JP2005116859A - Current-driven light-emitting element driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the power loss of a circuit while accurately controlling the light emission intensity of a current-driven light-emitting element. <P>SOLUTION: The drive circuit includes a rectifier circuit 1 for rectifying an AC input voltage, a smoothing capacitor Cc for smoothing the output of the rectifier circuit 1, a switching circuit 2 for turning on/off a voltage supplied from the rectifier circuit 1 to the smoothing capacitor Cc, in response to a switching control signal, a constant current circuit 3 for supplying a constant current to a light-emitting diode LED, while inputting a terminal voltage of the smoothing capacitor Cc, and a voltage comparator circuit 4 for controlling the ON/OFF of the switching circuit 2, by outputting the switching control signal so that the voltage drop of the constant current circuit 3 becomes smaller than or equal to a predetermined value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a current driven light emitting element driving circuit for lighting a current driven light emitting element such as a light emitting diode.

  In a current-driven light-emitting element such as a light-emitting diode, the amount of light and the drive current are approximately proportional. Therefore, constant current driving is desirable to keep the light quantity constant. In addition, the voltage-current characteristics of the current-driven light-emitting elements vary with temperature, and there are variations in voltage-current characteristics among the individual current-driven light-emitting elements. Based on the above conditions, a drive circuit using a constant current circuit is known as a circuit for stabilizing lighting of a current-driven light emitting element (see, for example, Patent Document 1). A configuration of a current-driven light-emitting element driving circuit disclosed in Patent Document 1 is shown in FIG. 3 is a constant current circuit for LED 102 that supplies a constant driving current to the light emitting diode 101, and a constant current circuit 103 for reference voltage that applies a reference voltage to the constant current circuit 102 for LED. And have.

  In this driving circuit, by controlling the reference voltage (driving voltage) applied to the LED constant current circuit 102 by the reference voltage constant current circuit 103, individual variations in the voltage-current characteristics of the light emitting diode 101 and the temperature fluctuations thereof. Alternatively, the fluctuation of the power supply voltage VCC is absorbed by the reference voltage constant current circuit 103. For this reason, the reference voltage constant current circuit 103 has a voltage drop that accounts for these variations and fluctuations. This voltage drop is a power loss that does not contribute to light emission as a drive circuit, and this power loss becomes very large especially in a display device having a large number of light emitting diodes used, and there is a problem that heat generation of the circuit increases.

  On the other hand, a switching type drive circuit using a DC / DC converter is known as another example of the current drive type light emitting element drive circuit (see, for example, Patent Document 2). FIG. 4 shows a configuration of a current-driven light-emitting element driving circuit disclosed in Patent Document 2. In the drive circuit in FIG. 4, the transistor 202 is turned on and off by a switching control signal output from the gate driver 201. When the transistor 202 is on, a current supplied from the battery 203 flows through the transistor 202 via the coil 204. When the transistor 202 is off, the current is supplied from the battery 203 via the coil 204 and includes a diode 205 and a capacitor 206. Current flows, and the DC voltage obtained here is applied to the light emitting diode 211 via the resistor 210.

On the other hand, the reference voltage Vref is input to the non-inverting input terminal of the error amplifier 212, and the voltage obtained by dividing the output voltage of the rectifier circuit by the resistor 207, the variable resistor 208, and the resistor 209 is input to the inverting input terminal of the error amplifier 212. Therefore, the differential voltage between the reference voltage Vref and the divided voltage is input to the PWM control circuit 213. The PWM control circuit 213 adjusts the pulse width of the switching control signal so that the differential voltage becomes zero and outputs it to the gate driver 201. As described above, in the drive circuit of FIG. 4, the loss of the drive circuit can be reduced by controlling the conduction angle of the pulse current.
The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
Japanese Patent Laid-Open No. 13-186001 Japanese Patent Laid-Open No. 14-203988

As described above, the drive circuit shown in FIG. 3 has a problem that the power loss of the circuit increases and heat generation increases.
On the other hand, in the driving circuit illustrated in FIG. 4, power loss can be reduced. However, since the current supplied to the light emitting diode 211 includes noise accompanying switching of the transistor 202, the light amount of the light emitting diode 211 is caused by switching noise. It fluctuates periodically. Therefore, when the drive circuit of FIG. 4 is applied to a display device that displays an image using a large number of light emitting diodes, the movement of the display image and the switching cycle interfere with each other, causing flickering or flickering. There was a problem such as.
The present invention has been made to solve the above problems, and provides a current-driven light-emitting element driving circuit capable of reducing the power loss of the circuit while accurately controlling the light emission intensity of the current-driven light-emitting element. The purpose is to do.

  The current-driven light-emitting element driving circuit of the present invention includes a smoothing capacitor that smoothes a DC input voltage, a switching circuit that turns on / off a DC input voltage supplied to the smoothing capacitor according to a switching control signal, and the smoothing A constant current circuit for supplying a constant current to the current-driven light emitting element with the capacitor terminal voltage as an input, and outputting the switching control signal so that the voltage drop of the constant current circuit is a predetermined value or less. And a voltage comparison circuit for controlling on / off of the switching circuit.

  According to the present invention, by controlling the on / off of the switching circuit so that the voltage drop of the constant current circuit is not more than a predetermined value, the input voltage of the constant current circuit is always not more than a certain value. The voltage drop of the circuit can be suppressed to the minimum voltage drop necessary for the ripple voltage of the smoothing capacitor and constant current. As a result, the AC input voltage fluctuates, the temperature fluctuates, the voltage-current characteristics of the current driven light emitting element fluctuate, or the output current of the constant current circuit changes the brightness of the current driven light emitting element. Even if the value is changed, the power loss of the constant current circuit can always be minimized, and the heat generation of the circuit can be suppressed. Further, even when the driving circuit of the present invention is applied to a display device that displays an image using a large number of light emitting diodes, the image does not flicker or flicker.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a current-driven light-emitting element driving circuit according to the first embodiment of the present invention.
The rectifier circuit 1 rectifies the AC input voltage, and the smoothing capacitor Cc smoothes the output of the rectifier circuit 1. The switching circuit 2 inserted between the rectifier circuit 1 and the smoothing capacitor Cc turns on / off the voltage supplied from the rectifier circuit 1 to the smoothing capacitor Cc according to the switching control signal.

  The constant current circuit 3 receives a terminal voltage of the smoothing capacitor Cc and supplies a constant current to the light emitting diode LED. Then, the voltage comparison circuit 4 controls the on / off of the switching circuit 2 by outputting a switching control signal so that the voltage drop Vd of the constant current circuit 3 is equal to or less than a predetermined value.

  As described above, in the present embodiment, the input voltage (smoothed) of the constant current circuit 3 is controlled by controlling the on / off of the switching circuit 2 so that the voltage drop Vd of the constant current circuit 3 is a predetermined value or less. Therefore, the voltage drop in the constant current circuit 3 can be the ripple voltage of the smoothing capacitor Cc and the minimum voltage drop required for constant current. Therefore, the AC input voltage fluctuates, the temperature fluctuates, the voltage-current characteristics of the light emitting diode LED fluctuate, or the output current value of the constant current circuit 3 is changed to change the brightness of the light emitting diode LED. Even in such a case, the power loss of the constant current circuit 3 can always be minimized. In addition, since the rectifier circuit 1 and the switching circuit 2 before the constant current circuit 3 do not have a portion that causes a large power loss, the power efficiency of the entire drive circuit is good and heat generation is extremely small.

  Further, switching is performed only for the smoothing capacitor Cc serving as a power supply unit for the constant current circuit 3, and the pulsating component is reduced to a sufficiently low (negligible) value by the pulsating component removing ability of the constant current circuit 3. Therefore, even if the drive circuit of this embodiment is applied to a display device or the like that displays an image using a large number of light emitting diodes, the image does not flicker or flicker.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 2 is a circuit diagram showing a configuration of a current drive type light emitting element drive circuit according to a second embodiment of the present invention. The present embodiment shows a specific example of the drive circuit of the first embodiment.
The rectifier circuit 1 includes a transformer T1 and diodes D1 and D2, and the switching circuit 2 includes a transistor Tr1. The resistor R1 and the Zener diode Zd1 constitute a regulator that divides and stabilizes the terminal voltage of the smoothing capacitor Cc.

  The constant current circuit 3 includes a transistor Tr2 and a resistor Rd connected in series with the light emitting diode LED, a resistor R2 and a variable resistor VR that divides the output voltage of the regulator, and a voltage divider generated by the resistor R2 and the variable resistor VR. It has an error amplifier AMP1 that outputs a voltage proportional to the error between the voltage and the terminal voltage of the resistor Rd, and a resistor R3 that connects the output terminal of the error amplifier AMP1 and the base of the transistor Tr2.

  When the divided voltage is higher than the terminal voltage of the resistor Rd, the error amplifier AMP1 increases the base current flowing through the transistor Tr2 through the resistor R3, and conversely, the divided voltage is lower than the terminal voltage of the resistor Rd. In this case, the base current flowing through the transistor Tr2 is reduced. Thus, the constant current circuit 3 supplies a constant current to the light emitting diode LED. Further, the output current value of the constant current circuit 3 can be adjusted by adjusting the variable resistor VR.

  The voltage comparison circuit 4 includes resistors Rr1 and Rr2 that divide the output voltage of the regulator, a comparator circuit including the resistors Ri and Rf and the comparator COMP1, a resistor Rsb that connects the output terminal of the comparator COMP1 and the base of the transistor Tr1, and Zener diode Zd1.

  The comparator circuit compares the collector voltage of the transistor Tr2 (voltage drop Vd of the constant current circuit 3) with the reference voltage Vdref generated by the resistors Rr1 and Rr2, and the differential voltage between the voltage drop Vd and the reference voltage Vdref. A switching control signal is output so that becomes zero. When the voltage drop Vd is equal to or lower than the reference voltage Vdref, the comparator circuit does not output a switching control signal. Therefore, the transistor Tr1 of the switching circuit 2 is kept on. On the other hand, when the voltage drop Vd is higher than the reference voltage Vdref, the comparator circuit outputs a switching control signal for turning off the transistor Tr1. This switching control signal is applied to the base of the transistor Tr1 via the Zener diode Zd2 and the resistor Rsb, and the transistor Tr1 is turned off.

  Thus, as described in the first embodiment, on / off of the transistor Tr1 of the switching circuit 2 can be controlled so that the voltage drop Vd of the constant current circuit 3 is equal to or less than a predetermined value (reference voltage Vref). it can.

  The present invention can be applied to a display device and the like that require stabilization of light emission intensity of a current driven light emitting element.

1 is a block diagram showing a configuration of a current-driven light emitting element driving circuit according to a first embodiment of the present invention. It is a circuit diagram which shows the structure of the current drive type light emitting element drive circuit used as the 2nd Embodiment of this invention. It is a block diagram which shows an example of the conventional current drive type light emitting element drive circuit. It is a block diagram which shows the other example of the conventional current drive type light emitting element drive circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Rectification circuit, 2 ... Switching circuit, 3 ... Constant current circuit, 4 ... Voltage comparison circuit, Cc ... Smoothing capacitor.

Claims (1)

A smoothing capacitor that smoothes the DC input voltage;
A switching circuit for turning on / off a DC input voltage supplied to the smoothing capacitor according to a switching control signal;
A constant current circuit for supplying a constant current to the current-driven light-emitting element using the terminal voltage of the smoothing capacitor as an input;
And a voltage comparison circuit that outputs the switching control signal to control on / off of the switching circuit so that a voltage drop of the constant current circuit is a predetermined value or less. Driving circuit.

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