WO2012119244A1

WO2012119244A1 - Single stage power factor corrected flyback converter with constant current multi-channel output power supply for led applications

Info

Publication number: WO2012119244A1
Application number: PCT/CA2012/000228
Authority: WO
Inventors: Donald Wesley Church; Alan Shawn Winters; Mark Adam Neary; Qiuning CHENAND; Jack Yitzhak Josefowicz
Original assignee: Led Roadway Lighting Ltd.
Priority date: 2011-03-07
Filing date: 2012-03-07
Publication date: 2012-09-13

Abstract

A method and a power supply for use with a light emitting diode (LED) module is provided. A power factor correction (PFC) circuit switches a flyback converter to provide power supplied to the LED module. A current controller is for monitoring voltage and/or current for operation of the LED module and determining a feedback for correcting the voltage and/or current to provide constant light output of the LED module. The feedback is provided to the PFC circuit via a feedback circuit having an optical isolation device for providing isolation between the primary and the secondary side of the flyback converter. The feedback may be adjusted by sinking or sourcing current to or from the feedback circuit.

Description

SINGLE STAGE POWER FACTOR CORRECTED FLYBACK CONVERTER WITH CONSTANT CURRENT MULTI-CHANNEL OUTPUT POWER SUPPLY FOR LED

APPLICATIONS

TECHNICAL FIELD

[0001] The present disclosure relates to power supplies and in particular to power supplies for powering light emitting diode (LED) modules in a lighting fixture.

BACKGROUND

[0002] Lighting fixtures, particularly outdoor or roadway lighting fixtures, are exposed to a range of environmental factors that impact performance and longevity of lighting fixtures. Existing roadway lighting commonly uses high-intensity discharge lamps, often high pressure sodium lamps (HPS). The power supply designs have been relatively simple but the light quality, efficiency and controllability of the fixtures has been less than ideal. The introduction next generation lighting fixtures such as light emitting diode (LED) based lighting fixtures provides greater efficiency, light quality and controllability however present challenges in ensuring reliable operation for the life of the lighting fixture. Factors such as thermal control, power efficiency, current regulation and packaging constraints must be accounted for to meet operation requirements. The temperature extremes and packaging restraints require an efficient design to ensure reliability. Providing a power supply that meets the demanding design requirements and cope with environmental extremes has not to date been achievable.

[0003] Accordingly, apparatus and methods that enable an improved power supply for light emitting diodes remains highly desirable.

SUMMARY

[0004] In accordance with the present disclosure, there is provided a power supply for use with a light emitting diode (LED) module, which includes: a rectifier for converting an alternating current (AC) from a source to a direct current (DC); a power factor correction (PFC) circuit for providing power supplied to the LED module by switching a flyback converter to control the DC voltage; a current controller for monitoring voltage and/or current for operation of the LED module and determining the feedback for correcting the voltage and/or current to provide constant light output of the LED module; and a feedback circuit for providing a feedback to the PFC circuit to control the PFC circuit and providing an optical isolation between the primary and the secondary side of the flyback converter.

[0005] In accordance with the present disclosure, there is provided a method operating a power supply for a light emitting diode (LED) module, which includes: converting an alternating current (AC) from a source to a direct current (DC); providing power to the LED module by a power factor correction (PFC) converter, including switching a flyback converter to control DC voltage; monitoring voltage and/or current for operation of the LED module by a current controller; determining feedback for correcting the voltage and/or current to provide constant light output of the LED module; and providing the feedback to the PFC circuit via a feedback circuit for providing an optical isolation between the primary and secondary side of the flyback converter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Further features and advantages will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Figure 1 depicts in a block diagram an example of a power supply system;

Figure 2 depicts in a block diagram a current controller and a load in the power supply system shown in Figure 1 ;

Figure 3 depicts in a schematic diagram an example of the current controller and the load shown in Figure 2;

Figure 4 depicts in a block diagram an example of a circuit between a current controller chip and a feedback circuit;

Figure 5 depicts in a schematic diagram an example of the feedback circuit;

Figure 6 depicts in a schematic diagram a single stage PFC flyback converter, an AC rectifier and an output rectifier; Figure 7 depicts in a schematic diagram a part of the flyback converter, the output rectifier and an output filter;

Figure 8 depicts in a flow chart an example of an operational flow for operating a LED module by the power supply system shown in Figure 1 ; and

Figure 9 depicts in a flow chart an example of a time of night dimming operation by the power supply system of Figure 1.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0007] Referring to Figure 1 , there is illustrated an example of a power supply system 100. The power supply system (or Advanced Constant Current PFC Driver system) 100 operates a load 118. The load 108 includes, for example, one or more light emitting diodes (LEDs) provided in strings or module comprising multiple lights. The power supply system provides the LED with controllable constant current that determines lumen output. The power supply system 00 with the load 118 may be mounted on a LED lighting fixture.

[0008] The power supply system 100 includes an electro-magnetic interference (EMI) filter 104, an AC rectifier (bridge rectifier) 106, a flyback converter 108, a power factor correction (PFC) circuit 110, an output rectifier 112, an output filter 114, a current controller block 116, an optically isolated feedback circuit 120, and a microprocessor 122. The microprocessor 122 communicates with the current controller block 116 to operate the current controller block 116. The microprocessor 122 communicates with external devices, such as, external sensor(s) 24 and a plug-in daughter board 26.

[0009] The power supply system 100 receives an alternating current (AC) from a source 102. The power supply system 100 is designed, for example, to accept input voltages typically ranging from 120 Vac-480 Vac, 50-60Hz nominal, including North American Voltage and European Voltage. The input AC current is fed to the EMI filter 104 to limit any conducted interference introduced by the power supply to the utility grid. The AC rectifiers (bridge rectifier) 106 converts the AC input to a DC input for the PFC circuit 110.

[0010] The flyback converter 108 and the PFC circuit 110 form a single-stage PFC flyback converter. In a non-limiting example, the single-stage PFC flyback converter includes a power factor correction integrated circuit (IC) operating in transition mode (discontinuous mode) to provide power factor correction and flyback switching all with one chip, reducing component count and increasing efficiency and power factor. The transition mode operation reduces the turn-on losses of the power switch which helps to improve efficiency and reduces the size of heat sink required. By using the PFC IC, high power factor correction (e.g., PF >.95) is achieved. Other similar PFC devices can also be used interchangeably. Because the power factor correction IC uses a large voltage ripple from the bridge rectifier for proper detection of the phase of the input voltage, the normally large, bulky, electrolytic input filter capacitors can be replaced with much smaller, more reliable, film or ceramic capacitors.

[0011] The PFC circuit 110 switches the flyback converter 108 with a converter metal- oxide-semiconductor field-effect transistor (mosfet) (e.g., Q6 of Figure 6) to produce the lower voltage required by the LED's. For example, the flyback converter 108 converts high DC voltage to lower DC voltage (Voltage step down, e.g., 165V to 36V). The PFC circuit switches the flyback converter 108 mosfet (metal-oxide-semiconductor field- effect transistor ) in sync with the AC line voltage, which keeps the current waveform in phase with the line voltage. This, in conjunction with limiting current spikes and peaks (harmonics), ensures good power factor meaning that the apparent power produced by the utility, and the real power consumed by the power supply, are very close to being the same resulting in minimal wasted energy as the power supply cannot make full use of the power generated by the out of phase voltage and current. The PFC chip in the PFC circuit 1 0 also provides over voltage and short circuit protection.

[0012] The output rectifier 1 2 and the output filter 114 are used to reduce voltage and current ripple in the output of the flyback converter 108. A large (e.g., 120Hz) ripple inherent with a PFC Flyback power supply is filter out by the output filter 114 capacitors. In a non-limiting example, automotive grade electrolytic capacitors are used in order to provide this amount of capacitance in a relatively small area and remain cost effective, 20 year life expectancy.

[0013] The current controller block 116 monitors LED voltage and/or current through the LED, and determines a feedback for correcting the LED voltage and/or current flowing in the LED strings to provide constant light output of the LED module. The feedback is provided to the PFC circuit 110 via the feedback circuit 120. The flyback converter relies on the feedback to provide the proper output. By sourcing or sinking current from or to the feedback loop, the flyback converter can be fooled into providing more or less output voltage, this achieves dynamic headroom control. The current controller block 116 also controls the current flowing in the LEDs by sensing the current with a current sense resistor.

[0014]The feedback circuit 120 allows the primary side of the system to monitor the output on the secondary side and adjust the primary mosfet switching of the converter as necessary to provide a constant output. Galvanic isolation between the primary and secondary sides is accomplished using an automotive grade optical isolation device as described below.

[0015] Referring to Figures 1 and 2, in a non-limiting example, the current controller block 116 includes one or more LED current controller modules 130a~130n for one or more LED modules 132a~132n. The LED module includes one or more LEDs or LED strings.

[0016] Referring to Figures 1 and 3, in a non-limiting example, the LED current controller module 130i includes a LED string current controller (or driver chip) 134i. The LED string current controller 134i is coupling to a plurality of LED strings 140a~140m via switches 142a~142m. The LED strings 140a~140m are arranged, for example, to endure the forward voltage of all LEDs remains below 42 V for UL class 2 operation. Each LED string current controller 134i may control up, for example, four strings of LEDs, which allows for tight current regulation of individual strings without requiring multiple switching ICs.

[0017] In this example, a dynamic headroom control (DHC) 148a, a pulse width modulated (PWM) signal 148b, a sensor input(s) 148c, and an enable/fault 148d are coupling to the LED string current controller 134i. The sensor input(s) 148c includes, for example, but not limited to, the outputs of temperature sensors, motion detectors, vibration detectors, global positioning receivers, ambient light sensors, or combinations thereof.

[0018]The LED string current controller 134i monitors voltage and/or current of the LED string, and determines a feedback 138i for correcting the voltage and/or current to provide constant light output of the LED module. The feedback 138i is adjusted and provided to the PFC circuit via the optically isolated feedback circuit 120. The LED string current controller 134i senses the current flowing in the LED string and operates on the switch coupling to the LED string.

[0019] In this example, the switches 142a~142m are mosfets (hereinafter referred to as controller mosfets). The controller mosfets 142a~142m may be operated in a fully on mode or in a non-fully on mode (linear region mode). The controller mosfets 142a~142m are coupling to current sensors 144a~144m, respectively, to dim the LEDs as required. In this example, the current sensors 144a~144m are resistors (hereinafter referred to as current sensing resistors).

[0020] LED current control is implemented by the LED string current controller 134Ί by sensing the current (e.g., Source a) with the current sensing resistor and adjusting the gate voltage of the controller mosfet by a gate driver in the current controller 134i, and/or determining the feedback 138i by sensing the drain voltage (e.g., Drain a) of the controller mosfet.

[0021] The precise amount of voltage required by the LEDs for a particular current can be set by changing the values of the current sensing resistors 144a~144m. This sets the maximum current that can flow through the LEDs when the mosfets are operating in the fully on mode.

[0022] The LED string current controller 134i implements linear current regulation to control the current in each LED string by operating the controller mosfets 142a~142m in their linear region (not in a fully on mode). By adjusting the gate voltage of the controller mosfet, the resistance of the controller mosfet during conduction can be changed so as to limit the amount of current flowing in the LED string. The on resistance of the mosfet has a linear response to the gate voltage when not operating in the fully on mode (minimum resistance). The current controller makes use of this linear mode of operation to control and monitor current flowing in the LEDs.

[0023] In a non-limiting example, the DHC 148a is used to define the minimum drain voltage of the controller mosfet and/or DHC response time, which provides functionality maximizes efficiency by adjusting the main flyback converter output to produce only the required voltage needed to drive the LEDs at a particular current.

[0024] Dimming of the LEDs is accomplished by supplying the PWM waveform 148b to the LED string current controller 134i, which in turn operates the controller mosfet, resulting in controlling the switching rate and average current of the LEDs. By varying the duty cycle of the switch using the PWM to drive the gate of the switch, the average current can be adjusted in the LED string without affecting the color of the LED as the peak current in the string remains constant. In this example, the microprocessor 122 provides the PWM signal 148b. It can be programmed to provide a constant PWM signal resulting in a constant average LED current as required by the end user.

[0025] As the LEDs age the light output decreases. To accommodate aging of the LEDs the PWM rate can be adjusted to the LED string current controllers based on values set in the microprocessor 122, which in turn regulate the current through the LEDs. The LED string current controller module 130Ί will adjust the feedback 138i to the PFC circuit 110 to maintain optimized efficiency. The compensation may be performed based upon a pre-defined aging schedule or based upon one or more sensor output used to monitor light output of one or more LEDs. The desired target light output can then be maintained across the lifespan of the light fixture.

[0026]A plurality of LED string current controllers may be connected each other via a sync pin of the LED string current controller in a master/slave mode to accommodate more LED strings. The LED string current controller 134i may be a National Semiconductor™ LM3464 series linear LED driver, or comparable device, having a linear current regulation, a fault handing device, a DHC startup control, and a thermal control circuit. For example, pins 16-19 of LM3464 are used to sense the voltage on the drain of the controller mosfets; pins 20, 24, 26, and 28 are used to sense the source current of the current mosfets; pins 21 , 23, 25, and 27 are used to drive the gates of the controller mosfets; pin 10 is used to output the feedback; pin 1 is used for master-slave configuration; pin 2 is used to receive the PWM 148b; and pin 15 is coupling to a LED rail voltage (which is supplied by, for example, D1 of Figure 7).

[0027] Referring to Figures 1 , 3 and 4, the current controller block 116 further includes a circuit 150. The input 152 of the circuit 150 is coupling to the feedback (e.g., 138i) from the LED string current controller. The output 154 of the circuit 150 is coupling to the feedback circuit 120. In a non-limiting example, the circuit 150 includes a diode D17 and resistors R64, R65, R98, and R100. R98 and R100 form a voltage divider network. The feedback 138i is adjusted by sourcing or sinking current by using D17 and R99 and the voltage divider network, resulting in increasing/decreasing the LED string voltage.

[0028] The diode D17 is, for example, a Schottkey barrier diode, which functions with one or two feedback signals, each being from one LED string current controller (e.g., 134i of Figure 3). The diode allows the two inputs to be connected to the same reference without interacting with each other.

[0029] An opt-coupler or isolated is used to provide feedback data from the secondary to the primary side while providing electrical isolation between the primary and secondary circuits in a flyback converter, which is not rated for long life and has a high degradation rate. Since galvanic isolation is required for class 2 power supplies, a solution was required to provide a long life solution for an isolated feedback loop.

[0030] Referring to Figures 1 and 5, an automotive rated digital opto-isolator 160 is employed in the feedback circuit 120 together with an operational amplifier 62. The opto-isolator 160 provides electrical isolation of the output from the utility grid for safety against shock and improved surge immunity for the LED modules. The opto-isolator 160 and the operational amplifier 162 are coupling to a power line 166 coupling to a voltage regulator (now shown). The opto-isolator 160 has pins for input voltages Vin+, Vin-, output voltages Vout+, Vout-, power supply VDD1 and VDD2, and grounds GND1 , GND2. The input voltage Vin+ pin is coupling to the output 154 from the circuit shown in Figure 4 via a resistor R67. The outputs Vout+ and Vout- are coupling to an operational amplifier 162. The operational amplifier 162 calculates the difference between Vout+ and Vout- of the opto-isolator 160 and converts it to the proper voltage reference required by the PFC circuit (PFC chip) 110. The output 164 of the operational amplifier 162 is provided to the PFC chip.

[0031]The opto-isolator 160 is, for example, an automotive isolation amplifier from Avago Technologies ™ ACPL 782T-000E. Using the digital opto-isolator, galvanic isolation can be accomplished is the feedback loop without compromising the lifetime rating of the power supply. The digital opto-isolator may be rated to 125°C and transfers information digitally across the optical barrier using precision analog to digital (A/D) converter technology. Because of the digital nature of the data being transferred, it is not as subject to the degradation of the internal infra-red LED like the opto-coupler which is an analog device and relies on the brightness of the LED to transfer information.

[0032] Referring to Figure 1 , the power supply incorporates the onboard microprocessor 122 to provide additional monitoring and control features. The microprocessor 122 receives various incoming digital or analog data and converts the incoming data to a corresponding PWM signal which will control the brightness of the LEDs from 0-100% based on its programming as required by the end user. The incoming signals include, for example, a 0-10Vdc analog input voltage for dimming of the LEDs, temperature sensors, motion detectors, vibration detectors, ambient light sensors, sun-set/sun-rise information, global positioning information, etc. Custom firmware is configured to receive and process the incoming data and generate a resultant PWM signal based on, for example, the required average LED current for a given input.

[0033] The onboard microprocessor 122 receives the incoming data through, for example, an I2C communications bus, UART (Universal Asynchronous Receiver/Transmitter), or direct analog voltages through several analog to digital (A/D) ports available on the microprocessor 122. The I2C communications link to an external board may be used to control LED current, on/off, error logging. The microprocessor 122 may also receive commands from other circuits (e.g., GPS module, wireless module, etc..) via several communication ports.

[0034]The microprocessor 122 is, for example, but not limited to, a Texas Instruments ™ MSP430F272 which has multiple communication protocols, analog to digital inputs. [0035] Figures 6 and 7 are schematic diagrams illustrating an example of the AC rectifier 116, the single-stage PFC flyback converter, and the output rectifier 112 and output filter stage 114. The AC rectifier 106 includes, for example, a diode bridge, which allows flexibility in choosing over rated components to exceed the designed current and voltage rating, ensuring the highest level of efficiency and life from the bridge.

[0036] The single-stage PFC flyback converter includes a PFC chip 180 having multiple pins (i.e., INV, COMP, MULT, CS, VCC, GD, GND and ZCD). The PFR chip 180 receives a feedback (e.g., 164 of Figure 5) from the feedback circuit 120 at a node 182. The flyback converter 108 has a primary winding 190, an auxiliary winding 192, a secondary winding 194 and a converter mosfet Q6 coupling to the PFC chip 180 and the primary winding 94. The PFC chip 80 may be a L6562DTR. A common snubber circuit 184 is provided to help filtering of noise generated by Q6. D20 connects to a 5V regulator 202 to supply the required voltage to the primary side of the opto-isolator. A rectifier D1 is an ultrafast, dual diode device to minimize power losses. The LED rail voltage 200 is supplied by D1. The 12V auxiliary voltage 204 is supplied by D6.

[0037] Referring to Figures 3, 5 and 6, once LED string controllers are running, the voltage on the drain of the controller mosfets is sensed by the LED string current controller 134i. As the current controller module 130i will adjust the amount of current it sinks from the opto-isolator 160, the 2.5V reference applied to the pin 1 of the PFC chip 180 begins to drop. The PFC chip 180 is designed to maintain pin 1 at 2.5V, and will increase its switching frequency to boost the LED string voltage to compensate for the feedback voltage drop until 2.5 V is restored to pin 1 of the PFC chip 180. In this way, the string current controllers can override the normal feedback voltage to keep the system optimized under a wide range of operating conditions.

[0038] The 2.5V reference voltage may be directly proportional to the sensed voltage on R100 of Figure 4. By sinking or sourcing current via the voltage divider network of R98/R 00, the 2.5V reference can be decreased or increased which will cause the PFC/Flyback converter to react and adjust the output to bring the voltage at R100 back to 2.5V.

[0039] In one example, the current controller 134i of Figure 3 is a LM3464MH, and the PFC chip 180 of Figure 6 is a L6562DTR. In this case, the voltage that has been filtered by the output filter is applied to pin 15 of LM3464MH. Once the string controllers are running, the voltage on the drain of the controller mosfets is sensed by LM3464MH at pins 16-19. The PWM signal from an external source is accepted via pin 2 of LM3464MH which in turn controls the gates of the controller mosfets (e.g., 142a~142m of Figure 3) controlling the switching rate and average current of the LEDs. Additional string current controller ICs can be added to increase the number of LED strings available by connecting pin 1 (sync pin) of LM3464MH to pin 2 of the next string current controller chip in the chain. The first string current controller will be the master and receive the external PWM signal and forward it to any successive string current controller IC which becomes the slaves. The slave ICs will sync their switching rate to the master, though each IC has the ability to independently adjust the feedback loop or shut down should a problem be detected.

[0040] Referring to Figures 1-6, to design the system, quality, p-n semiconductor devices (i.e., diodes, metal oxide semiconductor field effect transistors (mosfets), etc ..) with higher current ratings (for example, 2 or 3 times higher) may be selected to allow operation at the very low end of the devices limits, providing maximum efficiency for the rated input and load that leads to the highest efficiency.

[0041] The power supply has a life expectancy of 20+ years. The guidance for reliability is based on Telcordia SR332 and the selection of components is intended to provide a Mean Time Between Failure of greater than 1 million hours at 25°C operating temperature. This may be accomplished, for example, but not limited to, using the following design approach: i. High Reliability components (i.e., Automotive equivalent or better) with extended temperature ranges (+125°C minimum) that leads to maximum life

ii. Oversized for power and/or temperature over rated components to ensure components operate well with their design limits that leads to maximum life iii. Use of aircraft grade aluminum electrolytic capacitors (such as Evox PEG series), rated at 4 times the life of a conventional commercial electrolytic capacitor iv. Eliminating the use of standard opto-couplers for electrical isolation and substituting long life, automotive grade, opto-isolators which do not suffer from current transfer ratio (CTR) degradation

v. Designed for semiconductor junctions temperatures at or below 100°C for improved reliability and long life

[0042] Referring to Figures 1 and 8, an example of an operational flow for operating a LED module by the power supply system 100 is described. EMI filtering of incoming AC current is performed (302). The AC input is then rectified (304) to convert the AC to full- wave DC voltage and current. The rectified DC voltage is stepped down to lower voltage DC (306), for example voltage step-down is performed form 165 Volts to 36 Volts. The primary current is monitored (308) by the PFC circuit 110. If current exceeds design tolerances short circuit protection is performed by the PFC circuit 1 0 (310).

[0043] Based upon the desired output current, a PFC value can be applied to the PFC circuit 110 (312) to ensure the desired current is supplied. Secondary side filtering may then be performed (314) to reduce ripple from voltage and current to provide cleaner output. The stepped down, and possibly filtered output is then applied to the LED current controllers (316). The LED current is measured (318), and the LED voltage is measured (320). It is then determined if the voltage and current are correct (322). Based on the determination, feedback voltage is adjusted to provide required LED voltage and current (324). The feedback is provided to the optically isolated feedback current (326), and then the PFC is applied (312).

[0044] Referring to Figures 1 and 9, an example of a time of night dimming operation by the power supply system 00 is described when used in a outdoor application such as in a streetlight fixture. The power supply system 100 implements Time-of-Night dimming. The Time-of-Night dimming function is based on the principle that for a given geographic location, the actual time of day can be determined with good accuracy if the number of hours of between sunset and sunrise are known, and if the time relative to sunset is known. The logic program performs two primary functions, firstly, it uses a special logic algorithm to determine the time of day, secondly, it executes the preprogrammed dimming schedule based on the determined time of day. [0045] To determine the time of day, the microprocessor 122 records the time period from sunset to sunrise for every twenty-four (24) hour period of operation to the nearest 15 minutes. It does this by recording its hours of operation, which is defined by the Streetlight (Dusk-to-Dawn) Photo Control (e.g., sensor 124) that provides a switched source of power to the system. The microprocessor 122 averages these measurements over a period of up to eight (8) days to determine an averaged result. This result is then compared to a data table stored in a memory coupled to the microprocessor 122 or internally stored in the microprocessor. This data table includes, for example, sunrise and sunset times for all twelve (12) months of the year, for the specific geographic location in which the streetlight is located. By comparing the averaged result to the data in the table, the microprocessor 122 can determine in which part of the year it is operating, and the actual sunset time. When the microprocessor knows the sunset time, and knows that its operation starts at each sunset, it can execute the pre-programmed dimming schedule to within the system accuracy, which is its second primary task. The pre-programmed dimming schedule can provide a gradual increase of light output of the light fixture during sunset and a gradual decrease during sunrise. In addition, the maximum output of the light fixture can be adjusted based upon illumination conditions at a particular time of year.

[0046] The logic algorithm has been developed to execute the dimming schedule if at least two consecutive sunset-to-sunrise measurements have been made that are within measurement accuracy of each other. In the case of insufficient data to execute the dimming schedule, the streetlight will be set to full brightness as a fail-safe mechanism. In the case of a hardware malfunction of the dimming system, the streetlight is automatically set to full brightness as a fail-safe mechanism based on the hardware design.

[0047] The logic algorithm also includes special logic to handle the case of measurements that are beyond the allowed measurement accuracy. In this case, of measurements beyond the allowed measurement accuracy, the system will clear internal measurement data and set the streetlight to full brightness until at least two consecutive measurements within measurement accuracy have been recorded. This logic safely handles the case of data corruption due to a random power interruption, or a situation where a streetlight is stored for a period of time.

[0048]As shown in Figure 9, a sunset time (402) is determined by a sensor or a GPS receiver. At sunset, the mains power is applied to the streetlight micro-processor, through the normal operation of the standard photo control. The microprocessor 122 starts recording the time that passes, relative to the time of sunset (turn-ON). A sunrise time is determined by the light sensor or a GPS, based upon detection of sunrise (404). Length measurement of the night period is then determined. If the measurement is within tolerance in that the period is not excessive large or small, (YES at 406), the microprocessor determines is a predefined number of night periods has been stored, for examples, at least two night periods. If the predetermined number of periods is stored in memory, (YES at 408), the microprocessor 122 calculates the average time period (410) from sunset to sunrise for the past number of days, up to an upper threshold such as the previous eight (8) days known as the (average) sunset-sunrise-duration. Alternatively if sufficient night periods have not been stored to provide the required consecutive data the LED module illumination is set to full brightness (422) during the illumination period. Note that if the data for eight (8) days has not been gathered yet, the micro-processor will average only the data that has been gathered so far. Based on the (average) sunset-sunrise-duration value, the microprocessor 122 can make an estimate of the time of sunset to the nearest fifteen (15) minutes. A lookup is then performed (412) in a dimming schedule table to determine an associated dimming sequence defining PFC values and the time or period of time for applying the value for controlling the illumination output of the LED modules. The lookup will match the average night period to the desired dimming sequence. The output can then be controlled (414) by adjusting each LED string current controller (e.g., 134i of Figure 3) at the times defined in the sequence (414).

[0049]At each turn-ON, corresponding to sunset, the microprocessor 122 performs a check of each new sunset-sunrise-duration measurement before that measurement is stored within the set of eight (8) previous measurements. If the new measurement is not within measurement accuracy of the values, (NO at 406), in the measurement set, the microprocessor 122 will clear the previous measurements (402) and will set the streetlight to full brightness (without dimming) or a predetermined value for the current night (422). The microprocessor 122 will store the new measurement as the first potentially correct reading at continuing (402).

[0050] Knowing the time of sunset to the nearest fifteen (15) minutes, and the time since sunset (turn-ON), the microprocessor 122 can estimate the current time of day within an accuracy of 15 minutes. With the time of day determined within an accuracy of fifteen (15) minutes, the microprocessor 122 can control dimming start and dimming stop functions within an accuracy of fifteen (15) minutes (assuming that there is no error introduced by the photo control operation).

[0051]The microprocessor 122 includes logic that correctly handles the case of a power interruption. The logic is the same, regardless of whether the power interruption is short term (several seconds) or long term (up to years). If a power interruption as occurred, (YES at 416), the stored data is reset and the LED module is set to full brightness (422) via the current controller until sufficient data has been stored.

[0052] On the subsequent turn-ON, if the next measurement is within measurement accuracy of the previous reading, the microprocessor 122 will use the two (2) stored measurements to execute the programmed dimming schedule. If the new measurement is not within measurement accuracy of the data already stored, the reset logic described will repeat, and again the streetlight will be set for full brightness for the current night.

[0053] Inaccuracy in the dimming schedule will be caused by several factors as follows: Some degree of error will be introduced by the sunset and sunrise switching of the photo control. This error will depend on the exact installation and may be based on the photo control model, the photo control installation, and weather conditions. The microprocessor averages measurements of sunrise- sunrise-duration data over a period of eight (8) days to minimize the effects of random error such as that caused by weather conditions for example.

[0054] To ensure that any system inaccuracy does not affect streetlight safety, the microprocessor 122 is programmed so that system inaccuracy causes a loss of low light level only and never a loss of high light level. The inaccuracy, then, reduces energy savings but not safety. Each installation of a group of streetlights using Time-of-Night dimming is performed with an assessment of the actual system inaccuracies, so that the microprocessor is configured accordingly.

[0055] In some embodiments, any suitable computer readable media can be used for storing instructions for performing the methods described herein. For example, in some embodiments, computer readable media can be transitory or non-transitory. For example, non-transitory computer readable media can include media such as magnetic media (such as hard disks, floppy disks, etc.), optical media (such as compact discs, digital video discs, Blu-ray discs, etc.), semiconductor media (such as flash memory, electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), etc.), any suitable media that is not fleeting or devoid of any semblance of permanence during transmission, and/or any suitable tangible media. As another example, transitory computer readable media can include signals on networks, in wires, conductors, optical fibers, circuits, and any suitable media that is fleeting and devoid of any semblance of permanence during transmission, and/or any suitable intangible media.

[0056]Although the description discloses example methods, system and apparatus including, among other components, software executed on hardware, it should be noted that such methods and apparatus are merely illustrative and should not be considered as limiting. Although reference is made to specific components manufacturer and part numbers, the particular components may be substituted for one or more components or discrete parts to achieve similar functions or capability.

[0057] The embodiments described herein may include one or more elements or components, not illustrated in the drawings. The embodiments may be described with the limited number of elements in a certain topology by way of example only. Each element may include a structure to perform certain operations. Each element may be implemented as hardware, software, or any combination thereof.

[0058] One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

Claims

What is claimed is:

1. A power supply for use with a light emitting diode (LED) module, the power supply comprising:

a rectifier for converting an alternating current (AC) from a source to a direct current (DC);

a power factor correction (PFC) circuit for providing power supplied to the LED module by switching a flyback converter to control DC voltage;

a current controller for monitoring voltage and/or current for operation of the LED module and determining the feedback for correcting voltage and/or current to provide constant light output of the LED module; and

a feedback circuit for providing a feedback to the PFC circuit to control the PFC circuit and providing optical isolation between the primary and the secondary side of the flyback converter.

2. The power supply according to claim , wherein the feedback circuit comprises: an optical isolation device.

3. The power supply according to claim 2, wherein the current controller comprises: a circuit for adjusting the feedback by sinking current from the current controller to the optical isolation device.

4. The power supply according to claim 2, wherein the feedback is adjusted by adjusting the amount of current from/to the optical isolation device to keep the drain voltage on a metal-oxide-semiconductor field-effect transistor (mosfet) coupling to the LED module at a minimum value.

5. The power supply according to claim 1 , wherein the current controller comprises: a mosfet coupling to the LED module; and

a gate driver for controlling the gate voltage of the mosfet.

6. The power supply according to claim 5, wherein the current controller comprises: a current sensing resister coupling to the mosfet, for sensing the current of the LED.

7. The power supply according to claim 6, wherein the current controller implements a linear current regulation to operate the mosfet in its linear region.

8. The power supply according to claim 1 , wherein the current controller implements dynamic headroom control (DHC) to the flyback converter's output.

9. The power supply according to claim 1 , comprising:

a microprocessor for providing a control signal for controlling the current controller.

10. The power supply according to claim 9, wherein the control signal output from the microprocessor comprises at least a pulse width modulated (PWM) signal, to the current controller for regulating the LED current based on the PWM signal.

11. A method of operating a power supply for a light emitting diode (LED) module, the method comprising:

converting an alternating current (AC) from a source to a direct current (DC); providing power to the LED module by a power factor correction (PFC) converter, including switching a flyback converter to control DC voltage;

monitoring voltage and/or current for operation of the LED module by a current controller;

determining feedback for correcting the voltage and/or current to provide constant light output of the LED module; and

providing the feedback to the PFC circuit via a feedback circuit for providing optical isolation between the primary and secondary side of the flyback converter.

12. The method of claim 11 , wherein the feedback circuit comprises:

an optical isolation device.

13. The method according to claim 12, wherein the determining feedback further comprises: adjusting the feedback by sinking current from the current controller to the optical isolation device.

14. The method according to claim 12, wherein the determining feedback comprises: adjusting the amount of current from/to the optical isolation device to keep the drain voltage on a metal-oxide-semiconductor field-effect transistor (mosfet) coupling to the LED module at a minimum value.

15. The method according to claim 11 , wherein the monitoring voltage and/or current comprises:

monitoring the drain voltage of a mosfet coupling to the LED module to determine the feedback.

16. The method according to claim 11 , wherein the monitoring voltage and/or current comprises:

monitoring current for operation of the LED module via a current sensing resister coupling to a mosfet for operating the LED module.

17. The method according to claim 11 , wherein the determining feedback comprises: implementing a linear regulation by operating a mosfet coupling to the LED module in its linear region.

18. The method according to claim 11 , wherein the determining feedback comprises: implementing a dynamic headroom control (DHC) to adjust the flyback converter's output.

19. The method according to claim 11 further comprising determining a dimming schedule based upon a sensor or GPS input.

20. The method according to claim 19 further comprising adjusting the current controller based upon the determined dimming schedule to control the output of the LED module.

21. The method according to claim 11 further comprising determining an age of the LED module and adjusting a pulse width modulated (PWM) signal, to the current controller for regulating the LED current based on the PWM signal to increase the current to the LED module