12/6/2011

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Light Emitting Diodes -and-

Voltage Multipliers A crash lecture on those little lights we call LEDs, plus a brief illustration of voltage multipliers

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Topics

• Light Emitting Diode (LED) construction and operation

• LED Color table

• Voltage Multiplier circuits

• Voltage Doubler

• Voltage Tripler and Quadrupler

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The LED.

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Energy Release

• Recombination of holes and electrons (within the structure and the PN junction) occur during forward bias

• Recombination = transfer of energy from the previously unbound/free electrons

• In all semiconductors, energy is given off as heat and photons

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Energy Release

• In Si and Ge diodes, %heat >> %light

• However, for other constructs such as gallium arsenide phosphide (GaAsP) or gallium phosphide (GaP), the number of photons is sufficient enough for visible light.

• Electroluminescence – the process of giving off light by applying an electrical source of energy

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Construction and Operation

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12/6/2011

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LED Characteristics Typical LED Characteristics

Semiconductor Material

Wavelength Colour VF @ 20mA

GaAs 850-940nm Infra-Red 1.2v

GaAsP 630-660nm Red 1.8v

GaAsP 605-620nm Amber 2.0v

GaAsP:N 585-595nm Yellow 2.2v

AlGaP 550-570nm Green 3.5v

SiC 430-505nm Blue 3.6v

GaInN 450nm White 4.0v 7

Voltage Doubler and Tripler

• Employed to maintain a relatively low transformer peak voltage while stepping up the peak to 2-4x the amount of the peak rectified voltage

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Voltage Doubler Operation

• Works similar to clamper operation – charging and discharging of capacitors through conducting and non-conducting combinations of diodes

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Full-wave Voltage Doubler

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Voltage Tripler and Quadrupler

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