Chapter 33
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Thyristors and Optical Devices |
Introduction to
Thyristors
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Thyristors |
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Switch |
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On-state, off-state |
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Unilateral or bilateral |
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Latching |
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High power |
Introduction to
Thyristors
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Thyristors |
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Sinusoidal |
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Firing angle |
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Conduction angle |
Triggering Devices
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Used to pulse switching devices |
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Diac |
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3-layer |
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Bi-directional conduction |
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Breakover voltage |
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Blocking region |
Triggering Devices
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Unijunction Transistor (UJT) |
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3-terminal device |
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Intrinsic standoff ratio |
Triggering Devices
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UJT |
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0.5 < η < 0.9 |
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Emitter region heavily doped |
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VE – B1 = 0, p-n
junction reverse biased |
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Increase VE – B1,
reach peak point (maximum current) |
Triggering Devices
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UJT |
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Continue increase, reach valley point |
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Further increase VE – B1,
UJT is saturated |
Triggering Devices
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UJT relaxation oscillator |
Silicon Controlled
Rectifiers (SCRs)
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4-layer device, p-n-p-n |
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Anode (A) |
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Cathode (K) |
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Gate (G) |
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Unidirectional |
Silicon Controlled
Rectifiers (SCRs)
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High-power (I up to 2500 A, V up to
2500 V) |
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Phase control |
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Small VAK when On |
SCRs
SCRs
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Operation |
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IG = 0, no anode current |
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IG > IGT →
regenerative feedback → high IAK |
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IAK < IH →
turn off → IAK = 0 |
SCRs
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Can cause SCR turn-on |
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High temperature |
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High ∆V/∆t (noise) |
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Radiation |
SCRs
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Specifications |
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VDRM or VRRM Peak Repetitive Off-state Voltage |
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IT(RMS) On-State RMS current (maximum) |
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ITSM Peak Non-Repetitive Surge current |
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IGT Gate trigger current |
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IL Latching current |
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IH Holding current |
SCRs
SCRs
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Small R1 |
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Short RC time constant |
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SCR turns on rapidly, close to 0° |
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Large R1 |
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long RC time constant |
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SCR turns on slowly, close to 180° |
SCRs
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Too large R1 |
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SCR does not turn on |
Triacs
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3-terminal switch |
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Bi-directional current |
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Symbol |
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Gate trigger may be either + or – pulse |
Triacs
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Characteristics |
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Direct replacement for mechanical
relays |
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Trigger circuit for full-wave control |
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4 modes |
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Remains on in either direction until I
< IH |
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Blocking region, I ≈ μamps |
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Small voltage across Triac when On |
Triacs
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Specifications |
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Similar to SCR |
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PGM Peak Gate Power |
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PG(AV) Average Gate Power |
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VGM Peak Gate Voltage |
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VGT Gate trigger voltage |
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tgt Turn-On Time |
Triacs
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Phase control light dimmer |
Triacs
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Circuit operation |
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Turn-off due to small load current |
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Capacitor charges/discharges through
load |
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DIAC is bi-directional |
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RC time constant → 0° to 180°
turn on in each direction |
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Power Control
Fundamentals
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Review equations |
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Control |
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Lamp intensity |
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Heat from a resistive heater |
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Speed of a motor |
Power Control
Fundamentals
Power Control
Fundamentals
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Delayed turn-on, full-wave signal |
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Delayed turn-on, half-wave signal |
Power Control
Fundamentals
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V and P curves for full-wave control |
Introduction to Optical
Devices
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Opto-electronic devices λ =
wavelength |
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Current → light |
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Light → current |
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c = speed of light in a vacuum |
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c = 3 x 108 m/s |
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Introduction to Optical
Devices
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Electromagnetic spectrum |
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Visible (380 < λ(nm) < 750) |
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Infrared region (750 < λ(nm)
< 1000) |
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Introduction to Optical
Devices
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LED is a diode |
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When forward biased |
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Electron-hole recombination energy |
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Photons released: E = hf , h is
Planck’s constant |
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h = 6.626 ´ 10–34
Joules∙seconds |
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High energy → visible spectrum |
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Lower energy → IR spectrum |
Introduction to Optical
Devices
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LED advantages |
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Low voltage |
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Rapid change in light output with input
V change |
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Long life |
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LED output can be matched to
photodetector |
Introduction to Optical
Devices
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LED disadvantages |
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Easily damaged |
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Brightness dependent on temperature |
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Chromatic dispersion |
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Inefficient compared to LCDs |
Photodetectors
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R varies with light intensity |
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Photoresistors |
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Voltage or current varies with light
intensity |
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Photodiodes |
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Phototransistors |
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Light-Activated SCRs (LASCRs) |
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Photodetectors
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Photodiodes |
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Reverse biased |
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Low ambient light → very small
current, ID (small leakage current) |
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High ambient light → increased
current, ID (increase in minority carriers) |
Photodetectors
Photodetectors
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Phototransistor |
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Base open |
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Light on reverse-biased CB junction |
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Increase minority carriers |
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Increase IC |
Photodetectors
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Phototransistor |
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Usually used as a switch |
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Off → IC = 0 |
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On → IC > 0 |
Photodetectors
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LASCR |
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Light-Activated SCR or photo-SCR |
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Symbol |
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Light turns LASCR on |
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Open gate or resistor on gate to
control sensitivity |
Optocouplers
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Couple two circuits |
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LED and Photodetector in single circuit |
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Electrical isolation |
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Medical equipment |
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High voltage circuit to digital circuit |
Optocouplers
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Use as |
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Linear device |
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Digital buffer |
Optocouplers
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Phototransistor optocoupler |
Optocouplers
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Current transfer ratio |
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0.1 < CTR < 1 |
Optocouplers
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Operation |
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High diode current in input circuit
yields |
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High diode light output which yields |
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High collector current in output
circuit |
Semiconductor LASERs
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Light Amplification through Stimulated
Emission of Radiation |
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Operation |
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Similar to LEDs |
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Monochromatic (same frequency) |
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Coherent (same phase) output |
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Small pulse dispersion |