JP2012195251A - Led drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED drive circuit that eliminates variations in the quantity of light caused among LED strings by a forward voltage drop Vf and implements efficient drive in driving a plurality of LED strings.SOLUTION: The LED drive circuit for driving a plurality of LED strings simultaneously includes: a half bridge circuit for switching input DC power positively and negatively via switching elements under pulse width modulation control to generate AC power; a control IC for controlling the pulse width modulation of switching in the half bridge circuit; and as many transformers as the plurality of LED strings. The AC power output from the half bridge circuit is supplied to series-connected primary sides of the plurality of transformers, and the LED strings are connected to secondary sides of the transformers via rectifying/smoothing circuits.

Description

  The present invention relates to an LED drive circuit for simultaneously driving a plurality of LED strings (a plurality of LEDs connected in series) used for a backlight of a liquid crystal panel.

  In recent years, a light emitting diode (hereinafter referred to as an LED (Light Emitting Diode)) has been used for a backlight of a liquid crystal panel. When an LED is used for a backlight of a liquid crystal panel, a plurality of LEDs connected in series (hereinafter referred to as an LED string) is set as one set, and a plurality of LED strings 11 are arranged so that light is evenly applied to the liquid crystal panel. More and more are configured.

As a drive circuit for driving the plurality of LED strings 11, for example, a circuit as shown in FIG. The drive circuit shown in FIG. 2 has a plurality of LED strings (in the example of FIG. 2, eight LED strings 11a to 11a) that are converted into direct current by the AC / DC converter 12 and boosted by the boost converter 13. 11h) to supply power.
Another example of a driving circuit for driving a plurality of LED strings 11 is shown in FIG. In the drive circuit shown in FIG. 3, only one LED string 11 is driven by one boost converter 13. In the example of FIG. 3, eight boost converters 13a to 13h are used and eight LEDs are used. Each of the strings 11a to 11h is driven.

As such conventional LED driving circuits, for example, those shown in Patent Document 1 and Patent Document 2 have already been proposed. The LED drive circuit described in Patent Document 1 includes a step-up DC / DC converter, a voltage detection circuit, and a PWM control circuit, and controls the PWM signal so that the feedback voltage detected by the voltage detection circuit becomes a predetermined voltage. Thus, constant voltage control is realized.
Patent Document 2 discloses a PWM control unit that controls on / off of a switch with a switching pulse having a duty ratio determined in accordance with a preset internal reference voltage and a detection voltage detected by a voltage detection resistor. Thus, there is disclosed a backlight LED drive circuit capable of controlling the LED drive current to a constant constant current.
JP 2009-104848 A JP 2007-13183 A

  Here, there is a forward voltage drop Vf in the LED, and even if it is a product of the same standard, it is an element that varies depending on the LED, so that the same brightness can be obtained even if the same current is passed. There is a characteristic that is not necessarily. This is the same when the LED strings are compared with each other, and the Vf of the entire LED string also varies.

  Here, the conventional driving circuit shown in FIG. 2 is a system in which a plurality of LED strings 11a to 11h connected in parallel by one boost converter 13 are driven by FETs 14a to 14h connected in series to each other. In this case, since the output voltage of the boost converter 13 is adjusted to the LED string 11a having the highest total Vf, the LED string 11h having a low Vf needs to carry the voltage to the driving FET 14h, and the loss of the FET 14h is reduced. It became larger and caused a decrease in efficiency. Further, the light intensity of the LED string 11h having a low Vf has decreased.

  In addition, since the conventional drive circuit shown in FIG. 3 has a configuration in which one boost converter 13 is provided for one LED string 11, the problem of efficiency reduction in the drive circuit of FIG. The problem of variations in the amount of light generated between the strings 11 is not improved, and the boost converters 13 are required as many as the number of LED strings 11, which increases the cost.

  The problem of variation in the amount of light generated between the LED strings 11 in these conventional drive circuits cannot be solved even by using the drive circuits shown in Patent Document 1 and Patent Document 2.

  The present invention has been made in view of the above problems, and when driving a plurality of LED strings, variation in the amount of light generated between the LED strings due to the forward voltage drop Vf is eliminated, and the drive efficiency is good. An object of the present invention is to provide an LED driving circuit.

  Claim 1 of the present invention is an LED driving circuit for simultaneously driving a plurality of LED strings, and switching the switching element to the positive side and the negative side by pulse width modulation control of the input DC power to generate AC power. A half bridge circuit to be generated, a control IC for controlling pulse width modulation of switching in the half bridge circuit, and the same number of transformers as the plurality of LED strings are provided, and the primary sides of the plurality of transformers are connected in series. The LED drive is characterized in that the AC power of the output from the half-bridge circuit is supplied to the connection to the LED string, and the LED string is connected to the secondary side of each transformer through a rectifying and smoothing circuit. Circuit.

  According to a second aspect of the present invention, in addition to the first aspect, a means for detecting a current value and a voltage value is provided in a preceding stage of the LED string, and the detected current value and voltage value are fed back to the control IC. The LED drive circuit is characterized in that the pulse width modulation is controlled so that the output current and the output voltage are constant.

  According to a third aspect of the present invention, in addition to the first or second aspect, a pulse transformer is provided between the half bridge circuit and the control IC, and switching elements in the half bridge circuit are switched via the pulse transformer. This is an LED driving circuit.

  According to the first aspect of the present invention, the AC power of the output from the half bridge circuit is supplied to the primary side of the plurality of transformers connected in series, and the rectifying / smoothing circuit is provided on the secondary side of each transformer. Since the LED string is connected via the primary side, the primary side is connected in series, so that power is evenly supplied to a plurality of transformers, and thereby the power supply to the secondary side is also equal. In addition, due to the characteristics of the transformer (transformer), the secondary side functions so as to have a constant power, so that the current flowing on the secondary side varies according to the value of the forward voltage drop Vf of the LED string, In other words, the drive current increases when the voltage Vf is low, and the drive current decreases when the voltage Vf is high. Thereby, even if there is a variation in the Vf values of the plurality of LED strings, the light amounts of all the LED strings are made uniform and stabilized by the constant power control.

  According to the second aspect of the present invention, means for detecting the current value and voltage value is provided in the previous stage of the LED string, and the detected current value and voltage value are fed back to the control IC to output current and output voltage. Since the pulse width modulation is controlled so that the output voltage is constant, it is possible to control the output power to be constant even if the input voltage fluctuates. Can be controlled.

  According to the invention of claim 3, since the pulse transformer is provided between the half bridge circuit and the control IC, and the switching element in the half bridge circuit is switched via the pulse transformer, the control IC and the half bridge circuit are provided. Since a pulse transformer is provided and insulated from the control IC, there is no fear that a high voltage is applied to the control IC. Each LED string is connected to the secondary side of the transformer, and is insulated from the primary side to which a high voltage is applied, so that the LED string can be driven safely.

It is a block diagram showing the structure of the LED drive circuit by this invention. It is a block diagram showing the structure of the conventional LED drive circuit. It is a block diagram showing the structure of the conventional LED drive circuit.

  The LED drive circuit according to the present invention is an LED drive circuit for simultaneously driving a plurality of LED strings, and switches the switching element between the positive side and the negative side by pulse width modulation control of the input DC power to generate AC power. A half bridge circuit to be generated, a control IC for controlling pulse width modulation of switching in the half bridge circuit, and the same number of transformers as the plurality of LED strings are provided, and the primary sides of the plurality of transformers are connected in series. The AC power of the output from the half bridge circuit is supplied to the connection to the LED, and the LED string is connected to the secondary side of each transformer via a rectifying and smoothing circuit. is there. Details will be described below.

  An LED driving circuit according to the present invention will be described with reference to FIG. In FIG. 1, 15 is a DC input terminal, and 16 is a GND terminal. A direct current power source is connected between the DC input terminal 15 and the GND terminal 16. As the DC power source in this case, for example, as shown in FIG. 2 or FIG. 3 shown as a conventional circuit, an AC power source converted into DC by an AC / DC converter and boosted by a boost converter is used. This DC power supply is connected to the half bridge circuit 17.

The half bridge circuit 17 is formed by connecting two switching elements in a half bridge type, and the half bridge circuit 17 is connected to pulse transformers 18 and 19. By switching the two switching elements by 19, a DC input is output in the form of pulses respectively on the positive side and the negative side, and the pulse width is controlled by PWM control, and this is smoothed, thereby alternating current Generate power. This PWM control is performed by a control IC 20 connected to the pulse transformers 18 and 19.
The half bridge circuit 17 is an example, and may be a full bridge circuit, which can be changed as appropriate. Further, since the half bridge circuit 17 is configured to convert direct current into alternating current, it may be any inverter circuit that can control the final output by PWM control or the like, and widely includes such an inverter circuit. It shall be.

  The AC power generated in the half bridge circuit 17 is supplied to the primary side of the transformers T1 to T8. All of these eight transformers are of the same standard and have the same turn ratio, and AC power is supplied to the primary windings of these transformers T1 to T8 connected in series. As a result, the currents flowing in all the transformers are the same, and the voltages generated in the primary windings of the transformers are the same because they are the same standard. That is, the same power is supplied to all the transformers. In the present embodiment shown in FIG. 1, the number of transformers is set to eight in the case of an LED drive circuit that drives the eight LED strings 11a to 11h at the same time in accordance with the number of these eight LED strings. This is because the number of transformers is set. Of course, this is only an example, and the number of LED strings that are driven simultaneously can be appropriately changed in design, and the number of transformers can be changed accordingly. In this case, the value of the input DC voltage Will be changed accordingly. Further, the turns ratio of the transformer is appropriately determined depending on the amount of voltage applied to one transformer on the primary side, the amount of current desired on the secondary side, the amount of voltage, and the like.

The secondary side of each of the transformers T1 to T8 is connected to a rectifying / smoothing circuit 21 including two rectifier diodes, an inductor and a capacitor. To do.
Further, a current value and a voltage value are detected at the subsequent stage of the rectifying / smoothing circuit 21 using the current / voltage detecting means 22 and fed back to the control IC 20. The control IC 20 controls the pulse widths of the pulse transformers 18 and 19 based on the current value and voltage value fed back from the current / voltage detection means 22 so that the output becomes constant.

  The operation of the present invention in such a configuration will be described. As shown in FIG. 1, since the primary sides of the transformers T1 to T8 are connected in series, power is evenly supplied to each of the transformers T1 to T8, so that the power supply to the secondary side is also equal. It will be a thing. Here, due to the characteristics of the transformer (transformer), the secondary side functions so as to have a constant power, so that the current flowing to the secondary side varies according to the value of the forward voltage drop Vf of the LED string 11. It will be. In other words, the drive current increases when the voltage Vf is low, and the drive current decreases when the voltage Vf is high. As a result, even if there are variations in the Vf values of the eight LED strings 11a to 11h, the light amounts of all the LED strings are made uniform and stable by the constant power control. Further, in the drive circuits of FIGS. 2 and 3 which are the prior art, the efficiency is lowered because there is an FET that carries a wasteful voltage, but in the present invention, the drive portion of the secondary LED string 11 is not provided. Since there is no FET, high efficiency can be achieved.

In order to drive the plurality of LED strings 11a to 11h, a high voltage of, for example, 400V is applied to the DC input terminal 15, and this is supplied to the primary side of the plurality of transformers T1 to T8. However, the LED strings 11a to 11h are connected to the secondary sides of the transformers T1 to T8, and are insulated from the primary side to which a high voltage is applied. Can be driven.
Further, since the pulse transformers 18 and 19 are provided and insulated between the control IC 20 and the half-bridge circuit 17, there is no fear that a high voltage is applied to the control IC 20.

  In the said Example, although demonstrated as a circuit for driving the eight LED strings 11a-11h, this invention is not limited to this Example. The LED drive circuit of the present invention uniformly supplies power to each transformer by connecting the primary side of the transformer in series, and constant power control is performed on the secondary side according to the energy applied to the primary side. Thus, even if Vf of each LED string 11 is different, it is controlled so as to have the same power value. Therefore, the present invention is applicable when a plurality of LED strings 11 are driven, and the number of the LED strings 11 can be appropriately set as necessary.

  In the above embodiment, the pulse transformers 18 and 19 are used as means for controlling the switching timing in the half-bridge circuit 17 while maintaining insulation. However, the present invention is not limited to this. A coupler or the like can be substituted.

DESCRIPTION OF SYMBOLS 11, 11a-11h ... LED string, 12 ... AC / DC converter, 13, 13a-13h ... Boost converter, 14a-14h ... FET, 15 ... DC input terminal, 16 ... GND terminal, 17 ... Half bridge circuit, 18 ... Pulse transformer, 19 ... pulse transformer, 20 ... control IC, 21 ... rectifying / smoothing circuit, 22 ... current voltage detecting means.

Claims (3)

  An LED driving circuit for simultaneously driving a plurality of LED strings, and a half-bridge circuit that generates AC power by switching switching elements on the positive side and the negative side of input DC power by pulse width modulation control, and this A control IC for controlling switching pulse width modulation in a half-bridge circuit, and the same number of transformers as the plurality of LED strings are provided, and the half-bridge is connected to the primary side of the plurality of transformers in series. An LED driving circuit, wherein AC power output from a circuit is supplied, and one LED string is connected to a secondary side of each transformer through a rectifying / smoothing circuit.   A means for detecting the current value and voltage value is provided in the front stage of the LED string, and the detected current value and voltage value are fed back to the control IC, and pulse width modulation is performed so that the output current and output voltage are constant. The LED driving circuit according to claim 1, wherein the LED driving circuit is controlled.   3. The LED driving circuit according to claim 1, wherein a pulse transformer is provided between the half bridge circuit and the control IC, and switching elements in the half bridge circuit are switched via the pulse transformer.

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