Abstract:
The present invention discloses a direct current (DC) light emitting device control circuit with dimming function, and a method thereof, wherein the dimming function is provided in a feedback loop for feeding back a feedback signal from an output terminal to a power switch control circuit; the feedback signal relates to an output current supplied to the DC light emitting device. The present invention adjusts the feedback signal according to the desired brightness of the DC light emitting device. The present invention controls a power switch according to the adjusted feedback signal, such that the output current supplied to the DC light emitting device is adjusted, and accordingly the brightness of the DC light emitting device is adjusted below the full brightness.

Description:
CROSS REFERENCE 
       [0001]    The present invention claims priority to TW 099130931, filed on Sep. 13, 2010. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a direct current light emitting device control circuit with a dimming function and a method thereof, in particular to such direct current light emitting device control circuit with a dimming function which has a better power factor because it does not need to use a tri-electro AC switch (hereinafter referred to as the “TRIAC device”), and a method thereof. 
         [0004]    2. Description of Related Art 
         [0005]    LEDs (light emitting diodes) are one typical type of direct current light emitting devices. Referring to  FIG. 1 , in prior art, to drive the LEDs from an AC power source, the AC power is first rectified by a bridge rectifier  4 , and the rectified AC power is converted to a DC voltage by an AC-DC power regulator  10 ; next, an LED driver circuit  20  operates according to the DC voltage to control the currents of the LEDs. The AC-DC power regulator  10  includes a primary side circuit  11 , a secondary side circuit  12 , a transformer  13 , and other discrete devices such as capacitors and diodes (not shown). The secondary side circuit  12  detects the output voltage and the detection result is fed back to the switch control circuit PWM in the primary side circuit  11  by opto-coupling, to control the operation of the power switch P of the primary side circuit  11 . 
         [0006]    The drawback of the foregoing prior art is that it requires the AC-DC power regulator  10  to generate a regulated voltage, and the LED driver circuit  20  to control the currents of the LEDs according to the voltage, so at least three integrated circuit (IC) chips are needed: the primary side circuit  11 , the secondary side circuit  12 , and the LED driver circuit  20 ; the cost is high for such an arrangement. 
         [0007]    In addition, as an AC power source is used, if it is required to adjust the brightness (i.e., dimming) of the light emitting devices, the prior art usually utilizes a tri-electro AC switch (TRIAC) device  2  as shown in  FIG. 2  to extract the turn-ON angle of the inputted AC power, so as to adjust the output power for achieving the dimming function. However, one major disadvantage of using the TRIAC device is that it degrades the power factor because it changes the turn-ON angle. 
         [0008]    In view of above, the present invention overcomes the foregoing drawbacks by putting forth a direct current light emitting device control circuit with a dimming function and a corresponding method, which utilize a novel dimming control mechanism rather than a TRIAC device, so that the power factor is better and the power consumption is reduced. 
       SUMMARY OF THE INVENTION 
       [0009]    An objective of the present invention is to provide a direct current light emitting device control circuit with a dimming function. 
         [0010]    An objective of the present invention is to provide a control method for dimming a direct current light emitting device. 
         [0011]    To achieve the foregoing objective, the present invention provides a direct current light emitting device control circuit with a dimming function, comprising: at least one power switch; a switch control circuit controlling the power switch to convert an input voltage to an output current which is supplied to at least one direct current light emitting device; a feedback circuit generating a feedback signal according to a signal related to the output current; and a dimming circuit coupled to the feedback circuit, the dimming circuit adjusting the feedback signal to generate an adjusted feedback signal, wherein the switch control circuit receives the adjusted feedback signal and controls the power switch according to the adjusted feedback signal whereby the output current supplied to the direct current light emitting device is adjusted to achieve the dimming function. 
         [0012]    In the foregoing direct current light emitting device control circuit, the dimming circuit includes a variable resister connected to the feedback circuit in series or in parallel; or includes an adjustable gain amplifier having an input coupled to the feedback signal and an output coupled to the switch control circuit; further includes a parallel circuit coupled to an output terminal of the feedback circuit, the parallel circuit includes a switch, and the feedback signal is adjusted by turning on or off the switch. 
         [0013]    In another aspect, the present invention provides a control method for dimming a direct current light emitting device, comprising: controlling at least one power switch to convert an input voltage to an output current which is supplied to at least one direct current light emitting device; generating a feedback signal according to a signal related to the output current; adjusting the feedback signal to generate an adjusted feedback signal according to a brightness of the direct current light emitting device which is below full brightness; and controlling the power switch according to the adjusted feedback signal such that the output current supplied to the direct current light emitting device is adjusted, whereby the brightness of the direct current light emitting device is controllable below full brightness to achieve the dimming function. 
         [0014]    In the foregoing dimming control method, the power switch also converts the input voltage to an output voltage; in a kind of embodiments, the level of the output voltage is changed by adjusting the feedback signal, and the output current supplied to the direct current light emitting device is adjusted to achieve the dimming function. If the signal related to the output current is compared with a reference signal to generate the feedback signal, the feedback signal is adjusted according the change in the level of the output voltage. 
         [0015]    For example, in one embodiment, the level of the output voltage periodically varies between a high level and a low level and a duty ratio of each period determines the output current supplied to the direct current light emitting device. 
         [0016]    For example, in one embodiment, the level of the output voltage is at a normal level in a steady status, but the output voltage is raised to a level higher than the normal level when the feedback signal is adjusted, and a difference between the higher level and the normal level determines the output current supplied to the direct current light emitting device. 
         [0017]    For example, in one embodiment, the level of the output voltage is at a medium level in a steady status, but the output voltage is raised to a high level to generate a positive pulse or the output voltage is lowered to a low level to generate a negative pulse when the feedback signal is adjusted, and the step of adjusting the output current supplied to the direct current light emitting device according to the change in the level of the output voltage includes: upward adjusting the output current supplied to the direct current light emitting device one level according to the positive pulse or downward adjusting the output current supplied to the direct current light emitting device one level according to the negative pulse. 
         [0018]    The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a schematic diagram of a prior art arrangement utilizing an AC-DC power regulator  10  to convert an AC voltage to a DC voltage and an LED driver circuit  20  to control power supplied to LEDs. 
           [0020]      FIG. 2  shows a schematic diagram of a prior art arrangement utilizing a TRIAC device  2  for dimming control. 
           [0021]      FIG. 3  shows an embodiment of the present invention. 
           [0022]      FIGS. 4 and 5  show hardware embodiments of secondary side circuits  32  and also embodiments of dimming circuits  34 . 
           [0023]      FIGS. 6A and 6B  show two other embodiments of dimming circuits  34 . 
           [0024]      FIGS. 7A-7H  show several examples of DC-DC power regulators. 
           [0025]      FIGS. 8A-8B  show an embodiment of the present invention, illustrating a method of dimming brightness according to a feedback signal. 
           [0026]      FIGS. 9A-9B  show another embodiment of the present invention, illustrating another method of dimming brightness according to a feedback signal. 
           [0027]      FIGS. 9C-9D  show two hardware embodiments of the present invention, illustrating circuits fulfilling the dimming method in  FIGS. 9A-9B . 
           [0028]      FIGS. 10A-10B  show another embodiment of the present invention, illustrating another method of dimming brightness according to a feedback signal. 
           [0029]      FIG. 10C  shows a hardware embodiment of the present invention, illustrating a circuit fulfilling the dimming method in  FIGS. 10A-10B . 
           [0030]      FIGS. 11A-11B  show another embodiment of the present invention, illustrating another method of dimming brightness according to a feedback signal. 
           [0031]      FIG. 11C  shows a hardware embodiment of the present invention, illustrating a circuit fulfilling the dimming method in  FIGS. 11A-11B . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    The present invention is applicable to any DC controlled light emitting devices, not limited to LEDs. However, because the LEDs are the most common light emitting devices, the following embodiments are illustrated by using LEDs as examples. 
         [0033]      FIG. 3  shows an embodiment of the present invention. In this embodiment it does not need to provide two IC chips of the secondary side circuit  12  and the LED driver circuit  20 . As show in this figure, the light emitting device control circuit  30  of this embodiment receives an AC power rectified by a bridge rectifier  4  and generates an output current to be supplied to a load circuit  50 . The load circuit  50  may be an LED circuit, but it also may be any circuit requiring a controllable current. The light emitting device control circuit  30  comprises: a primary side circuit  11  receiving a rectified AC power; a transformer  13 , which is coupled to the primary side circuit  11 , for converting a primary side voltage to a secondary side voltage to be supplied to the load circuit  50 ; a secondary side circuit  32 , which is coupled to the transformer  13 , for directly controlling the current of the load circuit  50 , and the secondary side circuit  32  generates a feedback signal which is fed back to the primary side circuit  11  by opto-coupling; and a dimming circuit  34 , which is coupled to a path transmitting the opto-coupled feedback signal to the primary side circuit  11 , for adjusting the feedback signal received by the primary side circuit  11  to achieve the dimming function. 
         [0034]    More specifically, the switch control circuit PWM of the primary side circuit  11  controls the operation of the power switch P according to the feedback signal. Therefore, by changing the feedback signal, the output current of the secondary side circuit  32  can be adjusted. For example, assuming that the feedback signal and the output current Iout are positively correlated, if the dimming circuit  34  adjusts the feedback signal to 200% of its original value, then as the original feedback signal (the input of the dimming circuit  34 ) reaches 50% of the regulation target value, the feedback signal (the output value of the dimming circuit  34 ) received by the primary side circuit  11  reaches 100% of the regulation target value, so the switch control circuit PWM will reduce the duty ratio (or by other equivalent means, depending on the control mechanism of the switch control circuit PWM) of the power switch P to adjust the brightness of the LEDs downward. The foregoing embodiment only shows one of many possible dimming methods; other embodiments include providing mechanisms inside the switch control circuit PWM to control brightness in response to the variation of the feedback signal (the details will be described later). 
         [0035]    The detail configuration of the secondary circuit  32  will be described by way of example hereinafter. However, please note that the circuit has various equivalents and the scope of the present invention is not limited to the details of the figure. 
         [0036]    Referring to  FIG. 4 , in this embodiment, the secondary side circuit  32  comprises an IC  320 , an opto-coupler diode  322 , and a diode D. If the load circuit  50  includes multiple strings of LEDs, the IC  320  would include multiple current sources CS 1 -CSN to correspondingly control the current on each of the LED strings. The operation voltages of the current sources CS 1 -CSN must be higher than a minimum normal operation voltage. The IC  320  further includes a minimum selection circuit  321  which selects the lowest voltage among the corresponding nodes of the LED strings as the input of the error amplifier EA to be compared with a reference voltage Vref. The voltages of the foregoing corresponding nodes reflect the operation statuses of the corresponding current sources. The output of the error amplifier EA controls a transistor Q 1 , to thereby control a corresponding current to power the opto-coupler diode  322  to light. There are many ways to control the current through the opto-coupler diode  322  by the transistor Q 1 . For example, the transistor can directly form a controllable current source circuit, form a controllable current source circuit with other devices, or output a controllable voltage which is supplied to a resistor connected with it in series to control the current passing through the resistor. In the embodiment using a resistor connected with the transistor in series, the resistor can be embedded into the IC  320 , or provided externally to the IC  320  so that the parameters such as the current amount can be set and adjusted from outside of the IC  320 . What is shown in this is an embodiment using an external resister. The light emitted by the opto-coupler diode  322  is fed back to the phototransistor Q 2  of the primary side circuit by opto-coupling, such that the switch control circuit PWM controls the operation (in general, the opto-coupler diode  322  and the phototransistor Q 2  are integrated into one component referred to as an opto-coupler) of the power switch P according to the feedback signal. All of the operation voltages of the current sources CS 1 -CSN are accordingly higher than the minimum operation voltage through the feedback control mechanism. That is, the current sources CS 1 -CSN can operate normally to control the current of each LED string. In other words, the LED driver circuit  30  of the present invention directly controls the current of the load circuit  50 . If the load circuit  50  includes only one path (one single LED string) rather than a parallel circuit of multiple LED strings, then the IC  320  only needs a single current source and the minimum selection circuit  321  is not necessary. 
         [0037]    In this embodiment, the dimming circuit  34  may be a variable resistor which achieves the dimming function by adjusting the signal at the node A to change the feedback signal received by the primary side circuit  11 . Please note that the variable resistor is not limited to the location shown in the figure; in other embodiments, it can be coupled between the phototransistor Q 2  and the switch control circuit PWM in series, or connected in series below the phototransistor Q 2 , instead of connected with the phototransistor Q 2  in parallel. 
         [0038]    Another embodiment of the dimming circuit  34  is shown in  FIG. 5 . In this embodiment, the dimming circuit  34  receives a digital signal to control the transistors M 1  and M 2  such that the total resistance of the dimming circuit  34  is adjusted to change the value of the signal at the node A. Please note that the dimming circuit  34  of this embodiment can include only one path—the transistor switch M 1  and the resistor connected to the upper end of the transistor switch M 1 , for example. In this case, the dimming circuit  34  for example can adopt the dimming method exemplified in  FIGS. 8A-8B . 
         [0039]      FIG. 6A  shows another embodiment of the dimming circuit  34 . The dimming circuit  34  of this embodiment may be an adjustable gain amplifier so the value of the signal at the node A can be changed by adjusting the gain of the amplifier. 
         [0040]      FIG. 6B  shows another embodiment of the dimming circuit  34 . In this embodiment, the current of the phototransistor Q 2  can be controlled by the dimming circuit  34  to adjust the value of the signal at the node A. More specifically, the phototransistor Q 2  shown to be one single transistor in the foregoing embodiments is replaced with a composite phototransistor Q 2  including phototransistors Q 2   a -Q 2   c , and the dimming circuit  34  includes transistor switches M 1 -M 3 . By controlling the transistor switches M 1 -M 3 , the value of the signal at the node A can be changed. 
         [0041]    There are various other methods to adjust the feedback signal; when the feedback signal is a voltage signal, basically, any methods capable of adjusting voltages can be utilized to adjust the feedback signal for dimming. 
         [0042]    Although in the background section an AC-DC power regulator is introduced, and the description thus far describes the present invention by using an AC-DC power regulator as an example, the spirit of the present invention is not limited to the AC-DC power regulator; the same spirit, that is, to achieve the dimming function by adjusting the feedback signal, is also applicable to a DC-DC power regulator. Such DC-DC power regulator for example may be, but is not limited to, a synchronous or asynchronous power conversion circuit such as a buck converter, a boost converter, an inverting converter, or a buck-boost converter, as shown in  FIGS. 7A-7H . 
         [0043]    In addition to directly adjusting the value of the signal at the node A for dimming, in another embodiment, the dimming circuit  34  also can perform the so-called PWM dimming. Referring to  FIGS. 8A-8B , the dimming circuit  34  can control the feedback signal to switch between two levels, and the output voltage Vout is accordingly changed between a high level and a low level. The brightness of the LEDs can be adjusted through adjusting the duty ratio d % of each period, as shown in  FIG. 88 . The brightness of the LEDs is determined by the high level and the low level of the output voltage Vout, in which the lowest brightness of the LEDs does not have to be zero. 
         [0044]      FIGS. 9A-9B  shows another dimming method. In this embodiment, the output voltage Vout normally stays at a normal level instead of switching between the high level and low level as illustrated in the previous embodiment. However, when the dimming circuit  34  is dimming light, it controls the feedback signal such that the output voltage Vout is higher than the normal level. The difference Vo from the normal level determines the brightness of the LED as shown in  FIG. 9B . Similar to the previous embodiment, the lowest brightness of the LED is not necessarily zero. An example to perform the dimming control of  FIG. 9B  by hardware is shown in  FIG. 9C ; please refer to  FIG. 9C  in conjunction with  FIG. 4  ( FIG. 9C  omits some irrelevant circuits to simplify the drawing), a Vo level sample and hold circuit  323  can be provided in the IC  320  to sample the difference Vo. The difference Vo can be converted by an analog mapping table  324  to the reference voltage Vref, which serves as the reference input of the error amplifier EA. That is, the difference Vo determines the reference basis of the error amplifier EA. Alternatively, referring to  FIG. 9D  in conjunction  FIG. 4 , a Vo level sampling circuit  325  can be provided in the IC  320  to sample the difference Vo. Through an ADC (analog-to-digital converter) circuit  326 , the difference Vo can be converted to a digital signal which is stored in a latch circuit  327 . The value stored in the latch circuit  327  is converted by a DAC (digital-to-analog converter)  328  or a mapping table to the reference voltage Vref, which serves as the reference input of the error amplifier EA to determine the reference basis of the error amplifier EA. 
         [0045]      FIGS. 10A-10B  shows another dimming method. This embodiment uses a “pulse setting” method to diming light. More specifically, the output voltage Vout is controlled by the feedback signal to have three levels: a high level, a medium level, and a low level. When the output voltage Vout changes from the medium level to the high level to form a positive pulse, the brightness of the LEDs is adjusted upward to a next higher level; when the output voltage Vout changes from a medium level to a low level to form a negative pulse, the brightness of the LEDs is adjusted downward to a next lower level; in other cases the brightness of the LEDs keeps the same. An example to perform the dimming control of FIGS.  10 A- 10 B by hardware is shown in  FIG. 10C ; please refer to  FIG. 10C  in conjunction with  FIG. 4 , a first pulse generator  331  and a second pulse generator  332  are provided in the IC  320 ; the first pulse generator  331  generates a one-shot pulse each time a positive pulse is received, and the second pulse generator  332  generates a one-shot pulse each time a negative pulse is received. The one-shot pulse is inputted to the up/down counter  333  to conduct an up/down count. The count value of the up/down counter  333  is converted to the reference voltage Vref through a DAC  334  or a mapping table. The reference voltage Vref is inputted to an error amplifier EA to determine the reference basis of the error amplifier EA. 
         [0046]      FIGS. 11A-11B  shows another dimming method. In this embodiment, the output voltage Vout stays at a normal level during normal operation. When the dimming circuit  34  starts to dim light, the feedback signal makes the output voltage Vout to be higher than the normal level, and the period in which the output voltage Vout is higher than the normal level determines the brightness of the LED. An example to perform the dimming control of  FIG. 9B  by hardware is shown in  FIG. 11C ; please refer to  FIG. 9C  in conjunction with  FIG. 4 . A Vout high level detection circuit  335  is provided in the IC  320 . When it detects a high level, a counter  337  is reset to zero and a square wave generator  336  is enabled to generate pulses according a clock. The counter  337  counts the number of the pulses. The count value of the counter  337  is converted the reference voltage Vref through a DAC  334  or a mapping table. The reference voltage Vref is inputted to an error amplifier EA to determine the reference basis of the error amplifier EA. 
         [0047]    The foregoing embodiments illustrate two types of methods to dim light by the feedback signal. The first type adjusts the LED target current value according to the ratio between the original feedback signal (the original output of the feedback circuit) and the adjusted feedback signal (the feedback signal received by the switch control circuit PWM); the second type adjusts the output voltage Vout according to feedback signal, and the changes in the output voltage Vout determine the reference voltage Vref of the error amplifier EA. In addition to the above, other dimming methods can be conceived in light of the teachings by the present invention. For example, a switch can be provided on the path of each LED string, and a PWM signal is generated according to the feedback signal to control the duty of the switch, to perform PWM dimming. 
         [0048]    The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, the LEDs in each of the embodiments can be white LEDs, colored LEDs, or organic LEDs. The concept of the present invention is not limited to LEDs, and it can be extended to any DC controlled light emitting devices. For another example, the secondary side circuit  32  can be embodied in various ways other than the ones shown in  FIGS. 4 and 5 . For a further example, the dimming circuit  34  can be disposed in the feedback loop and connected to the feedback circuit in series or in parallel, or disposed out of the feedback loop and provided as an external circuit which controls the output signal of the feedback circuit, as long as the feedback signal can be controlled. Thus, the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.