Power Electronics Lecture(8)

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Power ElectronicsLecture(8)

• Prof. Mohammed Zeki Khedher
• Department of Electrical Engineering
• University of Jordan

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Thyristors

• Most important type of power semiconductor
  device.
• Have the highest power handling capability.they
  have a rating of 5000V / 6000A with switching
  frequencies ranging from 1KHz to 20KHz.

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• Is inherently a slow switching device compared to
  BJT or MOSFET.
• Used as a latching switch that can be turned on
  by the control terminal but cannot be turned off
  by the gate.

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SCR

• Symbol of
• Silicon Controlled Rectifier

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Structure
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Device Operation

• Simplified model of a thyristor

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Two Transistor Model of SCR

• ?

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V-I Characteristics
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Effects of gate current
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Turn-on Characteristics
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Turn-off Characteristics
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dv/dt Triggering
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Methods of Thyristor Turn-on

• Thermal Turn-on.
• Light.
• High Voltage.
• Gate Current.
• dv/dt.

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Thyristor Ratings
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Voltage Ratings
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Current Ratings
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Gate Specification
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Diodes

• Diode Product Range

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Phase Control Thyristors
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Fast switching Thyristors
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Thyristor Types

• Phase-control Thyristors (SCRs).
• Fast-switching Thyristors (SCRs).
• Gate-turn-off Thyristors (GTOs).
• Bidirectional triode Thyristors (TRIACs).
• Reverse-conducting Thyristors (RCTs).

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• Static induction Thyristors (SITHs).
• Light-activated silicon-controlled rectifiers
  (LASCRs).
• FET controlled Thyristors (FET-CTHs).
• MOS controlled Thyristors (MCTs).

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Phase Control Thyristor

• These are converter thyristors.
• The turn-off time tq is in the order of 50 to
  100?sec.
• Used for low switching frequency.
• Commutation is natural commutation
• On state voltage drop is 1.15V for a 600V device.

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• They use amplifying gate thyristor.

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Fast Switching Thyristors

• Also called inverter thyristors.
• Used for high speed switching applications.
• Turn-off time tq in the range of 5 to 50?sec.
• On-state voltage drop of typically 1.7V for
  2200A, 1800V thyristor.
• High dv/dt and high di/dt rating.

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Bidirectional Triode Thyristors (TRIAC)
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Triac Characteristics
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Gate Turn-off Thyristors

• Turned on by applying positive gate signal.
• Turned off by applying negative gate signal.
• On state voltage is 3.4V for 550A, 1200V GTO.
• Controllable peak on-state current ITGQ is the
  peak value of on-state current which can be
  turned-off by gate control.

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Advantages over SCRs

• Elimination of commutating components.
• Reduction in acoustic electromagnetic noise due
  to elimination of chokes.
• Faster turn-off, therefore can be used for higher
  switching frequencies.
• Improved efficiency of converters.

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Advantages over BJTs

• Higher voltage blocking capabilities.
• High on-state gain.
• High ratio of peak surge current to average
  current.
• A pulsed gate signal of short duration only is
  required.

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Disadvantages of GTOs

• On-state voltage drop is more.
• Due to multi cathode structure higher gate
  current is required.
• Gate drive circuit losses are more.
• Reverse blocking capability is less than its
  forward blocking capability.

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Reverse Conducting Thyristors
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• Anti-parallel diode connected across SCR on the
  same silicon chip.
• This diode clamps the reverse blocking voltage to
  1 or 2V.
• RCT also called Asymmetrical Thyristor (ASCR).
• Limited applications.

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Static Induction Thyristors

• Turned-on by applying positive gate voltage.
• Turned-off by applying negative gate voltage.
• Minority carrier device.
• Low on-state resistance low voltage drop.
• Fast switching speeds high dv/dt high di/dt
  capabilities.

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• Switching time in order of 1 to 6 ?sec.
• The rating can go upto 2500V / 500A.
• Process sensitive.

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Light-Activated Silicon Controlled Rectifiers

• Turned-on by direct light radiation on silicon
  wafer.
• Gate structure is sensitive for triggering from
  practical light sources.
• Used in high voltage and high current
  applications. Example HVDC transmission, Static
  reactive power compensation.

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• Offers complete electrical isolation between
  light triggering source power circuit.
• Rating could be has high as 4KV / 1500A.
• di/dt rating is 250A / ?sec.
• dv/dt rating is 2000V / ?sec.

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FET Controlled Thyristors

• Combines a MOSFET a thyristor in parallel as
  shown.
• High switching speeds high di/dt dv/dt.

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• Turned on like conventional thyristors.
• Cannot be turned off by gate control.
• Application of these are where optical firing is
  to be used.

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MOS-Controlled Thyristor

• New device that has become commercially
  available.
• Basically a thyristor with two MOSFETs built in
  the gate structure.
• One MOSFET for turning ON the MCT and the other
  to turn OFF the MCT.

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Structure
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Equivalent Circuit
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Features

• Low on-state losses large current capabilities.
• Low switching losses.
• High switching speeds achieved due to fast
  turn-on turn-off.
• Low reverse blocking capability.

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• Gate controlled possible if current is less than
  peak controllable current.
• Gate pulse width not critical for smaller device
  currents.
• Gate pulse width critical for turn-off for larger
  currents.

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MOSFET
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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Semiconductor Cross-section of IGBT
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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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IGBT
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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Advantages of IGBT

• Combines the advantages of BJT MOSFET
• High input impedance like MOSFET
• Voltage controlled device like MOSFET
• Simple gate drive, Lower switching loss
• Low on state conduction power loss like BJT
• Higher current capability higher switching
  speed than a BJT. ( Switching speed lower than
  MOSFET)

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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Applications of IGBT

• ac and dc motor controls.
• General purpose inverters.
• Uninterrupted Power Supply (UPS).
• Welding Equipments.
• Numerical control, Cutting tools.
• Robotics Induction heating.

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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Devices

• SITH Static Induction Thyristor
• GTO Gate Turn Off Thyristor
• MOS Metal Oxide Semiconductor
• MCT MOS Controlled Thyristor
• MTO MOS Turn Off Thyristor
• ETO Emitter Turn Off Thyristor
• IGCT Insulated Gate Controlled Thyristor
• TRIAC Triode Thyristor
• LASCR Light Activated SCR

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Devices..

• NPN BJT NPN Bipolar Junction Transistor
• IGBT Insulated Gate Bipolar Junction Transistor
• N-Channel MOSFET N-Channel Metal Oxide Silicon
  Field Effect Transistor
• SIT Static Induction Transistor
• RCT Reverse Conducting Thyristor
• GATT Gate Assisted Turn Off Thyristor

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Power Semiconductor Devices, their Symbols
Characteristics

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DEVICE SYMBOLS CHARACTERISTICS
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Prof. M. Madhusudhan Rao, EC Dept., MSRIT
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Comparison between different commonly used
Thyristors

• Line Commutated Thyristors available up to 6000V,
  4500A.
• Ex Converter grade (line commutated) SCR.
• V / I rating 5KV / 5000A
• Max. Frequency 60Hz.
• Switching time 100 to 400?sec.
• On state resistance 0.45m?.

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Example of Inverter Grade Thyristor Ratings

• V / I rating 4500V / 3000A.
• Max. Frequency 20KHz.
• Switching time 20 to 100?sec.
• On state resistance 0.5m?.

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Example of Triac Ratings

• Used in heat / light control, ac motor control
  circuit
• V / I rating 1200V / 300A.
• Max. Frequency 400Hz.
• Switching time 200 to 400?sec.
• On state resistance 3.6m?.

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Example of Power Transistor Ratings

• PT ratings go up to 1200V / 400A.
• PT normally operated as a switch in CE config.
• Max. Frequency 400Hz.
• Switching time 200 to 400?sec.
• On state resistance 3.6m?.

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Example of Power MOSFET Ratings

• Used in high speed power converters like
  inverters choppers.
• Ratings up to 1000V / 100A.
• Example MOSFET 800V / 7.5A rating.
• Max. Frequency 100KHz.
• Switching time 1.6?sec.
• On state resistance 1.2m?.

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Example of IGBT Ratings

• Used in high voltage / current high frequency
  switching power applications (Inverters, SMPS).
• Example IGBT 2500V / 2400A.
• Max. Frequency 20KHz.
• Switching time 5 to ?sec.
• On state resistance 2.3m?.

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• IGM w
• IG
• r p
• o
• n
• s
• x
• q
• y
• z
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• 4Load line
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• VG
• VGD
• FIGURE 18.2 Typical gate characteristics of an
  SCR.

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Photo-SCR coupled isolator
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Short pulse
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Long pulse
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Pulse train generator
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Pulse train with timer and AND gate
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Gate Triggering Methods

• - Efficient reliable method for turning on SCR.
• Types
• R - Triggering.
• RC - Triggering.
• UJT - Triggering.

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R-Triggering

• Resistance firing circuit

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RC Triggering

• RC half-wave trigger circuit

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Gate triggering characteristics
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Gate protection circuit
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Gate input characteristics
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High temperature due to
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Snubber
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Trajectory comparision with and without capacitor
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Turn off snubber circuit