1. Field of the Invention
The present invention generally relates to light emitting diodes (“LEDs”) and LED drivers. The present invention specifically relates to alternating current (“AC”) driven LEDs, LED circuits and AC drive circuits and methods.
2. Description of the Related Art
LEDs are semiconductor devices that produce light when a current is supplied to them. LEDs are intrinsically DC devices that only pass current in one polarity and historically have been driven by DC voltage sources using resistors, current regulators and voltage regulators to limit the voltage and current delivered to the LED. Some LEDs have resistors built into the LED package providing a higher voltage LED typically driven with 5V DC or 12V DC.
With proper design considerations LEDs may be driven more efficiently with AC than with DC drive schemes. LED based lighting may be used for general lighting, specialty lighting, signs and decoration such as for Christmas tree lighting. For example, U.S. Pat. No. 5,495,147 entitled LED LIGHT STRING SYSTEM to Lanzisera (hereinafter “Lanzisera”) and U.S. Pat. No. 4,984,999 entitled STRING OF LIGHTS SPECIFICATION to Leake (hereinafter “Leake”) describes different forms of LED based light strings. In both Lanzisera and Leake, exemplary light strings are described employing purely parallel wiring of discrete LED lamps using a step-down transformer and rectifier power conversion scheme. This type of LED light string converts input electrical power, usually assumed to be the common U.S. household power of 110 VAC, to a low voltage, rectified to nearly DC input.
Pat. Pending Application No. 0015968A1 entitled PREFERRED EMBODIMENT TO LED LIGHT STRING to Allen (hereinafter “Allen”) discloses AC powered LED-based light strings. Allen describes LED light strings employing series parallel blocks with a voltage matching requirement for direct AC drive placing fundamental restrictions on the number of diodes (LEDs) on each diode series block, depending on the types of diodes used. Allen discloses that for the forward voltage to be “matched,” in each series block, the peak input voltage must be less than or equal to the sum of the maximum forward voltages for each series block in order to prevent over-driving.
LEDs can be operated from an AC source more efficiently if they are connected in an “opposing parallel” configuration as shown by WO98/02020 and JP11/330561. More efficient LED lighting systems can be designed using high frequency AC drivers as shown by Patent Publication Number 20030122502 entitled Light Emitting Diode Driver (“Clauberg et. al.”) Clauberg et. al. discloses that higher frequency inverters may be used to drive an opposing parallel LED pair, an opposing parallel LED string and/or an opposing parallel LED matrix by coupling the LEDs to a high frequency inverter through a resonant impedance circuit that includes a first capacitor coupled in series to one or more inductors with the impedance circuit coupled in series to opposing parallel LEDs with each set of LEDs having a second series capacitor in series to the impedance circuit. In this system additional opposing parallel configurations of LEDs with capacitors may not be added to or removed from the output of the driver without effecting the lumens output of the previously connected LED circuits unless the driver or components at the driver and/or the opposing parallel LED capacitors were replaced with proper values. By adding or removing the opposing parallel LED circuits the voltage would increase or drop at the inductor and the current would increase or drop through the first series capacitor as the load changed therefore the inductor and all capacitors or entire driver would need to be replaced or adjusted each time additional LEDs were added to or removed from the system.
Patent application number US2004/0080941 entitled Light Emitting Diodes For High AC Voltage Operation And General Lighting discloses that a plurality of opposing parallel series strings of LEDs can be integrated into a single chip and driven with high voltage low frequency mains AC power sources as long as there are enough LEDs in each opposing parallel series string of LEDs to drop the total source voltage across the series LEDs within the chip. Patent numbers WO2004023568 and JP2004006582 disclose that a plurality of opposing parallel series strings or opposing parallel series matrix of LEDs can be integrated into a single chip and mounted on an insulating substrate and driven with a high drive voltage and low drive current as long as there are enough LEDs in each opposing parallel series string of LEDs to drop the total source voltage across the series LEDs within the chip. These patents and application disclose that for single chip or packaged LED circuits a plurality of opposing parallel series strings are required with the total number of LEDs in each series string needing to be equal to or greater than the AC voltage source in order to drop the total forward voltage and provide the required drive current when driven direct with low frequency AC mains power sources.
The present invention addresses the above-noted shortcomings of the prior art while providing additional benefits and advantages