JP2008103470A - Led (light emitting diode) driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an economical LED driving circuit capable of increasing a power supply voltage Vcc beyond the impressable maximum voltage Vceo of a switching element in relation to the increase of number of LEDs connected in series. <P>SOLUTION: In the driving circuit, in which a multitude of LEDs are connected in series and which is equipped with the switching element for controlling the ON/OFF of the conducted current of the LED, a circuit element 15, such as a resistor or the like, is connected to the switching element 13 in parallel. Current I flows through the circuit element 15 connected in parallel to the switching element 13 when the switching element 13 is off. Although this current I is too small to cause the LED to turn on, it causes voltage drop in the LEDs 11 connected in series, and due to this voltage drop, an impressing voltage Vsw on the switching element 13 is reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an LED drive circuit that turns on / off an energization current of an LED series connection circuit in which a large number of LEDs are connected in series to turn on / off a large number of LEDs at once.

2. Description of the Related Art Conventionally, in lighting devices and the like, LED series connection circuits in which a large number of LEDs are connected in series are connected in parallel in a plurality of rows, and the energization current is turned on / off using a switching element (transistor) of a drive circuit. The LED is turned on and off collectively (for example, Patent Documents 1, 2, 3, and 4).
JP 2001-15278 A Japanese Patent Laid-Open No. 2003-1000047 JP 2003-139712 A Japanese Patent Laying-Open No. 2005-50704

  FIG. 6 is an example of a conventional general LED drive circuit. A large number (n) of LEDs are connected in series to form an LED series connection circuit 11, and a DC power supply 12 and a switching element (transistor) 13 are connected in series thereto. A control circuit 14 is connected to the control terminal (base) of the switching element 13 and supplied with a control signal for controlling on / off of the switching element 13. When the ON signal voltage is supplied to the switching element 13, the collector and the emitter of the switching element 13 are turned on, an energizing current flows from the power supply 12, and a large number of LEDs are turned on collectively. When the off signal voltage is supplied to the switching element 13, the collector-emitter between the switching element 13 is turned off, the energization current from the power source 12 is cut off, and a large number of LEDs 13 are turned off collectively.

In the conventional LED driving circuit, when the switching element 13 is in the OFF state, a voltage substantially equal to the power supply voltage Vcc of the DC power supply 12 is applied between the collector and the emitter of the switching element 13. For this reason, if the number n of LEDs connected in series is increased for the purpose of making it brighter or the like, it is necessary to satisfy “power supply voltage Vcc> number of LEDs (n) × LED forward voltage (Vf)”. The power supply voltage Vcc increases. Here, the maximum voltage Vceo that can be applied to the switching element is
Vceo <Vcc
When the power supply voltage Vcc is increased until the above condition is satisfied, when the energization current is turned off, a voltage substantially equal to the power supply voltage Vcc is applied to the switching element 13 and exceeds the maximum applicable voltage Vceo of the switching element. There is a problem of destroying.

  The present invention has been made in view of the above-described circumstances, and the number of LEDs connected in series can be increased, and in this connection, the power supply voltage Vcc can be increased to be higher than the maximum voltage Vceo that can be applied to the switching element. An object is to provide an economical LED driving circuit.

  In a drive circuit having a switching element that connects a large number of LEDs in series and controls on / off of the LED current, a circuit element such as a resistor is connected in parallel to the switching element. A current I flows through the circuit element connected in parallel to the switching element when the switching element is turned off. Although this current I is a very small current that does not light the LED, it causes a potential drop in the LEDs connected in series, and this potential drop reduces the voltage applied to the switching element.

  In the LED drive circuit according to the present invention, the circuit element is additionally connected in parallel with the switching element, so that the voltage applied to the switching element at the time of OFF is reduced. Therefore, the power supply voltage Vcc higher than the maximum voltage Vceo that can be applied to the switching element. Can be applied to the LED drive circuit, and more LEDs can be connected in series than in the prior art to be turned on collectively. In addition, if the number of LEDs is the same, a switching element having a lower maximum applicable voltage Vceo can be selected as the switching element, so that options for the switching element are widened and effects such as cost reduction and circuit performance improvement are achieved. There is expected.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. In addition, in each figure, the same code | symbol is attached | subjected and demonstrated to the member or element which has the same function.

  FIG. 1 shows an LED drive circuit according to an embodiment of the present invention, and FIG. 2 shows an LED array to be driven. The LED drive circuit shown in FIG. 1 is a circuit that turns on / off all the LEDs in a lump by turning on and off the energization current to the LED array 11 in which a large number of LEDs shown in FIG. 2 are connected in series and parallel. The LED array 11 is a two-terminal circuit in which, for example, 30 columns of LED series connection circuits in which 20 LEDs are connected in series are connected in parallel, and a total of 600 LEDs are turned on / off collectively. The 600 LEDs are arranged in a matrix on a substrate in a planar shape to constitute a surface light source. In FIG. 1, only one row of LED series connection circuits is shown.

  In FIG. 2, when the number of LEDs connected in series is “n” and the number of parallel connections is “m”, “n, m” is an arbitrary natural number such as “1, 2, 3, 4,... The LED array can be configured. In FIG. 2, it is preferable to insert a resistor in each LED series connection circuit. As a result, even if there is a variation in the LED forward voltage Vf of each LED series connection circuit, a substantially uniform energization current can be passed through each LED series connection circuit, and the uniformity of luminance as a surface light source is ensured. be able to.

  The LED array 11 is connected in series to a DC power source 12 and a switching element (transistor) 13. When the switching element 13 is turned on, an energization current flows through each LED series connection circuit almost evenly, and all LEDs are turned on. When the switching element 13 is turned off, the energization current is interrupted and all the LEDs are turned off. The control circuit 14 receives an input signal such as a luminance signal and supplies an on signal voltage and an off signal voltage to the base electrode of the switching element 13. For this reason, the power supply voltage Vcc of the DC power supply 12 needs to be equal to or higher than the total voltage of the lighting voltages (forward voltage Vf × n) of n LEDs for one column and the ON voltage of the switching element 13. .

Here, assuming that the resistance value of the current setting resistor 16 connected to the emitter of the switching element 13 is R, the emitter current Ie of the switching element 13 is “Ie = (V _BON −V _BE ) / R "relationship is established.
Where V _BON = ON signal voltage,
V _BE = Base-emitter voltage of the switching element V _BE is an eigenvalue of the switching element. In the case of a bipolar transistor, it is about 0.7 to 1 V. Since the resistance value R is a circuit constant, the emitter current Ie _Can be controlled by the ON signal voltage V _BON .

  In the LED drive circuit of the present invention, an additional circuit element 15 is connected in parallel with the switching element 13. The circuit element 15 is, for example, a resistor, but a minute current flows through the circuit element 15 when the switching element 13 is turned off, and the voltage applied to the switching element 13 is reduced. That is, when a circuit element 15 such as a resistor is inserted in parallel with the switching element 13 at the output stage of the LED drive circuit, the LED series connection circuit 11 and the circuit element 15 are connected in series when the switching element 13 is turned off. A minute current I flows. Thereby, since “small current I> 0” is always obtained, the voltage V of each LED of the LED series connection circuit 11 can be set to “V> 0”, and the applied voltage Vsw to the switching element 13 can be reduced. .

  That is, as shown in FIG. 3 (cited from the product catalog of Nichia Corporation), the forward voltage-forward current characteristics of the blue LED are as follows: forward current is 1 mA, forward voltage is about 2.8 V, and μA A forward voltage of about 2 V per stage can be formed by flowing a very small current.

  According to the experiments by the present inventors, when 20 blue LEDs are connected in series, a 470 kΩ resistor is used as the circuit element 15 and the power supply voltage Vcc is 84 V, the switching element 13 is turned off. As a result, the applied voltage Vsw was 36V. From this result, if the leakage current of the switching element 13 is zero (for reference, the product catalog is 0.1 μA or less), the current I flowing through the circuit element 15 is about 76 μA, and the voltage across the LED series connection circuit is 48V, and the voltage per LED is about 2.4V.

  When the switching element 13 was turned on at the rated current, the applied voltage Vsw to the switching element 13 was 14V. From this result, the forward voltage when the LED was lit was 3.5 V per LED, which was consistent with the Vf type value of the product catalog.

  Therefore, when the circuit element 15 is not connected, the maximum voltage Vceo that can be applied to the switching element 13 needs to be equal to or higher than the power supply voltage Vcc (84 V or higher). However, the circuit element 15 including a 470 kΩ resistor is connected. It has been experimentally confirmed that this can be reduced to 36V. In addition, the resistance value of the circuit element 15 is set so that the LED does not light by the current flowing through the circuit element 15 additionally connected in parallel. As the current flowing through the circuit element 15 is as large as possible within the range in which the LED does not light, the voltage share of the LED series connection circuit portion can be increased, and the switching element 13 having a lower maximum applicable voltage can be employed. .

  Further, if the maximum voltage Vceo that can be applied to the switching element 13 is not changed, the voltage Vsw applied to the switching element 13 is reduced by additionally connecting the circuit element 15, so that a high power supply voltage Vcc can be applied. It becomes possible to connect more LEDs than in the past and light them. As a result, the choices of switching elements are widened, and effects such as cost reduction and brightness improvement are expected.

  An example of the circuit element 15 is not limited to a resistor, but may be one that allows a minute current to flow in an LED series connection circuit. For example, a constant voltage diode (zener diode) can be used. In the above experimental example, by using a Zener diode having a breakdown voltage of 36V, the applied voltage Vsw to the switching element 13 does not rise to 36V or higher, and the maximum voltage Vceo that can be applied to the switching element 13 is reduced to 36V or higher. be able to.

  Similarly, a constant current diode shown in FIG. 4 can be used as the circuit element 15. Constant current characteristics can be obtained by short-circuiting the source and gate electrodes of the FET. Further, a voltage limiting circuit shown in FIG. 5 may be used. In this circuit, a constant voltage diode and a transistor are combined, the Zener diode breaks down at a constant voltage, the transistor is turned on, and the transistor absorbs current. This voltage limiting circuit is suitable for a drive circuit such as a large-capacity LED array because it has a constant voltage diode characteristic and a larger current capacity than a Zener diode.

  In the above embodiment, an example in which a bipolar transistor is used as an example of the switching element has been described. However, a MOSFET or the like can also be used.

  Although one embodiment of the present invention has been described so far, it is needless to say that the present invention is not limited to the above-described embodiment, and may be implemented in various forms within the scope of the technical idea.

It is a circuit diagram of the LED drive circuit of one Embodiment of this invention. It is a circuit diagram which shows an example of an LED array. It is a figure which shows the forward direction voltage-current characteristic of blue LED. It is a figure which shows an example of a constant current diode. It is a figure which shows an example of a voltage limiting circuit. It is a circuit diagram of the conventional LED drive circuit.

Explanation of symbols

11 LED array (LED series connection circuit)
12 DC power supply 13 Switching element (transistor)
14 Control circuit 15 Circuit element 16 Current setting resistor

Claims (6)

In a drive circuit having a switching element that connects a large number of LEDs in series and controls on / off of the energization current of the LEDs,
An LED driving circuit comprising a circuit element connected in parallel to the switching element.   The LED drive circuit according to claim 1, wherein the circuit element is a resistor.   The LED driving circuit according to claim 1, wherein the circuit element is a constant voltage diode.   2. The LED driving circuit according to claim 1, wherein the circuit element is a constant current diode.   2. The LED driving circuit according to claim 1, wherein the circuit element is a voltage limiting circuit having a large current capacity above a certain voltage by combining a constant voltage diode and a transistor. In a drive circuit comprising n switching elements connected in series with a forward voltage Vf, and a switching element for controlling on / off of the current flowing through the LED,
A DC power supply with a power supply voltage Vcc of Vcc> Vf × n;
A circuit element connected in parallel to the switching element;
A switching element in which the maximum voltage Vceo that can be applied is Vceo <Vcc;
An LED driving circuit comprising:

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