JP2019096644A - LED drive circuit - Google Patents

Abstract

To surely detect abnormality in any series LED array in simple configuration in a circuit configured by connecting series LED arrays in parallel.SOLUTION: The present invention relates to an LED drive circuit comprising: multiple LED elements which are connected in series; series LED arrays each including a current control circuit for making a predetermined current flow to the LED elements and a current detection circuit for detecting the current flowing to the LED elements; a voltage output circuit for applying a level adjustable voltage to the multiple series LED arrays which are connected in parallel; and a control circuit for controlling the voltage that the voltage output circuit outputs. The control circuit detects start of electrification through the current detection circuit by gradually increasing the output voltage of the voltage output circuit when starting lighting each of the series LED arrays and, in a case where the applied voltage in the electrification start is not settled within a predetermined range, discriminates that the series LED array is abnormal.SELECTED DRAWING: Figure 11

Description

  The present invention uses a series LED array including a plurality of LED elements connected in series as a unit of drive, and in a circuit in which a plurality of the series LED arrays are connected in parallel, if any LED element has an abnormality The present invention relates to an LED drive circuit that detects an abnormality.

LED (Light Emitting Diode) elements are used as light sources of small size, long life, energy saving as compared with incandescent lamps and fluorescent lamps in various applications such as display, indoor and outdoor lighting.
In the case of illuminating a wide area, a plurality of LED elements are used in series or in parallel, and in that case, it is not easy to detect a failure of any of the LED elements.
In this regard, a voltage when a plurality of LEDs connected in series are normally lit is stored as an initial voltage, and it is determined that the LED array is abnormal if the voltage detected in the subsequent lighting state changes by a predetermined value or more with respect to the initial voltage. An LED drive circuit is known (see, for example, Patent Document 1).

  In addition, the following method is known as a method of detecting a failure of an LED array that constitutes a backlight of a liquid crystal monitor. Each LED array is sequentially lighted by sequentially switching the current switching switch provided between the power supply circuit and the LED array at the time of the initialization operation. Then, an initial current value flowing to each LED array is detected and stored. At the time of normal operation, the current changeover switches are sequentially switched to sequentially turn on the respective LED arrays. Then, a drive current value flowing for each LED array is detected, and a failure for each LED array is detected as compared with the initial current value (see, for example, Patent Document 2).

JP, 2011-181616, A International Publication No. 2014/174635

Taking a backlight of a liquid crystal monitor as in Patent Document 2 as an example, in recent years, with the development of large size, high definition, and light image quality of the liquid crystal monitor, the backlight tends to be required to have sufficient brightness. Therefore, instead of so-called dynamic drive in which the LED arrays are sequentially switched and lighted, there is a tendency to adopt a method in which each LED array is constantly lighted to obtain sufficient brightness.
In addition, in order to obtain sufficient brightness, a large number of LED elements need to be arranged, but a high voltage needs to be applied to a series of LED arrays connected in series as in Patent Document 1. Therefore, there is a tendency to adopt a configuration in which the series LED arrays are connected in parallel and the respective series LED arrays are individually current-driven. By driving each series LED array individually, the brightness of each LED element can be made uniform.

The present invention has been made in consideration of the above circumstances, and any one circuit is formed by connecting in parallel a series LED array including a plurality of LED elements connected in series and a current control circuit. The present invention provides a method for reliably detecting an abnormality of a series LED array with a simple configuration.
Although the application of the present invention is not limited to the backlight of the liquid crystal monitor, it can be said that the backlight used for the liquid crystal display device which has made remarkable progress is a preferable example for explaining the problem to be solved.

  The present invention is connected in parallel with a plurality of LED elements connected in series, a current control circuit that supplies a predetermined current to the LED elements, and a series LED array including a current detection circuit that detects the current flowing in the LED elements. And a control circuit for controlling a voltage output from the voltage output circuit, wherein the control circuit starts to light each series LED array. The output voltage of the voltage output circuit is gradually increased when causing the current detection circuit to detect the start of energization, and the series LED array is abnormal if the applied voltage at the start of energization is not within a predetermined range. To provide an LED drive circuit.

  In the LED drive circuit according to the present invention, the control circuit gradually increases the output voltage of the voltage output circuit when starting the lighting of each series LED array, detects the start of energization by the current detection circuit, and applies the voltage when starting energization. Since it is determined that the series LED array is abnormal when the voltage is not within the predetermined range, in a circuit in which a series LED array including a plurality of series connected LED elements and a current control circuit is connected in parallel Abnormality of any series LED array can be detected with simple configuration.

It is a block diagram which shows the structural example in the case of applying to the backlight of a liquid crystal display device as one Embodiment of the LED drive circuit by this invention. It is a block diagram which shows the detailed structure of the LED drive circuit shown in FIG. It is explanatory drawing which shows the connection form of the LED element in the LED drive circuit shown in FIG. 2, and the phenomenon which arises when one of the LED elements fails. FIG. 3 is an explanatory view that represents functions of a current control circuit and a current detection circuit shown in FIG. FIG. 4 is an explanatory diagram that represents the function of the DC / DC converter shown in FIG. 3 in the form of a circuit. It is explanatory drawing which shows the example of physical arrangement | positioning of each LED element in the backlight apparatus shown in FIG. It is explanatory drawing which shows the equivalent circuit of the LED element shown in FIG. FIG. 6 is an explanatory view showing a first conventional configuration example up to the configuration of the LED drive circuit shown in FIGS. 2 and 3; FIG. 6 is an explanatory view showing a second conventional configuration example up to the configuration of the LED drive circuit shown in FIGS. 2 and 3; It is explanatory drawing which shows the 3rd example of a conventional structure until it reaches the structure of the LED drive circuit shown to FIG. 2 and FIG. It is explanatory drawing which shows the structure of the LED drive circuit corresponding to FIG. 2 and FIG. In the LED drive circuit by this embodiment, it is a 1st graph which shows the waveform of the lighting process which served as abnormality detection of the LED element (waveform at the time of normal). In the LED drive circuit by this embodiment, it is a 2nd graph which shows the waveform of the lighting process which served as abnormality detection of the LED element (waveform at the time of abnormality). In this embodiment, it is a flowchart which shows the processing of the abnormality detection which CPU of a control circuit implements at the time of lighting processing of a LED element. In this embodiment, there is a graph showing how the CPU of the control circuit detects an abnormality based on a change in Vled while the LED element is on. It is a graph which shows the temperature characteristic of the forward voltage drop Vf of the LED element in this embodiment. Embodiment 4 It is a block diagram which shows the structure corresponding to FIG. 2 of the LED drive circuit in this embodiment. Embodiment 4 In this embodiment, it is a graph which shows the example of the difference of the normal range in case element temperature differs by the last abnormality determination and this abnormality determination (Embodiment 4) It is a flowchart corresponding to FIG. 12 which shows the process of the abnormality detection in this embodiment (Embodiment 4)

Hereinafter, the present invention will be described in more detail using the drawings. The following description is an exemplification in all respects, and should not be construed as limiting the present invention.
Embodiment 1
<< Example of application of LED drive circuit >>
FIG. 1 is a block diagram showing a configuration example in the case of being applied to a backlight of a liquid crystal display device as an embodiment of the LED drive circuit according to the present invention. In FIG. 1, the liquid crystal display device 10 controls the pixels of the liquid crystal panel 23 based on the liquid crystal panel 23, a backlight device 19 that emits light transmitted from the back side to the front side of the liquid crystal panel 23, and a video signal from the outside. And a display control circuit 13. The backlight device 19 is configured using a plurality of LED elements.

The liquid crystal display device 10 further includes a liquid crystal control circuit 21 that controls pixels of the liquid crystal panel 23, a backlight control circuit 15 that controls lighting of LED elements of the backlight device 19, and backlight abnormality detection that detects abnormalities of the LED elements. A circuit 17 is provided.
The backlight control circuit 15, the backlight abnormality detection circuit 17 and the backlight device 19 correspond to the LED drive circuit according to the present invention.
The functions of the backlight control circuit 15 and the backlight abnormality detection circuit 17 are different from each other, and therefore, they are shown as separate blocks, but may be an integral part of the circuit configuration. That is, the circuit configuration may be simplified by sharing a part of the lighting circuit of the LED element and the abnormality detection circuit.

  The liquid crystal display device 10 is a display monitor connected to a computer as a specific application, or is used as a signage display for displaying contents of guidance and advertisement. Of course, the present invention is not limited to those applications, and the backlight of the liquid crystal display device is only one application of the LED drive circuit.

FIG. 2 is a block diagram showing a detailed configuration of the LED drive circuit 11 shown in FIG. As shown in FIG. 2, in the LED drive circuit, a plurality of series LED arrays U1 to Un are connected in parallel, and the output voltage Vled from the voltage output circuit DCV is applied to the node portion of the parallel connection.
Each of the series LED arrays U1 to Un includes a series LED element group LEDs in which a plurality of LED elements are connected in series, a current control circuit DRV for causing a predetermined current to flow in the series LED element group, and a current flowing in the series LED element group It has a current detection circuit CS to detect. For example, the series LED array U1 includes a series LED element group LEDs1, a current control circuit DRV1, and a current detection circuit CS1, which are arranged in series. The same configuration is applied to the series LED arrays U2 to Un.

Further, as shown in FIG. 2, the LED drive circuit 11 includes a control circuit MC. The control circuit MC has hardware resources of the CPU 31, the memory 32, the timer 33, the current detection unit 34, the current control unit 35, and the voltage adjustment unit 36. The hardware resources may be mounted on the CPU 31 and other parts by using, for example, a general-purpose microcomputer, or may be mounted as a semiconductor element for specific application.
The memory 32 includes a non-volatile memory that stores control programs executed by the CPU 31 and data used for control. It also includes a RAM that provides a work memory used by the CPU 31 to execute processing.

The magnitudes of the currents detected by the current detection circuits CS1 to CSn of the series LED arrays U1 to Un are input to the control circuit MC via the current detection unit 34. The current detection unit 34 is an input interface circuit.
The control circuit MC indicates the magnitude of the current to be supplied to the series LED element groups LEDs 1 to LEDsn to which the current control circuits DRV 1 to DRVn of each series LED array U 1 to Un correspond. The current control unit 35 is an output interface circuit to each current control circuit.
Further, the control circuit MC indicates the magnitude of the voltage Vled to be output by the voltage output circuit DCV. The voltage adjustment unit 36 is an output interface circuit to the voltage output circuit DCV.

FIG. 3 is an explanatory view showing a connection form of LED elements in the LED drive circuit shown in FIG. 2 and a phenomenon that occurs when one of the LED elements breaks down.
In FIG. 3, a portion corresponding to the series LED element group LEDs 1 of FIG. 1 is configured of ten LED elements D0101 to D0110 connected in series. Further, in the circuit corresponding to the current control circuit DRV1 of FIG. 1, it is the FET element Qc01 that controls the current flowing to the LED element. For ease of understanding, only the FET element Qc01 is shown in FIG. 3 and the configuration of the other parts is omitted. The part corresponding to the current detection circuit CS1 of FIG. 1 mainly includes a current detection resistor Rc01. For easy understanding, only the current detection resistor Rc01 is shown in FIG. 3, and the configuration of the other parts is omitted.
Although the configuration of the series LED array U1 of FIG. 3 has been described in detail above, the same applies to the other series LED arrays U2 to Un.

FIG. 4 is an explanatory view showing functions of the current control circuit and the current detection circuit shown in FIG. The functional configurations of the current control circuit and the current detection circuit are shown in equivalent circuit diagrams.
The LEDs in FIG. 4 correspond to any of the series LED element groups LEDs 1 to LEDsn shown in FIG. 2, and in the case of the LEDs 1, for example, correspond to the LED elements D0101 to D0110 in FIG. The FET element Qc in FIG. 4 corresponds to, for example, the FET element Qc01 in FIG. The current detection resistor Rc in FIG. 4 corresponds to, for example, the current detection resistor Rc01 in FIG.
Although not shown in FIG. 3, the current control circuit has an error amplifier AMP1 or a function corresponding thereto. The voltage Vref1 from the reference power supply is input to one (inversion input) of the two inputs of the error amplifier AMP1. The reference voltage Vref1 is a voltage corresponding to the magnitude of the current to be targeted. A voltage Vr = If × Rc proportional to the magnitude of the current If is input to the other input (non-inverting input) of the error amplifier AMP1 from one end of the current detection resistor Rc. The output of the error amplifier AMP1 drives the gate of the FET element Qc. The FET element Qc is a voltage control element that causes a drain current of a size corresponding to the drive voltage of the gate to flow. That is, the FET element Qc is a voltage control element which tries to flow drain current with a characteristic of monotonically increasing with respect to the gate voltage in the active region.

The error amplifier AMP1 drives the FET element Qc so as to have a sufficiently large gain (ideally infinite) so that the potential difference between the two inputs is sufficiently small (ideally zero). That is, due to the negative feedback characteristic that targets the reference voltage Vref1, the FET element Qc flows the drain current If according to the reference voltage Vref1. Therefore, the drain current If is maintained at a constant magnitude corresponding to the reference voltage Vref1.
When the magnitude of the reference voltage Vref1 changes, the magnitude of the drain current If changes accordingly.
The functions and configurations of the current detection circuit and the current control circuit have been described above.

Further, in the mode shown in FIG. 3, the voltage output circuit DCV is specifically mounted as a DC / DC converter DDC, and adjusts the magnitude Vled of the DC voltage to be output. The configuration of the DC / DC converter DDC is, for example, as described below.
FIG. 5 is an explanatory view in circuit form of the function of the DC / DC converter shown in FIG.
A direct current voltage is supplied to the source of the FET element Qv shown in FIG. The resistors R1 and R2 divide the output voltage Vled from the DC / DC converter DDC, and input the divided voltage to one (non-inverting input) of two inputs of the error amplifier AMP2. The voltage Vo input to the non-inverting input of the error amplifier AMP2 is
Vo = Vled × R1 / (R1 + R2)
In a relationship of

The reference voltage Vref2 is input to the other input (inverting input) of the error amplifier AMP2.
The error amplifier AMP2 has a sufficiently large gain (ideally infinite) so that the potential difference between the two inputs Vref2 and Vo becomes sufficiently small (ideally zero). To drive. That is, at the input of the error amplifier AMP2,
Vref2 = Vo = Vled × R1 / (R1 + R2)
The FET element Qv controls the magnitude of the drain current Id so that the following relationship is established.
Therefore, output voltage Vled does not depend on the voltage on the source side of FET element Qv, and does not depend on the voltage-current characteristics of the load connected to the output side of DC / DC converter DDC and to which output voltage Vled is applied. The DC voltage has a magnitude corresponding to the magnitude of the reference voltage Vref2.

However, when the magnitude of the reference voltage Vref2 changes, the magnitude of the output voltage Vled changes accordingly. The magnitude of the reference voltage Vref2 is controlled by a control circuit MC not shown in FIG. The output voltage Vled can be output only to a voltage lower than the voltage on the source side.
The configuration shown in FIG. 5 shows an example in which the FET element Qv is analogically controlled in order to explain the function of the DC / DC converter in an easy-to-understand manner. In the configuration of FIG. 5, the voltage on the drain D side of the FET element Qv is smaller than the voltage supplied from the source S side, and the product of the potential difference between the source and drain and the drain current Id is the loss of the FET element Qv. .

  It is also possible to reduce the loss of the FET element Qv, that is, the conversion loss of voltage with respect to the analog drive of FIG. For example, an inductance is inserted between the drain D of the FET Qv of FIG. 5 and the load (not shown) connected to the end. Furthermore, a diode is inserted between the drain D and the ground (the drain D side is a cathode and the ground side is an anode). Then, the gate G of the FET element Qv is PWM-driven. With such a configuration, it is possible to cause the FET element Qv to perform switching operation. If the switching voltage from the drain D is smoothed by a capacitor and output, an output voltage Vled according to the reference voltage Vref2 can be obtained as in the analog operation of FIG.

FIG. 6 shows an example of the positions where the respective LED elements in FIG. 3 are disposed behind the liquid crystal panel 23. That is, FIG. 6 is an explanatory view showing an example of physical arrangement of each LED element in the backlight device shown in FIG. 1, and the code of each LED element in FIG. 3 corresponds to the code of each LED element in FIG. doing. In FIG. 6, lower-case alphabetic characters k, l (El), and m used for the symbols of the LED elements are shown as indicating numbers smaller than n. That is, m is synonymous with (n-1), 1 is synonymous with (n-2), and k is used as a thing synonymous with (n-2).
Although the aspect of FIG. 6 corresponds to a so-called direct type LED backlight, it is only one aspect, and may be, for example, a so-called edge type LED backlight in which an LED and a light guide member are combined.

«Light emission control of LED element»
In FIG. 3, the control circuit MC detects the current If1 flowing through the LED elements D0101 to D0110 by detecting, for example, the voltage of the current detection resistor Rc01 for the series LED array U1. That is, a voltage drop Vr1 = If1 × Rc01 of a magnitude proportional to the current If1 occurs in the current detection resistor Rc01. The current detection resistances Rc01 to Rcn all have the same resistance value.

A circuit corresponding to the current control circuit DRV1 of FIG. 1 controls the FET element Qc01 so that the magnitude of If1 becomes a predetermined target current value. The brightness of the LED element is approximately proportional to the magnitude of the current flowing through the LED element. When the current control circuit DRV1 passes a target current value to the LED elements D0101 to D0110, each LED element lights up (emits light) with a predetermined luminance.
By supplying currents If1 to Ifn of the same magnitude to the respective series LED arrays U1 to Un, the respective LED elements D0101 to Dn10 emit light uniformly with a predetermined luminance. The target values of If1 to Ifn may be changed according to the setting of the brightness of the liquid crystal display device 10, the brightness around the installed place, and the like.

The LED element emits light by passing a forward current, and a forward voltage drop Vf is generated at that time, but the magnitude of the voltage drop is proportional to the change in the magnitude of the forward current like a resistance It does not indicate the characteristics. However, the characteristics show that the forward voltage gradually increases monotonously with the increase of the current.
Generally, the magnitude of the forward voltage Vf generated at both ends of one LED element at the time of light emission is about 3V. However, it is known that in the LED elements, the magnitude of the forward voltage varies from element to element. In the numerical example appended to FIG. 3, the forward voltage of each of the LED elements D0101 to D0110 is 3.1 V (in order to simplify the description, there is no variation in the forward voltage among the LED elements D0101 to D0110. In the case of LED elements of the same production lot, the forward voltage is almost the same.) This is the largest forward voltage among the series-connected LED arrays connected in parallel. In the aspect of FIG. 3, since ten LED elements are connected in series, the magnitude of the forward voltage generated across the series LED element group LEDs 1 is 31V. The forward voltage of the other series LED element group is equal to or less than this.

Temporarily, the current control circuits DRV1 to DRVn and the current detection circuits CS1 to CSn of the series-connected LED arrays U1 to Un shown in FIG. 1 can be replaced by simple resistance elements R01 to Rn (where R01 = R02 = ... = Rn). Suppose that it replaced. In that case, although the same voltage Vled is applied to all the series LED arrays, the magnitudes of the currents If1 to Ifn flowing to the respective series LED arrays are different due to the variation of the forward voltage of the LED elements. As a result, variations in the brightness of the LED elements occur for each series LED array. The variation in luminance causes unevenness in the luminance of the liquid crystal display device 10 and degrades the quality of the image.
The serial LED arrays U1 to Un include the current control circuits DVR1 to DVRn so that even if there is a variation in the forward voltage of the LED elements, a predetermined forward current is caused to emit light with a predetermined luminance.

In FIG. 3, the FET elements Qc01 to Qcn play a role of absorbing the variation of the forward voltage of the LED element. That is, the variation in the forward voltage of the LED element is absorbed by the voltage Vds between the source and the drain of the FET element. In the numerical example appended to FIG. 3, the source-drain voltage Vds1 of the FET element Qc01 is 0.1V. This is at or near the on voltage in the saturation region of the FET element Qc01. That is, it is at or near the minimum value of the source-drain voltage that can be taken by the FET element Qc01.
Also, in the numerical example, the voltage Vr1 across the current detection resistor Rc01 is 0.3V.
According to those numerical examples, the voltage across the series LED array U1 when the current If1 is flowing, that is, Vled is
Vled = Vr1 + Vds + Vf = 31.4 (V)
It becomes.

On the other hand, it is assumed that the forward voltage of the LED elements of the series LED array Un is small, and the forward voltage Vfn of the series LED element group LEDsn is 27V. Note that the forward voltage of the LED element may vary by about 1 V depending on the element.
Since it is premised that the LED element is made to emit light at the same luminance, Ifn = If1 and Vrn = Vr1 = 0.3 V according to the condition.
Since Vfn and Vrn are determined as described above and the sum of them and the source-drain voltage Vdsn of the FET element Qcn must be Vled = 31.4 (V), the source-drain voltage Vdsn of the FET element Qcn is , Must be 4.1V.
In other words, the current control circuit DRVn sets the gate voltage of the FET element Qcn so that the source-drain voltage Vds of the FET element Qcn becomes 4.1 V so that the current Ifn flowing through the LED elements Dn01 to Dn10 becomes equal to If1. Control.

Here, considering the power (loss) consumed by the FET elements Qc01 to Qcn, the FET element is a voltage control element and the gate current can be neglected, and therefore the power consumption is the product of the drain current and the source-drain voltage. The drain currents If1 to Ifn of the respective FET elements are all equal to each other, which is If.
The power consumption of the FET element Qc01 is P1 = If × Vds1 = If × 0.1 (W)
It is.
The power consumption of the FET element Qc0n is Pn = If × Vdsn = If × 4.1 (W)
It is.
The same is true for the other FET devices, which is greater than or equal to P1 corresponding to the minimum source-drain voltage.

  As described above, since the current control circuits DRV1 to DRVn are controlled to flow the same target current If, in the series LED array in which the forward voltage of the series LED element group is large, the source-drain voltage of the FET element The loss is small, and for a direct current LED array with a small forward voltage, the source-drain voltage of the FET element and hence the loss become large.

The control circuit MC monitors the magnitude of the current detected by the current detection circuits CS1 to CSn. Then, the output voltage of the voltage output circuit DCV is controlled to output a voltage Vled voltage having a magnitude sufficient to flow the current If of a target size to the respective series LED arrays U1 to Un.
On the other hand, if Vled is too large, the loss of the FET elements Qc01 to Qcn will only increase. Therefore, the control circuit MC sets the magnitude of Vled so that the source-drain voltage of the series LED element group having the largest forward voltage can attain or close to the minimum voltage that can be taken (0.5 V in the numerical example of FIG. 3). It is preferable to control. By doing so, wasteful loss of the FET elements Qc01 to Qcn can be suppressed.

  The control circuit MC lowers the output voltage Vled of the DC / DC converter DDC when lighting the backlight in order to output the minimum output voltage Vled that can flow the target current If to each of the series LED arrays U1 to Un. Gradually increases from high voltage to high voltage. Then, when the target current If flows for any of the series LED arrays U1 to Un, the output voltage Vled thereafter is maintained at the voltage at that point, that is, the control circuit MC sets the target for the LED element The output voltage at the time when it lights up with the desired brightness is judged and maintained.

When the control circuit MC gradually raises the output voltage Vled, the series LED array in which the target current If flows finally flows is the series LED array with the largest forward voltage. Then, Vled when the target current If flows in the series LED array is a suitable output voltage Vled in which the target current If flows and the loss of each FET element is suppressed.
The lighting process also serves to detect an abnormality in the LED element. The process of abnormality detection will be described later.

The forward voltage of the LED element changes due to the aged deterioration of the LED element but does not change in a short time. Therefore, it is not necessary to obtain a suitable value of the output voltage Vled as described above each time the LED element is lighted, and once the optimum value of Vled is determined, it may be stored in the memory. Then, at the next lighting time, the output voltage Vled stored in the memory may be applied and lighted.
However, when the setting of the target current If changes, it is preferable to execute the above-described process again to determine a suitable value of the output voltage Vled. Furthermore, since it also serves to detect an abnormality in the LED element, a suitable value of the output voltage Vled may be determined as described above each time the LED element is turned on.

<< Phenomenon that occurs when an abnormality occurs in the LED element >>
Here, in the LED drive circuit shown in FIG. 3, a phenomenon will be described in the case where the LED element D 0203 is abnormal and does not emit light. When an abnormality occurs in the LED element for some reason, the LED element may be in a short state or an open state as a result.
If a failure occurs in a certain LED element and a short circuit occurs, the LED element does not emit light, but current flows. Therefore, the other LED elements connected in series to the LED element in which the abnormality has occurred continue to emit light. The forward voltage is zero for the LED element in which the abnormality has occurred. However, even if the forward voltage of the DC LED element group including the LED element in the short state decreases by the forward voltage of the LED element in the short state, the current limiting circuit keeps the current If constant. Since the forward voltage of one LED element is about 3 V, the possibility of adverse effects on other LED elements and other series LED arrays is small.
What is particularly problematic in the configuration of this embodiment is the case where the LED element is in the open state. In that case, the adverse effect may spread to other series LED arrays.

FIG. 7 is an explanatory view showing an equivalent circuit of each LED element shown in FIG. As a fact found by the inventor, most of the commercially available LED elements have a circuit configuration in which a light emitting diode portion as a light emitting portion and a Zener diode ZD are connected in antiparallel. The Zener voltage Vzd of the Zener diode ZD is 13 V in one example.
The equivalent circuit of FIG. 7 means that, when an abnormality occurs in the LED element and the light emitting diode part (LED shown in FIG. 7) is opened, a voltage higher than the Zener voltage Vzd is applied to the LED element. And current can flow through the zener diode ZD.

  Consider the case where the light emitting diode part of the LED element D 0203 is in the open state in FIG. In that case, with the output voltage Vled applied at the normal time, the current If2 flowing at the normal time does not flow. That is, no current flows in the current detection resistor Rc02, and as a result, the voltage drop Vr2 of the voltage detection resistor Rc02 becomes zero. The control circuit MC detects it. That is, the current control circuit DRV2 detects that If2 suddenly decreases or becomes zero. Correspondingly, the FET element Qc02 is controlled to make the source-drain voltage smaller than that in the normal state. However, the difference 10V between the normal voltage 3V and the Zener voltage 13V can not be extracted only by adjusting the source-drain voltage of the FET element Qc02. Therefore, the control circuit MC determines that the output voltage Vled of the DC / DC converter DDC is too low, and raises Vled. When Vled rises to the extent of the difference between the above-described Zener voltage Vzd and the normal voltage in the normal state, the anti-parallel Zener diode ZD of the LED element D 0203 becomes conductive. Then, the target current If2 flows through the series LED array U2 again. The output voltage Vled in that case is the sum of the forward voltage of the normal nine LED elements of the series LED element group LEDs 2 and the zener voltage Vzd.

In the numerical example appended to the series LED array U2 of FIG. 3, when the control circuit MC raises the output voltage Vled to 38.1 V, the target current If2 flows.
That is, the forward voltage of the series LED element group Leds2 is 3.0 V for the normal voltage of the LED element Vf2 = 2.7 × 9 + Vzd = 2.7 × 9 + 13 = 37.3 (V)
It is.
The source-drain voltage Vds2 of the FET element Qc02 is 0. Since the voltage drop Vr2 of the current detection resistor Rc02 is 0.3 V, the output voltage Vled when the target current value If2 flows at the time of abnormality is
Vled = Vf 2 + Vds 2 + Vr 2 = 37.7 (V)
And will be as described above.

If the output voltage Vled at the time of abnormality is extremely high compared to that at the normal time, the upper limit value of Vled may be determined in advance. However, since there is variation in the forward voltage of each LED element and that there is a range in setting of the target current If, it may not be possible to set a fixed upper limit value.
For example, in the above numerical example, the output voltage Vled should be 31.4 V in order to flow the target current If1 to the series LED array U1 at normal times. On the other hand, when the LED element D 0203 of the series LED array U 2 is abnormal, the output voltage Vled for flowing the target If 2 is 37.7 V. For example, it can not be considered to set 35 V, which is a voltage near the middle between the two, to the upper limit value of the output voltage Vled. However, if the output voltage Vled should be about 38 V when the target current is large even in the normal state, if the upper limit value is set to 35 V, erroneous detection occurs.

  On the other hand, if it is not possible to detect an abnormality of the LED element in the open state, the output voltage Vled higher by the amount of the Zener voltage Vzd of the antiparallel Zener diode ZD is maintained. Although the series LED array including the abnormal LED element causes the other LED elements to emit light, an excess output voltage is applied to the normal series LED array connected in parallel with that by the increase of the Zener voltage Vzd. Ru. The excess output voltage will be absorbed by the FET element Qc.

For example, it is assumed that the output voltage Vled = 37.7 (V) at the time of abnormality is applied to the series LED array U1. In order to make it easy to understand, it is assumed that the forward voltage of the series LED element group LEDs 1 is the same as that in the normal state. In that case, the FET element Qc01 must carry a voltage of 6.3 V obtained by subtracting the sum of the forward voltage Vf1 = 31 V of the series LED element group LEDs 01 and the current detection resistor Vr1 = 0.3 V from the above voltage as the source-drain voltage Vds1. You must. Similarly, for a series LED element group LEDsn whose forward voltage drop is smaller than that of the LEDs 01, the FET element Qcn has to carry a voltage of 10.3 (V) as the source-drain voltage Vdsn.
As described above, when an abnormality occurs in the LED elements of any of the series LED arrays and the open state is established, the output voltage Vled rises and the loss of the FET elements of the parallel series LED arrays becomes large. If the FET device can not withstand the voltage, a secondary failure or failure will occur, such as failure or burnout.
Therefore, as described later, the abnormality of the LED element is detected in the lighting process to prevent the occurrence of the secondary abnormality, that is, the failure of the FET element.

<< Details of the configuration of LED array and drive circuit >>
The process of the LED drive circuit of the backlight according to this embodiment reaching the configuration shown in FIGS. 3 and 6 will be described with reference to FIGS. 8A to 8C and FIG. 9 until the process of abnormality detection is described.
FIG. 8A is an explanatory view showing the configuration of a simple LED drive circuit having only one series LED element group LEDs. FIG. 1 is composed of one voltage output circuit DCV, one series LED array LEDs, one current control circuit DRV, one current detection circuit CS, and one control circuit MC.

In the case of a backlight of a small liquid crystal panel, the configuration shown in FIG. 8A is sufficient. However, in order to cope with the increase in size of the liquid crystal panel or the increase in brightness when the liquid crystal panel is installed outdoors, it is necessary to increase the number of LEDs. However, the number of LED elements that can be connected in series is limited. If the number is increased needlessly, a high voltage drive circuit corresponding to the forward voltage of a large number of LED elements is required, which is unreasonable in terms of safety and cost.
FIG. 8B shows a configuration in which the number of LEDs is increased by arranging a plurality of the circuits shown in FIG. 8A in parallel. However, in the configuration of FIG. 8B, the voltage output circuits DCV1, DCV2,... Are redundant configurations, which is disadvantageous in cost. Therefore, as shown in FIG. 8C, the idea of using a common voltage output circuit is created.

FIG. 9 is a configuration that embodies it, and corresponds to FIG. In the LED drive circuit shown in FIG. 9, a common voltage Vled is applied from the voltage output circuit DCV to the respective series LED arrays connected in parallel.
In the configuration of FIG. 9, as described with reference to FIG. 3, the FET elements Qc01 to Qcn corresponding to the current control circuits DRV1 to DRVn play a role of absorbing the variation of the forward voltage of the LED elements.

«Abnormal detection at the time of lighting»
The lighting process of the LED element which also serves as abnormality detection in this embodiment will be described.
FIG. 10 and FIG. 11 are graphs showing waveforms of lighting processing which also serves as detection of abnormality of the LED element in the LED drive circuit according to this embodiment. FIG. 10 shows the waveforms when all the LED elements are normal. On the other hand, FIG. 11 shows a waveform in the case where some of the LED elements are abnormal.

In FIGS. 10 and 11, the horizontal axis is the passage of time t. The vertical axis of the upper graph is the output voltage Vled of the voltage output circuit DCV. The vertical axis of the lower graph is the current If flowing through each series LED array.
As shown in FIGS. 10 and 11, when lighting the LED element, the CPU 31 of the control circuit MC raises the output voltage Vled from a low voltage to a high voltage gradually with the passage of time. The current control circuit DRV of each series LED array controls to flow the target current If, but can not flow current while the output voltage Vled is too low to meet the forward voltage Vf of the series LED element group. When the output voltage Vled gradually rises and reaches a level equal to the forward voltage Vf, forward current starts to flow in the series LED element group. When the voltage further rises, the forward current flowing through the series LED element group increases to reach the target current If. Furthermore, even if the output voltage Vled rises, the current control circuit DRV keeps the forward current at the target current If.

Since the forward voltage of each LED element has variation, the magnitude of the output voltage Vled at which the forward current reaches If differs depending on each series LED array.
Three current curves Ui, Uj and Uk shown in FIG. 10 indicate the rising of the current of the LED drive circuit composed of three series LED arrays Ui, Uj and Uk connected in parallel.
As shown in FIG. 10, among the three series LED arrays, the series LED array Ui corresponding to the waveform of Ui has the smallest forward voltage of the series LED element group, and hence the current starts to flow at the smallest output voltage Vled. The series LED array Uk corresponding to the waveform of Uk has the largest forward voltage of the series LED element group, and therefore the current starts to flow at the largest output voltage Vled.
In FIG. 10, the control circuit MC raises the output voltage Vled with time from the minimum voltage until the target current If flows in any of the three series LED arrays Ui, Uj and Uk. Keep rising.
After confirming that the target current If flows in the LED array Uk, the control circuit MC keeps the output voltage Vled of the subsequent voltage output circuit DRV at a constant voltage.

Subsequently, FIG. 11 will be described.
FIG. 11 shows the rising of the current of the LED drive circuit composed of five serial LED arrays U1, U2, Ui, Uj and Uk. Among them, Ui, Uj and Uk have the same characteristics as FIG. On the other hand, U1 and U2 assume that there is an abnormality in the LED element.
The series LED array U2 assumes that some of the LED elements included in the corresponding series LED element group LEDs 2 are in a shorted state. Therefore, the forward voltage Vf2 of the series LED element group is small, and the LED elements light up with the output voltage Vled smaller than the normal range in consideration of the variation.
However, the abnormality in which the LED element is in the short state will be described again in the second embodiment, and the abnormality detection related to U1 will be described. Since the abnormality detection related to U1 may cause a secondary failure, it is an essential abnormality detection item.

With regard to the series LED array U1, any one of the LED elements of the series LED element group LEDs 1 is in an open state due to an abnormality. The current If does not flow to the LED element in which the abnormality has occurred unless the Zener voltage Vzd of the anti-parallel Zener diode ZD is applied instead of the normal voltage in the normal state. Therefore, the current If does not flow in the LED element until the output voltage Vled larger than the normal range in consideration of the variation of the forward voltage of the LED element is applied.
As a method of detecting an abnormality, a normal range in which the variation of the forward voltage of the series LED element group is considered is determined in advance, and stored in the memory 32 of the control circuit MC. An output voltage Vled exceeding the normal range is applied to determine whether there is a series LED array in which current starts to flow. When such a series LED array is detected, the control circuit MC determines that any LED of the series LED array is in the open state and an abnormality has occurred. Then, the output voltage Vled is lowered to stop the driving of the LED element.

When the output voltage Vled rises during light emission of the LED element, the magnitude of the target current If varies depending on the setting and the use environment. Therefore, it is difficult to fixedly determine in advance the threshold value for dividing the normal range and the abnormal range of the output voltage.
According to this embodiment, the threshold value for dividing the normal range and the abnormal range is determined for the current at which the LED element starts lighting. That is, the threshold is determined based on the voltage at which current starts to flow in the LED element. In this way, the abnormality of the LED element can be determined more accurately.

«Processing procedure»
FIG. 12 is a flowchart showing an abnormality detection process performed by the CPU 31 of the control circuit MC in the lighting process of the LED element in this embodiment.
As shown in FIG. 12, when lighting the LED element, the CPU 31 controls the output voltage Vled of the voltage output circuit DCV through the voltage adjustment unit 36 to gradually increase from low voltage to high voltage with the passage of time. (Step S11).

  While raising the output voltage Vled, the CPU 31 monitors the currents If1 to Ifn of the series LED arrays U1 to Un detected by the current control circuits CS1 to CSn via the current detection unit 34 (step S13).

  Monitoring of the currents If1 to Ifn is continued while raising the output voltage Vled (No loop of step S15), and when the current If flows through any of the series LED arrays and the LED element emits light (Yes in step S15) And the value of the output voltage Vled at that time is acquired and stored in the memory 32 (step S17). Then, it is determined whether the acquired output voltage Vled is within the range of a predetermined normal value (step S19). If the acquired Vled is within the normal range (Yes in step S19), the CPU 31 checks whether current flows through all the series LED arrays, that is, whether or not there is a series LED array in which the LED elements have not yet emitted light. It determines (step S21). When the current If flows through all the series LEDs (Yes in step S21), the process of abnormality detection is ended. That is, it is determined that all the series LED arrays U1 to Un have been lit normally.

On the other hand, if there is a serial LED array in which the LED elements are not emitting light yet in step S21 (No in step S21), the routine returns to step S11. Then, the CPU 31 continues detecting the current while raising the output voltage Vled until the current flows through the series LED array in which the current does not flow yet and the LED element emits light.
When the acquired Vled is not in the normal range in the step S19 (No in the step S19), the CPU 31 determines that the LED element of the serial LED array is abnormal. Then, the output voltage Vled is lowered to stop the energization, and an overvoltage is applied to the FET elements Qc01 to Qcn of the normal series LED array so that a secondary failure does not occur.

The above is the procedure of abnormality detection at the time of lighting.
When an abnormality is detected, the CPU 31 detects it by means such as a display lamp (not shown) of the liquid crystal display device 10, a message display on the liquid crystal panel 23, or communication with an external device via a communication interface (not shown). It is preferable to notify the user or the management device of the detected abnormality.

Second Embodiment
In FIG. 11, when the output voltage Vled, at which the current If flows in the series LED array and lights up, exceeds the normal range, it is determined to be abnormal. This corresponds to the detection of “abnormal range 1” shown in FIG.
If the current If flows in the series LED array in the range where the output voltage Vled falls below the normal range, it may be determined as abnormal. This corresponds to the detection of an abnormality of the series LED array indicated by U2 in FIG. 11, that is, “abnormal range 2”.
This phenomenon corresponds to the case where the forward voltage of any of the LED elements is abnormally small, that is, when the light emitting diode part or the ZD part of the LED element is in a short state. If only one LED element in the series LED element group is shorted, it may be difficult to detect with this method because it falls within the range of the element variation of the forward voltage. However, when a large number of LED elements are shorted, an abnormality can be detected by this method.
The reason why the LED element does not set the short-circuited abnormality as an essential abnormality detection item is because it does not induce a secondary failure of another series LED array. In the case where the LED element is in the short state, there may be an aspect in which the abnormality detection is omitted.

Third Embodiment
As described in the first embodiment, in addition to detecting an abnormality at the time of lighting processing of the LED element, the occurrence of the abnormality may be detected during lighting of the LED element.
As shown in FIG. 13, it is assumed that an abnormality occurs in any of the LED elements at time tb while the LED elements are on, and the light emitting diode part is in the open state. Then, the control circuit MC raises the output voltage Vled of the voltage output circuit DCV in order to flow the target current value If through the series LED array including the abnormal LED element. The process of raising the output voltage Vled is executed by the CPU 31 of the control circuit MC.

Therefore, even if the target current value does not change while the LED element is on, the CPU 31 rapidly decreases the current of any series LED array, and as a result, the output voltage Vled has to be increased. If the error occurs, it may be determined as abnormal and the same process as step S23 of FIG. 12 may be performed.
Here, the determination as to whether or not there is an abnormality is based on the fact that the current detected during lighting of the LED element has a rapid change which decreases within a predetermined period beyond a predetermined threshold. It is also good. Alternatively, as a result of raising the output voltage Vled based on the abrupt phenomenon of current, based on the fact that the output voltage Vled has risen sharply beyond a predetermined threshold within a predetermined period. May be
Since the abnormality detection according to this embodiment is repeatedly performed while the LED element is lit, a processing load is applied to the CPU 31, but there is an advantage that the abnormality can be detected promptly if an abnormality occurs in the LED element.

Embodiment 4
In general, the LED element has a temperature characteristic in which the forward voltage drop Vf decreases as the temperature rises.
In this embodiment, a method of abnormality detection in consideration of the temperature characteristic of the above-mentioned LED element will be described.
The above-mentioned temperature characteristics slightly differ depending on the constituent material of the LED, but in one example, the temperature characteristics of the blue LED which is the base of the white backlight LED is −2 to −2.5 mV / ° C. That is, when the temperature of the element increases by 1 ° C., the magnitude of the forward voltage drop Vf decreases by 2 to 2.5 mV. For example, when the forward voltage drop Vf of the LED element at 20 ° C. is 3.1 V and the temperature characteristic is −2.5 mV / ° C., the forward voltage drop V f at 60 ° C. is 3.0 mV smaller than the Vf at 20 ° C. For example, the forward voltage drop of Vfn of the whole series LED element group becomes Vf1 = 30V.

FIG. 14 is a graph showing the temperature characteristics of the forward voltage drop Vf of the LED element in this embodiment.
FIG. 15 is a block diagram showing the configuration of the LED drive circuit in this embodiment. This corresponds to FIG. 2 described in the first embodiment, and is different from FIG. 2 in that the control circuit MC further includes a temperature sensor 37. The temperature sensor 37 detects the temperature of the LED element. As a specific hardware, for example, a thermistor can be applied. The temperature sensor 37 may be configured by one sensor, but may be configured by a plurality of sensors. For example, a sensor may be provided for each series LED array, and the judgment value of the normal range may be corrected for each series LED array based on the temperature detected by the sensor. Alternatively, each series LED array may have multiple sensors, and the correction may be made based on the average temperature of the sensors associated with the same series LED array.

FIG. 16 is a graph showing an example of the difference between the normal range when the element temperature is different between the previous abnormality determination and the current abnormality determination in this embodiment. The graph on the left half of FIG. 16 corresponds to FIG. 11, and shows the Vled normal range when the temperature detected by the temperature sensor 37 is Ta1 = 25.degree. The graph in the right half of FIG. 16 again corresponds to FIG. 11, but shows the case where the detected temperature is Ta2 = 45.degree.
When the threshold value of the normal range of Vled is compared between the graph on the left side and the graph on the right side of FIG. 16, it is understood that the threshold value of the graph on the right side is smaller than that of the graph on the left side. In accordance with the temperature characteristic of the forward voltage drop Vf of the LED element, the threshold value in the normal range is reduced as the temperature rises, and the abnormality determination is performed.

FIG. 17 is a flowchart corresponding to FIG. 12 and showing the process of abnormality detection in this embodiment.
As shown in FIG. 17, when performing processing for abnormality detection, the CPU 31 reads the temperature of the LED element detected by the temperature sensor 37 (step S31). Then, according to the read temperature, a shift amount of a threshold value for determining Vled as a normal value range is calculated (step S32). That is, the threshold is corrected by the difference between the reference temperature and the detected temperature with respect to the threshold set for the reference temperature, for example, 25 ° C. (step S33). The correction amount is calculated using a predetermined temperature characteristic.
Steps S41 to S53 correspond to S11 to S23 in FIG. 12, and show the same processes as those in FIG.

  As described above, by taking the temperature of the LED element into consideration, for example, when the abnormality is not detected for a long time and the temperature is near room temperature (corresponding to the left half in FIG. 16), the LED element emits light In the abnormality detection (corresponding to the right half of FIG. 16) in the case where the power of the LED drive circuit was turned on again (corresponding to the right half of FIG. 16) Can be realized.

As mentioned above,
(I) The LED drive circuit according to the present invention includes a plurality of LED elements connected in series, a current control circuit for causing a predetermined current to flow through the LED elements, and a series LED including a current detection circuit for detecting a current flowing through the LED elements. And a control circuit for controlling a voltage output from the voltage output circuit, the control circuit including: an array; a plurality of voltage output circuits for applying adjustable voltages to the plurality of series LED arrays connected in parallel; When starting lighting of the series LED array, the output voltage of the voltage output circuit is gradually increased and the current detection circuit detects the start of energization, and the applied voltage at the start of energization is not within a predetermined range. It is characterized in that the serial LED array is determined to be abnormal.

In the present invention, the voltage output circuit is a DC power supply circuit capable of changing the output voltage. As a specific aspect, for example, it is realized by a DC-DC converter circuit. However, other configurations may be employed.
The control circuit may be realized, for example, by a circuit including a memory and an input / output interface circuit centering on a microcomputer, but is not limited thereto and may be realized only by a hardware circuit that does not use a microcomputer.

  The suppression of the output voltage of the voltage output circuit within a range sufficient to flow a predetermined current when lighting the series LED array is described as follows. Since the LED element causes a voltage drop in the forward direction when current flows for light emission, a predetermined current can not flow when the voltage applied to the series LED array is too low. In addition, the magnitude of the voltage drop in the forward direction varies among the LED elements. Therefore, there is variation in each series LED array in which a plurality of LED elements are connected in series. The voltage output circuit needs to apply a voltage sufficient to flow a predetermined voltage to the series LED array with the largest sum of forward voltage drops.

  In other series LED arrays connected in parallel, the excess voltage is borne by the current control circuit and absorbed as a loss. If a large output voltage is applied, a predetermined current can be reliably supplied to each series LED array, but as the excess voltage exceeding the necessary and sufficient voltage is applied, the loss absorbed by the current control circuit increases and the current control The burden on the circuit increases. An excessively large voltage may cause a failure of the current control circuit. Therefore, the control circuit can supply necessary and sufficient voltages to the voltage output circuit so as to balance the contradictory conditions of flowing the target current to each series LED array and suppressing the load (loss) of the current control circuit. Make it output.

Furthermore, preferred embodiments of the present invention will be described.
(Ii) Each LED element includes a light emitting diode and a protective zener diode connected in parallel with the light emitting diode in the reverse direction, and the voltage output circuit is configured to turn on a predetermined current when lighting each series LED array Control may be performed to suppress the output voltage of the voltage output circuit within a range sufficient to flow, and the current control circuit may control the current by an operation including the active region of the FET element based on the detection of the current detection circuit. .

  If the LED element includes a zener diode connected in reverse parallel, even if the light emitting diode is abnormal and does not conduct, a current flows in the zener diode when a voltage exceeding the zener voltage is applied to the LED element. When there is an LED element that has fallen into such a state, the abnormality in the series LED array can be detected by gradually increasing the voltage applied to the series LED array at the time of lighting and detecting the applied voltage at the start of energization.

As a result of the current control circuit trying to flow a predetermined current, the output voltage of the voltage output circuit is adjusted, and a voltage exceeding the Zener voltage is applied to the failed LED element. As a result, excessive voltage is applied to the normal series LED array connected in parallel with the abnormal series LED array.
By performing the above-mentioned abnormality detection, a larger voltage is applied to the normal serial LED arrays connected in parallel than in the normal state, and the loss of the current control circuit of the normal serial LED array increases to cause a secondary failure Can be avoided.

(Iii) The control circuit may control the current control circuit so that a predetermined current may flow to any series LED array.
In this way, the control circuit can detect the current of each series LED array with the current detection circuit, and control the current control circuit and control the voltage output circuit based on the detection. Therefore, at least the current detection circuit is made common as compared with the configuration in which control of the current control circuit and control of the voltage control circuit are performed by another circuit, and a simple circuit configuration is realized.

(Iv) The control circuit monitors the current detected by the current detection circuit while lighting the LED elements of each series LED array, and a change in magnitude of the detected current exceeds a predetermined range If it does, the serial LED array may be determined to be abnormal.
In this way, it is possible to monitor the change in current not only when the series LED array is lit but also during lighting to detect an abnormality in the LED element.

(V) The series LED array may be used as a backlight of a liquid crystal display.
Preferred embodiments of the present invention also include combinations of any of the above-described plurality of embodiments.
Besides the embodiment described above, various modifications may be made to the present invention. Those variations are not to be construed as not falling within the scope of the present invention. The scope of the present invention should include the meanings equivalent to the scope of claims and all the modifications within the scope.

10: liquid crystal display device, 11: LED drive circuit, 13: display control circuit, 15: backlight control circuit, 17: backlight abnormality detection circuit, 19: backlight device, 21: liquid crystal control circuit, 23: liquid crystal panel 31 : CPU, 32: Memory, 33: Timer, 34: Current detection unit, 35: Current control unit, 36: Voltage adjustment unit, 37: Temperature sensors AMP1, AMP2: Error amplifiers CS, CS1, CS2 to CSn: Current detection circuit D0101 to Dn10: LED elements DCV, DCV1, DCV2: voltage output circuit DDC: DC / DC converters DRV, DRV1, DRV2 to DRVn: current control circuits LED: light emitting diodes LEDs, LEDs 1, LEDs2 to LEDsn: series LED element group MC, MC1, MC2: control circuits Qc, Qc01, Qc0 2 to Qcn, Qv: FET elements U1 and U2 to Un: series LED arrays R1 and R2: resistors Rc, Rc01, Rc02 to Rcn: current detection resistor ZD: Zener diode

Claims (5)

A series LED array including a plurality of LED elements connected in series, a current control circuit for supplying a predetermined current to the LED elements, and a current detection circuit for detecting a current flowing to the LED elements;
A voltage output circuit for applying a high and low adjustable voltage to the plurality of series connected LED arrays connected in parallel;
A control circuit that controls a voltage output from the voltage output circuit;
The control circuit gradually increases the output voltage of the voltage output circuit when starting lighting of each series LED array, detects the start of energization by the current detection circuit, and the applied voltage at the start of energization is predetermined. An LED drive circuit that determines that the series LED array is abnormal if it is out of range. Each LED element includes a light emitting diode and a protective zener diode connected in parallel in the reverse direction to the light emitting diode;
The voltage output circuit is controlled to suppress the output voltage of the voltage output circuit within a range sufficient to flow a predetermined current when lighting each series LED array,
The LED drive circuit according to claim 1, wherein the current control circuit controls the current by an operation including an active region of the FET element based on the detection of the current detection circuit.   3. The LED drive circuit according to claim 1, wherein the control circuit controls the current control circuit to cause a predetermined current to flow to any series LED array. 4.   The control circuit monitors the current detected by the current detection circuit while lighting the LED elements of each series LED array, and the magnitude of the detected current changes beyond a predetermined range. The LED drive circuit according to any one of claims 1 to 3, wherein the series LED array is determined to be abnormal.   The LED drive circuit according to any one of claims 1 to 4, wherein the series LED array is used as a backlight of a liquid crystal display.