JP2013047047A - Led lighting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost LED lighting circuit that prevents breakage of an LED due to overcurrent.SOLUTION: A control circuit 17, when the lighting state is changed from a high-beam state in which both a first light source block 14 and a second light source block 15 are turned on to a low-beam state in which only the first light source block 14 is turned on, starts a second short circuit 18 and short-circuits the LED 16 of the second light source block 15, and starts the first short circuit 19 in addition to the second short circuit 18, and short-circuits the LED 16 of the second light source block 15 after a first predetermined time t1 passes.

Description

  The present invention relates to an LED lighting circuit.

An automobile headlamp (headlamp) needs to be switchable between a high beam, which is a traveling headlamp, and a low beam, which is a passing headlamp.
Therefore, some headlamps using LEDs include an LED lighting circuit that controls the number of LEDs to be lit at the time of low beam to be smaller than that at the time of high beam.
Conventionally, there has been known an LED lighting circuit that controls to change the brightness by changing the number of LEDs to be lit.

  In Patent Document 1, a plurality of light source blocks composed of at least one LED are connected in series to a power source, and one part of the plurality of light source blocks is short-circuited by a short circuit, and this short circuit includes an integration circuit. In accordance with the output voltage from this integration circuit, the lighting circuit is such that the voltage applied to the light source block to be short-circuited reaches a short-circuit state with a slope in the direction of gradual decrease from the default value. Is disclosed.

JP 2008-126958 A

The integrating circuit of Patent Document 1 increases the voltage applied to the light source block on the side where the short circuit is not performed (the side where the light is not extinguished) to a light source block on the side where the short circuit is not performed. It is provided to prevent an overcurrent (inrush current) due to an overvoltage from flowing and breaking.
However, when the short circuit by the short circuit is repeated in a short time, the integrating circuit does not function sufficiently, and an overcurrent may flow through the light source block on the side where the short circuit is not performed, causing damage.
Therefore, when the technique of Patent Document 1 is applied to a vehicle headlamp, the LED may be damaged when the low beam and the high beam are frequently switched.

Further, in Patent Document 1, by providing a constant current control unit that controls a current flowing through a plurality of light source blocks to a constant value, it is possible to prevent an overcurrent from flowing through the light source block on the side where no short circuit is performed. It is disclosed.
However, even if a constant current control unit is provided, since an instantaneous overcurrent cannot be prevented, there is a possibility that the LED may be damaged when the short circuit by the short circuit is repeated in a short time.

By the way, it is conceivable to connect a plurality of light source blocks in parallel to a power source and switch the light source blocks to be lit.
However, if there is a difference in the currents flowing through the individual light source blocks due to variations in the threshold voltage of the LEDs that make up the light source block, the brightness of the individual light source blocks will also vary, resulting in reduced illumination quality. become.
Therefore, it is conceivable to provide a constant current circuit for each light source block so that the currents flowing in the individual light source blocks are the same. However, since the constant current circuit has a high component cost, the LED lighting circuit becomes expensive. End up.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an LED lighting circuit capable of preventing LED damage due to overcurrent at low cost.

  As a result of intensive studies in order to solve the above-mentioned problems, the present inventors have arrived at each aspect of the present invention as follows.

<First aspect>
The first aspect is
A first light source block comprising a plurality of LEDs connected in series;
A second light source block comprising a plurality of LEDs connected in series to the first light source block;
A constant current circuit for supplying a constant current to the first light source block and the second light source block;
A first short circuit connected in parallel to the second light source block;
A second short circuit connected in parallel to the second light source block and having a resistance value greater than that of the first short circuit;
An LED lighting circuit comprising a control circuit for controlling operations of the first short circuit and the second short circuit,
When the control circuit switches from a state in which both the first light source block and the second light source block are lit to a state in which only the first light source block is lit, first, the control circuit operates the second short circuit. The LED of the second light source block is short-circuited, and after the first predetermined time elapses, the LED of the second light source block is short-circuited by operating the first short-circuit circuit in addition to the second short-circuit circuit. Circuit.

When each short circuit is short-circuited, the output voltage of the constant current circuit is lowered, so that a peak in which the output current of the constant current circuit becomes instantaneously excessive occurs and an overcurrent flows.
However, if each short circuit is operated as in the first aspect, the output voltage decreases from the maximum voltage set by the constant current circuit to the predetermined voltage, and then after the first predetermined time has elapsed, the output voltage decreases from the predetermined voltage to the constant current. Since the output voltage decreases in two stages, such as when the voltage drops to the minimum voltage set by the circuit, the peak generated in the output current is reduced and the overcurrent is also reduced.
As a result, it is possible to prevent a large overcurrent from flowing through the LEDs constituting the first light source block and damage them.
Here, the predetermined voltage and the first predetermined time may be set by experimentally finding optimum values by cut-and-try so that the above-described operation and effect can be reliably obtained.
The predetermined voltage can be adjusted by appropriately setting the internal resistance of the second short circuit.

<Second aspect>
In a second aspect, in the first aspect, the control circuit switches from a state in which only the first light source block is lit to a state in which both the first light source block and the second light source block are lit. First, an LED lighting circuit that stops the short-circuit operation of the second light source block by the first short-circuit circuit, and stops the short-circuit operation of the second light source block by the second short-circuit circuit after a second predetermined time has elapsed. is there.

When each short circuit stops the short-circuit operation, the output voltage of the constant current circuit increases, so that a gap is generated in which the output current of the constant current circuit is momentarily reduced.
However, if each short circuit is operated as in the second aspect, the output voltage increases from the minimum voltage set by the constant current circuit to the predetermined voltage, and after the second predetermined time elapses, the output voltage increases from the predetermined voltage to the constant current. Since the output voltage increases in two stages, such as increasing to the maximum voltage set by the circuit, the gap generated in the output current is reduced.
As a result, the LED constituting the first light source block can be prevented from instantaneously turning off and flickering.
Here, the predetermined voltage and the second predetermined time may be set by experimentally finding optimum values by cut-and-try so that the above-described operation / effect can be reliably obtained.

<Third aspect>
A third aspect is the LED lighting circuit according to the first or second aspect, wherein the first short circuit is composed of a MOS transistor, and the second short circuit is an LED lighting circuit composed of a MOS transistor and a resistor connected in series.
Since the MOS transistor requires less driving power than other switching elements (for example, bipolar transistors), the power consumption of the LED lighting circuit can be reduced.
In addition, since the second short circuit has a simple configuration, the component cost can be reduced.

The circuit diagram of the LED lighting circuit 10 of one Embodiment which actualized this invention. 4 is a timing chart for explaining the operation of the LED lighting circuit 10. The graph which shows the relationship between the output voltage V of the DC-DC converter 12 of the LED lighting circuit 10, and the output current I.

  As shown in FIG. 1, an LED lighting circuit 10 according to an embodiment of the present invention includes a DC power supply 11, a DC-DC converter 12, a light source block 13 (first light source block 14, second light source block 15), The LED 16, the control circuit 17, the second short circuit 18 (resistor R, NMOS transistor Q 1), and the first short circuit 19 (NMOS transistor Q 2) are used for a vehicle headlamp.

The DC power supply 11 is an in-vehicle battery mounted on an automobile, and the steady-state voltage of the DC power supply 11 is 13.5V.
The DC-DC converter 12 is a constant current circuit that is supplied with power from the DC power supply 11 and generates and outputs a variable output voltage V (= 15 to 40 V) and a constant output current I (= 1 A). .
The light source block 13 includes a first light source block 14 and a second light source block 15 connected in series. The light source block 13 is connected to the DC-DC converter 12 and supplied with DC power.
Each light source block 14, 15 is composed of five LEDs 16 connected in series.
That is, the light source block 13 is composed of ten LEDs 16 connected in series, and the output voltage V of the DC-DC converter 12 is applied to the light source block 13 and the output current I flows.

The control circuit 17 is supplied with power from the DC power supply 11, and generates and outputs a control signal VG 1 for controlling the second short circuit 18 and a control signal VG 2 for controlling the first short circuit 19.
The second short circuit 18 includes a resistor R and a transistor Q1 connected in series, and is connected in parallel to the second light source block 15.
The first short circuit 18 includes a transistor Q2 and is connected to the second light source block 15 in parallel.
A control signal VG1 is applied to the gate of the transistor Q1, and a control signal VG2 is applied to the gate of the transistor Q2.

FIG. 2A is a timing chart for explaining the operation of the LED lighting circuit 10.
In the steady state of the high beam (Hi), the control signals VG1 and VG2 are both at a low level, and the transistors Q1 and Q2 are both turned off, so that the output voltage V = 40V is applied to the light source block 13 and the output current I = 1A flows, and both the light source blocks 14 and 15 are lit.

  When the high beam (Hi) is switched to the low beam (Lo), the control signal VG1 is switched from the low level to the high level, and the control signal VG2 is switched from the low level to the high level after a first predetermined time t1 from the switching of the control signal VG1. Switch to

  When the control signal VG1 is at a high level and the control signal VG2 is at a low level, the transistor Q1 is turned on and the transistor Q2 is turned off, so that both ends of the second light source block 15 are short-circuited by the second short circuit 18. The output voltage V drops to a predetermined voltage Va defined by the internal resistance value of the second short circuit 18, but both the light source blocks 14 and 15 remain lit and are the same as the high beam.

  When the control signals VG1 and VG2 are both high, the transistors Q1 and Q2 are both turned on, so that both ends of the second light source block 15 are short-circuited by the two short circuits 18 and 19, and the output voltage V is up to 15V. The light source block 14 is turned on and the light source block 15 is turned off to become a low beam.

  When the low beam (Lo) is switched to the high beam (Hi), the control signal VG2 is switched from the high level to the low level, and after the second predetermined time t2 from the switching of the control signal VG2, the control signal VG1 is switched from the high level to the low level. Switch to

  When the control signal VG1 is at a high level and the control signal VG2 is at a low level, the transistor Q1 is turned on and the transistor Q2 is turned off, so that both ends of the second light source block 15 are short-circuited by the second short circuit 18. The output voltage V increases to a predetermined voltage Va defined by the resistance R of the second short circuit 18, and both the light source blocks 14 and 15 are turned on to become a high beam.

When the control signals VG1 and VG2 are switched from the low level to the high level and the transistors Q1 and Q2 are switched from the off state to the on state, the output voltage V is decreased, and thus a peak is generated in which the output current I is instantaneously excessive. Overcurrent flows.
Further, when the control signals VG1 and VG2 are switched from the high level to the low level and the transistors Q1 and Q2 are switched from the on state to the off state, the output voltage V increases, so that there is a gap where the output current I becomes momentarily small. Arise.

FIG. 3A is a graph showing the relationship between the output voltage V and the output current I of the DC-DC converter 12.
When switching from the high beam (Hi) to the low beam (Lo), as shown by the arrow α, the output current I changes from a constant value = 1A to return to a constant value, and then increases again to return to a constant value. The change in the output current I corresponds to the instantaneous peak shown in FIG.
When switching from the low beam to the high beam, as indicated by an arrow β, the output current I changes from a constant value = 1A to a constant value and then returns to a constant value and then returns to a constant value. Corresponds to the instantaneous gap shown in FIG.

[Operations and effects of the embodiment]
According to the LED lighting circuit 10 of the present embodiment, the following operations and effects can be obtained.

  [1] When the control circuit 17 switches from a high beam (a state in which both the first light source block 14 and the second light source block 15 are turned on) to a low beam (a state in which only the first light source block 14 is turned on), Then, the second short circuit 18 is operated to short circuit the LED 16 of the second light source block 15, and after the first predetermined time t1, the first short circuit 19 is operated in addition to the second short circuit 18 to operate the second light source block. The 15 LEDs 16 are short-circuited.

  When the short circuits 18 and 19 are short-circuited (when the control signals VG1 and VG2 are switched from low level to high level and the transistors Q1 and Q2 are switched from off to on), the output of the DC-DC converter 12 is output. Since the voltage V decreases, a peak in which the output current I of the DC-DC converter 12 becomes instantaneously excessive occurs and an overcurrent flows.

  As shown in FIG. 2B, when only the short circuit 19 is operated without operating the short circuit 18, the output voltage V is changed from 40V (the maximum voltage set by the DC-DC converter 12) to 15V. Since the output current I decreases rapidly (the minimum voltage set by the DC-DC converter 12), a large peak occurs in the output current I, and the overcurrent also increases. ) May flow and be damaged.

FIG. 3B is a graph showing the relationship between the output voltage V and the output current I corresponding to FIG.
When switching from the high beam to the low beam, as indicated by an arrow α, the output current I changes so as to return to a constant value after increasing from a constant value = 1A, and the change of the output current I is the moment shown in FIG. This corresponds to a typical peak.

On the other hand, as shown in FIG. 2A and FIG. 3A, when the short circuit 18 is operated in addition to the short circuit 19, the output voltage V decreases from 40V to the predetermined voltage Va, Then, after the first predetermined time t1 has elapsed, the output voltage V decreases in two stages, such as when the voltage decreases from the predetermined voltage Va to 15V, so that the peak generated in the output current I decreases and the overcurrent also decreases.
As a result, according to the present embodiment, it is possible to prevent a large overcurrent from flowing through the LEDs 16 constituting the first light source block 14 and damage them.

Here, the predetermined voltage Va and the first predetermined time t1 may be set by experimentally finding optimum values by cut-and-try so that the above-described operation and effect can be reliably obtained.
The predetermined voltage Va is defined by the internal resistance of the second short circuit 18 and can be adjusted by appropriately setting the resistance value of the resistor R.

  [2] When the control circuit 17 switches from the low beam (the state where only the first light source block 14 is lit) to the high beam (the state where both the first light source block 14 and the second light source block 15 are lit), Then, the short circuit operation of the second light source block 15 by the first short circuit 19 is stopped, and the short circuit operation of the second light source block 15 by the second short circuit 18 is stopped after the second predetermined time t2.

  When each of the short circuits 18 and 19 stops the short circuit operation (when the control signals VG1 and VG2 are switched from the high level to the low level and the transistors Q1 and Q2 are switched from the on state to the off state), the DC-DC converter 12 Since the output voltage V of the DC-DC converter 12 increases, a gap in which the output current I of the DC-DC converter 12 becomes instantaneously small is generated.

As shown in FIG. 2 (B), when only the short circuit 19 is operated without operating the short circuit 18, the output voltage V increases from 15V to 40V at a stretch, and therefore there is a large gap in the output current I. As a result, the LED 16 constituting the first light source block 14 may be momentarily turned off to cause flickering.
As shown by the arrow β in FIG. 3B, when the low beam is switched to the high beam, the output current I changes so as to return to a constant value after decreasing from a constant value = 1A, and the change of the output current I is shown in FIG. This corresponds to the instantaneous gap shown in (B).

On the other hand, as shown in FIGS. 2A and 3A, when the short circuit 18 is operated in addition to the short circuit 19, the output voltage V increases from 15V to the predetermined voltage Va, Then, after the second predetermined time t2 has elapsed, the output voltage V increases in two stages, such as increasing from the predetermined voltage Va to 40V, so that the gap generated in the output current I is reduced.
As a result, according to the present embodiment, it is possible to prevent the LED 16 constituting the first light source block 14 from instantaneously turning off and causing flickering.
Here, the predetermined voltage Va and the second predetermined time t2 may be set by experimentally finding optimum values by cut-and-try so that the above-described operation / effect can be reliably obtained.

  [3] If the predetermined voltage Va and the predetermined times t1 and t2 are set to optimum values, even when the short circuit operation by the short circuits 18 and 19 is repeated in a short time (that is, when the low beam and the high beam are frequently switched) Damage to the LED 16 can be reliably prevented.

  [4] Since only one DC-DC converter (constant current circuit) 12 is provided, LED lighting is performed as compared with a case where two sets of light source blocks are provided in parallel and a constant current circuit is provided for each light source block. The cost of the circuit 10 can be reduced.

[5] The NMOS transistors Q1 and Q2 constituting the short circuits 18 and 19 require less driving power than other switching elements (for example, bipolar transistors), so that the LED lighting circuit 10 can be reduced in power consumption. Can be planned.
The short circuit 18 includes a transistor Q1 and a resistor R connected in series and has a simple configuration, so that the component cost can be reduced.

<Another embodiment>
The present invention is not limited to the above-described embodiment, and may be embodied as follows, and even in that case, operations and effects equivalent to or more than those of the above-described embodiment can be obtained.

  [A] The NMOS transistors Q1 and Q2 may be replaced with PMOS transistors or other switching elements (bipolar transistors, electrostatic induction transistors, thyristors, etc.).

[B] The number of LEDs 16 constituting the first light source block 14 and the second light source block 15 is not limited to five, and may be an appropriate number.
Further, the number of LEDs 16 constituting the first light source block 14 may be different from the number of LEDs 16 constituting the second light source block 15.

  [C] Not only an LED lighting circuit used for a vehicle headlight, but also any LED lighting circuit that controls to change brightness by changing the number of LEDs to be lit. Good.

  The present invention is not limited to the description of each aspect and the embodiment. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims. The contents of papers, published patent gazettes, patent gazettes, etc. specified in this specification are incorporated by reference in their entirety.

DESCRIPTION OF SYMBOLS 10 ... LED lighting circuit 11 ... DC power supply 12 ... DC-DC converter (constant current circuit)
13 ... light source block 14 ... first light source block 15 ... second light source block 16 ... LED
17 ... Control circuit 18 ... Second short circuit 19 ... First short circuit R ... Resistance Q1 ... NMOS transistor Q2 ... NMOS transistor

Claims (3)

A first light source block comprising a plurality of LEDs connected in series;
A second light source block comprising a plurality of LEDs connected in series to the first light source block;
A constant current circuit for supplying a constant current to the first light source block and the second light source block;
A first short circuit connected in parallel to the second light source block;
A second short circuit connected in parallel to the second light source block and having a resistance value greater than that of the first short circuit;
An LED lighting circuit comprising a control circuit for controlling operations of the first short circuit and the second short circuit,
When the control circuit switches from a state in which both the first light source block and the second light source block are lit to a state in which only the first light source block is lit, first, the control circuit operates the second short circuit. The LED of the second light source block is short-circuited, and after the first predetermined time has elapsed, in addition to the second short-circuit circuit, the first short-circuit circuit is operated to short-circuit the LED of the second light source block. Lighting circuit. In the LED lighting circuit according to claim 1,
When the control circuit switches from a state in which only the first light source block is lit to a state in which both the first light source block and the second light source block are lit, first, the control circuit performs the first short circuit by the first short circuit. An LED lighting circuit that stops the short-circuit operation of the two light source blocks and stops the short-circuit operation of the second light source block by the second short-circuit after the second predetermined time has elapsed. In the LED lighting circuit according to claim 1 or 2,
The first short circuit comprises a MOS transistor;
The second short circuit is an LED lighting circuit comprising a MOS transistor and a resistor connected in series.

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