KR101033364B1 - LED Driving Circuit - Google Patents

Abstract

The LED drive circuit of the present invention, at least two LED strings; A DC-AC converter for converting a DC voltage into an AC voltage; A transformer for transforming the converted AC voltage; A rectifier for rectifying the voltage output from the transformer and supplying the rectified voltage to the LED strings; A driving voltage detector for detecting driving voltages of the LED strings; And a PWM controller configured to generate a PWM signal for operating the DC-AC converter according to the measured driving voltages of the LED strings.

LED drive circuit, DC-AC conversion, PWM, backlight

Description

LED driving circuit {LED Driving Circuit}

The present invention relates to an LED driving circuit for supplying driving power to LEDs (Light Emitting Diodes).

LED light source devices consisting of a plurality of strings of light emitting diodes (LEDs) are rapidly spreading for a wide range of applications for lighting applications or backlights of LDC panels.

LEDs with high brightness as a whole can be used in many applications, including backlights of liquid crystal displays (LCDs) based on monitors and televisions (collectively referred to as monitors hereinafter). In large LCD monitors, LEDs are typically supplied to one or more strings of LEDs connected in series to share a common current.

To provide backlight for LCD monitors, one of two basic techniques is commonly used. The first technique uses one or more strings of white LEDs, which typically include blue LEDs with phosphors (materials). This phosphor absorbs the blue light generated by the LED and emits white light. In the second technique, one or more individual strings of colored LEDs are placed adjacent to each other so that the light combined with each other looks like white light.

Conventional LED drive circuits for driving LED strings may include a boost converter and / or a buck converter to convert an externally applied power source into a power source having electrical characteristics suitable for driving the LED strings. Converter for pressure reduction).

However, an LED driving circuit composed of a boost converter and / or a buck converter has disadvantages in that its output capacity is not sufficient and an electrical design of an externally applied power source is changed.

The present invention seeks to provide an LED driving circuit having a sufficient output capacity and a low manufacturing cost.

In another aspect, the present invention is to provide an LED driving circuit that is easy to change the design even if the electrical characteristics of the power applied from the outside is changed.

LED driving circuit according to an embodiment of the present invention, at least two LED strings; A DC-AC converter for converting a DC voltage into an AC voltage; A transformer for transforming the converted AC voltage; A rectifier for rectifying the voltage output from the transformer and supplying the rectified voltage to the LED strings; A driving voltage detector for detecting driving voltages of the LED strings; And a PWM controller configured to generate a PWM signal for operating the DC-AC converter according to the measured driving voltages of the LED strings.

By implementing the LED drive circuit of the present invention according to the above configuration, there is an advantage that the output capacity for driving the LED string is sufficient, but the manufacturing cost is low.

Alternatively, the LED drive circuit of the present invention has an advantage that the design can be easily changed even if the electrical characteristics of the power source applied from the outside are changed.

1 shows a structure of an LED driving circuit for smoothing the driving power of LED strings in a linear driving manner.

As shown, each LED string is supplied with driving power from a common power supply 11, and the bipolar transistor 13 and the op amp 12 are fixed in the current path formed by each of the LED strings 14. Current sources 19 are connected. In the illustrated circuit, the same current is supplied to each of the LED strings by the fixed current sources 19, so that the luminance of each of the LED strings 14 can be kept uniform even when there is a slight characteristic variation between the LED strings. have.

While the LED driving circuit of the illustrated method has the advantage of accurate current control and easy implementation of additional functions such as dimming, LED strings having different forward voltage drop values are unilaterally identical. By forcing a current of magnitude to flow, heat generation loss caused by the resistive component 16 on the current path occurs and the output capacity is limited.

2 illustrates a structure of an LED driving circuit for smoothing driving power of LED strings in a switching manner.

Each of the LED strings 24 of the illustrated LED driving circuit has switching driving DC-DC converters 21, respectively. As shown in the figure, time-sharing control of the switching transistor 32 of each DC-DC converter 21 according to the current detected by the switching control IC 31 sensing the output current of each LED string, the corresponding LED string 24 Adjust the average current flowing to).

While the LED driving circuit of the illustrated method has an advantage of suppressing heat loss due to a resistive component, an accurate current control process is relatively complicated, and costs are increased.

3 shows the structure of one embodiment of an LED drive circuit for driving LED strings in a split alternating current drive scheme.

The illustrated LED driving circuit includes a first LED string 130 and a second LED string 140; A DC-AC converter 110 for converting a DC voltage into an AC voltage; A transformer 120 for transforming the converted AC voltage; A rectifier 170 rectifying the voltage output from the transformer 120 and supplying the rectified voltage to the LED strings; A driving voltage detector configured to detect driving voltages of the first and second LED strings 130 and 140; And a PWM controller 190 generating the PWM signals PWM1 to 4 for operating the DC-AC converter 110 according to the detected driving voltage of the first / second LED string.

The driving voltage detector may include two voltage divider resistors R3 and R4 as input voltage detectors for detecting the output terminal voltage of the rectifier 170. In addition, the driving voltage detector is an output voltage detector for detecting the cathode voltages of the first and second LED strings 130 and 140, and the current passing through the first LED string 130 reaches ground. A first ground resistor R1 forming a path; A second ground resistor R2 forming a path through which the current passing through the second LED string 140 reaches ground; A first transfer diode D5 transferring a voltage applied to the first ground resistor R1 to the PWM controller 190; And a second transfer diode D6 transferring the voltage applied to the second ground resistor R2 to the PWM controller 190.

In the PWM control unit 190 of the figure, in determining the duty ratio of the PWM signals PWM1 to 4, as a basic information, a voltage applied to a connection node of the divided resistors R3 and R4 is applied, and As a basic information for blocking overcurrent from flowing through the first / second LED strings 130 and 140, a voltage applied through the first / second transfer diodes D5 and D6 may be applied.

That is, the PWM controller 190 adjusts the duty ratio of the PWM signals PWM1 to 4 according to voltages applied to the connection nodes of the divided resistors R3 and R4, and the first and second transfer diodes. Activate the LED device protection function according to the voltage applied via (D5, D6).

The illustrated LED driving circuit receives a voltage of a specific node and performs a PWM control of the DC-AC converter to constantly feedback-control the power output for driving the LED string. At the same time, to prevent the overcurrent from flowing to the LED string being driven by the breakdown lamp of one LED element of the LED string, the LED element protection function (e.g., driving stop or PWM frequency) Lowering).

Voltage control, rather than current control, is easier to implement in the ICized PWM controller 190, and devices for voltage detection are less expensive than devices for current detection. Therefore, the LED driving circuit according to the present embodiment having the feedback PWM control of the method of applying the voltage of a specific node and / or the LED element protection function of the method of applying the voltage of a specific node can reduce the manufacturing cost thereof. There is an advantage.

The PWM controller 190 may be implemented as a separate IC device, and may be electrically disconnected from the LED driving circuit. Applying the PWM control unit 190 in the form of an electrically disconnected IC, there is an advantage that can be blocked in advance by the malfunction caused by the noise generated in the LED driving circuit.

The illustrated transformer 120 may be provided with a turns ratio adjusting means to adjust the turns ratio between the primary side and the secondary side during design or manufacture. For example, a plurality of lead lines are provided in the middle of the coils of the primary side and / or the secondary side, and the number of turns of the coil is adjusted according to the two lead lines selected as ports among the plurality of lead lines, and thus, the primary side and the second side. It may have a structure of a method of adjusting the winding ratio between the vehicle side.

The DC-AC converter 110 may receive a DC power supply from an external DC voltage source (Vin voltage and ground voltage provided). The external DC voltage source may vary in electrical characteristics according to its implementation type. If the transformer 120 can adjust the winding ratio between the primary side and the secondary side during manufacture, the external DC voltage source may be configured according to the electrical characteristics of the external DC voltage source. By adjusting the winding ratio of the transformer 120, it can be easily matched.

The first LED string 130 is connected to one terminal of the secondary port of the transformer 120, and the second LED string 140 is connected to the other terminal. The rectifier 170 includes a first rectifying diode D1 and a first ground diode D3 as a half-wave rectifying element for the first LED string 130, and for the second LED string 140. As the half-wave rectifying device, a second rectifying diode D2 and a second ground diode D4 may be provided.

When the current IFb flows in the clockwise direction to the secondary side of the transformer 120 by the operation of the four diodes D1 to D4, the first LED string 130 is driven to operate the transformer 120. When the current IFa flows in the counterclockwise direction on the secondary side, the second LED string 140 is driven.

Although the technical spirit of the present invention has been described in detail according to the above embodiment, it should be noted that the above embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

For example, for convenience of description, the above-described embodiment is described as an LED driving circuit for driving two LED strings, but the idea of the present invention can be easily applied to an LED driving circuit for driving three or more LED strings. In addition, of course, it belongs to the scope of the present invention.

1 is a circuit diagram showing a structure of an LED driving circuit for smoothing the driving power of LED strings in a linear driving method.

2 is a circuit diagram showing a structure of an LED driving circuit for smoothing the driving power of the LED strings in a switching manner.

3 is a circuit diagram showing an LED driving circuit according to an embodiment of the present invention.

Claims (5)

At least two LED strings; A DC-AC converter for converting a DC voltage into an AC voltage; A transformer for transforming the converted AC voltage; A rectifier for rectifying the voltage output from the transformer and supplying the rectified voltage to the LED strings; A driving voltage detector for detecting driving voltages of the LED strings; And A PWM controller configured to generate a PWM signal for operating the DC-AC converter according to the detected driving voltages of the LED strings, The driving voltage detector, An input voltage detector for detecting an output terminal voltage of the rectifier; And An output voltage detector for detecting cathode voltages of the LED strings; The input voltage detector includes two voltage divider resistors R3 and R4 connected in series on a line connecting one side of an output terminal of the rectifier to a ground, The output voltage detector includes: a first ground resistor (R1) forming a path through which a current passing through the first LED string of the two LED strings reaches ground; A second ground resistor (R2) forming a path through which the current passing through the second LED string reaches ground; A first transfer diode (D5) for transmitting a voltage applied to the first ground resistor (R1) to the PWM controller; And a second transfer diode D6 transferring a voltage applied to the second ground resistor R2 to the PWM controller. The PWM control unit, Receiving voltages detected by the input voltage detector and the output voltage detector; The duty ratio of the PWM signal is adjusted according to the voltage detected by the input voltage detector, LED driving circuit to perform the LED element protection function according to the voltage detected by the output voltage detector. delete delete The method of claim 1, The transformer, LED driving circuit whose winding ratio is determined by the setting during the design process. The method of claim 1, The rectifying unit, LED driving circuit which is a half-wave rectifier circuit with a diode.

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