JP6747657B2 - LED drive circuit - Google Patents

Description

The present invention relates to an LED driving circuit.

FIG. 3 shows a conventional LED drive circuit. Reference numeral 51 is a voltage input terminal, 52 and 53 are an LED group in which a plurality of LEDs are connected in series, 54 is a PWM terminal to which a PWM signal is input, and 55 is a drive circuit.

When the voltage VIN of the voltage input terminal 51 is a low voltage, the circuit shown in FIG. 3A in which the LED groups 52 and 53 are connected in parallel is used, and when the voltage VIN of the voltage input terminal 51 is a high voltage, the LED is The circuit shown in FIG. 3B in which the groups 52 and 53 are connected in series is used.

However, this method requires two types of circuits. Alternatively, the wiring is different, and when the LEDs are formed on the same substrate, four LED groups are required, resulting in an increase in expensive parts and an increase in the substrate area.

On the other hand, in Patent Document 1, as shown in FIG. 4, when the forward voltage Vf of the LEDs of the LED groups 63 and 64 is small, the terminal 65 is connected by a resistor to connect the LEDs between the terminals 61 and 62. When the groups 63 and 64 are connected in series and the forward voltage Vf of the LEDs of the LED groups 63 and 64 is large, the terminal 65 is opened and the terminals 66 and 67 are connected by resistors, respectively. A configuration for connecting in parallel is described.

However, this is one in which one of the series connection and the parallel connection of the LED groups 63 and 64 is selected in accordance with the magnitude of the forward voltage Vf of the LED to be used. Not sure what to connect to. Further, it is not clear whether the forward voltage Vf of each LED of the LED groups 63 and 64 in series connection is determined from the terminal 61 side or the terminal 62 side. Furthermore, when connecting the terminal 65 with a transistor, it is not clear how to determine the bias setting. Therefore, there is a problem that the brightness cannot be designed.

Further, as shown in FIG. 5, a circuit has been proposed in which the LED groups 72 and 73 are connected in parallel or in series according to the level of the input voltage VIN. In FIG. 5, 71 is a voltage input terminal, 72 is a first LED group in which n (n is an integer of 1 or more) LEDs are connected in series, and 73 is a second LED in which n LEDs are connected in series. Group 74 is a PWM terminal. Reference numeral 75 denotes a control circuit, which includes a voltage detection circuit 751 that detects whether the voltage VIN input to the voltage input terminal 1 is high or low, and a switch control circuit 752 that outputs a control signal according to the detection result of the voltage detection circuit 751. Reference numeral 76 denotes a drive circuit, which includes a first drive unit 761 for driving the LED group 72 and a second drive unit 762 for driving a series circuit of the LED group 73 or the LED groups 72, 73, which have a predetermined current. The current is turned on/off by the PWM signal input to the PWM terminal 74 while setting the value. SW11 to SW14 are switch circuits whose ON/OFF is individually controlled by the switch control circuit 752.

In this LED drive circuit, the input voltage VIN is detected by the control circuit 75, and according to the detection result, the LED groups 72 and 73 are connected in series if the input voltage VIN is high, and are connected in parallel if the input voltage VIN is low. The drive currents at the time of series connection and parallel connection are set by the drive units 761 and 762 of the drive circuit 76, and the brightness of the LED groups 72 and 73 at the time of series connection and parallel connection is set to a predetermined value.

JP, 2009-059636, A

By the way, when the LED drive circuit is applied to a navigation light, the power supply voltage of the navigation light is 12V or 24V. Therefore, at 12V, the LED groups may be connected in parallel, and at 24V, they may be connected in series. In this case, a large difference occurs in the voltage applied to the switch circuit SW14.

When the power supply voltage VIN is as low as 12V, the control circuit 75 controls the switch circuit SW12 to be off and the switch circuits SW11, SW13 and SW14 to be on, and the LED groups 72 and 73 are connected in parallel. At this time, considering the path of the LED group 73, the voltage across the LED group 73 is 9V, the voltage across the switch circuit SW13 is 0.1V, and the voltage across the drive unit 762 of the drive circuit 76 is 0.5V. If added, a voltage of 12V-0.1V-9V-0.5V=2.4V is applied to both ends of the switch circuit SW14.

Since the power supply voltage VIN is expected to drop to 10V, the voltage across the switch circuit SW14 when it drops to 0.4V is 0.4V, and the voltage between the collector and the emitter when the switch circuit SW14 is composed of npn transistors. The voltage is low, and the amplification factor decreases, but it is maintained at a practical level.

On the other hand, when the power supply voltage VIN is as high as 24V, the control circuit 75 controls the switch circuits SW11 and SW13 to be off and the switch circuits SW12 and SW14 to be on, and the LED groups 72 and 73 are connected in series. At this time, considering the paths of the LED groups 72 and 73, the voltage across the LED groups 72 and 73 is 9 V, the voltage across the switch circuit SW12 is 0.1 V, and the voltage across the drive unit 762 of the drive circuit 76 is 0 V. If a voltage of 0.5V is applied, a voltage of 24V-9V-0.1V-9V-0.5V=5.4V is applied to both ends of the switch circuit SW14.

Since the power supply voltage VIN is expected to rise to 29V, the voltage across the switch circuit SW14 when it rises to 10.4V becomes 10.4V between the collector and emitter when the switch circuit SW14 is composed of npn transistors. The voltage is considerably high, and a transistor for high power is required.

As described above, it is expected that a widely varying voltage of 0.4 V to 5.4 V will be applied to the switch circuit SW14 commonly used during parallel connection and series connection. Required. Further, the transistor becomes large in size, resulting in high cost, and the circuit board becomes large.

An object of the present invention is to reduce the size of the switch circuit, reduce the cost, reduce the circuit board, etc. by preventing a high voltage from being applied to the switch circuit commonly used when the two LED groups are connected in parallel and connected in series. It is to provide a realized LED drive circuit.

In order to achieve the above object, the invention according to claim 1 is a first LED group which comprises a series circuit of n (n is an integer of 1 or more) LEDs, one end of which is connected to a voltage input terminal; A second LED group consisting of a series circuit of LED's, a drive circuit having first and second drive sections, and a first drive section connecting the other end of the first LED group to the first drive section. Switch circuit, a second switch circuit that connects one end of the second LED group to the other end of the first LED group, and connects one end of the second LED group to the voltage input terminal A third switch circuit and a fourth switch circuit that connects the other end of the second LED group to the second drive unit are provided, and when the voltage of the voltage input terminal is low, the second switch circuit is provided. The circuit is turned off, the first, third, and fourth switch circuits are turned on, the first LED group is connected to the first driving unit, and the second LED group is turned to the second LED group. When the voltage of the voltage input terminal is high, the first and third switch circuits are turned off, the second and fourth switch circuits are turned on, and the first and the third switch circuits are turned on. A second LED group is connected in series to the second driving unit, a first voltage is applied to the fourth switch circuit when the voltage of the voltage input terminal is low, and a voltage of the voltage input terminal is high. The voltage applied to the fourth switch circuit when the voltage of the voltage input terminal is low in the LED drive circuit where the second voltage higher than the first voltage is sometimes applied to the fourth switch circuit. Generates a voltage such that the voltage becomes the first voltage, and the voltage applied to the fourth switch circuit becomes the first voltage even when the voltage at the voltage input terminal is high. A generating circuit is provided, and when the first LED group is connected to the first drive section and the second LED group is connected to the second drive section, the first and second drive circuits are provided. Section controls the first and second LED groups to have the same brightness, and when the first and second LED groups are connected in series to the second drive section, the second drive section The first and second LED groups have the same brightness as when the first LED group is connected to the first drive unit and the second LED group is connected to the second drive unit. It is characterized in that it is controlled by.

According to a second aspect of the present invention, in the LED drive circuit according to the first aspect, the fourth switch circuit is composed of a transistor, and the voltage generating circuit is a variable voltage generating a voltage input to a base or a gate of the transistor. It is characterized in that it is composed of a voltage source.

The invention according to claim 3 is the LED drive circuit according to claim 1 or 2, further comprising a control circuit that detects the voltage of the voltage input terminal and controls the first to fourth switch circuits. And

According to the present invention, the voltage applied to the fourth switch circuit can be suppressed to a low voltage, and a small transistor can be used as a transistor forming the fourth switch circuit. Miniaturization can be realized.

It is a circuit diagram of the LED drive circuit of the first embodiment of the present invention. It is a circuit diagram of an LED drive circuit of the second embodiment of the present invention. It is a circuit diagram of a conventional LED drive circuit. It is a circuit diagram of another conventional LED drive circuit. It is a circuit diagram of another conventional LED drive circuit.

<First embodiment>
FIG. 1 shows an LED drive circuit of the first embodiment. 1 is a voltage input terminal, 2 is a first LED group in which n (n is an integer of 1 or more) LEDs are connected in series, 3 is a second LED group in which n LEDs are connected in series, and 4 is It is a PWM terminal. Reference numeral 5 denotes a control circuit, which includes a voltage detection circuit 51 that detects whether the voltage VIN input to the voltage input terminal 1 is high or low, and a switch control circuit 52 that outputs a control signal according to the detection result of the voltage detection circuit 51. Reference numeral 6 denotes a drive circuit, which includes a first drive unit 61 that drives the LED group 2 and a second drive unit 62 that drives a series circuit of the LED group 3 or the LED groups 2 and 3, and these have a predetermined current. A value is set and a PWM signal input to the PWM terminal 4 is used to perform dimming control by turning on/off the current. Reference numeral 7 is a voltage generation circuit which is controlled by the control circuit 5 and generates a voltage for setting the voltage applied to the switch circuit SW4. SW1 to SW4 are switch circuits whose ON/OFF is individually controlled by the switch control circuit 52.

Now, the high voltage or the low voltage VIN is input to the voltage input terminal 1. At this time, when the input voltage VIN is detected to be a low desired voltage by the power supply detection circuit 51 of the control circuit 5, the switch control circuit 52 controls the switch circuits SW1, SW3, SW4 to be turned on, The circuit SW2 is controlled to be off. As a result, the LED groups 2 and 3 are connected in parallel to the voltage input terminal 1. Then, the drive units 61 and 62 of the drive circuit 6 are set to the same current value and controlled by the same PWM signal. Thereby, each LED of the LED groups 2 and 3 emits light with the same brightness according to the PWM control. At this time, the switch circuit SW4 is controlled by the voltage generated in the voltage generation circuit 7 so that the voltage applied thereto becomes equal to that of the conventional one (about 0.4 V to 2.4 V).

As described above, when the input voltage VIN is low, the LED groups 2 and 3 are connected in parallel. At this time, since the number of LEDs connected in series to the voltage input terminal 1 is reduced, the dynamic range of the circuit can be increased.

On the other hand, when the input voltage VIN is detected by the power supply detection circuit 51 of the control circuit 5 to be a high desired voltage, the switch control circuit 52 controls the switch circuits SW2 and SW4 to turn on, and the switch circuits SW1 and SW1. SW3 is controlled to be off. As a result, the LED groups 2 and 3 are connected in series to the voltage input terminal 1. At this time, the drive unit 62 of the drive circuit 6 is PWM-controlled, so that the LED groups 2 and 3 emit light with the same brightness according to the PWM control.

At this time, the switch circuit SW4 is controlled by the voltage generating circuit 7 so that the voltage applied thereto becomes a predetermined voltage. For example, a voltage of 5.4 V to 10.4 V was applied to the switch circuit SW14 in the conventional circuit described in FIG. 5 when connected in series, but this voltage is low when applied in parallel (0.4 V to 0.4 V). It can be suppressed to about 2.4V).

As described above, a low voltage is applied to the switch circuit SW4 regardless of whether the LED groups 2 and 3 are connected in parallel or in series, so that the switch circuit SW4 is downsized, the cost is reduced, and the circuit board is downsized. Can be realized.

When the LED groups 2 and 3 are connected in series, the voltage applied to the switch circuit SW4 is reduced as compared with the conventional example, and therefore the voltage applied to the drive unit 62 of the drive circuit 6 is correspondingly increased. However, when the LED groups 2 and 3 are connected in series, the drive unit 61 has no relation and the drive unit 62 alone controls the current flowing through the LED groups 2 and 3. Therefore, the drive unit 62 is connected to the control circuit 5. The switch control circuit 52 may independently control and adjust the voltage there. Even if the flowing current may be different from that in the parallel connection, the brightness of the LED groups 2 and 3 may be adjusted to the same level as the brightness in the parallel by adjusting the PWM signal.

<Second embodiment>
FIG. 2 shows an LED drive circuit of the second embodiment. In this embodiment, in the LED drive circuit of FIG. 1, the switch circuit SW4 is composed of an npn transistor Q1 and the voltage generation circuit 7 is composed of a variable voltage source V1 for controlling the base voltage of the transistor Q1.

In this embodiment, when the input voltage VIN is a low voltage such as 10V to 12V, the control circuit 6 controls the voltage of the voltage source V1 so that the collector-emitter voltage of the transistor Q1 is 0.4V to 2.V. It is controlled to be about 4V. When the input voltage VIN is a high voltage such as 24V to 29V, the control circuit 6 controls the voltage of the voltage source V1 so that the collector-emitter voltage of the transistor Q1 is 0.4V to 2.4V. Control it to the extent.

As a result, the transistor Q1 of the switch circuit SW4 is applied with a voltage within the range of about 0.4V to 2.4V regardless of whether the LED groups 2 and 3 are connected in parallel or in series. A large capacity transistor is not required for the transistor Q1, an inexpensive and small transistor can be used, and the circuit board can be downsized.

Although the switch circuit SW4 is configured by the npn transformer in the above, it can be configured by the pnp transistor or the MOS transistor, and the driving units 61 and 62 can also be configured by the bipolar transistor or the MOS transistor.

1, 51, 61, 71: voltage input terminal 2, 52, 62, 72: LED group 3, 53, 63, 73: LED group 4, 74: PWM terminal 5, 75: control circuit 51, 751: voltage detection circuit 52, 752: switch control circuit 6, 76: drive circuit 61, 761: first drive unit 62, 762: second drive unit 7: voltage generation circuit

Claims (3)

a first LED group consisting of a series circuit of n LEDs (n is an integer of 1 or more), one end of which is connected to a voltage input terminal; a second LED group consisting of a series circuit of n LEDs; A drive circuit having first and second drive parts, a first switch circuit for connecting the other end of the first LED group to the first drive part, and one end of the second LED group for the first switch circuit. A second switch circuit connected to the other end of the first LED group, a third switch circuit connecting the one end of the second LED group to the voltage input terminal, and the other end of the second LED group And a fourth switch circuit for connecting to the second drive unit,
When the voltage of the voltage input terminal is low, the second switch circuit is turned off, the first, third, and fourth switch circuits are turned on, and the first LED group is driven by the first drive circuit. The second LED group is connected to the second drive unit and the voltage of the voltage input terminal is high, the first and third switch circuits are turned off and the second switch circuit is turned off. And a fourth switch circuit is turned on, the first and second LED groups are serially connected to the second drive unit,
A first voltage is applied to the fourth switch circuit when the voltage of the voltage input terminal is low, and a voltage higher than the first voltage is applied to the fourth switch circuit when the voltage of the voltage input terminal is high. In the LED drive circuit to which the voltage of 2 was applied,
A voltage is generated such that the voltage applied to the fourth switch circuit becomes the first voltage when the voltage of the voltage input terminal is low, and the fourth switch also generates when the voltage of the voltage input terminal is high. A voltage generating circuit for generating a voltage such that the voltage applied to the circuit becomes the first voltage,
When the first LED group is connected to the first drive section and the second LED group is connected to the second drive section, the first and second drive sections cause the first and second drive sections to operate. The first and second LED groups are controlled to have the same brightness,
When the first and second LED groups are connected in series to the second drive section, the second drive section causes the first and second LED groups to change to the first LED group. Is connected to the first drive unit, and the second LED group is controlled to have the same brightness as when connected to the second drive unit.
An LED drive circuit characterized by the above.
The LED drive circuit according to claim 1,
The LED drive circuit is characterized in that the fourth switch circuit is composed of a transistor, and the voltage generation circuit is composed of a variable voltage source that generates a voltage input to the base or gate of the transistor. The LED drive circuit according to claim 1 or 2,
An LED drive circuit comprising a control circuit for detecting the voltage of the voltage input terminal and controlling the first to fourth switch circuits.

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