JP2008152938A - Light emitting diode drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To normally light on the other LEDs except an LED in which malfunction such as disconnection is caused, and the effect of the disconnection is made to the minimum. <P>SOLUTION: A PWM output part 5 PWM drives LED groups 1 to 4 connected in parallel, and energizes peak electric currents around from several times to ten times the size of a rated current. If either one of the LED (1-1 to 1-4) to (4-1 to 4-4) is disconnected, by detecting a voltage of the LED or less has Low level, CPU6 specifies a disconnected LED. The CPU 6 controls an analog SW7, puts a switching element corresponding to the disconnected LED in a conductive state, shunts an electric current and normally lights on the other LEDs except the disconnected LED. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light emitting diode driving circuit used for a vehicle headlight or the like.

  Conventionally, in a vehicle, when a lamp of a headlight (halogen lamp or discharge lamp) is disconnected, the driver is notified by a disconnection alarm function. However, there was no method for effectively compensating for the lamp breakage, and the vehicle continued to run until the lamp was replaced. Therefore, lamp disconnection was detected and notified to the driver, or switched to another light.

  By the way, in recent years, lighting (stop lamps, flasher lamps, etc.) by LEDs (Light emitting diodes) having advantages such as power saving and long life has been increased and applied to vehicle headlights. It has become. Since not one LED but a plurality of LEDs are connected in series and used, when one LED is disconnected (open), all LEDs connected in series do not light up.

  Therefore, when driving a plurality of LEDs connected in series, a technique has been proposed in which the location of the LED is identified when the LED is defective, and the device current at the defective location is bypassed (see, for example, Patent Document 1). In the prior art, the drive control circuit bypasses drive currents that flow through LEDs other than any LED under measurement via switching elements connected in parallel to each of the plurality of LEDs connected in series, and the measurement undertaken. By supplying a driving current for measurement only to the LEDs, the driving currents flowing through the plurality of LEDs connected in series are detected by a detection circuit, and the defective LED is identified.

JP 2005-310998 A

  However, since the above-described prior art uses a bypass circuit for disconnection detection, there is a problem that only one LED is turned on and the other LEDs are turned off, and cannot always be monitored. In addition, there is a problem in that the brightness is reduced because only one LED is detected during disconnection detection.

  Therefore, the present invention normally turns on other LEDs other than the LED that caused the malfunction, even if one of the LEDs connected in series causes a malfunction such as disconnection, An object of the present invention is to provide a light emitting diode driving circuit capable of minimizing the influence of disconnection.

  A light emitting diode driving circuit according to the present invention is a light emitting diode driving circuit that performs lighting control of a light source in which a plurality of light emitting diodes are connected in series, and a driving unit that supplies a driving current to the plurality of light emitting diodes connected in series. When a current is supplied to the plurality of light emitting diodes, voltage detecting means for detecting a voltage state in each of the light emitting diodes is connected in parallel to each of the plurality of light emitting diodes, and each can be turned on / off. When an abnormality is detected in the voltage state of any of the light emitting diodes by the plurality of switching means and the voltage detecting means, the current is diverted to the switching means corresponding to the corresponding light emitting diode. Switching control means for controlling the operation of the switching means.

  In a preferred aspect of the present invention, a plurality of light emitting diode groups in which the plurality of light emitting diodes are connected in series are connected in parallel, and a pulse width modulated driving current from the driving unit is connected to each of the plurality of light emitting diode groups. And a drive control means for individually supplying the power.

  In a preferred aspect of the present invention, the drive control means supplies each of the plurality of light emitting diode groups with a phase shift of drive currents individually supplied by the drive control means. To do.

  The switching element is preferably an electronic device such as a thyristor or a transistor. In consideration of suppression of heat generation of the light emitting diode and improvement of the maximum luminance, it is desirable that the PWM duty ratio is about 1/8 to 1/10 (12.5 to 10%). Further, the number of light-emitting diodes connected in series may be changed according to the PWM voltage (power supply capacity).

  In the present invention, when a current is supplied to a plurality of light emitting diodes connected in series, the voltage state in each light emitting diode is detected, and when an abnormality is detected in the voltage state in any one of the light emitting diodes, The current of the light emitting diode is bypassed. Therefore, even if one of the plurality of light emitting diodes connected in series causes a problem such as disconnection, the other light emitting diodes can emit light as usual. At this time, if the voltage drop in the detour is reduced, the current flowing in the other light emitting diodes becomes larger than usual, and the brightness of the other light emitting diodes is increased. Therefore, the decrease in the overall brightness value is minimized. That's it.

  In addition, a plurality of light emitting diode groups in which a plurality of light emitting diodes are connected in series are connected in parallel, and a pulse-width modulated driving current is individually supplied to each of the plurality of light emitting diode groups, so that the number of rated currents It becomes possible to pass a peak current of about 10 to 10 times. As a result, it is possible to obtain peak luminance even in the case of short-time lighting by PWM driving.

  In addition, by supplying the drive current separately supplied to each of the plurality of light emitting diode groups, it is possible to reduce the power supply capacity, thereby reducing the size of the device and suppressing the temperature rise. The voltage drop can be reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing the configuration of the LED drive circuit according to the present embodiment. In the figure, the LED drive circuit is connected in series, LED group 1 consisting of LEDs 1-1 to 1-4, LED group 2 consisting of LEDs 2-1 to 2-4, and LED group consisting of LEDs 3-1 to 3-4. 3, LED group 4 consisting of LEDs 4-1 to 4-4 is provided. The LED groups 1 to 4 are connected in parallel.

  In the present embodiment, in order to effectively use the capacity of the power source, the voltage drops of the LEDs 1-1 to 1-4,. As shown in FIG. 2, the LEDs 1-1 to 1-4,..., 4-1 to 4-4 are housed in a lamp housing that also serves as a reflector for each LED group. In the example illustrated in FIG. 2, four lamps (for example, LEDs 1-1 to 1-4) are housed in the lamp housing 10.

  The LED driving circuit includes a PWM output unit 5 for driving the LEDs 1-1 to 1-4, 4-1 to 4-4, and the LEDs 1-1 to 1-4,. CPU 4 for detecting disconnection of ˜4-4, and analog SW 7 as a set of switching elements for bypassing the disconnected LED and lighting the other LEDs.

  The PWM output unit 5 PWM-drives the LED groups 1 to 4 connected in parallel, and supplies a peak current that is several times to 10 times the rated current. Due to the afterimage of the human eye, it is possible to obtain peak luminance even with a short time lighting by PWM driving.

  The CPU 6 has an A / D input port, and the cathodes of LEDs 1-1 to 1-4,..., 4-1 to 4-4 are connected to the A / D input port. The CPU 6 monitors the voltage drop of the A / D input port voltage values V1 to V4, that is, the LEDs 1-1 to 1-4,. An abnormal value is determined (disconnection detection). When the CPU 6 detects that one of the LEDs 1-1 to 1-4,..., 4-1 to 4-4 is disconnected, the analog SW 7 diverts the portion under the control of the CPU 6. Thus, LEDs other than the disconnected LED are normally turned on.

  Next, FIGS. 3 and 4 are circuit diagrams showing configuration examples of the analog SW 7. In the configuration example shown in FIG. 3, the analog SW 7 includes thyristors 7-1-1, 7-1-2,... Connected in series, and each thyristor 7-1-1, 7-1-2,. -1, 1-2,... In parallel. When any one of the LEDs 1-i (i = 1 to 4) is disconnected, the CPU 6 supplies the control signal S1- supplied to the gate of the thyristor 7-1-j (j = i) corresponding to the disconnected LED 1-i. When j (trigger signal) is turned on, the corresponding thyristor 7-1-j becomes conductive, and the current flows through the thyristor 7-1-j bypassing the broken LED1-i. . Thereby, it becomes possible to normally light other LEDs other than the disconnected LED. The other LED groups 2, 3, and 4 have the same configuration.

  4, the analog SW 7 includes transistors 7-2-1, 7-2-2,... Connected in series, and the transistors 7-2-1, 7-2-2,. Are connected in parallel to the LEDs 1-1, 1-2,. When any of the LEDs 1-i (i = 1 to 4) is disconnected, the CPU 6 supplies the control signal S2-j to the base of the transistor 7-2-j (j = i) corresponding to the disconnected LED 1-i. As a result, the transistor 7-2-j is turned on, and the current flows through the transistor 7-2-j bypassing the disconnected LED1-i. As described above, also in this case, it is possible to normally light other LEDs than the disconnected LED. The other LED groups 2, 3, and 4 have the same configuration.

  In the above-described configuration, conventionally, in each of the LED groups 1 to 4, LEDs i-j (i = 1 to 4, j = 1 to 4) are connected in series. ,..., 4-1 to 4-4, when any of them is disconnected, all the LEDs in the corresponding LED group cannot be turned on. In this embodiment, when any one of the LEDs 1-1 to 1-4,..., 4-1 to 4-4 is disconnected, the voltage below the LED becomes a low level. Whether or not is disconnected is specified. Next, the CPU 6 controls the analog SW 7 and diverts the portion to normally light other LEDs than the disconnected LED.

  At this time, when the voltage drop of the LEDs 1-1 to 1-4,..., 4-1 to 4-4 is compared with the voltage drop of the analog SW (for example, transistor) 7, the transistors 7-2-1 and 7-2 are compared. For example, the current If1 flowing through the LED group 1 is larger than usual, and the luminance of the other LEDs 1-2, 1-3, and 1-4 is increased. For this reason, the fall of the luminance value as the LED group 1 whole can be minimized.

  Next, FIG. 5 is a circuit diagram for explaining a modification according to the present embodiment. As described above, when a plurality of LEDs 1-1 to 1-4,..., 4-1 to 4-4 connected in series are connected in parallel, the A / D port of the CPU 6 may be insufficient. In this case, as shown in FIG. 5, the multiplexer 11 may switch the LED groups 1 and 2 connected to the A / D port of the CPU 6. The same applies to the LED groups 3 and 4.

  Next, FIG. 6 is a circuit diagram showing a configuration in which a power save circuit is added to the LED drive circuit of this embodiment. FIG. 7 is a timing chart for explaining the operation when the power save circuit is used. In the figure, the output of the PWM circuit 5 is supplied to one input terminal of AND circuits 12-1 to 12-4. Control signals S1 to S4 from the CPU 6 are input to the other input ends of the AND circuits 12-1 to 12-4. The control signals S1 to S4 are supplied to the AND circuits 12-1 to 12-4 with a quarter cycle of the output of the PWM circuit 5 and shifted in phase by one cycle.

  In the LED drive circuit according to the present embodiment, since the four LED groups 1 to 4 are connected in parallel, the maximum current value IfΣ of the PWM circuit 5 is an addition value from Ifa to Ifd in the configuration shown in FIG. . Therefore, as shown in FIG. 7, the output of the PWM circuit 5 is ¼ period of the output of the PWM circuit 5 and is shifted in phase by one period as shown in FIG. By outputting via -1 to 12-4, the maximum current value of the PWM circuit 5 is set to about 1/4 of the current value (Ifa, Ifb, Ifc, Ifd) of the series circuit. As a result, the power supply capacity of the PWM circuit 5 can be reduced, and the apparatus can be downsized, temperature rise can be suppressed, and voltage drop can be reduced.

  In consideration of suppression of heat generation of LEDs 1-1 to 1-4, 4-1 to 4-4 and improvement of maximum luminance, the duty ratio of PWM is from 1/8 to 1/10 (12. 5 to 10%) is desirable. Further, the larger the number of LEDs 1-1 to 1-4,..., 4-1 to 4-4 connected in series, the smaller the energization current. However, if the output voltage of the PWM circuit 5 is 12V, for example, 4 About the number is desirable (the number changes as the voltage changes).

It is a circuit diagram which shows the structure of the LED drive circuit by this embodiment. It is a schematic diagram which shows the structure of the lamp | ramp which consists of LED. It is a circuit diagram which shows the structural example of analog SW7. It is a circuit diagram which shows the structural example of analog SW7. It is a circuit diagram for demonstrating the modification by this embodiment. It is a circuit diagram which shows the structure which added the power save circuit to the LED drive circuit of this embodiment. It is a timing chart for demonstrating operation | movement at the time of using a power save circuit.

Explanation of symbols

1-4 LED group (light emitting diode group)
1-1 to 1-4, 4-1 to 4-4 LEDs (a plurality of light emitting diodes)
5 PWM output section (drive means)
6 CPU (voltage detection means, switching control means, drive control means)
7 Analog SW (switching means)
7-1-1, 7-1-2, ... Thyristor (switching element)
7-2-1, 7-2-2, ... Transistor (switching element)
11 Multiplexer 12-1 to 12-4 AND circuit (drive control means)

Claims (3)

A light emitting diode driving circuit that performs lighting control of a light source in which a plurality of light emitting diodes are connected in series,
Driving means for supplying a driving current to the plurality of light emitting diodes connected in series;
Voltage detection means for detecting a voltage state in each light emitting diode when current is supplied to the plurality of light emitting diodes;
A plurality of switching means connected in parallel to each of the plurality of light emitting diodes, each capable of on / off operation;
When an abnormality is detected in the voltage state of any of the light emitting diodes by the voltage detecting means, the operation of the switching means is controlled so that the current is diverted to the switching means corresponding to the corresponding light emitting diode. A light-emitting diode driving circuit comprising: a switching control unit; A plurality of light emitting diode groups in which the plurality of light emitting diodes are connected in series are connected in parallel.
2. The light emitting diode drive circuit according to claim 1, further comprising drive control means for individually supplying a pulse width modulated drive current from the drive means to each of the plurality of light emitting diode groups. . 3. The light emitting diode drive according to claim 2, wherein the drive control means supplies each of the plurality of light emitting diode groups with a phase shift of a drive current individually supplied by the drive control means. circuit.

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