AC LED lighting systems and control methods without flickering

The present disclosure relates generally to Light-Emitting Diode (LED) lighting systems, and more particularly to Alternating Current (AC) driven LED lighting systems and control methods the do not introduce flickering.

Light-Emitting Diodes or LEDs are increasingly being used for general lighting purposes. In one example, a set of LEDs is powered from an AC power source and the term “AC LED” is sometimes used to refer to such circuit. Concerns for AC LED lighting systems include manufacture cost, power efficiency, power factor, flicker, lifespan, etc.

FIG. 1 demonstrates AC LED lighting system 10 in the art, which, in view of electric circuit, simply has a LED module 12 and a current-limiting resistor 14 . The LED module 12 consists of two LED strings connected in anti-parallel. The AC LED lighting system 10 in FIG. 1 requires neither an AC-DC converter nor a rectifier. Even though a DC voltage is also compatible, an AC voltage V AC is typically supplied to power the AC LED circuit 10 directly. Simplicity in structure and low-price in manufacture are two advantages the AC LED lighting system 10 provides. Nevertheless, the AC LED lighting system 10 can only emit light in a very narrow time period in each AC cycle time, suffering in low average luminance.

FIG. 2 demonstrates another AC LED lighting system 15 in the art. Examples of the AC LED lighting system 15 can be found from U.S. Pat. No. 7,708,172. The AC LED lighting system 15 employs full-wave rectifier 18 to rectify an AC voltage V AC and provide a DC output power source across an input power line IN and a ground line GND. A string of LEDs are segregated into LED groups 20 1 , 20 2 , 20 3 , and 20 4 , each having one or more LEDs. An integrated circuit 22 as an LED controller has pins or nodes PIN 1 , PIN 2 , PIN 3 , and PIN 4 , connected to the cathodes of LED groups 20 1 , 20 2 , 20 3 , and 20 4 respectively. Inside integrated circuit 22 are channel switches SG 1 , SG 2 , SG 3 , and SG 4 , and a current controller 24 as well. When the rectified input voltage V IN at the input power line IN increases, current controller 24 can adjust the conductivity of channel switches SG 1 , SG 2 , SG 3 , and SG 4 , to make more LED groups join to emit light. Operations of integrated circuit 22 have been exemplified in U.S. Pat. No. 7,708,172 and are omitted here for brevity.

FIG. 3 illustrates the waveforms of signals in FIG. 2 when the AC input voltage V AC has a sinusoidal waveform. The upmost waveform in FIG. 3 shows the rectified input voltage V IN at the input power line IN. The second shows the total number of illuminating LEDs, meaning the number of LEDs that are illuminating. The four following waveforms regard with LED currents I LED4 , I LED3 , I LED2 and I LED1 , which, as shown in FIG. 2 , refer to the currents flowing through LED groups 20 4 , 20 3 , 20 2 and 20 1 , respectively. The total number of illuminating LEDs rises or descends stepwise, following the increase or decrease of the rectified input voltage V IN . When the rectified input voltage V IN increases, LED groups 20 1 , 20 2 , 20 3 , and 20 4 , one by one according to a forward sequence, join to illuminate. For example, when the rectified input voltage V IN increases to just exceed the forward voltage V TH1 , the voltage required for driving the LED group 20 1 to illuminate, the LED group 20 1 starts illuminating. When the rectified input voltage V REC decreases, LED groups 20 1 , 20 2 , 20 3 , and 20 4 darken, one by one according to a backward sequence. If, for example, the rectified input voltage V IN just falls below the forward voltage V TH4 , the voltage required for driving all the LED groups 20 1 , 20 2 , 20 3 and 20 4 to illuminate, then the channel switches SG 3 and SG 4 are switched ON and the channel switches SG 2 and SG 1 are OFF, such that the LED group 20 4 stops illuminating, leaving only the LED groups 20 1 , 20 2 and 20 3 to emit light. The AC LED lighting system 15 enjoys simple circuit architecture and, as can be derived, good power efficiency.

There in FIG. 3 however shows a dark period T DARK when no LEDs illuminate, because the rectified input voltage V IN is too low to drive the LED group 20 1 . If the rectified voltage V IN is a 120-Hertz signal, the voltage valley where the rectified voltage V IN is about zero volt appears at 120 Hz, causing the dark period T DARK to show up at the same frequency of 120 Hz. This phenomenon is sometimes referred to as flickering. Even though flickering might not be perceivable by human eyes, it is reported that people watching objects exposed under the luminance of the LED lighting system 15 could feel dizzy or discomfort. It is desired to have an AC LED lighting system that produces no flickering.

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

FIGS. 1 and 2 demonstrate two AC LED lighting systems in the art;

FIG. 3 illustrates the waveforms of signals in FIG. 2 ;

FIG. 4 demonstrates another AC LED lighting system;

FIG. 5 demonstrates an AC LED lighting system according to embodiments of the invention;

FIG. 6 shows waveforms of signals in FIG. 5 ;

FIG. 7 shows that the LED current I LED1 is in phase with the rectified input voltage V IN ; and

FIG. 8 demonstrates another AC LED lighting system according to embodiments of the invention.

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.

FIG. 4 demonstrates an AC LED lighting system 100 . The AC LED lighting system 100 has a full-wave rectifier 18 to rectify a sinusoid AC input voltage V AC , and provides a rectified input voltage V IN at the input power line IN and a ground voltage at the ground line GND. The LED groups 20 1 , 20 2 , 20 3 and 20 4 together compose a LED string connected in series between the input power line IN and the ground line GND. This LED string is deemed to have a main anode connected to the input power line IN and a main cathode connected to pin PIN 4 . Each LED group might have only one LED in some embodiments, or consist of several LEDs connected in parallel or in series, depending on its application. The LED group 20 1 is the most upstream LED group in FIG. 4 as its anode is connected to the highest voltage in the LED string, the rectified input voltage V IN . Analogously, the LED group 20 4 is the most downstream LED group in FIG. 4 .

An integrated circuit 102 as a LED controller has channel switches SG 1 , SG 2 , SG 3 and SG 4 , and a current controller 103 . Each of channel switches SG 1 , SG 2 , SG 3 and SG 4 helps connect one cathode of a corresponding LED group to the ground line GND. The current controller 103 controls the conductivity of each channel switch so as to regulate the LED current I LED1 . For example, if the rectified input voltage V IN is so low that the LED current I LED4 passing through the LED group 20 4 drops to about 0 A, then the current controller 103 turns on the channel switch SG 3 , coupling the cathode of the LED group 20 3 to the ground line GND. Meanwhile, the LED current I LED3 is monitored by the current controller 103 to control the conductivity of the channel switch SG 3 , so as to regulate the LED current I LED1 .

The AC LED lighting system 100 includes a power bank 104 coupled between the input power line IN and the ground line GND. The power bank 104 increases the electric energy stored in the capacitor 112 when the absolute value of the sinusoid AC voltage V AC , |V AC |, goes up along its way to maximums. The power bank 104 could be triggered by the integrated circuit 102 to release the electric energy, and to make the LED string illuminate when the rectified input voltage V IN is relatively low. With proper design, the AC LED lighting system 100 can illuminate continuously without flickering.

The capacitor 112 in the power bank 104 need sustain the high voltage at the input power line IN, however. For example, if the sinusoid AC input voltage V AC is 240 VAC, then the capacitor 112 must inevitably tolerate the stress of at least 240V. First of all, it is known in the art that high-voltage-tolerable devices are expensive. Second, the effective capacitance of a high-voltage-tolerable capacitor lowers when operating under a relatively-high voltage. For example, the effective capacitance of the capacitor 112 could be as large as 470 nF when the voltage stress across it is about 0V, but it becomes as low as 200 nF when the voltage stress increases to 260V. It is required to have the capacitor 112 with certain large capacitance, in order to avoid flickering. Therefore, the cost for assembling the AC LED lighting system 100 could be considerable.

FIG. 5 demonstrates an AC LED lighting system 200 according to embodiments of the invention. A full-wave rectifier 18 rectifies a sinusoid AC input voltage V AC to generate a DC input power source across an input power line IN and a ground line GND. The voltage at the input power line IN is referred to as a rectified input voltage V IN , and the voltage at the ground line GND is deemed to be a ground voltage, or 0V. The LED string in the embodiment of FIG. 5 has three LED groups 20 1 , 20 2 , and 20 3 , connected in series between the input power line IN and pin PIN 3 . The LED string in FIG. 5 as a whole could act as a diode with a main anode connected to the input power line IN and a main cathode connected to pin PIN 3 . A power bank 201 has two diodes D CHG and D DCHG , and a capacitor C AUX . As shown in FIG. 5 , the diode D CHG and the capacitor C AUX is connected in series between the main cathode (pin PIN 3 ) and pin PIN 4 , while the diode D DCHG is connected between the main anode (input power line IN) and the capacitor C AUX . It will become apparent later that the diode D CHG is for charging the capacitor C AUX and the diode D DCHG is for discharging the capacitor C AUX . The currents flowing through LED group 20 1 , 20 2 and 20 3 are denoted as LED currents I LED1 , I LED2 and I LED3 , respectively. The current passing through the capacitor C AUX to the ground line GND is denoted as a charge current I CHG .

An integrated circuit 202 performs as a LED controller, having channel switches MN 1 , MN 2 , MN 3 and MN 4 , and a current controller 204 . Channel switches MN 1 , MN 2 and MN 3 help connect LED group 20 1 , 20 2 and 20 3 to the ground line GND, respectively, while the channel switches MN 4 helps connect one terminal of the capacitor C AUX to the ground line GND. The current passing through the channel switches MN 1 , MN 2 , MN 3 , and MN 4 are denoted as driving currents I 1 , I 2 , I 3 and I 4 respectively. Similar with the function of the current controller 103 in FIG. 4 , the current controller 204 in FIG. 5 controls the conductivity of each channel switch so as to control the LED current I LED1 . For example, if the current controller 204 senses that the driving currents I 3 and I 4 both drop to 0 A, then the current controller 204 turns on the channel switch MN 2 , coupling the cathode of the LED group 20 2 to the ground line GND. The driving current I 2 , which is substantially equal to LED current I LED2 in amplitude in the meantime, is monitored by the current controller 204 , so as to control the conductivity of the channel switch MN 2 and to regulate both the LED currents I LED1 and I LED2 .

In one embodiment, the LED current I LED1 , the combination of the driving currents I 1 , I 2 , I 3 and I 4 , is regulated to be a target value. For instance, in case that the rectified input voltage V IN is high enough to make all the LED groups 20 1 , 20 2 and 20 3 illuminate, channel switches MN 1 and MN 2 are kept being OFF, and channel switches MN 3 and MN 4 are controlled to have the summation of driving currents I 3 and I 4 equal to the target value. In other words, the driving currents I 1 and I 2 are both 0 and the LED current I LED3 is regulated to be the target value. A portion of the LED current I LED3 could be diverted to become the charge current I CHG , which, as time goes by, charges the capacitor C AUX and increases the electric energy stored by the capacitor C AUX . The current controller 204 could sense the voltage V CS4 to determine the magnitude of the driving current I 4 , which represents the charge current I CHG in the present moment. The rest of the LED current I LED3 is led to become the driving current I 3 and flow through the channel switch MN 3 . As the capacitor C AUX is charged up over time, the driving current I 4 decreases due to increment of the voltage V CAP and the decrement of the charge current I CHG . The reduction in the driving current I 4 causes the current controller 204 to increase the conductivity of the channel switch MN 3 , so the driving current I 3 increases, and the LED current I LED3 , the combination of the driving current I 3 and the driving current I 4 , remains to be the target value.

FIG. 6 shows waveforms of signals in FIG. 5 . From top to bottom, the waveforms in FIG. 6 are the rectified input voltage V IN , the total number of illuminating LEDs, the LED currents I LED3 , I LED2 and I LED1 , the voltage V CAP on the capacitor C AUX , the charge current I CHG , the driving currents I 4 , I 3 , I 2 and I 1 , respectively. What is noted in FIG. 6 is that the total number of illuminating LEDs in FIG. 6 never drops to 0 all the time, implying the disappearance of the dark period T DARK of FIG. 3 . In other words, the AC LED lighting system 200 in FIG. 5 introduces no flickering.

For comparison, the waveform of the absolute value of the AC voltage V AC , or |V AC |, is also plotted as a dotted curve companying the waveform of the rectified input voltage V IN . Similarly, companying the waveform of the voltage V CAP are the waveforms of |V AC | and (|V AC |−V TH3 ), where the forward voltage V TH3 is the forward voltage required for making all the LED groups 20 1 , 20 2 and 20 3 illuminate. Similarly, forward voltage V TH2 is the voltage for making at least both the LED groups 20 1 and 20 2 illuminate, and forward voltage V TH1 the voltage for making the LED group 20 1 illuminate.

Shown in FIG. 6 , the LED groups 20 1 illuminates all the time and the reason why will be detailed later. As |V AC | ramps upward from a voltage valley where |V AC | is about 0V, the LED groups 20 2 and 20 3 join one by one to illuminate. When |V AC | ramps up further and (|V AC |−V TH3 ) surpasses the voltage V CAP , as what happens at the moment t CH in FIG. 6 , the diode D CHG is forward biased and the charge current I CHG starts to charge the capacitor C AUX in the power bank 201 . Accordingly, both the electric energy stored in the capacitor C AUX and the voltage V CAP start increasing at the moment t CH . This charging ends when (|V AC |−V TH3 ) is below the voltage V CAP , as it happens at the moment t CH-END . Demonstrated in FIG. 6 , during the charging, the charge current I CHG equals to the driving current I 4 , and the LED current I LED3 , the combination of the driving currents I 3 and I 4 , is regulated to be substantially constant.

The power bank 201 starts releasing the stored electric energy at moment t DCH when |V AC | drops below the voltage V CAP and the diode D DCHG becomes forward biased. Therefore, starting at moment t DCH , the rectified input voltage V IN follows the voltage V CAP , so its waveform departs from the waveform of |V AC |, as shown in FIG. 6 . The charge current I CHG ; becomes negative to discharge the capacitor C AUX , and this negative charge current I CHG flows from the ground line GND, via the body diode of the channel switch MN 4 , the capacitor C AUX , the diode D DCHG , and the input power line IN, to become the LED current I LED1 , which goes through the LED group 20 1 and the channel switch MN 1 to be driving current I 1 to the ground line GND. Meanwhile, the charge current I CHG is about a negative constant because the driving current I 1 is regulated to be constant. The driving current I 4 or the voltage V CS4 is slightly negative because channel switch MN 4 is kept ON and the voltage at pin PIN 4 is negative. Nevertheless, the current controller 204 could be designed to deem the negative voltage V CS4 as 0V, and still regulate the driving current I 1 to be about constant while both the driving currents I 2 and I 3 are zero. The voltage V CAP descends as the discharging of the capacitor C AUX continues. As |V AC | bounces back from 0V and surpasses the voltage V CAP at moment t DCH-END in FIG. 6 , the discharging stops and the rectified input voltage V IN starts following |V AC |.

Apparent from FIG. 6 , a portion of the LED current I LED3 is diverted during the period of time from moment t CH to t CH-END , to become the charge current I CHG , which flows through the diode D CHG and increases the electric energy stored in the capacitor C AUX of the power bank 201 . The electric energy stored in the capacitor C AUX is released via the diode D DCHG to make the LED group 20 1 illuminate during the period of time from moment t DCH to t DCH-END , so the AC LED lighting system 200 illuminates all the time. The period of time from moment t DCH to t DCH-END also means a period of time when the AC input voltage V AC is about 0.

The waveform of the voltage V CAP in FIG. 6 shows that the maximum voltage the capacitor C AUX tolerates is no more than the maximum of (|V AC |−V THS |), which normally is only tens volts. In comparison with the capacitor 112 in FIG. 4 , which need sustain a voltage as high as 240V, the capacitor C AUX in FIG. 6 could need to sustain only tens volt and could be a better selection in view of cost. The capacitor C AUX in FIG. 6 could also enjoy much higher effective capacitance in comparison with the capacitor 112 in FIG. 4 .

As the LED current I LED1 does not vary over time in FIG. 6 , the target value that the LED current I LED1 is regulated to be is a constant. The invention is not limited to, however. Some embodiments of the invention might have the target value varied, depending on some parameters. For example, an embodiment of the invention might change the target value when the channel switches MN 1 , MN 2 , MN 3 , and MN 4 switch. For example, when the current controller 204 turns channel switch MN 1 OFF, the current controller 204 adjusts the target value, making it slightly more. In one embodiment, the more channel switches turned OFF, the higher target value. In another embodiment, the target value is in association with the rectified input voltage V IN . The current controller 204 senses the rectified input voltage V IN via pin DET and resistor R DET in FIG. 5 to determine the target value. The higher the rectified input voltage V IN the more the target value, as demonstrated by FIG. 7 . As the LED current I LED1 is in phase with the rectified input voltage V IN , which follows |V AC | most of the time, total harmonic distortion (THD) and power factor (PF) that AC LED lighting system 200 performs could be very excellent. An embodiment of the invention has achieved power factor of 0.97 and THD of 19%.

According to embodiments of the invention, FIG. 8 demonstrates another AC LED lighting system 300 , which is capable of illuminating all the time without flickering. In FIG. 8 , LED groups 20 1A and 20 1B connected in series replaces LED group 20 1 in FIG. 5 . AC LED lighting system 300 further has a PNP bipolar junction transistor (BJT) BT with an emitter and a collector connected to the anode and the cathode of LED group 20 1A respectively. The base of BJT BT in FIG. 8 is connected to pin PAS of the integrated circuit 302 . The BJT BT acts as a bypass switch capable of letting the LED current I LED1 bypass the LED group 20 1A . Beside the devices commonly shown in FIG. 5 , the integrated circuit 302 , as a LED controller, further has a current controller 304 , two comparators 308 and 310 , and a SR register 306 . The current controller 304 in FIG. 8 is similar with the current controller 203 in FIG. 5 , modifying the conductivities of channel switches MN 1 , MN 2 , MN 3 and MN 4 in view of the driving currents I 1 , I 2 , I 3 and I 4 .

Comparator 310 compares the voltage V CS4 with 0V, where the voltage V CS4 somehow represents the driving current I 4 passing through channel switch MN 4 . Please have a look of FIG. 6 , where the driving current I 4 becomes negative only when the capacitor C AUX is discharging. Therefore, comparator 310 determines whether the capacitor C AUX is discharging.

Comparator 308 compares the voltage V CS1 with a reference voltage V REF , where the voltage V CS1 represents the driving current I 1 passing through channel switch MN 1 . In other words, comparator 308 determines whether the driving current I 1 is below a predetermined value, which in one embodiment of this invention is less than the target value that the LED current I LED1 is regulated to.

During the time when the capacitor C AUX is not discharging, the voltage V CS4 is not negative, so signal SB DCHG is logic 1 and SR register 306 is reset, having output signal SB PAS with logic 1. According, PNP BJT BT is turned OFF, so LED current I LED1 , if any, flows through both LED groups 20 1A and 20 1B .

Signal SB DCHG turns to be logic “0” when the capacitor C AUX discharges to make LED groups 20 1A and 20 1B illuminate. The capacitor voltage V CAP of the capacitor C AUX descends over time during the discharging. In the meantime, the current controller 304 adjusts the conductivity of the channel switch MN 1 so as to regulate the driving current I 1 to the target value. Once the capacitor voltage V CAP drops below the forward voltage required for driving both LED groups 20 1A and 20 1B , the driving current I 1 cannot be regulated any more, and starts falling. When the driving current I 1 drops further below the predetermined value represented by the reference voltage V REF , comparator 308 turns signal S TOO-LOW into logic “1”, setting the SR register 306 , so signal SB PAS becomes logic “0” and PNP BJT BT is turned ON. LED current I LED1 , if any, then bypasses LED 20 1A and flows through LED group 20 1B , to become the driving current I 1 , which can be regulated now because the capacitor voltage V CAP still exceeds the forward voltage for driving only the LED group 20 1B . The capacitor voltage V CAP can discharge further to make LED group 20 1B illuminate while the LED group 20 1A stops illuminating.

It is derivable that the capacitor C AUX in FIG. 8 can release its own electric energy until the capacitor voltage V CAP drops as low as the forward voltage required for driving only LED group 20 1B . The capacitor C AUX in FIG. 5 , however, stops discharging once the capacitor voltage V CAP drops below the forward voltage required for driving only LED group 20 1 . If the LED group 20 1 in FIG. 5 consists of LED groups 20 1A and 20 1B in FIG. 8 , the capacitor C AUX in FIG. 8 could release more electric power and operate more efficiently than the capacitor C AUX in FIG. 5 does.

FIG. 8 has PNP BJT BT capable of acting as a shunt to LED group 20 1A , but the invention is not limited to. Another embodiment of the invention could relocate PNP BJT BT of FIG. 8 to become a shunt to LED group 20 1B instead of LED group 20 1A .

The current controller 304 regulates the LED current I LED1 to be a target value. As demonstrated previously by another embodiment, this target value could be a constant, or is determined according to some parameters. For example, this target value is set to be about a constant if signal SB DCHG is “1” in logic, and becomes a relatively-less constant if signal SB DCHG is “0” in logic. A less target value when signal SB DCHG is “0” is beneficial in improving THD because the signal SB DCHG in “0” is also an indication that the AC input voltage V AC is about 0V.

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.