JP2013058670A - Led drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a motion break or flicker inconspicuous by eliminating or shortening a non-lighting period even after applying a voltage to an LED row by a low voltage phase of a full-wave rectification waveform.SOLUTION: A voltage detection circuit 110 measures a voltage of a full-wave rectification waveform and brings a switch 130 into conduction only while the voltage of the full-wave rectification waveform is low. The switch 130 is connected in series to a light-emitting circuit 120, which flows an electric current into the light-emitting circuit 120 only while the switch 130 is being brought into conduction. In the light-emitting circuit 120, an LED row 123 and a current limit circuit 124 are connected in series and a capacitor 125 is connected in parallel to the series circuit. Even while the switch 130 is being brought into non-conduction after the voltage of the full-wave rectification waveform becomes higher, the capacitor 125 is discharged to illuminate the LED row 123.

Description

  The present invention relates to an LED drive circuit including an LED array in which a plurality of LEDs are connected in series as a light source. More specifically, the number of LEDs included in the LED array is reduced, and the LED array is formed at a low voltage phase of a full-wave rectified waveform. The present invention relates to an LED drive circuit that does not apply a high voltage to this LED row in a high voltage phase while being lit.

  There is known a drive circuit that turns on an LED (also referred to as a light emitting diode) with a full-wave rectified waveform obtained by full-wave rectification of a commercial power supply or a voltage waveform close to the full-wave rectified waveform (hereinafter referred to as an LED drive circuit). In order to apply a full-wave rectified waveform directly to this LED, a large number of LEDs must be connected in series so that the LED can withstand a high voltage (hereinafter, a plurality of LEDs connected in series is referred to as an LED array). . For example, if the effective value of the commercial power supply is 100 V, the number of LED rows in series is often about 40.

  However, in the case of a small lighting device, there are cases where a large number of LEDs cannot be connected in series. There is a method of adding a current limiting element having a large voltage drop amount in series with the LED string in order to reduce the number of LED strings in series, but in this case, power loss due to the current limiting element becomes large, which is not preferable. Therefore, in order to efficiently drive the LED string with a small number of series stages, an LED drive circuit is known in which current is passed through the LED string only during the low voltage phase of the full-wave rectified waveform, and current is not passed through the LED string during the high voltage phase. (For example, Patent Document 1).

  In the AC LED lighting circuit (LED drive circuit) shown in FIG. 1 of Patent Document 1, LED 1 (LED array) is connected to an AC power source 3 via a rectifier diode 2 and a semiconductor switch 4. The LED 1 has a current flowing through the LED 1 only during an AC half-wave (one cycle of the full-wave rectification waveform) equal to or less than an arbitrary current determined by the semiconductor switch 4, and is lit twice in one AC half-wave.

  The operation of the circuit shown in FIG. 1 of Patent Document 1 will be described. Initially, the LED 1 is lit while the voltage of the full-wave rectified waveform is low. When the voltage of the full-wave rectified waveform rises and the current flowing through the semiconductor switch 4 reaches a predetermined value, the transistors 9 and 10 having a thyristor configuration become conductive and the semiconductor switch 4 is cut off. Further, even if the voltage of the full-wave rectified waveform rises, the LED 1 hardly illuminates because only a minute current determined by the resistor 8 flows. Thereafter, when the voltage of the full-wave rectified waveform starts to decrease and the emitter voltage of the transistor 9 decreases, the transistors 9 and 10 become non-conductive because no current flows. At this time, since the semiconductor switch 4 is turned on, the LED 1 is turned on again.

JP-A-8-148721 (FIG. 1)

  However, the LED driving circuit for lighting the LED string at a low voltage phase as shown in Patent Document 1 is in a non-lighting state when the voltage of the full-wave rectified waveform is high. As the non-lighting period becomes longer, motion breaks and flickers in which an object moving at high speed appears to be noticeable become more conspicuous.

Therefore, the present invention has been made in view of the above problems, and includes an LED array in which a plurality of LEDs are connected in series as a light source, and the number of LEDs included in the LED array is reduced to reduce the voltage of the full-wave rectified waveform. In an LED drive circuit that does not apply a high voltage to the LED string in the high voltage phase while lighting the LED string in the voltage phase, the purpose is to eliminate or shorten the non-lighting period to make the motion break and flicker inconspicuous. To do.

An LED drive circuit according to the present invention includes an LED array in which a plurality of LEDs are connected in series as a light source, and applies the voltage of the full-wave rectified waveform to the LED array when the full-wave rectified waveform is in a low voltage phase. In
A voltage detection circuit for measuring the voltage of the full-wave rectified waveform;
A switch controlled to conduct by the voltage detection circuit when the full-wave rectified waveform is in a low voltage phase;
A light emitting circuit connected in series with the switch,
The light emitting circuit includes the LED string and a capacitor, and the LED string and the capacitor are connected in parallel.
The voltage of the full wave rectified waveform is applied to a series circuit including the switch and the light emitting circuit.

(Function)
The voltage detection circuit measures the voltage of the full-wave rectified waveform and conducts the switch only during the period when the voltage of the full-wave rectified waveform is low. Since the switch is connected in series with the light emitting circuit, a current flows into the light emitting circuit when the switch is turned on. Inside the light emitting circuit, an LED array and a current limiting circuit are connected in series, and a capacitor is connected in parallel with the series circuit. As a result, during the period when the voltage of the full-wave rectified waveform is low and the switch is on, current flows into the light emitting circuit and the LED string is lit. At the same time, charge the capacitor. During the period when the voltage of the full-wave rectified waveform is high and the switch is non-conductive, the current does not flow into the light emitting circuit, but the capacitor is discharged and the LED string is lit. In this manner, the LED is lit even in the high voltage phase of the full-wave rectified waveform that has been in the non-lighting period so far, and the non-lighting period is eliminated or shortened.

  It is preferable to provide a diode for preventing backflow at the current input terminal of the light emitting circuit.

  The discharge time constant for the capacitor may be longer than the charge time constant.

  A parallel circuit including a resistor or a constant current element and a diode may be connected to one end of the capacitor.

  The LED row may be divided into a plurality, and a bypass circuit may be provided at a connection portion of the plurality of divided LED rows, and the bypass circuit may control the bypass current according to the current flowing through the LED row.

  As described above, the LED drive circuit of the present invention includes an LED string in which a plurality of LEDs are connected in series as a light source, and even if a voltage corresponding to a low voltage phase of a full-wave rectified waveform is applied to the LED string, a non-lighting period is obtained. It can be lost or shortened to make motion breaks and flicker less noticeable.

The block diagram which shows the LED drive circuit of 1st Embodiment of this invention. The circuit diagram of the LED drive circuit shown in FIG. The wave form diagram for demonstrating operation | movement of the LED drive circuit shown in FIG. The block diagram which shows the LED drive circuit of 2nd Embodiment of this invention. FIG. 5 is a circuit diagram of a light emitting circuit included in the LED drive circuit shown in FIG. 4. FIG. 5 is a waveform diagram for explaining the operation of the LED drive circuit shown in FIG. 4. The block diagram which shows the LED drive circuit of 3rd Embodiment of this invention. FIG. 8 is a circuit diagram of a light emitting circuit included in the LED drive circuit shown in FIG. 7. The wave form diagram for demonstrating operation | movement of the LED drive circuit shown in FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same or equivalent elements will be denoted by the same reference numerals, and redundant description will be omitted. For the sake of explanation, the partial scales of waveforms and the like are changed as appropriate. Furthermore, the relationship with the invention specific matter described in the claims is described in parentheses.
(First embodiment)

  The LED driving circuit 100 according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing the LED drive circuit 100, where (a) shows a case where the switch 130 is in a conductive state and (b) shows a case where the switch 130 is in a non-conductive state. The LED drive circuit 100 includes a full-wave rectified voltage source 105, a voltage detection circuit 110, a light emitting circuit 120, and a switch 130.

  The full-wave rectified voltage source 105 is connected to the + side terminal 111 of the voltage detection circuit 110 and the current input terminal 121 of the light emitting circuit 120 and applies a voltage having a full-wave rectified waveform. The voltage detection circuit 110 measures the voltage of the full-wave rectified waveform, makes the switch 130 conductive during the period when the voltage of the full-wave rectified waveform is low via the control output terminal 113 (a), and the voltage of the full-wave rectified waveform is high. The switch 130 is turned off during the period (b). The light emitting circuit 120 and the switch 130 are connected in series. In the example of this figure, the current output terminal 122 of the light emitting circuit 120 is connected to the upper end of the switch 130. The negative side terminal 112 of the voltage detection circuit 110 and the lower end of the switch 130 are connected to the lower end of the full-wave rectified voltage source 105 (terminal to which current returns).

  The light emitting circuit 120 includes an LED string 123 in which a plurality of LEDs 123a are connected in series, a current limiting circuit 124, and a capacitor 125. The LED string 123 and the current limiting circuit 124 are connected in series from the current input terminal 121 to the current output terminal 122. A capacitor 125 is connected in parallel with the series circuit.

  As shown in (a), when the switch 130 is conductive, if the voltage of the full-wave rectified waveform is higher than the threshold value of the LED string 123, the current I is the LED string 123, the current limiting circuit 124, and the switch 130. Flowing. At this time, the capacitor 125 is charged, and the voltage between both ends thereof becomes equal to the voltage applied to the series circuit composed of the LED string 123 and the current limiting circuit 124. When the voltage of the full-wave rectified waveform is lower than the threshold value of the LED string 123, the current I does not flow through the LED string 123.

  As shown in (b), when the switch 130 is non-conductive, current cannot flow from the current input terminal 121. At this time, the capacitor 125 starts discharging, and the current I flows through the LED string 123 until the voltage across the capacitor 125 reaches the threshold value of the LED string 123. When the switch 130 is non-conductive, the voltage applied to the series circuit composed of the LED string 123 and the current limiting circuit 124 is not the high-voltage phase voltage of the full-wave rectified waveform but the voltage across the capacitor 125.

Next, the LED drive circuit 100 of FIG. 1 will be described in more detail with reference to FIG. FIG. 2 is a circuit diagram of the LED driving circuit 100 shown in FIG. The full-wave rectified voltage source 105 (see FIG. 1) includes a bridge rectifier circuit 105a and a commercial power source 105b. The commercial power source 105b is connected to the AC input terminal of the bridge rectifier circuit 105a. The bridge rectifier circuit 105a includes four diodes 105c, and a terminal A and a terminal B are a current output terminal and a current input terminal for full-wave rectification, respectively.

  In the voltage detection circuit 110, the + side terminal 111 is a connection portion between the upper end of the resistor 110a and the upper end of the resistor 110c, and is connected to the terminal A and further connected to the current input terminal 121 of the light emitting circuit 120. The negative side terminal 112 is a connection portion between the lower end of the resistor 110b and the emitter of the transistor 110d, and is connected to the terminal B and the emitter of the transistor 130a (corresponding to the switch 130 in FIG. 1). Further, in the voltage detection circuit 110, the base of the transistor 110d is connected to the connection portion of the resistors 110a and 110b. Further, a connection portion (corresponding to the control output terminal 113 in FIG. 1) between the resistor 110c and the collector of the transistor 110d is connected to the base of the transistor 130a.

  The current input terminal 121 of the light emitting circuit 120 is connected to the terminal A, and the current output terminal 122 is connected to the collector of the transistor 130a. The light emitting circuit 120 includes a capacitor 125, an LED string 123 in which a plurality of LEDs 123a are connected in series, and a resistor 124a. The LED string 123 and the resistor 124a are connected in series from the current input terminal 121 to the current output terminal 122. Yes. A capacitor 125 is connected in parallel with a series circuit composed of the LED string 123 and the resistor 124a. The resistor 124a corresponds to the current limiting circuit 124 of FIG. 1, and the current limiting circuit 124 may be a constant current circuit or a constant current diode in addition to the resistor 124a.

  Next, the operation of the LED drive circuit 100 shown in FIGS. 1 and 2 will be described with reference to FIG. 3A and 3B are waveform diagrams showing the waveform of the current I flowing through the LED array 123 in FIG. 1A and FIG. Note that (a) and (b) have the same time axis. The full-wave rectified waveform is based on the terminal B (the same applies hereinafter). Further, the numbers of parts and terminals are referred to without referring to FIG. 1 and FIG.

  (A) has shown one period of the full wave rectification waveform. As will be described later, the voltage at the terminal A is transformed from the full-wave rectified waveform by the capacitor 125 during a period when the voltage of the full-wave rectified waveform is low. Therefore, an ideal full-wave rectified waveform is shown in FIG. In (b), one period is a period t1 in which the voltage of the full-wave rectified waveform is smaller than the threshold value of the LED string 123, and a period t2 until the voltage of the full-wave rectified waveform exceeds the threshold value of the LED string 123 and the switch 130 becomes non-conductive. The period 130 when the switch 130 is non-conductive, the period t4 when the voltage of the full-wave rectified waveform drops to the threshold value of the LED string 123 after the switch 130 is turned on again, and the voltage of the full-wave rectified waveform is less than the threshold value of the LED string 123. The period was divided into t5.

  In the period t1, the transistor 130a is in a conductive state, but the current I hardly flows through the LED string 123 because the voltage of the full-wave rectified waveform does not reach the threshold value of the LED string 123. Note that, as will be described later, there may be a current I due to the discharge of the capacitor 125, so that the current I is shown to be small in the period t1.

  Since the voltage of the full-wave rectified waveform exceeds the threshold value of the LED array 123 in the period t2, the current I increases as the voltage of the full-wave rectified waveform increases. The current I is limited by a current limiting resistor 124a. In the period t2, the capacitor 125 is charged.

At time t6, the voltage at the connection portion between the resistor 110a and the resistor 110b becomes 0.6V, and the transistor 110d is turned on. As a result, the transistor 130a is turned off, and current cannot flow from the current input terminal 121 of the light emitting circuit 120. At this time, the capacitor 125 starts discharging, and the current I flows until the voltage across the capacitor 125 becomes equal to the threshold value of the LED string 123 in the period t3. In the figure, the current I stops flowing in about half of the period t3.

  At time t7, the voltage at the connection portion between the resistor 110a and the resistor 110b is less than 0.6 V, the transistor 110d is turned off, and the transistor 130a is turned on. As a result, the current I starts to flow again through the light emitting circuit 120. In the period t4, the voltage of the full-wave rectified waveform is much higher than the threshold voltage of the LED string 123. Therefore, after the current I suddenly increases from time t7, the current I also decreases as the voltage of the full-wave rectified waveform decreases. Note that the discharge of the capacitor 125 with respect to the current I can be ignored in the period t4.

  When the voltage of the full-wave rectified waveform becomes near the threshold value of the LED string 123 at the first timing of the period t5, the current I flowing through the LED string 123 decreases. In the period t5, the voltage drop of the capacitor 125 becomes slower than the voltage drop of the full-wave rectified waveform. As a result, the current I slightly flows through the LED string 123 in the period t5. Note that since the voltage detection circuit 110 also serves as a discharge path in the period t5, the time constant is smaller than that in the period t3. When it is desired to eliminate the discharge path by the voltage detection circuit 110, a backflow prevention diode may be inserted into the current input terminal 121. In this way, even when the voltage of the full-wave rectified waveform falls below the threshold value of the LED string 123 from the period t5 to the period t1, the period during which the LED string 123 is not lit can be eliminated or shortened.

In the light emitting circuit 120, for example, the number of stages of the LED string 123 and the periods t2 and t4 are determined, and then the average current in the periods t2 and t4 is set to 60 mA, for example, and the average current in the period t3 is set to 20 mA. The value of is determined. As described above, when the effective value of the commercial power supply 105b is 100V, in many lighting devices, the number of series of LED strings is about 40, whereas the low voltage phase of the full-wave rectified waveform as in this embodiment. When there is a main light emission period, the number of LED rows 123 in series can be made about 10 to 20.
(Second Embodiment)

  In the LED drive circuit 100 according to the first embodiment, when the transistor 130a is turned off, the capacitor 125 is rapidly discharged, so that only about half of the high voltage phase (period t3) of the full-wave rectified waveform is lit (FIG. 3). reference). Motion breaks and flicker may become less noticeable if the lighting period is long. 4 to 6 show an LED drive circuit 400 capable of controlling the discharge time as a second embodiment of the present invention.

  FIG. 4 is a block diagram showing the LED drive circuit 400. In FIG. 4, only the case where the switch 130 is non-conductive is shown and simplified as compared with FIG. The LED drive circuit 400 includes a full-wave rectified voltage source 105, a voltage detection circuit 110, a light emitting circuit 420, and a switch 130 as in the LED drive circuit 100 of FIG. The full-wave rectified voltage source 105, the voltage detection circuit 110, and the switch 130 are the same as those shown in FIG.

  The light emitting circuit 420 includes a diode 426 and a resistor 427 in addition to an LED array 423 in which a plurality of LEDs 423a are connected in series, a current limiting circuit 424, and a capacitor 425. Similar to the light emitting circuit 120 in FIG. 1, the LED array 423 and the current limiting circuit 424 are connected in series from the current input terminal 421 toward the current output terminal 422. On the other hand, in this embodiment, a parallel circuit composed of a diode 426 and a resistor 427 is connected to a capacitor 425, and a parallel series circuit composed of a diode 426, a resistor 427 and a capacitor 425 is a series composed of an LED array 423 and a current limiting circuit 424. Connected in parallel with the circuit.

When the switch 130 is conductive, the current I flows through the LED string 423, the current limiting circuit 424, and the switch 130. At this time, the capacitor 425 is rapidly charged via the diode 426, and the voltage between both ends thereof becomes substantially equal to the voltage applied to the series circuit composed of the LED string 423 and the current limiting circuit 424. It is assumed that the voltage of the full-wave rectified waveform is higher than the threshold value of the LED array 423. When the voltage of the full-wave rectified waveform is lower than the threshold value of the LED string 423, the current I does not flow through the LED string 423.

  When the switch 130 is non-conductive, the current I cannot flow from the current input terminal 421. At this time, the capacitor 425 starts discharging. Unlike FIG. 1, in this embodiment, the current I flows from the terminal on the left side of the capacitor 425 through the resistor 427 through the LED array 423 and the current limiting circuit 424. Therefore, the value of the current I is reduced while the time constant is increased. Become. The discharge continues until the voltage across the capacitor 425 reaches the threshold value of the LED string 423. Since the resistor 427 limits the current I, it may be a constant current circuit or a constant current diode.

  Next, the LED drive circuit 400 of FIG. 4 will be described more specifically with reference to FIG. FIG. 5 is a circuit diagram of the light emitting circuit 420 included in the LED driving circuit 400 shown in FIG. In this embodiment, the full-wave rectified voltage source 105 (bridge rectifier circuit 105a and commercial power supply 105b), voltage detection circuit 110, and switch 130 (transistor 130a) of the first embodiment shown in FIGS. In the LED driving circuit 400, only the light emitting circuit 420 is shown, and description of other parts is omitted. In FIG. 5, the current limiting circuit 424 is merely a resistor 424 a specifically with respect to the light emitting circuit 420 shown in FIG. 4. The resistor 424a may be a constant current circuit or a constant current diode.

  Next, the operation of the LED drive circuit 400 will be described in more detail with reference to FIG. 6 is a waveform diagram for explaining the operation of the LED drive circuit 400 of FIG. 4 including the light emitting circuit 420 shown in FIG. 5, where (a) is a full-wave rectified waveform, and (b) is a current flowing through the LED array 423. 2 is a waveform diagram showing a waveform of I. FIG. Note that (a) and (b) have the same time axis, and the voltage of the full-wave rectified waveform in (a) is based on the terminal B (see FIG. 1). Further, the numbers of parts and terminals are referred to without referring to FIG. 4 and FIG.

  (A) has shown one period of the full wave rectification waveform. As will be described later, since the voltage at the terminal A is deformed on the low voltage side by the capacitor 425, an ideal full-wave rectified waveform is shown in FIG. In (b), one period is a period t1 in which the voltage of the full-wave rectified waveform is smaller than the threshold value of the LED string 423, and a period t2 until the voltage of the full-wave rectified waveform exceeds the threshold value of the LED string 423 and the switch 130 becomes non-conductive. The period 130 when the switch 130 is non-conductive, the period t4 when the voltage of the full-wave rectified waveform drops to the threshold value of the LED string 423 after the switch 130 is turned on again, and the voltage of the full-wave rectified waveform is less than the threshold value of the LED string 423. The period was divided into t5.

  In the period t1, the switch 130 is in a conductive state. However, since the voltage of the full-wave rectified waveform does not reach the threshold value of the LED array 423, no current flows from the current input terminal 421. On the contrary, since the capacitor 425 is discharged slowly, the capacitor is detected from the current input terminal 421 via current detection. 425 continues to discharge. Further, compared with the period t1 in FIG. 3B, the current I is slightly larger in the period t1 in this figure because the discharge is slow.

  Since the voltage of the full-wave rectified waveform exceeds the threshold value of the LED array 423 in the period t2, the current I increases as the voltage of the full-wave rectified waveform increases. Note that the operation is substantially the same as the period t2 in FIG. During this time, the current I is limited by the current limiting resistor 424a, and the capacitor 435 is charged.

At time t <b> 6, the switch 130 is turned off, and current does not flow from the current input terminal 421 of the light emitting circuit 420. At this time, the capacitor 425 starts discharging, and the current I flows until the voltage across the capacitor 425 becomes equal to the threshold value of the LED array 423. Compared with the period t3 in FIG. 3B, the current I in this figure is smaller than the current I in FIG. 3 in the first half of the period t3. I continues to flow. The capacitor 125 (see FIG. 1), the capacitor 425, the resistor 124a (see FIG. 1), and the resistor 424a are the same. Thus, in this embodiment, the LED string 423 continues to be lit even in the high voltage phase (period t3) of the full-wave rectified waveform.

  At time t7, the voltage of the full-wave rectified waveform decreases, the switch 130 is turned on again, and current starts to flow from the current input terminal 421 of the light emitting circuit 420. Similar to the period t4 in FIG. 3B, also in the period t4 in the figure, after the current I suddenly increases from the time t7, the current I also decreases as the voltage of the full-wave rectified waveform decreases. Note that the discharge of the capacitor 425 can be ignored for the current I in the period t4.

When the voltage of the full-wave rectified waveform becomes near the threshold value of the LED string 423 at the first timing of the period t5, the current I flowing through the LED string 423 is only due to the discharge of the capacitor 435. This current I is initially smaller than the current I in the period t5 in FIG. 3B, but gradually decreases. Note that in the case where the discharge is effectively used from the period t5 to the period t1, it is preferable to insert a backflow prevention diode as in the description of the period t5 in FIG.
(Third embodiment)

  In the first and second embodiments, it is an object to maintain lighting of the LED strings 123 and 423 in the high voltage phase (period t3) of the full-wave rectified waveform. For this reason, it is pointed out that there is a period in which lighting on the circuit is maintained for maintaining the lighting of the LED strings 123 and 423 in a period (periods t1 and t5) in which the voltage of the full-wave rectified waveform is lower than the threshold voltage of the LED strings 123 and 423. It only suggests that if the backflow prevention diode is inserted, the lighting maintenance is improved during this period t1 and t5. In the first and second embodiments, the LED rows 123 and 423 are one series circuit. However, the LED rows 123 and 423 may be brightened by dividing the LED rows 123 and 423 into two. Accordingly, an LED drive circuit 700 in which the backflow prevention diode is inserted and the LED array is divided into two parts will be described as a third embodiment with reference to FIGS.

  FIG. 7 is a block diagram showing the LED driving circuit 700. 7 also shows only the case where the switch 130 is non-conductive with respect to FIG. The LED drive circuit 700 includes a full-wave rectified voltage source 105, a voltage detection circuit 110, a light emitting circuit 720, and a switch 130, as in the LED drive circuits 100 and 400 of FIGS. The full-wave rectified voltage source 105, the voltage detection circuit 110, and the switch 130 are the same as those shown in FIG. 1 and FIG.

  The light emitting circuit 720 includes an LED array 723 in which a plurality of LEDs 723a are connected in series, a current limiting circuit 724, and a capacitor 725, a backflow prevention diode 729, an LED array 726 in which a plurality of LEDs 726a are connected in series, and a bypass circuit 727. Yes. The current input terminal 721 and the anode of the backflow prevention diode 729 are connected, and the LED string 723, the LED string 726, and the current limiting circuit 724 are connected in series from the cathode of the diode 729 toward the current output terminal 722. The bypass circuit 727 is connected so that current flows from the connection part of the LED string 723 and the LED string 726 and current also flows from the current limiting circuit 724, and a terminal for outputting these currents is the current of the light emitting circuit 720. The output terminal 722 is connected. The capacitor 725 has one end connected to the cathode of the diode 729 and the other end connected to the current output terminal 722 of the light emitting circuit 720. That is, the capacitor 725 is connected in parallel with a circuit including the LED strings 723 and 726, the current limiting circuit 724, and the bypass circuit 727.

If the switch 130 is turned on and the voltage of the full-wave rectified waveform is higher than the threshold value of the LED string 723, the current I flows from the current input terminal 721 through the current output terminal 722. At this time, when the voltage of the full-wave rectified waveform is higher than the threshold value of the LED string 723 and smaller than the sum of the threshold value of the LED string 723 and the threshold value of the LED string 726, the current I passes through the bypass circuit 727 without passing through the LED string 726. . The bypass circuit 727 monitors the inflowing current, and when the voltage of the full-wave rectified waveform becomes larger than the sum of the threshold value of the LED string 723 and the threshold value of the LED string 726 and the current I exceeds a predetermined value, all the currents I To flow through the LED string 726. At this time, the capacitor 725 is charged.

  When the voltage of the full-wave rectified waveform further increases and the switch 130 becomes non-conductive, current cannot flow from the current input terminal 721, and the capacitor 725 starts discharging. When the voltage of the full-wave rectified waveform decreases and the switch 130 is turned on again, the voltage of the full-wave rectified waveform is increased from the highest voltage to the vicinity of the threshold value of the LED array 723 in the reverse process of when the voltage of the full-wave rectified waveform increases. The current I decreases. Further, when the voltage of the full-wave rectified waveform decreases and becomes less than the threshold value of the LED array 723, the terminal voltage on the left side of the capacitor 725 becomes higher than the voltage of the full-wave rectified waveform, so that the diode 729 is cut off. Thereafter, the discharge current of the capacitor 725 goes to the bypass circuit 727 through the LED array 723.

  Next, the LED drive circuit 700 of FIG. 7 will be described more specifically with reference to FIG. FIG. 8 is a circuit diagram of a light emitting circuit 720 included in the LED driving circuit 700 shown in FIG. In this embodiment, the full-wave rectified voltage source 105 (bridge rectifier circuit 105a and commercial power supply 105b), voltage detection circuit 110, and switch 130 (transistor 130a) of the first embodiment shown in FIGS. In the LED driving circuit 700, only the light emitting circuit 720 is shown, and description of other parts is omitted. Compared with the light emitting circuit 720 shown in FIG. 7, the bypass circuit 727 and the current limiting circuit 724 are specifically shown in FIG.

  In the bypass circuit 727, the upper end of the resistor 727 a and the drain of the FET 727 b are connected to the connection portion between the LED string 723 and the LED string 726. The emitter of the transistor 727c and the lower terminal of the resistor 727d are connected to the current output terminal 722. Further, the connection portion of the source of the FET 727b, the base of the transistor 727c, and the upper end of the resistor 727d is connected to the emitter of the transistor 724c included in the current limiting circuit 724 and the lower end of the resistor 724d. Note that this connection portion is an addition portion for the bypass current and the current flowing in from the current limiting circuit 724 (shown by + in white circles in FIG. 7). Inside the bypass circuit 727, the lower end of the resistor 727a, the gate of the FET 727b, and the collector of the transistor 727c are connected.

  In the current limiting circuit 724, the upper end of the resistor 724a and the drain of the FET 724b are connected to the cathode of the LED string 726. Further, in the current limiting circuit 724, there are a connection portion between the source of the FET 724b, the base of the transistor 724c and the upper end of the resistor 724d, the lower end of the resistor 724a, the gate of the FET 724b and the collector of the transistor 724c.

  Next, the operation of the LED drive circuit 700 will be described in more detail with reference to FIG. FIG. 9 is a waveform diagram for explaining the operation of the LED drive circuit 700 of FIG. 7 including the light emitting circuit 720 shown in FIG. 8, where (a) is a full-wave rectified waveform, and (b) is a current flowing through the LED array 723. It is a wave form diagram which shows the waveform of I. FIG. Note that (a) and (b) have the same time axis, and the full-wave rectified waveform in (a) is based on the terminal B (see FIG. 1). Also, reference is made to component and terminal numbers without instruction to FIGS.

(A) has shown one period of the full wave rectification waveform. In this embodiment, since the diode 729 for preventing backflow is provided, the voltage waveform at the terminal A becomes a full-wave rectified waveform. In (b), one period is a period t8 in which the voltage of the full-wave rectified waveform is smaller than the threshold value of the LED string 723, and a period t9 until the voltage of the full-wave rectified waveform exceeds the threshold value of the LED string 723 and the switch 130 becomes non-conductive. The period 130 when the switch 130 is non-conducting, the period t10 when the voltage of the switch 130 is turned on again and the voltage of the full-wave rectified waveform falls to the threshold value of the LED string 723, and the voltage of the full-wave rectified waveform becomes less than the threshold value of the LED string 723. The period was divided into t11.

  In the period t8, the switch 130 is in a conductive state. However, since the voltage of the full-wave rectified waveform has not reached the threshold value of the LED array 723, no current flows from the current input terminal 721 to the light emitting circuit 720. Note that the discharge current of the capacitor 725 does not go to the current input terminal 721 due to the backflow prevention diode 729, and the threshold voltage of the LED string 723 is lower than the threshold voltage of the LED strings 123 and 423, so the period t8 is shown in FIGS. Due to being shorter than the shown period t1, the current I in the period t8 is larger than the current I in the period t1 in FIGS.

  In the period t9, the voltage of the full-wave rectified waveform exceeds the threshold value of the LED array 723, so that the current I increases as the voltage of the full-wave rectified waveform increases. Note that the current I increases rapidly at the beginning of the period t9, but a period during which the current I becomes flat appears. This is because the bypass circuit 727 operates at a constant current so as to keep the base voltage of the transistor 727c at 0.6 V in a specific current region. In the latter half of this period, the voltage of the full-wave rectified waveform exceeds the sum of the threshold value of the LED string 723 and the threshold value of the LED string 726, and current flows through the LED string 726. At this time, the sum of the bypass current and the current flowing through the LED array 726 (current I) becomes constant. Further, when the voltage of the full-wave rectified waveform rises, the transistor 727c is saturated, the bypass current disappears, and the current I rapidly rises again. Thereafter, the current limit circuit 724 determines the upper limit value of the current I. During this time, the capacitor 725 is charged.

  When the switch 130 is turned off at time t6, no current flows from the current input terminal 721 of the light emitting circuit 720, and the capacitor 725 starts discharging. Immediately after time t6, the current I becomes constant by the current limiting circuit 724, and then a discharge curve is drawn. Further, the current I becomes constant because the bypass circuit 727 operates at a constant value in the middle, and when the discharge progresses and the discharge current decreases, the discharge curve until the voltage across the capacitor 725 becomes equal to the threshold value of the LED array 723. Draw.

  At time t7, the voltage of the full-wave rectified waveform decreases, the switch 130 is turned on again, and current starts to flow from the current input terminal 721 of the light emitting circuit 720. In the period t10, the current I rapidly increases from the time t7, and then the current I decreases in the reverse process of the period t9. Note that the discharge of the capacitor 725 can be ignored for the current I in the period t10.

  When the voltage of the full-wave rectified waveform becomes lower than the threshold value of the LED string 723 in the period t11, the current I flowing through the LED string 723 is only due to the discharge of the capacitor 735. For the same reason as in the period t8, the current I in the period t11 is larger than the current I in the period t5 shown in FIGS. 3 and 6B.

  As described above, the present embodiment improves the lighting state during the period in which the voltage of the full-wave rectified waveform is lower than the threshold value of the LED array 723. That is, the LED drive circuit 700 according to the present embodiment has an LED string (an LED string 723 and an LED string 726 LED from the time when the voltage of the full-wave rectified waveform is lower than that of the LED drive circuits 100 and 400 according to the first and second embodiments. Since part of the (column) (LED column 723) starts to light, if the LED column has the same number of series stages, it becomes bright. Further, since the backflow prevention diode 729 is provided, all of the discharge current flows to the LED array 723 in the periods t8 and t11, so that it becomes bright.

In the present embodiment, the current limiting circuit 724 is a constant current circuit, but may be a resistor or a constant current diode. The discharge current can be used even if the low-voltage side terminal of the capacitor 725 is connected to the output side of the current limiting circuit 724. However, the lower limit of the discharge voltage is more connected to the output side of the bypass circuit 727 as in this embodiment. Since it becomes low, efficiency is good. The switch 130 may be located immediately before the current input terminal 721.

100, 400, 700 ... LED drive circuit,
105: full-wave rectified voltage source,
105a ... Bridge rectifier circuit,
105b ... commercial power supply,
105c, 426, 729 ... diode,
110: Voltage detection circuit,
110a, 110b, 110c, 124a, 424a, 427, 727a, 727d, 724a, 724d ... resistors,
110d, 130a, 727c, 724c ... transistors,
111 ... + side terminal,
112 ...-side terminal,
113 ... Control output terminal,
120, 420, 720 ... light emitting circuit,
121, 421, 721 ... current input terminals;
122,422,722 ... switch,
123, 423, 723, 726 ... LED row,
123a, 423a, 723a, 726a ... LED,
124, 424, 724 ... current limiting circuit,
125, 425, 725 ... capacitor,
130 ... switch,
724b, 727b ... FET,
727 ... Bypass circuit,
t1 to t5, t8 to t11 ... period,
t6, t7 ... time.

Claims (5)

In an LED drive circuit comprising an LED string in which a plurality of LEDs are connected in series as a light source, and applying a voltage of the full-wave rectified waveform to the LED string when the full-wave rectified waveform is in a low voltage phase,
A voltage detection circuit for measuring the voltage of the full-wave rectified waveform;
A switch controlled to conduct by the voltage detection circuit when the full-wave rectified waveform is in a low voltage phase;
A light emitting circuit connected in series with the switch,
The light emitting circuit has the LED row and a capacitor, and the LED and the capacitor are connected in parallel,
An LED driving circuit, wherein a voltage of the full-wave rectified waveform is applied to a series circuit including the switch and the light emitting circuit.   The LED driving circuit according to claim 1, further comprising a backflow preventing diode at a current input terminal of the light emitting circuit.   The LED driving circuit according to claim 1, wherein a discharge time constant to the capacitor is longer than a charge time constant.   4. The LED driving circuit according to claim 3, wherein a parallel circuit including a resistor or a constant current element and a diode is connected to one end of the capacitor. The LED array is divided into a plurality of parts, and a bypass circuit is provided at a connection portion of the LED arrays divided into the plurality, and the bypass circuit controls a bypass current according to a current flowing through the LED array. The LED drive circuit according to any one of 1 to 4.

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