Post on 01-Oct-2020
transcript
LED Lighting&
Visible Light Communication (VLC)
Dr. Karel L Sterckx
karel.s@bu.ac.th
http://bucroccs.bu.ac.th
EGTET 2020
6-7 March 2020
Outline
Semiconductor and Diode Basics
LED Lighting
High Brightness LEDs
LED Drivers
The LED Lighting Advantage
Eye Safety Issues
VLC Concept
Types of Optical Wireless Communication (OWC) and Their Applications
VLC Antennas
Merits of VLC
Drawbacks of VLC & Measures to mitigate these
OWC Standards
Slide 2
Semiconductor Basics
Material with an electrical conductivity between that of a metal and an insulator
Consists of either
Single elements that have 4 electrons on the outer shell
Compound of elements that have 3 and 5 electrons on the outer shell
Conductivity may be altered by introducing impurities (doping)
Donor impurities create N material
• Has free electrons at room temperature
Acceptor impurities create P material
• Leaves holes in the crystalline structure
• Since holes are able to attract electrons, they are considered positive
Figures on the next slide show
Silicon (Si) atom
Creation of P and N material
Slide 3
Semiconductor Basics (2)
Slide 4
E
Intrinsic (I) material P material N material
≡≡≡≡
Si14
Ge32
Al13
Ga31
In49
P15
As33
Sb51
III IV VAl = Aluminium
Ga = GalliumIn = Indium
Si = Silicon
Ge = Germanium
P= PhosphorusAs = Arsenic
Sb = Antimony
Intrinsic Si: ~5x1022 atoms/cm3
Doping Concentrations: 1013-1018 atoms/cm3
Diode Basics
Diode, Light Emitting Diode (LED) and Photodiode are essentially the same
Semiconductor device that consists of P and N-material
In reality, N and P regions are grown on a substrate
Electrons migrate to fill holes → electrons and holes combine
P and N material become negatively and positively charged, respectively
Migration stops after the voltage created by the migrated charge carriers reaches a certain value
Between P and N, a depletion zone (depleted of free charge carriers) is created
Slide 5
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depletion zone (intrinsic)
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Diode Basics
When electron and hole combine, a energy packet is released in the form of a photon (= elementary particle of light)
Diode in forward bias
Current-to-light conversion
LED
Diode in reverse bias
Light-to-current conversion
Photodiode
Slide 6
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Photodiode vs solar cell
In principle, they are the same
Photo Diode
Response time (speed) is important → Depletion zone is kept minimal to reduce junction capacitance
To further reduce the junction capacitance (to increase the response time), a photodiode may be negative biased (= photocurrent mode)
Solar cell
Response time is of no importance
The amount of captured sunlight is important → Depletion zone is very large
As it makes no sense to apply a voltage over a solar cell, it is never negative biased (= photovoltaic mode)
Slide 7
LED
Slide 8
Wavelength (colour) depends on the semiconductor compound
Forward voltage drop over depletion zone increases with emitted frequency
http://lednique.com/tag/led
High Brightness LEDs (HBLEDs)
Divide between LED and HBLED is somewhat arbitrary →→→→Efficacy > 50 lumens/watt is often quoted
Efficiency
Blue and UV: About 80%
Red: About 60%
Green: About 30%
White HBLEDs, in fact, emit blue light
Part of the light converted by a yellow phosphor
Converted part and non-converted part mixed to obtain white light of desired colour temperature (warm or pure white)
Material: InAlGaN
Slide 9
https://ledheadgrowlights.com/blogs
/news/red-white-or-blue-choosing-
the-right-led-color
High Brightness LEDs (2)
Coloured HBLEDs
InAlGaN emitting UV light
All light converted by a phosphor to obtain the desired colour
RGB HBLED
R = InAlGaP
G and B = InAlGaN
Due to limited efficiency of R and G used in displays only
Slide 10
LED Drivers
Constant current source
LEDs convert current to light!
Up to saturation, this conversion is linear
Requirements
Power Effective
Isolate LEDs from the mains (220V/50Hz in India)
Driver that fulfils both requirements
Flyback
Drivers that fulfil only the first requirement (used when powered by low DC voltage, e.g. certain spotlights)
Buck (Supply voltage higher than total forward voltage over LEDs)
Boost (Supply voltage lower than total forward voltage over LEDs)
Buck/Boost (Both lower and higher supply voltages)
SEPIC (Both lower and higher supply voltages)
Slide 11
The LED Lighting Advantage
More power efficient: 80% vs 50% compared to Compact Fluorescent Lamps (CFLs)
Larger life times: 5-10 times longer (25-50 years)
No degrading in performance over time
No hazardous materials such as mercury and halides
No burning hazard as LED lamps become lukewarm only
Only drawback used to be acquisition price →→→→ Issue has been addressed and prices have become competitive with those of gas discharge lamps
Slide 12
Eye Safety Issues
Slide 13
https://en.m.wikipedia.org/wiki/Eye
Eye Safety Issues (2)
With regard to visible light, the retina is a risk
Retina is the human photodetector
Consists of cones that detect Red, Green and Blue (RGB) light, respectively
Retina is most sensitive to shorter wavelengths, i.e. blue light
For LED lighting, the blue component is the primary concern
Iris absorbs visible light →→→→ Closes more at higher intensities
Lens absorbs blue light
Lenses of young people pass about 65% of blue light
Around 25 years, only 20% of blue light is passed → adult eye filters blue light a lot more
In aging eyes, scattering may increase →→→→ blurred vision
Particular problem for blue light
Older people may not see blue LED displays clearly
Slide 14
Eye Safety Issues (3)
According to a recent (2017) 92 page study commissioned by the European Union, there is no evidence of direct adverse health effects from LED emission in normal use (lamps and displays)
However, it is advisable to closely monitor long terms effects as this data is not yet available
In environments populated by children, it is advisable to use warm white light instead of cold white light
Slide 15
https://ec.europa.eu/health/sites/health/files/scientific_committees/scheer/docs/scheer_o_011.pdf
Visible Light Communication (VLC)
LED lights can be switched rapidly
Human eye detects average intensity
Photodetector sees zeros and ones
LED lighting can also wirelessly transmit data
Concept is known as VLC
Slide 16
http://visiblelightcomm.com/wp-
content/uploads/2013/03/VLC.png
Types of OWC and Their Applications
OWC = Optical Wireless Communication
Wireless Connectivity via a light carrier
Carrier = Infrared (Ir), visible light of any colour, or Ultra Violet (UV)
VLC = Visible Light Communication (downlink only)
Slide 17
Types of OWC and Their Applications (2)
OWC = Optical Wireless Communication
Wireless Connectivity via a light carrier
Carrier = Infrared (Ir), visible light of any colour, or Ultra Violet (UV)
VLC = Visible Light Communication (up- and downlink)
Slide 18
www.naka-lab.jp/product/uvlc_feature_e.html
Types of OWC and Their Applications (3)
OWC = Optical Wireless Communication
Wireless Connectivity via a light carrier
Carrier = Infrared (Ir), visible light of any colour, or Ultra Violet (UV)
VLC = Visible Light Communication
IrWC = Infrared Wireless Communication
Slide 19
VLC
IrWC
Types of OWC and Their Applications (4)
OWC = Optical Wireless Communication
Wireless Connectivity via a light carrier
Carrier = Infrared (Ir), visible light of any colour, or Ultra Violet (UV)
VLC = Visible Light Communication
IrWC = Infrared Wireless Communication
FSO = Free Space Optics
Slide 20
Wireless ‘Last Mile’
Fixed Line
Types of OWC and Their Applications (5)
OWC = Optical Wireless Communication
Wireless Connectivity via a light carrier
Carrier = Infrared (Ir), visible light of any colour, or Ultra Violet (UV)
VLC = Visible Light Communication
IrWC = Infrared Wireless Communication
FSO = Free Space Optics
UVWC = Ultraviolet Wireless Communication
Slide 21
http://spectrum.ieee.org/aerospace/military/ultraviolet-radios-beam-to-life
VLC Antennas
Slide 22
Transmitter (TX)
Light Emitting Diode (LED)
Until saturation
Almost every recombination of an electron–hole pair excites a photon
Current-to-intensity conversion is linear
Wavelength (colour) depends on semiconductor compound
Bandwidth limiting factor: Junction Capacitance
Receiver (RX)
Large Area Photodiode (Si-PIN)
Until saturation
Almost every photon that is absorbed excites an electron hole-pair
Intensity-to-current conversion is linear
Converts near Ir (sensitivity peak around 850 nm) and visible light
Bandwidth limiting factor: Junction Capacitance
Modulation Format: Intensity Modulation (IM)/Direct Detection (DD)BEWARE: Unipolar
Infrared Emitting Diode (IrED)
Used in Infrared Wireless Communication (IrWC), including uplink in LiFi Systems
Emission level restricted by eye safety standards
Material: GaAs or AlGaAs
Threshold voltage about 1.6 V
Slide 23
Only practical photo detector for VLC and IrWC to date
Intrinsic layer between P and N layer to enlarge the photosensitive junction →→→→ Also increases the junction capacitance
Receives both visible light and near Ir
Si PIN Photodiode
www.repairfaq.org/sam/sipdresp.gif
Merits of VLC
Larger bandwidth (factor 1,000) and entirely license free
BEWARE: License free ≠≠≠≠ Unregulated
Does not cause and is not affected by EMI
Cheaper transceiver components and less complex transceiver designs
Light cannot penetrate walls and other opaque objects →→→→Security at the physical layer
Footprint can be more accurately designed
No fading as the photosensitive detection area is much larger than the wavelength of the carrier
Slide 25
Drawbacks of VLC & Measures To Mitigate
Capacitances of LED and photodiode limit attainable bandwidth
Equalisation at TX (active) or RX (passive)
Multicarrier transmission
At RX, TIA isolates, to a certain extend, the capacitance of photodiode
Transmission power restricted by eye safety regulations,
Sensitive RX required (especially for Ir light)
Background Noise caused by ambient light
Mitigated via HPF
Multipath dispersion (MPD) as the signal reaches the receiver through various paths of different length
Multicarrier Transmission
High dynamic range
Automatic Gain Control (AGC)
Drive voltage amplifier of RX into saturation (digital signals only)
Slide 26
OWC Standards
IEEE 802.15.7-2018
IEEE Standard for Local and metropolitan area networks - Part 15.7: Short-Range Optical Wireless Communications
Defines a physical layer (PHY) and medium access control (MAC) sublayer for short-range OWC
https://ieeexplore.ieee.org/document/8697198
IEEE 802.15 WPAN Task Group 13 (TG13) Multi-Gigabit/s Optical Wireless Communications
www.ieee802.org/15/pub/TG13.html
Standard (IEEE 802.15.13) targeted for publication in Q4 2020
Slide 27
Thank you for your attention!Questions?
Dr. Karel L SterckxDirector BU-CROCCS
karel.s@bu.ac.th
http://bucroccs.bu.ac.th