EE3053 - Power Electronics & Applications I

ASSIGNMENT 1 Semester V

NAME : Ediriweera W.E.P.S

INDEX NO : 120138A

FIELD : Electrical Engineering

DATE OF SUB: 23/10/2015

BJT

Transistor is a unidirectional semiconductor device with three terminal named gate, collector and emitter .It can behave as either an insulator or a conductor according to gate terminal current. And it can operate within three different regions named active region, saturation region and cutoff region. With these three region it has two basic functions: switching and amplification. When it is been used as an amplifier, it us kept in active region. But unlike amplifier case, switching from one region to another, it can be made operate as a solid state switch.

Switching Characteristics

The area of operation for a transistor switch are known as saturation region and the cut off region. In figure 1 shows that ignoring Q point biasing and driving it back and forth between its cutoff and saturation region, transistor behaves as a switch.

Switch off characteristics

Here basically we keep the base current as zero and it will result transistor in off state. So no current flow through the device. And output collector current will be zero. We can see a maximum collector voltage as it behave as a mega ohms resistor with a large depletion layer. In following I have listed characteristics of a fully off transistor.

Figure 1: Characteristics curve of the transistor

Figure 2: The input and base are grounded

Base-Emitter voltage VBE < 0.7v

Base-Emitter junction is reverse biased

Base-Collector junction is reverse biased

Transistor is fully-OFF (Cut-off region)

No Collector current flows

Transistor operates as an open switch

Switch on characteristics

Here what we expected is that transistor will behave as a conductor with zero resistor. But in practical term it must conduct the maximum current it can. Here we biased the transistor by applying a maximum amount of base current to ensure that it operates in saturate region. This will result in a small depletion layer allowing to flow a large conduction current. So transistor is Fully-ON. In following I have listed characteristics of fully-ON transistor.

Base-Emitter voltage VBE > 0.7v

Base-Emitter junction is forward biased

Base-Collector junction is forward biased

Transistor is fully-ON (saturation region)

Max Collector current flows

VCE = 0 (ideal saturation)

Transistor operates as a closed switch

By apply a zero signal to the base of the transistor we can drive it to the cutoff region and turn it off (block the conduction current).Now it behavior is just like an open switch. And with required positive signal to the base, we can fully on the transistor ensuring closed switch characteristics. Transistor switches can be used to switch and control lamps, relays or even motors. If an inductive load has to be operated then a flywheel diode must be used with parallel with the load to drive the inductive current after transistor is off.

Drive Circuits

To turn on the BJT, We must supply a positive current continuously to the gate. In the driver angle, it should supply forward biasing voltage between the base and the emitter. To turn off the BJT base current must be made zero. For a faster turn off we must apply a negative current to the gate. In Driver circuit case it should

Figure 3: The input and base are connected to the power supply

supply negative base-emitter voltage. But we know that BJT are current controlled devices. So By changing the base-emitter voltage, we cannot control the base current. Then it makes drive circuits somewhat complex.

When considering about the entire requirement for being a good driver circuit, the driver circuits for Power BJTs should have capability to

Supply positive voltage to the base with respect to the emitter

Supply the correct collector current by controlling base current

Adapt the base current to the collector current

Extract a reverse current from base to speed up the device blocking

Protection Schemes

Most critical damages that can happen to the BJT are at its turn on and turn off states. At a faster turn off we apply a large negative base current. And this current may avalanche the base emitter junction leading to destruction.

1. Temperature reduction method

Heat sinks and automatic thermal cutoff methods are used avoid excessive temperature in the device.

2. Turn on protection

Figure 4: BJT Drive circuit 1

Figure 5: BJT Drive circuit 2

Figure 6: BJT Drive Circuit 3

Using the turn-on snubber we can reduce the voltage across the BJT as collector current increases.

Figure 7:Turn-ON Snubber circuit

The reduction in the voltage across the transistor during turn-on is due to the voltage drop across the snubber inductance LS. When the transistor turns off, the energy stored in the snubber inductance.

3. Turn-Off protection

Using the turn-off snubber we can provide a zero voltage across the transistor when the collector current reduces to zero.

Figure 8:Turn-OFF Snubber Circuit

At turn-off in the presence of this snubber, the transistor current ic decreases with a constant di/dt flows into the capacitor through the snubber diode Ds.

4. Overvoltage Protection

When a inductance present in a BJT circuit, due to its stored energy there may be over voltage at turn off. That can be minimized using an overvoltage snubber.

Figure 9: Over Voltage Snubber Circuit

At turn off, no conduction is possible through the transistor. Then the inductance current circulate through the free-wheeling diode.

Applications

There are main two type of applications of BJTs. They are switching and amplification.

1. Transistor as a Switch

Transistor can be used as a switch in logic gates will be operated in extreme regions of input output characteristics in which both regions will be forward biased(On state) or reverse biased(Off state) which are called saturation and cut off regions of operation simultaneously.

2. Transistor as an amplifier

Transistor when used as amplifier is operated in active region in which input junction will be forward biased and output junction will be reverse biased. There are three types of operating modes of amplifier i.e. Common Base (CB) amplifier, Common Emitter (CE) amplifier and Common Emitter (CE) amplifier. Other region inverse active region is of less importance in practice.

References

[1] Lecture Slides [2] http://www.electrical4u.com/application-of-bipolar-junction-transistor-or-bjt-history-of-bjt/ [3] Mohan Undeland Riobbins, Power Electronics [4] http://www.completepowerelectronics.com/base-drive-circuits-driving-power-bjt/ [5] http://www.electronics-tutorials.ws/transistor/tran_4.html [6] Google Pictures

MOSFET

MOSFET is a three terminal named gate, source and drain, unidirectional voltage control semi-conductor device. And it has a metal oxide gate electrode. Using a very thin layer of insulating material most of the cases silicon dioxide, it is electrically insulated from the main semiconductor channel So that gate is just like a one plate of a capacitor making gate or input resistance is extremely high. As there is always an infinite gate resistance, there will not be any current flow in to the gate and hence to the main channel like in BJT. The amount of current flowing through the channel that is between the drain and the source, is proportional to the input voltage. That is why we call the Mosfet as a voltage control device. MOSFETs are available in two basic forms as depletion type and enhancement type.

Switching Characteristics

To discuss about the switching characteristics of the MOSFET, I thought it is better to look at enhancement-mode mosfet as its operation is similar to transistor operation .That means it requires a positive gate voltage to turn ON and a zero voltage to turn OFF making them easily understood as switches and also easy to interface with logic gates. When gate voltage is zero, Mosfet does not conduct any current and output voltage is equal to supply voltage. At that moment it is within cutoff region as in figure 1

Switch-off Characteristics

Here we apply a zero gate voltage to the MOSFET. The channel resistance become very high so that it acts like an open circuit and no current flow through the channel. Now the device is closed. In the following I have listed characteristics of a switch-off mosfet.

Figure 10: Characteristics Curve of an enhancement type MOSFET

Figure 11: The Input and gate are grounded.

Gate-source voltage less than threshold voltage VGS < VTH

MOSFET is OFF ( Cut-off region )

No Drain current flows ( ID = 0 )

VOUT = VDS = VDD

MOSFET operates as an open switch

To keep the e- mosfet in off state or as a switch off mode, we have lower the gate source voltage below a thresh hold value and that will ensure that the drain current is zero. But for a p channel enhancement mosfet, gate potential must be more positive than the source to drive it to cut off region.

Switch-on characteristics

Here we apply a maximum amount of gate voltage driving it to saturation region. Then channel resistance will be reduced as small as possible allowing maximum drain current flowing through the channel. Therefore conductive channel is open and the device is switch on. In the following I have listed characteristics of a switch-on mosfet.

Gate-source voltage is much greater than threshold voltage VGS > VTH

MOSFET is ON ( saturation region )

Maximum Drain current flows

VDS = 0V (ideal saturation)

Minimum channel resistance RDS(on) < 0.1

VOUT = VDS = 0.2V due to RDS(on)

MOSFET operates as a low resistance closed switch

When e-mosfet is being used as a switch, by keeping the gate source voltage larger than thresh hold voltage, we can drive it to saturation region and get a maximum drain current. But for a p-channel enhancement mosfet the gate potential must be more negative with respect to the source to drive it t saturation region. Mosfet can be turned on and off faster or slower so that it is an effective switch with high switching applications and it can pass high or low currents.

Figure 12: The input and the gate are connected to the supply

Drive Circuits

To turn on the mosfet we should apply a positive gate pulse with respect to the source and to turn off a negative or zero voltage. But for a faster turn off and to avoid false triggering, it is better to keep the negative gate voltage to drive it to off condition.Mosfets are voltage control devices so drive circuit are more simpler than current controlled devices.

When wev consider about the Power MOSFETs operation, a good driver circuit for Power MOSFET should have capability to

Supply positive voltage to gate with respect to source

Supply negative voltage to gate with respect to source

The drive circuit in the following figure is suitable low switching speed applications as this circuit is supposed to switch off the mosfet by keeping the base source voltage zero,

The drive circuit in the following figure is suitable high switching speed applications as this circuit is supposed to switch off the mosfet by keeping the base source voltage negative ,

Figure 13: MOSFET Drive Circuit 1

Figure 14: MOSFET Drive Circuit 2

Figure 16:Turn-OFF snubber circuit

Protection Schemes

When a high current flow through the mosfet, it may damage the semiconductor device. In most of the application current flow may exceed their current handling capacities so that there are over current protection methods for their protection.

1. Over current protection

This is done using an integrated current feedback controlling methods .In simple terms it uses a comparator. The Current through the mosfet (drain current) is measured continuously and compared with a reference value using a comparator. If the drain current exceeds the safe value, controller turn off the mosfet and protect it

2. Current and voltage spike protection

MOSFET need a protection for over voltages and gate protection. During the device is turning off, to prevent voltage spikes and voltage oscillations a RC turn off snubber can be used

3. Protection against temperature

Heat sinks and automatic thermal cutoff systems are used to avoid damages caused due to excessive temperature.

4. Over voltage Protection

Here, charging and discharging of capacitor C, through diode and the resistor, avoids the over voltage.

Over current Comparator

Drive Circuit Power

Electronic

device

Feedback

Drive signal Gate

Figure 15: Integrated current feedback controller

Figure 18: Over voltage protection for a MOSFET

Applications

A Voltage-controlled resistor

A switch

A chopper

References

[1] Lecture Slides [2] Mohan Undeland Riobbins, Power Electronics [3] https://wiki.analog.com/university/courses/electronics/text/chapter-15 [4] http://www.electronics-tutorials.ws/transistor/tran_7.html [5] http://electronicdesign.com/power/power-mosfet-gate-drivers [6] Google pictures

Figure 17: Gate over voltage protection

THYRISTOR

Thrysistors is unidirectional 4 layer semiconductor device. It has three terminal name gate, anode and cathode. In off state it has high resistance. In on state, it has a small forward resistance. The Conductor happens between anode and cathode and the gate is used to triggers the thyristors in to conduction by applying pluses. In the figure 1, I have shown the characteristics curve of a thyristors.

We can see there are three operating regions.

1. Reverse blocking mode-Voltage is applied in the negative direction

2. Forward blocking mode-Voltage is applied in correct direction that would cause the thyristor, but the thyristor has not been triggered into conduction.

3. Forward conducting mode - The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current".

Switching characteristics

So we can imagine that it behaves as an off switch in reverse blocking mode and as an on switch in forward conduction mode. The switching characteristics of the thyristor are important to get an idea about its switching speed between conduction mode and blocking mode.

The switching characteristics are important particularly at high-frequency, to define the device velocity in changing from conduction state to blocking state and vice versa. Switching losses (losses occurring in the device during its switching on state to off state and off state to on state) can be calculated by studying switching characteristics.

Figure 19: Characteristics curve of a thyristor

Switch-ON characteristics

When a positive gate pulse is applied to forward blocking mode thyristors, it goes to forward conduction mode. Figure 3 shows the basic construction of a thyristors. A Positive pulse at the gate turn on Q2 transistor providing a current path for IB1.Then in turn Q1 turn on automatically providing more base current to Q2 transistor.

Figure 20: thyristors switching characteristics

Following I have listed few state during its switch on operation and few important terms.

Turn The time taken for SCR to traverse from the blocking state to conducting state is called as turn on time.

on time is divided into 3 periods.

tON = td + tr + tp

td = delay time, tp or ts = peak time (or) spread time

when the gate current reaches 0.9IG the anode current IA starts increasing and reaches 0.1IA (10% of its max value)

The time taken for anode current to reach 0.1IA is called as delay time (td).

In other words, it is the time taken for anode voltage to fall from VA to 0.9VA

The anode current further increases and reaches 0.9IA.

The time taken by the anode current to increases from 0.1IA to 0.9IA is called as rise time (tr).

In other words, it is the time taken by the anode voltage to fall from 0.9VA to 0.1VA

Switching-OFF characteristics

We cannot switch thyristor from its conduction mode to reverse blocking mode by applying a negative gate pulse. Thyristor will be in conduction state as long as forward current exceed threshold current.

To stop the thyristor, the following conditions must be satisfied.

1. Bring the anode current below its holding current

2. Then hold the thyristor in the reverse blocking mode for a time not less than tQ (thyristor turn-off time)

When the anode current is zero, we need to remove forward voltage as soon as possible. Because excess charge carriers are still present after it come in to forward blocking mode from conduction mode, thyristor will not be able to hold the forward voltage and start conduction without a gate pulse. So to remove excess charge carriers from the thyristor, thyristor is reverse biased after reducing conduction current to zero.

Figure 21: Basic construction of a thyristors

In some circuits above two conditions are satisfied automatically .but in some circuits we have to intervene and provide these conditions by force. There are three methods to provide these conditions known as commutation methods.

1. Line commutation- In this method the conditions are satisfied due to the natural reversal of line voltage

2. Load commutation- In this method, the conditions for turn-off are satisfied by the reversal of voltage induced by the load.

3. Forced commutation- This method is chosen if other methods of commutation are not available.

Following I have listed few state during its switch off operation and few important terms.

The turn OFF time is defined as the time from the instant the anode current becomes zero to the instant SCR reaches its forward blocking ability.

Turn off time tOFF = trr + tgr

Where trr = Reverse recovery time tgr =Gate recovery time

Reverse recovery process is the removal of excessive charge carries from the top and bottom layers of SCR.

Consider at t=0, conduction current is zero. As time increases conduction current build up in the reverse direction due to the charge carriers stored in the four layers. That means reverse recovery removes the carries present in each layer.

.

Figure 22: Behavior of a thyristor during switch off Figure 23: Current and voltage waveform during its switch off

Figure 24: thyristor gate drive circuit 1

Drive Circuits

1. Gate Driver circuit

D1 To circulate inductive energy after transistor T is off.

D2 To stop reverse gate current

D3 To stop reverse gate voltage

S To suppress noise spikes

Figure 25: Thyristor gate drive circuit 2

In above circuit load is connected to the AC power supply through the thyristor. The thyristor automatically drives in to the reverse blocking region in negative half cycle of the power supply. And using above drive circuit we can control gate pulse width and the firing angle and control power flow in positive half cycle.

2. Commutation circuits

When we work with DC voltages, it is not easy to stop the conduction of the thyristor (In AC case, due to natural phase reversal, it goes to reverse blocking region).To stop the conduction, we use commutation circuits

Assume the switch S to be closed at t = 0 to initiate commutation. Thyristor current i is reduced as load current is now diverted through the capacitor.

Thyristor overall operation drive circuit

When consider about the drive circuit requirement for this type of drives for the thyristor, the drive circuit should have

Commutation circuit when working with dc

Capability to change the firing angle when working with ac

Figure 26: Block diagram of a gate drive circuit with commutation circuit to work with DC

Protection Schemes

Thyristor is a soft device and for reliable and successful functionality, there must be method to protect it from all disturbances. There are several types of protection circuits dedicate to specific disturbance.

1) dv/dt, di/dt and switching over-voltage protection

This circuit

limits di/dt at turn on

limits dv/dt in the off state

Limits over-voltage spikes occurring at instance of turn-off.

limits dv/dt across the thyristor when recovering inverse parallel diode, in case such a diode is part of the circuit

Following figure shows over voltage spike occurring at switching off without a protection scheme

Now we can see the way the LCR snubber suppress the transient spike and smooth the waveform.

Figure 27: snubber circuit

Figure 28: Behavior at turn off without a protection

2) Over-current protection

Thyristors have a very good overcurrent current capability so that they can be protected against surge current using normal semiconductor fuse.

Figure 29: behavior at turn off with a snubber circuit

Figure 30: fuse Characteristics against a surge current

3) Over-voltage (power line surges) protection

Against voltage surges, a surge arrestor is used parallel with the thyristor. Normally we use one arrestor at supply side and one arrestor for each thyristor.

4) Circuit over voltage protection

It is hard and not a good idea to change the semiconductor fuse every time when a fault occurs in the system and fuse blow up As I describe above if we have used surge arrestor, some time we may have to replace all the arrestor at an over voltage. Alternatively we can arrange an outer protection cover for the whole system known as Crowbar protection. Crowbar thyristor is turned on automatically if over-voltage exceeds a preset level and trigger the thyristor. This resulting current through the thyristor will signal input breaker in supply side to be tripped off, interrupting the supply connection to the loads.

Figure 31: A surge arrester parallel with a snubber

Figure 32: Crow Bar circuit

Applications

Thyristors are used in pulse mode or half wave bridge circuits and they are available in a large range of RMS current rating up to 50A and voltage rating up to 1200 V with through hole and surface mount packages. Due to their low current gate triggering, they are easy to drive. Thyristors are ideal for general purpose switching application. Some of them can be stated as follows.

Stepper motor control applications

Stepper motors are generally used for applications where high precision movement are required. Stepper motor moves in discrete steps as commanded, rather than rotating continuously like a conventional motor. Rotor moves one step at a time when drive waveforms are changed. By changing the firing angle of the thyristor we can easily change the drive waveform

DC motor control application

Thyristor can rectify and control. Thyristor is triggered on the positive cycle and turns off on the negative cycle.A circuit in the following figure can be used for speed control for fan or power tools and other related applications.

Fluorescent lamp lighting

Conventional fluorescent lamp with filament can be ignited easily and quickly by using thyristors

Battery chargers

The thyristor-controlled power supplies and battery chargers can be used to rectify and control electric power. The advantages of thyristor-controlled units are given by a simple technical concept resulting in robustness and reliability.

Figure 33: DC motor control circuit

Light dimmers

A light dimmer works by essentially chopping parts out of the AC voltage. This allows only parts of the waveform to pass to the lamp. The brightness of the lamp is determined by the power transferred to it, so the more the waveform is chopped, the more it dims.

Over-Voltage Protection

Thyristors can be used for protecting other equipment from over-voltages due to their fast switching action. Here the thyristor which is employed is connected in parallel with the load.

Automatic night lamp designs

Fiber optical controlling applications.

References

[1] Lecture Slides [2] Mohan Undeland Riobbins, Power Electronics [3] http://www.completepowerelectronics.com/scr-thyristor-switching-characteristics/ [4] http://www.electronics-tutorials.ws/power/thyristor-circuit.html [5] http://www.circuitstoday.com/scr-applications [6] Google pictures

IGBT

IGBT is a unidirectional semiconductor device with three terminals named gate, collector and emitter. It is a combination of bipolar transistor and mosfet. It is the most popular switching semiconductor device at the moment. Figure 1 shows the characteristics of an IGBT.

Switching characteristics.

To explain the switching characteristics of the IGBT, I am going to use following model.

Switch-ON characteristics

The voltage VGE is normally negative. When we increase gate voltage respect to emitter ,VGE increases. At the moment when gate emitter voltage exceeds the thresh hold value, a collector current start to flow. And collector current start to increase and reach its maximum value. Now IGBT is in its on state and the voltage across the device start to fall to a value of nearly zero (actual value is called VCE(ON)).Following I have listed few states during switch on and important terms.

Time taken for VGE to rise and reach VGE(th) (or) The time for IC to start increasing is called as turn-on delay time tdn.

Figure 34: Characteristics curve of an IGBT

The time for IC to rise and reach its max value is called as rise time for current (tri).

The time taken for voltage VCE to start to fall and reach its saturated value VCES is called as fall time for voltage (tfv).

Therefore the turn-on time tON = tdn + tri + tfv

These time delays are due to two reasons.

Gate-Collector capacitance will increase in MOSFET portion of IGBT at low VCE.

PNP transistor portion of IGBT travels (or) moves to the ON state more slowly than the MOSFET portion of IGBT.

Switch-OFF Characteristics

We can turn off the IGBT by removing the gate voltage. When the gate voltage is removed, Gate-emitter voltage starts to fall and in turn collector-emitter voltage increases. When gate-emitter voltage reaches its thresh hold value, collector current reduces to zero (actually the current is not zero, but a small current flows due to the stored charge in n-drift region. This is the internal BJT current).Following I have listed few states during switch on and important terms.

Turn-off delay time (tdf) is the time between the VGE starts to decrease and the VCE starts to increase.

The time taken for VCE to rise and reach its full value is called as rise time for voltage (trv).

Time interval tfi1 is the fall time for current. It is the turn-off interval of the MOSFET section of IGBT.

A tailing of current due to BJT internal current takes place during the interval tfi2. It is the turn-off interval of the BJT section of IGBT.

The turn-off time TOFF = tdf + trv + tfi1 + tfi2

Figure 35: Switch ON and OFF characteristics of an IGBT

Drive Circuits

When we apply a positive gate voltage with respect to the emitter, IGBT will turn on. And the gate current control the collector current. To turn off the IGBT a negative or zero gate emitter voltage must be applied. But as in the mosfet case, for a faster switching operation and to minimize false triggering it is better to keep the gate-emitter voltage negative.

IGBT has a high switching speed compared to other semiconductor devices. To use the switching capability of the MOSFET successfully, drives should be designed properly. The delay between the time when we apply the switching signal and the time gate signal is generated in response to the switching signal, must be minimized.

When considering about the entire requirement for operation, the driver circuits for IGBTs should have capability to

Control the gate current

Supply positive voltage for gate to turn on

Supply negative voltage for gate with respect to the emitter for fast turn off

Minimize the delay between control signal and gate signal

Supply sufficient peak current capability to provide the required gate current for zero current switching and zero voltage switching.

Protection Schemes.

IGBTs are voltage controlled devices. Then if the gate voltage is higher than the rated value, IGBT can be damaged. Therefore to avoid the damages happens due to high gate voltages, there must be a way to keep the gate voltage below the threshold value.

1. Over voltage protection

Simple way to keep the gate-emitter voltage below rated value is to connect zenor diode between the gate and the emitter. The zenor diode clamp the over voltages and supply correct voltage to the gate emitter junction.

Figure 37: Modern drive circuit which reduce switching delay

Figure 36: Conventional Drive circuit

2. Voltage and Current spikes

To protect the IGBTs from these voltage and current spikes there is need to use turn on and turn off snubber circuits for complete protection.

Applications

White goods.

Small appliances.

Lighting controls.

Motor drives.

Meter readers.

Small off-line power supplies.

References

[1] Lecture Slides [2] Mohan Undeland Riobbins, Power Electronics [3] http://www.completepowerelectronics.com/igbt-switching-characteristics/

Figure 38: Over voltage protection

MCT

MOS control thyristor (MCT) is a unidirectional semiconductor device with three terminals named gate, anode and cathode. The device is basically thyristor with two mosfet built into the gate structure. One mosfet is used to turn on the MCT and other one too switch off it. It has a high switching frequency and a low conduction drop as it is a combination of thyristors and mosfets. . In this device, all the gate signals are applied with respect to anode, which is kept as the reference. Figure 1 shows the characteristics curve of a MCT.

Switching characteristics

Switching-ON Characteristics

The device is turn on by applying negative pulse at the gate with respect to the anode when the device is forward biased. Initially both FETS are off. With the gate pulse, ON-FET gets turned ON whereas OFF-FET is already OFF. Then a current starts to flow from anode and then as the base current and emitter of n-p-n transistor and then to cathode. This turns on n-p-n transistor. This causes the collector current to flow in n-p-n transistor. As OFF FET is OFF, this collector current of npn transistor acts as the base current of p-n-p transistor. Subsequently, p-n-p transistor is also turned ON. If both the transistors are ON, regenerative action of the connection scheme takes place and the MCT is turned ON.

Switching-OFF Characteristics

The device is turned OFF by applying a positive voltage pulse at the gate. The positive voltage pulse causes the OFF-FET to turn ON and ON-FET to turn OFF. After OFF-FET is turned ON, emitter based terminals of p-n-p transistor are short circuited by OFF-FET. So, now anode current begins to flow through OFF-FET and thus base current of p-n-p transistor begins to decrease. The device has the disadvantage of reverse voltage blocking capability.

Figure 39: VI characteristics curve

Drive Circuits

Gate drive circuits

The MCT has a MOS gate similar to MOSFET and IGBT so that the control is not complex. As the MCT need a range of gate voltages between -5V to 10V special gate drivers are needed. One of them is a push pull pair with discrete NMOS-PMOS devices. A breakers clamp push-pull can also be used to generate pulses of negative and positive pulses.

Protection Schemes

1. Snubber protection

The turn on snubber consist of Ls and DLS and the turn off snubber consist of Rs, Cs and Dcs. The series connected turn on snubber reduces di/dt while the turn off snubber helps to reduce the peak power and the total power dissipated by the MCT by reducing voltage across the MCT when the anode current decays to zero. There are some freewheeling and other protections as well.

Figure 40: Protection with turn on and off snubbers

Applications

Parallel operation for proper current sharing at high current applications

References

[1] http://www.completepowerelectronics.com/mos-controlled-thyristor-mct/ [2] https://en.wikipedia.org/wiki/MOS-controlled_thyristor [3] http://www.circuitstoday.com/mos-controlled-thyristor-mct