CN108924989B

CN108924989B - Lighting circuit and vehicle lamp using the same

Info

Publication number: CN108924989B
Application number: CN201810502386.4A
Authority: CN
Inventors: 市川知幸
Original assignee: Koito Manufacturing Co Ltd
Current assignee: Koito Manufacturing Co Ltd
Priority date: 2015-07-31
Filing date: 2016-07-29
Publication date: 2020-07-21
Anticipated expiration: 2036-07-29
Also published as: JP6821835B2; CN108924989A; JP6687425B2; JP2017034970A; JP2020064877A; CN106413178B; CN106413178A

Abstract

The invention provides a lighting circuit capable of suppressing overcurrent.A buck converter (20d) has an output inductor (L2) and drives a current (I) through the output inductor (L2)_DRV) Supplied to the light source (2) and feedback-controlled to drive the current (I)_DRV) Close to the target current (I)_REF). The protection circuit (60) stops the switching of the buck converter (20d) during a stop time if it detects that the output terminal of the buck converter (20d) has returned from an open state to a normal state.

Description

Lighting circuit and vehicle lamp using the same

The present application is a divisional application based on chinese national application No. 201610615937.9 (lighting circuit and vehicle lamp using the same) filed on 29/7/2016, and the contents thereof are incorporated below.

Technical Field

The present invention relates to a vehicle lamp used in an automobile or the like, and particularly to failure detection thereof.

Background

Heretofore, as a light source of a vehicle lamp, particularly a headlamp, a halogen lamp and an hid (high intensity discharge) lamp have been the mainstream, but in recent years, development of a vehicle lamp using a semiconductor light source such as L ED (light emitting diode) or a laser diode has been advanced instead of these lamps.

A vehicle lamp using a semiconductor light source is required to detect an open failure due to open failure of the semiconductor light source, falling of a wire harness, disconnection of a wire, or the like, and to notify the vehicle side of the open failure. Fig. 1(a) and (b) are circuit diagrams of a vehicle lamp having a lighting circuit having an open circuit abnormality detection function. Further, they are circuits that the present inventors have previously studied, and are not considered to be known techniques. The lighting circuit 10r in fig. 1(a) includes a Buck converter (Buck converter) 20 and an open circuit detection circuit 30 r. Via a switch 6 from the accumulator 4Voltage V of_BATIs supplied to the lighting circuit 10 r. Buck converter 20 to voltage V_BATStep-down is carried out to output voltage V_OUTTo the light source 2. The buck converter 20 is feedback-controlled by a converter controller, not shown, so that the drive current I flowing through the light source 2_DRVClose to a target value I defining a target light quantity of the light source 2_REF。

The open circuit detection circuit 30R of fig. 1(a) includes a comparator 32R and a sense resistor R for current detection_S. Sensing resistor R_SInserted into the drive current I_DRVOn the path of (A), a driving current I is generated and driven between the two ends of the path_DRVProportional voltage drop (current detection signal) V_IS. The comparator 32r detects the current signal V_ISWith a predetermined threshold voltage V_THA comparison is made. When the vehicle lamp 1r shown in fig. 1(a) is normal, the normal drive current I is set_DRVFlows through the sensing resistor R_SGenerating a voltage exceeding a threshold voltage V_THVoltage drop V of_IS. On the contrary, if an open circuit abnormality occurs, it is caused by the driving current I_DRVBecomes no longer flowing and therefore a voltage drop V_ISBecomes substantially zero and becomes a specific threshold voltage V_THLow. Thus, the output signal of comparator 32r is at V_IS＞V_THHas a 1 st level (e.g., high level) indicating normal, at V_IS＜V_THHas a 2 nd level (e.g., low level) indicating an abnormality.

The open circuit detection circuit 30s of fig. 1(b) has resistors R11, R12, and a comparator 32 s. Resistors R11, R12 for the output voltage V of the buck converter 20_OUTPartial pressure is carried out. The comparator 32s divides the output voltage (voltage detection signal) V_VSAnd a threshold voltage V_THA comparison is made.

When the vehicle lamp 1s of fig. 1(b) is normal, the output voltage V is set_OUTIs feedback-controlled to be optimum for supplying the target current I to the light source 2_REFThe voltage level of (c). If an open-circuit abnormality occurs, the current I is driven_DRVBecomes no longer flowing, but the controller of the buck converter 20 drives the current I_DRVIs close to the target value I_REFMake and break (switch)g) Is increased, thereby outputting the voltage V_OUTAnd (4) rising. As a result, the voltage detection signal V_VSOver a threshold voltage V_TH。

Thus, the output signal of comparator 32s is at V_VS＜V_THHas a 1 st level (e.g., high level) indicating normal, at V_VS＞V_THAnd has the 2 nd level (low level) indicating an open circuit abnormality.

Patent document 1: japanese patent laid-open No. 2004-134147

1. The present inventors have studied the

lighting circuits

10r and 10s shown in fig. 1(a) and (b), and as a result, have come to recognize the following problems.

In a vehicle lamp in which a laser diode is a light source 2, a low-luminance mode (test mode) in which the light source 2 emits light at low luminance is required for maintenance or testing. In this case, in the lighting circuit 10r of fig. 1(a), it is necessary to set the threshold voltage V_THSet to be lower than the voltage detection signal V in the low brightness mode_ISLow but due to the driving current I flowing through the light source 2 in the low brightness mode_DRVIs weak, voltage detection signal V_ISExtremely small, and therefore the threshold voltage V must be set_THSet very low and are susceptible to errors.

The light source 2 may be constituted by a plurality of L EDs connected in series and a plurality of bypass switches provided in parallel with a plurality of L EDs, and if this light source 2 is used, the lighting and extinction of L EDs connected in parallel with the bypass switches can be controlled in accordance with the turning on or off of the bypass switches, and when the number of L EDs that are lighting is N, the output voltage V of the step-down converter 20 is determined by the output voltage V of the step-down converter 20_OUTBy

V_OUT≈V_F×N

Given, therefore, the output voltage V_OUTDynamically varies according to the number of lighting N. In the lighting circuit 10s of fig. 1(b), the output voltage V is set_OUTWhen the threshold voltage V varies dynamically, it is difficult to appropriately determine the threshold voltage V_TH。

2. In addition, the present inventors have recognized the following problems regarding open circuit failures.

Since a semiconductor light source is weak to be subjected to an overcurrent, particularly in a laser diode, which may cause a failure of cod (capacitive optical damage), it is necessary to prevent a current exceeding an absolute maximum rated value from flowing even if it is instantaneous, and a more severe overcurrent protection is required as compared with other light sources.

Chattering may occur in which contact (normal state) and non-contact (open state) are repeated at the connector contacts of the lighting circuit 10r (or 10s) and the light source 2. In the open state, the current detection signal V is generated by the lighting circuit 10r (10s)_ISIs zero, so the duty cycle is increased to drive the current I_DRVIs close to the target value I_REF. As a result, the voltage of the output capacitor increases. Then, if the connector contact is restored to the contact state, the electric charge excessively accumulated in the output capacitor flows into the light source 2, and an overcurrent is generated.

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lighting circuit capable of appropriately detecting an open circuit abnormality. Another object of the present invention is to provide a lighting circuit capable of suppressing an overcurrent.

1. One aspect of the present invention relates to a lighting circuit. The lighting circuit includes: a step-down converter that supplies a drive current to the light source and performs feedback control so that the drive current approaches a target current; and an open circuit detection circuit that compares a potential difference between an input voltage and an output voltage of the buck converter with a predetermined threshold voltage.

The drive current becomes zero if the load of the buck converter becomes an open circuit abnormality, feedback is applied in a direction in which the output voltage rises in order to increase the drive current, and the potential difference between the input and output of the buck converter approaches zero if the open circuit abnormality occurs. According to this aspect, the open-circuit abnormality can be detected based on the potential difference between the input and output of the buck converter.

The open circuit detection circuit may include a PNP bipolar transistor having an emitter connected to the input terminal of the buck converter and a base connected to the output terminal of the buck converter. Since the on/off of the bipolar transistor corresponds to the result of the abnormality detection, the voltage converter is not necessary, and therefore, the cost can be reduced.

The open circuit detection circuit may further include a 1 st resistor, and the 1 st resistor may be provided between the collector of the bipolar transistor and the ground.

The open circuit detection circuit may further include a P-channel FET (field Effect transistor) having a source connected to an input terminal of the buck converter and a gate connected to an output terminal of the buck converter. In this case, since the on/off of the FET corresponds to the result of the abnormality detection, the voltage converter is not necessary, and thus the cost can be reduced.

The open circuit detection circuit may further include a clamping element disposed between the gate and the source of the FET. This can suppress the voltage between the gate and the source to be lower than the withstand voltage.

The open circuit detection circuit may also have a 2 nd resistor, the 2 nd resistor being disposed between the drain of the FET and ground.

2. Another aspect of the present invention also relates to a lighting circuit. The lighting circuit includes: a converter having an output inductor, supplying a drive current to the light source via the output inductor, and performing feedback control so that the drive current approaches a target current; and a protection circuit that stops the switching of the converter during a stop time if it is detected that the output terminal of the converter is restored from the open state to the normal state.

If the state is changed to an open state, the detection value of the drive current becomes zero, and therefore the duty ratio of the converter increases, and the output voltage rises. Then, if the normal state is restored, the excessive charge accumulated in the output inductor is supplied to the light source via the output inductor. Since the output inductor forms a resonance circuit together with the output capacitor, a limited resonance current flows through the light source, and thus an overcurrent is suppressed.

However, when the open state is returned to the normal state, the resumption of the on/off operation of the on/off converter is delayed, and the drive current is generated after the current of the resonant circuit decreases, so that the overcurrent can be suppressed.

The protection circuit may determine the recovery to the normal state when the output voltage of the converter decreases sharply. The "case where the output voltage abruptly decreases" includes the following cases: the slope of the output voltage exceeds a predetermined threshold, the width of change of the output voltage for a predetermined time exceeds a predetermined threshold, and the time required for the output voltage to change for a predetermined width is shorter than the predetermined threshold. This makes it possible to detect the return from the open state to the normal state.

The protection circuit may also gradually increase the duty cycle of the on/off of the converter after the lapse of the stop time. Thus, after the on/off is restarted, the drive current is slowly increased, and thus the overcurrent can be further suppressed.

The protection circuit may set the target current to zero during the stop time, and may gradually increase the target current after the stop time elapses.

The protection circuit may stop switching of the converter during the stop time if it is detected that the output terminal of the converter has returned from the short-circuited state to the normal state.

Thus, when the short-circuit state is restored to the normal state, the resumption of the on-off operation of the on-off converter is delayed, so that the drive current is generated after the current of the resonance circuit decreases, and thus the overcurrent can be suppressed.

The protection circuit may determine that the short-circuit state is restored to the normal state when the output voltage of the converter rises sharply. This makes it possible to detect the return from the normal state from the short-circuited state.

The protection circuit may also include a differentiating circuit or a high pass filter that receives the output voltage. The protection circuit may determine that the state is recovered to the normal state if the output signal of the differentiating circuit or the high-pass filter exceeds a predetermined value.

The protection circuit may also include: a capacitor having one end grounded; a charging resistor connected to the other end of the capacitor, for applying a target voltage, which defines the target current in a normal state, to the capacitor; and a discharge switch provided in parallel with the capacitor. Alternatively, the discharge switch may be turned on if the protection circuit detects a return to the normal state.

The converter may also be of the buck type. The lighting circuit may further include an open circuit detection circuit that compares a potential difference between the input voltage and the output voltage of the converter with a predetermined threshold voltage.

Another aspect of the invention relates to a vehicle lamp. The vehicle lamp includes a light source and the arbitrary lighting circuit for driving the light source.

ADVANTAGEOUS EFFECTS OF INVENTION

According to one aspect of the present invention, an open circuit abnormality can be appropriately detected. In addition, according to one aspect of the present invention, an overcurrent can be suppressed.

Drawings

Fig. 1(a) and (b) are circuit diagrams of a vehicle lamp having a lighting circuit having an open circuit abnormality detection function.

Fig. 2 is a circuit diagram of the vehicular lamp according to embodiment 1.

Fig. 3 is an operation waveform diagram of the lighting circuit of fig. 2.

Fig. 4(a) and (b) are circuit diagrams showing specific configuration examples of the vehicle lamp.

Fig. 5(a) and (b) are circuit diagrams showing another configuration example of the vehicular lamp.

Fig. 6 is a circuit diagram of the vehicular lamp according to embodiment 2.

Fig. 7 is an operation waveform diagram of the lighting circuit of fig. 6.

Fig. 8 is a specific circuit diagram of the lighting circuit of fig. 6.

Fig. 9 is an operation waveform diagram of the lighting circuit of fig. 8.

Fig. 10 is a circuit diagram showing a specific configuration example of the protection circuit.

Fig. 11 is a waveform diagram showing the return from the short-circuit state to the normal state.

Fig. 12 is a circuit diagram of a protection circuit according to modification 2.2.

Fig. 13 is a perspective view of a lamp unit including the vehicle lamp according to the embodiment.

Description of the reference numerals

1 … vehicle lighting device, 2 … light source, 4 … battery, 6 … switch, 10 … lighting circuit, 12 … connector, 14 … resonant circuit, 20 … buck converter, 22 … controller, 30 … open circuit detection circuit, 32 … comparator, 40 … open circuit detection circuit, 42 … bipolar transistor, R1 … first 1 resistor, R2 … second resistor, R3 … base resistor, R4 … gate resistor, 44 … FET, 46 … clamp element, 48 … voltage comparator, 500 … lighting unit, 502 … cover, 504 … high beam unit, 506 … low beam unit, 508 … frame, M … switch transistor, … output inductor, C … input capacitor, C … output capacitor, 60 … protection circuit, 62 … first 1 differential circuit, 64 … target current controller, 66 … buffer, 70 … current sensing amplifier, … current comparison amplifier, … comparison capacitor, … charging switch, … Q switch, … discharge switch.

Detailed Description

The present invention will be described below based on preferred embodiments with reference to the accompanying drawings. The same or equivalent constituent elements, members, and processes shown in the respective drawings are denoted by the same reference numerals, and overlapping descriptions are appropriately omitted. The embodiments are illustrative and not restrictive, and all the features and combinations described in the embodiments are not necessarily essential to the invention.

In this specification, the term "state in which the component a and the component B are connected" includes a case in which the component a and the component B are indirectly connected via another component in addition to a case in which the component a and the component B are physically and directly connected, and the indirect connection does not substantially affect the electrical connection state thereof or impair the function or effect achieved by the connection thereof.

Similarly, the "state in which the component C is provided between the components a and B" includes a case in which the components a and C, or the components B and C are indirectly connected via another component in addition to a case in which the components a and C, or the components B and C are directly connected, and the indirect connection does not substantially affect the electrical connection state thereof or impair the function or effect achieved by the combination thereof.

In the present specification, reference numerals used to designate circuit elements such as voltage signals, current signals, resistors, and capacitors are used to indicate voltage values, current values, resistance values, and capacitance values, as necessary.

Further, those skilled in the art can understand that a Bipolar Transistor, a MOSFET, or an IGBT (Insulated Gate Bipolar Transistor) can be replaced, a P-channel (PNP type) and an N-channel (NPN type) of a Transistor can be exchanged, and the power supply and the ground can be inverted from top to bottom.

(embodiment 1)

Fig. 2 is a circuit diagram of the vehicle lamp 1 according to embodiment 1. The vehicle lamp 1 includes a light source 2 and a lighting circuit 10. The lighting circuit 10 includes a buck converter 20, a controller 22, and an open circuit detection circuit 40.

Via a switch 6, the voltage V from the battery 4_BATIs supplied to the lighting circuit 10. Step-down converter 20 pair and battery voltage V_BATCorresponding input voltage V_INStep-down is carried out to output voltage V_OUTTo the light source 2. The buck converter 20 performs feedback control by the converter controller 22 so that the driving current I flowing through the light source 2_DRVIs close to the target value I_REFThe target value I_REFThe buck converter 20 includes an input capacitor C1, an output capacitor C2, a switching transistor M1, a rectifying diode D1, and an inductor L1, the controller 22 controls the driving current I_DRVIs close to the target value I_REFTo generate a pulse signal S with a variable duty ratio_PWMDrive switch crystalBody tube M1. The control method of the controller 22 is not particularly limited, and may be hysteresis control (Bang-Bang control) or feedback control using an error amplifier.

The open circuit detection circuit 40 reduces the input voltage V of the buck converter 20_INAnd an output voltage V_OUT△ V and a predetermined threshold voltage V_THA comparison is then made at △ V > V_THWhen the abnormal state is judged to be normal, the abnormal state detection signal S1 is set to the 1 st level (for example, high level) and is set to △ V < V_THIf it is determined to be abnormal, the abnormality detection signal S1 is set to the 2 nd level (e.g., low level).

The above is the basic configuration of the lighting circuit 10. Next, the operation will be described. Fig. 3 is an operation waveform diagram of the lighting circuit 10 of fig. 2. Before time t0, the vehicle lamp 1 is normal, and the drive current I_DRVIs stabilized to the target quantity I_REF. At this time, the output voltage V_OUTIs stabilized to a certain voltage level.

At time t0, if an open-circuit abnormality occurs, the current I is driven_DRVIs turned off to become zero. The controller 22 drives the current I_DRVIs close to the target value I_REFSo that the pulse signal S_PWMThe duty cycle of (a) is increased. In response thereto, the output voltage V_OUTRises to the input voltage V shortly after_IN. Open circuit detection circuit 40 pairs V_INAnd V_OUT△ V-V_IN－V_OUTMonitoring is performed, at time t1, if △ V < V_THThen, the abnormality detection signal S1 is set to low level.

The above is the operation of the lighting circuit 10, and according to this lighting circuit 10, it is possible to detect an open circuit abnormality based on the potential difference △ V between the input and output of the buck converter 20.

When the lighting circuit 10 is used in the vehicle lamp 1 in which the light source 2 is a laser diode, the driving current I is set to the low-luminance mode even when the lighting circuit 10 is set to the low-luminance mode_DRVEven when the value is a small value, the open circuit abnormality can be appropriately detected. Alternatively, the lighting circuit 10 may be used in series with the light source 2Even in the case of a plurality of L EDs connected, in the case of a vehicle lamp in which the turning on or off of the lamp is controlled by a bypass switch, the output voltage V can be controlled in accordance with the output voltage V_OUTThe open circuit abnormality is appropriately detected regardless of the dynamic variation of (2).

The present invention is grasped as a block diagram or a circuit diagram of fig. 2, or a configuration equivalent to various devices and circuits derived from the above description, and is not limited to a specific configuration. In the following, a more specific configuration example will be described in order to facilitate understanding of the essence of the invention and circuit operation and to clarify them, but not to narrow the scope of the invention.

Fig. 4(a) and (b) are circuit diagrams showing a specific configuration example of the vehicle lamp 1. The open circuit detection circuit 40a of fig. 4(a) includes a PNP bipolar transistor 42, a 1 st resistor R1, and a base resistor R3. The emitter of the bipolar transistor 42 is connected to the input terminal of the buck converter 20, and the base thereof is connected to the output terminal of the buck converter 20 via the base resistor R3. The 1 st resistor R1 is disposed between the collector of the bipolar transistor 42 and ground. The 1 st resistor R1 may be omitted and used as an open collector output. In addition, the base resistor R3 may be omitted.

The potential difference △ V between the input and output of the buck converter 42 is input between the base and emitter of the bipolar transistor 42, and in normal operation, the bipolar transistor 42 is turned on and the abnormality detection signal S1 is at a high level (V1) because the potential difference △ V is sufficiently large_IN) When an open circuit abnormality occurs and the potential difference △ V between the input and output becomes smaller than the threshold voltage (0.6-0.7V) between the base and emitter of the bipolar transistor 42, the bipolar transistor 42 is turned off and the abnormality detection signal S1 becomes low level, that is, the turning on and off of the bipolar transistor 42 corresponds to the detection of an abnormality without detection.

The open circuit detection circuit 40b in fig. 4(b) is understood as a structure in which the bipolar transistor 42 in fig. 4(a) is replaced with a P-channel FET 44. The FET 44 has a source connected to the input terminal of the buck converter 20, and a gate connected to the output terminal of the buck converter 20 via a gate resistor R4. The 2 nd resistor R2 is disposed between the drain of FET 44 and ground. The clamp element 46 is provided between the gate and the source of the FET 44, and clamps the gate-source voltage so as not to exceed a predetermined value. For example, the clamping element 46 can be formed by a zener diode, a schottky diode, or the like.

A potential difference △ V between the input and output of the buck converter 20 is inputted between the gate and source of the FET 44, and in a normal state, since the potential difference △ V is sufficiently large, the FET 44 is in a conduction state, and the abnormality detection signal S1 is at a high level (V)_IN) If an open circuit abnormality occurs, the potential difference △ V between the input and output becomes larger than the threshold voltage V of the FET_GS(TH)When (for example, 1.5V) is small, the FET 44 is turned off, and the abnormality detection signal S1 becomes low. That is, the on/off of the FET 44 corresponds to the presence or absence of an abnormality. According to the vehicle lamp 1b of fig. 4(b), since the voltage converter is not required, the circuit cost can be reduced.

Fig. 5(a) and (b) are circuit diagrams showing another configuration example of the vehicular lamp 1 c. In the vehicle lamp 1c shown in fig. 5(a), the open circuit detection circuit 40c is configured using the voltage comparator 48. The voltage comparator 48 may also be coupled to the input voltage V_INIs shifted to the low potential side by V_THThe latter voltage, and the output voltage V_OUTA comparison is made. Voltage offset V_THIs introduced by level shifter 49. Fig. 5(b) is a circuit diagram showing a configuration example of the level shifter 49. For example, level shifter 49 includes resistor R5 and current source 50. The resistor R5 has one end connected to the input terminal of the buck converter 20 and the other end connected to the current source 50. The current source 50 generates a predetermined constant current I_C. At the junction of resistor R5 and current source 50, V is generated_IN－R5×I_CI.e., R5 × I_CBecomes V_TH. According to the vehicle lamp 1c of fig. 5(a), since the voltage comparator is used, accurate voltage comparison can be performed although the cost increases. In addition, when a comparator circuit including a plurality of voltage comparators is used and the voltage comparators are redundant, the cost is not increased.

(embodiment 2)

Fig. 6 is a circuit diagram of a vehicle lamp 1d according to embodiment 2. For example, the connector 12 is provided between the light source 2 and the lighting circuit 10d, and the light source 2 and the lighting circuit 10d are detachably connected. The lighting circuit 10d includes a buck converter 20d, a controller 22, and a protection circuit 60. Note that, since the technique described in embodiment 2 can be used in combination with the technique described in embodiment 1, the lighting circuit 10d may further include the open circuit detection circuit 40 described above, although it is omitted in fig. 6.

The lighting circuit 10d includes an output inductor L2, an output inductor L2 interposed between the output capacitor C2 and the light source 2, and the lighting circuit 10d of fig. 2, and the protection circuit 60 stops the turning on and off (switching) of the buck converter 20d for a stop time τ 1 if it detects that the output terminal of the buck converter 20d is returned from the open state to the normal state.

For example, the protection circuit 60 may also be based on the output voltage V of the buck converter 20d_OUTThe recovery from the open state to the normal state is detected. The protection circuit 60 may gradually increase the on/off duty ratio of the converter 20d from zero (soft start) after the elapse of the stop time τ 1.

The above is the basic configuration of the lighting circuit 10 d. The operation thereof will be described next. Fig. 7 is an operation waveform diagram of the lighting circuit 10d of fig. 6. Before time t1, connector 12 is in an open state. In the open state, drive current I_DRVIs zero. The controller 22 sets the drive current I to zero by feedback control_DRVClose to the target current I_REFAs a result, the current of the inductor L1 flows into the output capacitor C2, and the output voltage V is output_OUTHaving a higher voltage level than normal.

At time t1, the open state of the connector 12 is canceled, returning to the contact state (i.e., normal state) — thereby, the excessive charge accumulated in the output capacitor C2 is supplied to the light source 2 via the output inductor L2, since the output inductor L2 forms the L C resonant circuit 14 together with the output capacitor C2, the limited one isResonant current I_RESIn addition, it is desirable to note that, in the absence of the output inductor L2, the lamp current I flowing through the light source 2, as indicated by the one-dot chain line, is due to_LAMPRises without restriction and thus becomes an overcurrent.

Lamp current I_LAMPIs a drive current I generated by the buck converter 20d based on feedback control_DRVAnd a resonance current I flowing through the resonance current 14_RESThe sum of the currents. Resonant current I_RESFlows in a loop formed by the output capacitor C2 and the output inductor L2, and is therefore in the current detection signal V to the controller 22_ISIn (1), does not contain a resonance current I_RES. Therefore, if it is assumed that the protection circuit 60 immediately restarts the on/off operation of the buck converter 20d without the stop time τ 1 when returning from the open state to the normal state, the resonance current I_RESIs superimposed on the drive current I generated by feedback control_DRVLamp current I flowing through the light source 2_LAMPMay become an overcurrent.

In contrast, in the present embodiment, when the protection circuit 60 returns from the open state to the normal state, the on/off operation of the buck converter 20d is restarted after the elapse of the stop time τ 1. With respect to the stop time τ 1, the resonance current I is considered_RESThe relaxation time to be sufficiently short may be determined. Thereby, the resonance current I in the resonance circuit 14_RESAfter becoming smaller, generates drive current I_DRVTherefore, overcurrent can be suppressed.

In addition, if the soft-start control is not performed when the on-off operation is restarted after the elapse of the stop time τ 1, an overcurrent may occur due to resonance of the inductor L1, the output capacitor C2, and the output inductor L2_DRVThe increase is gradual, so that the overcurrent can be suppressed.

Next, a specific configuration example of the lighting circuit 10d of fig. 6 will be described. Fig. 8 is a specific circuit diagram of the lighting circuit 10d of fig. 6. The controller 22 is a hysteresis control (Bang-Bang control)) Has a current sense amplifier 70, a hysteresis comparator 72, and a driver 74. For example, the detection resistor R_SIs inserted into the driving current I generated by the buck converter 20d_DRVOn the path of (c). The current sense amplifier 70 generates a current in the sense resistor R_SVoltage drop V of_ISAmplification is performed. The hysteresis comparator 72 will drop the voltage V_ISAnd a threshold voltage V which varies 2 in accordance with its output_H、V_LAnd comparing to generate the modulated control pulse. Threshold voltage V_H、V_LIs based on a reference voltage V_REFTo define the reference voltage V_REFIndicating the drive current I_DRVTarget value of_REF. The driver 74 drives the switching transistor M1 based on the control pulse generated by the hysteresis comparator 72. The controller 22 may be controlled by feedback control using an error amplifier.

As shown in fig. 7, at time t1, if the state is returned from the open state to the normal state, the output voltage V is output_OUTInstantaneously dropping. The protection circuit 60 can also detect recovery from the normal state by utilizing this phenomenon. That is, the protection circuit 60 may output the voltage V_OUTWhen the voltage drops sharply, the recovery from the open state to the normal state is determined.

For example, the protection circuit 60 can include a 1 st differentiation circuit 62 or a low pass filter. E.g. with respect to the output signal V of the 1 st differentiating circuit 62_AOutput voltage V_OUTBecomes larger, the output signal V is output_AThe more increased. Then, the signal V is output_AReturns to 0 with a slope corresponding to the internal time constant TC1 of the 1 st differentiating circuit 62. The stop time τ 1 is defined by the time constant TC 1.

Output V of the target current controller 64 and the 1 st differentiating circuit 62_AAccordingly, for the reference voltage V_REFIs regulated, the reference voltage V_REFFor the drive current I_DRVTarget value of_REFThe specification is carried out. Specifically, the target current controller 64 outputs the signal V at the 1 st differentiating circuit 62_AIs less than a predetermined threshold value V_BAt low time, the reference voltage V is set_REFIs set to a normal value V_NORM. The output signal V of the target current controller 64 at the 1 st differentiating circuit 62_AExceeds a threshold value V_BIn the state of (1), the reference voltage V is set_REFI.e. the target current I_REFIs set to zero. Thereby, the switching of the buck converter 20d is stopped.

Regarding the target current controller 64, if the output signal V of the 1 st differentiating circuit 62_AIs less than a threshold value V_BThen the reference voltage V is set_REFI.e. the target current I_REFTowards the normal value V_NORMSlowly increasing. With this, after the stop time τ 1 has elapsed, the soft start control can be realized.

Fig. 9 is an operation waveform diagram of the lighting circuit 10d of fig. 8. Fig. 9 shows the operation of the connector 12 when the contacts are reset. At time t1, if the contacts of connector 12 are reset, voltage V is output_OUTFalls sharply, the output signal V of the 1 st differentiating circuit 62_ARises above a threshold value V_B. Thus, the reference voltage V_REFFrom the normal value V_NORMDropping to zero, the switching of the buck converter 20d is stopped.

And, at a voltage V_AWhen the time constant TC1 in the 1 st differentiating circuit 62 falls, it becomes higher than the threshold V at time t2_BLow. Thus, the target current controller 64 makes the reference voltage V_REFSlowly rising. That is, the delay time from time t1 to t2 is the stop time τ 1.

Fig. 10 is a circuit diagram showing a specific configuration example of the protection circuit 60. The 1 st differentiating circuit 62 mainly includes a bipolar transistor Q11, a capacitor C21, and a resistor R21. With this structure, the voltage V is generated and output_OUTCorresponding to the slope of the downward slope of the signal V_A. The time constant TC1 of the 1 st differentiating circuit 62 is specified by a resistor R21 and a capacitor C21. The 1 st differentiating circuit 62 can also be understood as a high-pass filter.

The target current controller 64 mainly includes a capacitor C22, a charging resistor R22, and a discharging switch Q12. One end of the capacitor C22 is grounded. The charging resistor R22 converts the voltage V_CNTIs applied to the capacitor C22 and,the voltage V_CNTFor reference voltage V_REFNormal value of (V)_NORMThe specification is carried out. When the discharge switch Q12 is turned off, the voltage V of the capacitor C22_C22And voltage V_CNTAre equal. Voltage V of capacitor C22_C22The reference voltage V is applied to the voltage dividing resistors R23 and R24 via the buffer 66 to generate a reference voltage V_REF。

Output signal V of 1 st differentiating circuit 62_A' is input to the base of a discharge switch Q12 which is an NPN-type bipolar transistor. If the base voltage V of the transistor Q11 in the 1 st differentiating circuit 62_ALess than the on/off threshold voltage (threshold value) V of the transistor_BOutput signal V of 1 st differentiating circuit 62_A' exceeding threshold V between base and emitter_BEThen, the discharge switch Q12 is turned on, and the voltage V of the capacitor C22_C22Becomes zero, i.e. reference voltage V_REFBecomes zero. The discharge switch Q12 is a voltage comparison unit and has a reference voltage V_REFReset to zero function.

If the base voltage V of the transistor Q11_AExceeds a threshold value V_BThen, the transistor Q11 is turned off, and the discharge switch Q12 becomes off. Thus, the capacitor C22 is charged via the resistor R22. At this time, the voltage V of the capacitor C22_C22Rising with a CR time constant TC 1. Thereby, the above-described soft start can be realized. In addition, since the transistor Q11 is a PNP type bipolar transistor, the input voltage V_INIs supplied to its emitter and is thus supplied with an input voltage V_INThe operation is performed as a reference. It is therefore desirable to keep track of the situation if the voltage V is_ALess than threshold voltage V_BThen transistor Q11 becomes conductive if voltage V_AOver a threshold voltage V_BThen the transistor Q11 becomes off.

When the response delay of the buffer 66 is large, the transistor Q13 is added. As transistor Q13, if signal V_AExceeds a threshold value V_BE(＝V_B) Then, the voltage is turned on, and the reference voltage V generated at the node of the voltage dividing resistors R23 and R24_REFPull down directly to zero. In addition, when the buffer 66 is at a high speed, the crystal can be omittedThe transistor Q13 may further omit the resistors R23 and R24.

(modification of embodiment 2)

(modification 2.1)

In the above description, the technique of suppressing the overcurrent when the open state is returned to the normal state has been described, but the technique can also be used to suppress the overcurrent generated when the short state is returned to the normal state. In this case, if the protection circuit 60 detects that the output terminal of the buck converter 20d has returned from the short-circuited state to the normal state, the on/off of the buck converter 20d may be stopped for the stop time τ 2. The stop time τ 2 may be the same as or different from the stop time τ 1.

Fig. 11 is a waveform diagram showing the return from the short-circuit state to the normal state. In the short-circuit state, the output voltage V_OUTFixed near zero. Drive current I generated by buck converter 20d_DRVIs stable to a target value I even in a short-circuit state_REF. At time t1, if the short-circuit state is recovered to the normal state, the voltage V is output_OUTGreatly increased. Therefore, the protection circuit 60 may be configured to control the output voltage V of the buck converter 20d_OUTWhen the voltage rises sharply, it is determined that the short-circuit state has returned to the normal state. The protection circuit 60 may include a 2 nd differentiation circuit 62s (for example, as shown in fig. 12) or a low-pass filter in place of the 1 st differentiation circuit 62 described above. With respect to the output signal of the 2 nd differentiating circuit 62s, the output voltage V_OUTThe larger the slope of the upward inclination becomes, the more the output signal increases. Then, the output signal returns to zero with a slope corresponding to the internal time constant TC2 of the 2 nd differentiation circuit 62 s. According to this modification, the overcurrent at the time of recovery from the short-circuit state can be suppressed.

(modification 2.2)

Further, the circuit can be configured to cope with both recovery from an open state and recovery from a short-circuited state. For example, for open circuits and for short circuits, two systems of protection circuits 60 can be provided. Alternatively, in fig. 8, 2 systems of the 1 st differentiating circuit 62 for open circuit and the 2 nd differentiating circuit 62s for short circuit may be provided, and the target current controller 64 may be shared.

Fig. 12 is a circuit diagram of a protection circuit 60e according to modification 2.2. The protection circuit 60e further includes a capacitor C23 in addition to the protection circuit 60 of fig. 10. The capacitor C23 forms the 2 nd differentiating circuit 62s for short circuit together with the base resistor R12 of the transistor Q12 and the base resistor R13 of the transistor Q13. The 2 nd differentiating circuit 62s generates and outputs a voltage V_OUTVoltage V corresponding to the slope of the positive edge of_C1、V_C2. Regarding the transistors Q12, Q13, if the output signal V of the 2 nd differentiating circuit 62s_C1、V_C2Exceeds a predetermined value V_BAnd then conducting.

Output signal V for 2 nd differentiating circuit 62s_C1、V_C2Output voltage V_OUTBecomes larger, the output signal V is output_C1、V_C2The more increased. Then, the signal V is output_C1、V_C2Returns to 0 with a slope corresponding to the internal time constant TC2 of the 2 nd differentiating circuit 62 s. The time constant TC2 defines a stop time τ 2 at the time of recovery of the short circuit.

According to the protection circuit 60e of fig. 12, overcurrent can be suppressed for both the return from the open state to the normal state and the return from the short-circuited state to the normal state. If the 1 st differentiating circuit 62 is omitted from fig. 12, the overcurrent at the time of return from the short-circuited state to the normal state can be suppressed.

(use)

Finally, the use of the vehicular lamp 1 is explained. Fig. 13 is an oblique view of a lamp unit (lamp assembly) 500 including the vehicle lamp 1 according to

embodiment

1 or 2. The lamp unit 500 includes a transparent cover 502, a high beam unit 504, a low beam unit 506, and a frame 508. The vehicle lamp 1 described above can be used in, for example, the high beam unit 504. Instead of the high beam unit 504, the vehicle lamp 1 may be used in the low beam unit 506, or the vehicle lamp 1 may be used in both the high beam unit 504 and the low beam unit 506.

The present invention has been described based on the embodiments using specific terms, but the embodiments merely show the principle and application of the present invention, and it is considered that many modifications and changes in arrangement are possible in the embodiments without departing from the scope of the idea of the present invention defined by the claims.

Claims

1. A lighting circuit includes:

a step-down converter that supplies a drive current to a light source and performs feedback control so that the drive current approaches a target current; and

and an open circuit detection circuit that compares a potential difference between an input voltage and an output voltage of the buck converter with a predetermined threshold voltage, and outputs an abnormality detection signal indicating an abnormality when the potential difference is smaller than the threshold voltage.

2. The lighting circuit according to claim 1, wherein,

the open circuit detection circuit includes a PNP bipolar transistor, an emitter of the PNP bipolar transistor is connected to the input terminal of the buck converter, and a base of the PNP bipolar transistor is connected to the output terminal of the buck converter.

3. The lighting circuit according to claim 2, wherein,

the open circuit detection circuit further comprises a 1 st resistor, wherein the 1 st resistor is arranged between the collector of the bipolar transistor and the ground.

4. The lighting circuit according to claim 1, wherein,

the open circuit detection circuit includes a P-channel FET having a source connected to an input terminal of the buck converter and a gate connected to an output terminal of the buck converter, wherein the FET is a field effect transistor.

5. The lighting circuit according to claim 4, wherein,

the open circuit detection circuit further includes:

a clamping element disposed between the gate and source of the P-channel FET; and

a 2 nd resistor disposed between the drain of the P-channel FET and ground.

6. A lamp for a vehicle, characterized by comprising:

a light source; and

the lighting circuit of any one of claims 1 to 5, which drives the light sources.