Control method capable of preventing flicker effect and light emitting device thereof

A control method capable of preventing flicker effect for a light source module includes detecting variation situations of a driving current passing through the light source module to generate a current detection signal, adjusting a variable reference voltage according to the current detection signal, obtaining a feedback voltage from the light source module, generating a voltage control signal according to the feedback voltage and the variable reference voltage, and generating an output voltage according to the voltage control signal to drive the light source module.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to control method and device thereof, and more particularly, to a control method capable of preventing flicker effect and a related light emitting device.

2. Description of the Prior Art

Light emitting diodes (LEDs) offer advantages of energy savings, long device lifetime, no mercury used, high achievable color gamut, without idle time, and fast response speed, so that LED technology is widely applied in fields of display and illumination. In addition, compared with a conventional light source device, light emitting diodes are suitable for fabrication as a tiny device or an array device, such as in traffic lights, outdoor displays, backlight modules of liquid crystal displays, PDAs, notebooks, or mobile phones with features of small size, shock resistance, ease of mass production, and high applicability.

Please refer toFIG. 1, which is a schematic diagram of an LED driving device10according to the prior art. The LED driving device10is utilized for driving a light source module102which includes a plurality of LED groups C1to Cmarranged in parallel. The LED driving device10includes a voltage converter104, a current source106, a pulse modulation unit108, and a control unit110. The voltage converter104is utilized for providing an output voltage VDto the light source module102. The current source106is utilized for providing driving currents ID1to IDMfor LED groups C1to Cmto drive the light source module102. The pulse modulation unit108is utilized for dimming according to a dimming signal SD. In general, a plurality of headroom voltages VHR1to VHRmexist on each path of the LED groups C1to Cm. The headroom voltages VHR1to VHRmrepresent the voltage value across the current source106on each path of the LED groups C1to Cm, i.e. available voltage value for the current source106on each LED group path. In practice, the currents passing through the LEDs can usually be kept constant, i.e. the driving currents ID1to IDMare fixed, for steady brightness control and power consumption of the LEDs. However, the voltages across the LEDs may not be all the same due to non-ideal factors in the manufacturing process or other reasons, and the headroom voltages VHR1to VHRmare not the same correspondingly. In such a condition, the headroom voltage may be too high or too low, and will result in some unwanted effects. For example, if the headroom voltage is too high, the power consumption of the current source will increase, and the power conversion efficiency will be reduced. If the headroom voltage is not high enough, the current source will operate in an improper state, and cannot keep constant current sink, even to the point of not being able to provide the required driving current to the LED, and the LED will not conduct.

Therefore, as shown inFIG. 1, in the conventional technology, the voltage converter104may be controlled to change the output voltage VDby the control unit110in negative feedback form in order to obtain appropriate headroom voltages. The control unit110includes a voltage selector112, an error amplifier114, and a conversion controller116. The voltage selector112is coupled to the output terminal of each LED group C1to Cmfor selecting one of the headroom voltages VHR1to VHRmas the feedback voltage VFB. Again, the feedback voltage VFBand a predetermined reference voltage VREFare inputted to the positive end and negative end of respectively. The error amplifier114generates an error voltage signal SEaccording to the difference between the feedback voltage VFBand the predetermined reference voltage VREF. Furthermore, the conversion controller116generates a voltage control signal SCaccording to the error voltage signal SEfor control the conversion process of the voltage converter104. Thus, as the headroom voltages VHR1to VHRmcorresponding to each LED group C1to Cmare too low, the error amplifier114generates the error voltage signal SEsent to the conversion controller116, and the conversion controller116generates the voltage control signal SCaccordingly to control the voltage converter104to increase the output voltage VD. As a result, as the driving currents ID1to IDMare fixed, the headroom voltages VHR1to VHRmwill not vary accordingly. On the other hand, the headroom voltages VHR1to VHRmare proportional to the output voltage VD. Therefore, the control unit110is able to control the output voltage VDto be increased so that the headroom voltages VHR1to VHRmincrease correspondingly, and vice versa. Therefore, under the steady driving currents ID1to IDMprovided, the LED driving circuit10can lock the headroom voltages VHR1to VHRmwithin an appropriate range, such as the predetermined reference voltage VREF, by the control unit110.

However, current variation situations may occur often in the currents passing through the LEDs in many cases. For example, during the dimming process, the brightness of the LEDs can be changed by adjusting the currents passing through the LEDs (i.e. by adjusting the driving currents ID1to IDM), so that the voltages across the LEDs vary correspondingly. But, the LED driving circuit10adjusts the output voltage VDby only comparing the output voltage VDwith a fixed predetermined reference voltage, which results in consuming too much feedback tracking time for adjusting the output voltage VD. In other words, the output voltage VDcan not be arranged to an appropriate voltage level immediately, and the headroom voltages of the current source106become too low to provide sufficient driving currents, so that flicker effects occur.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide a control method capable of preventing flicker effect and light emitting device.

The present invention discloses a control method capable of preventing flicker effect for a light source module. The control method includes detecting variation situations of a driving current passing through the light source module to generate a current detection signal; adjusting a variable reference voltage according to the current detection signal; obtaining a feedback voltage from the light source module; generating a voltage control signal according to the feedback voltage and the variable reference voltage; and generating an output voltage according to the voltage control signal to drive the light source module.

The present invention further discloses an LED device which includes a voltage converter, a light source module, a variable current source, and a control unit. The voltage converter is utilized for converting an input voltage into an output voltage according to a voltage control signal. The light source module is coupled to the voltage converter. The variable current source is coupled to the light source module for providing a driving current to drive the light source module. The control unit is coupled to the light source module and the voltage converter for obtaining a feedback voltage from the light source module and detecting variation situations of the driving current passing through the light source module to generate a current detection signal. The control unit adjusts a variable reference voltage according to the current detection signal and generates the voltage control signal according to the feedback voltage and the variable reference voltage to the voltage converter.

The present invention further discloses a control method capable of preventing flicker effect for a light source module. The control method includes detecting variation situations of a driving current provided by a variable current source to generate a current detection signal; adjusting a variable reference voltage according to the current detection signal; obtaining a feedback voltage from the light source module; generating a voltage control signal according to the feedback voltage and the variable reference voltage; and generating an output voltage according to the voltage control signal to drive the light source module.

The present invention further discloses an LED device which includes a voltage converter, a light source module, a variable current source, and a control unit. The voltage converter is utilized for converting an input voltage into an output voltage according to a voltage control signal. The light source module is coupled to the voltage converter. The variable current source is coupled to the light source module for generating a driving current to drive the light source module. The control unit is coupled to the variable current source and the voltage converter for obtaining a feedback voltage from the light source module and detecting variation situations of the driving current provided by the variable current source to generate a current detection signal. The control unit adjusts a variable reference voltage according to the current detection signal and generates the voltage control signal according to the feedback voltage and the variable reference voltage to the voltage converter.

DETAILED DESCRIPTION

Please refer toFIG. 2, which is a schematic diagram of an LED device20according to an embodiment of the present invention. The LED device20can be applied to any kind of light source, which includes a voltage converter202, a light source module204, a variable current source206, and a control unit208. The voltage converter202is utilized for converting an input voltage VINinto an output voltage VDaccording to a voltage control signal SCfor the light source module204. The light source module204is coupled to the voltage converter202. Note that, in the embodiment of the present invention, the light source module204includes a plurality of LED groups C1to Cm, and this should not be a limitation of the present invention. In other words, the light emitting component102can also have one LED group only. On the other hand, since the LED is a current driven component, the brightness of the LED is proportional to the driving current. Therefore, each LED group includes at least one LED in series, such as having n LEDs in each LED group, and the number of the LEDs included in each LED group must be the same in order to allow the current through each LED to be identical and result in the same brightness. As shown inFIG. 2, the variable current source206is coupled to the light source module204for providing load currents IL1to ILmfor LED groups C1to Cmto drive the light source module204. The control unit208is coupled to the light source module204and the voltage converter202for obtaining a feedback voltage VFBfrom the light source module204and detecting variation situations of the load currents IL1to ILMpassing through the LED groups C1to Cmto generate a current detection signal SL. Furthermore, the control unit208adjusts a variable reference voltage VREF—Vaccording to the current detection signal SLand generates the voltage control signal SCsent to the voltage converter202according to the feedback voltage VFBand the variable reference voltage VREF—V. As can been seen, the control unit208can detect variation situations of the load currents IL1to ILMpassing through the light source module204in real-time and dynamically adjust the variable reference voltage VREF—Vaccordingly to control the voltage converter202to convert to the appropriate output voltage VDfor the light source module204.

The following further elaborates the control unit208shown inFIG. 2. Please further refer toFIG. 2. The control unit208includes a voltage selector210, a current detector212, a processing unit214, a reference voltage converter216, an error amplifier218, and a conversion controller220. The voltage selector210is coupled to the light source module204for selecting the feedback voltage VFBfrom a plurality of headroom voltages VHR1to VHRMcorresponding to the LED groups C1to Cm. The current detector212is coupled to the light source module204for detecting variation situations of the load currents IL1to ILmto generate the current detection signal SL. The processing unit214is coupled to the current detector212for generating a reference voltage converting signal SVaccording to the current detection signal SL. The reference voltage converter216is coupled to the processing unit214for generating the variable reference voltage VREF—Vaccording to the reference voltage converting signal SV. Therefore, as the current detection signal SLindicates the current variations of the load currents IL1to ILmoccur, the processing unit214is capable of informing the reference voltage converter216of variation situations via the reference voltage converting signal SVso that the reference voltage converter216generates the required variable reference voltage VREF—Vaccordingly. For example, when the current detection signal SLindicates the current variations of the load currents IL1to ILmbecome greater, the processing unit214is able to notify the reference voltage converter216. After that, the reference voltage converter216can increase the variable reference voltage VREF—Vaccordingly. When the current detection signal SLindicates the current variations of the load currents IL1to ILMbecome smaller, the processing unit214is able to notify the reference voltage converter216, so that the reference voltage converter216can decrease the variable reference voltage VREF—Vaccordingly.

Moreover, a positive end and a negative end of the error amplifier218are coupled to the reference voltage generator216and the voltage selector210respectively. The error amplifier218generates an error voltage signal SEaccording to the feedback voltage VFBand the variable reference voltage VREF—Vand outputs the error voltage signal SEthrough an output end of the error amplifier218. The conversion controller220is coupled to the output end of the error amplifier218and the voltage converter202for generating the voltage control signal SCaccording to the error voltage signal SEfor the voltage converter202. In such a condition, regardless of whether the feedback voltage VFBis greater or less than the variable reference voltage VREF—V, the error amplifier218generates the error voltage signal SEaccording to the difference between the feedback voltage VFBand the variable reference voltage VREF—Vin order to inform the conversion controller220. The conversion controller220then generates the corresponding voltage control signal SCfor increasing or decreasing the output voltage VDaccordingly. As can been seen, the control unit208can detect in real-time variation situations of the load currents IL1to ILMof the light source module204, and further adjust the variable reference voltage VREF—Vdynamically for instantaneously tracking the proper output voltage VDthrough feedback.

In the prior art, when current variation occurs, the headroom voltage may be constricted to be too small, so that the variable current source206can not provide enough load current and a flicker effect occurs, or the headroom voltage be constricted to be too high so that the variable current source206consumes too much power through the variable current source206. Therefore, the present invention can detect in real-time variation situations of the load currents IL1to ILMpassing through the LED groups C1to Cmand dynamically adjust the variable reference voltage VREF—Vaccordingly to control the voltage converter202to convert to the appropriate output voltage VD. As a result, the present invention can prevent the headroom voltage from being constricted to be too small to avoid the flicker effect, and the present invention can also prevent the headroom voltage from being constricted to be too high to enhance voltage conversion efficiency.

As to the operating method of the LED device20, please refer toFIG. 3.FIG. 3is a schematic diagram of a procedure30according to an embodiment of the present invention. The procedure30comprises the following steps:

Step302: Detect variation situations of load currents IL1to ILMpassing through light source module204to generate current detection signal SL.

Step304: Adjust variable reference voltage VREF—Vaccording to current detection signal SL.

Step306: Obtain feedback voltage VFBfrom light source module204.

Step308: Generate voltage control signal SCaccording to feedback voltage VFBand variable reference voltage VREF—V.

Step310: Generate output voltage VDaccording to voltage control signal SCto drive light source module204.

The procedure40is utilized for illustrating the implementation of the LED device20. Related variations and the detailed description can be referred from the foregoing description, so as not to be narrated herein.

In addition, the control unit can also detect current variation situations of the variable current source and dynamically adjust the variable reference voltage VREF—Vaccordingly to control the voltage converter to convert to the appropriate output voltage VD. Please refer toFIG. 4, which is a schematic diagram of an LED device40according to an embodiment of the present invention. Please note that elements of the LED device40shown inFIG. 4with the same reference numerals as those in the LED device20shown inFIG. 2have similar operations and functions and further description thereof is omitted for brevity. The interconnections of the units are as shown inFIG. 4. The LED device40includes a voltage converter402, a light source module404, a variable current source406, and a control unit408. The control unit408includes a voltage selector410, a current detector412, a processing unit414, a reference voltage converter416, an error amplifier418, and a conversion controller420. Different from the LED device20shown inFIG. 2is that current detector412shown inFIG. 4is coupled to the variable current source406. The current detector412is utilized for detecting variation situations of the load current generated by the variable current source406to generate the current detection signal SLand further to adjust the variable reference voltage VREF—V. Furthermore, please refer toFIG. 5. The variable current source406further includes a variable current mirror502and a current driving element504. The variable current mirror502is coupled to the current detector412for generating the load currents IL1˜ILm. The current driving element504is coupled to the variable current mirror502and the LED groups C1to Cmof the light source module404for controlling the load currents IL1˜ILmto be provided to the LED groups C1to Cm. In other words, the control unit408can directly monitor the current variation on the variable current source406in order to convert to the appropriate output voltage VDat once. On the other hand, in the embodiment of the present invention, the control unit408can be directly coupled to the variable current mirror502for detecting the current variation of the load currents IL1˜ILm, and this should not be limited. The control unit408can also be directly coupled to other components of the variable current source406(such as the current driving element504) and detect the current variation at other components of the variable current source406. Thus, the control unit408can detect the current variation at any component of the variable current source406to obtain the variation situations of the load currents IL1˜ILm. Moreover, regarding implementation, the control unit408and the variable current source406can be implemented on the same chip, so that the above mentioned operation method will be realized in the chip without external circuits. In such a condition, the purpose of preventing the flicker effect may be achieved more immediately.

As to the operating method of the LED device40, please refer toFIG. 6.FIG. 6is a schematic diagram of a procedure60according to an embodiment of the present invention. The procedure60comprises the following steps:

Step602: Detect variation situations of load currents IL1to ILMprovided by variable current source406to generate current detection signal SL.

Step604: Adjust variable reference voltage VREF—Vaccording to current detection signal SL.

Step606: Obtain feedback voltage VFBfrom light source module204.

Step608: Generate voltage control signal SCaccording to feedback voltage VFBand variable reference voltage VREF—V.

Step610: Generate output voltage VDaccording to voltage control signal SCto drive light source module204.

The procedure60is utilized for illustrating the implementation of the LED device40. Related variations and the detailed description can be referred from the foregoing description, so as not to be narrated herein.

Note that the above mentioned embodiments are exemplary embodiments of the present invention, and those skilled in the art can make alternations and modifications accordingly. For example, the reference voltage converters216and416can provide various voltage values by any method in accordance with requirements for providing the proper variable reference voltage VREF—V. As shown inFIG. 7andFIG. 8, the reference voltage converters216and416can be multiplexers702and802, which switch to the corresponding variable reference voltage VREF—Vfrom predetermined reference voltages VREF—1to VREF—Zaccording to the reference voltage converting signal SV. In addition, the voltage selectors210and410can select the feedback voltage VFBamong the headroom voltages VHR1to VHRMaccording to any rule, such as the voltage selectors210and410can select the lowest headroom voltage from the headroom voltages VHR1to VHRmas the feedback voltage VFB. On the other hand, the processing units214and414can calculate a suitable reference voltage value with arithmetic and logical operations according to the current variations of the load currents IL1to ILM. For example the processing units214and414are able to estimate a corresponding reference voltage value according to amount of LED groups having current variation, amount of current variation, or amount of average variation of overall load currents. Moreover, the processing units214and414may generate the current detection signal SLaccording to the current variation every specific time interval or whenever at least one load has a current variation situation. The variable current source can vary the current provided to the light source module according to a dimming signal to adjust the brightness of the LED on the light source module.

In summary, the present invention can detect in real-time variation situations of the load currents IL1to ILMof the light source modules204,404and further adjust the variable reference voltage VREF—Vdynamically for converting the appropriate output voltage VDfor the light source modules204,404instantaneously. As a result, when current variation occurs, the present invention can prevent the headroom voltage from being constricted to be too small to avoid the flicker effect, and also prevent the headroom voltage from being constricted to be too high to enhance voltage conversion efficiency.