Lighting control device, lighting device, and vehicle lamp

Abstract

To reduce losses in an LED driver due to a decrease in an LED forward voltage.SOLUTION: A lighting control device 3 includes an LED driver 32 that supplies a drive current to an LED, a power supply circuit that supplies power to the LED driver 32, and a temperature sensor that detects the ambient temperature. The power supply circuit controls the output voltage applied to the LED driver 32 on the basis of the ambient temperature detected by the temperature sensor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lighting control device, a lighting device, and a vehicle lamp. [Background technology]

[0002] In lighting control devices for vehicle lamps, in order to drive a light-emitting string made up of light-emitting elements such as LEDs connected in series, a drive voltage is supplied to the light-emitting elements by a voltage supply circuit such as a DC-DC converter. For example, in a step-down power supply circuit, the voltage is stepped down by a DC-DC converter, and lighting control is performed by an LED driver. Here, the DC-DC converter monitors the output voltage from the feedback (FB) terminal and controls the output to a constant voltage.

[0003] For example, Patent Document 1 discloses that a step-down DC-DC converter with an LED (light-emitting diode) as a load is provided with a feedback control circuit for maintaining a constant output voltage against fluctuations in the input voltage. Also, Patent Document 2 discloses that a DC-DC converter with a Peltier element as a load uses a thermistor for feedback control to satisfy required performance that changes with the ambient temperature. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-225360 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-50151 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the lighting device disclosed in Patent Document 1 does not have an LED driver circuit, so it cannot control the on / off of any LED. The LED driver has an internal electrical switch that controls the on / off of LEDs. The LED driver also controls the connected LEDs to provide a constant current that is set arbitrarily. The LED load, the forward voltage, decreases as the temperature increases. Because the power supply to the LED driver is constant voltage, the product of the decreased forward voltage and the set current becomes surplus power. This surplus power is dissipated as heat by the LED driver. In particular, in automotive lighting fixtures, losses increase under harsh conditions at high temperatures, making it necessary to ensure the reliability of the LED driver and the entire circuit board, which increases costs. Patent Document 2 also discloses control in response to changes in ambient temperature, but it does not provide feedback based on the temperature characteristics of the connected load itself; rather, it provides feedback based on the ambient temperature, since the output required by the load changes depending on the ambient temperature. Therefore, it does not contribute to the power conversion efficiency of the circuit.

[0006] The present invention has been made in view of the above, and has an object to reduce losses in an LED driver that accompany a decrease in the forward voltage of an LED. [Means for solving the problem]

[0007] The lighting control device of the present invention comprises an LED driver that supplies a drive current to an LED, a power supply circuit that supplies power to the LED driver, and a temperature sensor that detects the ambient temperature, and the power supply circuit controls the output voltage applied to the LED driver based on the ambient temperature detected by the temperature sensor. [Effects of the Invention]

[0008] According to the present invention, by controlling the output voltage applied to the LED driver based on the ambient temperature detected by the temperature sensor, it is possible to reduce losses in the LED driver that accompany a decrease in the forward voltage of the LED. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram showing a configuration of a vehicle lamp according to a first embodiment of the present invention. [Figure 2] 10A and 10B are graphs showing the output and loss of a DC-DC converter versus ambient temperature, where (a) is a graph showing the output voltage of the DC-DC converter, and (b) is a graph showing the loss due to a decrease in the forward voltage of an LED. DETAILED DESCRIPTION OF THE INVENTION

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A vehicle lamp according to an embodiment of the present invention will now be described with reference to the accompanying drawings. In the drawings, the same or equivalent parts are designated by the same reference numerals.

[0011] Fig. 1 is a block diagram showing the configuration of a vehicle lamp 1. The vehicle lamp 1 includes a light source 2 and a lighting control device 3 that controls the lighting of the light source 2. The light source 2 is configured by connecting LED strings 21 and 22 in parallel, each of which has a plurality of LEDs connected in series. Each LED string only needs to include at least one LED, and the number of LED strings only needs to be one or more, and can be set appropriately depending on the application.

[0012] The lighting control device 3 includes a step-down DC-DC converter 31, which is a voltage supply circuit that supplies a drive voltage to the light source 2, and an LED driver 32 that controls the light source 2 to be turned on and off.

[0013] The step-down DC-DC converter 31 is a power supply circuit that receives power from a DC power supply 4, generates an output voltage, and applies it to the light source 2 via an LED driver 32 to supply drive power to the LED strings 21 and 22. The DC power supply 4 is connected to a pair of power input terminals 3a and 3b of the lighting control device 3, and outputs a DC power supply voltage V in is input to the step-down DC-DC converter 11. The step-down DC-DC converter 31 converts the power supply voltage V in The desired DC output voltage Vout The step-down DC-DC converter 31 steps down this output voltage V out is fed back to the output voltage V out The output voltage V out The voltage supply circuit is not limited to a step-down type, but may be a step-up type.

[0014] The LED driver 32 has an internal switching element and controls the on / off of the light source 2 by turning the switching element on / off. It also controls the drive current flowing through the light source 2 to be a constant current value that is set arbitrarily. The light source 2 is connected to a pair of output terminals 3c, 3d of the lighting control device 3, and a drive voltage is applied to the light source 2 from the output terminals 3c, 3d. The anodes of the LED strings 21, 22 of the light source 2 are connected to the common output terminal 3c. In contrast, multiple output terminals 3d are provided, and the LED strings 21, 22 are connected separately. The LED driver 32 has a switching element for each of the multiple output terminals 3d, and controls the on / off of each LED string 21, 22 by turning these switching elements on / off.

[0015] The light source 2 is mounted on a light source board, and the lighting control device 3 is mounted on a separate control board, and the light source 2 and the lighting control device 3 are electrically connected by connecting a harness to connectors provided on both boards.

[0016] As described above, the step-down DC-DC converter 31 converts this output voltage V out is fed back to the output voltage V out This allows for control to stabilize the output voltage V out is divided by resistors R1 and R2 and fed back to the feedback terminal FB provided in the step-down DC-DC converter 31, out However, as the temperature of the LED constituting the light source 2 increases, the forward voltage V FTherefore, when the LED gets hot, the forward voltage V F Even if the voltage V out is a constant voltage, so the forward voltage V F The excess power is generated due to the drop in the output voltage V. This excess power is consumed as heat by the LED driver 32. out By detecting the ambient temperature using a temperature sensor whose resistance changes with temperature as a feedback resistor for feeding back the F The output voltage V out The signal is controlled to be output.

[0017] In Figure 1, the output voltage V out In addition to the first resistor R1 and the second resistor R2, a third resistor R3 and a thermistor TH1, which is a temperature sensor, are connected between the resistors R1 and R2 to form a voltage divider circuit. The resistor R3 and thermistor TH1 are connected in parallel with each other. The divided output voltage V is fed to the feedback terminal FB from the connection point between the resistor R3 and thermistor TH1 and resistor R2. out Thermistor TH1 is an NTC (Negative Temperature Coefficient) thermistor that exhibits a negative resistance change with respect to the ambient temperature. The resistance value of thermistor TH1 changes according to the ambient temperature, and the voltage division ratio changes accordingly.

[0018] Here, the voltage V required by the light source 2 and the LED driver 32 is o is given by the following equation (1): V o =(V F +ΔV LED )×N LED_Series +V LED Driver Drop (1) In equation (1), V F is the forward voltage of the LED, ΔV LED is the V of the temperature conditions FChange, N LED_Series is the number of LEDs in series, V LED Driver Drop is the voltage drop in the LED driver 32.

[0019] On the other hand, the output voltage V output from the step-down DC-DC converter 31 is controlled by feedback control. out is given by the following equation (2): V out ={[th1(r1+r2+r3)+r3(r1+r2)] / r2(th1+r3)}×V REF (2) In equation (2), th1 is the resistance value of the thermistor TH1, r1 is the resistance value of resistor R1, r2 is the resistance value of resistor R2, r3 is the resistance value of resistor R3, and V REF is the reference voltage of the control IC of the step-down DC-DC converter 31. The above output voltage formula changes depending on the control IC of the step-down DC-DC converter 31.

[0020] From equation (2), as the ambient temperature increases, the resistance of the thermistor TH1 decreases, and the output voltage V out This reduces the LED forward voltage V F This can reduce losses due to a decrease in power consumption.

[0021] Since the thermistor TH1 detects the temperature of the LED, it is desirable to place it near the light source 2. As mentioned above, the light source 2 is mounted on a light source board, and the lighting control device 3 is mounted on a separate control board. Therefore, it is desirable to place the thermistor TH1 on the light source board. However, since the light source 2 and the lighting control device 3 are electrically connected by connecting harnesses to connectors on both boards, placing the thermistor TH1 on the light source board requires running two additional harnesses to connect the thermistor TH1 to the lighting control device 3. This increases the number of harnesses and connector pins, resulting in cost disadvantages. Furthermore, connecting multiple light source boards to the lighting control device 3 is difficult because the ambient temperatures of each board are different. Therefore, the thermistor TH1 is placed on the control board rather than on the light source board, and the resistance constant of the voltage-dividing resistor is set taking into account the temperature difference between the ambient temperature on the light source board and the ambient temperature on the control board. Considering the case where there is a difference in ambient temperature between the control board and the light source board, the worst case scenario is when the ambient temperature of the control board is higher than the ambient temperature of the light source board. For example, when the ambient temperature of the control board is 80°C and the ambient temperature of the light source board is 75°C, which is 5°C lower than that, the output voltage V out is a voltage value feedback-controlled by equation (2) at an ambient temperature of 80°C. In contrast, the voltage V required by the light source 2 and the LED driver 32 is o is the voltage value required at an ambient temperature of 75°C. At this time, the output voltage V out The voltage value of V is controlled to be smaller by the amount that the ambient temperature is 5°C higher than the ambient temperature of the light source board. This voltage value is the voltage V required by the light source 2 and the LED driver 32 at an ambient temperature of 75°C. o If the voltage is smaller than this value, the light source 2 cannot be turned on.

[0022] Therefore, when the ambient temperature of the control board is Ta°C, and the maximum possible temperature difference between this and the ambient temperature of the light source board, which is lower than this, is Δt, the resistance constants of resistors R1, R2, R3 and thermistor TH1 are set so as to satisfy the following equation (3). V oTa℃-Δt <V outTa℃ (3) In equation (3), V outTa℃ is the output voltage of the step-down DC-DC converter 31 when the ambient temperature of the control board is Ta°C, V oTa℃-Δt is the voltage required by the light source 2 and the LED driver 32 when the ambient temperature of the light source board is Ta° C.-Δt.

[0023] Next, the output of the step-down DC-DC converter 31 and the forward voltage V of the LED with respect to changes in the ambient temperature when the vehicle lamp 1 shown in FIG. 1 is used will be described. F The waveform of the loss due to the decrease in output voltage V of the DC-DC converter 31 is shown in FIG. out 2(b) is a graph showing the forward voltage V F 1 is a graph showing losses due to a decrease in

[0024] In FIG. 2(a), the horizontal axis represents the ambient temperature, and the vertical axis represents the output voltage V of the DC-DC converter 31. out The solid line indicates the output voltage V of the step-down DC-DC converter 31 relative to changes in ambient temperature when the vehicle lamp 1 of the present invention is used. out. In contrast, the dotted line, for comparison, shows the output voltage of the step-down DC-DC converter with respect to changes in ambient temperature when the thermistor TH1 is not provided as a voltage dividing resistor for dividing the output voltage. In the case of a vehicle lamp that does not use the thermistor TH1, the output voltage divided according to a fixed voltage dividing ratio is fed back, so the feedback-controlled output voltage is constant regardless of changes in ambient temperature. In contrast, in the case of the vehicle lamp 1 of the present invention, the voltage dividing ratio changes depending on the ambient temperature and the output voltage is fed back, so the feedback-controlled output voltage changes in accordance with changes in ambient temperature. As the ambient temperature rises, the resistance value of the thermistor TH1 decreases, so the voltage dividing ratio increases and the feedback output voltage becomes larger than in the past. Therefore, the feedback-controlled output voltage decreases as the ambient temperature rises. In this way, the output voltage V of the step-down DC-DC converter 31 decreases as the ambient temperature rises. out is decreasing, and the forward voltage V F This corresponds to the change in the voltage required by the LEDs and the LED driver 32 due to the temperature drop.

[0025] 2(b), the horizontal axis represents the ambient temperature and the vertical axis represents the loss. The loss is calculated based on the output voltage V out and the required voltage of the LED and LED driver 32. The solid line indicates the loss due to changes in ambient temperature when the vehicular lamp 1 of the present invention is used. In contrast, the dotted line indicates the loss due to changes in ambient temperature when the thermistor TH1 is not provided as a voltage dividing resistor for dividing the output voltage, as in FIG. 2(a). In the case of a vehicular lamp that does not use the thermistor TH1, the output voltage is constant regardless of changes in ambient temperature, as in FIG. 2(a). In contrast, the required voltage of the LED and LED driver 32 changes as the forward voltage V FAs the ambient temperature rises, the output voltage V decreases, and as the ambient temperature rises, the difference between the two voltages increases. As a result, the loss also increases. Therefore, the loss is large at high temperatures. On the other hand, in the case of the vehicle lamp 1 of the present invention, the output voltage decreases as the ambient temperature rises, as shown in FIG. 2(a). Similarly, the required voltage of the LED and the LED driver 32 also decreases as the ambient temperature rises, and the forward voltage V F This decrease in the voltage reduces as the ambient temperature rises. Therefore, the difference between the two voltages does not change significantly as the ambient temperature rises, and the loss does not change significantly overall, remaining flat especially at high temperatures. Therefore, compared to conventional vehicle lamps, which experience high losses at high temperatures, the loss decreases as the temperature rises.

[0026] As described above, according to this embodiment, the thermistor TH1 detects the ambient temperature, and based on the feedback voltage obtained by dividing the output voltage of the step-down DC-DC converter 31 at a voltage division ratio corresponding to the detected temperature, the output voltage is controlled to a voltage corresponding to the voltage drop in the LED strings 21, 22 and the LED driver 32 that occurs due to a decrease in the LED forward voltage caused by temperature, thereby reducing losses in the LED driver 32 that occur due to a decrease in the LED forward voltage caused by an increase in ambient temperature.

[0027] In the above embodiment, thermistor TH1, whose resistance value changes according to temperature, is used as a temperature sensor, and the output voltage input to feedback terminal FB, which feeds back the output voltage of step-down DC-DC converter 31, is changed according to the detected ambient temperature, thereby controlling the output voltage. However, other types of sensors may be used as the temperature sensor, or the output voltage of step-down DC-DC converter 31 may be controlled by directly inputting the detected temperature detection signal.

[0028] Furthermore, in the above embodiment, a DC-DC converter has been described as an example of a power supply circuit, but the present invention is not limited to this and can be applied to other types of power supply circuits such as a regulator.

[0029] Furthermore, in the above embodiment, a description has been given of a vehicle lamp, but the present invention is not limited to this, and can also be used as a lighting device for other purposes, such as amusement equipment, as long as the load has a similar characteristic of decreasing resistance as the temperature rises or being controlled by constant current.

[0030] Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to the above-described embodiments, but includes the scope of the invention described in the claims and its equivalents. [Explanation of symbols]

[0031] 1...vehicle lighting fixture, 2...light source, 3...lighting control device, 3a, 3b...power supply input terminal, 3c, 3d...output terminal, 4...DC power supply, 21, 22...LED string, 31...step-down DC-DC converter, 32...LED driver, FB...feedback terminal, R1, R2, R3...resistor, TH1...thermistor

Claims

1. an LED driver that supplies a drive current to the LED; a power supply circuit for supplying power to the LED driver; a temperature sensor for detecting an ambient temperature; Equipped with The power supply circuit includes: controlling an output voltage applied to the LED driver based on the ambient temperature detected by the temperature sensor; Lighting control device.

2. the temperature sensor is a thermistor whose resistance value changes depending on the ambient temperature, The power supply circuit includes: a voltage divider circuit including a resistor and the thermistor divides the output voltage, and feeds back the divided voltage to control the output voltage; The lighting control device according to claim 1 .

3. The voltage divider circuit the resistors include a first resistor, a second resistor, and a third resistor connected in series in this order; the second resistor is connected in parallel with the thermistor; a voltage is divided by the first resistor, the second resistor, the thermistor, and the third resistor; The lighting control device according to claim 2 .

4. the thermistor is provided on a substrate separate from the substrate on which the LED is mounted; setting the resistance constants of the resistor and the thermistor in accordance with the maximum expected temperature difference between the temperature detected by the thermistor and the ambient temperature of the LED; The lighting control device according to claim 2 or 3.

5. The resistance constant is set so that the output voltage at a temperature detected by the thermistor that is lower than the ambient temperature of the LED by the maximum temperature difference is lower than a voltage corresponding to a voltage drop of the LED and the LED driver that accompanies a temperature-dependent decrease in the forward voltage of the LED at the ambient temperature of the LED. The lighting control device according to claim 4 .

6. A lighting device comprising the lighting control device according to claim 1 and the LED.

7. A vehicle lamp constructed using the lighting device according to claim 6.

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