LED lighting systems, LED controllers and LED control methods for a string of LEDs

The present disclosure relates generally to LED lighting systems and LED control methods therefor.

LED lights have several advantages. For example, LEDs have been developed to have lifespan up to 50,000 hours, about 50 times as long as a 60-watt incandescent bulb. Furthermore, an LED requires minute amount of electricity, having luminous efficacy about 10 times higher than an incandescent bulb and 2 times higher than a florescent light. As power consumption and conversion efficiency are big concerns in the art, LED lights are expected to replace several kinds of lighting fixtures in the long run.

A LED is a current-driven device. As commonly known in the art, the brightness of a LED is substantially determined by its driving current, and the voltage drop across the LED when illuminating, commonly referred to as forward voltage, is about a constant. FIG. 1 shows LED lighting system 20 according to US patent application publication 20120217887, which is incorporated herein by reference in its entirety. LED lighting system 20 in FIG. 1 has LED string 14 with LEDs 15 a , 15 b and 15 c connected in series. Bridge rectifier 12 , connected to a branch circuit providing an alternative-current (AC) voltage V AC , generates input voltage V IN as an input power source to power LED string 14 . Switch controllers C a , C b , and C c control path switches S a , S b , and S c , respectively, where each path switch is connected to a cathode of a LED. Mode decider 32 decides the operation modes of the operational amplifiers (C a /C b , and C c ), in response to current sense voltages VCS a , VCS b , and VCS c . Line waveform sensor 28 determines current-setting voltage V SET based on the present input voltage V IN , while current-setting voltage V SET substantially determines the target value of the current passing a LED in the LED string when that LED shines.

FIGS. 2A and 2B demonstrate two different luminance intensity results when LED lighting system 20 is powered by branch circuits of 200 ACV and 100 ACV, respectively, where threshold voltages V TH1 , V TH2 and V TH3 are the forward voltages of the LED string with only LED 15 a , the LED string with LEDs 15 a and 15 b , and the LED string with LEDs 15 a , 15 b and 15 c , respectively. FIGS. 3A and 3B demonstrate the input current I IN from input voltage V IN to the LED string 14 of FIG. 1 when LED lighting system 20 is powered by branch circuits of 200 ACV and 100 ACV, respectively. Input current I IN in FIG. 3B is almost a constant when the LED string 14 is driven to illuminate. Recess 26 in FIG. 3A , which causes the happening of recess 24 in FIG. 2A , occurs, nevertheless, because there is a period of time when input voltage V IN exceeds reference voltage V IN-REF . Recess 24 helps the shadowed area in FIG. 2A to be as large as that in FIG. 2B , such that the average luminance intensity of the LED lighting system 20 could be independent to the voltage magnitude of the branch circuit.

The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 shows a LED lighting system in the art;

FIGS. 2A and 2B demonstrate two different luminance intensity results when the LED lighting system of FIG. 1 is powered by branch circuits of 200 ACV and 100 ACV, respectively;

FIGS. 3A and 3B demonstrate the input current I IN from input voltage V IN to the LED string of FIG. 1 when the LED lighting system is powered by branch circuits of 200 ACV and 100 ACV, respectively;

FIG. 4 shows a LED lighting system 60 according to embodiments of the invention;

FIGS. 5A and 5B demonstrate two different luminance intensity results when the LED lighting system of FIG. 4 is powered by branch circuits of 200 ACV and 100 ACV, respectively;

FIGS. 6A and 6B demonstrate the input current I IN from input voltage V IN to the LED string of FIG. 4 when LED lighting system is powered by branch circuits of 200 ACV and 100 ACV, respectively;

FIG. 7 illustrates some circuits in the line waveform sensor and the mode decider of FIG. 4 according to one embodiment of the invention;

FIG. 8 demonstrates some signal waveforms relevant to FIGS. 4 and 7 ;

FIG. 9 illustrates a LED controller, which in another embodiment of the invention could embody the LED controller in FIG. 4 ;

FIGS. 10A and 10B demonstrate the input current I IN from input voltage V IN to the LED string 14 of FIG. 4 when LED lighting system 60 employs the circuits in FIG. 9 and is powered by branch circuits of 200 ACV and 100 ACV, respectively; and

FIGS. 11 and 12 show two exemplary LED lighting systems.

The following embodiments are described in sufficient detail to enable those skilled in the art to make and use the invention. It is to be understood that other embodiments would be evident based on the present disclosure, and that improves or mechanical changes may be made without departing from the scope of the present invention.

In the following description, numerous specific details are given to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. In order to avoid obscuring the present invention, some well-known configurations and process steps are not disclosed in detail.

Even though recess 26 in FIG. 3A provides constant brightness control to LED lighting system 20 , it deteriorates power factor (PF) and electromagnetic interference (EMI) of LED lighting system 20 , however. An excellent power factor requires an input current to an electronic appliance substantially in phase with an input voltage supplied. At the time when recess 26 happens, the input current I IN is adversely about out of phase with the input voltage V IN , because the higher input voltage V IN the lower input current I IN . It could be derived that the power factor exhibited in FIG. 3A is worse than that exhibited in FIG. 3B . Furthermore, in comparison with the waveform of input current I IN in FIG. 3B , recess 26 in FIG. 3A introduces two additional corners at about the time points of t 1 and t 2 , which distribute more energy to radiation signals in view of frequency spectrum, resulting worse EMI.

In one embodiment of the invention, the peak voltage V IN-PEAK of input voltage V IN is sensed and a representative voltage V PSTV is accordingly provided to represent the peak voltage V IN-PEAK . This representative voltage V PSTV is held, by a capacitor for example, substantially unchanged when any one of the LEDs in a LED string shines. In another point of view, the representative voltage V PSTV is about the same during the cycle time of the input voltage V IN , where the input voltage V IN might be, for example, 220V or 110V of magnitude, and 120 Hz or 110 Hz of frequency. The representative voltage V PSTV determines the target value to which the driving current flowing through an illuminating LED is controlled to approach. The higher the representative voltage V PSTV , the lesser the driving current and the darker the illuminating LED. As will be detailed later, the dependence of the driving current to the representative voltage V PSTV according to one embodiment of the invention could also provide substantially-constant average luminance intensity control.

Different from the driving current in FIG. 3A , which varies in a cycle time in response to the present magnitude of input voltage V IN and forms the recess 26 , the driving current in one embodiment of the invention is about a constant in one cycle time, such that the recess 26 occurs no more, resulting in well-controlled power factor and EMI.

In one embodiment, although the representative voltage V PSTV is about a constant in one cycle time, it is slightly reduced when the input voltage V IN is at about a valley, in order to track the peak voltage V IN-PEAK which might go down in a following cycle time. The timing when the representative voltage V PSTV slightly reduces could be at the moment when a most upstream LED is switched OFF due to a too-low input voltage V IN .

Some embodiment detects directly the peak voltage V IN-PEAK by using a resistor connected to the input voltage V IN . Other embodiment detects the peak voltage V IN-PEAK indirectly by using a resistor connected to a cathode of an LED in a LED string. In some other embodiments, the resistor could be replaced by a capacitor to sense a maximum differentiation value of the input voltage V IN , which in a way represents the peak voltage V IN-PEAK too.

FIG. 4 shows a LED lighting system 60 according to embodiments of the invention. Similar with LED lighting system 20 in FIG. 1 , LED lighting system 60 in FIG. 4 has LED string 14 with LEDs 15 a , 15 b and 15 c connected in series. Each LED in LED string 14 represents a LED group, which in one embodiment includes only one micro LED, and in some other embodiments includes several micro LEDs connected in series or in parallel. In one non-limiting embodiment, each LED has the same number of micro LEDs connected in series. In one embodiment, the micro LEDs in the LED string 14 are of the same color, which is red, green, blue, or white, for example. Nevertheless, some embodiments have the LED string 14 consisting of different-color micro LEDs. The LED string according to the invention is not limited to have only 3 LEDs, and could have any number of LEDs in other embodiments.

Bridge rectifier 12 , connected to a branch circuit providing an AC voltage V AC , generates input voltage V IN as an input power source to power LED string 14 . The AC voltage V AC could be of 100 VAC, 110 VAC, 220 VAC, or 230 VAC with a frequency of 50 Hz or 60 Hz. As a result, input voltage V IN could be of an M-shaped waveform with a frequency of 100 Hz or 120 Hz.

LED controller 61 could be embodied in an integration circuit with several pins. In one embodiment, one pin of LED controller 61 , referred to as pin CPS, is directly connected to input voltage V IN by resistor R SENSE to sense the waveform of input voltage V IN . Pins N a , N b , N c are respectively connected to the cathodes of LEDs 15 a , 15 b and 15 c , providing separate conduction paths to drain current to ground. Inside LED controller 61 are path switches S a , S b , and S c , line waveform sensor 66 and management center 63 .

Path switches S a , S b , and S c respectively control conduction paths from pins N a , N b , N c , to the ground, and are controlled by management center 63 , which includes switch controllers C a , C b , C c and mode decider 62 . The control circuit for one path switch is similar with the one for another. Taking the control for path switch S a as an example, switch controller C a , which is an operational amplifier in this embodiment, could operate in one of several modes, including but not limited to fully-ON, fully-OFF, and constant-current modes, depending upon the signal sent from mode decider 62 . For example, when switch controller C a is determined to operate in the constant-current mode, switch controller C a controls the impedance of path switch S a to make current sense voltage VCS a approach current-setting voltage V SET . Current sense voltage VCS a is the detection result representing the current passing path switch S a . When switch controller C a is determined to operate in the fully-ON mode, path switch S a is always ON, performing a short circuit, disregarding current sense voltage VCS a . On the other hand, when switch controller C a is determined to operate in the fully-OFF mode, path switch S a is always OFF, performing an open circuit, disregarding current sense voltage VCS a . In one instant when input voltage V IN is high enough to turn on the LED string with only LEDs 15 a and 15 b , for example, switch controllers C a , C b and C c could operate in the fully-OFF, constant-current and fully-ON modes, respectively, such that the current passing through LEDs 15 a and 15 b are the same, corresponding to current-setting voltage V SET , and that current passing through LED 15 c is about zero. If later on input voltage V IN ramps down and mode decider 62 finds current sense voltage VCS b cannot increase to approach current-setting voltage V SET , then mode decider 62 changes the operation modes of switch controllers C a and C b to be constant-current and fully-ON modes, respectively. Therefore, the current passing through LED 15 a stays at a value determined by current-setting voltage V SET , and those passing through LEDs 15 b and 15 b are zero. In the opposite, if later on input voltage V IN ramps up and current sense voltage VCS c indicates that the current passing through LED 15 c turns to be more than zero, switch controllers C b and C c are switched to operate in the fully-OFF and constant-current modes, respectively. From the teaching above, it can be concluded that current-setting voltage V SET substantially determines the target value of the current passing a LED in the LED string when that LED shines.

In one embodiment, line waveform sensor 66 detects the waveform of input voltage V IN via resistor R SENSE , and accordingly provides current-setting voltage V SET . Line waveform sensor 66 , for example, holds a representative voltage V PSTV representing the peak voltage V IN-PEAK of the input voltage V IN . The operational amplifier turns on an NMOS in line waveform sensor 66 to raise the representative voltage V PSTV if the representative voltage V PSTV is less than a divided voltage of the input voltage V IN at pin CPS, such that representative voltage V PSTV represents the peak voltage V IN-PEAK . The representative voltage V PSTV substantially stays unchanged during a cycle time of the input voltage V IN , and determines current-setting voltage V SET and the current passing a LED as well. For instance, in case that the AC voltage V AC is 220 VAC, the representative voltage V PSTV corresponds to 220V. In case that the AC voltage is 110 VAC, the representative voltage V PSTV corresponds to 110V.

The representative voltage V PSTV substantially determines the current-setting voltage V SET provided. In one embodiment, if the peak voltage V IN-PEAK of the input voltage V IN is below a threshold value V FOLD , the current-setting voltage V SET is a constant. If the peak voltage V IN-PEAK exceeds the threshold value V FOLD , the higher the peak voltage V IN-PEAK , the lower the current-setting voltage V SET . FIGS. 5A and 5B demonstrate two different luminance intensity results when LED lighting system 60 is powered by branch circuits of 200 ACV and 100 ACV, respectively, where threshold voltages V TH1 , V TH2 and V TH3 are the forward voltages of the LED string with only LED 15 a , the LED string with LEDs 15 a and 15 b , and the LED string with LEDs 15 a , 15 b and 15 c , respectively. FIGS. 6A and 6B demonstrate the input current I IN from input voltage V IN to the LED string 14 of FIG. 4 when LED lighting system 60 is powered by branch circuits of 200 ACV and 100 ACV, respectively. FIGS. 5B and 6B are similar with FIGS. 2B and 3B , respectively, such that their explanation is omitted for brevity. Different with the waveforms in FIGS. 2A and 3A , those of FIGS. 5A and 5B have no recesses. Please note that when at least one LED is ON the input current I IN in FIG. 6A is smaller than that in FIG. 6B , because the peak voltage V in-PEAK in FIG. 5A is 200 ACV, higher than that in FIG. 5B . The instant luminance intensity of FIG. 5A is less than that of FIG. 5B simply because the input current I IN of FIG. 6A is less than that of FIG. 6B . The shadowed areas in FIGS. 5A and 5B represent two average luminance intensities that human eyes could conceive when the LED string 14 is powered by 200 VAC and 100 VAC, respectively. In comparison with that in FIG. 5B , the shadowed area in FIG. 5A is lower but wider, and could have the same in volume if fine-tuned. In other words, it is possible for the average luminance intensity of the LED lighting system 60 to be substantially independent to the voltage magnitude of the branch circuit.

Unlike the waveform of FIG. 3A , which has a recess and two additional corners, the waveform of FIG. 6A has neither the recess nor the two additional corners, implying better PF and EMI results.

FIG. 7 illustrates some circuits in line waveform sensor 66 and mode decider 62 of FIG. 4 according to one embodiment of the invention.

Shown in FIG. 7 , line waveform sensor 66 has peak-hold circuit 68 , transferring circuit 70 and refreshing circuit 72 , while mode decider 62 has valley detector 74 . Peak-hold circuit 68 can generate and hold representative voltage V PSTV over capacitor C HOLD , to represent peak voltage V PEAK-in of input voltage V IN . Transferring circuit 70 provides current-setting voltage V SET in response to representative voltage V PSTV , based upon a predetermined transferring function. In the non-limiting embodiment shown in FIG. 7 , the transferring function defines that current-setting voltage V SET is about a constant if representative voltage V PSTV is below a threshold value V FOLD , and that the more the representative voltage V PSTV exceeds the threshold value V FOLD the less the current-setting voltage V SET is. Since representative voltage V PSTV and current-setting voltage V SET correspond to the peak voltage V IN-PEAK and the target value of input current I IN , respectively, the target value of input current I IN is about a constant when the peak voltage V IN-PEAK is below a predetermined threshold, but decreases when the peak voltage V IN-PEAK exceeds the predetermined threshold.

The peak voltage V IN-PEAK of the input voltage V IN in a flowing cycle time might be different to that in the present cycle, and to track the change in the peak voltage V IN-PEAK of the input voltage V IN , the representative voltage V PSTV might be refreshed once every cycle or every several cycles. It is a good timing to perform the refreshing when the input voltage V IN is so low that none LED in the LED string 14 shines, or when the input voltage V IN is about at a valley. In one embodiment, valley detector 74 in mode decider 62 generates a pulse S FRESH at the moment when input voltage V IN enters a valley. Upon receiving the pulse S FRESH , refreshing circuit 72 refreshes the representative voltage V PSTV .

In one embodiment, when none of current sense voltages VCS a , VCS b , and VCS c can be manipulated to be as high as current-setting voltage V SET , valley detector 74 deems it as the occurrence of the input voltage V IN having entered a valley. When at least one of current sense voltages VCS a , VCS b , and VCS c is about the same as the current-setting voltage V SET , the input voltage V IN exits the valley. In another embodiment, valley detector 74 could use other means to determine whether input voltage V IN enters or exits a valley. Normally, input voltage V IN enters and exits a valley once every cycle, and the signal S FRESH could, but is not limited to, be provided once whenever the input voltage V IN enters or exits a valley. The signal S FRESH could be provided once when every two valleys have be passed, for example.

In the embodiment shown in FIG. 7 , the pulse S FRESH triggers a constant current source to discharge the capacitor C HOLD for a very short period of time, such that the representative voltage V PSTV is slightly reduced upon the receiving of the pulse S FRESH .

According to one embodiment of the invention, FIG. 8 demonstrates some signal waveforms relevant to FIGS. 4 and 7 . Input voltage V IN , as being a power source rectified from a sinusoidal AC voltage, has a M-shaped waveform as shown at the top of FIG. 8 . Representative voltage V PSTV is about a constant all the time, but tracks the increment of input voltage V IN at about the middle of a cycle time. Accordingly, representative voltage V PSTV represents the peak voltage V IN-PEAK . Input current I IN , even though being about constant when any one LED of LED string 14 shines, reduces slightly at about the middle of a cycle time in response to the slight increment in representative voltage VPSTV. In FIG. 8 , the pulse S FRESH is generated once every time when input current I IN drops to about zero, causing slight reduction to representative voltage V PSTV . In other words, at the moment when management center 63 turns off the most upstream LED 15 a , representative voltage V PSTV is refreshed.

FIG. 9 illustrates a LED controller 61 a , which in another embodiment of the invention could embody the LED controller 61 in FIG. 4 . Comparison with FIG. 7 , FIG. 9 additionally has adder 90 and attenuator 92 . kV IN , outputted by attenuator 92 and being in proportion to input voltage V IN , is a small factor to slightly increase the current-setting voltage V SET . FIGS. 10A and 10B demonstrate the input current I IN from input voltage V IN to the LED string 14 of FIG. 4 when LED lighting system 60 employs the circuits in FIG. 9 and is powered by branch circuits of 200 ACV and 100 ACV, respectively. FIGS. 10A and 10B could achieve less total harmonic distortion (THD), having less radioactive signal generated to other electric devices via the branch circuit.

The foregoing embodiments of the invention have resistor R SENSE coupled between pin CPS and bridge rectifier 12 to directly sense the waveform of input voltage V IN . The invention is not limited thereto, however. Pin CPS could be coupled to any connection nodes in driven LED string 14 of FIG. 4 , for example, to indirectly sense the waveform of input voltage V IN . FIG. 11 shows an exemplary LED lighting system 200 , which is the same with the LED lighting system of FIG. 4 but has resistor R SENSE coupled to pin N c , the cathode of LEDs 15 c . In other embodiments, resistor R SENSE could be coupled from pin CPS to pin N b or pin N a , instead.

Line waveform sensors according to embodiments of the invention are not limited to sense the voltage at pin CPS to determine the peak voltage V IN-PEAK of input voltage V IN . In some embodiments, it is the current flowing through resistor R SENSE and into pin CPS that a line waveform sensor senses to determine the peak voltage V IN-PEAK of input voltage V IN . In other embodiment, it is the differentiation of input voltage V IN that a line waveform sensor senses to determine the peak voltage V IN-PEAK . FIG. 12 shows an exemplary LED lighting system 300 , which is the same with the LED lighting system of FIG. 4 but has resistor R SENSE replaced by capacitor C SENSE . The differentiation of input voltage V IN could induce a current into pin CPS. The larger the maximum differentiation of input voltage V IN , the larger the magnitude of input voltage V IN , the higher the peak voltage V IN-PEAK . In other embodiments, capacitor C SENSE could be connected between pin CPS and any one of pins N a , N b , and N c .

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.