JP2006216304A - Driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct scatter of brightness of a group of light-emitting elements generated by scatter of voltages of the respective light-emitting elements in a forward direction. <P>SOLUTION: A voltage value at a cathode side of an LED group 30 is denoted as A voltage. An IC 50 finds the voltage impressed on a resistor R1 by reading in the A voltage by making it synchronized with a high level of a switching signal outputted to a transistor TR, generates the switching signal by calculating a duty ratio so that an average current flows through the LED group 30, and outputs the generated switching signal to the transistor TR. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a drive circuit that keeps the luminance of a light emitting element constant.

  Conventionally, light emitting diodes (LEDs) have been used for illumination in the interior of a vehicle or illumination of instruments at night. FIG. 4 is a diagram showing a state in which LEDs are installed in the switches of the vehicle. As shown in FIG. 4, in in-vehicle lighting, a power source is obtained from a battery that is a vehicle power source, and an LED is turned on based on the battery voltage.

  However, the battery voltage varies depending on the running state of the vehicle and the operating state of auxiliary devices (brake, A / C, blinker, etc.). In general, the luminance of an LED changes substantially linearly with respect to the value of the current flowing through the LED. For this reason, the illumination brightness (brightness) of the LED that obtains the power from the battery also changes according to the fluctuation of the battery voltage.

  Further, when a plurality of LEDs are connected in series, the following problems occur. First, when a plurality of LEDs are connected in series, FIG. 5A is a diagram showing the relationship between the vehicle power supply and the luminance of the LED, and FIG. 5B is a diagram illustrating the forward voltage Vf of the LED and the luminance of the LED. It is the figure which showed the relationship.

  As shown in FIG. 5 (a), the value of the current flowing through the LED changes greatly even if there is a slight change in the vehicle power supply (for example, blinking of the blinker, ON / OFF of the headlight, etc.). The brightness of the LED also changes greatly. This becomes more noticeable as the number of LEDs connected in series increases.

  In addition to the above-described fluctuations in battery voltage, as shown in FIG. 5B, variations in the forward voltage (Vf) of the LEDs also become a problem. That is, even if the difference in forward voltage between individual LEDs is small, the variation in forward voltage is increased by connecting a plurality of LEDs in series. Thereby, the brightness | luminance of LED also varies.

  Therefore, an LED drive circuit that detects the fluctuation of the battery voltage and corrects the brightness of the LED is mounted on the vehicle. FIG. 6 is a diagram illustrating a conventional LED driving circuit. As shown in FIG. 6, the LED driving circuit includes an ILL switch 100 for turning on the LED, a vehicle power supply input circuit 110 that detects the voltage of the vehicle power supply, and an output that corrects the detected voltage of the vehicle power supply. IC 120 to be configured, an LED group 130 in which a plurality of LEDs are connected in series, and a transistor 140 that drives the LED group 130 based on a switching signal input from the IC.

  In such a circuit, when the ILL switch 100 is turned on, the voltage value of the vehicle power supply is input to the IC 120 via the vehicle power supply input circuit 110. Then, a switching signal for causing the average current to flow through the LED group 130 according to the vehicle power supply is generated and output by the IC 120. Thereafter, when a switching signal is input to the transistor 140, the LED group 130 is driven according to the signal. In this way, fluctuations in the vehicle power supply are fed back so that an average current flows through the LED group 130, and variations in luminance of the LED group 130 are reduced.

  However, although the vehicle power source can be monitored by the vehicle power source input circuit 120 in the above conventional technique, the luminance due to the variation in the forward voltage Vf of each LED cannot be corrected in the LED group 130. That is, as shown in FIG. 5B, when many LEDs are connected in series, the sum of variations in the forward voltage Vf of each LED becomes larger. Correction cannot be made so that the luminance of 130 is constant.

  For example, in the red, yellow, and green LEDs, the forward voltage Vf is defined as 2.0 V, but actually the forward voltage Vf varies (about ± 0.3 V) depending on the individual LEDs. Therefore, as the number of LEDs connected in series increases, the sum of variations in Vf increases. As a result, the brightness of the LED group 130 varies.

  In view of the above points, the present invention provides a drive circuit capable of correcting a variation in luminance of a light emitting element group caused by a variation in forward voltage of each light emitting element when a plurality of light emitting elements are connected in series. For the purpose.

  In order to achieve the above object, according to the first aspect of the present invention, when the voltage value on the cathode side of the light emitting element group is the low side voltage, the control unit synchronizes with the high of the switching signal output to the switching element. The low-side voltage is used to determine the potential difference applied to the light emitting element group, the duty ratio is calculated so that the average current flows through the light emitting element group, and the switching signal is generated. It is characterized by output.

  In this way, the low side voltage is read in synchronization with the high state of the switching signal, and a switching signal is generated and output so that an average current flows through the light emitting element group. As a result, an average current can always flow through the light emitting element group. Since the luminance of the light emitting element linearly changes to the current value, the luminance of the light emitting element group can be kept constant by making the current flowing through the light emitting element group constant. In this way, it is possible to correct not only the voltage variation of the vehicle power supply but also the luminance variation in consideration of the forward voltage variation of each light emitting element of the light emitting element group.

  Since one cycle of the switching signal is a time interval (for example, on the order of milliseconds) that cannot be recognized by human eyes, the light emitting element group is turned on or off by the switching element. Is the average luminance. For this reason, even if the duty ratio of the switching signal is changed, the average luminance of the light emitting element is recognized by human eyes, and therefore, variation in luminance can be corrected by changing the duty ratio.

  The invention according to claim 2 is characterized in that the switching element is provided on the anode side or the cathode side of the light emitting element group. As described above, the light emitting element group can be driven regardless of whether the switching element is provided on the anode side or the cathode side of the light emitting element group.

  The invention described in claim 3 is characterized in that a constant voltage circuit is provided on the anode side of the light emitting element group. As described above, even when the constant voltage circuit is provided, the variation in the forward voltage Vf of the light emitting element can be corrected, and a constant current value can always be supplied to the light emitting element group.

  In addition, the code | symbol in the bracket | parenthesis of each said means shows the correspondence with the specific means as described in embodiment mentioned later.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram illustrating an LED driving circuit according to a first embodiment of the present invention. As shown in FIG. 1, the drive circuit includes an ILL switch 10, a vehicle power input circuit 20, an LED group 30, a resistor R1, a transistor TR, a voltage input circuit 40, and an IC 50. Has been.

  The ILL switch 10 is a nighttime illumination switch for turning on an LED group 30 described later. Such an ILL switch 10 is turned on or off in response to, for example, turning on or off of a small light of the vehicle.

  The vehicle power input circuit 20 is an interface for inputting a signal to the IC 50 when the ILL switch 10 is turned on. The vehicle power supply input circuit 20 includes a plurality of resistors R5 and R6 and a diode D2.

  The LED group 30 includes a plurality of LEDs that are light emitting elements, and the LEDs are connected in series. Each LED of the LED group 30 is individually arranged in, for example, switches (see FIG. 4), and lights up when the ILL switch 10 is turned on. The resistor R1 limits the current flowing through the LED group 30.

  The LED corresponds to the light emitting element of the present invention, and the LED group 30 corresponds to the light emitting element group of the present invention. In addition, the anode side of the LED group 30 refers to the anode side of the LED on the highest side among the plurality of LEDs connected in series. Similarly, the cathode side of the LED group 30 refers to the cathode side of the lowest-side LED among a plurality of LEDs connected in series.

  The transistor TR is a switching element that turns on or off each LED of the LED group 30 in accordance with a switching signal input from an IC 50 described later. The transistor TR corresponds to the switching element of the present invention.

  Here, a connection point between the LED group 30 and the resistor R1 is A, and a voltage value at the connection point is an A voltage. The point A is a voltage value obtained by subtracting a voltage drop value (the number of LEDs × Vf) in the LED group 30 from the voltage value of the vehicle power supply of the vehicle. The A voltage corresponds to the low side voltage of the present invention.

  The voltage input circuit 40 is an interface for inputting the A voltage to the IC 50. In this embodiment, the voltage input circuit 40 includes a plurality of resistors R2 and R3, a capacitor C, and a diode D3, and inputs the A voltage on the cathode side of the LED group 30 to the IC 50.

  Thus, by inputting the A voltage value to the IC 50 by the voltage input circuit 40, it becomes possible to simultaneously monitor the fluctuation of the vehicle power supply and the variation in the forward voltage Vf of the LED group 30.

  The IC 50 is a control circuit having a function of keeping the average value of the current flowing through the LED group 30 constant. The IC 50 is supplied with an illumination control signal (hereinafter referred to as an illumination controller signal) for adjusting the illumination brightness in the passenger compartment. The illuminator is controlled by the IC 50 so as to be effective when the ILL switch 10 is turned on. The IC 50 includes a logic circuit or software that performs PWM control for feeding back the value of the A voltage to the output of the illumination controller signal so that an average current flows through the LED group 30. The IC 50 corresponds to the control unit of the present invention.

  Such an IC 50 is already mounted on the vehicle as another control component, and is used in the LED drive circuit shown in FIG. Thereby, parts can be shared, new parts are not required, and the LED drive circuit can be configured at low cost without increasing the number of parts.

  The above is the configuration of the LED drive circuit.

  Next, the operation of the LED drive circuit will be described. First, an illuminator signal is input to the IC 50. And in IC50, the brightness | luminance of each LED of the LED group 30 is set based on this illumination controller signal, and the electric current value which flows into the LED group 30 is calculated | required in order to implement | achieve the brightness | luminance.

  Further, a switching signal having a duty ratio corresponding to the current value of the current flowing through the LED group 30 is generated by the IC 50 and output to the transistor TR. In the transistor TR, a current flows through the LED group 30 in accordance with the duty ratio of the input switching signal. Thus, each LED of the LED group 30 is turned on.

  As described above, when the ILL switch 10 is in the ON state and the LEDs are lit, the luminance variation of each LED is corrected by the following PWM control.

  First, when the ILL switch 10 is turned on by the user, the voltage value of the vehicle power supply is input to the IC 50 via the vehicle power supply input circuit 20. On the other hand, since the value of the A voltage has a value when the transistor TR is turned on, the voltage A is synchronized with the high state of the transistor TR, that is, the high state of the switching signal output from the IC 50. The data is read into the IC 50 through 40.

  The voltage A at the point A of the circuit shown in FIG. 1 is a voltage obtained by subtracting the total value of each Vf of the LED group 30 from the voltage value of the vehicle power supply. Therefore, the fluctuation of the vehicle power supply and the forward voltage Vf of the LED group 30 Can be monitored simultaneously.

  For this reason, when the total value of the standard values of each LED of the LED group 30 is used as a reference, if the value of the A voltage input to the IC 50 is smaller than this reference, a switching signal with a longer on-time of the transistor TR is generated. To do. On the other hand, when the value of the A voltage is larger than the reference, a switching signal in which the ON time of the transistor TR is shortened is generated.

  As described above, when a switching signal whose duty ratio is changed according to the value of the A voltage is generated, the switching signal is output to the transistor TR. In this way, control is performed so that the average current flowing through the LED group 30 does not change, that is, by changing the duty ratio of the switching signal, the fluctuation of the vehicle power supply and the variation in the forward voltage Vf of the LED group 30 are corrected. The brightness of the LED group 30 is kept constant.

  Note that one cycle of the switching signal is a time interval (for example, on the order of milliseconds) that cannot be recognized by human eyes. For this reason, the human eye recognizes the average luminance of the LED group 30 when the LED group 30 is turned on or off by the transistor TR. Therefore, even if the duty ratio of the switching signal is changed, the average brightness of each LED is recognized by human eyes, so that the average brightness can be realized by changing the duty ratio.

  FIG. 2 is a table showing current change rates when a plurality of LEDs are connected in series. FIG. 2 shows a change in current flowing through the LED group 30 particularly when the vehicle power supply is changed from 13V to 12V. As shown in FIG. 2, when the number of connected LEDs increases, the current change rate also changes greatly due to the variation in Vf. Therefore, the luminance of the LED group 30 is controlled by controlling the average current to flow through the LED group 30. Can be kept constant.

  As described above, in the present embodiment, the A voltage is read in synchronization with the high state of the switching signal, and a switching signal that causes the average current to flow through the LED group 30 based on the vehicle power supply and each voltage value of the A voltage is obtained. Generate and output. Thereby, an average current can flow through the LED group 30.

  Since the luminance of each LED linearly changes to the current value, the luminance of the LED group 30 can be kept constant by making the current flowing through the LED group 30 constant. In this way, it is possible to correct not only the voltage fluctuation of the vehicle power supply but also the luminance variation in consideration of the variation of the forward voltage Vf of each LED of the LED group 30. Therefore, by adopting the configuration as described above, even when a large number of LEDs are connected in series, it is possible to drive without a change in luminance.

(Second Embodiment)
In the present embodiment, only different parts from the first embodiment will be described. FIG. 3 is a diagram illustrating an LED driving circuit according to a second embodiment of the present invention. 3, parts that are the same as or equivalent to the LED drive circuit diagram shown in FIG. 1 are denoted by the same reference numerals in FIG. 3 for the sake of simplicity.

  In the present embodiment, as shown in FIG. 3, the first LED group 31 and the second LED group 32 form a line in the circuit. A first resistor R11 and a first transistor TR1 are connected to the first LED group 31 in series. The second LED group 32 is connected to the second resistor R12 and the second transistor TR2 in series.

  The first and second LED groups 31 and 32 are equivalent to the LED group 30 of the first embodiment. The first and second resistors R11 and R12 correspond to the resistor R1 of the first embodiment. Further, the first and second transistors TR1 and TR2 are equivalent to the transistor TR of the first embodiment.

  In the present embodiment, the connection point between the first LED group 31 and the resistor R11 is A1, and the voltage value at this point A1 is the A1 voltage. A connection point between the second LED group 32 and the resistor R12 is A2, and a voltage value at the point A2 is an A2 voltage. The A1 and A2 voltages are input to the IC 50 via the voltage input circuit 40, respectively.

  Therefore, the IC 50 performs PWM control of the LED groups 31 and 32 independently. In this embodiment, since the voltage input circuit 40 is shared by the LED groups 31 and 32, each switching signal is such that when one of the switching signals is on, the other switching signal is off. Is generated. That is, for example, when the A1 voltage of the first LED group 31 is read, the second LED group 32 is turned off.

  Thus, even when there are a plurality of LED groups, it is possible to suppress variations in the luminance of the LEDs for each LED group 31 and 32 column.

(Other embodiments)
The LED drive circuits shown in the first and second embodiments are examples, and are not limited to these.

  In the first and second embodiments, the timing of reading the A voltage, the A1 voltage, or the A2 voltage is around the center value while the transistors TR, TR1, and TR2 are turned on (each transistor TR, TR1, In order to avoid transient effects due to the operation of the alternator and other components at the time when TR2 is turned on and the A voltage, A1 voltage, or A2 voltage is stabilized), a capacitor is provided in the circuit, or IC50 The number of samplings may be increased and averaging processing may be performed.

  In the second embodiment, the voltage input circuit 40 is shared by the LED groups 31 and 32, but the voltage input circuit 40 may be prepared for each of the LED groups 31 and 32.

  In the first and second embodiments, by providing a constant voltage circuit on the anode side of the LED group to make the voltage applied to the LED group constant, the fluctuation of the vehicle power supply can be made constant. Thereby, in the IC 50, it is only necessary to correct the variation in the forward voltage Vf of the LED group.

  In the first and second embodiments, the transistors TR, TR1, and TR2 are provided on the cathode side of the LED groups 30 to 32, but are provided on the anode side of the LED groups 30 to 32. May be.

  In the second embodiment, two light emitting element groups are prepared. However, the number of light emitting element groups may be any number.

It is the figure which showed the LED drive circuit which concerns on 1st Embodiment of this invention. It is the chart which tabulated the rate of current change when a plurality of LEDs were connected in series. It is the figure which showed the LED drive circuit which concerns on 2nd Embodiment of this invention. It is the figure which showed the state by which LED was installed in switches of vehicles. (A) is the figure which showed the relationship between a vehicle power supply and the brightness | luminance of LED, (b) is the figure which showed the relationship between the forward voltage Vf of LED, and the brightness | luminance of LED. It is the figure which showed the conventional LED drive circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... ILL switch, 20 ... Vehicle power input circuit, 30 ... LED group,
31 ... 1st LED group, 32 ... 2nd LED group, 40 ... Voltage input circuit, 50 ... IC,
TR ... transistor, TR1 ... first transistor, TR2 ... second transistor.

Claims (3)

A light emitting element group (30) in which a plurality of light emitting elements are connected in series;
A switching element (TR) for turning on or off the plurality of light emitting elements;
A controller (50) configured to read a voltage of a vehicle power source supplied to the light emitting element group and generate and output a switching signal for controlling the switching element.
When the voltage value on the cathode side of the light emitting element group is a low side voltage, the control unit reads the low side voltage in synchronization with the high of the switching signal output to the switching element, and uses the low side voltage to perform the light emission. A potential difference applied to the element group is obtained, a duty ratio is calculated so that an average current flows through the light emitting element group, a switching signal is generated, and the generated switching signal is output to the switching element. A drive circuit characterized by that. The drive circuit according to claim 1, wherein the switching element is provided on an anode side or a cathode side of the light emitting element group. The drive circuit according to claim 1, wherein a constant voltage circuit is provided on an anode side of the light emitting element group.

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