in symmetrical configuration.

The operation of this converter mainly consists of three modes:

Mode 1 :(α to π):

During positive half-cycle of AC input voltage, thyristor T₁ and diode D₁ conduct and thyristor T₁ is fired at ωt = α. Therefore the average output voltage is equal to the instantaneous supply voltage and load current flows through T₁ → R → D₁ and back supply again.

Mode 2 : (π to π + α):

At instant ωt = π, the supply goes through zero and after π supply voltage reverses its polarity. Due to reverse supply across conducting devices T₁ and D₁, they turned off at ωt = π and this type of turn-off is called as “natural” or “line commutation”. Therefore, the average output voltage is zero (i.e. V₀ = 0 volt) and load current is also zero.

Mode 3: (π + α to 2π):

At instant ωt = π + α, the supply voltage becomes completely negative i.e. during negative half cycle of AC input voltage, thyristor T₂ and diode D₂ are forward biased, thyristor T₂ is fired at ωt = π + α. Thus the average output is equal to instantaneous supply voltage because due to conduction of T₂ and D₂, the load is directly connected to supply (i.e. V₀ = V_S). The load current flows through T₂ → R → D₂. Thyristor T₂ and diode D₂ conduct upto 2 π, at ωt = 2π commutation takes place due to natural zero appears across supply.

Advantages of Single Phase Semi Converter

1. Improved power factor.
2. Cost is reduced as 2 SCRs are replaced by 2 diodes.
3. Reactive input power is reduced.
4. Average output d.c. voltage is higher for same firing angle.

Disadvantages of Single Phase Semi Converter

1. It can be operated only in rectification mode.
2. Energy feedback from load to source is not possible.

Applications of Single Phase Semi Converter

Since it has a higher power factor, it can be used in high power applications. It is also widely used in main line a.c. traction where large d.c. motors are applied from a single-phase a.c. supply.

Formulas of Single Phase Semi Converter

Average value of output voltage (V₀)

\[{{\text{V}}_{\text{0}}}=\frac{{{\text{V}}_{\text{m}}}}{\text{  }\text{ }\!\!\pi\!\!\text{ }\text{    }}\left( 1+\cos \alpha  \right) \]

RMS value of output voltage (V_rms)

\[{{\text{V}}_{\text{rms}}}=\frac{{{\text{V}}_{\text{m}}}}{\sqrt{2}}{{\left[ \frac{\text{1}}{\text{  }\pi \text{  }}\left( \text{  }\pi \text{ }-\alpha +\text{ }\frac{\text{sin 2}\alpha }{\text{ 2    }}\text{ } \right) \right]}^{\text{1/2}}}\]

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Types of Single Phase Full Wave Controlled Rectifier

The controlled bridge type converters are classified into two classes:

1. Fully-controlled converter (Full converter).
2. Half-controlled converter (Semi converter).

Single-Phase Fully Controlled Bridge Rectifier with R Load

(a)  circuit diagram

 (b) Input and output voltage and current waveforms

Fig. 1: Single-phase fully controlled rectifier with R load

Fig. 1 (a) shows the typical circuit diagram of single-phase fully controlled bridge converter. It consists of four SCRs T₁ to T₄ and they are connected in bridge type configuration driving the resistive load.

The operation of single-phase fully controlled converter consists of three modes:

Mode 1 :(α to π):

At ωt = 0 instant, the supply voltage goes through zero, after ωt = 0, the supply goes towards positive, i.e. during positive half cycle of AC input voltage, thyristors T₁ and T₂ are fired at ωt = α, thus the average output voltage is equal to the supply voltage (i.e. V₀ = V_S). The current flows from point L through thyristor T₁ through load resistance through T₂ to point N. The load current is positive and has the same shape as that of AC mains input voltage. The load voltage and load current are in phase. At ωt = π instant, the supply voltage goes through zero, the conducting SCRs T₁ and T₂ are turned off due to natural commutation. At this instant, both load voltage and load currents are zero.

Mode 2 : (π to π + α):

At ωt = π, the supply becomes zero, after ωt = π the supply voltage reverses polarities. Therefore, in this mode of operation, no SCR conducts. Both load voltage and load currents are zero.

Mode 3: (π + α to 2π):

At instant ωt = π + α i.e. during negative half cycle of AC input voltage, the SCRs T₂ and T₄ are fired at ωt = π + α. Therefore, the load is directly connected to supply voltage and average output voltage is equal to the instantaneous supply voltage (i.e. V₀ = V_S). The load voltage is positive and load current is continuous positive. The SCRs T₂ and T₄ continue to conduct upto 2π. At ωt = 2π, the supply goes through zero, so conducting thyristors T₂ and T₄ will turn-off at ωt = 2π, due to line natural commutation.

Formulas of Single Phase Full Wave Controlled Rectifier

Average value of output voltage (V₀)

\[{{\text{V}}_{\text{0}}}=\frac{{{\text{V}}_{\text{m}}}}{\text{ }\text{ }\!\!\pi\!\!\text{ }\text{   }}\left[ 1+\cos \alpha  \right] \]

RMS value of output voltage (V_rms)

\[{{\text{V}}_{\text{rms}}}=\frac{{{\text{V}}_{\text{m}}}}{\sqrt{2}}{{\left[ \frac{\text{1}}{\text{ }\text{ }\!\!\pi\!\!\text{ }\text{ }}\left( \text{ }\text{ }\!\!\pi\!\!\text{ }\text{ }-\alpha +\text{ }\frac{\text{sin 2}\alpha }{\text{ 2    }}\text{ } \right) \right]}^{\text{1/2}}}\]

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(a) Circuit diagram.

(b) Input and output voltage and current waveforms.

Fig. 1: Single Phase Full Wave Controlled Rectifier with RL load

Single Phase Full Wave Controlled Rectifier with RL load Operation

The operation of single-phase fully controlled converter consists of four modes as follows:

Mode 1 α to π):

During positive half cycle of AC input voltage, leave point L is positive with respect to cathode, therefore thyristors T₁T₂ are fired at ωt = α. Thus, the average output voltage is equal to the instantaneous supply voltage.

In this mode of operation, the shape of load voltage is identical to that of supply voltage. The load voltage is positive and constant. The load current i₀ is also positive as that of the supply current. Both load voltage and load current are positive, the inductive load will store energy.

Mode 2: (π to π + α):

In this step of operation, at instant ωt = π, the supply goes through zero and after π radians supply reverses its polarities and it becomes negative. Therefore, the conducting thyristors T₁ and T₂ will try to turn-off due to natural reversal of supply voltage (i.e. natural commutation or line commutation). But due to stored energy in inductive load, it will oppose any change in the current flow through load. So thyristors T₁ and T₂ will continue to conduct in negative half for some period. In this mode of operation, the load voltage becomes negative and load current is always positive, continuous and constant. Both load voltage and load currents are opposite in polarities. So the stored energy in inductive load will return back to the supply again.

Mode 3: (π + α to 2π):

At instant ωt = π + α, the conducting thyristors T₁ and T₂ are turned off due to natural or line commutation, at the same time other pair of SCRs T₃ and T₄ are fired at ωt = π + α. Therefore, the average output voltage is equal to the instantaneous supply voltage. The load current is instantaneously transferred from one pair of SCRs (T₁, T₂) to other pair of SCR (T₃, T₄). In this mode of operation, both load voltage and load currents are positive, the inductive load will again store energy.

Mode 4: (2π to 2π + α):

At instant ωt = 2π radians, the input voltage goes through zero after 2π it becomes positive. i.e. during positive half cycle of AC input, the conducting thyristors T₃, T₄ try to turn off, the inductive load will oppose any change in current through it, in order to maintain the load current constant and in some direction, a self-induced voltage appears across the load. This maintains conducting thyristors T₃ and T₄ forward biased, in spite of the change in the polarity of supply voltage. The load voltage becomes negative and equal to the supply voltage whereas the load current continues positive. Therefore, load acts as a source and the stored energy in inductive load will be returned back to supply again.

Formulas of Single Phase Full Wave Controlled Rectifier with RL load

Average value of output voltage (V₀)

\[{{\text{V}}_{\text{0}}}=\frac{\text{2}{{\text{V}}_{\text{m}}}}{\text{ }\!\!\pi\!\!\text{ }\text{  }}\cos \alpha \]

RMS value of output voltage (V_rms)

\[{{\text{V}}_{\text{rms}}}=\frac{{{\text{V}}_{\text{m}}}}{\sqrt{2}}\]

The post What is Single Phase Full Wave Controlled Rectifier with RL load? Working, Circuit Diagram & Waveform appeared first on ElectricalWorkbook.

Fig. 1: Circuit diagram of complementary voltage commutation (class C commutation).

The circuit diagram for the complementary voltage commutation is as shown in Fig. 1. Both the thyristors SCR₁ and SCR₂ are capable of handling the load current. C is the commutating capacitor and the voltage across it is used to commutate the conducting SCR. In this way, class C commutation is voltage commutation.

The resistors R₁ and R₂ are load resistors and let I_R1 and I_R2 be the corresponding load currents such that

\[{{\text{I}}_{\text{R1}}}=\text{ }\frac{{{\text{V}}_{\text{S}}}}{{{\text{R}}_{\text{1}}}}\]

And

\[{{\text{I}}_{\text{R2}}}=\text{ }\frac{{{\text{V}}_{\text{S}}}}{{{\text{R}}_{\text{2}}}}\]

We assume that both the SCRs are ideal. Therefore the on state voltage drop across them will be zero.

Class C Commutation Operation of the circuit

The operation of complementary commutation circuit can be explained by dividing one cycle of operation into four intervals or modes of operation.

(a) Equivalent circuit for mode I

(b) Equivalent circuit for mode II

Fig 2: Equivalent circuit

Before SCR₁ is triggered at instant t₀ assume that SCR₂ is conducting. The voltage across the commutating capacitor is equal to V_S with the polarities of voltage as shown in Fig. 2(a). The load current I_R2 flows through SCR₂.

Mode I (t₀ to t₁):

At the instant t = t₀, SCR₁ is triggered. This connects the commutating capacitor C across the conducting SCR₂. The voltage across C reverse biases the conducting SCR₂ and it is turned off due to voltage commutation. The load current I_R1 which is equal to V_S / R₁ starts flowing through SCR₁. The charging current of the capacitor I_C also flows through SCR₁ as shown in Fig. 2(a). At t = t₁ the capacitor is fully charged with polarities as shown in Fig. 2(b), and I_C = 0. SCR₁ continues to conduct.

Mode II (t₁ to t₂):

At t = t₁, the charging current of the capacitor C goes to zero. The voltage on C is equal to V_C with the polarities as shown in Fig. 1.26.6(b). SCR₁ continues to conduct the load current I_R1 = V_S / R₁. At the end of this mode i.e. at t = t₂ the other thyristor SCR₂ is triggered to turn off SCR₁.

Mode III (t₂ to t₃):

(a) Equivalent circuit for mode III

(b) Equivalent circuit for mode IV

Fig 3: Equivalent circuit

At instant t = t₂, SCR₂ is turned on. This will connect the commutating capacitor C across SCR₁ via SCR₂. The voltage on C reverse biases SCR₁ and turns it off, due to voltage commutation. The current through SCR₂ i.e. I_SCR2 is the sum of the load current I_R2 and the charging current of capacitor I_C as shown in Fig. 3 (a). The voltage across SCR₁ will follow the variations in the capacitor voltage. At the end of this mode (t = t₃), the capacitor is fully charged, I_C = 0 and V_SCR1 = V_S.

Mode IV (t₃ to t₄):

In this mode of operation, SCR₂ continues to carry the load current I_R2 = V_S / R₂. The voltage on C is held constant equal to V with the polarities as shown in Fig. 3 (b). The charging current of the capacitor I_C is equal to zero. At instant t₄, SCR₁ is triggered again to turn off SCR₂ and the cycle of operation repeats itself.

For successful commutation, the time for which the outgoing SCR is reverse biased (t_c) as shown in Fig. 4, must be greater than the turn off time of the SCR i.e. (t_q).

Class C Commutation Waveforms

The input and output waveforms are as shown in Fig. 4.

Fig. 4: Waveforms of class-C commutation.

Class C Commutation Advantages

The advantage of class-C commutation method of an SCR is as under:

1. The charging current flowing through the capacitor does not flow through the load.

Class C Commutation Drawbacks

The drawbacks of class-C commutation method of an SCR are as under:

1. The capacitor C can be charged to the voltage limited to the supply voltage and not more than that of it.
2. The turn-ON time of the main SCR is limited due to limit on supply voltage. In high current applications, it requires high value of turn-ON time.

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Types of 3 Phase Converter

Three-phase converters are mainly classified into three classes like single-phase converters (rectifiers).

1. Half-wave converters.
2. Full-wave converters.
3. Dual converter.

Full-wave converters are further classified into two classes:

1. Fully-controlled converters.
2. Half-controlled converters.

Three-Phase Half-Wave Controlled Converter (With Highly Inductive Load)

Fig. 1: Circuit diagram of three-phase half-wave controlled converter with highly inductive load.

Fig. 1 shows the circuit diagram of three-phase half wave controlled converter with highly inductive load. This circuit provides the load current i₀ is ripple free and continuous. The conduction period for each SCR is 2π/3 radians or 120°. Three phases V_AN, V_BN and V_CN are applied for SCRs T₁, T₂ and T₃ respectively. The load is connected between common cathode point of the three SCRs i.e. point D and neutral point N.

Fig. 2: Input, output voltage and current waveforms of three-phase half-wave controlled converter with RL load.

The average output voltage is controlled with the help of firing angle (α), from positive maximum to zero. Unlike the reference point of α, being the zero-crossover instant for single-phase system, here the reference or neutral point NP is the crossover point of voltage value of two phases as shown in Fig. 2. Therefore, the NP (neutral point) for phase A, B and C are ωt = π /6, 5 π /6 and 9 π /6 respectively. Since there are three voltage pulses, NP for each phase is displaced by 120°.

3 Phase Converter Operation:

The operation of three-phase half-wave controlled converter with R_L load consists of three modes:

Mode 1:

At instant ωt = (π/6 + α) or 30° + α, the SCR T₁ triggered, the phase voltage V_AN reaches to the load. The thyristor acts as a mechanical switch and V₀ is same as V_AN. As the SCR T₁ is turned ON, the conducting thyristor T₂ is turned OFF due to line commutation.

In this mode of operation, the load voltage V₀ and load current I₀ both positive, so the inductive load will store energy. After some time at ωt = π, the supply voltage V_AN goes through zero, and after π it becomes negative. The conducting thyristor T₁ try to turn-off due to reverse voltage appeared across the thyristor T₁. But due to stored energy in inductive load, the thyristor T₁ continues to conduct in negative half cycle for some period.

Mode 2:

At instant ωt = (5π /6 + α) or (150° + α), SCR T₂ is triggered, the load current immediately transferred from T₁ to T₂. The load voltage is equal to phase voltage V_BN i.e. V₀ = V_BN. Since the conduction of thyristor T₂ continues upto 9π /6 + α, even negative voltage reaches to load (as commutation does not take place at ωt = π due to stored energy in inductive load).

SCR T₂ commutate at ωt = 9π /6 + α

Mode 3:

At instant ωt = (9π /6 + α) or (330° + α), the SCR T₃ is triggered, the line voltage V_CN appears across the load. This mode of operation is very similar to previous two modes.

Formulas of 3 Phase Converter

Average value of output voltage

\[{{\text{V}}_{\text{o}}}=\frac{\text{3}\sqrt{3}{{\text{V}}_{\text{m}}}}{\text{2 }\text{ }\!\!\pi\!\!\text{ }\text{ }}\cos \alpha \]

RMS value of output voltage

\[{{\text{V}}_{\text{rms}}}=\sqrt{3}{{\text{V}}_{\text{m}}}{{\left( \frac{\text{1}}{\text{ }6\text{ }}\text{ }+\text{ }\frac{\sqrt{3}}{\text{ 8}\text{ }\text{ }\!\!\pi\!\!\text{ }\text{ }}\text{cos 2 }\alpha  \right)}^{\text{1/2}}}\]

Difference between 3 Phase Converter and 1 Phase Converter

The post What is 3 Phase Converter? Types, Working & Circuit Diagram appeared first on ElectricalWorkbook.

(a) Structure.

(b) Equivalent structure.

(c) Equivalent circuit

Fig. 1: Two transistor model of an SCR.

Two transistor model of an SCR is obtained by splitting two middle layers into two separate parts as shown in Fig. 1 (b). The corresponding symbolic representation of two transistor model of an SCR is shown in Fig. 1 (c). It may be noted that the anode is connected to the emitter of a PNP transistor and the cathode is connected to the emitter of an NPN transistor. The collector of each transistor is connected to the base of other transistor. Both the transistors are connected in common base configuration. A current loop is formed in the inner layers of the SCR and this loop is responsible for regenerative feedback action.

Two Transistor Model of SCR Operating principle

Fig. 2: Equivalent circuit for two transistor model of an SCR.

The operating principle of an SCR, i.e. how the SCR turns ON by the internal regenerative feedback, can be best understood by two transistor model of an SCR.

The electrical equivalent circuit for two transistor model of an SCR is shown in Fig. 2. The SCR is forward biased by applying a supply voltage V_S. Assume that the supply voltage is less than the breakover voltage V_BO, i.e. V_S < V_BO and the SCR is not triggered. As both the transistors Q₁ and Q₂ are connected in CB configuration, the collector current I_C is given by

\[{{\text{I}}_{\text{C}}}=\text{  }\!\!\alpha\!\!\text{  }{{\text{I}}_{\text{E}}}+\text{ }{{\text{I}}_{\text{CBO}}}\]

A small gate current will increase the loop currents, i.e. base currents and collector currents (I_B1, I_B2, I_C  and I_C2) of both transistors cumulatively so that the anode through the SCR will increase sharply. This can be explained as below:

By Kirchhoffs current law (KCL), the anode current can be written as

\[{{\text{I}}_{\text{a}}}=\text{ }{{\text{I}}_{\text{C1}}}+\text{ }{{\text{I}}_{\text{C2}}}\]

Using CB configuration for both transistors, we get

\[{{\text{I}}_{\text{C1}}}=\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}{{\text{I}}_{\text{E1}}}+\text{ }{{\text{I}}_{\text{CBO1}}}\]

\[{{\text{I}}_{\text{C2}}}=\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{E2}}}+\text{ }{{\text{I}}_{\text{CBO2}}}\]

where α₁ and α₂ are the current gains for transistors Q₁ and Q₂. I_CBO1 and I_CBO2 are the reverse saturation currents for transistors Q₁ and Q₂.

The anode current can be written as

\[{{\text{I}}_{\text{a}}}=\text{ }\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}{{\text{I}}_{\text{E1}}}+\text{ }{{\text{I}}_{\text{CBO1}}} \right)\text{ }+\text{ }\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{E2}}}+\text{ }{{\text{I}}_{\text{CBO2}}} \right)\]

We know that I_E1 = I_a and I_E2 = I_k.

\[{{\text{I}}_{\text{a}}}=\text{ }\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}{{\text{I}}_{\text{a}}}+\text{ }{{\text{I}}_{\text{CBO1}}} \right)\text{ }+\text{ }\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{k}}}+\text{ }{{\text{I}}_{\text{CBO2}}} \right)… (1)\]

By applying KCL, cathode current can be written as

\[{{\text{I}}_{\text{k}}}=\text{ }{{\text{I}}_{\text{B2}}}+\text{ }{{\text{I}}_{\text{C2}}}\]

As

\[{{\text{I}}_{\text{B2}}}=\text{ }{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{C1}}}\]

\[{{\text{I}}_{\text{k}}}=\text{ }\left( {{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{C1}}} \right)\text{ }\!\!~\!\!\text{ }+\text{ }{{\text{I}}_{\text{C2}}}\]

Also

\[{{\text{I}}_{\text{a}}}=\text{ }{{\text{I}}_{\text{C1}}}\text{ }\!\!~\!\!\text{ }+\text{ }{{\text{I}}_{\text{C2}}}\]

\[{{\text{I}}_{\text{k}}}=\text{ }{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{a}}}… (2)\]

Substituting equation (2) in equation (1), we get

\[{{\text{I}}_{\text{a}}}=\text{ }\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}{{\text{I}}_{\text{a}}}+\text{ }{{\text{I}}_{\text{CBO1}}} \right)\text{ }+\text{ }\left[ {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}\left( {{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{a}}} \right)\text{ }+\text{ }{{\text{I}}_{\text{CBO2}}} \right]\]

\[{{\text{I}}_{\text{a}}}=\text{ }\left[ {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}{{\text{I}}_{\text{a}}}+\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{a}}} \right]\text{ }+\text{ }\left[ {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{CBO1}}}+\text{ }{{\text{I}}_{\text{CBO2}}} \right]\]

Also

\[{{\text{I}}_{\text{a}}}-\text{ }{{\text{I}}_{\text{a}}}\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}+\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}} \right)\text{ }=\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{CBO1}}}+\text{ }{{\text{I}}_{\text{CBO2}}}\]

where I_CBO = I_CBO1 + I_CBO2 is the total reverse leakage current in collector-base junction.

Thus

\[{{\text{I}}_{\text{a}}}-\text{ }{{\text{I}}_{\text{a}}}\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}+\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}} \right)\text{ }=\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{CBO}}}\]

Thus

\[{{\text{I}}_{\text{a}}}=\text{ }\frac{{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}}{{\text{I}}_{\text{g}}}+\text{ }{{\text{I}}_{\text{CBO}}}}{1-\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}+\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}} \right)}\]

where (α₁ + α₂) is the loop gain.

In silicon transistors, the current gain α depends upon the emitter current and its value is approximately or less than 0.5. The SCR is made up of silicon, the current gain α is about 0.5 at low emitter current. When the SCR is not triggered, i.e. I_g = 0, the emitter current is negligibly small and hence α₁ and α₂ are very small, i.e. (α₁ + α₂) < 1.

So, the anode current can be written as

\[{{\text{I}}_{\text{a}}}=\text{ }\frac{{{\text{I}}_{\text{CBO}}}}{1-\left( {{\text{ }\!\!\alpha\!\!\text{ }}_{\text{1}}}+\text{ }{{\text{ }\!\!\alpha\!\!\text{ }}_{\text{2}}} \right)}…(3)\]

As (α₁ + α₂) is less than unity, the anode current will be negligibly small. The SCR does not conduct the current and remains OFF. This is a forward blocking state. When the forward biased SCR is triggered by applying a positive gate current pulse, the emitter currents in both transistors increase the current gains α₁ and α₂. When the loop gain (α₁ + α₂) increases and reaches the unity, the anode current increases rapidly to a high value as per equation (3). The SCR now conducts the current and turns it ON. This is a forward conduction (ON) state. Thus the SCR can be turned ON by passing a positive gate current so that the loop gain (α₁ + α₂) approaches to unity.

The post What is the Two Transistor Model (Analogy) of SCR (Thyristor)? Derivation, Diagram & Working appeared first on ElectricalWorkbook.

Fig. 1: Single Phase Half Wave Controlled Rectifier Circuit Diagram.

Fig. 1 (a) shows the simple circuit of Single phase half wave controlled rectifier with resistive load. The supply voltage is AC (V_S = V_m sin ωt) as shown in Fig. 1 (a). If positive gate voltage is applied to gate-cathode junction and anode voltage is positive then only SCR conducts.

Single Phase Half Wave Controlled Rectifier Working:

Fig. 2: Single Phase Half Wave Controlled Rectifier Waveforms.

The operation of this converter is in two modes.

Mode 1 :(α to π):

During positive half cycle of AC in put voltage (see Figure 2), the thyristor anode is more positive with respect to cathode and thyristor conducts (said to be forward biased). Thyristor fired (conduct) at ωt = α (α is called as delay angle or firing angle), the load is directly connected to supply i.e. V_o = V_S. Thyristor conducts upto ωt = π and commutation takes place at ωt = π. The average value of output voltage and current is positive. The converter operates in the first quadrant of V₀ – i₀ characteristics.

Mode 2: (π to π + α):

During negative half-cycle of AC input voltage, the cathode of thyristor is more positive with respect to anode, the thyristor is said to be reverse biased condition (OFF state). Therefore the average average value of output voltage V_o = 0.

For resistive load, the current i_o is in phase with V_o as shown in Fig. 2. Firing angle of a thyristor is measured from the instant at which SCR starts conducting. If thyristor is replaced by diode, it would begin conduction at ωt = 0, 2π, 4 π etc., firing angle is measured from this instant. “A Firing Angle is defined as the Instant at which SCR Conducts”.

Fig. 2 shows the output and input current waveforms, where the thyristor conducts from ωt = α to π, (2π + α) to 3 π and so on. Over the firing angle delay α, load voltage V₀ = 0 but during conduction angle (π – α), V_o = V_S. As firing angle is increased from 0 to π, the average load voltage decreases from the largest value to zero.

Formulas of Single phase half wave controlled rectifier

Average value of output voltage

\[{{\text{V}}_{\text{o}}}=\frac{{{\text{V}}_{\text{m}}}}{\text{2 }\!\!\pi\!\!\text{ }}\left[ 1+\cos \alpha  \right]\]

RMS value of output voltage

\[{{\text{V}}_{\text{rms}}}=\frac{{{\text{V}}_{\text{m}}}}{\text{2}}{{\left[ \frac{\text{1}}{\text{ }\!\!\pi\!\!\text{ }}\left( \text{ }\!\!\pi\!\!\text{ }-\text{ }\!\!\alpha\!\!\text{ }+\frac{\text{sin2 }\!\!\alpha\!\!\text{ }}{\text{2}} \right) \right]}^{\text{1/2}}}\]

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Circuit Detail of Natural Commutation (or Line Commutation)

Fig. 1: Natural commutation Circuit diagram.

Fig. 1 shows the circuit diagram of a single-phase controlled rectifier using SCR T. The SCR T is triggered into the positive half cycles of the supply voltage. The firing angle α of a triggered circuit can be controlled from 0° to 180°. The resistive load RL is connected in series with the SCR T.

Principle of Operation of Natural Commutation (or Line Commutation)

Suppose an SCR T is triggered at the firing angle α by the external trigger circuit and it is conducting the load current during the interval from α to (π – α). At the end of positive half cycle, i.e. when the anode current is zero and the SCR goes to reverse biased, the SCR automatically turns OFF due to natural behaviour of line (a.c.) voltage.

Fig. 2: Natural commutation Waveforms.

The input and output waveforms for natural commutation of SCR Fig. 2 (i) and (ii) respectively. Again during the positive half-cycle of line (a.c.) voltage, the SCR is triggered and the SCR is naturally turned OFF, when the anode current of an SCR goes to zero at the end of every positive half-cycle and the reverse voltage appears across it. The SCR can be naturally turn-OFF by applying the line (a.c.) voltage without using any trigger circuit. Thus, the natural reversal of a.c. supply voltage turns-OFF the conducting SCR.

Advantages of Natural Commutation (or Line Commutation)

The advantages of natural (or line) commutation of an SCR are as under:

1. It is very simple.
2. It is very reliable method.

Applications of Natural Commutation (or Line Commutation)

The applications of natural (i.e. line) commutation of an SCR are as under:

1. It is used in a.c. to d.c. converters.
2. It is used in a.c. regulators.
3. It is used in cycloconverters.
4. It can be used in light dimmers.
5. It can be used in fan speed regulators

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Construction of Silicon Controlled Switch

Fig. 1 (a) shows the basic structure of a SCS.

(a) Basic structure

(b) Symbol

Fig. 1: Construction of SCS

It consists of four semiconductor layers PNPN. It has four terminals such as anode A, cathode K, anode gate G_A and cathode gate G_K. It has three junctions. Its construction is similar to that of a SCR. The SCS can also be manufactured using planer technique but in that case the anode is a ring of P-type diffused material surrounding all NPN transistor. The circuit symbol of a SCS is as shown in Fig. 1 (b). It is available at low power ratings.

Principle of Operation of Silicon Controlled Switch

The equivalent circuit with two transistor analogy of a SCS is as shown in Fig. 2.

Fig. 2: Operation of SCS

The SCS is considered as consisting of a complementary pair of transistor with regenerative feedback. The operation of SCS is like a SCR about it can be turned ON by either a positive pulse at gage G_K or a negative pulse of gate G_A. Its construction is similar to that of a SCR.

The SCS is forward biased by applying a positive voltage at anode A with respect to cathode and no voltage is applied at the gates G_K and G_A. As the gate current is zero, both the transistors will be OFF. If a positive pulse is applied at the gate G_K, the gate current will flow through the base of transistor Q₁. Due to regenerative feedback action, the transistors, Q₁ and Q₂ are driven into saturation and heavy current will flow through the device from anode to cathode. This is forward ON (conduction) state of a SCS. The SBS can also be turned ON by applying a negative pulse at the anode gate G_A. Thus the SCS can be turned ON by either a positive pulse at gate G_K or a negative pulse at gate G_A. The SCS can be turned OFF from its ON state by applying a positive pulse at anode G_A or a negative pulse at cathode gate G_K. Once the SCS is turned ON, it behaves like an SCR. The SCS can be turned OFF like SCR by reducing anode current below the holding current I_H.

V-I Characteristics of Silicon Controlled Switch

The V-I characteristics of a SCS is shown in Fig. 3.

Fig. 3: V-I characteristic of SCS

Under forward biased condition, when a positive pulse voltage at the gate G_K or a negative gate pulse voltage at the gate G_A is below the breakover voltage, the SCS does not conduct the current. But the reverse saturation (leakage) current flows through the SCS. The SCS is now in forward blocking (OFF) state. Under forward biased condition, when a positive pulse voltage at the gate G_K or a negative pulse voltage at the gate G_A exceeds the breakover voltage, the SCS starts conducting current through it from anode to cathode. The anode current rises suddenly but the voltage across it drops to low voltage about 1 V. This is a forward conduction (ON) state of a SCS. The forward OFF and ON states are shown in Fig. 3.

Advantages of Silicon Controlled Switch

The advantages of SCS over SCR are as given below:

1. It has fast turn-off.
2. It can be turned-off with positive or negative pulse at either gate.

Applications of Silicon Controlled Switch

Some of the important applications of SCS are as given below:

1. It is used mainly in low power sensing circuits.
2. It is used in timers, registers and counters.
3. It is used in digital logic circuits.
4. It is used in pulse generators.
5. It is used in oscillators.

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