JP5893873B2 - LED drive circuit - Google Patents

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.

  2. Description of the Related Art An LED drive circuit that lights an LED (also referred to as a light emitting diode) with a pulsating current obtained by full-wave rectification of a commercial AC power supply or a voltage waveform close to the pulsating current is known. The LED driven by this LED drive circuit has a configuration in which a plurality of LEDs are connected in series so as to withstand a high voltage (hereinafter referred to as an LED array). This LED string has a threshold value, and when the threshold value is exceeded, a current flows through the LED string and lights up. Therefore, if the effective value of the commercial power supply is 100V, the threshold value is set to about 100 to 120V. Individual LEDs have a threshold value called a forward voltage Vf, and when a voltage higher than the forward voltage Vf is applied, a current flows and lights up. Since the threshold value of the LED string is the sum of the forward voltage Vf of each LED included in the LED string, for example, when the forward voltage is 3 V and the effective value of the commercial power supply is 100, the number of series stages of the LED string is 40. To a degree.

  When a pulsating voltage is simply applied to the LED string, the LED string is lit only during a period when the pulsating voltage exceeds the threshold voltage. For this reason, not only does it become dark and flickering becomes conspicuous, but the power factor and distortion rate also deteriorate. If the number of LED rows in series is reduced in order to shorten the non-lighting period, the power loss of the current limiting circuit inserted in series with the LED rows increases, which is not preferable. In view of this, there has been an attempt to solve the above-described problem by switching the number of series of LED strings that are turned on in accordance with the voltage applied to the LED string (for example, Patent Document 1).

  In FIG. 1 of Patent Document 1, the light-emitting diode circuit 15 (LED array) is divided into six diode circuits 17 to 22, and the drive switches 30 to 35 are switched based on the pulsating voltage, and the light-emitting diode 11 that is lit is turned on. A light-emitting diode lighting device (LED driving circuit) for adjusting the number (the number of series stages) is shown. However, in the LED driving circuit that switches the current path based on the pulsating voltage as in Patent Document 1, the current flowing through the LED array is greatly reduced or greatly increased at the moment when the path is switched. That is, the current value becomes discontinuous, causing various problems such as an increase in harmonic noise.

  On the other hand, there is an LED drive circuit in which the current value changes smoothly when the number of series stages is switched (for example, Patent Document 2). The LED drive circuit shown in FIG. 26 of Patent Document 2 measures the current flowing through the LED string, and when the current exceeds a predetermined value, the number of series stages in the LED string increases and at the same time the current continuously increases. .

  The circuit of FIG. 26 of Patent Document 2 will be described with reference to FIGS. FIG. 4 is a circuit diagram of the LED drive circuit, and for the sake of explanation, the circuit of FIG. 26 in Patent Document 2 is modified without departing from the spirit of the LED drive circuit. 5A is a waveform diagram showing a pulsating voltage V applied to the LED array in the circuit of FIG. 4, and FIG. 5B is a waveform diagram showing a current I flowing in the LED drive circuit 400. In FIG. 5, the time axes t of (a) and (b) coincide.

First, the circuit diagram of FIG. 4 will be described. The LED drive circuit 400 includes a bridge rectifier circuit 405, a bypass circuit 420, and a current limiting resistor 433 along with the LED strings 410 and 430. The commercial power source 406 is connected to the AC input terminal of the bridge rectifier circuit 405. The bridge rectifier circuit includes diodes 401, 402, 403, and 404, and terminal A and terminal B are a current output terminal and a current input terminal, respectively. The terminal A is connected to the anode of the LED array 410, and the terminal B is connected to the negative terminal 426 of the bypass circuit 420. The cathode of the LED array 410 is connected to the + side terminal 425 of the bypass circuit 420 and the anode of the LED array 430. The cathode of the LED array 430 is connected to the upper end of the current limiting resistor 433, and the lower end of the resistor 433 is connected to the current detection terminal 427 of the bypass circuit 420.

  In the bypass circuit 420, the + side terminal 425 is a connection portion between the upper end of the resistor 421 included in the bypass circuit 420 and the drain of the n-MOS field effect transistor 422 (hereinafter referred to as FET). The − side connection terminal 426 is a connection portion between the emitter of the NPN bipolar transistor 423 (hereinafter referred to as a transistor) included in the bypass circuit 420 and the lower end of the resistor 424. The current detection terminal 427 is a connection portion between the source of the FET 422, the base of the transistor, and the upper end of the resistor 424 in the bypass circuit. In the bypass circuit 420, the resistor 421, the collector of the transistor 423, and the gate of the FET 422 are connected.

  When the effective value of the commercial power source 406 is 100 V, the total number of LEDs 411, 412, 431, 432 included in the LED rows 410, 430 is set to about 40. When the number of serial stages of the LED array composed of the LED array 410 and the LED array 430 is 40, for example, the number of serial stages of the LED array 410 and the LED array 430 is 20, respectively.

  Next, the operation of the circuit of FIG. 4 will be described with reference to FIG. When the commercial power source 406 is rectified by the bridge rectifier circuit 405, a sine wave pulsating voltage V appears at the terminal A. Among them, (a) shows the pulsating voltage V for one cycle. At this time, the current I includes a period t6 in which no current flows, a period t7 in which the current increases rapidly, a period t8 in which the current becomes constant, and a period t9 in which the current changes in a sine wave shape.

  In the period t6, the current I does not flow because the pulsating voltage V is below the threshold value of the LED array 410. In the period t7, the pulsating voltage V exceeds the threshold value of the LED array 410, and the current I increases rapidly. Note that in the period t7, the current I is still small, and the base voltage of the transistor 423 is less than 0.6 V, so that the FET 422 is in an ON state. When the pulsating voltage V further rises and the period t8 is entered, the bypass circuit 420 operates at a constant current so that the base voltage of the transistor 423 is maintained at 0.6V. In the last part of the period t8, since the pulsating voltage V exceeds the sum of the threshold value of the LED string 410 and the threshold value of the LED string 430, the sum of the current flowing through the FET 422 and the current flowing through the LED string 430 is made constant. In the period t9, the pulsating voltage V further increases, the transistor 423 is saturated, and the FET 422 is cut off. At this time, the current I is limited by the current limiting resistor 433 and has a shape similar to the pulsating voltage V. When the pulsating voltage V decreases, the waveform of the current I decreases on the reverse path.

  As described above, the LED drive circuit 400 of FIG. 4 increases the current in synchronization with the voltage increase (absolute value) of the commercial power supply 406, and further, the FET 422 switches in an analog manner to smoothly change the current I. It is characterized by good power factor and distortion and low harmonic noise.

Japanese Patent No. 4581646 (FIG. 1) WO2011 / 020007 (FIG. 26)

  However, since the current waveform shown in FIG. 5B rises rapidly in the period t7, a harmonic component having a relatively low order appears strongly. For example, in ICE61000-3-2, a product of 25 W or more may not reach the standard (Class C) that should be met.

  Accordingly, the present invention has been made in view of the above problems, and in an LED drive circuit including an LED array in which a plurality of LEDs are connected in series as a light source, a relatively low-order harmonic component included in a circuit current waveform. An object of the present invention is to provide an LED drive circuit with reduced noise.

The LED drive circuit of the present invention includes a bridge rectifier circuit and an LED array in which a plurality of LEDs are connected in series as a light source, and an LED drive circuit that applies a pulsating voltage to the LED array,
A first bypass circuit connected to the current output terminal of the bridge rectifier circuit;
The first bypass circuit includes:
A circuit in which a switch element and a resistor are connected in series, and a first bypass current flows from the current output terminal of the bridge rectifier circuit to the current input terminal of the bridge rectifier circuit through the switch element and the resistor;
A transistor, an upper end connected to the current output terminal of the bridge rectifier circuit, a lower end connected to the collector of the transistor, an upper end connected to the base of the transistor, and a lower end connected to the emitter of the transistor And a first current detection terminal connected to the base of the transistor, and a detection circuit for detecting a current flowing from the first current detection terminal,
The emitter of the transistor is connected to the current input terminal of the bridge rectifier circuit;
Based on the voltage of the collector of the transistor, the detection circuit controls to switch on and cut off the switch element,
In a period in which no current flows through the LED string, no current flows between the collector and emitter of the transistor, the switch is turned on, and the first bypass current increases or decreases along with the pulsating voltage,
In the period in which current flows through the LED string, current flows between the collector and emitter of the transistor when the current flowing through the LED string and flowing into the first current detection terminal exceeds a first predetermined value. The switch is cut off, and the first bypass current is eliminated.

(Function)
In the LED drive circuit of the present invention, when the pulsating voltage starts to increase from 0V, a first bypass current that increases with the pulsating voltage flows. Subsequently, when current starts to flow through the LED string, the first bypass current disappears, and the circuit current is only that flowing through the LED string. When the pulsating voltage decreases and the current flowing through the LED array decreases drastically, the first bypass current flows again and decreases with the decrease of the pulsating voltage. As a result, the current waveform has no portion that suddenly rises from the state where there is no current, and the overall shape approaches a sine wave (absolute value), so that harmonic components of a relatively low order are reduced.

The LED row has a first LED row and a second LED row,
A connecting portion of the second bypass circuit and the first LED row second LED string is inserted between the first current detection terminal,
The second bypass circuit includes a second current detection terminal,
The second bypass current may be cut off when the current flowing through the second LED array and flowing into the second current detection terminal exceeds a second predetermined value.

  As described above, the LED drive circuit of the present invention can reduce a relatively low-order harmonic component included in the circuit current waveform while including an LED array in which a plurality of LEDs are connected in series as a light source.

The circuit diagram of the LED drive circuit in embodiment of this invention. The wave form diagram for demonstrating operation | movement of the LED drive circuit shown in FIG. The graph which shows the harmonic component of the current waveform shown in FIG. The circuit diagram of the LED drive circuit in a prior art example. FIG. 5 is a waveform diagram for explaining the operation of the LED drive circuit shown in FIG. 4.

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 scale of the members is changed as appropriate. Furthermore, the relationship with the invention specific matter described in the claims is described in parentheses.

  An LED driving circuit 100 shown as an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram of the LED drive circuit 100. The LED drive circuit 100 includes a bridge rectifier circuit 105, a first bypass circuit 110, a first LED string 120, a second bypass circuit 130, a second LED string 140, and a constant current circuit 150. The commercial power source 106 is connected to the AC input terminal of the bridge rectifier circuit 105. The bridge rectifier circuit includes four diodes 101, 102, 103, and 104, and a terminal A and a terminal B are a current output terminal and a current input terminal, respectively. The terminal A is connected to the anode of the first LED row 120, and the terminal B is connected to the − side terminal 116 of the first bypass circuit 110. The LED drive circuit 100 shown in FIG. 1 is obtained by adding a bypass circuit 110 to the conventional LED drive circuit 400 shown in FIG. 4 and replacing the current limiting resistor 433 with a constant current circuit 150.

  In the first bypass circuit 110, the + side terminal 115 is a connection portion between the upper end of the resistor 111 and the drain of the n-MOS field effect transistor 112 (hereinafter referred to as FET), and the terminal A and the first LED row 120. Connected to the anode. The negative terminal 116 is a connection portion between the emitter of the NPN bipolar transistor 113 (hereinafter referred to as a transistor) included in the first bypass circuit 110 and the lower ends of the resistors 114 and 118. The current detection terminal 117 (first current detection terminal) of the first bypass circuit 110 is a connection portion between the upper end of the resistor 114 and the base of the transistor 113 and is connected to the negative terminal 136 of the second bypass circuit 130. doing. In the first bypass circuit 110, there is a series circuit of an FET 112 (switching element) and a resistor 118. The gate of the FET 112 is connected to the lower end of the resistor 111 and the collector of the transistor 123. Here, the resistors 111 and 114 and the transistor 113 constitute a current detection circuit, and the first bypass current flowing through the FET 112 is controlled by the collector voltage of the transistor 113.

  In the second bypass circuit 130, the + side terminal 135 is a connection portion between the upper end of the resistor 131 and the drain of the FET 132, and is connected to the cathode of the first LED row 120 and the anode of the second LED row 140. The negative terminal 136 is a connection portion between the lower end of the resistor 134 and the emitter of the transistor 133. A current detection terminal 137 (second current detection terminal) of the second bypass circuit 130 is a connection portion between the upper end of the resistor 134, the source of the FET 132, and the base of the transistor 133, and is connected to the negative terminal 156 of the constant current circuit 150. Connected. In the second bypass circuit 130, the gate of the FET 132 is connected to the lower end of the resistor 131 and the collector of the transistor 133.

  In the constant current circuit 150, the + side terminal 155 is a connection portion between the upper end of the resistor 151 and the drain of the FET 152, and is connected to the cathode of the second LED row 140. The negative terminal 156 is a connection portion between the lower end of the resistor 154 and the emitter of the transistor 153. In the constant current circuit 150, the gate of the FET 152, the lower end of the resistor 151, and the collector of the transistor 153 are connected. Further, the upper end of the resistor 154, the source of the FET 152, and the base of the transistor 153 are connected.

  When the effective value of the commercial power supply 106 is 100 V, the total number of LEDs 121, 122, 141, 142 included in the first LED array 120 and the second LED array 140 is set to about 40. When the number of serial stages of the LED array composed of the first LED array 120 and the second LED array 140 is 40, for example, the number of serial stages of the first LED array 120 and the second LED array 140 is 20 each. .

  Next, the operation of the LED drive circuit 100 of FIG. 1 will be described with reference to FIG. 2A shows the pulsating voltage V applied to the first LED string 120 in the circuit of FIG. 1, and FIG. 2B shows the current I output from the terminal A when the first bypass circuit 110 is not provided. FIG. 4C is a waveform diagram showing the current I output from the terminal A when the first bypass circuit 110 is present. In FIG. 2, the time axes t of (a), (b), and (c) are the same.

  When the commercial power source 106 is rectified by the bridge rectifier circuit 105, a sinusoidal pulsating voltage V appears at the terminal A. Among them, (a) shows a pulsating flow for one cycle. As shown in (b), without the first bypass circuit 110, the waveform of the current I has a period t1 in which no current flows, a period t2 in which the current increases rapidly, a period t3 in which the current becomes constant, and a current further A period t4 that increases, becomes constant after that, and finally decreases appears. On the other hand, as shown in (c), when there is the first bypass circuit 110, the waveform of the current I has a period t5 in which the current increases with the pulsating voltage V, and the current I is similar to (b). A period t3 in which the current becomes constant, and a period t4 in which the current further increases and then becomes constant and then decreases appear. In the period in which the pulsating voltage V decreases, the current I takes the reverse path.

  In other words, when there is the first bypass circuit 110, the waveform of the current I looks like a fillet-shaped bypass current added to the current waveform shown in FIG. As a result, the overall shape of the waveform of the current I approaches a sine wave (absolute value), and harmonic components of a relatively low order are reduced.

  The operation of the LED drive circuit 100 will be described in detail with reference to FIGS. 1, 2A, and 2C. In the period t5, since the pulsating voltage V is equal to or lower than the threshold value of the first LED string 120, no current flows through the first LED string. Therefore, since the current detection terminal 117 is 0 V (referenced to the terminal B), the FET 112 is turned on, and the current I (which is equal to the first bypass current at this time) is determined by the quotient of the pulsating voltage V and the resistance 118. When the pulsating voltage V exceeds the threshold value of the first LED string 120, a current flows through the first LED string 120 and the second bypass circuit 130. When the voltage of the current detection terminal 117 exceeds 0.6 V due to this current (first predetermined value), the FET 112 is cut off and the first bypass current disappears.

  In the period t3, the pulsating voltage V exceeds the threshold value of the first LED string 120, and the second bypass current flows through the second bypass circuit 130. In most part of the period t3, the pulsating voltage V is smaller than the sum of the threshold value of the first LED string 120 and the threshold value of the second LED string 140. At this time, the first bypass current is cut off, and no current flows through the second LED row 140. Therefore, the second bypass current becomes the current I. At this time, the second bypass circuit 130 operates at a constant current so that the base voltage of the transistor 133 is maintained at 0.6V. In the last part of the period t <b> 3, the pulsating voltage V becomes larger than the sum of the threshold value of the first LED string 120 and the threshold value of the second LED string 140, and current starts to flow through the second LED string 140. At this time, the second bypass circuit 130 keeps the sum of the second bypass current and the current flowing through the second LED string 140 (second predetermined value) constant so that the base voltage of the transistor 133 is maintained at 0.6V. To.

  Further, when the pulsating voltage rises and the period t4 starts and the current flowing through the second LED string increases and the transistor 133 is saturated, the FET 132 is cut off and the second bypass current disappears. As a result, the current I becomes equal to the current flowing through the second LED string 140. Note that the constant current circuit limits the upper limit of the current I so that the base voltage of the transistor 153 becomes 0.6V. The period in which the pulsating voltage V decreases follows the reverse path of the period in which the pulsating voltage V increases.

Next, how the harmonic component is improved by the LED drive circuit 100 will be described with reference to FIG. FIG. 3 is a graph showing harmonic components of the current waveform shown in FIG. The vertical axis represents the ratio of the high-frequency current component, and the horizontal axis represents the harmonic order. In the figure, a thick solid line indicates a limit in IEC61000-3-2 class C. In FIG. 2, (b) does not include the first bypass circuit 110.
The harmonic component of is shown. The circles indicate the harmonic components of FIG. 2C provided with the first bypass circuit 110. If the first bypass circuit 110 is not provided, harmonic components around the 10th order cannot satisfy the IEC61000-3-2 standard. On the other hand, if the first bypass circuit 110 is provided as in the LED driving circuit 100 of the present embodiment, the standard of IEC61000-3-2 can be satisfied.

  Note that the first bypass circuit 110 of this embodiment is cut off after a current starts to flow through the second bypass circuit 130. For this reason, a thin pulse-like current increase appears at the boundary between the periods t5 and t3 in FIG. 2C (not shown, and the same when the pulsating voltage V decreases). However, this pulsed current increase only affects the higher harmonics. The LED drive circuit 100 of the present embodiment originally had a small high-order harmonic component without the first bypass circuit 110 (see FIG. 2B). Class C of IEC61000-3-2 is satisfied even if there is a pulse-like current increase.

  The first bypass circuit 110 in the present embodiment employs an FET 112 as a switch element, and includes a circuit in which a switch element and a resistor are connected in series. The switch element is not limited to the FET 112, and may be a p-MOS field effect transistor, a bipolar transistor, a thyristor, a triac, or the like.

  In this embodiment, the LED array is divided into a first LED array 120 and a second LED array 140, and the second bypass circuit 130 is connected to the connection portion between the first LED array 120 and the second LED array 140, and 1 current detection terminal 117. The second bypass circuit 130 cuts off the bypass current (second bypass current) according to the current flowing into the second current detection terminal 137. In this way, only the first LED string 120 is lit during a period when the pulsating voltage is relatively low, and the first and second LED strings 120 and 140 are lit simultaneously during a period when the pulsating voltage is high. . For this reason, in addition to being able to be bright, it is possible to suppress high-order harmonic components low by changing the circuit current in an analog manner while suppressing power consumption. However, the LED string does not necessarily have to be divided into two. For example, when the circuit is simplified, the low-order harmonic components can be reduced only by combining a single LED string and the first bypass circuit. For finer control, the LED array may be divided into three or more.

  In the LED drive circuit 100 of the present embodiment, the LED string 140 is automatically turned on when the current flowing through the LED string 120 increases. However, the pulsating current voltage may be measured, and the LED row to be lit may be selected from among the divided LED rows. Similarly, the pulsating voltage may be measured to control the first bypass circuit.

  In the LED drive circuit 100 of this embodiment, the constant current circuit 150 is provided on the cathode side of the second LED row 140. Since the constant current circuit 150 can limit the upper limit value of the current even when the pulsating voltage is unstable, the operation of the entire circuit is stabilized. If the stability of the circuit may be somewhat deteriorated, circuit components can be reduced by replacing the constant current circuit 150 with a current limiting resistor as in the LED driving circuit 400 shown in FIG. When temperature compensation is required, a circuit including a thermistor may be used.

100, 400 ... LED drive circuit,
101, 102, 103, 104, 401, 402, 403, 404 ... diode,
105, 405 ... Bridge rectifier circuit,
106,406 ... commercial power supply,
110: first bypass circuit;
111,114,118,131,134,151,154,421,424,433 ... resistance,
112 ... FET (switch element),
113, 133, 153, 423 ... transistors,
115, 135, 155, 425... + Side terminal,
116, 136, 156, 426... -Side terminal,
117, 137, 427 ... current detection terminals,
120, 140, 410, 430 ... LED row,
121,122,141,142,411,412,431,432 ... LED,
130 ... second bypass circuit,
132, 152, 422 ... FET,
150 ... constant current circuit,
t1 to t9: Period.

Claims (2)

In an LED drive circuit comprising a bridge rectifier circuit and an LED string in which a plurality of LEDs are connected in series as a light source, and applying a pulsating voltage to the LED string,
A first bypass circuit connected to the current output terminal of the bridge rectifier circuit;
The first bypass circuit includes:
A circuit in which a switch element and a resistor are connected in series, and a first bypass current flows from the current output terminal of the bridge rectifier circuit to the current input terminal of the bridge rectifier circuit through the switch element and the resistor;
A transistor, an upper end connected to the current output terminal of the bridge rectifier circuit, a lower end connected to the collector of the transistor, an upper end connected to the base of the transistor, and a lower end connected to the emitter of the transistor And a first current detection terminal connected to the base of the transistor, and a detection circuit for detecting a current flowing from the first current detection terminal,
The emitter of the transistor is connected to the current input terminal of the bridge rectifier circuit;
Based on the voltage of the collector of the transistor, the detection circuit controls to switch on and cut off the switch element,
In a period in which no current flows through the LED string, no current flows between the collector and emitter of the transistor, the switch is turned on, and the first bypass current increases or decreases along with the pulsating voltage,
In the period in which current flows through the LED string, current flows between the collector and emitter of the transistor when the current flowing through the LED string and flowing into the first current detection terminal exceeds a first predetermined value. The LED drive circuit, wherein the switch is cut off and the first bypass current is eliminated. The LED row has a first LED row and a second LED row,
A second bypass circuit is inserted between the connection between the first LED row and the second LED row, and the first current detection terminal;
The second bypass circuit includes a second current detection terminal,
2. The LED drive according to claim 1, wherein the second bypass current is cut off when a current flowing through the second LED array and flowing into the second current detection terminal exceeds a second predetermined value. circuit.

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