Lighting circuit

Abstract

A lighting circuit includes: a drive circuit that drives a light source; a first boost converter that includes a first switching element and generates a power supply voltage of the drive circuit; a second boost converter that includes a second switching element and generates the power supply voltage; a first voltage detection circuit that detects a first voltage corresponding to a voltage of a first node on a high potential side in the first switching element; a second voltage detection circuit that detects a second voltage corresponding to a voltage of a second node on a high potential side in the second switching element; and a determination circuit that determines whether there is an abnormality in either the first or second boost converter on the basis of the first and second voltages.

Description

Lighting circuit

[0001] The present invention relates to a lighting circuit.

[0002] 2. Description of the Related Art Known lighting circuits used in lamps (e.g., vehicle lamps) use a boost converter that boosts an input voltage based on the switching operation of a switching element (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2018-32553

[0004] However, when a load is driven by one boost converter, if the current flowing through the load is large, the boost converter may generate heat. If two boost converters are arranged in parallel and operated alternately, the load is halved and heat generation can be suppressed, but if an abnormality occurs in one of them (for example, an abnormality due to an open circuit in an element constituting the boost converter), the load will be solely on the other, and heat generation cannot be suppressed.

[0005] An object of the present invention is to provide a lighting circuit that can prevent abnormal heat generation.

[0006] The main invention for achieving the above-mentioned object is a lighting circuit comprising: a drive circuit for driving a light source; a first boost converter including a first switching element and generating a power supply voltage for the drive circuit; a second boost converter including a second switching element and generating the power supply voltage; a first voltage detection circuit for detecting a first voltage corresponding to the voltage of a first node on the high potential side of the first switching element; a second voltage detection circuit for detecting a second voltage corresponding to the voltage of a second node on the high potential side of the second switching element; and a determination circuit for determining whether or not there is an abnormality in either the first or second boost converter based on the first and second voltages.

[0007] According to the present invention, it is possible to provide a lighting circuit that can prevent abnormal heat generation.

[0008] FIG. 1 is a diagram showing the configuration of a vehicular lamp 1. FIG. 2 is a diagram showing the configuration of a lighting circuit 10 of the first embodiment. FIG. 3 is a flowchart showing the operation of abnormality determination in the lighting circuit 10 of the first embodiment. FIG. 4 is a diagram showing waveforms of voltages V1 and V2 when the first boost circuit 21 and the second boost circuit 22 are normal. FIG. 5 is a diagram showing waveforms of voltages V1 and V2 when the coil L2 of the second boost circuit 22 is open. FIG. 6 is a diagram showing waveforms of voltages V1 and V2 when the NMOS transistor Q2 of the second boost circuit 22 is open. FIG. 7 is a diagram showing the configuration of a lighting circuit 10A of the second embodiment. FIG. 8 is a flowchart showing the operation of abnormality determination in the lighting circuit 10A of the second embodiment. FIG. 9 is a diagram showing waveforms when the first boost circuit 21 and the second boost circuit 22 are normal in the second embodiment. FIG. 10 is a diagram showing waveforms when the coil L2 of the second boost circuit 22 is open. FIG. 11 is a diagram showing waveforms when the NMOS transistor Q2 of the second boost circuit 22 is open.

[0009] <Cross-reference to related applications> This application claims priority to Japanese patent application No. 2024-122318, filed on July 29, 2024, the contents of which are incorporated by reference.

[0010] At least the following matters will become clear from the description of this specification and the accompanying drawings.

[0011] In the following description, the same or equivalent components, members, etc. shown in each drawing will be denoted by the same reference numerals, and redundant explanations may be omitted as appropriate.

[0012] In this embodiment, "connection" refers to an electrically connected state unless otherwise specified. Therefore, "connection" includes not only a case where two components are connected via wiring, but also a case where two components are connected via, for example, a resistor.

[0013] First Embodiment <<Configuration of Vehicle Lamp 1>> Fig. 1 is a diagram showing the configuration of a vehicle lamp 1. Fig. 2 is a diagram showing the configuration of a lighting circuit 10 of the first embodiment.

[0014] As shown in FIG. 1, the vehicle lamp 1 includes a light source 4 and a lighting circuit 10 .

[0015] The light source 4 includes a light-emitting element, and is lit by a drive current (constant current) supplied from a drive circuit 30 (described later) of the lighting circuit 10. In this embodiment, a light-emitting diode (LED) is used as the light-emitting element, but the light-emitting element is not limited to a light-emitting diode. For example, other semiconductor light-emitting elements such as a laser diode (LD) or an organic EL element, or an incandescent lamp may also be used. Furthermore, the number of light-emitting elements in the light source 4 is not limited to one, and multiple light-emitting elements may be connected in series or in parallel. Furthermore, multiple light sources for different purposes may be provided in parallel. These light sources may then be configured to be lit appropriately.

[0016] The lighting circuit 10 is a circuit that lights the light source 4 based on a voltage Vbat (e.g., 12 V) supplied from the vehicle battery 2, and is a module in which multiple circuits for lighting the light source 4 (light-emitting element) are attached to a board.

[0017] In this embodiment, the voltage drop due to elements (e.g., diodes) (not shown) provided in the supply line of the voltage Vbat from the battery 2 to the lighting circuit 10 is ignored. Therefore, the voltage Vbat is input to the lighting circuit 10. In other words, the voltage Vbat corresponds to the "input voltage."

[0018] <<Configuration of the lighting circuit 10>> As shown in Figures 1 and 2 , the lighting circuit 10 includes a first boost circuit 21, a second boost circuit 22, a capacitor C, a drive circuit 30, a first voltage detection circuit 41, a second voltage detection circuit 42, a microcomputer 50, and a switching control circuit 60.

[0019] <First Boost Circuit 21> The first boost circuit 21 is a circuit that boosts a voltage Vbat (e.g., 12 V) to generate a power supply voltage Vcc (e.g., 45 V) for the drive circuit 30. The first boost circuit 21 corresponds to a "first boost converter." As shown in FIG. 2 , the first boost circuit 21 of this embodiment includes a coil L1, a diode D1, and an NMOS transistor Q1.

[0020] A voltage Vbat is applied to one end of the coil L1. The other end of the coil L1 is connected to the high-potential electrode (drain) of the NMOS transistor Q1. This connection point is referred to as node N1. The coil L1 corresponds to the "first inductor," and the node N1 corresponds to the "first node."

[0021] The anode of the diode D1 is connected to the node N1, and the cathode is connected to the drive circuit 30 via the capacitor C. In other words, the diode D1 has an anode connected to the node N1 and a cathode that outputs the power supply voltage Vcc to the drive circuit 30. The diode D1 corresponds to a "third diode."

[0022] The source of the NMOS transistor Q1 is grounded, and a signal is input to the gate from the switching control circuit 60. The NMOS transistor Q1 turns on and off depending on the level of the input signal. Specifically, when the signal is high level (hereinafter referred to as H level), the NMOS transistor Q1 turns on, and when the signal is low level (hereinafter referred to as L level), the NMOS transistor Q1 turns off. The NMOS transistor Q1 corresponds to a "first switching element."

[0023] <Second Boost Circuit 22> Similar to the first boost circuit 21, the second boost circuit 22 is a circuit that boosts the voltage Vbat (e.g., 12 V) to generate the power supply voltage Vcc (e.g., 45 V) for the drive circuit 30. The second boost circuit 22 corresponds to a "second boost converter." The second boost circuit 22 of this embodiment includes a coil L2, a diode D2, and an NMOS transistor Q2.

[0024] A voltage Vbat is applied to one end of the coil L2. The other end of the coil L2 is connected to the high-potential electrode (drain) of the NMOS transistor Q2. This connection point is referred to as node N2. The coil L2 corresponds to a "second inductor," and the node N2 corresponds to a "second node."

[0025] The anode of the diode D2 is connected to the node N2, and the cathode is connected to the drive circuit 30 via the capacitor C. In other words, the diode D2 has an anode connected to the node N2 and a cathode that outputs the power supply voltage Vcc to the drive circuit 30. The diode D2 corresponds to a "fourth diode."

[0026] The source of the NMOS transistor Q2 is grounded, and a signal from the switching control circuit 60 is input to the gate. Like the NMOS transistor Q1, the NMOS transistor Q2 is turned on and off depending on the level of the input signal. The NMOS transistor Q2 corresponds to a "second switching element." In this embodiment, NMOS transistors (NMOS transistors Q1 and Q2) are used as the switching elements of the first boost circuit 21 and the second boost circuit 22, but are not limited to NMOS transistors. For example, PMOS transistors or bipolar transistors may also be used.

[0027] 1 and 2, the first boost circuit 21 and the second boost circuit 22 are provided in parallel and controlled to operate alternately. This makes it possible to suppress heat generation compared to when a single boost circuit boosts the voltage Vbat and supplies current to the drive circuit 30.

[0028] The configuration of the boost circuits (first boost circuit 21 and second boost circuit 22) is not limited to the above, as long as they can boost the input voltage using at least a switching element (transistor, etc.).

[0029] <Drive Circuit 30> The drive circuit 30 is a circuit that receives the output voltages of the first boost circuit 21 and the second boost circuit 22 (here, the charging voltage of the capacitor C), and generates a voltage according to the load while generating a predetermined drive current (constant current) for driving the light emitting elements of the light source 4. The drive circuit 30 is, for example, a step-down DC-DC circuit (step-down converter) controlled to output a constant current.

[0030] <First Voltage Detection Circuit 41> The first voltage detection circuit 41 is a circuit that detects a voltage corresponding to the voltage of the node N1 (hereinafter also referred to as voltage VN1) of the first boost circuit 21. The first voltage detection circuit 41 of this embodiment includes a diode Da and a first voltage divider circuit 71, as shown in FIG.

[0031] The diode Da is a backflow prevention element. The anode of the diode Da is connected to the node N1 of the first voltage boost circuit 21, and the cathode is connected to the first voltage divider circuit 71.

[0032] The first voltage divider circuit 71 divides the voltage VN1 at the node N1 (more specifically, the voltage resulting from the forward voltage drop across the diode Da from the voltage VN1) to a voltage level (here, 5 V or less) that can be input to the microcomputer 50. The first voltage divider circuit 71 includes resistors Ra and Rb connected in series. The resistor Ra is connected to the node N1 via the diode Da, and the resistor Rb is grounded. The voltage (hereinafter referred to as voltage V1) generated at the connection node between the resistors Ra and Rb is the output of the first voltage detection circuit 41.

[0033] The voltage V1 corresponds to the voltage VN1 at the node N1 and corresponds to the “first voltage” and the “first divided voltage.” The diode Da has an anode connected to the node N1 and a cathode connected to the first voltage divider circuit 71, and corresponds to the “first diode.”

[0034] <Second voltage detection circuit 42>

[0035] The second voltage detection circuit 42 is a circuit that detects a voltage corresponding to the voltage of the node N2 (hereinafter also referred to as voltage VN2) of the second boost circuit 22. The second voltage detection circuit 42 includes a diode Db and a second voltage divider circuit 72.

[0036] The diode Db is a backflow prevention element. The anode of the diode Db is connected to the node N2 of the second boost circuit 22, and the cathode is connected to the second voltage divider circuit 72.

[0037] The second voltage-dividing circuit 72 divides the voltage VN2 at the node N2 (more specifically, the voltage resulting from the forward voltage drop across the diode Db from the voltage VN2) to a voltage level (here, 5 V or less) that can be input to the microcomputer 50. The second voltage-dividing circuit 72 includes resistors Rc and Rd connected in series. The resistor Rc is connected to the node N2 via the diode Db, and the resistor Rd is grounded. The voltage (hereinafter, voltage V2) generated at the connection node between the resistors Rc and Rd is the output of the second voltage detection circuit 42. In this embodiment, the first voltage-dividing circuit 71 and the second voltage-dividing circuit 72 have the same configuration (i.e., the resistance values ​​of the resistors Ra and Rc are the same, and the resistance values ​​of the resistors Rb and Rd are the same).

[0038] Voltage V2 corresponds to the voltage at node N2 and corresponds to the “second voltage” and the “second divided voltage.” Diode Db has an anode connected to node N2 and a cathode connected to second voltage divider circuit 72, and corresponds to the “second diode.”

[0039] <Microcomputer 50> The microcomputer 50 acquires the detected voltage (voltage V1) of the first voltage detection circuit 41 and the detected voltage (voltage V2) of the second voltage detection circuit 42. Then, based on the acquired voltages, the microcomputer 50 instructs the switching control circuit 60 to perform the switching operation of the NMOS transistors Q1 and Q2 so that the power supply voltage Vcc (voltage of the capacitor C) of the drive circuit 30 becomes a predetermined level (e.g., 45 V).

[0040] Furthermore, the microcomputer 50 determines whether or not there is an abnormality in either the first boost circuit 21 or the second boost circuit 22 based on the acquired voltages V1 and V2. The microcomputer 50 corresponds to a "determination circuit." The operation of this determination will be described later.

[0041] <Switching Control Circuit 60> The switching control circuit 60 appropriately turns on and off the NMOS transistor Q1 of the first boost circuit 21 and the NMOS transistor Q2 of the second boost circuit 22 (outputs a signal to control on and off) in response to instructions from the microcomputer 50.

[0042] <Regarding Boosting Operation> The boosting operation is the same for the first boosting circuit 21 and the second boosting circuit 22. The boosting operation will be described below using the first boosting circuit 21 as an example.

[0043] When the signal from the switching control circuit 60 goes high and turns on the NMOS transistor Q1, a current flows from the battery 2 to the coil L1, node N1, NMOS transistor Q1, and then to ground. At this time, energy is stored in the coil L1 from the battery 2. Note that, because a diode D1 is provided between the node N1 and the capacitor C, no current flows from the capacitor C to the NMOS transistor Q1 (the capacitor C is not discharged).

[0044] Thereafter, when the signal from the switching control circuit 60 goes low and the NMOS transistor Q1 turns off, the coil L1 tries to continue to pass current, generating a voltage equivalent to the stored energy. This voltage is applied to the capacitor C via the diode D1.

[0045] By repeating this operation, a voltage is superimposed on the capacitor C, and the voltage of the capacitor C becomes higher than the input voltage (voltage Vbat).

[0046] In the lighting circuit 10 of this embodiment, the first boost circuit 21 and the second boost circuit 22 are provided in parallel and operated alternately, thereby reducing the burden (load) on each boost circuit. This reduces the occurrence of abnormal heat generation. However, if an abnormality occurs in either the first boost circuit 21 or the second boost circuit 22, only the other boost circuit is used for voltage boosting. This may result in abnormal heat generation. Therefore, in this embodiment, it is possible to determine whether an abnormality has occurred (abnormality determination).

[0047] <<Abnormality Determination Operation>> Fig. 3 is a flow chart showing an example of the abnormality determination operation in the lighting circuit 10 of the first embodiment. Fig. 4 is a diagram showing the waveforms of the voltages V1 and V2 when the first boost circuit 21 and the second boost circuit 22 are normal, and Fig. 5 is a diagram showing the waveforms of the voltages V1 and V2 when the coil L2 of the second boost circuit 22 is open. The horizontal axis of Figs. 4 and 5 represents time, and the vertical axis represents voltage magnitude. In Figs. 4 and 5, the voltage V1 is indicated by a solid line, and the voltage V2 is indicated by a dashed line.

[0048] First, the microcomputer 50 receives the voltages V1 and V2 from the first voltage detection circuit 41 and the second voltage detection circuit 42, respectively (FIG. 3: S01). As described above, the voltages V1 and V2 are voltages (divided voltages) obtained by dividing the voltages (voltages VN1 and VN2) at the nodes N1 and N2, respectively.

[0049] When the first booster circuit 21 and the second booster circuit 22 are normal, the voltages V1 and V2 have a pulse-like waveform that alternately rises as shown in FIG.

[0050] For example, from time t0 to time t1, both the NMOS transistor Q1 and the NMOS transistor Q2 are on, and both the voltages V1 and V2 are at the L level.

[0051] At time t1, NMOS transistor Q1 turns off, causing voltage V1 to go high. Then, at time t2, NMOS transistor Q1 turns on, causing voltage V1 to go low. During this time (between times t1 and t2), NMOS transistor Q2 is on, and voltage V2 remains low.

[0052] After that, at time t3, NMOS transistor Q2 turns off, causing voltage V2 to go high. Then, at time t4, NMOS transistor Q2 turns on, causing voltage V2 to go low. During this time (between times t3 and t4), NMOS transistor Q1 remains on, and voltage V1 remains low.

[0053] In this way, by alternately switching between the first boost circuit 21 and the second boost circuit 22, the input voltage (voltage Vbat) of approximately 12 V is boosted to approximately 45 V. In this case, the load is halved compared to when boosting is performed with one boost circuit, so abnormal heat generation can be suppressed.

[0054] However, for example, if the coil L2 of the second boost circuit 22 is open, the voltage V2 becomes zero (V) and only the voltage V1 fluctuates, as shown in Figure 5. In other words, the boost operation is performed only by the first boost circuit 21. This increases the load on the first boost circuit 21, which may cause abnormal heat generation.

[0055] Therefore, the microcomputer 50 determines whether there is an abnormality in the waveforms of the acquired voltages V1 and V2 (FIG. 3: S02). Here, the fluctuation of the waveforms of voltages V1 and V2 is evaluated (for example, the presence or absence of a pulse edge is detected). If only one of the voltages (voltage V1) rises as shown in FIG. 5, it can be determined that the other (voltage V2: i.e., the second boost circuit 22) is abnormal.

[0056] If no abnormality is found in step S02 (NO in S02), the process returns to step S01 and the same operation is performed.

[0057] On the other hand, in step S02, if there is an abnormality in the waveform as shown in Figure 5 (YES in S02), it is determined that an abnormality has occurred in either the first boost circuit 21 or the second boost circuit 22 (here, the second boost circuit 22 is abnormal) (Figure 3: S03), and the switching control circuit 60 is caused to stop the switching operation of the NMOS transistors Q1 and Q2 (Figure 3: S04).

[0058] In this way, if an abnormality occurs, the operation of both boost circuits is stopped, thereby suppressing abnormal heat generation caused by driving only one of the first boost circuit 21 and the second boost circuit 22.

[0059] <When the transistor is open>

[0060] In the above embodiment, the evaluation was described for the case where the coil L2 of the second boost circuit 22 becomes open. However, for example, the evaluation can be similarly performed for the case where the NMOS transistor Q2 of the second boost circuit 22 becomes open.

[0061] FIG. 6 is a diagram showing the waveforms of the voltages V1 and V2 when the NMOS transistor Q2 of the second booster circuit 22 is open.

[0062] When the NMOS transistor Q2 of the second boost circuit 22 becomes open, no switching operation is performed by the NMOS transistor Q2, and the voltage at node N2 (voltage VN2) becomes the input voltage (Vbat: for example, 12 V). Therefore, voltage V2 becomes the voltage obtained by dividing voltage Vbat by the second voltage divider circuit 72. In this case as well, it is possible to determine that an abnormality exists from the waveform. Note that an open coil and an open transistor can be distinguished from the voltage level of the abnormal waveform (see voltage V2 in Figures 5 and 6).

[0063] 7 is a diagram showing the configuration of a lighting circuit 10A according to the second embodiment. The lighting circuit 10A includes a first step-up circuit 21, a second step-up circuit 22, a capacitor C, a drive circuit 30, a first voltage detection circuit 41A, a second voltage detection circuit 42A, a microcomputer 50, and a switching control circuit 60. That is, in the second embodiment, the configurations of the voltage detection circuits (first voltage detection circuit 41A, second voltage detection circuit 42A) are different from those of the first embodiment (first voltage detection circuit 41, second voltage detection circuit 42).

[0064] For convenience, the output voltages of the first voltage detection circuit 41A and the second voltage detection circuit 42A are denoted by the voltages V1 and V2, the same as in the first embodiment.

[0065] The first voltage detection circuit 41A includes a diode Da, a first voltage dividing circuit 71, a first filter circuit 81, and a coil La.

[0066] The first filter circuit 81 is a filter circuit (low-pass filter circuit) that passes low-frequency components of a signal and blocks high-frequency components. Specifically, it is a circuit that smooths the waveform of a signal generated by switching at a high frequency (e.g., about 350 kHz) and converts it into a waveform that matches the sampling frequency (e.g., 3.5 kHz) of the microcomputer 50 (the same applies to the second filter circuit 82 described later). The first filter circuit 81 includes a resistor R1 and a capacitor C1.

[0067] One end of the resistor R1 is connected to the connection node between the resistors Ra and Rb that make up the first voltage divider circuit 71, and the other end of the resistor R1 is grounded via the capacitor C1. The resistor R1 corresponds to the "first resistor," and the capacitor C1 corresponds to the "first capacitor."

[0068] The coil La is a noise removal component that removes noise before outputting the signal. One end of the coil La is connected to the capacitor C1, and the output (voltage V1) of the first voltage detection circuit 41A is output from the other end. Note that the voltage V1 (output of the first filter circuit 81) in the second embodiment corresponds to the "first output."

[0069] The second voltage detection circuit 42A includes a diode Db, a second voltage dividing circuit 72, a second filter circuit 82, and a coil Lb.

[0070] The second filter circuit 82 is a filter circuit (low-pass filter circuit) similar to the first filter circuit 81, and includes a resistor R2 and a capacitor C2.

[0071] One end of the resistor R2 is connected to the connection node between the resistors Rc and Rd that make up the second voltage divider circuit 72, and the other end of the resistor R2 is grounded via the capacitor C2. The resistor R2 corresponds to the "second resistor," and the capacitor C2 corresponds to the "second capacitor."

[0072] One end of the coil Lb is connected to the capacitor C2, and the other end outputs the output (voltage V2) of the second voltage detection circuit 42A. Note that the voltage V2 (output of the first filter circuit 81) in the second embodiment corresponds to the "second output."

[0073] <<Abnormality Determination Operation>> Fig. 8 is a flow chart showing an example of an abnormality determination operation in the lighting circuit 10A of the second embodiment. Fig. 9 is a diagram showing waveforms when the first boost circuit 21 and the second boost circuit 22 are normal in the second embodiment, and Fig. 10 is a diagram showing waveforms when the coil L2 of the second boost circuit 22 is open.

[0074] 9 and 10 show the waveforms of voltages VN1 and VN2, voltages V1 and V2, and the waveform of the absolute value of the difference between voltages V1 and V2, respectively. In each of the diagrams in Fig. 9 and 10, the horizontal axis represents time, and the vertical axis represents the magnitude of the voltage.

[0075] First, the microcomputer 50 receives the voltages V1 and V2 from the first voltage detection circuit 41A and the second voltage detection circuit 42A, respectively (FIG. 8: S11).

[0076] When the first boost circuit 21 and the second boost circuit 22 are normal, the voltage VN1 at the node N1 and the voltage VN2 at the node N2 have waveforms that alternately rise, as shown in Fig. 9. The voltages VN1 and VN2 in Fig. 9 have waveforms similar to those of Fig. 4 (voltages V1 and V2) in the first embodiment, except that the voltage at the H level is approximately 45 V (voltage amplitude).

[0077] In the second embodiment, the voltages V1 and V2 taken in by the microcomputer 50 are the outputs of the divided voltages at the nodes N1 and N2 that have been passed through the first filter circuit 81 and the second filter circuit 82, respectively, and therefore have gradually changing waveforms as shown in the middle diagram of FIG. 9 .

[0078] For example, when the voltage VN1 is at the H level, the output of the first voltage dividing circuit 71 (the divided voltage of the voltage VN1) is applied to the capacitor C1 via the resistor R1, and the capacitor C1 is charged. At this time, the voltage V1 rises with a time constant determined by the resistor R1 and the capacitor C1.

[0079] Furthermore, when voltage VN1 is at the L level, the charge stored in capacitor C1 is discharged via resistor R1 and resistor Rb of first voltage divider circuit 71. At this time, voltage V1 drops at a rate that depends on the resistance values ​​of resistors R1 and Rb. In this way, voltage V1 repeatedly rises and falls in response to changes in voltage V1 (in other words, the switching operation of NMOS transistor Q1). The same is true for voltage V2.

[0080] As a result, the voltages V1 and V2 have triangular waveforms with the same amplitude but different phases.

[0081] The microcomputer 50 then calculates the difference between the voltages V1 and V2 (FIG. 8: S12) and determines whether the difference (absolute value) is greater than a predetermined value (e.g., 100 mV) (FIG. 8: S13). The reason for using 100 mV will be explained below.

[0082] When both the first boost circuit 21 and the second boost circuit 22 are normal, the values ​​of voltage V1 and voltage V2 are close to each other, and the absolute value of the difference between them is very small (at the level of a few mV: at most 7.7 mV in the figure), as shown in the lower diagram of Figure 9.

[0083] On the other hand, if either the first boost circuit 21 or the second boost circuit 22 is abnormal (here, coil L2 of the second boost circuit 22 is open), the difference between voltages V1 and V2 becomes large, as shown in the middle diagram of FIG. 10. Specifically, voltage V1 is a triangular wave close to 1.4 V, and voltage V2 is zero (V). Therefore, the absolute value of this difference exceeds 1 V (approximately 1.39 V), as shown in the bottom diagram of FIG. 10.

[0084] Therefore, the presence or absence of an abnormality can be determined depending on whether the absolute value of the difference between voltage V1 and voltage V2 is greater than a predetermined value (here, 100 mV).

[0085] If it is determined that the absolute value of the difference between voltages V1 and V2 is smaller than the predetermined value (NO in S13), the microcomputer 50 returns to step S11 and performs the same process.

[0086] On the other hand, if it is determined that the absolute value of the difference between voltages V1 and V2 is greater than the predetermined value (YES in S13), the microcomputer 50 determines that either the first boost circuit 21 or the second boost circuit 22 is abnormal (FIG. 8: S14), and causes the switching control circuit 60 to stop the switching operation of the NMOS transistors Q1 and Q2 (FIG. 8: S15).

[0087] In this way, also in the second embodiment, it is possible to detect the occurrence of an abnormality in either the first boost circuit 21 or the second boost circuit 22, and to prevent abnormal heat generation.

[0088] <When a Transistor is Open> In the second embodiment as well, it can be evaluated that the transistors (NMOS transistors Q1 and Q2) are open.

[0089] FIG. 11 is a diagram showing waveforms when the NMOS transistor Q2 of the second booster circuit 22 is open.

[0090] When the NMOS transistor Q2 of the second boost circuit 22 is open, no switching operation is performed by the NMOS transistor Q2, and therefore the voltage VN2 at the node N2 becomes the input voltage (Vbat: for example, 12 V). Therefore, the voltage V2 becomes a voltage obtained by dividing the voltage VN2. In this case, too, it is possible to determine that an abnormality has occurred from the waveform, as in the first embodiment.

[0091] Summary The lighting circuit 10 of the first embodiment has been described above. The lighting circuit 10 of the first embodiment includes a drive circuit 30 that drives the light source 4, a first boost circuit 21, a second boost circuit 22, a first voltage detection circuit 41, a second voltage detection circuit 42, and a microcomputer 50. The first boost circuit 21 includes an NMOS transistor Q1 and generates a power supply voltage Vcc for the drive circuit 30. The second boost circuit 22 includes an NMOS transistor Q2 and generates a power supply voltage Vcc for the drive circuit 30. The first voltage detection circuit 41 detects a voltage V1 corresponding to the voltage at the drain (node ​​N1) of the NMOS transistor Q1. The second voltage detection circuit 42 detects a voltage V2 corresponding to the voltage at the drain (node ​​N2) of the NMOS transistor Q2. The microcomputer 50 determines whether an abnormality exists in either the first boost circuit 21 or the second boost circuit 22 based on the voltages V1 and V2. As a result, if an abnormality occurs in either the first booster circuit 21 or the second booster circuit 22, it is possible to prevent only one of them from operating, thereby preventing abnormal heat generation.

[0092] The first voltage detection circuit 41 includes a first voltage divider circuit 71 connected to the node N1, and the second voltage detection circuit 42 includes a second voltage divider circuit 72 connected to the node N2. This allows the voltage at the node N1 and the voltage at the node N2 to be divided and input to the microcomputer 50 (the voltages can be divided to voltages that can be input to the microcomputer 50). The divided voltages can then be used to determine whether or not an abnormality exists.

[0093] The first voltage detection circuit 41 further includes a diode Da having an anode connected to the node N1 and a cathode connected to the first voltage divider circuit 71. The second voltage detection circuit 42 further includes a diode Db having an anode connected to the node N2 and a cathode connected to the second voltage divider circuit 72. This makes it possible to prevent backflow in the path from the microcomputer 50 to the first booster circuit 21 and the second booster circuit 22.

[0094] Furthermore, the microcomputer 50 determines whether or not there is an abnormality in either the first boost circuit 21 or the second boost circuit 22, based on the divided voltage (voltage V1) at the node N1 and the divided voltage (voltage V2) at the node N2. This makes it possible to determine whether or not there is an abnormality from the waveforms of the voltage V1 and the voltage V2.

[0095] Furthermore, the first voltage detection circuit 41A of the second embodiment has a first filter circuit 81 including a resistor R1 and a capacitor C1, and the second voltage detection circuit 42A has a second filter circuit 82 including a resistor R2 and a capacitor C2. The microcomputer 50 determines whether or not there is an abnormality in either the first step-up circuit 21 or the second step-up circuit 22 based on the output (voltage V1) of the first filter circuit 81 and the output (voltage V2) of the second filter circuit 82. This makes it possible to determine whether or not there is an abnormality from the output (voltage V1) of the first filter circuit 81 and the output (voltage V2) of the second filter circuit 82.

[0096] Furthermore, when the difference between the output (voltage V1) of the first filter circuit 81 and the output (voltage V2) of the second filter circuit 82 is equal to or greater than a predetermined value, the microcomputer 50 determines that there is an abnormality in either the first boost circuit 21 or the second boost circuit 22. This makes it possible to determine that there is an abnormality in either the first boost circuit 21 or the second boost circuit 22 based on the calculation of the difference between the output (first output) of the first filter circuit and the output (second output) of the second filter circuit.

[0097] The first boost circuit 21 includes a coil L1 and a diode D1. One end of the coil L1 is applied with a voltage Vbat, and the other end is connected to a node N1. The diode D1 has an anode connected to the node N1 and a cathode that outputs the power supply voltage Vcc for the drive circuit 30. Similarly, the second boost circuit 22 includes a coil L2 and a diode D2. One end of the coil L2 is applied with a voltage Vbat, and the other end is connected to a node N2. The diode D2 has an anode connected to the node N2 and a cathode that outputs the power supply voltage Vcc for the drive circuit 30. This allows the input voltage (Vbat) to be boosted using two boost circuits, thereby preventing abnormal heat generation.

[0098] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. Furthermore, the present invention may be modified or improved without departing from the spirit thereof, and the present invention includes equivalents thereof.

[0099] REFERENCE SIGNS LIST 1 Vehicle lamp 2 Battery 4 Light source 10, 10A Lighting circuit 21 First voltage boost circuit 22 Second voltage boost circuit 30 Drive circuit 41, 41A First voltage detection circuit 42, 42A Second voltage detection circuit 50 Microcomputer 60 Switching control circuit 71 First voltage divider circuit 72 Second voltage divider circuit 81 First filter circuit 82 Second filter circuit L1, L2, La, Lb Coils D1, D2, Da, Db Diodes Q1, Q2 NMOS transistors R1, R2, Ra, Rb, Rc, Rd Resistors C, C1, C2 Capacitors

Claims

1. A lighting circuit comprising: a drive circuit for driving a light source; a first boost converter including a first switching element and generating a power supply voltage for the drive circuit; a second boost converter including a second switching element and generating the power supply voltage; a first voltage detection circuit for detecting a first voltage corresponding to the voltage of a first node on the high potential side of the first switching element; a second voltage detection circuit for detecting a second voltage corresponding to the voltage of a second node on the high potential side of the second switching element; and a determination circuit for determining whether or not there is an abnormality in either the first or second boost converter based on the first and second voltages.

2. A lighting circuit according to claim 1, wherein the first voltage detection circuit includes a first voltage divider circuit connected to the first node, and the second voltage detection circuit includes a second voltage divider circuit connected to the second node.

3. A lighting circuit according to claim 2, wherein the first voltage detection circuit further includes a first diode having an anode connected to the first node and a cathode connected to the first voltage divider circuit, and the second voltage detection circuit further includes a second diode having an anode connected to the second node and a cathode connected to the second voltage divider circuit.

4. A lighting circuit according to claim 2 or 3, wherein the determination circuit determines whether or not there is an abnormality in either the first or second boost converter based on the first divided voltage of the first voltage divider circuit and the second divided voltage of the second voltage divider circuit.

5. A lighting circuit according to claim 1, wherein the first voltage detection circuit has a first filter circuit including a first resistor and a first capacitor, the second voltage detection circuit has a second filter circuit including a second resistor and a second capacitor, and the determination circuit determines whether or not there is an abnormality in either the first or second boost converter based on a first output of the first filter circuit and a second output of the second filter circuit.

6. A lighting circuit according to claim 5, wherein the determination circuit determines that there is an abnormality in either the first or second boost converter when the difference between the first output of the first filter circuit and the second output of the second filter circuit is equal to or greater than a predetermined value.

7. A lighting circuit according to claim 1, wherein the first boost converter includes: a first inductor having one end to which an input voltage is applied and the other end connected to the first node; and a third diode having an anode connected to the first node and a cathode for outputting the power supply voltage; and the second boost converter includes: a second inductor having one end to which an input voltage is applied and the other end connected to the second node; and a fourth diode having an anode connected to the second node and a cathode for outputting the power supply voltage.

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