"Power Electronics & Drives : Power Semiconductor Devices
1. HISTORY OF POWER SEMICONDUCTOR DEVICES
The first semiconductor device used in power circuits was the electrolytic rectifier - an early version was described by a French experimenter, A. Nodon, in 1904. These were briefly popular for the application of industries using aluminium sheets, and in domestic household appliances because they can withstand low voltages and have good efficiency.
The first solid-state power semiconductor devices were copper oxide rectifiers, used in early battery chargers and power supplies for radio equipment, announced in 1927 by L.O. Grundahl and P. H. Geiger.
The first germanium power semiconductor device appeared in 1952 with the introduction of the power diode by R.N. Hall. It had a reverse voltage blocking capability of 200 V and a current rating of 35 A.
Later, germanium bipolar transistors with substantial power handling capabilities (up to 100 mA collector current) were introduced around 1952. Silicon power transistors were introduced in 1957 and had made a big change in the industries because silicon power devices have better frequency response than germanium devices and could operate up to 150⁰C junction temperature.
In 1957, thyristors were appeared & is able to withstand very high reverse breakdown voltage and is also capable of carrying high current. However, one disadvantage of the thyristor in switching circuits is that once it becomes 'latched-on' in the conducting state; it cannot be turned off by external control, as the thyristor turn-off is passive, i.e., the power must be disconnected from the device. Thyristors which could be turned off without any external control mechanism are called gate turn-off thyristors (GTO) which were introduced in late 1960’s. These overcome some limitations of the ordinary thyristor, because they can be turned on or off with an applied gate signal.
- POWER ELECTRONICS & ITS APPLICATION
Power Electronics is a subject that concerns with the application of electronic principles into situations that are rated at power level rather than signal level.
In the modern era, power electronics has various application such as
- Commercial – Uninterruptible power supply (UPS)
- Aerospace – Aircraft power systems
- Industrial –Textile mills, cement mills, welding
- Residential –Personal computers, vacuum cleaners
- Transportation-Street cars, trolley buses.
- Utility systems-HVDC, static circuit breakers.
2.1. Basic Power Electronic Circuit Block Diagram:
The figure below shows a basic power electronic system. The output of the power electronic circuit may be variable DC/ variable AC voltage/ variable frequency. The feedback component measures parameters of load like speed in case of a rotating machine. The difference between the target speed and measured speed controls the behavior of power electronic circuit.
Figure 1: Block diagram of a typical power electronic system
- POWER SEMI-CONDUCTOR DEVICES
Power semi-conductor devices should ideally constitute following characteristics:
- Must be able to carry large current.
- ON resistance should be lower (ideally 0) to reduce heat dissipation.
- OFF resistance should be higher (ideally ∞) to withstand switching transients.
- They must carry large currents with uniform distribution of current over device’s area to avoid localized heating and breakdown.
- Device should be capable of high switching speed.
- Faulty switching due to applied voltage transient should not happen.
3.1. Classification of Power Semi-conductor devices based on Operating Characteristics:
3.1.1. Uncontrolled Devices:
Uncontrolled devices are the power semi-conductor devices whose V-I characteristics cannot be controlled. Their on and off states are controlled by power supply. These are typically used in uncontrolled rectifiers. For Example: Power diodes.
3.1.2. Fully Controlled Devices:
These devices can be switched ON/OFF by using a control signal. For Example: Power transistors, MOSFET’s.
3.1.3. Semi-Controlled Devices:
These devices can be partially controlled using a control signal. For Example: SCR can be turned ON using gate signal but can’t be turned OFF.
3.2. Classification of Power Semi-conductor devices based on Polarity & Direction of Current:
3.2.1. Unipolar switch:
The switch can block only one polarity of voltage when it is in OFF state.
3.2.2. Bipolar switch:
This switch can block both polarity of voltage when it is in blocking state.
3.2.3. Unidirectional switch:
This switch can carry current in only one direction when it is in conduction state.
3.2.4. Bidirectional switch:
This switch can carry current in both the direction when it is in conduction state.
- POWER SEMI-CONDUCTOR DIODE
Power diodes belong to the class of uncontrolled power semiconductor devices. They are like p-n junction diodes but having large voltage and current rating. It has 3-layer diode which makes it suitable for high power application as they are constructed with n- layer between p+ and n+ layers to support large blocking voltage by controlling the width of depletion region.
4.1. Basic Structure and Symbol of the Power Diode:
Figure 2: Constructional Structure of Power diode
Figure 3: Symbol of Power diode
4.2. Transfer Characteristics of a Power Diode:
When anode is positive with respect to cathode, diode is forward biased. When forward voltage across diode is slowly increased from 0 to cut-in voltage, diode current is almost zero. Above cut-in voltage, the diode current rises rapidly and the diode is said to conduct. When anode is negative with respect to cathode, diode is reverse biased.
Figure 4: Transfer characteristics of power diode
4.3. Reverse recovery characteristics of Diode:
4.3.1. Reverse recovery time (trr):
When a diode is changed from forward biased state to reverse biased state, the diode continues to conduct in the reverse direction because of stored charges in two layers. The reverse current flows for reverse recovery time, trr. The reverse recovery time is defined as the time between the instant forward current becomes zero and the instant reverse recovery current decays to 25% of reverse peak value, IRM.
Reverse recovery time, trr = ta + tb
Where ta is time for diode current to reach IRM from 0,
tb is the time for diode current to reach IRM from IRM.
In time ta, charge in depletion region is removed, hence the current through diode decays thereafter. During tb, charge from two semiconductor layers is removed.
4.3.2. Softness factor (S):
Softness factor (S) is defined as ratio of tb and ta. Softness factor is a measure of voltage transients that occur during the time diode recovers.
Class of diode
Nature of voltage
Soft recovery diode
Less voltage transient
Fast recovery diode
High voltage transient
Table 1: Classification of diode based on softness factor
4.3.3. Classification of Power Diodes:
Power diodes can be classified as below based on their use case. This classification is different from that of classification based on softness factor.
- General Purpose Diodes: These diodes have high reverse recovery time, trr. Application of this type of diodes include battery charging electric traction and uninterruptible power supplies (UPS).
- Fast Recovery Diodes: These diodes have low trr. To get low trr, platinum or gold doping is done while manufacturing these diodes. Hence these diodes have more forward voltage drop. These diodes are mainly used in choppers, commutation circuits and switched mode power supplies (SMPS).
- Schottky Diodes: These diodes use metal to semi-conductor junction. Hence these diodes have lesser trr and lesser forward voltage drop. In these diodes, current flow is by majority carriers only and hence there is no turn off delay due to absence of minority carries combination.
4.3.4. Points to Remember in Power Semiconductor Diodes
- Power diodes are constructed with a vertically oriented structure that includes a ‘n-’ drift region to support large blocking voltages.
- The breakdown voltage is approximately inversely proportional to the doping density of the drift region, and the required minimum length of the drift region scales with the desired breakdown voltage.
- Achievement of large breakdown voltages requires special depletion layer boundary shaping techniques.
- Conductivity modulation of the drift region in the on state keeps the losses in the diode to manageable levels even for large on state currents.
- Low on-state losses require long carrier lifetimes in the diode drift region.
- Minority-carrier devices have lower on-state losses than majority-carrier devices such as MOSEFTs at high blocking voltage ratings.
- During the turn-on transient the forward voltage in a diode may have a substantial overshoot, on the order of tens of volts.
- Short turn-off times require short carrier lifetimes, so a trade-off between switching times and on-state losses must be made by the device designer.
- During turn-off, fast reverse recovery may lead to large voltage spike because of stray inductance.
- The problems with the reverse-recovery transient are most severe in diodes with large blocking voltage ratings.
- Schottky diodes turn on and off faster than p-n junction diodes and have no substantial reverse-recovery transient.
- Schottky diodes have lower on-state losses than p-n junction diodes but also have low breakdown voltage ratings, rarely exceeding 100 V.
- POWER BIPOLAR JUNCTION TRANSISTOR: (POWER BJT)
Power transistor is a current controlled device and the control current is made to flow through base terminal. Thus, the device can be switched ON or OFF by applying a positive/negative signal at base. The transistor remains in on-state if control signal is present. The need for a large blocking voltage in the off state and a high current carrying capability in the on state means that a power Bipolar junction Transistor (BJT) must have a substantially different structure than its logic level counterpart.
- POWER MOSFET
MOSFET is a voltage-controlled device. It has three terminals, called drain, source and gate. As its operation is based on flow of majority carriers only, MOSFET is unipolar device. A metal oxide semiconductor field effect transistor (MOSFET) has three terminals called drain (D), source (S) and gate (G).
As power MOSFET is unipolar device, there is no minority storage effect so that high switching speed is possible. Here switching speed is limited by inherent capacitance only. Also due to large drain area, secondary breakdown and thermal runaway that destroy the device do not occur.
The basic structure and circuit symbol of power MOSFET is,
- Generally, MOSFET are low voltage and high current devices.
- These are very popular in dc to dc conversion (choppers).
- These are very fast devices compared to BJT.
- BJT is a minority carrier device where MOSFET is a majority carrier device.
- MOSFET has a very high input impedance.
- Gate is insulated from the rest of the device.
- No steady current flows through the gate. (Only displacement current like in parallel plate capacitor will flow.)
- MOSFET is in cutoff region when gate to source voltage (VGS) is less than threshold value.
- When VGS> Threshold (VTh). It converts silicon surface below the gate into an N-type channel.
- The threshold value depends upon oxide layer and it can be reduced by reducing the thickness of SiO2
- A BJT is a current controlled device whereas a power MOSFET is a voltage-controlled device.
- The control signal, or base current in BJT is much larger than the control signal (or gate current) required in a MOSFET. This is because gate circuit impedance in MOSFET gate to be driven directly from microelectronic circuits.
SILICON CONTROLLED RECTIFIER (SCR) OR THYRISTOR
The first SCR was developed in late 1957. Silicon controlled rectifier which is shortly referred as SCR belongs to thyristor family. It’s a solid-state device like transistor. It is a four-layer three junction p-n-p-n device and has three terminals; anode, cathode and gate.
SCR can be turned on by using a gate signal controlling the charge near the p-n junction. Hence SCR is a charge-controlled device. However, SCR can’t be turned off by using gate signal.Thus,
SCR belongs to the class of semi-controlled semi-conductor device.
SCR is made up of silicon, it acts as a rectifier. It has very low resistance in the forward direction and high resistance in the reverse direction. It is unidirectional device.
The schematic diagram and circuit symbol of thyristor are shown in figure below:
1.1. Operating Modes of Thyristor:
Depending on polarity of Anode (A) and Cathode (K) voltage and gate signal, SCR can be operated in following modes:
1.1.1. Reverse Blocking Mode:
In this mode, terminal K is positive with respect to terminal A and also the gate terminal is open. Hence the junctions, J1 and J3 are reverse biased and J2 is forward biased. If reverse voltage is increased above VBR, avalanche breakdown occurs at J1 and J3 and reverse current increases rapidly. A large current associated with VBR gives rise to more losses in SCR. This may lead to thyristor damage as the junction temperature may exceed permissible limit. Therefore, it should be ensured that maximum reverse voltage doesn’t exceed VBR.
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.
1.1.2. Forward Blocking Mode:
In this mode, terminal A is positive with respect to terminal K and gate terminal is open. Hence junction J1 and J3 are forward biased and J2 and SCR starts conducting. But this method of triggering the SCR is not preferred as it may damage the device. Hence in forward blocking state, thyristor can be treated as an open switch. 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.
1.1.3. Forward Conduction Mode:
A thyristor is brought from forward blocking mode to forward conduction mode by increasing VAK above VBO or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed-switch.
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
- By applying a gate pulse between gate and
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.
- STATIC V-I CHARACTERISTICS OF SCR
An elementary circuit diagram for obtaining static V-I characteristics of SCR is shown in figure
- The Anode and Cathode are connected to main source through the load.
- The Gate and Cathode are fed from another source ‘Eg’
- The static V-I characteristics of SCR are shown below:
Va=Anode voltage : Ia=Anode current
VBO = Forward breakover voltage
VBR= Reverse breakdown voltage
- SWITCHING CHARACTERISTICS OF SCR
- SCR voltage and current waveforms during turn-on and turn–of process.
- Switching characteristics are also known as dynamic characteristics or transient characteristics.
- The time variations of the voltage across the SCR and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics.
3.1. Switching Characteristics During Turn-on:
SCR turn on time, is defined as the time during which SCR changes from forward blocking mode to final on state.
Total turn on-time can be divided into three intervals;
- Delay time (td)
- Rise time (tr)
- Spread time (tp)
3.1.1. Delay Time (td):
The delay time (td) is the time between the instant at which gate current reaches 0.9 Ig to the instant at which anode current reaches 0.1 Ia. Here Ig and Ig are respectively the final values of gate and anode currents or the delay time (td) may also be defined as the time during which anode voltage falls from Va to 0.9 Va where Va =initial value of anode voltage. The time during which anode current rises from forward leakage current to 0.1 Ia=final value of anode current.
3.1.2. Rise Time (tr):
The time taken by the anode current to rise from 0.1Ia to 0.9Ia. The rise time is also defined as the time required for the forward blocking off state voltage to fall from 0.9 to 0.1 of its initial value OA. During rise time, turn-on losses in the thyristor are high due to high anode voltage (Va) and large anode current(Ia) occurring together in the thyristor.
3.1.3. Spread Time(tp):
The time taken by the anode current to rise from 0.9Ia to Ia. It is also defined as the time for the forward blocking voltage to fall from 0.1 of its initial value to the on-state voltage drop.
3.2. Switching Characteristics During Turn-off:
SCR turn-off means that it has changed from on to off state and can block the forward voltage. The dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.
Note: If forward voltage is applied to the SCR now its anode current falls to zero, the device will not be able to block this forward voltage, as the carriers (holes and electrons) in the four layers are still favorable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. So to solve this problem it is essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.
Turn-off time (tq):
It is the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.
During this time (tq) all the excess carriers from four layers of SCR must be removed.
The turn-off time is divided into two intervals:
- Reverse recovery time (trr)
- Gate recovery time (tgr)
After t1: anode current builds up in the reverse direction with the same di/dt slope. The reason for the reversal of anode current is due to the presence of charge carriers stored in the four layers.
At instant t3 when reverse recovery current has fallen to nearly zero value, end junction J1 and J3 recover and SCR is able to block the reverse voltage.
At the end of reverse recovery period t3: the middle junction J2 still has charges, therefore, the thyristor is not able to block the forward voltage at t3
The charge carriers at J2 cannot flow to the external circuit, therefore they must decay only by recombination. This is possible if a reverse voltage is maintained across SCR. The time taken for this (t4 - t3) is called gate recovery time (tgr).
The thyristor turn-off time tq is depended upon magnitude of forward current, di/dt at the time of commutation and junction temperature.
Circuit Turn-off Time ‘tc’:
It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero.
Note: tc > tq for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.
- Thyristors with slow turn-off time are called converter grade SCR’s.
EX: Phase controlled rectifiers, cyclo-converters and ac voltage controllers.
- SCR with fast turn-off time are called inverter grade SCR’s.
EX: Inverters, choppers and forced commutation converters.
4. THYRISTOR TURN-ON METHODS
All the thyristor methods involve increasing carriers near junction J2. When anode is positive with respect to cathode, thyristor can be turned on by any of the methods listed below:
4.1. Forward Voltage Triggering:
When anode to cathode forward voltage is increased with gate circuit open, the reverse biased junction, J2 will break due to avalanche breakdown. But this mechanism is never employed as it may damage the device.
A forward voltage is applied between anode and cathode with gate circuit open.
- Junction 𝐽1 and 𝐽3 is forward
- Junction 𝐽2 is reverse
- 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 phenomenon 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.
4.2. Gate Triggering:
By applying the current at gate terminal, forward break- over Voltage, VBO can be reduced. Thus, SCR is made to conduct. With gate current, charges are injected into inner P layer and voltage at which forward break over occurs is reduced. Figure below shows a basic circuit for gate triggering and variation of break over voltage with gate current.
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.
First step is to choose a thyristor with 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. This means that 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 low applied voltage is required for triggering.
4.3. dv/dt Triggering:
When a SCR in forward blocking state, junction J2 acts a capacitor so large anode current flows when is higher. Thus, SCR starts conducting. Anode current, Ia=Cj= where Cj is function capacitance.
This is a turning ON method but it may lead to the 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. So, 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.
4.4. Temperature Triggering:
By increasing the temperature at junction J2 (when a SCR is in forward blocking state) carriers at J2 are increased. Above a limit, SCR starts conducting.
During forward biased, 𝐽2 is reverse biased so a leakage forward current always associated with SCR. Now as we know the leakage current is temperature dependent, so if we increase the temperature the leakage current will also increase and heat dissipation of junction 𝐽2 occurs. When this heat reaches a enough 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.
Latching current: The latching current may be defined as the minimum value of anode current which it 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.
4.5. Light triggering:
Increase carriers near junction J2, by applying light signal near J2. This mechanism is used is a class of SCRs called light activated SCR (LASCR).
5. THYRISTORS FAMILY
5.1. Programmable Unijunction Transistor (PUT):
PUT is like SCR, but gate is connected to n terminal. It is mainly used in time delay, logic and SCR trigger circuits. The circuit symbol and V-I characteristics of PUT are shown in figure below.
In PUT, G is always biased positive with respect to cathode. If Va> (Vg+0.7), junction J1 gets forward biased and PUT turns on. If Va < (vg+0.7), PUT is turned off.
5.2. Silicon Unilateral Switch (SUS):
SUS is like PUT with an inbuilt avalanche diode between G & K. Here diode is to compensate for changes in ambient atmosphere. So SUS gets turned on for a fixed anode to cathode voltage. Its mainly used in timing, logic and trigger circuits. Its circuit symbol, equivalent circuit and V-I characteristics are shown in figure below.
5.3. Silicon Controlled Switch (SCS):
CSC has two gates, anode and cathode gates as shown in figure below and it can be turned on by either gate. When a negative pulse is applied at AG, junction J1 gets forward biased and SCS is turned on. Similarly: a positive pulse at AG will reverse bias junction J1 and turns off SCS.
Similarly, a positive pulse at KG turns on the device and a negative pulse at KG turns it off. Its application includes pulse generators, voltage sensors and oscillators. Its schematic diagram, circuit symbol, equivalent circuit and V-I characteristics are shown in figure below.
5.4. Light Activated Thyristors (or LASCR):
Light-activated thyristors can be turned on by throwing pulse of light on gate terminal of thyristor. Due to the generation of excess electron-hole pairs due to radiation light activated thyristor gets turned on. The circuit symbol and V-I characteristics of LASCR are shown in figure below.
5.5. Static Induction Thyristor (SITH):
SITH is like SCR. It can be turned on by applying positive pulse and turned off by applying negative pulse between gate and cathode. The circuit symbol of SITH is as shown in figure below.
DIAC refers to bidirectional thyristor diode. It has symmetrical breakdown characteristics. A DIAC is similar to gateless TRIAC. The circuit symbol and V-I characteristics of DIAC are shown in figure below:
TRIAC is like SCR but can conduct in both directions. Thus, it’s bidirectional thyristor with three terminals. Its operation is similar to two SCRs connected in antiparallel. The circuit symbol and V-I characteristics of TRIAC are as shown in figure below.
GTO refers to gate turn off thyristor. It’s like SITH in operation. It can be turned off by a negative pulse at gate terminal. Hence, it’s used in inverter and chopper circuits. The circuit symbol of GTO is as shown in figure below.
6. THYRISTOR COMMUTATION TECHNIQUES
Commutation is defined as process of turning off a thyristor. Thyristor commutation is a necessary mechanism for obtaining the controlled output in many of the thyristor circuits. As thyristor is a semi-controlled device, it can’t be turned off directly by gate signal. Hence external means are required for turning off thyristor. As discussed, thyristor turn-off requires that anode current falls below holding current and a reverse voltage is applied to thyristor for sufficient time to enable it to recover to blocking state.
Various thyristor commutation techniques are discussed in detail below. In these techniques, it’s assumed that holding current is zero and load draws a constant current of I0.
6.1. Class-A or Load Commutation:
Circuit shown below corresponds to load commutation. Here the elements L & C are chosen such that, circuit is underdamped. Hence, the current through the load decays to zero in finite-time and Thyristor gets turned-off. But this technique can be only applied to circuits which are energized from dc source. This type of commutation is also called resonant or self commutation. For low values of R, elements L and C are connected in series for commutation. For high values for R, capacitor element is connected in parallel with R for commutation.
6.2. Class-B or Resonant Pulse Commutation:
The figure shown below is circuit diagram for resonant pulse commutation. Here main Thyristor T1 is triggered at t = 0 and auxiliary Thyristor, TA is triggered at t = t1. Also, VC(0-) = Vs. Based on these, following analysis is done and the corresponding waveforms are shown.
6.3. Class –C or Complementary Commutation:
The figure shown below demonstrates class –C commutation.
Circuit-turn-off time for T1 is obtained by
Similarly, circuit turn-off time for T2,
tC2=R2C ln 2
Based on above analysis, the waveforms corresponding to class –C commutation are shown in figure below.
6.4. Class-D or Impulse Commutation:
Figure shown below demonstrates impulse commutation along with corresponding waveforms.
6.5. Class-E or External Pulse Commutation:
In this type of commutation, a pulse of current from a separate voltage source is used to turn-off SCR. Here the peak value of current must be more than load current for commutation. Figure shown below demonstrates external pulse commutation.
Initially, T3 is triggered to make C to charge to 2V1. Later T2 is triggered to make T1 reverse biased with a voltage of (VS-2V1<0⇔VS<2V1
6.6. Class –F or Line Commutation:
This type of commutation is also known as natural commutation. Here thyristor carrying load current is reverse biased by a.c. supply voltage and device is turned off hen anode current falls below holding current. This is mainly used in phase-controlled rectifiers and line-commutated inverters. Line commutation is demonstrated in the figure shown below along with corresponding waveforms.