In-vehicle LED lighting circuit and in-vehicle lighting device

Abstract

To provide a technology that can suppress fluctuations in LED brightness in an in-vehicle LED lighting circuit.SOLUTION: An in-vehicle LED lighting circuit includes a constant current circuit for an LED current flowing through one or more LEDs. The constant current circuit includes a Zener diode, a first transistor for controlling the Zener current flowing through the Zener diode, a diode for controlling the drive of the first transistor with a forward voltage, and a second transistor for controlling the LED current with a reverse voltage of the Zener diode.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an in-vehicle LED lighting circuit and an in-vehicle lighting device. [Background technology]

[0002] Conventionally, as described in Patent Document 1, an in-vehicle LED (light emitting diode) lighting circuit is provided with a constant current circuit that keeps the current flowing through the LED constant. In the following description, the current flowing through the LED is referred to as the LED current. Since the in-vehicle LED lighting circuit is powered by a battery, it is affected by fluctuations in the battery voltage. In this case, the LED current fluctuates according to the voltage fluctuations. Fluctuations in the LED current cause fluctuations in brightness, which are a cause of flickering. The constant current circuit in the in-vehicle LED lighting circuit suppresses fluctuations in the LED current, thereby suppressing fluctuations in the LED brightness.

[0003] The constant current circuit described in Patent Document 1 uses a Zener diode, and utilizes the constant voltage characteristics of the Zener diode in the reverse direction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-100633 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, even when a constant current circuit is used, the LED current may fluctuate. For example, the LED current fluctuates depending on the characteristics of the elements used in the constant current circuit. In a Zener diode, the constant voltage characteristic in the reverse voltage is not strictly constant because it actually fluctuates according to the current. Therefore, even in a constant current circuit using a Zener diode, fluctuations in LED brightness occur. Therefore, there was a demand for technology that could further suppress fluctuations in LED brightness in automotive LED lighting circuits. [Means for solving the problem]

[0006] The present disclosure can be realized in the following forms.

[0007] (1) According to a first aspect of the present disclosure, there is provided an in-vehicle LED lighting circuit. The LED lighting circuit includes a constant current circuit for an LED current flowing through one or more LEDs. The constant current circuit includes a Zener diode, a first transistor for controlling a Zener current flowing through the Zener diode, a diode for controlling the drive of the first transistor by a forward voltage, and a second transistor for controlling the LED current by a reverse voltage of the Zener diode. With this configuration, the constant current circuit controls the LED current by the second transistor with the reverse voltage of the Zener diode. The Zener current flowing through the Zener diode is controlled by the first transistor. Furthermore, the driving of the first transistor is controlled by the forward voltage of the diode. That is, the Zener current is controlled based on the forward voltage of the diode having constant voltage characteristics. Therefore, the reverse voltage of the Zener diode is stabilized by controlling the Zener current. That is, the constant current circuit can stably control the LED current by the second transistor. Therefore, the LED lighting circuit 11 can further suppress fluctuations in the brightness of the LED. (2) In the above embodiment, the diode may be an LED included in the one or more LEDs. By adopting such a configuration, the LED lighting circuit can be configured with the first LED as a diode in the constant current circuit, thereby reducing the number of components compared to a case in which a separate diode is provided. (3) In the above embodiment, the constant current circuit may further include a first resistor which determines the Zener current and a second resistor which determines the LED current, the Zener diode may be connected in a reverse direction with respect to a supply voltage of the LED lighting circuit and connected in series with the emitter and collector of the first transistor, the diode may be connected in a forward direction with respect to the supply voltage and connected in series with the emitter and collector of the second transistor, the base of the first transistor may be connected between the diode and the second transistor, and the base of the second transistor may be connected between the Zener diode and the first transistor. In this configuration, the first transistor is controlled by a forward voltage because its base is connected to the first LED as a diode in the constant current circuit. The second transistor is controlled by a reverse voltage because its base is connected to a Zener diode. In other words, the LED lighting circuit can easily control the first and second transistors by utilizing the characteristics of the diodes. (4) In the above embodiment, the cathode of the diode may be connected to the ground of the LED lighting circuit. By adopting such a configuration, the LED lighting circuit can control the second transistor more stably than when the reference for the reverse voltage of the Zener diode is not set to ground. (5) In the above embodiment, the anode of the Zener diode may be connected to the ground of the LED lighting circuit. With such a configuration, the LED lighting circuit can be applied even when, for example, the first LED has a common cathode and the cathode needs to be connected to ground. (6) According to a second aspect of the present disclosure, there is provided an in-vehicle lighting device, the lighting device including: a first LED lighting circuit according to the above aspect, in which the one or more LEDs are LEDs that emit red light; a second LED lighting circuit according to the above aspect, in which the one or more LEDs are LEDs that emit blue light; and a third LED lighting circuit according to the above aspect, in which the one or more LEDs are LEDs that emit green light. With this configuration, the lighting device can express the color tone of the three-color lighting with greater precision because the LED lighting circuit suppresses fluctuations in LED brightness. (7) In the above aspect, in the first LED lighting circuit, the second LED lighting circuit, and the third LED lighting circuit, the second resistor may be a variable resistor. With this configuration, each LED lighting circuit can change the brightness of the LEDs by the second resistor, which is a variable resistor. Therefore, the lighting device of the present disclosure can emit light of various colors by adjusting the second resistor of the first LED lighting circuit, the second resistor of the second LED lighting circuit, and the second resistor of the third LED lighting circuit. Furthermore, the lighting device of the present disclosure can reduce the variation in the color of the emitted light even when the supply voltage fluctuates. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing a configuration of an in-vehicle LED lighting circuit. [Diagram 2] 1 is a graph showing the relationship between LED current and supply voltage. [Diagram 3] FIG. 11 is a circuit diagram showing an LED lighting circuit according to a second embodiment. [Figure 4] FIG. 11 is a circuit diagram showing an LED lighting circuit according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] A. First embodiment: A-1. Circuit configuration: 1 is a circuit diagram showing the configuration of an in-vehicle LED lighting circuit 11. The in-vehicle LED lighting circuit 11 lights an LED (light emitting diode). More specifically, the LED lighting circuit 11 is provided in a vehicle V in order to light an LED used in the interior of the vehicle V. For example, the LED lighting circuit 11 is used to light LEDs used for interior lighting, a speedometer, an indicator lamp, and the like.

[0010] The LED lighting circuit 11 includes a battery 110, a first LED 120, a second LED 130, a second resistor 140, and a constant current circuit 150. In FIG. 1, the area surrounded by a dashed line is the portion of the constant current circuit 150. Furthermore, the constant current circuit 150 includes a second transistor 153, a Zener diode 154, a first transistor 152, a first resistor 151, and a diode 120. In the first embodiment, the diode 120 is configured by the first LED 120 included in the LEDs serving as a load of the LED lighting circuit 11. First, the connection of each element will be described.

[0011] The first LED 120 is connected in the forward direction to the battery 110 that outputs the supply voltage V4. Furthermore, the first LED 120 is connected in series to the emitter and collector of the second transistor 153. More specifically, the anode of the first LED 120 is connected to the positive side of the power supply. The cathode of the first LED 120 is connected to the base of the first transistor 152 and the anode of the second LED 130.

[0012] The second LED 130 is forward-connected to the battery 110 that outputs the supply voltage V4. Furthermore, the second LED 130 is connected in series to the emitter and collector of the second transistor 153. More specifically, the cathode of the second LED 130 is connected to the collector of the second transistor 153. The anode of the second LED 130 is connected to the base of the first transistor 152 and the cathode of the first LED 120.

[0013] The second resistor 140 is connected in series to the LED. More specifically, one terminal of the second resistor 140 is connected to the emitter of the second transistor 153. The other terminal of the second resistor 140 is connected to the negative electrode of the battery 110. In other words, the other terminal of the second resistor 140 is connected to the ground of the LED lighting circuit 11.

[0014] The second transistor 153 has a collector and an emitter connected in series with the LED and the second resistor 140. The base of the second transistor 153 is connected between the Zener diode 154 and the first transistor 152. The connection between the collector and the emitter of the second transistor 153 has been explained in the section on connections of other elements, and therefore will not be described here.

[0015] The Zener diode 154 is connected in the reverse direction with respect to the supply voltage V4 of the LED lighting circuit 11. Furthermore, the Zener diode 154 is connected in series with the emitter and collector of the first transistor 152. More specifically, the anode of the Zener diode 154 is connected to the ground of the LED lighting circuit 11. That is, the anode of the Zener diode 154 is connected to the negative side of the power supply. The cathode of the Zener diode 154 is connected to the base of the second transistor 153 and the collector of the first transistor 152.

[0016] The first resistor 151 is connected in series with the Zener diode 154. More specifically, one terminal of the first resistor 151 is connected to the emitter of the first transistor 152. The other terminal of the first resistor 151 is connected to the positive electrode of the battery 110.

[0017] The first transistor 152 has a collector and an emitter connected in series with the Zener diode 154 and the first resistor 151. The base of the first transistor 152 is connected between the first LED 120 and the second LED 130. The connection between the collector and the emitter of the first transistor 152 has been explained in the section on connections of other elements, and therefore will not be described here.

[0018] A-2. Element Functions: Next, the function of each element will be described.

[0019] The battery 110 supplies power to the LED lighting circuit 11. More specifically, the battery 110 is a voltage source that applies a supply voltage V4 to the LED lighting circuit 11. For example, the battery 110 is an auxiliary battery that supplies power to electrical components of the vehicle V, such as lamps and audio equipment.

[0020] The voltage applied by the battery 110 is designed to have a nominal voltage of, for example, 14 V. The voltage applied by the battery 110 may vary within a range of approximately 9 V to 16 V depending on fluctuations in the output current and the state of charge.

[0021] The first LED 120 emits light by power supplied from the battery 110. The first LED 120 is a light-emitting diode that emits one color. That is, the first LED 120 is an element having the characteristics of a diode. The forward voltage Vf of the first LED 120 is, for example, 3.5 V. Note that the symbol Vf of the "forward voltage Vf" is added to indicate that it is a voltage characteristic of the element, and is not used in the drawings.

[0022] Furthermore, the first LED 120 is also included in the constant current circuit 150, which will be described later. The first LED 120 in the constant current circuit 150 is a diode that controls the driving of the first transistor 152 by a forward voltage V1. In this embodiment, the driving of the first transistor 152 means that the base current I6 conducts between the emitter and the collector. The total voltage of the voltage V7 of the first resistor 151 and the voltage V8 between the base and the emitter of the first transistor 152 is determined by the forward voltage Vf of the first LED 120. Therefore, the first LED 120 controls the driving of the first transistor 152 by controlling the base current I6 of the first transistor 152.

[0023] By adopting such an embodiment, the LED lighting circuit 11 can use the LED as a component of the constant current circuit 150, and therefore the number of components can be reduced compared to a case where a diode is separately provided.

[0024] The second LED 130 emits light using power supplied from the battery 110. The second LED 130 is a light emitting diode that emits the same color as the first LED 120. That is, the second LED 130 has the same forward voltage Vf as the forward voltage Vf of the first LED 120.

[0025] The first LED 120 and the second LED 130 are described as LEDs having the characteristics of a real diode, not as LEDs having the characteristics of an ideal diode. Ideally, the forward voltage of an LED is completely constant in a range where the forward voltage exceeds the forward voltage Vf. However, in this embodiment, the LED is described as having a characteristic where the forward voltage also increases slightly in a range where the forward voltage exceeds the forward voltage Vf. For example, the forward current I3 of the first LED 120 fluctuates in response to fluctuations in the forward voltage V1, as in a real element, even in a voltage range where the forward voltage Vf as a characteristic is exceeded. The brightness of the LED depends on the magnitude of the current. Therefore, the brightness of the first LED 120 fluctuates in response to fluctuations in the forward voltage V1.

[0026] The second resistor 140 determines the LED current I1. More specifically, the second resistor 140 determines the LED current I1 flowing through the emitter when the second transistor 153 is driven to pass the LED current I1 between the emitter and collector of the second transistor 153. The LED current I1 will be described in detail later.

[0027] The constant current circuit 150 controls the LED current I1 flowing through the LED. The constant current circuit 150 includes a second transistor 153, a Zener diode 154, a first transistor 152, a first resistor 151, and a diode 120. In the first embodiment, the diode 120 is configured by a first LED 120 included in the LED as a load of the LED lighting circuit 11. Therefore, a description of the diode 120 will be omitted.

[0028] In addition, in a state where the forward current of the LED is flowing, the forward current I3 of the first LED 120, the forward current I4 of the second LED 130, and the emitter current I5 of the second transistor 153 are collectively referred to as the LED current I1. The state where the forward current of the LED is flowing is a state in mode M3 of the LED lighting circuit 11 described later. Strictly speaking, each current in the first line L1 is different. For example, the forward current I3 of the first LED 120 and the forward current I4 of the second LED 130 are different currents due to the influence of the base current I6 of the first transistor 152. However, the base current I6 is very small compared to the forward current I3 of the first LED 120 and the forward current I4 of the second LED 130. Therefore, the forward current I3 of the first LED 120 can be regarded as the forward current I4 of the second LED 130. Similarly, the emitter current I5 of the second transistor 153 can be regarded as the forward current I4 of the second LED 130.

[0029] The second transistor 153 controls the LED current I1. The second transistor 153 is an NPN bipolar transistor. The second transistor 153 is driven by a base current I7 controlled by a reverse voltage V3 of the Zener diode 154. Therefore, the second transistor 153 controls the LED current I1 flowing between the emitter and the collector.

[0030] The Zener diode 154 controls the driving of the second transistor 153. The Zener diode 154 controls the driving of the second transistor 153 by a reverse voltage V3. In a state where the voltage of the battery 110 is applied, the reverse voltage V3 of the Zener diode 154 reaches a Zener voltage Vz as a characteristic. Therefore, the Zener diode 154 regulates the voltage V5 between the base and emitter of the second transistor 153 and the voltage V6 of the second resistor 140 to the Zener voltage Vz. Therefore, the Zener diode 154 controls the driving of the second transistor 153 by controlling the base current I7 of the second transistor 153. Note that the symbol Vz of the "Zener voltage Vz" is added to indicate that it is a voltage as a characteristic of the element, and is not used in the illustration.

[0031] The Zener diode 154 is described as having the characteristics of a real Zener diode, not as an ideal Zener diode. For example, the reverse voltage V3 of the Zener diode 154 is ideally completely constant in a range exceeding the Zener voltage Vz. However, in this embodiment, the reverse voltage V3 of the Zener diode 154 is described as having a characteristic of slightly increasing in accordance with an increase in the reverse current I10, even in a range exceeding the Zener voltage Vz.

[0032] The first transistor 152 controls a Zener current I2 flowing through a Zener diode 154. More specifically, the first transistor 152 controls the Zener current I2 flowing between the emitter and the collector by being driven by a base current I6 controlled by a forward voltage V1 of the first LED 120. The first transistor 152 is a PNP-type bipolar transistor.

[0033] In addition, in a state where the reverse current I10 of the Zener diode 154 flows, the current in the second line L2 is collectively called the Zener current I2. The state where the reverse current I10 of the Zener diode 154 flows is a state in mode M3 of the LED lighting circuit 11 described later. That is, the emitter current I8 of the first transistor 152, the collector current I9 of the first transistor 152, and the reverse current I10 of the Zener diode 154 are collectively called the Zener current I2. Strictly speaking, each current in the second line L2 is different. However, the influence of the base current I7 of the first transistor 152 and the second transistor 153 on the current in the second line L2 is very small. Therefore, the emitter current I8 of the first transistor 152, the collector current I9 of the first transistor 152, and the reverse current I10 of the Zener diode 154 can be regarded as equivalent currents.

[0034] The first resistor 151 determines the Zener current I2 flowing through the Zener diode 154. More specifically, the first resistor 151 determines the Zener current I2 flowing through the emitter in a state in which the Zener current I2 is conducted between the emitter and collector of the first transistor 152 by driving the first transistor 152.

[0035] A-3.LED lighting circuit operation: Fig. 2 is a graph showing the relationship between the LED current I1 and the supply voltage V4. In Fig. 2, a range of modes M1 to M3 is illustrated based on the characteristics of the LED current I1 with respect to the supply voltage V4.

[0036] When the LED lighting circuit 11 lights up, it operates in the range of mode M3. That is, the voltage applied by the battery 110 is included in the range of mode M3. For example, the voltage applied by the battery 110, which fluctuates between about 9V and 16V, occurs in the range of mode M3. That is, the LED lighting circuit 11 controls the LED current I1 by the constant current circuit 150 in the range of mode M3.

[0037] In the following description, in order to facilitate understanding of the technology, the operation of the LED lighting circuit 11 in modes M1 and M2 other than mode M3 will also be described. However, as described above, the supply voltage V4 in modes M1 and M2 is lower than the voltage applied by the battery 110.

[0038] The operation of the LED lighting circuit 11 in FIG. 1 will be described for each of the modes M1 to M3, dividing it into the first line L1 and the second line L2.

[0039] Mode M1 in Figure 2 represents the state of the LED current I1 up to a range based on the LED forward voltage Vf. Voltage Va is a voltage based on the LED forward voltage Vf.

[0040] In mode M1 in Fig. 2, in the first line L1 in Fig. 1, the forward voltage V1 of the first LED 120 and the forward voltage V2 of the second LED 130 increase in response to an increase in the supply voltage V4. However, in mode M1, the voltages V1 and V2 do not reach the forward voltages Vf of the LEDs, so the LED current I1 is not conducted. That is, the first LED 120 and the second LED 130 are not lit.

[0041] 2 is illustrated to include a range M1a in which the LED current I1 flows, but the range M1a represents a forward current required in the definition of the forward voltage Vf.

[0042] In the mode M1 of FIG. 2, the second transistor 153 starts to be driven in the first line L1 of FIG. 1. This is because the reverse voltage V3 of the Zener diode 154 also rises in response to the rise in the supply voltage V4. More specifically, the rise in the reverse voltage V3 causes the voltage V6 of the second resistor 140 and the voltage V5 between the base and emitter of the first transistor 152 to rise. The voltage V5 between the base and emitter of the second transistor 153 exceeds the forward voltage Vf between the base and emitter, causing the base current I7 of the second transistor 153 to flow. Thus, the second transistor 153 starts to be driven in response to the rise in the reverse voltage V3. The rise in the reverse voltage V3 of the Zener diode 154 will be described later.

[0043] In mode M1 of FIG. 2, in the second line L2 of FIG. 1, the driving of the first transistor 152 is controlled by the forward voltage V1 of the first LED 120. More specifically, the voltage V7 of the first resistor 151 and the voltage V8 between the base and emitter of the first transistor 152 increase in response to an increase in the forward voltage V1. When the voltage V8 between the base and emitter of the first transistor 152 exceeds the forward voltage Vf between the base and emitter, the base current I6 is conducted. Therefore, when the first transistor 152 is driven, the emitter current I8 of the first transistor 152 is conducted.

[0044] The emitter current I8 is calculated by formula (1). In formula (1), R is the resistance value of the first resistor 151. That is, the relationship is V7=R×I8. As formula (1) shows, the emitter current I8 increases in response to an increase in the forward voltage V1 of the first LED 120. I8 = (V1 - V8) / R ... (1)

[0045] The emitter current I8 can be regarded as the Zener current I2 in a state where the reverse current I10 of the Zener diode 154 flows. The state where the reverse current I10 of the Zener diode 154 flows is the state in mode M3 of FIG. 2, which will be described later. In this case, the Zener current I2 can also be calculated by formula (1). However, in mode M1 of FIG. 2, the reverse voltage V3 of the Zener diode 154 is less than the Zener voltage Vz. That is, the reverse current I10 of the Zener diode 154 does not flow. Therefore, in mode M1, the emitter current I8 is not regarded as the Zener current I2.

[0046] 2 represents a state in which the LED current I1 increases according to the supply voltage V4 in a range based on the forward voltage Vf of the LED and the Zener voltage Vz of the Zener diode 154. The voltage Vb is a voltage based on the Zener voltage Vz.

[0047] In the mode M2 ​​in Fig. 2, the forward voltage V1 of the first LED 120 and the forward voltage V2 of the second LED 130 exceed the forward voltage Vf in the first line L1 in Fig. 1, so that the LED current I1 flows. Furthermore, the LED current I1 increases in response to an increase in the supply voltage V4.

[0048] 2 is determined by the second resistor 140. More specifically, when the second transistor 153 is driven, the LED current I1 flows between the emitter and the collector of the second transistor 153. Therefore, the LED current I1 is determined by the second resistor 140.

[0049] In mode M2 ​​in FIG. 2, in the second line L2 in FIG. 1, the reverse voltage V3 of the Zener diode 154 increases in response to an increase in the supply voltage V4. However, in mode M2, the reverse voltage V3 of the Zener diode 154 is lower than the Zener voltage Vz. Therefore, the reverse current I10 of the Zener diode 154 is not conducted. That is, in mode M2, the emitter current I8 is not regarded as the Zener current I2.

[0050] 2 is a state in which the LED current I1 is controlled by the constant current circuit 150 in a range exceeding the Zener voltage Vz of the Zener diode 154. First, the second line L2 in FIG. 1 will be described.

[0051] In mode M3 in FIG. 2, in the second line L2 in FIG. 1, the reverse voltage V3 of the Zener diode 154 exceeds the Zener voltage Vz, so that the reverse current I10 of the Zener diode 154 flows. That is, the Zener current I2 flows. At this time, the emitter current I8 can be regarded as the Zener current I2. Therefore, the Zener current I2 is calculated by the formula (1).

[0052] In the mode M3 of FIG. 2, the reverse voltage V3 of the Zener diode 154 is determined by the Zener voltage Vz. As described above, the Zener diode 154 is not an ideal Zener diode. For this reason, for example, in a circuit without the first transistor 152, when the Zener current I2 increases due to an increase in the supply voltage V4, the reverse voltage V3 of the Zener diode 154 also increases slightly. However, as shown in the formula (1), the emitter current I8 as the Zener current I2 is controlled by the first transistor 152 based on the forward voltage V1 of the first LED 120. The forward voltage V1 of the first LED 120 is a constant voltage because it exceeds the forward voltage Vf. Therefore, even if the supply voltage V4 increases, the Zener current I2 does not increase. Therefore, the reverse voltage V3 of the Zener diode 154 does not increase either.

[0053] In mode M3 in Fig. 2, in the first line L1 in Fig. 1, the reverse voltage V3 of the Zener diode 154 does not increase, so that the base current I7 of the second transistor 153 becomes constant. That is, the LED current I1 becomes constant by the second transistor 153 in response to an increase in the supply voltage V4. Therefore, the first LED 120 and the second LED 130 are lit with constant brightness.

[0054] In the mode M3 of FIG. 2, the LED current I1 increases slightly with an increase in the supply voltage V4. This is due to the characteristics of the actual elements, since the Zener diode 154 and the first LED 120 are not ideal elements. That is, since the drive of the first transistor 152 is controlled by the forward voltage V1 of the first LED 120, the Zener current I2 increases with a slight increase in the forward voltage V1. Furthermore, since the drive of the second transistor 153 is controlled by the reverse voltage V3 of the Zener diode 154, the LED current I1 increases with a slight increase in the reverse voltage V3.

[0055] However, in the LED lighting circuit 11, the Zener current I2 and the LED current I1 are controlled, thereby suppressing the voltage fluctuation due to the characteristics of the real elements of the Zener diode 154 and the first LED 120. Therefore, in the mode M3 in Fig. 2, the increase in the LED current I1 due to the characteristics of the real elements of the Zener diode 154 and the first LED 120 is suppressed compared to the case where the Zener current I2 is not controlled.

[0056] In the mode M3 in Fig. 2, the LED lighting circuit 11 suppresses the increase in the LED current I1, thereby suppressing heat generation from the elements. That is, the LED lighting circuit 11 can reduce heat generation from the circuit compared to when the Zener current I2 is not controlled, and therefore can reduce the number of mechanisms for heat dissipation.

[0057] With this configuration, the constant current circuit 150 controls the LED current I1 by the second transistor 153 with the reverse voltage V3 of the Zener diode 154. The Zener current I2 flowing through the Zener diode 154 is controlled by the first transistor 152. Furthermore, the driving of the first transistor 152 is controlled by the forward voltage V1 of the diode 120, which is also the first LED 120. That is, the Zener current I2 is controlled based on the forward voltage V1 of the diode 120 having constant voltage characteristics. Therefore, the reverse voltage V3 of the Zener diode 154 is stabilized by controlling the Zener current I2. Therefore, the constant current circuit 150 can stably control the LED current I1 by the second transistor 153. Therefore, the LED lighting circuit 11 can further suppress the fluctuation of the LED brightness.

[0058] Furthermore, in the LED lighting circuit 11, the first LED 120 can be configured as a diode of the constant current circuit 150, so that the number of components can be reduced compared to the case where a separate diode is provided.

[0059] Moreover, the first transistor 152 is controlled by a forward voltage V1 by having its base connected to the first LED 120 serving as the diode 120 in the constant current circuit 150. The second transistor 153 is controlled by a reverse voltage V3 by having its base connected to a Zener diode 154. In other words, the LED lighting circuit 11 can easily control the first transistor 152 and the second transistor 153 by utilizing the characteristics of the diodes.

[0060] Furthermore, the LED lighting circuit 11 generates the reverse voltage V3 of the Zener diode 154 with the ground as a reference. Therefore, the LED lighting circuit 11 can control the second transistor 153 more stably than when the reverse voltage V3 of the Zener diode 154 is not based on the ground.

[0061] B. Second embodiment: FIG. 3 is a circuit diagram showing the LED lighting circuit 21 of the second embodiment. The area surrounded by the dashed line in FIG. 3 is the part of the constant current circuit 250 in the second embodiment. In the second embodiment, some of the elements and their connections are different from those in the first embodiment. In the LED lighting circuit 21 shown in FIG. 3, the configuration corresponding to the configuration of the LED lighting circuit 11 of the first embodiment is shown by adding a code in which the first digit of the three-digit code added to the configuration of the LED lighting circuit 11 is replaced with a 2 instead of 1. For example, the first transistor 252 corresponds to the first transistor 152 and has the same function as the first transistor 152.

[0062] In the second embodiment, the first transistor 252 is an NPN bipolar transistor. In the second embodiment, the second transistor 253 is a PNP bipolar transistor. The other elements are the same as those in the first embodiment.

[0063] Furthermore, in the second embodiment, the connections of the elements are different from those in the first embodiment. The connections of the elements will be described in detail below.

[0064] The first LED 220 is connected in the forward direction to the battery 210 that outputs the supply voltage V4a. Furthermore, the first LED 220 is connected in series to the emitter and collector of the second transistor 253. More specifically, the cathode of the first LED 220 is connected to the ground of the LED lighting circuit 21. The anode of the first LED 220 is connected to the base of the first transistor 252 and the cathode of the second LED 230.

[0065] The second LED 230 is connected in the forward direction to the battery 210 that outputs the supply voltage V4a. Furthermore, the second LED 230 is connected in series to the emitter and collector of the second transistor 253. More specifically, the anode of the second LED 230 is connected to the collector of the second transistor 253. The cathode of the second LED 230 is connected to the base of the first transistor 252 and the anode of the first LED 220.

[0066] The second resistor 240 is connected in series with the LED. More specifically, one terminal of the second resistor 240 is connected to the emitter of the second transistor 253. The other terminal of the second resistor 240 is connected to the positive electrode of the battery 210.

[0067] The second transistor 253 has a collector and an emitter connected in series with the LED and the second resistor 240. The base of the second transistor 253 is connected between the Zener diode 254 and the first transistor 252. The connection between the collector and the emitter of the second transistor 253 has been explained in the section on connections of other elements, and therefore will not be described here.

[0068] The Zener diode 254 is connected in the reverse direction to the supply voltage V4a of the LED lighting circuit 21. Furthermore, the Zener diode 254 is connected in series to the emitter and collector of the first transistor 252. More specifically, the cathode of the Zener diode 254 is connected to the positive side of the power supply. The anode of the Zener diode 254 is connected to the base of the second transistor 253 and the collector of the first transistor 252.

[0069] The first resistor 251 is connected in series to the Zener diode 254. More specifically, one terminal of the first resistor 251 is connected to the emitter of the first transistor 252. The other terminal of the first resistor 251 is connected to the ground of the LED lighting circuit 21.

[0070] The first transistor 252 has a collector and an emitter connected in series with the Zener diode 254 and the first resistor 251. The base of the first transistor 252 is connected between the first LED 220 and the second LED 230. The connection between the collector and the emitter of the first transistor 252 has been explained in the section on connections of other elements, and therefore will not be described here.

[0071] In the second embodiment, the characteristics of the LED current I1a with respect to the supply voltage V4a are the same as those in the first embodiment.

[0072] With such a configuration, the LED lighting circuit 21 can be applied even when, for example, the first LED 220 has a common cathode and needs to have its cathode connected to ground.

[0073] C. Third embodiment: FIG. 4 is a circuit diagram showing an LED lighting circuit 31 of the third embodiment. The LED lighting circuit 31 of the third embodiment is a configuration in which the second LED 130 is removed from the LED lighting circuit 11 of the first embodiment. A constant current circuit 350 corresponds to the constant current circuit 150 of the first embodiment. Other configurations are the same as those of the first embodiment. The second LED 130 is not an element that contributes to the constant current circuit 350. Therefore, even in the case of a single LED, the LED lighting circuit 31 of the third embodiment can suppress fluctuations in the brightness of the LED.

[0074] D. Other embodiments: In the first and second embodiments, the diode in the constant current circuit is constituted by the first LED included in the LED as the load in the LED lighting circuit. However, the diode does not have to be constituted by an LED. The diode may be another element having diode characteristics. For example, the first LED may be a rectifier diode or a Zener diode.

[0075] E. Other embodiments: The LED lighting circuit of the above embodiment may be used to configure an in-vehicle lighting device. For example, the in-vehicle lighting device includes three LED lighting circuits of the first or second embodiment. The first LED lighting circuit is configured with the first LED 120 and the second LED 130 being LEDs that emit red light. The second LED lighting circuit is configured with the first LED 120 and the second LED 130 being LEDs that emit blue light. The third LED lighting circuit is configured with the first LED 120 and the second LED 130 being LEDs that emit green light. That is, the in-vehicle lighting device is configured with three LED lighting circuits that configure three colors. Note that, in the three LED lighting circuits, a supply voltage is applied from the same battery to the battery.

[0076] By adopting this configuration, the in-vehicle lighting device can express the color tone of the three-color lighting with greater precision because the LED lighting circuit suppresses fluctuations in LED brightness.

[0077] F. Other Embodiments: In the above-mentioned vehicle-mounted lighting device, the second resistor in each of the first LED lighting circuit, the second LED lighting circuit, and the third LED lighting circuit may be configured with a variable resistor. More specifically, each of the LED lighting circuits may control its LED current by a variable resistor.

[0078] With this configuration, each LED lighting circuit can change the brightness of the LEDs by the second resistor, which is a variable resistor. Therefore, the lighting device of the present disclosure can emit light of various colors by adjusting the second resistor of the first LED lighting circuit, the second resistor of the second LED lighting circuit, and the second resistor of the third LED lighting circuit. Furthermore, the lighting device of the present disclosure can reduce the variation in the color of the emitted light even when the supply voltage fluctuates.

[0079] G. Other embodiments: In the above embodiment, the first transistor and the second transistor are bipolar transistors. However, the first transistor and the second transistor are not limited to bipolar transistors. The first transistor and the second transistor may be MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors).

[0080] The present disclosure is not limited to the above-mentioned embodiments, and can be realized in various configurations without departing from the spirit of the present disclosure. For example, the technical features of the embodiments corresponding to the technical features in each form described in the Summary of the Disclosure can be appropriately replaced or combined to solve some or all of the above problems or to achieve some or all of the above effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0081] 11, 21, 31 ... LED lighting circuit, 110, 210 ... battery, 120, 220 ... diode, first LED, 130, 230 ... second LED, 140, 240 ... second resistor, 150, 250, 350 ... constant current circuit, 151, 251 ... first resistor, 152, 252 ... first transistor, 153, 253 ... second transistor, 154, 254 ... Zener diode, I1, I1a...LED current, I2...Zener current, I3,I4,I10...current, I5,I8...emitter current, I6,I7...base current, I9...collector current, L1...first line, L2...second line, M1~M3...mode, M1a...range, V...vehicle, V1~V3...voltage, V4,V4a...supply voltage, V5~V8...voltage, Va,Vb...voltage, Vf...forward voltage, Vz...Zener voltage

Claims

1. An in-vehicle LED lighting circuit, A constant current circuit for controlling an LED current flowing through one or more LEDs is provided, The constant current circuit is A Zener diode, a first transistor for controlling a Zener current flowing through the Zener diode; a diode that controls the driving of the first transistor by a forward voltage; an LED lighting circuit comprising a second transistor that controls the LED current by a reverse voltage of the Zener diode;

2. 2. The in-vehicle LED lighting circuit according to claim 1, The LED lighting circuit, wherein the diode is an LED included in the one or more LEDs.

3. 3. The in-vehicle LED lighting circuit according to claim 2, The constant current circuit further comprises: a first resistor defining the Zener current; a second resistor that determines the LED current; the Zener diode is connected in a reverse direction with respect to a supply voltage of the LED lighting circuit, and is connected in series with the emitter and collector of the first transistor; the diode is forward connected to the supply voltage and connected in series to the emitter and collector of the second transistor; the base of the first transistor is connected between the diode and the second transistor; a base of the second transistor is connected between the Zener diode and the first transistor.

4. 4. The in-vehicle LED lighting circuit according to claim 3, The cathode of the diode is connected to the ground of the LED lighting circuit.

5. 4. The in-vehicle LED lighting circuit according to claim 3, The anode of the Zener diode is connected to the ground of the LED lighting circuit.

6. An in-vehicle lighting device, 6. The in-vehicle LED lighting circuit according to claim 3, wherein the at least one LED is an LED that emits red light; and A second LED lighting circuit in the vehicle, the one or more LEDs being LEDs that emit blue light; and a third LED lighting circuit which is an in-vehicle LED lighting circuit and in which the one or more LEDs are LEDs that emit green light.

7. The vehicle-mounted lighting device according to claim 6, a second resistor in each of the first LED lighting circuit, the second LED lighting circuit, and the third LED lighting circuit, which is a variable resistor;

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