Thyristor is a four layer three junction pnpn semiconductor switching device. It has 3 terminals these are anode, cathode and gate. SCRs are solid state device, so they are compact, possess high reliability and have low loss. SCR is made up of silicon, it act as a rectifier; it has very low resistance in the forward direction and high resistance in the reverse direction. It is a unidirectional device.
Anode and cathode are connected to main source voltage through the load. The gate and cathode are fed from source ES. A typical SCR V-I characteristic is as shown below:
V-I characteristic of SCR
VBO=Forward breakover voltage
VBR=Reverse breakover voltage;Ig=Gate current
Va=Anode voltage across the thyristor terminal A,K; Ia=Anode current
It can be inferred from the static V-I characteristic of SCR.
SCR have 3 modes of operation:
- Reverse blocking mode
- Forward blocking mode ( OFF state)
- Forward conduction mode (ON state)
Reverse Blocking Mode
When cathode of the thyristor is made positive with respect to anode with switch open thyristor is reverse biased. Junctions J1 and J2 are reverse biased where junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them.
- A small leakage current of the order of few mA only flows. As the thyristor is reverse biased and in blocking mode. It is called as acting in reverse blocking mode of operation.
- Now if the reverse voltage is increased, at a critical breakdown level called reverse breakdown voltage VBR,an avalanche occurs at J1 and J3 and the reverse current increases rapidly. As a large current associated with VBR and hence more losses to the SCR.
- This results in Thyristor damage as junction temperature may exceed its maximum temperature rise.
Forward Blocking Mode
- When anode is positive with respect to cathode, with gate circuit open, thyristor is said to be forward biased.
- Thus junction J1 and J3 are forward biased and J2 is reverse biased. As the forward voltage is increases junction J2 will have an avalanche breakdown at a voltage called forward breakover voltage VBO.
- When forward voltage is less then VBO thyristor offers high impedance. Thus a thyristor acts as an open switch in forward blocking mode.
Forward Conduction Mode
Here thyristor conducts current from anode to cathode with a very small voltage drop across it. So a thyristor can be brought from forward blocking mode to forward conducting mode:
- By exceeding the forward breakover voltage.
- By applying a gate pulse between gate and cathode.
During forward conduction mode of operation thyristor is in on state and behave like a close switch. Voltage drop is of the order of 1 to 2mV. This small voltage drop is due to ohmic drop across the four layers of the device.
Turn ON methods for SCR
- Forward voltage triggering
- Gate triggering
- dv/dt triggering
- Light triggering
- Temperature triggering
Forward voltage triggering
A forward voltage is applied between anode and cathode with gate circuit open.
- Junction J1 and J3 is forward biased.
- Junction J2 is reverse biased.
- As the anode to cathode voltage is increased breakdown of the reverse biased junction J2 occurs. This is known as avalanche breakdown and the voltage at which this phenomena occurs is called forward breakover voltage.
- The conduction of current continues even if the anode cathode voltage reduces below VBO till Ia will not go below Ih. Where Ih is the holding current for the thyristor.
This is the simplest, reliable and efficient method of firing the forward biased SCRs. First SCR is forward biased. Then a positive gate voltage is applied between gate and cathode. In practice the transition from OFF state to ON state by exceeding VBO is never employed as it may destroy the device. The magnitude of VBO, so forward breakover voltage is taken as final voltage rating of the device during the design of SCR application.
- Make the thyristor forward breakover voltage (say 800V) higher than the normal working voltage. The benefit is that the thyristor will be in blocking state with normal working voltage applied across the anode and cathode with gate open.
- When we require the turning ON of a SCR a positive gate voltage between gate and cathode is applied.
- The point to be noted that cathode n- layer is heavily doped as compared to gate p-layer. So when gate supply is given between gate and cathode gate p-layer is flooded with electron from cathode n-layer. Now the thyristor is forward biased, so some of these electron reach junction J2 .
- As a result width of J2 breaks down or conduction at J2 occur at a voltage less than VBO.As Ig increases VBO reduces which decreases then turn ON time. Another important point is duration for which the gate current is applied should be more then turn ON time.
- If the gate current is reduced to zero before the anode current reaches a minimum value known as holding current, SCR can’t turn ON. In this process power loss is less and also low applied voltage is required for triggering.
This is a turning ON method but it may lead to destruction of SCR and so it must be avoided.
- When SCR is forward biased, junction J1 and J3 are forward biased and junction J2 is reversed biased so it behaves as if an insulator is place between two conducting plate.
- Here J1 and J3 acts as a conducting plate and J2 acts as an insulator. J2 is known as junction capacitor.
- Now if we increase the rate of change of forward voltage instead of increasing the magnitude of voltage. Junction J2 breaks and starts conducting. A high value of changing current may damage the SCR. So SCR may be protected from high dv/dt.
Ia = C(dv/dt)
Ia α dv/dt
- During forward biased, J2 is reverse biased so a leakage forward current always associated with SCR.
- Since leakage current is temperature dependant,so if we increase the temperature the leakage current will also increase and heat dissipation of junction J2 occurs. When this heat reaches a sufficient value J2 will break and conduction starts.
- This type of triggering causes local hot spot and may cause thermal run away of the device.
- This triggering cannot be controlled easily.
- It is very costly as protection is costly.
First a new recess niche is made in the inner p-layer. When this recess is irradiated, then free charge carriers (electron and hole) are generated. Now if the intensity is increased above a certain value then it leads to turn ON of SCR. Such SCR are known as Light activated SCR (LASCR).
Latching current: The latching current may be defined as the minimum value of anode current which at must attain during turn ON process to maintain conduction even if gate signal is removed.
Holding Current:It is the minimum value of anode current below which if it falls, the SCR will turn OFF.
Switching characteristics of Thyristors
The time variation of voltage across the thyristor and current through it during turn on and turn off process gives the dynamic or switching characteristic of SCR.
Switching characteristic during turn on:
Turn on time: It is the time during which it changes from forward blocking state to ON state. Total turn on time is divided into 3 intervals.
Delay time If Ig and Ia represent the final value of gate current and anode current. Then the delay time can be explained as time during which the gate current attains 0.9 Ig to the instant anode current reaches 0.1 Ig or the anode current rises from forward leakage current to 0.1 Ia.
- 1. Gate current 0.9Ig to 0.1 Ia.
- Anode voltage falls from Va to 0.9Va.
- Anode current rises from forward leakage current to 0.Ia.
Rise time (tr): Anode current rises from 0.1Ia to 0.9Ia. Forward blocking voltage falls from 0.9Va to 0.1Va.Va is the initial forward blocking voltage.
Spread time (tp):Time taken by the anode current to rise from 0.9Ia to Ia.Time for the forward voltage to fall from 0.1Vo to on state voltage drop of 1 to 1.5V. During turn on, SCR is considered to be a charge controlled device.
Switching Characteristics During Turn Off
Thyristor turn off means it changed from ON to OFF state. Once thyristor is ON there is no role of gate. As we know thyristor can be made turn OFF by reducing the anode current below the latching current. Here we assume the latching current to be zero ampere. If a forward voltage is applied across the SCR at the moment it reaches zero then SCR will not be able to block this forward voltage.
So now the turn off time can be different as the instant anode current becomes zero to the instant when SCR regains its forward blocking capability.
tq is the turn off time,trr is the reverse recovery time, tqr is the gate recovery time.
The turn off time is affected by:
- Junction temperature
- Magnitude of forward current di/dt during commutation.
- Turn off time decreases with the increase of magnitude of reverse applied voltage.
Over Voltage Protection: Over voltage occurring during the switching operation causes the failure of SCR.
Internal Overvoltage Protection: It is due to the operating condition of SCR.During the commutation of SCR ,when the anode current decays to zero anode current reverses due to stored changes. First the reverse current rises to peak value, then reverse current reduces abruptly with large di/dt. During series inductance of SCR large transient large voltage i.e Ldi/dt. is generated.
External Overvoltage: This is due to external supply and load condition. This is because of
- The interruption of current flow in an inductive circuit.
- Lightening strokes on the lines feeding the thyristor systems.
This overvoltages cause random turn ON of a SCR. The effect of overvoltage is minimized using
- RC circuits
- Non linear resistor called voltage clamping device.
When voltage surge appears voltage clamping device offers a low resistance and it create a virtual short circuit across the SCR. Hence voltage across SCR is clamped to a safe value. When surge condition over voltage clamping device returns to high resistance state. e.g. of voltage clamping device
- Seleniumthyrector diodes
- Metal Oxide varistors
- Avalanche diode supressors
Over Current Protection: Long duration operation of SCR, during over current causes junction temp. of SCR to rise above the rated value,causing permanent damage to device.
SCR is protected from overcurrent by using
- Circuit breakers
- Fast acting fuses Proper co-ordination is essential because Fault current has to be interrupted before SCR gets damaged.
- Only faulty branches of the network has to be replaced.
Electronic Crowbar Protection
For overcurrent protection of power converter using SCR, electronic crowbar are used. It provide rapid isolation of power converter before any damage occurs.
HEAT PROTECTION- To protect the SCR
1. From the local spots 2. Temp rise SCRs are mounted over heat sinks.
GATE PROTECTION-Gate circuit should also be protected from
1. Overvoltages 2. Overcurrents
Overvoltage across the gate circuit causes the false triggering of SCR Overcurrent raise the junction temperature. Overvoltage protection is by zener diode across the gate circuit.
Series and Parallel operation of SCR
SCR are connected in series for h.v demand and in parallel for fulfilling high current demand. Sting efficiency can be defined as measure of the degree of utilization on SCRs in a string.
String efficiency < 1.
- Derating factor (DRF) = 1 – string efficiency.
- If DRF more then no. of SCRs will more, so string is more reliable.
Series Operation: Let the rated blocking voltage of the string of a series connected SCR is 2V1 as shown in the figure below, But in the string two SCRs are supplied a maximum voltage of V1+V2.
Significance of String Efficiency
- Two SCRs are have same forward blocking voltage ,When system voltage is more then the voltage rating of a single SCR.
- SCRs are connected in series in a string. There is a inherent variation in characteristics.
- The voltage shared by each SCR may not be equal. Suppose, SCR1 leakage resistance > SCR2 leakage resistance. For same leakage current I0 in the series connected SCRs. For same leakage current SCR1 supports a voltage V1, SCR2 supports a voltage V2.
So string η for two SCRs = V1+V2/ 2V2
= 1/2(1+V2/V1) which is < 1
so V1 > V2
- The above operation is when SCRs are not turned ON. But in steady state of operation,A uniform voltage distribution in the state can be achieved by connect a suitable resistance across each SCRs , so that parallel combination have same resistance.
- But this is a cumbersome work. During steady state operation we connect same value of shunt resistance across each SCRs. This shunt resistance is called state equalizing circuit.
Let SCR1 has lower leakage current Ibm , It will block a voltage comparatively larger than other SCRs.
- Voltage across SCR1 is Vbm =I1R.
- Voltage across (n-1)SCR is (n-1) I2R,
- The voltage equation for the series circuit is
Vs = I1R+(n−1)I2R =Vbm+ (n-1)R(I − Ibmx)
as I1 = I− Ibmn
So, Vs = Vbm+(n-1)R[I1−(Ibmx−Ibmn)]
If ΔIb= Ibmx−Ibmn
Then Vs = Vbm+(n-1)R (I1 −ΔIb)
Vs = Vbm+ (n-1)R I1-(n-1)R ΔIb
So, Vs = Vbm+ (n-1) Vbm -(n-1)R ΔIb
= n Vbm- (n-1)R ΔIb
- SCR data sheet usually contain only maximum blocking current , Ibmx.
so we assume Ibmn= 0
So the value of R calculated is low than actually required.
When current required by the load is more than the rated current of single thyristor , SCRs are connected in parallel in a string.
For equal sharing of current, SCRs must have same V-I characteristics during forward conduction.VT across them must be same. For same VT,SCR1 share I1 and SCR2 share I2. If I1 is the rated current I2< I1 The total current I1+I2 and not rated current 2I1.Type equation here.
Thus string efficiency ,
- Middle conductor will have more inductance as compared to other two nearby conductor. As a result less current flow through the middle conductor. Another method is by magnetic coupling.
Thyristor Gate Characteristics
- Vg = +ve gate to cathode voltage. Ig= +ve gate to cathode current. As the gate cathode characteristic of a thyrister is a p-n junction.
- Curve 1 the lowest voltage values that must be applied to turn on the SCR.
- Curve 2 highest possible voltage values that can be safely applied to get circuit.
- Vgm= Maximum limit for gate voltage ;
- Igm= Maximum limilt for gate current.
- Pgav = Rated gate power dissipation for each SCR.
- These limits should not be crossed in order to avoid the permanent damage of the device junction J3.
- OY = Minimum limit of gate voltage to turn ON . OX = minimum limit of gate current to turn ON.
- If Vgm, Igm,Pgav are exceeded the thyristor will damage so the preferred gate drive area of SCR is bcdefghb.
- oa = The non triggering gate voltage , If firing circuit generates +ve gate signal prior to the desired instant of triggering the SCR.It should be ensured that this un wanted signal should be less than the non–triggering voltage oa.
- Es = Vg+IgRs Gate Source Voltage; Vg = Gate Cathode Voltage.
- Rs = Gate source resistance
- If Igmn , Vgmn are the minimum gate current and gate voltage to turn ON the SCR.
Es = (Igmn+ Vgmn/R1)Rs +Vgmn.
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