LOW-FLICKERLIGHT-EMITTING DIODE LIGHTING DEVICE HAVING MULTIPLE DRIVING STAGES

Abstract

An LED lighting device includes multiple luminescent devices driven by a rectified AC voltage. The multiple luminescent devices are turned on flexibly in a multi-stage driving scheme using multiple current control units. At least one charge storage unit is coupled in parallel with at least one luminescent device. When the rectified AC voltage is still insufficient to turn on the at least one luminescent device, the at least charge storage unit is configured to discharge energy to the at least one luminescent device, thereby keeping the at least one luminescent device turned on.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an LED lighting device having multiple driving stages, and more particularly, to an LED lighting device having multiple driving stages for providing wide effective operational voltage range without causing flicker and uniformity issue.

2. Description of the Prior Art

An LED lighting device directly driven by a rectified alternative-current (AC) voltage usually adopts a plurality of LEDs coupled in series in order to provide required luminance. As the number of the LEDs increases, a higher forward-bias voltage is required for turning on the LED lighting device, thereby reducing the effective operational voltage range of the LED lighting device. As the number of the LEDs decreases, the large driving current when the rectified voltage is at its maximum level may impact the reliability of the LEDs.

An LED lighting device is configured to modulate luminous flux and intensity. This time variation is commonly referred to as flicker. LED flicker, whether perceptible or not, has been a concern of the lighting community because of its potential human impacts, which range from distraction, mild annoyance to neurological problems. Therefore, there is a need for an LED lighting device capable of improving the effective operational voltage range, the reliability and the flicker phenomenon.

SUMMARY OF THE INVENTION

The present invention provides an LED lighting device having a first driving stage and a second driving stage. The first driving stage includes a first luminescent device driven by a rectified AC voltage for providing light according to first current; a second luminescent device driven by the rectified AC voltage for providing light according to second current; a first current controller coupled in series to the first luminescent device and configured to regulate the first current so that the first current does not exceed a first value; a second current controller coupled in series to the second luminescent device and configured to regulate the second current so that the second current does not exceed a second value; a first charge storage unit coupled in parallel with at least the first luminescent device and configured to discharge energy to the first luminescent device when the rectified AC voltage is insufficient to turn on the first luminescent device, thereby keeping the first luminescent device turned on; and a path-controller configured to conduct third current and having a first end coupled between the first luminescent device and the first current controller and a second end coupled to the second current controller. The second driving stage includes a third current controller coupled in series to the first driving stage and configured to conduct fourth current and regulate the fourth current so that the fourth current does not exceed a third value.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1˜4 are diagrams of LED lighting devices according to embodiments of the present invention.

FIGS. 5˜9 are diagrams illustrating the operation of the multiple driving stages in the LED lighting device of the present invention.

FIG. 10 is a diagram illustrating the current-time characteristic of the luminescent device in the LED lighting device of the present invention.

FIG. 11 is a diagram illustrating the overall operation of an LED lighting device according to embodiments of the present invention.

FIG. 12 is a diagram illustrating the overall operation of an LED lighting device.

FIGS. 13˜16 are diagrams of LED lighting devices according to other embodiments of the present invention.

DETAILED DESCRIPTION

FIGS. 1-4 are diagrams of LED lighting devices 101-104 according to embodiments of the present invention. Each of the LED lighting devices 101-104 includes a power supply circuit 110 and (N+1) driving stages ST₁˜ST_N+1. The power supply circuit 110 is configured to receive an AC voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage V_AC, whose value varies periodically with time, for driving the (N+1) driving stages. In another embodiment, the power supply circuit 110 may receive any AC voltage VS, perform voltage conversion using an AC-AC converter, and rectify the converted AC voltage VS using the bridge rectifier 112, thereby providing the rectified AC voltage V_AC, whose value varies periodically with time. The configuration of the power supply circuit 110 does not limit the scope of the present invention.

In the LED lighting devices 101˜103, each of the 1^st to N^th driving stages ST₁˜ST_N includes a plurality of luminescent devices, a path controller, a first-type current controller, a second-type current controller, and M charge storage units CH₁˜CH_M, wherein N is a positive integer larger than 1, and M is a positive integer smaller or equal to 2N. The (N+1)^th driving stage ST_N+1 includes a third-type current controller.

In the LED lighting device 104, the 1^st₁ driving stage ST₁ includes a plurality of luminescent devices, while each of the 2^nd to N^th driving stages ST₂˜ST_N includes a plurality of luminescent devices, a path controller, a first-type current controller, a second-type current controller, and M charge storage units CH₁˜CH_M, wherein N is a positive integer larger than 1, and M is a positive integer smaller or equal to 2N. The (N+1)^th driving stage ST_N+1 includes a third-type current controller.

Each first-type current controller includes an adjustable current source and a current detection and control unit. Each second-type current controller includes an adjustable current source and a voltage detection and control unit. The third-type current controller includes an adjustable current source and a detection and control unit.

For illustrative purposes, the following symbols are used to represent each device in the LED lighting devices 101-104 throughout the description and figures. A₁˜A_N and B₁˜B_N represent the luminescent devices in the corresponding driving stages ST₁˜ST_N, respectively. D₁˜D_N represent the path-controllers in the corresponding driving stages ST₁˜ST_N, respectively. CCA₁˜CCA_N represent the first-type current controllers in the corresponding driving stages ST₁˜ST_N, respectively. CCB₁˜CCB_N represent the second-type current controllers in the corresponding driving stages ST₁˜ST_N, respectively. CC_N+1 represents the third-type current controller in the (N+1)^th driving stage ST_N+1. ISA₁˜ISA_N represent the adjustable current sources in the corresponding first-type current controllers CCA₁˜CCA_N, respectively. ISB₁˜ISB_N represent the adjustable current sources in the corresponding second-type current controllers CCB₁˜CCB_N, respectively. IS_N+1 represents the adjustable current source in the third-type current controller CC_N+1. UNA₁˜UNA_N represent the current detection and control units in the corresponding first-type current controllers CCA₁˜CCA_N respectively. UNB₁˜UNB_N represent the voltage detection and control units in the corresponding second-type current controllers CCB₁˜CCB_N, respectively. UN_N+1 represents the detection and control unit in the (N+1)^th driving stage ST_N+1.

For illustrative purposes, the following symbols are used to represent related current/voltage in the LED lighting devices 101˜104 throughout the description and figures. V_IN1˜V_INN represent the voltages established across the 1^st to N^th driving stages ST₁˜ST_N, respectively. V_AK1˜V_AKN represent the voltages established across the corresponding first-type current controllers CCA₁˜CCA_N, respectively. V_BK1˜V_BKN represent the voltages established across the corresponding second-type current controllers CCB₁˜CCB_N, respectively. V_CK represents the voltage established across the third-type current controller CC_N+1. I_AK1˜I_AKN represent the current flowing through the corresponding first-type current controllers CCA₁˜CCA_N, respectively. I_BK1˜I_BKN represent the current flowing through the corresponding second-type current controllers CCB₁˜CCB_N, respectively. I_A1˜I_AN represent the current flowing through the corresponding luminescent devices A₁˜A_N, respectively. I_B1˜I_BN represent the current flowing through the corresponding luminescent devices B₁˜B_N, respectively. I_D1˜I_DN represent the current flowing through the corresponding path controllers D₁˜D_N, respectively. I_SUM1˜I_SUMN represent the current flowing through the corresponding driving stages ST₁˜ST_N, respectively. The overall current of the LED lighting devices 101˜104 may be represented by I_SUM(N+1).

In the 1^st to N^th driving stages ST₁˜ST_N of the LED lighting devices 101˜103, the current detection and control units UNA₁˜UNA_N, respectively coupled in series to the corresponding luminescent devices A₁˜A_N and the corresponding adjustable current sources ISA₁˜ISA_N, are configured to regulate the values of the adjustable current sources ISA₁˜ISA_N according the current I_AK1˜I_AKN, respectively. The voltage detection and control units UNB₁˜UNB_N, respectively coupled in series to the corresponding luminescent devices B₁˜B_N and in parallel with the corresponding adjustable current sources ISB₁˜ISB_N, are configured to regulate the values of the adjustable current sources ISB₁˜ISB_N according the voltages V_BK1˜V_BKN respectively.

In the 2^nd to N^th driving stages ST₂˜ST_N of the LED lighting device 104, the current detection and control units UNA₂˜UNA_N respectively coupled in series to the corresponding luminescent devices A₂˜A_N and the corresponding adjustable current sources ISA₂˜ISA_N, are configured to regulate the values of the adjustable current sources ISA₂˜ISA_N according the current I_AK2˜I_AKN, respectively. The voltage detection and control units UNB₂˜UNB_N, respectively coupled in series to the corresponding luminescent devices B₂˜B_N and in parallel with the corresponding adjustable current sources ISB₂˜ISB_N, are configured to regulate the values of the adjustable current sources ISB₂˜ISB_N according the voltages V_BK2˜V_BKN respectively.

In the (N+1)^th driving stage ST_N+1 of the LED lighting devices 101˜104, the adjustable current source IS_N+1 is coupled in series to the 1^st to N^th driving stages ST₁˜ST_N. In a first configuration, the detection and control unit UN_N+1 of the third-type current controller CC_N+1 may be coupled in series to the adjustable current source IS_N+1 and is configured to regulate the value of the adjustable current source IS_N+1 according the current I_SUMN. In a second configuration, the detection and control unit UN_N+1 of the third-type current controller CC_N+1 may be coupled in parallel with the adjustable current source IS_N+1 and is configured to regulate the value of the adjustable current source IS_N+1 according the voltage V_CK. FIGS. 1˜4 depict the embodiments adopting the first configuration, but do not limit the scope of the present invention.

In the embodiment of the present invention, each of the luminescent devices A₁˜A_N and B₁˜B_N may adopt a single LED or multiple LEDs coupled in series. FIGS. 1-4 depict the embodiments using multiple LEDs which may consist of single-junction LEDs, multi-junction high-voltage (HV) LEDs, or any combination of various types of LEDs. However, the types and configurations of the luminescent devices A₁˜A_N and B₁˜B_N do not limit the scope of the present invention. In a specific driving stage, the dropout voltage V_DROP for turning on the corresponding current controller is smaller than the cut-in voltage V_CUT for turning on the corresponding luminescent device. When the voltage established across a specific luminescent device exceeds its cut-in voltage V_CUT, the specific luminescent device may be placed in a conducting ON state; when the voltage established across the specific luminescent device does not exceed its cut-in voltage V_CUT, the specific luminescent device may be placed in a non-conducting OFF state. The value of the cut-in voltage V_CUT is related to the number or type of the LEDs in the corresponding luminescent device and may vary in different applications.

In the embodiment of the present invention, each of the M charge storage units CH₁˜CH_M may adopt a capacitor, or one or multiple devices which provides similar function. However, the types and configurations of the charge storage units CH₁˜CH_M do not limit the scope of the present invention.

In the embodiment of the present invention, each of the path-controllers D₁˜D_N may adopt a diode, a diode-connected field effect transistor (FET), a diode-connected bipolar junction transistor (BJT) or other devices having similar function, or one or multiple devices which provides similar function. However, the types and configurations of the path controllers D₁˜D_N do not limit the scope of the present invention. When the voltage established across a specific path controller exceeds its turn-on voltage, the specific path controller is forward-biased and functions as a short-circuited device; when the voltage established across the specific path controller does not exceed its turn-on voltage, the specific path controller is reverse-biased and functions as an open-circuited device.

FIGS. 5˜8 are diagrams illustrating the operation of the 1^st to N^th driving stages ST₁˜ST_N in the LED lighting devices 101-103 according to embodiments of the present invention. The driving stage ST₁ in the LED lighting devices 101-103 is used for illustrative purpose, wherein FIG. 5 illustrates the current-voltage curve (I-V curve) of the first-type current controller CCA₁, FIG. 6 illustrates the I-V curve of the second-type current controller CCB₁, FIG. 7 illustrates the equivalent circuits of the 1^st driving stage ST₁ during different phases of operation, and FIG. 8 illustrates the I-V curve of the 1^st driving stage ST₁. FIG. 9 is a diagram illustrating the operation of the current controller CC_N+1 in the (N+1)^th driving stages ST_N+1 of the LED lighting devices 101-104. V_DROPA, V_DROPB and V_DROPC represent the drop-out voltages for turning on the first-type current controller CCA₁, the second-type current controller CCB₁ and the third-type current controller CC_N+1, respectively. V_OFFA, V_OFFB and V_ONB represent the threshold voltages based on which the first-type current controller CCA₁ or the second-type current controller CCB₁ switch operational modes. I_SETA1, I_SETB1 and I_SETC are constant values which represent the current settings of the first-type current controller CCA₁, the second-type current controller and the third-type current controller CC_N+1, respectively. An arrow R indicates the rising period of the voltage V_AK1, V_BK1 or V_CK. An arrow L indicates the falling period of the voltage V_AK1, V_BK1 or V_CK.

In FIG. 5, during the rising and falling periods of the voltage V_AK1 when 0<V_AK1<V_DROPA, the first-type current controller CCA₁ is not completely turned on and operates as a voltage-controlled device in a linear mode in which the current I_AK1 changes with the voltage V_AK1 in a specific manner. For example, if the first-type current controller CCA₁ is implemented using metal-oxide-semiconductor (MOS) transistors, the relationship between the current I_AK1 and the voltage V_AK1 may correspond to the I-V characteristic of an MOS transistor when operating in the linear region.

During the rising and falling periods of the voltage V_AK1 when V_AK1>V_DROPA, the current I_AK1 reaches I_SETA1, and the first-type current controller CCA₁ switches to a constant-current mode and functions as a current limiter. The current detection and control unit UNA₁ is configured to clamp the current I_AK1 at I_SETA1. For example, in response to an increase in the current I_D1, the current detection and control unit UNA₁ may decrease the value of the adjustable current source ISA₁ accordingly. Similarly, in response to a decrease in the current I_D1, the current detection and control unit UNA₁ may increase the value of the adjustable current source ISA₁ accordingly. Therefore, the current I_AK1 (=I_D1+ISA₁) flowing through the 1^st driving stage ST₁ may be maintained at the constant value I_SETA1 instead of changing with the voltage V_AK1.

During the rising period of the voltage V_AK1 before the current I_D1 reaches I_SETA1, the current detection and control unit UNA₁ turns on the adjustable current source ISA₁ and the current controller CCA₁ functions as a current limiter in the constant-current mode in which the current I_AK1 (=IS_ETA1+I_D1) is clamped at a constant value of I_SETA1. When the current I_D1 reaches I_SETA1, the current detection and control unit UNA₁ turns off the adjustable current source ISA₁ and the current controller CCA₁ switches to a cut-off mode in which the current I_AK1 increases with the current I_D1.

During the falling period of the voltage V_AK1 before the current I_D1 drops I_SETA1, the current detection and control unit UNA₁ turns off the adjustable current source ISA₁ and the current controller CCA₁ operates in the cut-off mode in which the current I_AK1 decreases with the current I_D1. When the current I_D1 drops to I_SETA1, the current detection and control unit UNA₁ turns on the adjustable current source ISA₁ and the current controller CCA₁ functions as a current limiter in the constant-current mode in which the current I_AK1 is clamped at a constant value of I_SETA1.

In FIG. 6, during the rising and falling periods of the voltage V_BK1 when 0<V_BK1<V_DROPB, the second-type current controller CCB₁ is not completely turned on and operates as a voltage-controlled device in the linear mode in which the current I_BK1 changes with the voltage V_BK1 in a specific manner. For example, if the second-type current controller CCB₁ is implemented using MOS transistors, the relationship between the current I_BK1 and the voltage V_BK1 may correspond to the I-V characteristic of an MOS transistor when operating in the linear region.

During the rising period of the voltage V_BK1 when V_BK1>V_DROPB, the current I_BK1 reaches I_SETB1, and the current controller CCB₁ switches to the constant-current mode and functions as a current limiter. The voltage detection and control unit UNB₁ is configured to clamp the current I_BK1 at I_SETB1.

During the rising period of the voltage V_BK1 when V_BK1>V_OFFB, the voltage detection and control unit UNB₁ is configured to turn off the adjustable current source ISB₁ and the second-type current controller CCB₁ switches to the cut-off mode. In other words, the second-type current controller CCB₁ functions as an open-circuited device. During the falling period of the voltage V_BK1 when V_BK1<V_ONB, the voltage detection and control unit UNB₁ is configured to turn on the adjustable current source ISB₁ and the current controller CCB₁ switches to the constant-current mode and functions as a current limiter, thereby clamping the current I_BK1 at I_SETB1. The threshold voltage V_ONB is larger than or equal to the threshold voltage V_OFFB. In an embodiment, a non-zero hysteresis band (V_ONB−V_OFFB) may be provided in order to prevent the second-type current controller CCB₁ from frequently switching operational modes due to fluctuations in the voltage V_BK1.

In FIG. 7, when the 1^st driving stage ST₁ operates in a first phase with V1<V_IN1<V2, the luminance device A₁ is coupled in parallel with the luminance device B₁, as depicted on the left of FIG. 7. When the 1^st driving stage ST₁ operates in a second phase with V_IN1>V3, the luminance device A₁ is coupled in series to the luminance device B₁, as depicted on the right of FIG. 7.

In FIG. 8, during the rising period when the voltage V_IN1 is low, the luminance device A₁, the luminance device B₁ and the path-controller D₁ remain off. During the rising period as the voltage V_IN1 reaches a turn-on voltage V_A1 which is the sum of the cut-in voltage for turning on the luminance device A₁ and the cut-in voltage for turning on the first-type current controller CCA₁, the first-type current controller CCA₁ and the luminance device A₁ are turned on, allowing the current I_A1 to gradually increase with the voltage V_IN1 until reaching I_SETA1; during the rising period as the voltage V_IN1 reaches a turn-on voltage V_B1 which is the sum of the cut-in voltage for turning on the luminance device B₁ and the cut-in voltage for turning on the second-type current controller CCB₁, the second-type current controller CCB₁ and the luminance device B₁ are turned on, allowing the current I_B1 to gradually increase with the voltage V_IN1 until reaching I_SETB1. With the path controller D1 still off, the current I_SUM1 is equal to the sum of the current I_A1 and the current I_B1, wherein the current I_A1 is regulated by the current controllers CCA₁ and the current I_B1 is regulated by the current controllers CCB₁. The value of the turn-on voltage V_A1 may be equal to or different from that of the turn-on voltage V_B1. In other words, the current I_SUM1 starts to increase at a voltage V1 which is equal to the smaller one among the turn-on voltage V_A1 and the turn-on voltage V_B1.

During the rising period when the voltage V_IN1 reaches V2 so that V_BK1=V_OFFB, the second-type current controller CCB₁ switches to the cut-off mode in which the current I_B1 is directed towards the path-controller D₁, thereby turning on the path-controller D1. The current I_SUM1 is equal to the current I_B1 and I_A1, wherein both the current I_A1 and the current I_B1 are regulated by the first-type current controller CCA₁. As the current I_B1 flows through the path-controller D₁, the current I_D1 gradually increases with the voltage V_IN1. In response, the first-type current controller CCA₁ decreases the value of the adjustable current source ISA accordingly, so that the overall current I_AK1 is still maintained at the constant value I_SETA1. When the value of the current source ISA₁ drops to zero at V_IN1=V3, the first-type current controller CCA₁ switches to the cut-off mode. The current I_SUM1 is now regulated by the subsequent driving stage.

In FIG. 9, during the rising and falling periods of the voltage V_CK when 0<V_CK<V_DROPC, the third-type current controller CC_N+1 is not completely turned on and operates as a voltage-controlled device in the linear mode in which the current I_CK changes with the voltage V_CK in a specific manner. For example, if the third-type current controller CC_N+1 is implemented using MOS transistors, the relationship between the current I_CK and the voltage V_CK may correspond to the I-V characteristic of an MOS transistor when operating in the linear region. During the rising and falling cycles of the voltage V_CK when V_CK>V_DROPC, the current I_CK reaches I_SETC, and the third-type current controller CC_N+1 switches to the constant-current mode and functions as a current limiter.

Similarly, the operation of the 2^nd to N^th driving stages ST2˜STN in the LED lighting device 104 may also be illustrated in FIGS. 5˜8, while the operation of the current controller CC_N+1 in the (N+1)th driving stages ST_N+1 of the LED lighting device 104 may also be illustrated in FIG. 9.

In the present invention, the charge storage units CH₁˜CH_M may be coupled in parallel with one or multiple luminescent devices among the luminescent devices A₁˜A_N and B₁˜B_N, respectively. The charge storage units CH₁˜CH_M can reduce the flicker of the LED lighting devices 101˜104, wherein M may be smaller than or equal to 2N.

In an embodiment when M=2N, each of the luminescent devices A₁˜A_N and B₁˜B_N is coupled in parallel with a corresponding charge storage unit. For illustrative purpose, FIG. 1 depicts the above-mentioned embodiment of N=2 and M=4 in which the LED lighting device 101 includes 4 luminescent devices A₁˜A₂ and B₁˜B₂ coupled in parallel with the charge storage units CH₁˜CH₄, respectively. However, the number and configuration of the charge storage units do not limit the scope of the present invention.

In an embodiment when M<2N, each of the luminescent devices B₁˜B_N is coupled in parallel with a corresponding charge storage unit. For illustrative purpose, FIG. 2 depicts the above-mentioned embodiment of N=2 and M=2 in which the LED lighting device 102 includes 4 luminescent devices A₁˜A₂ and B₁˜B₂ among which the luminescent devices B₁˜B₂ are coupled in parallel with the charge storage units CH₁˜CH₂, respectively. However, the number and configuration of the charge storage units do not limit the scope of the present invention.

In an embodiment when M<2N, the M charge storage units CH₁˜CH_M may be coupled in parallel with the luminescent devices which have the longest turn-on time among the luminescent devices A₁˜A_N and B₁˜B_N. For illustrative purpose, FIG. 3 depicts the above-mentioned embodiment of N=2 and M=2 in which the LED lighting device 103 includes 4 luminescent devices A₁˜A₂ and B₁˜B₂ among which the luminescent devices A₁ and B₁ are coupled in parallel with the charge storage units CH₁˜CH₂, respectively. However, the number and configuration of the charge storage units do not limit the scope of the present invention.

In an embodiment when M=1<2N, the charge storage unit CH₁ may be coupled in parallel with multiple luminescent devices which have the longest turn-on time among the luminescent devices A₁˜A_N and B₁˜B_N. For illustrative purpose, FIG. 4 depicts the above-mentioned embodiment of N=2 and M=1 in which the LED lighting device 104 includes 3 luminescent devices A₂ and B₁˜B₂ among which the luminescent devices B₁˜B₂ are coupled in parallel with the charge storage unit CH₁. However, the number and configuration of the charge storage units do not limit the scope of the present invention.

FIG. 10 is a diagram illustrating the current-time characteristic of the luminescent devices in the LED lighting devices 101˜104. The diagram in the middle of FIG. 10 represents the current-time characteristic of a luminescent device adopting a first configuration, and the diagram at the bottom of FIG. 10 represents the current-time characteristic of a luminescent device adopting a second configuration. In FIG. 10, I_LED represents the current flowing through the luminescent device adopting the first configuration and I_LED represents the current flowing through the luminescent device adopting the second configuration. The luminescent device adopting the first configuration is coupled in parallel with a corresponding charge storage unit, such as the luminescent device A₁, A₂, B₁ or B₂ in the LED lighting device 101, the luminescent device B₁ or B₂ in the LED lighting device 102, the luminescent device A₁ or B₁ in the LED lighting device 103, or the luminescent device B₁ or B₂ in the LED lighting device 104. The luminescent device adopting the second configuration is not coupled in parallel with any charge storage unit, such as the luminescent device A₁ or A₂ in the LED lighting device 102, the luminescent device A₂ or B₂ in the LED lighting device 103, or the luminescent device A₂ in the LED lighting device 104.

During the rising period before the rectified AC voltage V_AC becomes sufficiently large to turn on the luminescent device, the luminescent device adopting the second configuration remains in OFF state, while the luminescent device adopting the first configuration may be maintained in ON state by the energy discharged from the corresponding charge storage unit. The corresponding path controller is arranged to prevent the energy stored in the corresponding charge storage unit from being discharged through the corresponding current controller.

During the rising period or the falling period when the rectified AC voltage V_AC becomes sufficiently large, the luminescent device adopting the first configuration or the luminescent device adopting the second configuration may be maintained in ON state by the rectified AC voltage V_AC, which is now charging the corresponding charge storage unit.

During the falling period after the rectified AC voltage V_AC is no longer sufficiently large to turn on the luminescent device, the luminescent device adopting the second configuration remains in OFF state, while the luminescent device adopting the first configuration may still be maintained in ON state by the energy discharged from the corresponding charge storage unit. The corresponding path controller is arranged to prevent the energy stored in the corresponding charge storage unit from being discharged through the corresponding current control unit.

As depicted in FIG. 10, the introduction of the charge storage unit allows the luminescent device adopting the second configuration to have longer turn-on time than the luminescent device adopting the first configuration.

FIG. 11 is a diagram illustrating the overall operation of the LED lighting device 103 when two of the 4 luminescent devices A₁˜A₂ and B₁˜B₂ (N=2 and M=2) are coupled in parallel to respective charge storage units CH₁˜CH₂ or coupled in parallel to one communal charge storage unit CH₁. FIG. 12 is a diagram illustrating the overall operation of the LED lighting device 103 when no charge storage unit is adopted. E₁˜E₃ represent the overall intensity/flux of the present LED lighting device 103. It is to be noted that FIG. 12 is used as a comparison to FIG. 11 for illustrating how flicker can be improved using the present charge storage units as depicted in FIGS. 1˜4, but is by no means an intended operation of present invention.

Since the voltages V_AK1˜V_AK2 and V_BK1˜V_BK2 are associated with the rectified AC voltage V_AC whose value varies periodically with time, a driving cycle of t₀-t₇ is used for illustration, wherein the period between t₀-t₃ belongs to the rising period of the rectified AC voltage V_AC and the period between t₄-t₇ belongs to the falling period of the rectified AC voltage V_AC. The following Table 1 lists the operational modes of the luminescent devices A₁˜A₂ and B₁˜B₂ in accordance with the configuration depicted in FIG. 11. The following Table 2 lists the operational modes of the luminescent devices A₁˜A₂ and B₁˜B₂ in accordance with the configuration depicted in FIG. 12.

	TABLE 1
	luminescent	t0~t1/	t1~t2/	t2~t3/
	device	t6~t7	t5~t6	t4~t5	t3~t4
	A1	ON (P)	ON (P)	ON (S)	ON (S)
	B1	ON (P)	ON (P)	ON (S)	ON (S)
	A2	OFF	ON (P)	ON (P)	ON (S)
	B2	OFF	ON (P)	ON (P)	ON (S)

	TABLE 2
	luminescent	t0~t1/	t1~t2/	t2~t3/
	device	t6~t7	t5~t6	t4~t5	t3~t4
	A1	OFF	ON (P)	ON (S)	ON (S)
	B1	OFF	ON (P)	ON (S)	ON (S)
	A2	OFF	ON (P)	ON (P)	ON (S)
	B2	OFF	ON (P)	ON (P)	ON (S)

In FIG. 12 and Table 2, at the beginning of the rising period and at the end of the falling period, the rectified AC voltage V_AC is insufficient to turn on the luminescent devices A₁˜A₂ and B₁˜B₂.

Without the present charge storage units, the luminescent devices A₁˜A₂ and B₁˜B₂ remain in the OFF state between t0˜t1 and t6˜t7. Between t1˜t6, the luminescent devices A₁˜A₂ and B₁˜B₂ are sequentially turned on as the rectified AC voltage V_AC increases or decreases, and the 1^st driving stage ST₁ and the 2^nd driving stage ST₁ may operates in the first phase in which the two turned-on luminance devices are coupled in parallel (designated by “P” in Table 1 and Table 2) as depicted on the left of FIG. 7 or in the second phase in which the two turned-on luminance devices are coupled in series (designated by “S” in Table 1 and Table 2) as depicted on the right of FIG. 7. More specifically, the overall intensity/flux of the LED lighting device 103 varies stepwise and reaches E₃ between t3˜t4 when all the luminescent devices A₁˜A₂ and B₁˜B₂ operate in the ON state in the series configuration.

In FIG. 11 and Table 1, at the beginning of the rising period and at the end of the falling period, the rectified AC voltage V_AC is insufficient to turn on the luminescent devices A₁˜A₂ and B₁˜B₂. With the present charge storage units, the luminescent devices A₁ and B₁ may be kept in the ON state during the entire driving period between t0˜t7 regardless of the rectified AC voltage V_AC. More specifically, the overall intensity/flux of the present LED lighting device 103 may be maintained at E₁ between t0˜t1 and t6˜t7 when the rectified AC voltage V_AC is still small.

As well-known to those skilled in the art, LED flicker is periodic, with its waveforms characterized by variations in amplitude, average level, periodic frequency, shape, and/or duty cycle. Percent Flicker and Flicker Index are metrics historically used to quantify flicker, as represented by the following formula:

$\begin{array}{cc}\mathrm{Percent}\mathrm{Flicker}=100\%\times \frac{\mathrm{MAX}-\mathrm{MIN}}{\mathrm{MAX}+\mathrm{MIN}}& \left(1\right)\\ \mathrm{Flicker}\mathrm{Index}=\frac{\mathrm{AREA}1}{\mathrm{AREA}1+\mathrm{AREA}2}& \left(2\right)\end{array}$

In formula (1), MAX represents the maximum intensity/flux of the LED lighting devices 101˜104, while MIN represents the minimum intensity/flux of the LED lighting devices 101˜104. In formula (2), AREA1 represents the summation of intensity/flux within a duration of a driving cycle when the intensity/flux of the LED lighting devices 101˜104 is above its average, while AREA2 represents the summation of intensity/flux within a duration of the driving cycle when the intensity/flux of the LED lighting devices 101˜104 is below its average.

As can be seen in FIG. 11, the introduction of the charge storage units can increase MIN in formula (1) and AREA2 in formula (2), thereby lowering the Percent Flicker and Flicker Index of the LED lighting devices 101˜104.

FIGS. 13˜16 are diagram of LED lighting devices 105˜108 according to other embodiments of the present invention. Similar to the LED lighting devices 101˜104 depicted in FIGS. 1˜4, each of the LED lighting devices 105˜108 also includes a power supply circuit 110 and (N+1) driving stages ST₁˜ST_N+1 (N is a positive integer). However, the LED lighting devices 105˜107 differ from the LED lighting devices 101˜103 in that each of the 1^st to N^th driving stages ST₁˜ST_N includes a plurality of luminescent devices, a path controller, and two first-type current controllers. The LED lighting device 108 differs from the LED lighting device 104 in that each of the 2^nd to N^th driving stages ST₂˜ST_N includes a plurality of luminescent devices, a path controller, and two first-type current controllers.

Each first-type current controller in the LED lighting devices 105˜108 includes an adjustable current source and a current detection and control unit, and its I-V curve may also be shown in FIG. 5. In the first-type current controllers represented by CCA₁˜CCA_N, the current detection and control units UNA₁˜UNA_N, respectively coupled in series to the corresponding luminescent devices A₁˜A_N and the corresponding adjustable current sources ISA₁˜ISA_N, are configured to regulate the values of the adjustable current sources ISA₁˜ISA_N according the current I_AK1˜I_AKN, respectively. In the first-type current controller represented by CCA_1′˜CCA_N′, the current detection and control units UNA_1′˜UNA_N′, respectively coupled in series to the corresponding luminescent devices B₁˜B_N and the corresponding adjustable current sources ISA_1′˜ISA_N′, are configured to regulate the values of the adjustable current sources ISA_1′˜ISA_N′ according the current I_BK1˜I_BKN, respectively.

With the above-mentioned multi-stage driving scheme, the present invention may turn on multiple luminescent devices flexibly using multiple current control units. With the above-mentioned charge storage units, the present invention may reduce luminous variation of the LED lighting device. Therefore, the present invention can provide an LED lighting device capable of improving the effective operational voltage range, the reliability and the flicker phenomenon.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

Claims

1. A light-emitting diode (LED) lighting device having multiple driving stages, comprising: a first driving stage including: a first luminescent device driven by a rectified alternative-current (AC) voltage for providing light according to first current;a second luminescent device driven by the rectified AC voltage for providing light according to second current;a first current controller coupled in series to the first luminescent device and configured to regulate the first current so that the first current does not exceed a first value;a second current controller coupled in series to the second luminescent device and configured to regulate the second current so that the second current does not exceed a second value;a first charge storage unit coupled in parallel with at least the first luminescent device and configured to discharge energy to the first luminescent device when the rectified AC voltage is insufficient to turn on the first luminescent device, thereby keeping the first luminescent device turned on; anda first path-controller configured to conduct third current and comprising: a first end coupled between the first luminescent device and the first current controller; anda second end coupled to the second current controller; anda second driving stage including: a third current controller coupled in series to the first driving stage and configured to conduct fourth current and regulate the fourth current so that the fourth current does not exceed a third value.

2. The LED lighting device of claim 1, wherein the first charge storage unit is further configured to stop discharging the energy to the first luminescent device and start to be charged by the rectified AC voltage when the rectified AC voltage become sufficient to turn on the first luminescent device.

3. The LED lighting device of claim 1, further comprising: a second charge storage unit coupled in parallel with the second luminescent device and configured to discharge energy to the second luminescent device when the rectified AC voltage is insufficient to turn on the second luminescent device, thereby keeping the second luminescent device turned on.

4. The LED lighting device of claim 1, further comprising: a third driving stage coupled between the rectified AC voltage and the first driving stage and including: a third luminescent device driven by the rectified AC voltage for providing light, wherein the first charge storage unit is coupled in parallel with the first luminescent device and the third luminescent device and configured to discharge energy to the first luminescent device and the third luminescent device when the rectified AC voltage is insufficient to turn on the first luminescent device and the third luminescent device, thereby keeping the first luminescent device and the third luminescent device turned on.

5. The LED lighting device of claim 4, wherein the first charge storage unit is further configured to stop discharging the energy to the first luminescent device and the third luminescent device and start to be charged by the rectified AC voltage when the rectified AC voltage become sufficient to turn on the first luminescent device and the third luminescent device.

6. The LED lighting device of claim 1, wherein: during a rising period or a falling period of a rectified AC voltage when a voltage established across the first current controller does not exceed a first voltage, the first current controller operates in a first mode in which the first current changes with the voltage established across the first current controller;during the rising period when the voltage established across the first current controller exceeds the first voltage but does not exceed a second voltage, the first current controller operates in a second mode in which the first current is maintained at the first value; andduring the rising period when the voltage established across the first current controller exceeds the second voltage, the first current controller operates in a third mode in which the first current controller is turned off.

7. The LED lighting device of claim 6, wherein: during the falling period when the voltage established across the first current controller exceeds the second voltage but does not exceed a third voltage, the first current controller operates in the second mode in which the first current is maintained at the first value, and the third voltage is larger than or equal to the second voltage.

8. The LED lighting device of claim 1, wherein: during a rising period or a falling period of the rectified AC voltage when the voltage established across the second current controller does not exceed a fourth voltage, the second current controller operates in a first mode in which the second current changes with the voltage established across the second current controller;during the rising period or the falling period when the third current does not exceed the second value, the second current controller operates in a second mode in which the second current is maintained at the second value; andduring the rising period or the falling period when the third current exceeds the second value, the second current controller operates in a third mode in which the second current controller is turned off.

9. The LED lighting device of claim 1, wherein: during a rising period or a falling period of the rectified AC voltage when the voltage established across the third current controller does not exceed a sixth voltage, the third current controller operates in a first mode in which the fourth current changes with the voltage established across the third current controller; andduring the rising period or the falling period when the voltage established across the third current controller exceeds the sixth voltage, the third current controller operates in a second mode in which the fourth current is maintained at the third value.

10. The LED lighting device of claim 1, wherein the first current controller includes: a first adjustable current source configured to conduct fifth current; anda first detection and control unit coupled in parallel with the first adjustable current and configured adjust the fifth current according to a voltage established across the first current controller.

11. The LED lighting device of claim 1, wherein the first current controller includes: a first adjustable current source configured to conduct fifth current, and comprising: a first end coupled to the first luminescent device; anda second end coupled to the second luminescent device; anda first detection and control unit coupled in series to the first adjustable current source and configured adjust the fifth current according to the first current and the second current.

12. The LED lighting device of claim 1, wherein the second current controller includes: a second adjustable current source configured to conduct sixth current; anda second detection and control unit configured adjust the sixth current according to the second current or the third current, and comprising: a first end coupled to the second end of the first path-controller and the second adjustable current source; anda second end coupled to the second luminescent device.

13. The LED lighting device of claim 1, wherein: the first current controller includes: a first adjustable current source configured to conduct fifth current; anda first detection and control unit coupled in parallel with the first adjustable current source and configured adjust the fifth current according to a voltage established across the first current controller; andthe second current controller includes: a second adjustable current source configured to conduct sixth current; anda second detection and control unit configured adjust the sixth current according to the second current or the third current, and comprising: a first end coupled to the second end of the first path-controller and the second adjustable current source; anda second end coupled to the second luminescent device.

14. The LED lighting device of claim 1, wherein: the first current controller includes: a first adjustable current source configured to conduct fifth current, and comprising: a first end coupled to the first luminescent device; anda second end coupled to the second luminescent device; anda first detection and control unit coupled in series to first adjustable current source and configured adjust the fifth current according to the first current and the second current; andthe second current controller includes: a second adjustable current source configured to conduct sixth current; anda second detection and control unit configured adjust the sixth current according to the second current or the third current, and comprising: a first end coupled to the second end of the first path-controller and the second adjustable current source; anda second end coupled to the second luminescent device.

15. The LED lighting device of claim 1, wherein the third current controller includes: a third adjustable current source configured to conduct the fourth current; anda third detection and control unit coupled in series to the third adjustable current source and configured to control the third adjustable current source according to the fourth current.

16. The LED lighting device of claim 1, wherein the first path-controller includes a diode, a diode-connected field effect transistor (FET), or a diode-connected bipolar junction transistor (BJT).

17. The LED lighting device of claim 1, wherein: the first luminescent device is coupled in parallel with the second luminescent device when the first path-controller is turned off; andthe first luminescent device is coupled in series to the second luminescent device when the first path-controller is turned on.

18. The LED lighting device of claim 1, wherein: when the first path-controller is turned off, the third current is zero, and the fourth current is equal to a sum of the first current and the second current; andwhen the first path-controller is turned on, the first current, the second current, the third current and the fourth current is equal.