JP7295706B2 - LED driving device, display device, and control device for LED driving device - Google Patents

Description

The present disclosure relates to an LED driving device, a display device including the same, and a control device for the LED driving device.

The use of LEDs (Light Emitting Diodes) as energy-saving light sources has been expanding more and more in recent years. An LED driver for supplying a constant current to the LED is used to drive the LED from the forward voltage-current characteristics and light emission characteristics of the LED.

For example, Japanese Patent Laying-Open No. 2018-19498 (Patent Document 1) discloses an LED driver using a switching power supply circuit. This LED driver includes a DC/DC converter that supplies current to the LED and a current driver that adjusts the current flowing through the LED. A DC/DC converter is composed of an asynchronous rectification chopper circuit including a coil, a switching element, and a diode. With such a configuration, the LED can be driven with high efficiency (low loss) (see Patent Document 1).

JP 2018-19498 A

In the case where the chopper circuit that constitutes the DC/DC converter is of a step-up type, the following problems occur when a diode that constitutes the chopper circuit is short-circuited. That is, when the switching element is turned on, the power stored in the capacitor on the output side flows to the switching element through the short-circuited diode. This may cause ASO (Area of Safe Operation) breakdown of the switching element, and the LED will not light up. JP-A-2003-200002 does not particularly consider such a problem.

The present disclosure has been made to solve such a problem, and an object of the present disclosure is to provide an LED driving device capable of avoiding that an LED cannot be lit when a short-circuit failure of a diode occurs, and a display including the same. The object is to provide a device as well as a control device for an LED driver.

The LED driver of the present disclosure comprises a DC/DC converter configured to power a plurality of LEDs connected in series, and a driver configured to drive the DC/DC converter. The DC/DC converter is an asynchronous rectification boost chopper circuit including a coil, a first switching element, and a diode. The LED driver further comprises a second switching element connected in parallel with a portion of the plurality of LEDs. The second switching element is off when no diode short fault has occurred. The driving device turns off the first switching element and turns on the second switching element when a short-circuit failure occurs.

In the LED driving device described above, when a short-circuit failure occurs in the diode of the DC/DC converter, the first switching element is turned off. As a result, although the boost function of the DC/DC converter is lost, it is possible to output power from the DC/DC converter to a plurality of LEDs. Here, since the second switching element is turned on, some of the plurality of LEDs are not lit, but the total forward voltage due to the remaining LEDs is reduced. As a result, a current flows through the remaining LEDs, and the LEDs can be lit.

Some of the LEDs to which the second switching element is connected in parallel may include the most downstream LED among the plurality of LEDs.

Alternatively, some of the LEDs to which the second switching element is connected in parallel may include the most upstream LED among the plurality of LEDs.

The second switching element may be provided within the driving device.
The LED driver may further comprise a detector configured to detect the drain voltage or drain current of the first switching element or the current through the diode. Then, the driving device may determine whether or not a short-circuit failure has occurred based on the output of the detector.

The diodes may be Schottky barrier diodes.
A display device of the present disclosure includes any one of the LED driving devices described above and a plurality of LEDs connected in series and driven by the LED driving device.

Also, the control device of the present disclosure is a control device for an LED driving device. The LED driver includes a DC/DC converter configured to power multiple LEDs connected in series. The DC/DC converter is an asynchronous rectification boost chopper circuit including a coil, a first switching element, and a diode. The control device includes a second switching element connected in parallel to part of the plurality of LEDs, and a controller. The second switching element is off when no diode short fault has occurred. The controller is configured to turn off the first switching element and turn on the second switching element when a short fault occurs.

According to the LED driving device, the display device, and the control device of the LED driving device according to the present disclosure, it is possible to prevent the LED from being unable to light when a diode short failure occurs in the DC/DC converter.

1 is a diagram showing the overall configuration of a display device using LED drivers according to an embodiment of the present disclosure; FIG. 2 is a timing chart schematically showing an operation example of the LED driver shown in FIG. 1; 4 is a flow chart showing an example of a procedure of processing executed by a controller; FIG. 10 is a diagram showing the overall configuration of a display device using LED drivers according to a modification;

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

<Configuration of LED driver>
FIG. 1 is a diagram showing the overall configuration of a display device using LED drivers according to an embodiment of the present disclosure. Referring to FIG. 1 , display device 1 includes LED array 10 , DC/DC converter 20 , and drive device 50 . The DC/DC converter 20 and the driving device 50 constitute an LED driver that drives the LED array 10 .

LED string 10 includes a plurality of LEDs 12 connected in series. Although this example shows a single LED string 10 containing more than four LEDs 12, the number of LED strings may be more than one, with each LED string including four LEDs 12. may be one or less.

DC/DC converter 20 includes a power supply node 22 , coil 24 , switching element Q<b>1 , diode 26 and capacitor 28 . Coil 24 is connected between power supply node 22 and switching element Q1. Switching element Q1 is connected between coil 24 and the ground node. A gate of the switching element Q1 is connected to an external terminal T2 of the drive device 50 . In this example, the switching element Q1 is an N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor), but may be an NPN-type bipolar transistor or the like.

The diode 26 has an anode connected to the connection point between the coil 24 and the switching element Q<b>1 , and a cathode connected to the anode side of the LED array 10 . In this example, the diode 26 is a Schottky barrier diode (hereinafter referred to as "SBD (Schottky Barrier Diode)"). Since the SBD has a small forward voltage, it has a small forward loss, high efficiency, and high speed switching. It should be noted that it is not essential that the diode 26 be an SBD, and it is also possible to adopt a diode other than an SBD. Capacitor 28 is connected between the power line to which the cathode of SBD 26 is connected and the ground node.

These power supply node 22, coil 24, switching element Q1, SBD 26, and capacitor 28 form a so-called asynchronous rectification boost chopper circuit. The driving device 50 controls the switching of the switching element Q1, and the power supplied from the power supply node 22 is stepped up and supplied to the LED array 10. FIG.

Driving device 50 includes a current driver 60 , a controller 70 , a switching element Q 2 and a detector 80 . The driving device 50 is configured, for example, as an IC (Integrated Circuit) in which these elements are integrated. The driving device 50 further includes external terminals T1 to T5 for electrical connection with external elements.

Current driver 60 includes a switching element 62 , a resistive element 64 and an error amplifier 66 . The switching element 62 is connected between the external terminal T5 to which the cathode side of the LED array 10 is connected and the resistance element 64 . In this example, the switching element 62 is an N-type MOSFET, but may be an NPN-type bipolar transistor or the like. Resistance element 64 is connected between switching element 62 and the ground node.

A connection point between the switching element 62 and the resistance element 64 is connected to the inverting input terminal (−) of the error amplifier 66 . A reference voltage Vref is applied to the non-inverting input terminal (+) of the error amplifier 66 . The output terminal of the error amplifier 66 is connected to the gate of the switching element 62 .

A detection signal (voltage) obtained by converting the current flowing through the switching element 62 (that is, the current flowing through the LED array 10 ) into a voltage by the resistance element 64 is input to the inverting input terminal (−) of the error amplifier 66 . The reference voltage Vref input to the non-inverting input terminal (+) of the error amplifier 66 is a signal obtained by converting the target value of the current flowing through the LED array 10 into a voltage using the resistance value of the resistance element 64 .

The error amplifier 66 drives the switching element 62 by outputting to the gate of the switching element 62 a signal obtained by amplifying the difference between the reference voltage Vref and the detection signal (voltage). Thereby, the current flowing through the switching element 62, that is, the current flowing through the LED array 10 is adjusted to the current target value corresponding to the reference voltage Vref. Note that the current driver 60 may be provided outside the IC.

Controller 70 receives voltage Vfb (voltage on the cathode side of LED array 10) input from external terminal T5, and drives DC/DC converter 20 based on voltage Vfb. Specifically, the controller 70 includes an error amplifier and a PWM (Pulse Width Modulation) circuit (both not shown). The error amplifier amplifies and outputs the difference between the voltage Vfb and a predetermined value so that the voltage Vfb becomes a predetermined value (for example, 0.5 V). The PWM circuit generates a PWM signal based on the output of the error amplifier, and outputs the generated PWM signal to the external terminal T2 connected to the gate of the switching element Q1.

Note that the controller 70 is not limited to being implemented by hardware (electronic circuit), and may be implemented by software. Specifically, the controller 70 includes a CPU (Central Processing Unit), memory (ROM (Read Only Memory) and RAM (Random Access Memory)), an input/output buffer for inputting/outputting various signals, and the like. may The CPU develops a program stored in the ROM into the RAM or the like and executes it. The program stored in the ROM is a program in which processing procedures of the controller 70 are described. The controller 70 may be configured to execute various processes according to this program.

In this LED driver, the current driver 60 adjusts the current flowing through the LED string 10 to a current target value corresponding to the reference voltage Vref. Further, the DC/DC converter 20 is driven by the controller 70 based on the voltage Vfb so that the voltage Vfb on the cathode side of the LED array 10 becomes a predetermined value. As a result, the current flowing through the LED array 10 is controlled to a target.

<SBD26 short failure>
In DC/DC converter 20, when SBD 26 is short-circuited, when switching element Q1 is turned on, power stored in capacitor 28 flows to switching element Q1 through short-circuited SBD 26. FIG. As a result, there is a possibility that ASO breakdown will occur in the switching element Q1, and the LED array 10 will not light up.

Therefore, in the LED driver according to the present embodiment, switching element Q2 connected in parallel to part of LED array 10 is provided. Switching element Q2 is provided in drive device 50 and driven by controller 70 . In this example, the switching element Q2 is connected in parallel to two LEDs 12 including the most downstream LED 12 among the plurality of LEDs 12, but the number of the LEDs 12 to which the switching element Q2 is connected in parallel may be one. , three or more. Also, in this example, the switching element Q2 is an N-type MOSFET, but may be an NPN-type bipolar transistor or the like.

Then, the controller 70 turns off the switching element Q2 when the SBD 26 is not short-circuited. When the SBD 26 is short-circuited, the controller 70 turns off the switching element Q1 and turns on the switching element Q2.

When a short-circuit failure occurs in the SBD 26, the switching element Q1 is turned off, so that although the DC/DC converter 20 loses its boosting function, the power can be output from the DC/DC converter 20 to the LED array 10. becomes possible.

When a short failure occurs in the SBD 26, the switching element Q2 is turned on so that the LEDs 12 (two in this example) to which the switching element Q2 is connected in parallel do not light up. The forward voltage of 10 becomes smaller. As a result, a current flows through the LED row 10, and the remaining LEDs 12 other than the LEDs 12 to which the switching element Q2 is connected in parallel can be lit.

The number of LEDs 12 to which the switching element Q2 is connected in parallel depends on the output voltage of the DC/DC converter 20 when the DC/DC converter 20 is used with the switching element Q1 turned off, the characteristics of each LED 12 (forward voltage etc.), is appropriately determined based on the adjustment value of the voltage Vfb on the cathode side of the LED array 10 or the like.

A short circuit failure of the SBD 26 is determined based on the detected value of the detector 80 . The detector 80 is connected to the external terminal T1 to which the drain of the switching element Q1 is connected, and detects the drain voltage Vd of the switching element Q1. Controller 70 receives the detected value of voltage Vd from detector 80, and determines that a short failure has occurred in SBD 26 when voltage Vd does not sufficiently decrease while switching element Q1 is on.

In this example, the drain voltage Vd of the switching element Q1 is used to determine whether or not the SBD 26 has a short-circuit failure. may be detected by a detector, and the short-circuit failure of the SBD 26 may be determined based on the detected value. Specifically, when the drain current of the switching element Q1 abnormally rises when the switching element Q1 is turned on, or when a reverse current flows through the SBD 26, it is determined that the SBD 26 has a short failure. be able to.

FIG. 2 is a timing chart schematically showing an operation example of the LED driver shown in FIG. Referring to FIG. 2, drain voltage Vd of switching element Q1 varies depending on whether switching element Q1 is turned on or off, and voltage Vd decreases when switching element Q1 is turned on.

It is assumed that a short failure of SBD 26 occurs at time t1. Before a short-circuit failure occurs, when switching element Q1 is turned on, voltage Vd drops to a level lower than threshold value Vth (for example, about 0.1 V). Note that the switching element Q2 is turned off before time t1 when no short-circuit failure occurs.

At time t1, SBD 26 is short-circuited, and then at time t2, when switching element Q1 is turned on, current flows from capacitor 28 on the cathode side of SBD 26 to switching element Q1 through SBD 26 in which the short-circuit has occurred. As a result, a current exceeding the maximum rating flows into the switching element Q1, and as a result, the voltage Vd does not drop to the level of the threshold value Vth.

When the voltage Vd is higher than the threshold value Vth (the voltage Vd does not drop below the threshold value Vth) while the switching element Q1 is on, it is determined that the SBD 26 has a short failure. , the short fault flag is turned on. This short-circuit fault flag remains on until the short-circuit fault is resolved by repairing (replacement, etc.) the SBD 26 .

When the short fault flag is turned on, the LED driver operates in fault mode. That is, the switching element Q1 is turned off and the switching element Q2 is turned on. This state continues until the short-circuit failure flag is turned off, that is, until the SBD 26 is repaired (replaced, etc.) to eliminate the short-circuit failure. As a result, it is possible to prevent the LED array 10 from not lighting due to a short failure of the SBD 26 .

FIG. 3 is a flowchart showing an example of a procedure of processing executed by the controller 70. As shown in FIG. A series of processes shown in this flowchart are repeatedly executed at predetermined intervals.

Referring to FIG. 3, controller 70 determines whether or not the short failure flag of SBD 26 is on (step S10). When the short-circuit failure flag is ON (YES in step S10), controller 70 proceeds to step S50 (described later).

When it is determined in step S10 that the short-circuit failure flag is off (NO in step S10), controller 70 determines whether drain voltage Vd of switching element Q1 is higher than threshold value Vth while switching element Q1 is on. (step S20). Voltage Vd is detected by detector 80 (FIG. 1).

If it is determined in step S20 that the voltage Vd has fallen below the threshold value Vth while the switching element Q1 is on (NO in step S20), the SBD 26 is not short-circuited, and the controller 70 performs the switching operation. The element Q2 is turned off (step S30). Note that the switching element Q1 is switching-driven based on the voltage Vfb.

On the other hand, when it is determined in step S20 that voltage Vd is higher than threshold value Vth (voltage Vd does not decrease below threshold value Vth) while switching element Q1 is on (YES in step S20), controller 70 determines that a short-circuit failure has occurred in the SBD 26, and turns on the short-circuit failure flag (step S40).

When the short-circuit failure flag is turned on, the controller 70 turns off the switching element Q1 and turns on the switching element Q2 (step S50). As a result, the LED array 10 can be lit even though the number of lit LEDs 12 is smaller than when no short-circuit failure occurs.

As described above, in this embodiment, when a short-circuit failure occurs in SBD 26, switching element Q1 is turned off. As a result, power can be output from the DC/DC converter 20 to the LED array 10, although the boost function of the DC/DC converter 20 is lost. Since the switching element Q2 is turned on, a part of the LED row 10 is not lit, but the forward voltage of the LED row 10 due to the remaining LEDs 12 is reduced. As a result, a current flows through the LED row 10, and the remaining LEDs 12 other than the LEDs 12 to which the switching element Q2 is connected in parallel can be lit.

In the above embodiment, when the SBD 26 is short-circuited, the switching element Q2 is connected in parallel to the LED 12 on the downstream side of the LED array 10. The switching elements may be connected in parallel.

FIG. 4 is a diagram showing the overall configuration of a display device using LED drivers according to such a modification. Referring to FIG. 4, display device 1A includes drive device 50A instead of drive device 50 in display device 1 shown in FIG. Drive device 50A includes switching element Q3 in place of switching element Q2 in drive device 50 .

Switching element Q3 is also provided in drive device 50 and driven by controller 70, like switching element Q2. The switching element Q3 is connected in parallel to two LEDs 12 including the most upstream LED 12 among the plurality of LEDs 12 . The number of LEDs 12 to which the switching element Q3 is connected in parallel may be one, or three or more. Also, in this example, the switching element Q3 is a P-type MOSFET, but may be a PNP-type bipolar transistor or the like.

Even with such a configuration, the same effects as those of the above-described embodiment can be obtained.
In the above-described embodiment and the modification shown in FIG. 4, one LED row 10 is provided, but a plurality of LED rows 10 may be arranged in parallel. In this case, a switching element Q2 or Q3 is provided for each LED row 10, and in each LED row 10, the switching element Q2 or Q3 is connected in parallel to the LED 12 at the same position as another LED row 10.

The display device 1 (1A) described above can be applied, for example, as a light source for a liquid crystal display (LCD), and is particularly suitable as a light source for an in-vehicle display that displays the driving status of the vehicle to the driver. is. According to the display device 1 (1A) described above, even if the SBD 26 is short-circuited, it is possible to prevent the in-vehicle display from turning off completely and to perform the limp-home run.

The embodiments disclosed this time should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is indicated by the scope of the claims rather than the description of the above-described embodiments, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

1, 1A display device, 10 LED row, 20 DC/DC converter, 22 power supply node, 24 coil, 26 SBD, 28 capacitor, 50, 50A drive device, 60 current driver, 62, Q1, Q2, Q3 switching element, 64 Resistive element, 66 error amplifier, 70 controller, 80 detector, T1 to T5 external terminals.

Claims (7)

a DC/DC converter configured to power a plurality of LEDs connected in series;
a driving device configured to drive the DC/DC converter;
The DC/DC converter is an asynchronous rectification boost chopper circuit including a coil, a first switching element, and a diode, and
A second switching element connected in parallel to a portion of the plurality of LEDs,
The second switching element is off when the short-circuit failure of the diode does not occur,
The drive device turns off the first switching element and turns on the second switching element when the short-circuit failure occurs,
further comprising a detector configured to detect the drain voltage or drain current of the first switching element, or the current flowing through the diode;
The LED driving device, wherein the driving device determines whether or not the short circuit failure occurs based on the output of the detector . 2. The LED driving device according to claim 1, wherein some of said plurality of LEDs include the most downstream LED among said plurality of LEDs. 2. The LED driving device according to claim 1, wherein some of said plurality of LEDs include an uppermost LED among said plurality of LEDs. 4. The LED driving device according to any one of claims 1 to 3, wherein said second switching element is provided within said driving device. 5. The LED driving device according to any one of claims 1 to 4 , wherein said diode is a Schottky barrier diode. The LED driving device according to any one of claims 1 to 5 ;
A display device comprising a plurality of serially connected LEDs driven by the LED driver. A control device for an LED driving device,
wherein the LED driver includes a DC/DC converter configured to power a plurality of LEDs connected in series;
The DC/DC converter is an asynchronous rectification boost chopper circuit including a coil, a first switching element, and a diode,
The control device is
a second switching element connected in parallel to a portion of the plurality of LEDs;
a controller;
The second switching element is off when the short-circuit failure of the diode does not occur,
The controller is configured to turn off the first switching element and turn on the second switching element when the short failure occurs ,
The LED driving device further includes a detector configured to detect the drain voltage or drain current of the first switching element or the current flowing through the diode,
A control device for an LED driving device, wherein the controller is configured to determine whether the short-circuit fault has occurred based on the output of the detector.

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