Lighting circuit and vehicular lamp

A drive circuit supplies a drive current to a second light source. A dummy load circuit is connected to a control line to which a lighting control signal, which instructs the second light source 304 to be turned on and off, is input, and the dummy load circuit sinks a dummy load current IDUMMYLOAD which decreases as a temperature increases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from Japanese Patent Application No. 2017-054962, filed on Mar. 21, 2017 with the Japan Patent Office, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present disclosure relates to a lamp used for an automobile or the like.

BACKGROUND

In the related art, a halogen lamp or a high intensity discharge (HID) lamp has been mainly used as a vehicular lamp, particularly, a light source of a headlamp, but recently, a vehicular lamp using a semiconductor light source such as a light emitting diode (LED) and a semiconductor laser (LD) is being developed instead of the halogen lamp or the high intensity discharge (HID) lamp.

Multiple light sources, which are controlled to be individually turned on and off, are mounted in the vehicular lamp. For example, in some instances, a light source for a low beam and a light source for a high beam are mounted in the vehicular lamp.FIGS. 1A and 1Bare circuit diagrams of the vehicular lamp that is provided with the multiple light sources studied by the present inventors. In the drawings, a first light source302corresponds to the low beam, and a second light source304corresponds to the high beam.

A lighting circuit400R of a vehicular lamp300R inFIG. 1Ais provided with a first drive circuit410and a second drive circuit412which correspond to the first light source302and the second light source304, respectively. The respective drive circuits410and412are configured with (i) a converter for outputting constant current, or (ii) a combination of a converter for outputting constant voltage and a constant current circuit.

Power source voltage VLOis input to an LO terminal through a mechanical relay RY1. When the mechanical relay RY1is turned on and the power source voltage VLOis supplied to the LO terminal, the first drive circuit410supplies drive current (lamp current) ILAMP1to the first light source302. Power source voltage VHIis input to an HI terminal via a mechanical relay RY2. When the mechanical relay RY2is turned on and the power source voltage VHIis supplied to the HI terminal, the second drive circuit412supplies drive current ILAMP2to the second light source304.

In a vehicular lamp300S inFIG. 1B, the two light sources302and304are connected in series. A common drive circuit414supplies common drive current ILAMPto a series connection circuit of the light sources302and304. A bypass switch430is provided in parallel with the second light source304, and a switch driver432turns off the bypass switch430when high-level voltage is input to the HI terminal. In this case, the drive current ILAMPis supplied to the second light source304such that the second light source304is turned on. When the HI terminal is at a low level, the switch driver432turns on the bypass switch430. In this case, the drive current ILAMPis applied to the bypass switch430and the second light source304is turned off.

While the combination of the high beam and the low beam has been described here, the same problem may occur even in respect to a combination of other light sources. See, for example, Japanese Patent Application Laid-Open No. 2016-082691.

SUMMARY

The lowest energizing current (the lowest guarantee current) is defined for a relay because an oxide film is formed on a surface of a contact in an OFF state, and there is concern that a conduction failure occurs because the contact is oxidized when a current higher than the lowest energizing current is not supplied in an ON state (an electric conduction state). In the vehicular lamp300R inFIG. 1A, both of the relays RY1and RY2are provided on power source lines via which a somewhat high current flows, and as a result, it is ensured that the current higher than the lowest energizing current flows in the respective relays.

Meanwhile, in the vehicular lamp300S inFIG. 1B, an impedance for an interior of a lighting circuit400S as seen from the HI terminal is high. That is, the relay RY2is not disposed on the power source line, but on a signal line. For this reason, there is concern that the current flowing in the relay RY2is lower than the lowest energizing current when the relay RY2is turned on for a period of time for which the high beam is turned on.

The present disclosure has been made in consideration of the aforementioned situations, and one of the exemplary objects of the aspect of the present disclosure is to provide a lighting circuit capable of inhibiting deterioration of a relay.

An aspect of the present disclosure relates to a lighting circuit that operates a light source. The lighting circuit includes: a drive circuit configured to supply a drive current to the light source; and a dummy load circuit connected to a control line into which a lighting control signal, which instructs the light source to be turned on and off, is input, and configured to sink a dummy load current which decreases as a temperature increases.

The lighting circuit may further include a bypass switch provided in parallel with the light source. The lighting control signal may be a signal that controls the bypass switch.

The lighting circuit may further include a constant current source provided in series with the light source. The lighting control signal may be a signal that controls the constant current source.

Another aspect of the present disclosure relates to a lighting circuit that operates a first light source and a second light source connected in series. The lighting circuit includes: a bypass switch provided in parallel with the second light source; a drive circuit configured to apply a drive current to a series connection circuit including the first light source and the second light source; and a dummy load circuit connected to a control line to which a lighting control signal, which instructs the second light source to be turned on and off, is input, and configured to sink a dummy load current which decreases as a temperature increases.

According to the aspect, it is ensured that a current higher than the dummy load current flows in an electric conduction state in an outer relay connected to the control line, and as a result, it is possible to inhibit deterioration of the contact of the relay. In addition, the dummy load circuit is considered as a heat source in the lighting circuit such that the lighting circuit itself is easily and thermally designed by decreasing the amount of generated heat by decreasing the dummy load current in a state in which a temperature is high, and as a result, the degree of freedom in terms of choosing components of configuration elements of the dummy load circuit is enhanced.

The dummy load circuit may include: a transistor and a resistor sequentially provided in series between the control line and the ground; and a bias circuit configured to apply a bias voltage to a control terminal of the transistor. The bias voltage is substantially constant within a first temperature range and decreases together with a temperature within a second temperature range higher than the first temperature range.

The bias circuit may include: a thermistor having a positive temperature characteristic and provided between the control line and the control terminal of the transistor, and a Zener diode provided between the control terminal of the transistor and the ground. According to the configuration, it is possible to maintain a constant dummy load current in a room temperature region and in a temperature region lower than the room temperature region, and it is possible to decrease the dummy load current in a temperature region higher than the room temperature region as a temperature increases.

The transistor may be a bipolar transistor, and the bias circuit may further include a diode which is provided in series with the Zener diode between the control terminal of the transistor and the ground. It is possible to cancel an influence by a temperature on the forward voltage of the diode and on the base-emitter voltage of the transistor, and as a result, it is possible to generate the dummy load current in proportion to Zener voltage in the room temperature region.

Another aspect of the present disclosure relates to a vehicular lamp. The vehicular lamp may include: a first light source and a second light source which are connected in series; and one of the aforementioned lighting circuits configured to operate the first light source and the second light source. The second light source may be a high beam.

Any combinations of the aforementioned constituent elements or substitutions of the constituent elements and expressions of the present disclosure between the method, the apparatus, the system, and the like are also effective as aspects of the present disclosure.

According to the aspect of the present disclosure, it is possible to inhibit deterioration of the relay.

DESCRIPTION OF EMBODIMENT

Hereinafter, based on suitable exemplary embodiments, the present disclosure will be described with reference to the drawings. The same or equivalent constituent elements, members, processes illustrated in the respective drawings are denoted by the same reference numerals, and duplicated descriptions thereof will be appropriately omitted. In addition, the exemplary embodiment does not limit the invention, and all the features or combinations thereof, which are disclosed in the exemplary embodiment as an example, do not limit that the invention is necessarily essential.

In the present specification, “a state in which a member A and a member B are connected to each other” includes not only a case in which the member A and the member B are physically and directly connected to each other, but also a case in which the member A and the member B are indirectly connected to each other without substantially affecting an electrically connected state therebetween or causing damage to a function or an effect exhibited by the engagement therebetween, or through other members.

Similarly, “a state in which a member C is provided between a member A and a member B” includes not only a case in which the member A and the member C or the member B and the member C are directly connected to each other, but also a case in which the member A and the member C or the member B and the member C are indirectly connected to each other without substantially affecting an electrically connected state therebetween or causing damage to a function or an effect exhibited by the engagement therebetween, or through other members.

In the present specification, the symbols, which denote electrical signals such as voltage signals and current signals, or circuit elements such as resistors and capacitors, indicate, as necessary, voltage values, current values, resistance values, and capacitance values.

FIG. 2is a block diagram of a vehicular lamp300including a lighting circuit400according to an exemplary embodiment. The vehicular lamp300includes a first light source302, a second light source304, and a lighting circuit400. The first light source302and the second light source304include a single or multiple LEDs connected in series, respectively. The first light source302and the second light source304are connected in series, and the lighting circuit400operates the first light source302and the second light source304connected in series.

In the present exemplary embodiment, the first light source302is, but not exclusively, a light source for a low beam, and the second light source304is, but not exclusively, a light source for a high beam. When a power source voltage VLO(e.g., the voltage VBATof a non-illustrated battery) is supplied to an LO terminal, the lighting circuit400turns on the first light source302. In addition, the lighting circuit400turns on the second light source304when a high-level voltage is input to an HI terminal, and the lighting circuit400turns off the second light source304when a low-level voltage is input to the HI terminal. A control signal, which instructs the first light source302to be turned on and off, may be input in addition to the supply of the power source voltage VLOto the LO terminal.

The power source voltage VLOis input to the LO terminal through a mechanical relay RY1. A lighting control signal VHI, which instructs the second light source304to be turned on and off, is input to the HI terminal through a mechanical relay RY2. The lighting circuit400includes a drive circuit414, a bypass switch430, a switch driver432, and a dummy load circuit450. The bypass switch430is provided in parallel with the second light source304. The drive circuit414supplies a drive current ILAMPto a series connection circuit including the first light source302and the second light source304. The drive circuit414may be configured with a constant current converter. The switch driver432turns off the bypass switch430when the lighting control signal VHIis at a high level, and the switch driver432turns on the bypass switch430when the lighting control signal VHIis at a low level.

The dummy load circuit450is connected to a control line434to which the lighting control signal VHIis input, and the dummy load circuit450sinks a dummy load current IDUMMYLOADfrom the control line434. The dummy load circuit450is configured to decrease the dummy load current IDUMMYLOADwhen a temperature is increased. Therefore, the dummy load circuit450may include a temperature detecting element452.

FIG. 3is a circuit diagram of the dummy load circuit450according to the exemplary embodiment. A transistor TR101and a resistor R103are sequentially provided in series between the control line434and the ground. A bias circuit454provides a control terminal of the transistor TR101with a bias voltage Vbwhich is substantially constant within a first temperature range and decreases together with the temperature within a second temperature range higher than the first temperature range. For example, the transistor TR101is an NPN type bipolar transistor, and the emitter voltage thereof is Vb−Vbe. Vbeis the base-emitter voltage of the transistor TR101. When the emitter voltage is applied to the resistor R103, the dummy load current IDUMMYLOADindicated by Equation 1 flows in the series connection circuit of the transistor TR101and the resistor R103.
IDUMMLOAD=(Vb−Vbe)/R103  (1)

An element having appropriate impedance is inserted between the control line434and a collector of the transistor TR101. In the present exemplary embodiment, a diode D101and a resistor R101are inserted, but the present disclosure is not limited thereto. The diode D101prevents the dummy load current IDUMMYLOADfrom flowing reversely.

The bias circuit454includes a thermistor TH101which is the temperature detecting element452. The thermistor TH101is a positive thermal coefficient (PTC) thermistor, and a resistance value thereof indicates a constant resistance value in a room temperature region or in a temperature region lower than the room temperature region, and the resistance value is increased together with the temperature when the temperature exceeds a predetermined constant temperature. The thermistor TH101is provided in series with a resistor R102between the control line434and a control terminal (base) of the transistor TR101. The resistor R102may be omitted in accordance with the resistance value of the thermistor TH101.

A Zener diode ZD101is a constant voltage diode. A diode D102and the Zener diode ZD101are provided in series between the control terminal (base) of the transistor TR101and the ground.

The aforementioned configuration is a configuration of the vehicular lamp300. An operation of the vehicular lamp300will be subsequently described.FIG. 4is a view for explaining an operation of the dummy load circuit450inFIG. 3. R.T. indicates the room temperature. The bias voltage Vbis indicated by Equation 2 within a first temperature range A in which an ambient temperature Tais lower than a predetermined constant value TTH, and a resistance value of the thermistor TH101is constant.
Vb=VF+VZD(2)

VFindicates the forward voltage of the diode D102, and VZDindicates the Zener voltage of the Zener diode ZD101.

Equation 3 is obtained by substituting Expression 2 into Expression 1.
IDUMMLOAD=(VF+VZD−Vbe)/R103  (3)

That is, within the first temperature range, a constant dummy load current I0DUMMLOAD, which does not depend on the ambient temperature Ta, may be generated. The constant dummy load current I0DUMMLOADmay be set to be equal to the lowest energizing current of the relay RY2.

Within a second temperature range B in which the ambient temperature Tais higher than the predetermined constant value TTH, the resistance value RPTCof the thermistor TH101is increased in accordance with an increase in temperature. By the resistance value RPTCof the thermistor TH101, the base current Ibof the transistor TR101is throttled, and the dummy load current IDUMMYLOADis decreased.

The aforementioned operation is an operation of the vehicular lamp300. Subsequently, an advantage of the vehicular lamp300will be described.

According to the lighting circuit400inFIG. 2, it is ensured that a current higher than the dummy load current IDUMMYLOADflows in an electric conduction state in the outer relay RY2connected to the control line434. Therefore, it is possible to inhibit deterioration of a contact of the relay RY2by setting the amount of the dummy load current IDUMMYLOADto the amount equal to or higher than the lowest energizing current.

A further advantage of the lighting circuit400inFIG. 2becomes clear by comparison with a comparative technology. In the comparative technology, a constant dummy load current, which does not depend on a temperature, is generated by a dummy load circuit. This comparative technology corresponds to a configuration in which the thermistor TH101inFIG. 3is omitted. The dummy load circuit acts as a heat source in the lighting circuit, and as a result, when the dummy load circuit further generates heat in a state in which the ambient temperature is high, the temperature of the lighting circuit is further increased. Therefore, it is necessary to improve heat dissipation properties of the lighting circuit, and constituent components of the dummy load circuit need to be chosen in consideration of an operation in a high temperature region. In general, the temperature of the lighting circuit400is increased by self-heating of the lighting circuit400which includes consumption of a dummy current as time is elapsed from the start of lighting.

In contrast, the dummy load circuit450of the present exemplary embodiment decreases the dummy load current IDUMMYLOADin a high temperature state, and decreases the amount of generated heat. This acts in a direction in which a temperature of the lighting circuit400is decreased. Therefore, the lighting circuit400itself is easily and thermally designed, and the degree of freedom in terms of choosing constituent components of the dummy load circuit450is enhanced. Specifically, in a case in which the dummy load circuit450is configured as illustrated inFIG. 3, the sizes of the resistors R101and R103and the transistor TR101may be decreased and inexpensive components may be chosen.

When the second light source304is turned on, the lighting circuit400comes into a high temperature state by self-heating caused by consumption of dummy current immediately after the second light source304is turned on, and when the second light source304is turned off in this state and then turned on immediately, a defect of the contact does not occur because an oxide film is not yet formed on the contact of the relay even though passing current of the mechanical relay RY2at the time of turning on the second light source304again is lower than lowest passing current.

While the present disclosure has been described using specific words and phrases based on the exemplary embodiment, the exemplary embodiment just describes the principle and the application of the present disclosure, and many modified examples and changes in arrangement may be conceived from the exemplary embodiment without departing from the spirit of the present disclosure defined in claims.

Modified Example 1

FIG. 5is a block diagram of a vehicular lamp300A that includes a lighting circuit400A according to Modified Example 1. A first constant current source460and a first light source302are connected in series, and a second constant current source462and a second light source304are connected in series. A drive circuit414A outputs a constant voltage, and supplies a common drive voltage Voutto the first light source302and the second light source304provided in parallel two paths. A control line434is connected to the second constant current source462, and the second constant current source462is controlled to be turned on and off by a lighting control signal VHI. Even in this modified example, it is possible to obtain an effect similar to the effect of the exemplary embodiment.

Modified Example 2

A field effect transistor (FET) may be used instead of the bipolar transistor as the transistor TR101, and in this case, the base may be read as a gate, the emitter may be read as a source, and the collector may be read as a drain. Further, in this case, the diode D102may be omitted, and instead, the FET, which connects the gate and the drain, may be inserted. Therefore, it is possible to cancel an influence by a temperature on the gate-source voltage of the transistor TR101of the FET.

Modified Example 3

The configuration of the dummy load circuit450is not limited to the configuration inFIG. 3. A person ordinarily skilled in the art may design a current source capable of creating the current IDUMMYLOADhaving temperature dependency as illustrated inFIG. 4using a PTC thermistor, an NTC thermistor, a thermocouple, and the like.

Modified Example 4

The light sources302and304are not limited to the LED, and an LD or an organic electro luminescence (EL) may be used. In addition, the drive circuit414is not limited to the switching converter, and the drive circuit414may be configured with a linear regulator or other circuits.

Modified Example 5

In the exemplary embodiment, the combination of the high beam and low beam has been described, but the present disclosure is not limited thereto, and may be applied to (i) a combination of a main low beam and an additional low beam, (ii) a combination of a clearance lamp and a fog lamp, and (iii) a combination of a turn lamp and daytime running lamps (DRL).

Modified Example 6

In the exemplary embodiment, the two light sources302and304are connected in series, but three or more light sources may be connected in series. In contrast, the multiple light sources are not essential, and the present technology may also be applied to a lighting circuit which operates a single light source. For example, a configuration in which the first light source302inFIG. 2is omitted is allowable, and a configuration in which the first light source302and the first constant current source460inFIG. 5are omitted is allowable.

That is, the present disclosure may be widely applied to a configuration in which the lighting control signal is input through the mechanical relay, and the mechanical relay is not disposed on a power line in which a high current flows, but disposed on a control line in which minute current (several mA or less) flows.