1. UNIT 3
THYRISTORS
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2. Topic details INTRODUCTION I-V CHARACTERSITICS TWO TRANSISTOR ANALOGY
GATE CHARACTERISTICS
THYRISTOR FIRING CIRCUITS
THYRISTOR PROTECTION
APPLICATIONS
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3. INTRODUCTION
SCR is a two state device used for efficient power control .
An SCR is similar to conventional rectifier but controlled by a gate signal.
It is a 4-layered 3-terminal device (Anode , Cathode , Gate)with alternate PNPN structure .
When the gate to cathode voltage exceeds a certain threshold, the device turns 'on' and conducts
current.
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4. VOLT/AMPERE CHARACTERISTICS
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5. VOLT/AMPERE CHARACTERISTICS(Contn.,)
Anode-to-Cathode Characteristics
Forward characteristics:
Blocking state
Avalanche region
Breakover point
Negative resistance region
Conducting state
Holding current point
Reverse characteristics:
Blocking region
Critical parameters
IT (On-state Current)
VRRM (Repetitive Peak Reverse Voltage)
VDRM (Repetitive Peak Off-state Voltage)
IH (Holding current)
IGT (Gate Trigger Current)
Caution About Gate Current and Voltage
Gate current must be limited to the rated value to avoid damage to device
Reverse gate voltage must be limited to the rated value to avoid damage to device
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6. MODES OF OPERATION
The operation of a SCR can be understood in terms of a pair of tightly coupled Bipolar Junction Transistors
SCR has three states:
Reverse blocking mode, forward blocking mode and forward conducting mode
Forward blocking mode : When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place.
Reverse blocking mode : When cathode is more positive with respect to anode junctions J1 and J3 are reversed biased, while junction J2 is forward biased hence SCR do not carry any current except for the reverse leakage current.
Forward conducting mode : Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting.
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.
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7. TWO TRANSISTOR ANALOGY
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8. SCR GATE CHARACTERISTICS
For SCR to turn on satisfactorily the gate current and gate voltage should be
Ig (min) < Ig < Ig (max)
Vg (min) < Vg < Vg (max)
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9. GATE TRIGGER METHODS
Different Methods for Gate Triggering are as follows:
DC triggering
AC triggering
Pulse triggering
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10. THYRISTOR FIRING CIRCUITS – RESISTANCE TRIGGERING
In this method, the variable resistance R is used to control the gate current.
Depending upon the value of R, when the magnitude of the gate current reaches the sufficient value the SCR starts to conduct.
The diode D is called as blocking diode. It prevents the gate cathode junction from getting damaged in the negative half cycle. 
By considering that the gate circuit is purely resistive, the gate current is in phase with the applied voltage.
By using this method we can achieve maximum firing angle up to 90°.
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11. THYRISTOR FIRING CIRCUITS – RESISTANCE CAPACITANCE TRIGGERING
By using this method firing angle can be achieved for more than 90°.
In the positive half cycle, the capacitor is charged through the variable resistance R up to the peak value of the applied voltage.
The variable resistor R controls the charging time of the capacitor.
Depends upon the voltage across the capacitor, when sufficient amount of gate current will flow in the circuit, the SCR starts to conduct.
In the negative half cycle, the capacitor C is charged up to the negative peak value through the diode D2.
Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half cycle.
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12. dv/dt is the rate of change of voltage in SCR.
THYRISTOR PROTECTION
dv/dt Protection:-
dv/dt is the rate of change of voltage in SCR.
As iC=C.dv/dt. ie, when dv/dt is high, iC is high.
This high current(iC) may turn ON SCR even when gate current is
zero. This is called as dv/dt turn ON or false turn ON of SCR.
To protect the thyristor against false turn ON or against high dv/dt a
"Snubber Circuit" is used
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13. APPLICATIONS
Thyristors are mainly used in single phase converters and AC voltage controllers and the family of devices finds its use in UPS, Industrial applications such as welding, heating etc., HVDC systems and as power supplies for communication systems, telephone exchangea nd satellite systems.
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