JP4506593B2 - Lighting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an illumination device not lowering control voltage of a voltage control circuit even when an input voltage is dropped, restraining generation of large current after the input voltage gets back to a normal voltage. <P>SOLUTION: The illumination device is composed of a light emitting element driving circuit 2 constituted by serially connecting a plurality of light emitting diodes LED1 to LEDn and driving elements T1 to Tn, a voltage control circuit 1 boosting input voltage of a power source 4, and impressing the boosted prescribed voltage on the light emitting diode driving circuit 2, and a current control circuit 3 controlling the driving current of the light emitting element driving circuit 2 in constant value by impressing a first prescribed voltage on the driving elements T1 to Tn when the input voltage is same with a set voltage or higher, and limiting the driving current by impressing a second voltage corresponding to the input voltage on the driving elements T1 to Tn when the input voltage becomes lower than the set voltage. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to an illuminating device for stably lighting and driving a light-emitting element employed for illumination of a meter, for example.

2. Description of the Related Art Conventionally, as an example of this type of lighting device, a circuit device that lights and drives a plurality of light emitting diodes (light emitting elements) as an illumination light source for a vehicle meter is known (for example, see Patent Document 1).
Japanese Patent Laid-Open No. 2003-1000047

  In this device, the on-board battery voltage is used as it is as a power source to drive the light-emitting diodes, and it is not sufficient to drive and drive a large number of light-emitting diodes connected in series for illumination.

  Therefore, the present inventors, as shown in FIG. 5, adjust the voltage of the on-vehicle battery to a predetermined voltage in order to drive a large number of light emitting diodes LED1 to LEDn and transistors T1 to Tn connected in series. The provision of the circuit 1 was examined.

  This voltage adjustment circuit 1 is constituted by a so-called switching regulator, and is a large capacitor for charging the electrolytic capacitor 11, the choke coil 12, the switching transistor 13, the rectifying diode 14, the control circuit 100, and the input power supply when the input power is turned on. A soft start circuit 200 is provided for preventing current from flowing. The voltage obtained by boosting the battery voltage Vb (input voltage), which is the primary voltage, by the intermittent operation of the choke coil 12 and the transistor 13 is charged in the electrolytic capacitor 11 and is output as the secondary voltage Vs.

  However, when the soft start circuit 200 is activated, such as when the input power is turned on, the generation of a large current can be effectively suppressed. However, even if the input voltage fluctuates to such an extent that the soft start circuit 200 does not operate. It was found that when the drive current of the light emitting diode is large, the secondary side voltage Vs cannot be maintained at a predetermined voltage. For this reason, when the input voltage is lowered, the supply of the drive current cannot be sufficiently handled and the secondary side voltage Vs is lowered. However, when the input voltage returns to a normal voltage in this state, the electrolytic capacitor 11 is not subjected to soft start. There is a possibility that a large current for charging the battery will flow. This large current may cause circuit elements in the current path to exceed their ratings.

  The present invention has been made in view of the above points, and prevents the adjustment voltage of the voltage adjustment circuit from being lowered even when the input voltage is lowered, and suppresses the generation of a large current immediately after the input voltage returns to the normal voltage. An object of the present invention is to provide a lighting device that enables the above.

  In order to achieve the above object, the present invention employs the technical means of claims 1 to 8.

According to the lighting device of the present invention described in claim 1,
A light emitting element driving circuit in which a plurality of light emitting elements and a driving element are connected in series;
A voltage adjusting circuit that boosts an input voltage from a power source and applies the boosted predetermined voltage to the light emitting element driving circuit;
When the input voltage is equal to or higher than a set voltage, a constant first voltage is applied to the drive element to adjust the drive current of the light emitting element drive circuit to be constant, and when the input voltage becomes lower than the set voltage, A current adjusting circuit that limits the drive current by applying a second voltage that changes according to an input voltage to the drive element ;
The current adjustment circuit includes a constant voltage generation circuit that generates the first voltage and a voltage response circuit that generates the second voltage, and an output terminal of the constant voltage generation circuit and an output terminal of the voltage response circuit And connected through a resistor,
The voltage responsive circuit includes a diode that conducts and outputs the second voltage when the second voltage is lower than the first voltage, and connects an anode of the diode to an output side of the constant voltage generation circuit. It is characterized by that.

Accordingly, in the present invention, since the input voltage is boosted, even if a large number of light emitting elements are connected in series, it is possible to stably drive the lighting for illumination. In addition, when the input voltage becomes lower than the set voltage, by applying a second voltage corresponding to the input voltage to the drive element to limit the drive current, it is possible to suppress a decrease in the output voltage of the voltage adjustment circuit. In the case of the configuration, it is possible to effectively suppress the generation of a large current that tends to occur immediately after recovery from a decrease in input voltage.
In the present invention, since the constant voltage generating circuit and the voltage responsive circuit are provided, it is possible to appropriately generate the first voltage and the second voltage of both circuits. Furthermore, in the present invention, the first voltage and the second voltage can be switched only by connecting to the output side of the constant voltage generation circuit via a diode.

  According to the second aspect of the present invention, the driving element is a transistor, and the light emitting element driving circuit includes a current limiting resistor connected in series with the transistor.

  Thus, in the present invention, by applying a constant voltage to the base side of the transistor, a constant current circuit can be configured, and the luminance of the light emitting element can be stably controlled.

According to a third aspect of the present invention, the voltage adjusting circuit has an output-side capacitor that charges a predetermined voltage obtained by boosting the input voltage via a rectifying diode.

  Accordingly, in the present invention, it is possible to form a stable output voltage by charging and smoothing the voltage boosted by the output side capacitor.

According to the present invention described in claim 4, wherein the plurality of light emitting elements is composed of a plurality of light emitting diodes connected in series, is driven to be lit plurality of light emitting diodes by the same driving current using a common drive element It becomes possible.

According to this invention of Claim 5 , the said constant voltage generation circuit generates the said 1st voltage fixed according to the light control signal which adjusts the brightness | luminance of a vehicle-mounted instrument, and thereby raises the brightness | luminance of a light emitting element. It is possible to stably adjust the brightness to meet the requirements.

According to this invention of Claim 6 , it applies to the backlight of a vehicle-mounted instrument, and it becomes possible to set the brightness | luminance of a vehicle-mounted instrument stably.

  Embodiments of the present invention will be described below with reference to the drawings.

  The present invention relates to an illuminating device that stably lights and drives a plurality of light emitting elements. Hereinafter, an illuminating device for a backlight of an in-vehicle instrument will be described, but the present invention can also be applied to uses other than the in-vehicle instrument. .

(First embodiment)
FIG. 1 is a circuit diagram showing the overall configuration of the illumination device of the present invention.

  The voltage adjustment circuit 1 is composed of a switching regulator, and similarly to the circuit shown in FIG. 5, the electrolytic capacitor 11 having a relatively large capacity, the choke coil 12, the switching NPN transistor 13, the rectifying diode 14, the control circuit 100, And a soft start circuit 200 for preventing a large current for charging the electrolytic capacitor 11 from flowing when the input power is turned on. The battery voltage Vb (primary voltage) that is the input voltage of the choke coil 12 is boosted by the intermittent operation of the choke coil 12 and the transistor 13. The boosted predetermined voltage is rectified by the diode 14, charged in the electrolytic capacitor 11, and smoothed to output the secondary voltage Vs that is an adjustment voltage.

  Here, the control circuit 100 includes a comparator 101 that compares the secondary side voltage Vs with a predetermined reference voltage Vref and a drive control circuit 102, and the secondary side voltage Vs is a predetermined voltage (for example, 30v) determined by the reference voltage Vref. Thus, the output duty (DUTY) ratio of the transistor 13 is controlled. The magnitude of the predetermined voltage is determined by the number of light emitting diodes LED1 to LEDn connected in series (in this case, 6).

  The soft start circuit 200 is configured to operate when the input voltage Vb is equal to or lower than the set voltage. Once activated, the soft start circuit 200 does not operate again unless the input voltage Vb becomes lower than the set voltage again. For example, the control circuit 100 is operated to gradually increase the output duty ratio of the transistor 13 from 0 for a certain period T0 immediately after the input power is turned on by turning on the ignition switch 5, and this gradual change is performed. The generation of a large current can be suppressed by providing the period T0 for an appropriate period. In this case, the output duty ratio of 0 means that the transistor 13 is completely turned off, and the current for boosting the electrolytic capacitor 11 is 0.

  FIG. 2 (a) shows the operation when the soft start is not applied when the input power is on. The output duty ratio becomes maximum (MAX) when the input power is on, and the input current to the electrolytic capacitor 11 is large. Shows the current state. FIG. 2B shows the operation when the soft start is applied when the input power is turned on. When the input power is turned on, the output duty ratio gradually increases during a certain period T0, and the input current to the electrolytic capacitor 11 is increased. Indicates a state that can be kept low.

  The light emitting element driving circuit 2 has a configuration in which a large number (in this case, six) of light emitting diodes LED1 to LEDn, NPN transistors T1 to Tn, and current limiting resistors R1 to Rn are connected in series. The adjustment circuit 3 controls the drive currents of the light emitting diodes LED1 to LEDn. In this case, by applying a constant voltage to the base side of the transistors T1 to Tn, a constant current circuit can be configured, and the luminance of the light emitting diodes LED1 to LEDn can be stably controlled.

  In this example, n light emitting element driving circuits 2 are arranged, but the values are determined according to the required performance as the backlight of the vehicle-mounted instrument. The light emitting diodes LED1 to LEDn are white light emitting diodes in this example.

  The current adjustment circuit 3 generates a second voltage corresponding to the input voltage Vb in order to limit the drive current of the constant voltage generation circuit 31 that generates a constant first voltage, the resistor 32, and the light emitting diodes LED1 to LEDn. The voltage response circuit 33 is connected to each other, and the output terminals of both the circuits 31 and 33 are connected via a resistor 32. Specifically, the anode of the diode 35 in the voltage response circuit 33 is connected to the output side of the constant voltage generation circuit 31.

  The voltage responsive circuit 33 includes a voltage conversion circuit 34 that generates a second voltage corresponding to the input voltage Vb, and a forward direction of the second voltage and the diode 35 when the second voltage is lower than the first voltage. And a diode 35 that conducts when the sum of the voltage drops is lower than the first voltage) and enables output to the base side of the transistors T1 to Tn. That is, when the input voltage Vb becomes lower than the set voltage, a second voltage is generated on the anode side of the diode 35, and this second voltage is applied to the bases of the transistors T1 to Tn. As described above, the first voltage and the second voltage can be switched only by connecting to the output side of the constant voltage generation circuit 31 via the diode 35.

  Here, the operation characteristics set in the current adjustment circuit 3 will be described with reference to FIG.

  FIG. 3A shows an input voltage Vb that exhibits a limit characteristic A in which the secondary voltage (adjusted voltage) Vs of the voltage adjusting circuit 1 does not decrease due to the LED current that is the sum of the currents flowing to all the light emitting element driving circuits 2. -LED current characteristic diagram, (b) is an input voltage Vb showing a limit characteristic B in which the secondary side voltage (adjusted voltage) Vs does not decrease due to the LED current to the light emitting element driving circuit 2 -base voltages of the transistors T1 to Tn The characteristic diagram, (c) is the input voltage Vb showing the allowable characteristic C allowed for the voltage conversion circuit 34-the base voltage characteristic diagram of the transistors T1 to Tn, (d) is set in the current adjustment circuit 3 Input voltage Vb indicating the overall characteristics, switching the allowable characteristics with the set voltage Vk, and indicating the allowable characteristics that are not reduced by the LED current to the light emitting element driving circuit 2 -the base voltages of the transistors T1 to Tn It is a characteristic diagram. In any of the limit characteristics A to D, the upper region indicates an operation region in which the secondary side voltage Vs decreases, and the lower region indicates a normal operation region in which the secondary side voltage Vs does not decrease.

  In this way, the limit characteristic B that does not decrease the secondary side voltage (adjusted voltage) Vs is obtained, and the allowable characteristic C of the voltage conversion circuit 34 is set slightly safer than the limit characteristic B. That is, when the input voltage Vb decreases, the LED current can be reduced by lowering the base voltages of the transistors T1 to Tn. On the other hand, in the normal use region where the input voltage Vb is stable (input voltage Vb ≧ Vk), a constant first voltage from the constant voltage generation circuit 31 is applied to the bases of the transistors T1 to Tn in order to make the LED current constant. It will be.

  The voltage adjustment circuit 1 and the current adjustment circuit 3 are supplied with an input voltage (battery voltage) Vb from an in-vehicle battery 4 serving as an input power supply via an ignition switch 5 and a circuit protection fuse 6.

  The operation of the above configuration will be described.

  First, when the user turns on the ignition switch 5, the input power of the lighting device of the vehicle-mounted instrument is turned on. Then, the voltage adjustment circuit 1 gradually increases the output duty ratio and gradually changes the input current to the electrolytic capacitor 11 as shown in FIG.

  When the fixed period T0 has passed after the input power is turned on and the input voltage Vb is stable at the set voltage Vk or higher, the voltage adjustment circuit 1 stably provides the secondary side voltage Vs and the current adjustment circuit 3 is also constant. A constant first voltage is provided from the voltage generation circuit 31 to the bases of the transistors T1 to Tn. Thus, the transistors T1 to Tn and the current limiting resistors R1 to Rn operate as a constant current circuit, and the light emitting diodes LED1 to LEDn can stably emit light with a predetermined luminance.

  On the other hand, when the input voltage Vb decreases below the set voltage Vk, the second voltage of the voltage conversion circuit 34 also decreases. When the voltage is lower than the first voltage of the constant voltage generating circuit 31 (specifically, when the sum of the second voltage and the forward voltage drop of the diode 35 is lower than the first voltage), the diode 35 becomes conductive and the second voltage is reached. Is provided to the base side of the transistors T1 to Tn. For this reason, the transistors T1 to Tn perform current control according to the second voltage, flow an LED current at a level at which the secondary side voltage Vs does not decrease, and cause the light emitting diodes LED1 to LEDn to emit light with a little lower luminance.

  Thereafter, even immediately after the input voltage Vb becomes higher than the set voltage Vk and normalizes, the secondary side voltage Vs does not decrease due to the current limiting operation, so that a large current does not flow to the electrolytic capacitor 11.

  Thus, in this embodiment, since the input voltage is boosted, even if a large number of light emitting diodes LED1 to LEDn are connected in series, it is possible to stably drive the lighting for illumination. In addition, when the input voltage Vb becomes lower than the set voltage Vk, the output voltage of the voltage adjustment circuit 1 (by applying the second voltage corresponding to the input voltage to the transistors T1 to Tn as drive elements to limit the drive current) It is possible to suppress a decrease in the secondary voltage), and it is also possible to effectively suppress the generation of a large current that tends to occur immediately after the recovery from the decrease in the input voltage in the case of the boosting configuration. Therefore, it is possible to reliably prevent the fuse 6, choke coil 12, transistor 13 and the like in the current path from exceeding the rating due to a large current and leading to element deterioration or damage.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG. This second embodiment shows an example of a circuit that embodies the first embodiment, and is different from the first embodiment in that the current adjustment circuit 3 includes the constant voltage generation circuit 31 and the voltage response circuit 33. This is a point showing a typical circuit, and a point in which an operational amplifier 352 is added to eliminate the influence of the forward voltage drop due to the diode 351.

  In FIG. 4, a constant voltage generation circuit 31 receives a dimming signal for adjusting and instructing the luminance of the meter for meter illumination from a dimming signal generation circuit 36 in an in-vehicle instrument (not shown). The first voltage (constant voltage) applied to the light emitting element driving circuit 2 is adjusted so that the LED current is determined by the voltage level.

  That is, the constant voltage generation circuit 31 determines the operating state of the transistors 311 and 312 according to the voltage level of the dimming signal, and applies a constant first voltage determined by the dimming signal to the connection point 313 of the resistors 316 and 317. generate. In addition, since the output terminal of the voltage response circuit 33 is directly connected to the connection point 313, when the output voltage (second voltage) of the voltage response circuit 33 is lower than the first voltage, the voltage at the connection point 313 is Pulled down to the lower second voltage. In this configuration, an output voltage corresponding to the voltage generated at the connection point 313 is provided to the light emitting element driving circuit 2 through the circuit of the output side transistor 314 and the resistor 315 and the base resistors Rb1 to Rbn.

  The voltage response circuit 33 includes a voltage conversion circuit 34 and an output circuit 35a. This voltage conversion circuit 34 includes an operational amplifier 341 and an NPN transistor 342, constitutes a differential amplifier circuit of an input voltage Vb (battery voltage) and a reference voltage Vref1, and outputs an output voltage (first voltage) corresponding to the input voltage Vb. 2 voltage) is generated at the connection point 343.

  The output circuit 35 a includes a diode 351 and an operational amplifier 352. The output circuit 35 a is configured to remove the influence of the forward voltage drop due to the diode 351 using the operational amplifier 352 and generate the second voltage generated at the connection point 343 on the anode side of the diode 351. The diode 351 has the same function as the diode 35 shown in FIG. 1, and affects the connection point 313 of the constant voltage generation circuit 31 only when the second voltage <the first voltage (that is, the voltage at the connection point 313). The lower side).

  Since the operation of the second embodiment is the same as that of the first embodiment, description thereof is omitted. In the second embodiment, since the influence of the forward voltage drop due to the diode 351 can be removed in the output circuit 35a, the allowable characteristic C when the input voltage Vb shown in FIG. It is possible to set the allowable characteristic C closer to the limit characteristic B where the secondary side voltage Vs does not decrease. This makes it possible to reduce the LED current of the light emitting element driving circuit 2 as much as possible even when the input voltage Vb is reduced, and to reduce the luminance of the light emitting diodes LED1 to LEDn.

(Modification)
In the said embodiment, although white light emitting diode LED1-LEDn was mentioned as an example as a light emitting element, the light emitting diode of another luminescent color may be sufficient, and the light emitting element of another type may be sufficient. Further, although NPN transistors T1 to Tn are taken as examples of driving elements, PNP transistors and MOS transistors may be used as long as they are elements capable of constant current control.

It is a circuit diagram which shows the whole structure of the illuminating device used as 1st Embodiment of this invention. 3 is an operation explanatory diagram of a soft start circuit 200. FIG. It is explanatory drawing which shows the limit characteristic and tolerance characteristic which the secondary side voltage of the voltage adjustment circuit 1 does not fall. It is a circuit diagram which shows the whole structure of the illuminating device used as 2nd Embodiment of this invention. It is a circuit diagram which shows the whole structure of the illuminating device which the present inventors examined beforehand.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Voltage adjustment circuit 2 Light emitting element drive circuit 3 Current adjustment circuit 4 Car-mounted battery 5 Ignition switch 11 Electrolytic capacitor (output side capacitor)
12 Choke coil 13 NPN transistor for switching 14 Diode for rectification 31 Constant voltage generating circuit 33 Voltage response circuit 34 Voltage conversion circuit 35, 351 Diodes LED1 to LEDn Light emitting diode (light emitting element)
T1-T2 transistor (drive element)
R1 to Rn Current limiting resistors

Claims (6)

A light emitting element driving circuit in which a plurality of light emitting elements and a driving element are connected in series;
A voltage adjusting circuit that boosts an input voltage from a power source and applies the boosted predetermined voltage to the light emitting element driving circuit;
When the input voltage is equal to or higher than a set voltage, a constant first voltage is applied to the drive element to adjust the drive current of the light emitting element drive circuit to be constant, and when the input voltage becomes lower than the set voltage, A current adjusting circuit that limits the drive current by applying a second voltage that changes according to an input voltage to the drive element ;
The current adjustment circuit includes a constant voltage generation circuit that generates the first voltage and a voltage response circuit that generates the second voltage, and an output terminal of the constant voltage generation circuit and an output terminal of the voltage response circuit And connected through a resistor,
The voltage responsive circuit includes a diode that conducts and outputs the second voltage when the second voltage is lower than the first voltage, and connects an anode of the diode to an output side of the constant voltage generation circuit. A lighting device characterized by that. The drive element is a transistor;
The lighting device according to claim 1, wherein the light emitting element driving circuit includes a current limiting resistor connected in series with the transistor.   The lighting device according to claim 1, wherein the voltage adjustment circuit includes an output-side capacitor that charges a predetermined voltage obtained by boosting the input voltage via a rectifying diode.   The lighting device according to claim 1, wherein the plurality of light emitting elements are composed of a plurality of light emitting diodes connected in series. 2. The lighting device according to claim 1 , wherein the constant voltage generation circuit generates the constant first voltage that is determined according to a dimming signal that adjusts luminance of an on-vehicle instrument. The lighting device according to claim 5 , wherein the lighting device is applied to a backlight of a vehicle-mounted instrument.

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