Patent Publication Number: US-9420659-B2

Title: LED power supply device

Description:
This application is based on Japanese Patent Application No. 2013-204639 filed on Sep. 30, 2013, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an LED (light emitting diode) power supply device equipped with a dimming function. 
     2. Description of Related Art 
     In recent years, LED power supply devices have been required not only to be energy saving but also to be capable of producing stylish illumination, and advanced dimming systems have been attracting increasing attention. It is easy to differentiate a good dimming function from a bad one, and thus, to make a product more attractive to consumers, it is necessary to achieve high-level dimming control. 
     As examples of the conventional technology related to the foregoing, JP-A-2011-108668 and JP-A-2011-187205 can be cited. 
     SUMMARY OF THE INVENTION 
     However, a minimum value of a dimming ratio (=a rate of a target output current to a maximum output current) that can be set by means of conventional LED power supply devices is about 5%. Thus, in comparison of dimming between an LED lighting apparatus and an incandescent light bulb, the LED lighting apparatus has a problem that it goes out more steeply than the incandescent light bulb when fully turned off, and dimming of the LED lighting apparatus is not smooth (see  FIG. 21 ). Note that, in order to achieve dimming of an LED lighting apparatus that is similar to the dimming of an incandescent light bulb, a dimming ratio is required to be smaller (for example, about 0.1%) than is conventionally achieved. 
     Moreover, LED power supply devices have been required to have a high efficiency (for example, 80% or higher), a high power factor (for example, 0.9 or higher [AC 100-200 V]), and an increased maximum output current value (for example, 1050 mA), and to acquire the PSE (product safety electrical appliance and materials) mark, for example. 
     In view of the above described problems found by the inventor of the present application, an object of the invention disclosed in the present specification is to provide an LED power supply device capable of performing fine dimming control. 
     To achieve the above object, an LED power supply device disclosed herein includes a DC dimmer circuit that performs dimming control of an LED such that the higher a reference voltage variably controlled according to a dimming signal is, the smaller an output current flowing in the LED is. 
     Other features, components, steps, advantages, and characteristics of the present invention will be disclosed in the following detailed description of the best mode for carrying out the present invention and relevant attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a comparison diagram for comparing a one-converter method with a two-converter method; 
         FIG. 2  is a comparison diagram for comparing DC dimming with burst dimming; 
         FIG. 3  is a block diagram showing a schematic configuration of an LED power supply device; 
         FIG. 4  is a circuit diagram showing a first configuration example of a current monitor; 
         FIG. 5  is a waveform diagram showing a behavior of a reference voltage in the first configuration example; 
         FIG. 6  is a circuit diagram showing a second configuration example of the current monitor; 
         FIG. 7  is a waveform diagram showing a behavior of the reference voltage in the second configuration example; 
         FIG. 8  is a correlation table of duty, output current, and dimming ratio in the second configuration example; 
         FIG. 9  is a correlation diagram of dimming signal ratio and output current ratio in the second configuration example; 
         FIG. 10  is a correlation diagram of dimming signal ratio and output current ratio in each of a plurality of products; 
         FIG. 11  is a correlation table of power supply voltage and output current; 
         FIG. 12  is a circuit diagram for illustrating CTR variation of a photo coupler; 
         FIG. 13  is a block diagram showing an LED power supply device equipped with a software trimming function; 
         FIG. 14  is a schematic diagram showing an example of a flow of trimming; 
         FIG. 15  is a correlation diagram of dimming signal ratio and output current ratio before and after trimming; 
         FIG. 16  is a correlation diagram of dimming signal ratio and output current ratio after trimming in each of a plurality of products; 
         FIG. 17  is a comparison diagram for comparing behaviors of an LED lighting apparatus and an incandescent light bulb; 
         FIG. 18  is a specifications table of an LED power supply device; 
         FIG. 19  is a circuit diagram showing a detailed configuration (a first example) of an LED power supply device; 
         FIG. 20  is a circuit diagram showing a detailed configuration (a second example) of an LED power supply device; and 
         FIG. 21  is a comparison diagram for comparing a behavior of a conventional LED lighting apparatus with a behavior of an incandescent light bulb. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     &lt;Selection of Power Supply System&gt; 
       FIG. 1  is a comparison diagram for comparing a one-converter method with a two-converter method. An LED lighting apparatus  100  has an LED power supply device  1  and an LED  2  that is driven by receiving power supply from the LED power supply device  1 . The LED power supply device  1  is a fly-back power supply device that generates an output voltage Vo from an input voltage Vdc, with a primary circuit system  1   p  and a secondary circuit system is insulated from each other by using a transformer TR 1 . The LED power supply device  1  is equipped also with an AC/DC conversion function to convert an AC voltage Vac supplied from a commercial AC power supply  3  into a DC input voltage Vdc. 
     In the one-converter method (see an upper section of the figure), a power factor improvement circuit, a constant current circuit, and a dimming function portion are all provided together in the primary circuit system  1   p . Merits of the one-converter method include its high power supply efficiency and its low cost (a simple circuit configuration). On the other hand, demerits of the one-converter method include that dimming is difficult therewith. 
     In the two-converter method (see a lower section of the figure), the power factor improvement circuit is provided in the primary circuit system  1   p , and the constant current circuit and the dimming function portion are provided in the secondary circuit system  1   s . Merits of the two-converter method include that dimming is easy therewith. On the other hand, demerits of the one-converter method include its low power supply efficiency and its high cost (a complicated circuit configuration). 
     In view of these facts, it can be said that, for highly efficient dimming, the one-converter method should be selected to be adopted in the power supply system of the LED power supply device  1 . 
     &lt;Selection of Dimming Method&gt; 
       FIG. 2  is a comparison diagram for comparing DC dimming with burst dimming. In the DC dimming (see an upper section in the figure), dimming of the LED  2  is performed by increasing/decreasing a current value of an LED current (=an output current Io flowing in the LED  2 ). That is, when the current value of the LED current is large, the LED  2  becomes bright, and when the current value of the LED current is small, the LED  2  becomes dim. Merits of the DC dimming include that it generates no noise. On the other hand, demerits of the DC dimming include that it makes fine dimming difficult. 
     In the burst dimming (see a lower section in the figure), by periodically turning on/off an LED current having a constant current value to increase/decrease a time average of the LED current, and thereby the dimming of the LED  2  is performed. That is, when an on-duty (rate of ON time in a cycle) of the LED current is large, the LED  2  becomes bright, and when the on-duty of the LED current is small, the LED  2  becomes dim. Merits of the burst dimming include that it makes fine dimming control easy. On the other hand, demerits of the burst dimming include that it generates a stroboscopic effect and noise, and existence of the PSE standard. 
     In view of these facts, it can be said that, for higher versatility, as the dimming method for LED power supply devices, it is desirable to select the DC dimming which has fewer demerits. 
     &lt;LED Power Supply Device&gt; 
       FIG. 3  is a block diagram showing a schematic configuration of the LED power supply device  1 . The LED power supply device  1  has a DC dimmer circuit  10  as means for performing DC dimming of the LED  2 . As shown in an upper section of the figure, the DC dimmer circuit  10  includes a current monitor  11  that monitors the output current Io flowing in a sense resistor Rs to generate a feedback signal FB, a driver IC  12  that performs constant current control of the output current Io according to the feedback signal FB, and a microcomputer  13  that performs PWM (pulse width modulation) driving of a dimming signal DIM by using a PWM signal source  4 , and sends out the dimming signal DIM to the current monitor  11 . 
     In the one-converter LED power supply device  1 , the driver IC  12  and the microcomputer  13  are both provided in the primary circuit system  1   p , and the current monitor  11  alone is provided in the secondary circuit system  1   s.    
     As shown in a lower section of the figure, it is also possible to use a microcomputer  14  where the driver IC  12  and the microcomputer  13  are integrated to achieve integrated digital control of power supply and dimming. Adoption of these configurations makes it possible not only to enhance ability to respond to users&#39; needs regarding the power supply specifications but also to reduce the number of components and cost. 
     Current Monitor 
     First Configuration Example 
       FIG. 4  is a circuit diagram showing a first configuration example of the current monitor  11 . The current monitor  11  of the present configuration example includes a sense resistor Rs, resistors R 1  and RDIM, and an operational amplifier AMP. The resistors R 1  and RDIM are connected in series between an internal power supply terminal (VREF) and a ground terminal, and a reference voltage V+(={RDIM/(R 1 +RDIM)}×VREF) is output from a connection node between the two resistors. Resistance of the resistor RDIM is variably controlled according to the dimming signal DIM. At a high-potential terminal of the sense resistor Rs, there appears a detection voltage Vs (=Io×Rs) in accordance with the output current Io. The operational amplifier AMP amplifies a difference between the reference voltage V+ applied to its non-inverting input terminal (+) and the detection voltage Vs applied to its inverting input terminal (−) to generate the feedback signal FB. 
     On receiving the input of the feedback signal FB, the driver IC  12  (not shown in  FIG. 4 ) performs dimming control of the output current Io (on/off control of an output switch that is connected to a primary coil of the transformer TR 1 ) such that the feedback signal FB becomes small. As a result, in the DC dimmer circuit  10 , an output feedback is applied such that the reference voltage V+ and the detection voltage Vs are equal to each other (imaginary short), and thus the output current Io is matched to a target value (=V+/Rs) in accordance with the reference voltage V+. 
     Here, in the DC dimmer circuit  10  using the current monitor  11  of the first configuration example, dimming is performed by means of voltage control, and the output current Io becomes smaller according as the reference voltage V+ is lowered. However, a lower reference voltage V+ invites increased liability to be influenced by noise, and this makes it difficult to set the dimming ratio to a small value (see  FIG. 5 ). 
     For example, when VREF=1.2V, R 1 =1 kΩ, and RDIM=1 kΩ (max), if RDIM is reduced to 1% (=10Ω), the reference voltage V+ falls to 12 mV and becomes more liable to be influenced by noise, and accordingly, flickers occur in the LED  2 . Thus, it can be said that, with the DC dimmer circuit  10  using the current monitor  11  of the first configuration example, it is difficult to perform fine dimming control. 
     Current Monitor 
     Second Configuration Example 
       FIG. 6  is a circuit diagram showing a second configuration example of the current monitor  11 . The current monitor  11  of the present configuration example includes a sense resistor Rs, resistors R 1 -R 6 , a photo coupler PC 1 , a capacitor C 1 , a diode D 2 , a buffer BUF, and an operational amplifier AMP. 
     An anode of a photo diode that forms the photo coupler PC 1  is connected to an output terminal of the buffer BUF. A cathode of the photo diode is connected to a ground terminal. A collector of a photo-transistor that forms the photo coupler PC 1  is connected via the resistor R 6  to a second internal power supply terminal (=VREF 2 ). An emitter of the photo-transistor is connected to an anode of the diode D 2 , a first terminal of the resistor R 5 , and to a first terminal of the capacitor C 1 . Second terminals of the resistor R 5  and the capacitor C 1  are both connected to the ground terminal. 
     The photo coupler PC 1  performs current output in accordance with the dimming signal DIM input thereto from the microcomputer  13  via the buffer BUF. The capacitor C 1  smooths the current output form the photo coupler PC 1  to generate a dimming current IDIM. That is, in the current monitor  11  of the present configuration example, the photo coupler PC 1 , the capacitor C 1 , and the resistors R 5  and R 6  function as a current DAC (digital to analog converter) that converts the dimming signal DIM which is PWM driven into the dimming current IDIM which is an analog current. 
     The sense resistor Rs is provided on a path through which the output current Io flows. The resistors R 1  and R 2  are connected in series between a first internal power supply terminal (=VREF 1 ) and a low-potential terminal (=−Io×Rs) of the sense resistor Rs, and a current I 1  flows through the resistors R 1  an R 2  via the path. As a result, at a connection node between the resistors R 1  and R 2 , there appears a detection voltage Vs (=I 1 ×R 2 −Io×Rs) in accordance with the output current Io. That is, in the current monitor  11  of the present configuration example, the sense resistor Rs and the resistors R 1  and R 2  function as a detection voltage generation portion that generates the detection voltage Vs according to the output current Io. Here, the smaller the output current Io is, the higher a voltage value of the detection voltage Vs becomes, and the larger the output current Io is, the lower the voltage value of the detection voltage Vs becomes. 
     The resistors R 3  and R 4  are connected in series between the first internal power supply terminal and a high-potential terminal (=GND) of the sense resistor Rs, and a current I 2  flows in the resistors R 3  an R 4  via the path. Moreover, the connection node between the resistor R 3  and the resistor R 4  is connected also to a cathode of the diode D 2  (corresponding to an output terminal of the current DAC), and, in the resistor R 4 , there flows a sum current (=I 2 +IDIM) that is obtained by adding the current I 2  to the dimming current IDIM. As a result, at the connection node between the resistor R 3  and the resistor R 4 , there appears a reference voltage V+(=(I 2 +IDIM)×R 4 ) in accordance with the dimming current IDIM. That is, in the current monitor  11  of the present configuration example, the resistors R 3  and R 4  function as a reference voltage generation portion that generates the reference voltage V+ according to the dimming current IDIM. 
     As in the above-described first configuration example, the operational amplifier AMP amplifies a difference between the reference voltage V+ applied to its non-inverting input terminal (+) and the detection voltage Vs applied to its inverting input terminal (−) to generate a feedback signal FB. 
     Furthermore, as in the above-described first configuration example, on receiving input of the feedback signal FB, the driver IC  12  (not shown in  FIG. 6 ) performs dimming control of the output current Io such that the feedback signal FB becomes small. As a result, in the DC dimmer circuit  10 , an output feedback is applied such that the reference voltage V+ and the detection voltage Vs are equal to each other (imaginary short), and thus the output current Io is matched to a target value (=(I 1 ×R 2 −V+)/Rs) in accordance with the reference voltage V+. 
     An important point here is that the dimming control of the LED  2  is performed not such that the lower the reference voltage V+ variably controlled according to the dimming signal DIM is, the smaller the output current Io becomes, but such that the higher the reference voltage V+ is, the smaller the output current Io becomes. 
     Thus, in the DC dimmer circuit  10  using the current monitor  11  of the second configuration example, dimming is performed by means of current control using the photo coupler PC 1 , and the output current Io becomes smaller according as the reference voltage V+ is raised. Consequently, even when the dimming ratio is set small, the reference voltage V+ becomes less liable to be influenced by noise (see  FIG. 7 ). As a result, in comparison with the above-described first configuration example, it is possible to achieve a very fine dimming (with a dimming ratio of 0.1%, for example) (see  FIG. 8  and  FIG. 9 ). 
     &lt;Software Trimming&gt; 
     Where the current monitor  11  of the second configuration example is used, there may be a trade-off such that it is possible to achieve a dimming ratio of 0.1%, but on the other hand, due to the introduction of the buffer BUF and the photo coupler PC 1 , the dimming signal ratio (duty value) when the output current Io is at a zero value varies from product to product (see  FIG. 10 ). 
     A first factor responsible for the trade-off is variation in a power supply voltage VDD supplied to the buffer BUF. If the power supply voltage VDD varies, the pulse peak value of the dimming signal DIM transmitted to the photo coupler PC 1  from the buffer BUF varies and a forward current IF of the photo coupler PC 1  also varies, and this in turn causes variation in the dimming current IDIM, and furthermore, the output current Io is caused to greatly vary in value. For example, if the power supply voltage VDD varies by 10 mV (0.3%), it causes the output current Io to vary by about 7 mA (see  FIG. 11  and  FIG. 12 ). A second factor is the variation in a current transfer ratio (CTR (=IDIM/IF×100)) that the photo coupler PC 1  has. The CTR of the photo coupler PC 1  generally has a wide range of variation (50% to 300%), and thus the current value of the output current Io also has a wide range of variation. 
     Unfortunately, however, it is difficult to eliminate these factors by taking measures in terms of hardware. Thus, here, variation in dimming (±10%), which cannot be eliminated by taking measures in terms of hardware, is eliminated by software trimming by using the microcomputer  13 . 
       FIG. 13  is a block diagram showing the LED power supply device  1  equipped with a software trimming function. The microcomputer  13  generates the dimming signal DIM using a dimming table TBL 1  that defines correlation between dimming signal ratio and output current ratio. 
     In doing so, the microcomputer  13  detects, by using an initial version of the dimming table, a dimming signal ratio when the output current ratio is zero, and then corrects the dimming table TBL 1  such that the detected value is equal to a target value (see  FIG. 14  and  FIG. 15 ). 
     Through such software trimming, it is possible to match the dimming signal ratio when the output current Io is at a zero value to the target value (90%, for example) with respect to all products, and this makes it possible to significantly reduce the variation in dimming (±10%→±3%) (see  FIG. 16 ) 
     For example, when a plurality of LED light sources are turned on simultaneously, the LED light sources actually start to shine at different timings before the software trimming, and this makes them look awkward in comparison with incandescent light bulbs. On the other hand, after the software trimming, the timings at which the LED light sources start to shine are almost the same, and this allows the LED light sources to demonstrate a behavior similar to that of incandescent light bulbs. An LED lighting apparatus using these LED light sources are the most suitable as a substitute for a large number of incandescent light bulbs provided for stage effects in a large hall. 
     Conclusion 
     Adoption of the above configurations makes it possible to achieve dimming that is smoother than conventionally performed dimming and that is close to dimming performed with incandescent light bulbs (see  FIG. 17 ). Moreover, it is possible to achieve performance excellent in all of the following: dimming ratio, power factor, efficiency, noise terminal voltage, and disturbance power (see  FIG. 18 ). 
     &lt;Detailed Configuration&gt; 
       FIG. 19  is a circuit diagram showing a detailed configuration (a first example) of the LED power supply device  1 . Here, detailed descriptions will be given focusing mainly on the portion corresponding to the current monitor  11  of the second configuration example (see  FIG. 6 ). Among the various circuit elements illustrated in the present figure, IC 31 , R 41 -R 46 , and R 48 -R 50 , R 32 , R 132 , R 232 , R 332 , C 46 , D 35  and PC 32  correspond to the circuit elements that form the current monitor  11  of the second configuration example. 
     Specifically, IC 31  in  FIG. 19  corresponds to the operational amplifier AMP in  FIG. 6 . A resistor R 42  in  FIG. 19  corresponds to the resistor R 1  in  FIG. 6 . A resistor R 41  in  FIG. 19  corresponds to the resistor R 2  in  FIG. 6 . Resistor R 46  in  FIG. 19  corresponds to the resistor R 3  in  FIG. 6 . Resistors R 43 -R 45  in  FIG. 19  correspond to the resistor R 4  in  FIG. 6 . Resistors R 49  and R 50  in  FIG. 19  correspond to the resistor R 5  in  FIG. 6 . A resistor R 48  in  FIG. 19  corresponds to the resistor R 6  in  FIG. 6 . A photo coupler PC 32  in  FIG. 19  corresponds to the photo coupler PC 1  in  FIG. 6 . A resistor R 32 , a resistor R 132 , a resistor R 232 , and a resistor  332  in  FIG. 19  correspond to the sense resistor Rs in  FIG. 6 . A capacitor C 46  in  FIG. 19  corresponds to the capacitor C 1  in  FIG. 6 . A diode D 35  in  FIG. 19  corresponds to the diode D 2  in  FIG. 6 . 
     The dimming signal DIM output from an RC 1  pin of IC 101  (the microcomputer  13 ) is input to the photo coupler PC 32 . The feedback signal FB output from a PC pin of IC 31  (the operational amplifier AMP) is input to an FB pin of IC 81  (corresponding to the driver IC  12 ) via the photo coupler PC 31 . IC 81  outputs a gate signal from its OUT pin according to the feedback signal FB, and thereby performs on/off control of an output switch Q 1  connected to primary coils  34 T and  35 T of a transformer T 1  (corresponding to the transformer TR 1 ). 
       FIG. 20  is a circuit diagram showing a detailed configuration (a second example) of the LED power supply device  1 . The circuit configuration of the second example, which is basically the same as the above first example, is different from the first example in some points, one of which is that the diode D 35  (the diode D 2  in  FIG. 6 ) for reverse current prevention connected to the output terminal of current DAC is omitted in the second example. 
     With this configuration, it is possible to eliminate variation in dimming resulting from variation in components of the diode D 35 . 
     Other Modified Examples 
     In addition to the above embodiments, it is possible to add various modifications to the various technical features disclosed in the present specification without departing the spirit of the technological creation. In other words, it should be understood that the above embodiments are examples in all respects and are not limiting; the technological scope of the present invention is not indicated by the above description of the embodiments but by the claims; and all modifications within the scope of the claims and the meaning equivalent to the claims are covered. 
     INDUSTRIAL APPLICABILITY 
     The invention disclosed herein is applicable to, for example, lighting equipment for use in facilities where the dimming function is required for saving energy and lighting equipment for household use where sophisticated dimming is required.