Dimming device

A dimming device includes: a constant current generator for generating a current to a main LED unit; a state generator for generating a state counting output having a counting value that is equal to one of continuous values respectively associated with different light-mixing states of a secondary LED unit; and an RGB modulation circuit operable to generate, based on the state counting output, multiple PWM signals with the same PWM cycle for driving respectively R, G, and B LEDs of the secondary LED unit such that the secondary LED unit is sequentially operated in the light-mixing states. The secondary LED unit emits a mixed light with an individual color in each of the different light-mixing states to compensate lighting of the main LED unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Taiwanese Application No. 100139372, filed on Oct. 28, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a dimming device, and more particularly to a dimming device for adjusting color temperature and color rendition index (CRI).

2. Description of the Related Art

Dimming methods for a light emitting diode (LED) include an analog dimming method and a digital dimming method.

According to the analog dimming method, a voltage across the LED is adjusted to non-linearly raise or reduce a current flowing through the LED such that the brightness of the LED is adjusted to achieve different lighting effects. However, since variation of an amount of the current may result in the wavelength drift of light emitted from the LED, color temperature cannot be exactly controlled.

According to the digital dimming method, for example a pulse width modulation (PWM) dimming method, the LED is driven by a PWM signal with a fixed amplitude and various duty cycles. In other words, the brightness of the LED is adjusted by controlling a ratio of on-off time of the LED, i.e., the duty ratio of the LED, in response to the PWM signal. In this case, due to the persistence of vision, light emitted from the LED has a relatively stable wavelength, thereby facilitating control of color temperature as compared to the analog dimming method. However, for a white light LED formed by means of a blue light LED coated with a yellow phosphor, although such a white light LED is driven by a PWM signal, desired color temperature may not be achieved.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a dimming device that can overcome the aforesaid drawbacks of the prior art.

According to one aspect of the present invention, there is provided a dimming device for a lighting device including a main light emitting unit and a secondary light emitting unit. The secondary light emitting unit includes at least red-light-emitting element, at least one green-light-emitting element and at least one blue-light-emitting element. The dimming device of the present invention comprises:

a constant current generator adapted for outputting a current with a fixed amplitude to the main light emitting unit;

a state generator operable to repeatedly count based on an input frequency signal so as to generate a state counting output having a counting value that is equal to one of a plurality of continuous counting values and that changes from a current one of the continuous values to a next one of the continuous values after a time period associated with the input frequency signal, the values being respectively associated with a plurality of different light-mixing states of the secondary light emitting unit; and

an RGB modulation circuit connected electrically to the state generator for receiving the state counting output therefrom, and operable to generate, based on the state counting output from the state generator, a plurality of periodic pulse width modulation (PWM) signals with the same PWM cycle, each of the PWM signals being adapted to drive a corresponding one of the red-light-emitting, green-light-emitting and blue-light-emitting elements of the secondary light emitting unit such that the secondary light-emitting unit is sequentially operated in the different light-mixing states.

The secondary light-emitting unit emits a mixed light with an individual color in each of the different light-mixing states to compensate lighting of the main light emitting unit.

According to another aspect of the present invention, there is provided a dimming device for a light emitting unit. The dimming device of the present invention comprises:

a pulse-width modulation (PWM) signal generator for generating a plurality of periodic pulse width modulation (PWM) singles with the same PWM cycle; and

an off-pulse generator connected electrically to the PWM signal generator for generating a periodic off-pulse signal based on an external dimming control input, and outputting the off-pulse signal to the PWM signal generator, the off-pulse signal having a frequency higher than that of the PWM signals, and a pulse width determined by the external dimming control input.

The PWM signal generator further adjusts the duty cycle of each of the PWM signals based on the off-pulse signal from the off-pulse generator, and then outputs the PWM signals adjusted thereby to the light emitting unit to drive the light emitting unit to emit light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, the preferred embodiment of a dimming device for a lighting device9according to the present invention is shown to include a constant current generator1, a frequency generator2, a state generator3, a step generator4, an RGB modulation circuit5, an input unit6, and an OFF-pulse generator7. The lighting device9includes a main light emitting unit91, and a secondary light emitting unit92. In this embodiment, the main light emitting unit91includes a white light LED (not shown), which is formed by means of a blue light LED coated with a yellow phosphor. The secondary light emitting unit92includes a red-light-emitting element, such as a red light LED921, a green-light-emitting element, such as a green light LED922, and a blue-light-emitting element, such as a blue light LED923(seeFIG. 6). In other embodiments, the secondary light emitting unit92can merely include a red light LED and a green light LED. The dimming device is adapted to drive the main light emitting unit91by a driving signal from the constant current generator1, and drive the secondary light emitting unit92by a plurality of periodic pulse-width modulation (PWM) signals with the same PWM cycle that are to be generated by a PWM signal generator consisting of the frequency generator2, the state generator3, the step generator4and the RGB modulation circuit5. It is noted that, in this embodiment, the PWM signals are applied to cathodes of the red, green and blue light LEDs921,922,923of the secondary light emitting unit92(seeFIG. 6). As a result, when any one of the PWM signals has a low level, a corresponding one of the red, green and blue light LEDs921,922,923conducts. When any one of the PWM signal has a high level, a corresponding one of the red, green and blue light LEDs921,922,923does not conduct. In this case, the high level serves as a non-conduction level, and the low level serves as a conduction level. As such, each of the red, green and blue light LEDs921,922,923is activated to emit light during the conduction level of a corresponding PWM signal, and is deactivated to cease to emit light during the non-conduction level of the corresponding PWM signal.

The constant current generator1is adapted to be connected electrically to the main light emitting unit91for outputting a current with a fixed amplitude to the main light emitting unit91such that the main light emitting unit91emits light in response to the current from the constant current generator1. In this embodiment, the current serves as the driving signal

Referring further toFIG. 2the frequency generator2is configured to generate a first set of frequency-divided signals and a second set of frequency-divided signals based on a clock input (CLK) that is generated by an oscillator (not shown). In this embodiment, the frequency generator2includes a ripple counter consisting of a plurality of D-type flip flops. The clock input (CLK) has a frequency of 8 MHz. Frequencies of the first set of frequency-divided signals have 32 KHz, 16 KHz, 8 KHz, 4 KHz, 2 KHz, 512 Hz, 256 Hz and 128 Hz, respectively. Frequencies of the second set of frequency-divided signals have 32 Hz, 16 Hz, 8 Hz, 4 Hz, 2 Hz, 1 Hz, ½ Hz, ¼ Hz and ⅛ Hz, respectively. In addition, the frequency generator2is further operable to hold the second set of frequency-divided signals upon receipt of a pause signal.

The state generator3is connected electrically to the frequency generator2. The state generator3is configured to repeatedly count based on an input frequency signal so as to generate a state counting output having a counting value that is equal to one of a plurality of continuous counting values and that changes from a current one of the continuous values to a next one of the continuous values after a time period associated with the input frequency signal. In this embodiment, referring further toFIG. 3, the state generator3includes a ripple counter31consisting of five T-type flip flops, a decoder32, and an S-R latch33. The input frequency signal is the frequency-divided signal of ¼ Hz from the frequency generator2. Thus, the time period is a period of 4 seconds. The state counting output is a five-bit output (A0A1A2A3A4). In addition, the continuous values are 0, 1, . . . , and 17. Thus, the continuous values of 0, 1, . . . , and 17 are respectively associated with 18 different light-mixing states of the secondary light emitting unit92. It is noted that, when the second set of frequency-divided signals are held in response to the pause signal, the state generator3ceases to count to thereby unchange the counting value. The relationships between the counting value of the state counting output and the light-mixing states of the secondary light emitting unit92are shown in the following Table 1:

As may be seen from Table 1 when the counting value of the state counting output changes from 0 to 3, the duty cycles of corresponding PWM signals for respectively driving the red and blue light LEDs921,923remain to be 100% and 0, respectively, and the duty cycle a corresponding PWM signal for driving the green light LED922changes from 0% to 100%. When the counting value state of the counting output changes from 4 to 8, the duty cycle of the corresponding PWM signal for driving the red light LED921changes from 75% to 0%, the duty cycle of the corresponding PWM signal for driving the green light LED922remains to be 100%, and the duty cycle of the corresponding PWM signal for driving the blue light LED923changes from 0% to 75%. When the counting value changes from 9 to 14, the duty cycle of the corresponding PWM signal for driving the red light LED921changes from 0% to 100%, the duty cycle of the corresponding PWM signal for driving the green light LED922changes from 75% to 0%, and the duty cycle of the corresponding PWM signal for driving the blue light LED923remain to be 100%. When the counting value of the state counting output changes from 15 to 17, the duty cycles of the corresponding PWM signals for respectively driving the red and green light LEDs921,922remain to be 100% and 0, respectively, and the duty cycle of the corresponding PWM signal for driving the blue light LED923changes from 75% to 0%.

Referring further toFIG. 4, the step generator4is connected electrically to the frequency generator2. The step generator4is configured to generate a step output based on the first set of frequency-divided signals and the second set of the frequency-divided signals from the frequency generator2. The step output includes first and second step-down signals (AW, BW) associated with pulse-width increment of the PWM signals, and first and second step-down signals associated with pulse-width decrement of the PWM signals. Each of the first and second step-down signals (AW, BW) and the first and second step-up signals (CW, DW) has a pulse component within each PWM cycle.

In this embodiment, since the frequency of the PWM signals is 128 HZ, a counting value (p) of a 9-bit code corresponding to the first set of frequency-divided signals changes from 0 to (29−1) within each PWM cycle, where G=29, and g represents a counting value of a 9-bit code corresponding to the second set of frequency-divided signals, where 0≦g≦(G−1). Therefore, when the counting value (p) reaches g, the pulse component of the first step-down signal (AW) occurs. When the counting value (p) reaches (g−G/2) mod G, the pulse component of the second step-down signal (BW) occurs. Therefore, the time difference between the pulse components of the first and second step-down signals (AW, BW) within the same PWM cycle is half the PWM cycle. On the other hand, when the counting value (p) reaches [(G−1)+G/2−g] mod G, the pulse component of the first step-up signal (CW) occurs. When the counting value (p) reaches [(G−1)−g] mod G, the pulse component of the second step-up signal (DW) occurs. Therefore, the time difference between the pulse components of the first and second step-up signals (CW, DW) within the same PWM cycle is half the PWM cycle. For example, during the counting value (g) being 510 (=2′b111111110), the pulse component of the first step-down signal (AW) occurs when the counting value (P) reaches 510 (=2′b111111110), the pulse component of the second step-down signal (BW) occurs when the counting value (P) reaches 254(=2′b011111110), the pulse component of the first step-up signal (CW) occurs when the counting value (P) reaches 257(=2′b100000001), and the pulse component of the second step-up signal (DW) occurs when the counting value (P) reaches 1(=2′b000000001).

It is noted that, since the counting value (g) gradually increases with time, for each of the first and second step-down signal (AW, BW), the appearance time of the pulse component within each PWM cycle is later than the appearance time of the pulse component within a previous PWN cycle, and for each of the first and second step-up signals (CW, DW), the appearance time of the pulse component within each PWM cycle is earlier than the appearance time of the pulse component within a previous PWN cycle. Furthermore, when the second set of frequency-divided signals is held in response to the pause signal, the step generator4ceases generation of the step output, i.e., the first and second step-down signals (AW, BW) and the first and second step-up signals (CW, DW).

The RGB modulation circuit5is connected electrically to the state generator3, the step generator4, and is adapted to be connected electrically to the secondary light emitting unit92of the lighting device9. The RGB modulation circuit5receives the state counting output from the state generator3, and the step output from the step generator4. The RGB modulation circuit5is operable to generate the PWM signals based on the state counting output and the step output received thereby. In this embodiment, the RGB modulation circuit5includes an encoding unit51, a cycle adjusting unit52, and a duty-cycle adjusting unit53.

Referring further toFIG. 5, the encoding unit51is connected electrically to the state generator3and the step generator4for receiving the state counting output and the step output therefrom. The encoding unit51is operable to encode the state counting output based on the step output into an encoding output. In this embodiment, the encoding output includes first, second and third encoding components (ROUT, GOUT, BOUT) that correspond to the PWM signals for driving the red, green and blue light LEDs921,922,923, respectively. According to configuration of the encoding unit51showFIG. 5, the relationships between the state counting output and the encoding output can be obtained and are shown in the following Table 2:

Table 2 is related to Table 1. As such, any one of the first, second and third encoding components (ROUT, GOUT, BOUT) being equal to 1 represents the duty cycle of a corresponding PWM signal being 100%. Similarly, any one of the first, second and third encoding components (ROUT, GOUT, BOUT) being equal to 0 represents the duty cycle of a corresponding PWM signal being 0%.

Referring further toFIG. 6, the cycle adjusting unit52is connected electrically to the encoding unit51for receiving the encoding output therefrom. The cycle adjusting unit52is operable to generate the PWM signals based on the encoding output received thereby. According to configuration of the cycle adjusting unit52shown inFIG. 6, the cycle adjusting unit52is operable to enable each PWM signal to have the non-conduction level, i.e., the high level at the beginning of each PWM cycle, and determines whether each PWM signal switches from the non-conduction level to the conduction level, i.e., the low level, based on a corresponding one of the first, second and third encoding components (ROUT, GOUT, BOUT) of the encoding output from the encoding unit51. In detail, the cycle adjusting unit52determines that each PWM signal switches from the non-conduction level to the conduction level upon detecting that the corresponding one of the first, second and third encoding elements (ROUT, GOUT, BOUT) is identical to one of the first and second step-down signals (AW, BW) and the first and second step-up signals (CW, DW).

The duty-cycle adjusting unit53is connected electrically to the cycle adjusting unit52, and receives the PWM signals from the cycle adjusting unit52and a periodic off-pulse signal. It is noted that the off-pulse signal has a frequency higher than that of the PWM signals, and a pulse width. The duty-cycle adjusting unit53is operable to adjust the duty cycle of each PWM signal at the conduction level, i.e., the low level, based on the off-pulse signal, and then outputs the PWM signals adjusted thereby to the cathodes of the red, green and blue LEDs921,922,923of the secondary light emitting unit92, as shown inFIG. 6. For example, the PWM signal has the frequency of 128 Hz, as shown inFIG. 8a. The off-pulse signal has the frequency of 16 KHz, as shown inFIG. 8b. In this case, the PWM signal ofFIG. 8ais adjusted by the duty-cycle adjusting unit53based on the off-pulse signal ofFIG. 8bto obtain the adjusted PWM signal that has a reduced low-level period in each PWM cycle, as shown inFIG. 8c, thereby reducing the duty cycle of the PWM signal. In addition, the larger the pulse width of the off pulse signal, the smaller will be the duty cycle of each PWM signal.

The off-pulse signal generator7is connected electrically to the duty-cycle adjusting unit53, and is operable to generate the periodic off-pulse signal based on an external dimming control input, and outputs the off-pulse signal to the duty-cycle adjusting unit53. The pulse width of the off-pulse signal is determined by the dimming control input. The dimming control input consists of eight input signals (S1, S2, . . . , S8). In this embodiment, referring further toFIG. 7, the off-pulse signal generator7includes an 8-bit programmable counter. In is noted that the pulse width of the off-pulse signal is determined based on the dimming control input.

The input unit6is connected electrically to the frequency generator2. The input unit6is operable to generate the pause signal, and outputs the pause signal to the frequency generator2. Thus, the frequency generator2holds the second set of frequency-divided signals upon receipt of the pause signal such that the state generator3ceases to count to thereby unchange the counting value. As a result, the counting value of the state counting output generated by the state generator3is remained to a target one of the values such that the secondary light emitting unit92is remained in a desired one of the light-mixing states that corresponds to the counting value of the state counting output with the target one of the values through driving of the PWM signals generated by the dimming device.

In use, during activation of the main light emitting unit91through driving of the current generated by the constant current generator1, the secondary light emitting unit92is periodically switched among the light-mixing states through driving of the PWM signals generated by the PWM signal generator such that light generated by the lighting device has various color temperatures and various color rendition indexes for each a period of 72 (=4×18) seconds. Then, once the input unit6is manually operated to generate the pause signal, the secondary light emitting unit92will be in a desired light-mixing state through driving the PWM signals such that light generated by the lighting device has desired color temperature and color rendition index. On the other hand, during deactivation of the main light emitting unit91, the secondary light emitting unit92is driven by the PWM signals generated by the dimming device of the present invention to emit light with various colors corresponding respectively to the light-mixing states.