Abstract:
When an LED lighting apparatus which is a lighter load than an incandescent lamp or halogen lamp is connected to a dimmer, a malfunction may occur. The invention prevents the occurrence of such malfunction without defeating the purpose of low power consumption of the LED lighting apparatus. More specifically, the LED lighting apparatus includes a rectifier circuit, a light-emitting circuit connected to the rectifier circuit and containing a single or a plurality of LEDs in which current begins to flow when an output voltage of the rectifier circuit exceeds a threshold voltage, and a bypass circuit having a bypass path for making the current flow to the rectifier circuit without passing through the light-emitting circuit, and a detecting unit for detecting the current flowing through the light-emitting circuit, and wherein when the current detected by the detecting unit exceeds a predetermined value, the bypass circuit shuts off the current flowing through the bypass path.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an LED lighting apparatus for lighting an LED by using an output of a dimmer. 
       BACKGROUND 
       [0002]    A lighting apparatus (hereinafter called an LED lighting apparatus) is known which is connected to an AC commercial power supply and used for lighting an LED (also called a light-emitting diode). Such LED lighting apparatus commonly operates by rectifying the power supplied from the AC commercial power supply. In particular, a pulsating or near-pulsating voltage may be applied across an LED array constructed by connecting a large number of LEDs in series without requiring the use of large capacitors. 
         [0003]    If a pulsating voltage is directed applied to the LED array, the light emission period becomes short; to address this, it is known to provide a circuit for adjusting the number of series-connected LED stages by detecting the current flowing through the LED array (for example, refer to patent document 1). 
         [0004]      FIG. 7  is a diagram showing an LED lighting apparatus illustrated in  FIG. 26  in patent document 1. For convenience,  FIG. 7  includes numbers, currents, etc. where necessary. 
         [0005]    The LED lighting apparatus shown in  FIG. 7  includes an AC commercial power supply  712 , a bridge rectifier circuit  705  constructed from four diodes, a first LED group and a second LED group arranged in parallel, a third LED group connected in series to the first and second LED groups, resistors R 1 , R 2 , and R 3 , an n-type MOS transistor (FET) Q 1 , and an NPN transistor Q 2 . 
         [0006]    The resistors R 2  and R 3  and the transistors Q 1  and Q 2  together constitute a bypass circuit  717 . A current output terminal A of the bridge rectifier circuit  705  is connected to the parallel-connected first and second LED groups. The cathode side of the parallel-connected first and second LED groups is connected to the bypass circuit  717  as well as to the anode side of the third LED group. A current I 3  passing through the bypass circuit  717  and a current I 4  passing through the third LED group flow into the current sensing resistor R 3  and the base of the transistor Q 2  contained in the bypass circuit  717 . 
         [0007]      FIG. 8  is a diagram showing a voltage versus current relationship for the LED lighting apparatus of  FIG. 7 .  FIG. 8(   a ) shows an example of a voltage waveform for one pulsating cycle that appears at the terminal A with respect to the terminal B of the bridge rectifier circuit  705 , and  FIG. 8(   b ) is an example of a current waveform for one pulsating cycle that flows in the bridge rectifier circuit  705 . The current waveform shown in  FIG. 8(   b ) is approximately equal to the sum of the currents I 3  and I 4 . 
         [0008]    The currents I 3  and I 4  are both equal to 0 A during a period t 1  when the voltage at the terminal A is lower than the threshold voltage of the parallel-connected first and second LED groups. When the voltage at the terminal A subsequently rises and exceeds the threshold voltage of the parallel-connected first and second LED groups, the current increases rapidly for a short period t 2 . When the voltage at the terminal A further rises, there appears a period t 3  during which the sum of the currents I 3  and I 4  is constant. In the first half of the period t 3 , only the current I 3  flows through the bypass circuit  717 , and in the second half of the period t 3 , the current I 4  flows not only through the bypass circuit  717  but also through the third LED group. At this time, the currents I 3  and I 4  are regulated so that the base-emitter voltage of the transistor Q 2  is maintained at 0.6 V. 
         [0009]    Next, when the voltage at the terminal A rises, entering a period t 4  which contains the peak of the voltage waveform, the transistor Q 2  is saturated, and the bypass circuit  717  is cut off, so that the current I 3  no longer flows. In the period t 4 , the overall current varies substantially linearly with the voltage of the terminal A, since the current I 4  is only limited by the current-limiting resistor R 3 . The period during which the voltage of the terminal A falls is the reverse of the period during which the voltage rises. 
         [0010]    The LED lighting apparatus of  FIG. 7  has the advantage that, since the period t 1  during which all the LEDs are turned off is short, not only does flicker decrease, but power factor and distortion factor both improve and harmonic noise also decreases. 
         [0011]    In the prior art, it is also known to provide an LED lighting apparatus that includes a dimmer circuit between the AC commercial power supply and the bridge rectifier circuit (for example, refer to patent document 2). In the LED lighting apparatus disclosed in patent document 2, a pulsating voltage output from the bridge rectifier circuit is smoothed using a large-capacitance capacitor, and the thus smoothed voltage is used for lighting an LED. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0000]    
       
         Patent document 1: WO2011/020007 (FIG. 26) 
         Patent document 2: Japanese Unexamined Patent Publication No. 2011-3467 (FIG. 1) 
       
     
       SUMMARY 
       [0014]      FIG. 9  is a diagram showing an example in which a dimmer  901  is inserted between the AC commercial power supply and the bridge rectifier circuit  705  in the LED lighting apparatus shown in  FIG. 7 . 
         [0015]    The dimmer  901  shown in  FIG. 9  is a leading-edge type dimmer, which varies the intensity of LED light by controlling the phase of the voltage waveform being output from the AC commercial power supply  712 . For example, the dimmer  901  operates as if the voltage is present only in the second half portion by truncating the first half portion of the pulsating voltage shown in  FIG. 8(   a ), and varies the intensity of LED light by adjusting the length of the period during which the voltage is present. 
         [0016]      FIG. 10  is a diagram showing a voltage versus current relationship for the LED lighting apparatus of  FIG. 9 .  FIG. 10(   a ) shows an example of a voltage waveform for one pulsating cycle that appears at the terminal A with respect to the terminal B of the bridge rectifier circuit  705  for an ideal load, and  FIG. 10(   b ) is an example of a voltage waveform for one pulsating cycle that the bridge rectifier circuit  705  outputs in the circuit shown in  FIG. 9 . 
         [0017]    In the voltage waveform of  FIG. 10(   a ), the first half portion of the pulsating voltage shown in  FIG. 8(   a ) is truncated by the action of the dimmer  901 . As shown in  FIG. 10(   b ), a gradually increasing voltage appears at the output of the bridge rectifier circuit  705  during the first half period when no voltage should be present. In the second half period, a plurality of sharp peaks appear on the voltage being output at the terminal A of the bridge rectifier circuit  705 , as shown in  FIG. 10(   b ). Here, if the current flowing through the parallel-connected first and second LED groups is increased up to a certain point, the peaks appearing as shown in  FIG. 10(   b ) can be made to disappear, but the abnormal voltage in the first half period does not disappear. 
         [0018]    The reason that a faulty operation such as shown in  FIG. 10(   b ) occurs is believed to be that there is a need to flow a certain amount of current in order to properly operate the dimmer  901 . In actuality, however, in the period during which the voltage waveform of  FIG. 10(   a ) is substantially held to zero, the current minimum necessary for proper operation does not flow to the dimmer  901 . 
         [0019]    A faulty operation such as shown in  FIG. 10(   b ) can occur not only when the LED lighting apparatus shown in  FIG. 7  is connected to the dimmer  901 , but also when the LED lighting apparatus which is a lighter load than an incandescent lamp or halogen lamp is connected to any dimmer other than the above dimmer. If the load is increased by forming a current path in parallel with the light load LED apparatus, the above faulty operation may be able to be resolved. However, increasing the load in such a manner would defeat the purpose of low power consumption of the LED lighting apparatus. 
         [0020]    By contrast, the LED lighting apparatus disclosed in patent document 2 is provided with a load circuit  7  for holding a minimum current necessary for the proper operation of the dimmer circuit  2 . However, the LED lighting apparatus disclosed in patent document 2 is further provided with a smoothing circuit  4  which includes a capacitor, and the voltage output from the rectifier circuit  3  is first smoothed and then supplied to a lighting circuit  5  for lighting the load  6  such as an LED. 
         [0021]    As a result, in the LED lighting apparatus disclosed in patent document 2, the load  6  such as an LED is DC driven. To adjust the intensity of LED light in DC driving, the lighting circuit  5  detects the phase with which the dimmer circuit  2  supplies power and, in accordance with the thus detected phase, controls the DC voltage to be supplied to the load  6  such as an LED. Such lighting control requires not only complicated control circuitry but also a stable DC voltage supply. This therefore requires the provision of a large-capacitance capacitor in the smoothing circuit  4 , and such a large-capacitance capacitor becomes an obstacle to reducing the circuit size. Furthermore, if an electrolytic capacitor, for example, is used as the large-capacitance capacitor, there arise problems such as reduced lifetime due to the effects of the heat generated by the LED, reducing the lifetime of the LED lighting apparatus itself or requiring frequent maintenance. 
         [0022]    Accordingly, it is an object of the present invention to provide an LED lighting apparatus that uses an LED as a light source, and that can operate properly even when operated using an output of a dimmer and can yet reduce power consumption. 
         [0023]    It is another object of the present invention to provide an LED lighting apparatus that can be implemented with simple circuitry without using a smoothing circuit and that does not cause malfunction of a dimmer. 
         [0024]    An LED lighting apparatus includes a rectifier circuit, a light-emitting circuit connected to the rectifier circuit and containing a single or a plurality of LEDs in which current begins to flow when an output voltage of the rectifier circuit exceeds a threshold voltage, and a bypass circuit having a bypass path for making the current flow to the rectifier circuit without passing through the light-emitting circuit, and a detecting unit for detecting the current flowing through the light-emitting circuit, and wherein when the current detected by the detecting unit exceeds a predetermined value, the bypass circuit shuts off the current flowing through the bypass path. 
         [0025]    Preferably, in the LED lighting apparatus, the bypass circuit maintains the sum of the current flowing through the bypass circuit and the current flowing through the light-emitting circuit constant. 
         [0026]    Preferably, in the LED lighting apparatus, the bypass circuit includes a current detecting resistor and a depletion-type FET placed in the bypass path, wherein the depletion-type FET controls opening and closing of the bypass path by detecting the current flowing through the light-emitting circuit by the current detecting resistor. 
         [0027]    Preferably, in the LED lighting apparatus, the bypass circuit includes a current detecting resistor and an enhancement-type FET placed in the bypass path, a bipolar transistor for controlling the enhancement-type FET, and a pull-up resistor, wherein the bipolar transistor detects the current flowing through the light-emitting circuit by the current detecting resistor, and controls opening and closing of the bypass path by using the enhancement-type FET. 
         [0028]    Preferably, the LED lighting apparatus further includes a second bypass circuit connected to the light-emitting circuit, a second light-emitting circuit connected to the second bypass circuit and containing a single or a plurality of LEDs in which current begins to flow when the output voltage of the rectifier circuit exceeds a threshold voltage, and a current limiting circuit for limiting the current flowing into the second light-emitting circuit. 
         [0029]    Preferably, the LED lighting apparatus further comprises a filter circuit connected in parallel with the bypass circuit and constructed from a series connection of a resistor and a capacitor. 
         [0030]    Preferably, in the LED lighting apparatus, the filter circuit is placed after the bypass circuit but before the light-emitting circuit. 
         [0031]    An LED lighting apparatus includes a rectifier circuit, a light-emitting circuit containing a single or a plurality of LEDs, the light-emitting circuit having a first power supply terminal and a second power supply terminal, and a bypass circuit having a third power supply terminal, a fourth power supply terminal, and a current detecting terminal, wherein the first power supply terminal and the third power supply terminal are connected to one end of the rectifier circuit, the second power supply terminal is connected to the current detecting terminal, and the fourth power supply terminal is connected to the other end of the rectifier circuit, and wherein when the voltage developed between the one end and the other end of the rectifier circuit is low, current flows through the third power supply terminal, and when the current flowing through the current detecting terminal exceeds a predetermined value, the current flowing through the third power supply terminal no longer flows, while when the voltage at the one end of the rectifier circuit exceeds the threshold voltage of the single LED or the threshold voltage of an LED array of the plurality of LEDs connected in series, the current flows through the single LED or the LED array into the current detecting terminal. 
         [0032]    A dimmer receives a voltage from an AC commercial power supply, and modifies the voltage waveform in such a manner that the voltage is present only in a specific period and no voltage is present in the remaining period. However, even in the no-voltage period, the voltage is not completely zero but a slight amount of voltage is present. Therefore, in the LED lighting apparatus, current is allowed to flow through the bypass circuit in the no-voltage period in order to stabilize the operation of the dimmer. In the no-voltage period, no current flows to the light-emitting circuit because there is a threshold voltage for the operation of the LEDs. Even when current begins to flow into the light-emitting circuit immediately after the output of the dimmer transitions to the voltage period, the stable operation of the dimmer is maintained. When the output of the dimmer transitions to the voltage period, and the current flowing through the light-emitting circuit exceeds a predetermined value, the bypass circuit is cut off, and the current thus flows only through the light-emitting circuit. Therefore, the LED lighting apparatus of the invention can operate properly even when operated using the output of the dimmer and can yet reduce power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a schematic block diagram of an LED lighting apparatus  100 . 
           [0034]      FIG. 2  is a circuit diagram of the LED lighting apparatus  100  shown in  FIG. 1 . 
           [0035]      FIG. 3(   a ) is a diagram depicting the voltage measured at terminal A with respect to terminal B in the LED lighting apparatus  100  shown in  FIG. 1 . 
           [0036]      FIG. 3(   b ) is a diagram depicting the waveform of current I flowing through terminal A in response to the voltage of  FIG. 3(   a ). 
           [0037]      FIG. 4  is a circuit diagram of an alternative LED lighting apparatus  400 . 
           [0038]      FIG. 5(   a ) is a diagram depicting the voltage measured at terminal A with respect to terminal B in the LED lighting apparatus  400  shown in  FIG. 4 . 
           [0039]      FIG. 5(   b ) is a diagram depicting the waveform of current I flowing through terminal A in response to the voltage of  FIG. 5(   a ). 
           [0040]      FIG. 6  is a circuit diagram of a further alternative LED lighting apparatus  500 . 
           [0041]      FIG. 7  is a diagram showing an LED lighting apparatus illustrated in  FIG. 26  in patent document 1. 
           [0042]      FIG. 8(   a ) is a diagram showing an example of a voltage waveform for one pulsating cycle that appears at terminal A with respect to terminal B of a bridge rectifier circuit  705  in the LED lighting apparatus shown in  FIG. 7 . 
           [0043]      FIG. 8(   b ) is an example of a current waveform for one pulsating cycle that flows in the bridge rectifier circuit  705  in the LED lighting apparatus shown in  FIG. 7 . 
           [0044]      FIG. 9  is a diagram showing an example in which a dimmer  901  is inserted between an AC commercial power supply and the bridge rectifier circuit  705  in the LED lighting apparatus shown in  FIG. 7 . 
           [0045]      FIG. 10(   a ) is a diagram showing an example of a voltage waveform for one pulsating cycle that appears at terminal A with respect to terminal B of the bridge rectifier circuit  705  for an ideal load. 
           [0046]      FIG. 10(   b ) is an example of a voltage waveform for one pulsating cycle that the bridge rectifier circuit  705  outputs in the LED lighting apparatus shown in  FIG. 9 . 
       
    
    
     DESCRIPTION 
       [0047]    LED lighting apparatus will be described below with reference to the drawings. It will, however, be noted that the technical scope of the present invention is not limited by any particular embodiment described herein but extends to the inventions described in the appended claims and their equivalents. Further, in the description of the drawings, the same or corresponding component elements are designated by the same reference numerals, and the description of such component elements, once given, will not be repeated thereafter. It will also be noted that the scale to which each component element is drawn is changed as needed for illustrative purposes. 
         [0048]      FIG. 1  is a schematic block diagram of an LED lighting apparatus  100 . 
         [0049]    The LED lighting apparatus  100  is connected to the power output end of a dimmer  109 , and the power input end of the dimmer  109  is connected to an AC commercial power supply  108 . The LED lighting apparatus  100  comprises a rectifier circuit  105 , a bypass circuit  106 , and a light-emitting circuit  107 . 
         [0050]    The rectifier circuit  105  is a diode bridge constructed from four diodes  101  to  104 , and the upper end and lower end of the diode bridge are connected to the power output end of the dimmer  109 . A terminal A is the terminal at the current output end of the rectifier circuit  105 , and a terminal B is the terminal at the current input end. While the rectifier circuit  105  is shown here by way of example as being a diode bridge constructed from four diodes, the configuration of the rectifier circuit  105  is not limited to this particular example, but any other suitable configuration may be employed. For example, the rectifier circuit  105  may be constructed from a single diode. 
         [0051]    The bypass circuit  106  includes a positive power supply terminal  111  (third power supply terminal), a negative power supply terminal  112  (fourth power supply terminal), a current detecting terminal  113 , a current limiting unit  116 , and a current detecting unit  117 . The positive power supply terminal  111  is connected at one end to the terminal A and at the other end to the upper end of the current limiting unit  116 , while the negative power supply terminal  112  is connected at one end to the terminal B and at the other end to the lower end of the current detecting unit. Current flows into the current detecting unit  117  from the current limiting unit  116 , and current also flows into it from the light-emitting circuit  107  via the current detecting terminal  113 . 
         [0052]    When the voltage measured between the terminals A and B of the rectifier circuit  105  is low (hereinafter, the voltage measured at the terminal A with respect to the terminal B is referred to as the voltage of the terminal A), the current flows from the positive power supply terminal  111  to the terminal B by passing through the current limiting unit  116 , the current detecting unit  117 , and the negative power supply terminal  112 . When the voltage of the terminal A rises and reaches a point where the current also flows into the light-emitting circuit  107 , feedback is applied so that the current flowing in the current detecting unit  117  is maintained substantially constant. When the voltage of the terminal A further rises, and the current passing through the current detecting terminal  113  exceeds a predetermined value, feedback is applied so as to reduce the current flowing into the bypass circuit  106  through the positive power supply terminal  111 . 
         [0053]    The light-emitting circuit  107  contains therein a single or a plurality of light-emitting diodes (hereinafter called the LEDs), and is provided with a positive power supply terminal  114  (first power supply terminal) and a negative power supply terminal  115  (second power supply terminal). The positive power supply terminal  114  is connected to the positive power supply terminal  111  of the bypass circuit  106  and hence to the terminal A. The negative power supply terminal  115  is connected to the current detecting terminal  113  of the bypass circuit  106 . 
         [0054]      FIG. 2  is a circuit diagram of the LED lighting apparatus  100  shown in  FIG. 1 . In  FIG. 2 , the bypass circuit  106  and light-emitting circuit  107  contained in the LED lighting apparatus  100  of  FIG. 1  are shown at the device level. 
         [0055]    The bypass circuit  106  includes resistors  121  and  124 , an n-channel enhancement-type MOS transistor  122  (hereinafter called the FET), and an NPN bipolar transistor  123  (hereinafter called the transistor). The light-emitting circuit  107  includes an LED array constructed from a series connection of a large number of LEDs including LEDs  126  and  127 , and a resistor  128 . 
         [0056]    The positive power supply terminal  111  of the bypass circuit  106  is connected to the upper end of the resistor  121  and the drain of the FET  122 , while the negative power supply terminal  112  is connected to the emitter of the transistor  123  and the lower end of the resistor  124 . The current detecting terminal  113  is connected to a connection node at which the source of the FET  122 , the base of the transistor  123 , and the upper end of the resistor  124  are connected. The current I 1  passing through the FET  122  and the current I 2  flowing in from the light-emitting circuit  107  are directed toward the terminal B of the rectifier circuit  105  by passing through the resistor  124  and the transistor  123 . 
         [0057]    In  FIG. 1 , the functions of the current limiting unit  116  and current detecting unit  117  are depicted in block diagram form; here, the FET  122  substantially corresponds to the current limiting unit  116 , and the resistor  124  corresponds to the current detecting unit. The resistor  121  and the transistor  123  together work to implement a feedback function for maintaining the current flowing through the resistor  124  at a constant level. 
         [0058]    In the light-emitting circuit  107 , when the forward voltage of each of the LEDs, including the LEDs  126  and  127 , contained in the LED array  125  is about 3 V, the number of series-connected LED stages forming the LED array  125  is determined by the root-mean-square value of the AC commercial power supply  108 . When the root-mean-square value of the AC commercial power supply  108  is 100 to 120 V, the number of series-connected LED stages is, for example, 30 to 40, and when the root-mean-square value of the AC commercial power supply  108  is 200 to 240 V, the number of series-connected LED stages is, for example, 60 to 80. The resistor  128  limits the current flowing into the LED array  125 . The positive power supply terminal  114  of the light-emitting circuit  107  is connected to the anode of the LED array  125 , and the negative power supply terminal  115  is connected to the lower end of the resistor  128 . 
         [0059]    The operation of the bypass circuit  106  will be described below. For convenience, it is assumed that the voltage of the terminal A starts at 0 V and rises as the time elapses. 
         [0060]    When the voltage of the terminal A of the rectifier circuit  105  is 0 V, the current I 1  does not flow. When the voltage of the terminal A subsequently rises, the current I 1  begins to flow through the positive power supply terminal  111 , and thereafter, the current I 1  maintained at a constant level flows so as to hold the base-emitter voltage of the transistor  123  at about 0.6 V. 
         [0061]    When the voltage of the terminal A further rises, and the current I 2  begins to flow into the light-emitting circuit  107 , the current I 1  is regulated so that the product of the sum of the currents I 1  and I 2  and the resistor  124  becomes equal to about 0.6 V. That is, there exists a voltage range over which the sum of the current I 1  flowing in through the positive power supply terminal  111  and the current I 2  flowing in through the current detecting terminal  113  is constant. In this voltage range, the transistor  123  in the bypass circuit  106  is in a non-saturated condition, and the sum of the currents I 1  and I 2  is maintained constant by reference to the base-emitter voltage. 
         [0062]    When the voltage of the terminal A further rises, and the current passing through the current detecting terminal  113  exceeds a predetermined value, the transistor  123  is saturated, and the FET  122  is cut off. As a result, the current no longer flows through the positive power supply terminal  111 , and the current flowing back to the terminal B of the rectifier circuit  105  through the current detecting terminal  113  is only the current I 2  flowing through the light-emitting circuit  107 . Here, the magnitude of the current flowing through the resistor  121  is small enough that it can be neglected. The current I 2  is limited by the resistor  128 , but increases as the voltage of the terminal A rises. 
         [0063]      FIG. 3  is a waveform diagram for the case where the circuit shown in  FIG. 2  is operated by using the output of the dimmer  109 .  FIG. 3(   a ) is a diagram depicting the voltage measured at the terminal A with respect to the terminal B in the LED lighting apparatus  100  shown in  FIG. 1 , and  FIG. 3(   b ) is a diagram depicting the waveform of the current I flowing through the terminal A in response to the voltage of  FIG. 3(   a ). 
         [0064]    As shown in  FIG. 3(   a ), the dimmer  109  produces an output voltage by truncating a portion of the pulsating voltage, and when the output voltage is full-wave rectified by the rectifier circuit  105 , the resulting waveform is such that the truncated portion is held at 0 V. The dotted line in  FIG. 3(   a ) indicates the pulsating voltage when no dimming control was applied. 
         [0065]    As shown in  FIG. 3(   b ), the current I first rises from 0 A and reaches a constant value. Since, in actuality, a slight amount of voltage (a few volts) is present even in the portion where the voltage of the terminal A is shown as being 0 V in  FIG. 3(   a ), the current I 1  is allowed to flow through the bypass circuit  106 , thereby stabilizing the operation of the dimmer  109  during the period when only a slight amount of voltage (a few volts) is present. 
         [0066]    Next, when the voltage of the terminal A sharply rises, the current I 2  flows into the light-emitting circuit  107 , and the current waveform also rises sharply (see t 10 ). At this time, since the current rises above the limit below which the bypass circuit  106  can maintain the sum of the currents I 1  and I 2  constant, the transistor  123  is saturated, and the FET  122  is cut off. As a result, the current I 1  drops to 0 A, and the current I becomes equal to the current I 2 . Then, the waveform of the current I varies substantially linearly with the voltage waveform of the terminal A (see  FIG. 3(   a )). 
         [0067]    After that, the voltage of the terminal A drops, and there appears a period during which the current I is constant (see t 11 ). In the period t 11 , the base voltage of the transistor  123  drops, and the feedback path is again formed to maintain the sum of the currents I 1  and I 2  constant. In the first half of the period t 11 , the current I 2  is still flowing, but in the second half, only the current I 1  flows. After the period t 11 , the current I 1  finally drops to 0 A, and the current I no longer flows. The dotted line in  FIG. 3(   b ) indicates the waveform of the current I when no dimming control was applied. 
         [0068]    The dimmer  109  is a leading-edge type dimmer which operates so as to truncate the first half portion of the pulsating voltage, and comprises, for example, a triac  200 , a diac  201 , a potentiometer  202 , a resistor  203 , and a capacitor  204 . Alternatively, the dimmer  109  may be configured as a trailing-edge type dimmer which operates so as to truncate the second half portion of the pulsating voltage. Further alternatively, the dimmer  109  may be configured to operate so as to truncate the first half and the second half of the pulsating voltage in alternating fashion. Regardless of the type of the dimmer, it becomes possible to stabilize the operation of the dimmer by flowing a bypass current through the bypass circuit during the period corresponding to the truncated portion of the voltage waveform. 
         [0069]      FIG. 4  is a circuit diagram of an alternative LED lighting apparatus  400 . 
         [0070]    The light-emitting circuit  107  contained in the LED lighting apparatus  100  shown in  FIGS. 1 and 2  was a simple one that contained only one LED array  125 . In this case, the light emission period becomes short compared with one pulsating cycle, and hence, flicker and motion breaks may become noticeable. An effective method to lengthen the light emission period is to change the number of series-connected stages of LED arrays according to the voltage or the current. In the LED lighting apparatus  400 , the number of series-connected stages of LED arrays is changed according to the current, with provisions made not to cause a faulty operation even when the output of the dimmer is used. 
         [0071]    In  FIG. 4 , the AC commercial power supply  108 , the dimmer  109 , the rectifier circuit  105 , and the bypass circuit  106  are the same as those shown in  FIG. 2 . The LED lighting apparatus  400  of  FIG. 4  differs from the LED lighting apparatus  100  of  FIG. 2  in that the light-emitting circuit  407  in the LED lighting apparatus  400  has multiple stages and in that a filter circuit  403  is inserted in parallel with the bypass circuit  106 . 
         [0072]    When  FIG. 4  is compared with  FIG. 1 , the light-emitting circuit  407  in  FIG. 4  corresponds to the light-emitting circuit  107  in  FIG. 1 , the positive power supply terminal  414  of the light-emitting circuit  407  in  FIG. 4  corresponds to the positive power supply terminal  114  of the light-emitting circuit  107  in  FIG. 1 , and the negative power supply terminal  415  of the light-emitting circuit  407  in  FIG. 4  corresponds to the negative power supply terminal  115  of the light-emitting circuit  107  in  FIG. 1 . 
         [0073]    The light-emitting circuit  407  comprises an LED array  435  constructed from LEDs  436  and  437  and an LED array  445  constructed from LEDs  446  and  447 . A second bypass circuit  408  is connected between the LED arrays  435  and  445 , and a current limiting circuit  409  is connected to the cathode side of the LED array  445 . When the root-mean-square value of the AC commercial power supply  108  is 100 to 120 V, the number of series-connected stages may be, for example, 25 for the LED array  435  and  15  for the LED array  445 , and when the root-mean-square value of the AC commercial power supply  108  is 200 to 240 V, the number of series-connected LEDs may be, for example, 50 for the LED array  435  and  30  for the LED array  445 . 
         [0074]    The second bypass circuit  408  comprises a resistor  431 , an FET  432 , a transistor  433 , and a resistor  434 , and is thus identical in circuit configuration to the bypass circuit  106 , but the value of the resistor  434  differs from the value of the resistor  124  in the LED lighting apparatus  100  shown in  FIG. 2 . Similarly, the current limiting circuit  409  comprises a resistor  441 , an FET  442 , a transistor  443 , and a resistor  444 , and is thus identical in circuit configuration to the bypass circuit  106 , but the value of the resistor  444  differs from the value of the resistor  124  in the LED lighting apparatus  100  shown in  FIG. 2 . Here, the value of the resistor  444  is smaller than the value of the resistor  434  which is smaller than the value of the resistor  124 . 
         [0075]    The operation of the light-emitting circuit  407  will be described below. For convenience, it is assumed that the voltage of the terminal A starts at 0 V and rises as the time elapses. 
         [0076]    When the voltage of the terminal A of the rectifier circuit  105  is 0 V, the current I does not flow. When the voltage of the terminal A subsequently rises and exceeds the threshold value of the LED  435 , the current I begins to flow into the light-emitting circuit  407 , and there appears a voltage range where a constant current flows so as to maintain the base-emitter voltage of the transistor  433  at about 0.6 V. In the first half of this voltage range, the current flows only into the FET  432  contained in the bypass circuit  408 , and in the second half, the current passing through the LED array  445  also flows. In this voltage range, the sum of the current flowing through the FET  432  contained in the bypass circuit  408  and the current flowing through the LED array  445  is maintained constant. 
         [0077]    When the voltage of the terminal A further rises, the current flowing through the LED array  445  and through the current limiting circuit  409  increases, and the transistor  433  saturates; as a result, the bypass circuit  408  is cut off, and the current no longer flows to the FET  432 . When the bypass circuit  408  is cut off, if the voltage of the terminal A further rises the current flowing through the LED array  445  is limited by the current limiting circuit  409 . Since the current flowing through the light-emitting circuit  407  can thus be prevented from increasing above its upper limit value, the current limiting circuit  409  can ensure stable operation of the light-emitting circuit  407  even when the AC commercial power supply  108  or the output voltage of the dimmer  109  is unstable. 
         [0078]    If the bypass circuit  106  and the filter circuit  403  formed from a series connection of a resistor  401  and a capacitor  402  were removed from the LED lighting apparatus  400 , the waveform of the voltage at the terminal A in  FIG. 4  would be as shown in  FIG. 10(   b ). That is, abnormal voltage would appear during the period when the voltage should normally be 0 V and, at the same time, sharp peaks would appear during the period when a portion of the pulsating voltage should normally appear. On the other hand, if the bypass circuit  106  alone were removed from the LED lighting apparatus  400 , the peaks occurring in the second half portion in  FIG. 10(   b ) would disappear from the waveform of the voltage at the terminal A in  FIG. 4 , but the abnormal voltage in the first half portion would not disappear. If, for example, an LED lighting apparatus consisting only of the bypass circuit  106  and light-emitting circuit  407  were connected to the dimmer  109 , the load balance would be disrupted, causing oscillations in the voltage of the terminal A, even during the period when the LED lighting apparatus should normally cause the LEDs to light (see  FIG. 10) . By contrast, in the LED lighting apparatus  400 , since the filter circuit  403  is inserted, such oscillations can be suppressed, serving to achieve stable operation. In particular, when the amount of current to be supplied to the LED array is small, the effect of inserting the filter circuit  403  is enormous. 
         [0079]    Further, when it is attempted to reduce the current flowing to the bypass circuit  106 , the stability of the LED lighting apparatus  400  to the dimmer degrades but, by inserting the filter circuit  403 , the stability can be recovered. That is, it can be seen that the filter circuit  403  formed by connecting the resistor  401  and the capacitor  402  in series serves to stabilize the operation of the LED lighting apparatus  400 . In the filter circuit  403 , the value of the resistor  401  may be set, for example, to 1 kΩ, and the value of the capacitor  402  may be set, for example, to 0.047 μF. 
         [0080]      FIG. 5  is a waveform diagram for the case where the circuit shown in  FIG. 4  is operated by using the output of the dimmer  109 .  FIG. 5(   a ) is a diagram depicting the voltage measured at the terminal A with respect to the terminal B in the LED lighting apparatus  400  shown in  FIG. 4 , and  FIG. 5(   b ) is a diagram depicting the waveform of the current I flowing through the terminal A in response to the voltage of  FIG. 5(   a ). 
         [0081]    The dimmer  109  produces an output by truncating a portion of the pulsating wave, the output waveform being such that the truncated portion is held at 0 V; therefore, when the output waveform is full-wave rectified by the rectifier circuit  105 , the resulting waveform is such that there is no voltage in the first half and a portion of the pulsating voltage appears in the second half, as shown by a solid line in  FIG. 5(   a ). In  FIG. 5(   a ), the dotted line indicates the pulsating voltage when no dimming control was applied. The operation of the bypass circuit  106  is basically the same as that described for the LED lighting apparatus  100 , but the operation will be described in detail below for the LED lighting apparatus  400  shown in  FIG. 4 . 
         [0082]    As shown in  FIG. 5(   b ), the current I first rises from 0 A and reaches a constant value. This is because, in actuality, a slight amount of voltage (a few volts) is present even in the portion where the voltage of the terminal A is shown as being 0 V in  FIG. 5(   a ), and as a result, the current flows through the bypass circuit  106 . Next, when the voltage of the terminal A rises, the current flows through the LED array  435 , and the current waveform rapidly rises (see time t 20 ). At time t 20 , the bypass circuit  106  is cut off, the current flowing through the FET  122  drops to 0 A, and the current I is equal to the current flowing through the LED array  435 . In  FIG. 5(   b ), the dotted line indicates the pulsating current when no dimming control was applied. 
         [0083]    As earlier described, there are three voltage ranges in the operation of the light-emitting circuit  407 : the first voltage range in which the bypass circuit  106  and the second bypass circuit  408  are both cut off and the current flowing through the LED array  445  is limited by the current limiting circuit  409 ; the second voltage range over which the sum of the current flowing through the second bypass circuit  408  are the current flowing through the LED array  445  is maintained constant according to the voltage of the terminal A; and the third voltage range over which the sum of the current flowing through the bypass circuit  106  are the current flowing through the LED array  435  is maintained constant. Accordingly, the waveform of the current I has three levels as shown in  FIG. 5(   b ), that is, the first level (L 1 ) corresponding to the first voltage range, the second level (L 2 ) corresponding to the second voltage range, and the third level (L 3 ) corresponding to the third voltage range.  FIG. 5  shows the case where the LEDs being to light in the voltage range in which the voltage of the terminal A is current-limited; generally, the waveform of the current I subjected to dimming is obtained by removing a portion from the source waveform (indicated by the dotted line plus the succeeding portion of the solid line) not subjected to dimming. 
         [0084]    In the LED lighting apparatus  400 , the number of series-connected stages of LED arrays is changed by detecting the current, but the number of series-connected stages of LED arrays may be changed by detecting the voltage. However, with the method that changes the number of series-connected stages of LED arrays by detecting the voltage, the current value may change abruptly so as to produce a sharp peak when changing the number of series-connected stages of LED arrays, and this can result in the generation of harmonic noise. By contract, in the LED lighting apparatus  400  that changes the number of series-connected stages of LED arrays by detecting the current, since the current can be made to change so as to follow voltage changes, it becomes possible to prevent harmonic noise and to maintain good power factor and good distortion factor. 
         [0085]    In the LED lighting apparatus  400 , the number of series-connected stages is changed by switching between two LED arrays, but the number of series-connected stages to be changed is not limited to two. For example, when connecting five LED arrays in series, five sets of circuits are provided, each set being identical in configuration to the set comprising the LED array  435  and the second bypass circuits  408 . Then, the thus provided five sets of circuits are connected in cascade in a manner similar to the manner in which the set comprising the LED array  445  and the current limiting circuit  409  is connected to the set comprising the LED array  435  and the second bypass circuits  408  in the LED lighting apparatus. The value of the resistor connected to the source of the FET is different for each set. 
         [0086]      FIG. 6  is a circuit diagram of a further alternative LED lighting apparatus  500 . 
         [0087]    In  FIG. 6 , the AC commercial power supply  108 , the dimmer  109 , and the rectifier circuit  105  are the same as those shown in  FIG. 4 . The LED lighting apparatus  500  of  FIG. 6  differs from the LED lighting apparatus  400  of  FIG. 4  in the circuit configuration of the bypass circuit  506 , second bypass circuit  508 , and current limiting circuit  509  and in the position of the filter circuit  503 . 
         [0088]    In the LED lighting apparatus  400  of  FIG. 4 , the bypass circuit  106 , the second bypass circuit  408 , and the current limiting circuit  409  are each constructed using two resistive elements, an n-channel enhancement-type MOS transistor (FET), and an NPN bipolar transistor. On the other hand, in the LED lighting apparatus  500  of  FIG. 6 , the corresponding circuits are each constructed using a depletion-type FET and a single resistor. 
         [0089]    In the bypass circuit  506 , the drain of the FET  512  is connected to the output terminal A of the rectifier circuit  105 , the gate is connected to one end of the resistor  511 , and the source is connected to the other end of the resistor  511 . When a current Ix flow through the resistor  511 , a voltage drop occurs, and a potential difference develops between the gate voltage VG and source voltage VS of the FET  512 . The depletion-type FET operates so as to turn off when the VG-VS potential difference becomes lower than an offset value. Accordingly, in the bypass circuit  506 , when the current Ix flowing through the resistor  511  increases due to the current flowing through the light-emitting circuit  507 , the FET  512  turns off, and the current flowing between the drain and source of the FET  512  is shut off. 
         [0090]    The second bypass circuit  508  and the current limiting circuit  509  operate in the same manner as the above bypass circuit  506 . The bypass circuit  506 , second bypass circuit  508 , and current limiting circuit  509  provided in the LED lighting apparatus  500  of  FIG. 6  function in the same manner as the bypass circuit  106 , second bypass circuit  408 , and current limiting circuit  409  provided in the LED lighting apparatus  400  of  FIG. 4 . That is, the bypass circuit  506 , second bypass circuit  508 , and current limiting circuit  509  switch the output current path of the rectifier circuit  105  and restrict the upper limit value. 
         [0091]    Accordingly, in the operation of the light-emitting circuit  507 , as in the operation of the light-emitting circuit  407  shown in  FIG. 4 , there are three voltage ranges: the first voltage range in which the bypass circuit  506  and the second bypass circuit  508  are both cut off and the current flowing through the LED array  445  is limited by the current limiting circuit  509 ; the second voltage range over which the sum of the current flowing through the second bypass circuit  508  are the current flowing through the LED array  445  is maintained constant according to the voltage of the terminal A; and the third voltage range over which the sum of the current flowing through the bypass circuit  506  are the current flowing through the LED array  435  is maintained constant. 
         [0092]    In the LED lighting apparatus  500  of  FIG. 5 , the filter circuit  503  is placed directly after the bypass circuit  506 . The bypass circuit  506 , like the bypass circuit  106  (see  FIG. 4 ), has the function of preventing malfunction of the dimmer  109  by continuing to flow a small amount of current to the dimmer  109  throughout the period during which the voltage is substantially held at 0 V. Further, in the LED lighting apparatus  500 , the filter circuit  503  suppresses voltage oscillations that may occur due to mismatching between the load and the dimmer  109 . To feed back the current flowing through the filter circuit  503  to the bypass circuit  506 , the filter circuit  503  is placed directly after the bypass circuit  506 . This arrangement serves to reduce the current flowing through the filter circuit  503 . The filter circuit  503  is identical in configuration and function to the filter circuit  403  (see  FIG. 4 ). 
         [0093]    The LED lighting apparatuses  100 ,  400 , and  500  described above properly operate at low power consumption even when they are connected to the AC commercial power supply without using the dimmer  109 .