Power Factor Improvement in the Converter

Power factor of the Phase Controlled Converter

The operation of phase-controlled converter is very simple, reliable and less costly. The commutation circuit is not necessary in this type of converter. The power factor becomes low when the output voltage less than the supply voltage (particularly when the firing angle is high).

Gate Turn off Thyristor ( GTO )

• The main drawback of the SCR is that once it turns on, it will be not turned off in spite of gate signal removes. 
• The GTO is upgrade version of the SCR in which it is turned off by negative pulse however the turn off current gain ratio is 4 – 5. ( If the anode current is 200 Amp then turn off current must be is in the range of 200/4 to 200/5 )

Insulated Gate Bipolar Transistor ( IGBT )

IGBT

• The BJT has low power loss but large turn off time therefore the switching speed of the IGBT is slow. 
• The MOSFET has higher power loss due to high on state resistance but small turn off time therefore the switching speed of the MOSFET is high.
• The IGBT has combine characteristics of IGBT and MOSFET. 
• The configuration of IGBT consists of BJT and MOSFET in the Darlington configuration. 
• The input of the IGBT consists of MOSFET whereas the output consists of BJT. 
• As the current gain of the BJT has low, the voltage rating of the MOSFET should be high for higher power rating. 
• The input characteristics of the IGBT should be similar to that of power MOSFET whereas the output characteristics similar to that of BJT.

Full Form of IGBT

The full form of IGBT is insulated gate bipolar transistor

Structure of IGBT

Why vertical structure?

• The basic structure of the IGBT is shown in the figure A. 
• The structure of the IGBT should be vertical in order to provide maximum area for flow of current. 
• As the on-state resistance decreases, the power loss also reduces.

Types of IGBT

When we compare the structure of the IGBT with N- channel MOSFET there is additional P+ layer substrate over N+ layer. This layer makes PN junction diode with drift layer, this layer is called as inject layer because it injects holes in the n- layer. There are two types of impurities in the n type drift layer.

Non-Punch Through IGBT

• It consists of Lightly doped n- layer. It is also called as drift layer. 
• The forward blocking voltage in the IGTB depends upon this layer’s doping level and width. 
• It does not affect on state voltage drop due to conductivity modulation.

Punch Through IGBT

• It consists of heavily doped n+ layer. 
• It is not necessary for the operation of this layer. 
• It lies between P+ layer and n- layer. 
• The on-state voltage drop, turn off time and also reverse voltage blocking capacity reduces due to this layer

Symbol and Equivalent Circuit of IGBT

• It consists of three terminals namely collector, emitter and gate. 
• The one cell of IGBT consists of PNP transistor, NPN transistor and driver MOSFET.

PNP transistor

• P+ injecting layer as emitter
• N- drift layer as base
• P body layer as collector

NPN transistor

• N+ region emitter
• P body layer base
• N- drift layer collector

Body Spreading Resistance

• In the NPN transistor base is shorted with emitter but due to lower quality of short, resistance between emitter and base is called body spreading resistance. 
• If the output current is high, due to high voltage drop across body spreading resistance and PNPN latching process IGBT turns on. 
• The parasitic thyristor is used to solve problem of latching in which doping of the body layer is changed.

Operation of IGBT

• The operation of the IGBT is explained as follows

(A) Inversion layer

OFF State

• When gate-emitter voltage less than the threshold voltage, inversion layer is not created.  
• The forward voltage between collector to emitter reverse biased across junction j2 and only leakage current flow

ON State

• When gate- emitter voltage greater than threshold voltage inversion layer is created. 
• Due to this inversion layer conduction channel N+NN- is created, therefore flow of current is possible.

(B) Conducting Modulation

• When forward voltage is given to collector to emitter, junction j3 becomes forward biased therefore holes from P+ layer moves towards n+ buffer layer. 
• The conducting channel is created due to inversion layer resulting there is creation of space charge region in the n- drift layer. 
• Therefore holes are attracted from n+ buffer layer. 
• The double injection takes place (holes from left and electrons from right) in the n- drift layer therefore its conductivity increases and resistance is decreased. 
• Therefore due to conducting modulation on state voltage drop across device is reduced.

Latching problem

• The drift region (electric field or movement of charge) of the IGBT works as a base of the PNP transistor . 
• The current gain of the transistor depends upon the width and doping level of the transistor. 
• If this current gain is kept low, most of the current flow through MOSFET resulting voltage drop across body spreading resistance is decreased and latching problem is solved. 

Static Characteristics of IGBT

The static characteristics of the IGBT is similar to BJT. The controlling parameter in the BJT is base current whereas gate to emitter voltage in the IGBT.

Cut off Region

• When the gate-emitter voltage is less than the threshold voltage, the collector to emitter voltage is equal to supply voltage and it is called as cut off region. 
• The semiconductor device which can withstand forward voltage between collector to emitter is called as forward breaker over voltage BV_CE. 
• Its value depends upon avalanche breakdown voltage (high reverse voltage across the junction, it is moderately operated) of the body – drain PN junction diode and only leakage current flows through the device.
• The forward breakdown voltage depends upon collector current in the IGBT.

Active Region

• When the gate- emitter voltage greater than the threshold voltage, the IGBT operates in the active region. 
• The collector current depends upon transfer characteristics of the IGBT. As the gate – emitter voltage increases, the collector current also increases. 
• The characteristics becomes linear for higher value of collector current.
• The ratio of collector current to the gate – emitter voltage is called as forward transconductance.
• Forward transconductance: I_c / V_ge Mho

Saturation Region

• When the gate - emitter voltage increases, the collector current also increases as shown in the transfer characteristics of the IGBT.  
• The collector to emitter voltage decreases for a given load resistance R_L.
• The collector – emitter voltage ( V_CE ) becomes less than the gate – emitter voltage ( V_GE ) for a given specific collector current therefore the MOSFET enter in the ohmic region and P⁺N^-P transistor in the saturation region. This is called as saturation region of the device. 
• The voltage drop across device becomes constant in this region and this voltage drop decreases as the gate – emitter voltage ( V_GE ) increases.
• The secondary breakdown does not occur in the IGBT as that of in the BJT. The reverse voltage blocking capability of the punch through IGBT ( PT IGBT ) is in tens of volt due to heavily doped n⁺ layer. 
• The Non punch through IGBT ( NPT IGBT ) can withstand maximum reverse voltage V_RB. The IGBT always operate in the saturation or cut off region.

Applications of IGBT

• UPS
• Motor controllers
• Choppers
• Inverters
• SMPS

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MOS Controlled Thyristors ( MCT )

Introduction

• The commutation circuit of SCR is costly in order to turn off in the DC circuit. 
• The main disadvantage of the GTO is that it requires very high negative current in order to turns it off. 
• ( If the anode current of the SCR is 100 amp, the negative gate current requires to turn off the GTO should be 20 to 25 amp ).

Negative gate current = Anode current / Turn off current gain

• The power MOSFET is used for low voltage and low power application because of high input impedance, higher switching speed, simple control circuit, low current density and higher forward voltage drop. 
• The IGBT and MCT are semiconductor devices which combines characteristics of both BJT and FET. 
• The characteristic of the IGBT is much superior to the BJT whereas the characteristic of MCT is superior to the SCR. 
• The MOSFET in the MCT is used to turn on and turn off the device.
•  The MCT consists of two MOSFET, one MOSFET is used to turn on the device and other MOSFET is used to turn off the device.

Symbol and Equivalent Circuit

• The symbol of MCT is shown in the Figure. 
• It has three terminals anode, cathode and gate. 
• The equivalent circuit of MCT consists of two transistors and two MOSFET. 
• The P channel MOSFET is connected between collector and emitter terminal of PNP transistor whereas N channel transistor is connected between emitter and base terminal of PNP transistor. 
• The P channel MOSFET is associated with turn on process whereas N channel MOSFET is associated with turn off process.

Structure

• The single cell structure of the P – MCT is shown in the figure A.
• There are thousands of cell connected in parallel in order to increase current capacity and its fabrication is done on single silicon wafer. The P buffer layer lies between P^- base layer and n⁺ substrate layer in the punch through MCT (  P – MCT  ). 
• There is no P type buffer layer in the non – punch through MCT. 
• The single cell MCT consists of one SCR and two MOSFET.  One MOSFET is used to switch on the device whereas the other MOSFET is used to switch off the device. 
• The P channel MOSFET is connected between collector and emitter of the PNP transistor and N channel MOSFET is connected between base and emitter of the PNP transistor. 
• The P channel MOSFET is used to turn on the device whereas the N channel MOSFET is used to turn off the device. 
• The MCT is called as complementary MCT because the gate is made negative with respect to anode instead of the cathode. 

Turn on process

• The gate is made positive with respect to anode to turn off the device.
•  When the gate is made negative with respect to anode, the P - MOSFET is turned on therefore drain current flows through the base of the NPN transistor resulting it turns on. 
• The collector of the NPN transistor is connected to the base of the PNP transistor resulting it turns on. 
• Therefore the MCT is switched on. It remains in ON condition in spite of removing gate pulse. 
• The conductivity of the PNP transistor is high as compared to P channel MOSFET therefore the most of the current flows through PNP transistor. 
• The N channel MOSFET remains off during this period.

Turn off process

• When the gate is made positive with respect to anode, the N channel MOSFET is switched on. 
• This MOSFET is in parallel with base – emitter of the PNP transistor resulting the base current decreases and the MCT is turned off in the short time. 
• The current gain of the PNP transistor is high, it is connected with parallel to N channel MOSFET. 
• All cells of MCT must be turned off at a time in order to turn off MCT otherwise high current density of the one cell may damage the device.
•  The drain – source voltage of the device is less than 0.7 voltage in order to turn off the device

Static Characteristics

• The static characteristic of the MCT is similar to that of the SCR and GTO. 
• The MCT has low reverse breaking capability ( 20 - 30 V ). 
• The ON state voltage drop of the MCT is greater than the SCR but less than GTO.

Features

• High switching speed due to two MOSFET (1 Micro second )
• Temp co-efficient low at negative current and high at positive current
• Low on state voltage drop ( 1.1 volt )
• Simple Gate drive circuit due to high input impedance
• Low switching power loss
• MCT is connected in parallel when high power is required
• High current density as compared to IGBT

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Pulse Transformer

Function of Pulse Transformer

• The function of pulse transformer is to generate pulse for semi conductor device and provide electrical isolation.

Speed Control of DC Motor by Solid State Devices

Basic Equation of DC Motor

Back emf  E_b = ZNP / 60A

                    = V – I_aR_a …….….(1)

                         OR

Speed N α E_b / Ф…................…(2)

Speed Control of Three Phase Induction Motor

• The three phase induction motor or synchronous motor is used as AC drive. 
• The three phase induction motor has following advantages : 

              1. Low cost

              2. Low maintenance