POWER ELECTRONICS

BTech IN ELECTRICAL ENGINEERING

4^TH SEMESTER

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GANDHI INSTITUTE FOR EDUCATION AND TECHNOLOGY, BANIATANGI, BHUBANESWAR

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INTRODUCTION:

Classification of power semiconductor switches

• Based on the degree of controllability Power Semiconductor switches are divided into three types
• 1.UNCONTROLLABLE SWITCHES:DIODES
• 2.SEMI-CONTROLLABLE SWITCHES:THYRISTORS
• 3.FULLY CONTROLLED SWITCHES: BJT,MOSFET,JFET,IGBT,GTO,MCT

• A power diode is a type of diode that is commonly used in power electronics circuits.
• In order to increase the power handling capacity we use power diode by small changes in the structure of simple p-n junction diode.
• The power diode is a simple semiconductor device that includes three layers,two terminals & single junction.
• It can be used as a rectifier in converter circuits,voltage regulation circuits,flyback/freewheeling diode,reverse voltage protection etc.
• Switching speed of power diode is low as compared to that of the low power diode.

VI CHARACTERSTICS:

Dynamic Switching Characteristics:

3 types of signal are used for gate triggering:

• DC supply is connected between Gate and cathode .
• Electrons from the cathode are injected into Gate P layer.
• Injected electrons in gate P layer reach junction J2 and hence reduces the width of the depletion region.
• SCR triggers at lower forward voltage less than Vbo.
• More gate current and less will be Vbo.

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• Provides proper isolation between power and control circuit.
• Firing angle control is obtained very conveniently by changing the phase angleof the control signal.
• Gate drive is maintained for one half cycle after the device is turned ON.
• Reverse voltage is applied between gate and catode during negative half cycle.

• Gate drive consists of a single pulse appearing periodically,or a sequence of high-frequency pulses.
• Known as carrier frequency gating.
• Pulse transformer used for isolation.
• Electrical isolation provided between main device supply and its gating signal.
• No need of applying gate signal continously hence gate loss reduced.

Voltage and Current commutation in SCR:

1.Forced commutation

2.Natural commutation

There are 5 types of forced voltage co

mmutation in SCR:

a.Class A or self commutated

b.Class B

c.Class C

d.Class D

e.Class E

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Class A commutation:

• This is known as self commutation,or resonant commutation,or load commutation.
• The load is underdamped.
• The thyristor is triggered by angle α and SCR IS ON.Meanwhile the capacitor is in charging position upto supply voltage.
• When we remove the supply a negative current flows through SCR and capacitor star discharging upon the resistor and SCR will off.

Class B commutation:

Class C type:

Class D Commutation:

Class E commutation:

Natuaral Commutation or Class F type:

Resistance firing circuit:

RC firing circuit:

MOSFET:

• Metal Oxide Semiconductor Field Effect Transistor.
• Power mosfet can having high switching from upto about 100khz.

Features of power MOSFET:

Low input current

It is a voltage controlled device.

High switching speed.

Active device.

Symbol and structure of mosfet:

• Mosfet are of two types:

1.N-type

2.P-type

Working:

Transfer characteristics of mosfet:

BJT:

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IGBT:

• It has a best combination quality of BJT and MOSFET.
• It has high input impedance like mosfet.
• It has low ON state power loss as a BJT.

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GATE DRIVE CIRCUIT FOR FOSFET & IGBT:

The gate drive circuit for MOSFET should satisfy the following requirements:

1.The gate source input capacitance should be charged quickly.

2.Mosfet turns on when gate source input capacitance is charged to sufficient level.

3.To turn off Mosfet quickly,the negative gate current should be sufficiently high to discharge gate source input capacitance.