JP2593079Y2 - Cold cathode tube lighting device - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-cathode tube lighting device, and more particularly to a cold-cathode tube lighting device for stabilizing the luminous intensity of a cold-cathode tube.

[0002]

2. Description of the Related Art It is conceivable to use a switching power supply circuit known as model number FA7612 as a conventional cold cathode tube lighting device of this type. Figure 2 shows model number FA7612
FIG. 3 is a circuit diagram showing a configuration example of a cold-cathode tube lighting device using the circuit of FIG.

In FIG. 1, reference numeral 1 denotes a DC power supply;
DC conversion circuit, 3 is an inverter circuit, 4 is a dimming circuit, 5
Is a cold cathode tube.

The DC-DC conversion circuit 2 includes a PNP-type transistor 21 as a switching element for outputting a chopping voltage signal, a diode 22, a choke coil 23 for smoothing the chopping voltage signal, and a model number FA.
It comprises a switching power supply circuit 24 which is commercially available at 7612 and controls the switching of the transistor 21 based on the adjustment voltage of the dimming circuit 4 and the like.

[0005] REF is a reference voltage section constituting a DC constant voltage power supply of the power supply circuit 24, and receives the voltage of the DC power supply 1 to stabilize the output reference voltage VREF. C1
Is a capacitor for smoothing the voltage of the DC power supply 1. OS
C is an oscillator unit that continuously generates a sawtooth wave of a predetermined frequency. ER. AMP is an error amplifier that inverts and amplifies a divided voltage Vi2 of a load current detection voltage VS2 described later and outputs the result.
PWM is a PWM comparison unit, which includes an error amplifier ER. C. AMP output level and constant current circuit C. The lower level of C is compared with the output level of the oscillator section OSC, and when these corresponding levels are higher than the level of the oscillator section OSC, an H level signal is output. G is an output selection gate, which is usually an error amplifier ER. An output control signal consisting of the output of the AMP is passed, and the passage of the output control signal is prohibited when a prohibition signal OFF described later is received. CS is a capacitor charged for soft start and short circuit protection; C is a constant current circuit that supplies a charging current to the capacitor CS.
The capacitor CS has a constant current circuit C.C. C, or a reference voltage VREF via a resistor RC1, the voltage is divided by resistors RC1 and RC2 and charged to a divided voltage. In response to the charging current from C, the charging current is charged to a voltage sufficiently higher than the divided voltage. SCP is a comparator for detecting a short circuit, and includes an error amplifier ER. When the output level of the AMP reaches a predetermined abnormal level, the constant current circuit C.P. Activate C. LO is a comparator for short-circuit protection.
When the charging voltage VCS reaches a predetermined short-circuit-corresponding potential, the output selection gate G is given an inhibition signal OFF. Also, UVLO
Is a constant voltage malfunction prevention circuit. When the reference voltage VREF drops to a predetermined abnormal voltage, the transistor Q1 is activated to discharge the capacitor CS, stop the operation of the PWM comparison unit PWM and the like, and turn off the inhibition signal to the output selection gate G. give. Q2 is an output transistor which is turned on for a period during which the transistor 21 is turned on.
And outputs a chopping voltage signal of a level according to the voltage of.

The inverter circuit 3 comprises a well-known resonant push-pull multivibrator. That is,
A primary winding 311 having an intermediate tap 311c, and a secondary winding 312
And a tertiary winding 313, NPN transistors 32 and 33, resistors 34 and 35, and capacitors 36 and 37. A capacitor 36 is connected in parallel with the primary winding 311. One end of the primary winding 311 is connected to the collector of the transistor 32, and the primary winding 311
Are connected to the other end of the transistor 33, respectively. Further, the emitters of the transistors 32 and 33 are grounded, and the base of the transistor 32 is connected to one end of the tertiary winding 313 and to the transistor 3
The base 3 is connected to the other end of the tertiary winding 313, respectively. Furthermore, the intermediate tap 311c of the primary winding 311 is
Connected to the other end of the coil 23 and a resistor 34
To the base of transistor 32 and to resistor 3
5 are connected to the bases of the transistors 33, respectively. One end of the secondary winding 312 is connected to a capacitor 37.
Is connected to one end of the cold cathode tube 5, and the other end is grounded.

The dimming circuit 4 includes a load current detecting resistor RS connected between the other end of the cold cathode tube 5 and the ground, a diode D0 for rectifying the load current detecting voltage VS2 detected by the resistor RS. A smoothing capacitor Co and voltage dividing resistors R1 and R2 for dividing the rectified load current control voltage Va2.
And a fixed resistor R0 and a dimming variable resistor VR connected in parallel with the load current detecting resistor RS. The adjusted voltage at the voltage dividing point (divided voltage Vi2) is supplied to the error amplifier ER. AMP-
It is given to the terminal.

In the configuration shown in FIG. 2, when the DC power supply 1 is turned on, a voltage is applied to the primary winding 311 of the transformer 31 via the transistor 21 and the choke coil 23, and the transistors 32, 33 Due to the difference in amplification rate,
Either turns on first. For example, the transistor 32 is turned on, and the current flows through the tertiary winding 313 to the transistor 3.
A voltage is generated in a direction that promotes the ON state of transistor 2, and transistor 32 is fully turned on. During the ON period, the capacitors 36,
37 and the inductance of the transformer 31 and the resonance condition. When the polarity of the resonance waveform is inverted, the transistor 32 is turned off and the transistor 33 is turned on, and the oscillation continues. As a result, an AC voltage is generated in the secondary winding 312, and the cold cathode tube 5 is turned on. The value of the current Io flowing through the cold-cathode tube 5 is detected as a load current detection voltage Vs2 by a current detection resistor RS and a variable resistor VR, and the rectified load current control voltage Va2 is determined by the resistors R1 and R2. The voltage is divided and output as a divided voltage Vi2. The divided voltage Vi2 is supplied to the error amplifier ER. The voltage is compared with the voltage VREF by AMP, and the PWM comparator P
The output of the WM is adjusted, and the output of the transistor 21 is adjusted corresponding to the output. Here, when the resistance value of the variable resistor VR for dimming is adjusted, the load current detection voltage V
The output of the transistor 21 is adjusted by adjusting s2, and thus the brightness of the cold cathode fluorescent lamp 5 is adjusted.

[0009]

However, the switching power supply circuit used as the above-mentioned cold cathode tube lighting device is expensive because it has an oscillator section for generating a sawtooth wave, and the cost of the entire device increases. In addition, since the frequency of the sawtooth wave cannot be synchronized with the oscillation frequency of the inverter circuit, a beat or oscillation may occur, and stable control may not be performed.

An object of the present invention is to provide a cold-cathode tube lighting device having a simple structure and capable of obtaining stable luminance control.

[0011]

In order to achieve the above object, the present invention comprises a DC-DC conversion circuit for chopping and controlling a DC power supply voltage by a switching element, and converts an output voltage of the DC-DC conversion circuit to a DC voltage. A cold-cathode tube lighting device for driving a cold-cathode tube by performing AC conversion, load current detecting means for detecting a load current of the cold-cathode tube and outputting a load current detection voltage proportional to the load current; A first comparator which can be adjusted by the load current detection voltage and which is compared with the load current detection voltage and which is set as a comparison reference voltage with respect to the load current detection voltage, and outputs a mutual deviation signal; A voltage detecting means for detecting an output voltage of the DC conversion circuit and outputting a voltage detection voltage proportional to the output voltage; and superposing a predetermined voltage on an output signal of the first comparator. A second comparator for comparing a load voltage reference value generated as a comparison reference voltage for the voltage detection voltage with the voltage detection voltage and outputting a mutual deviation signal to the switching element as a negative feedback control signal. Was.

[0012]

According to the present invention, the first comparator compares the load current reference value with the load current detection voltage and outputs a mutual deviation signal, and the second comparator outputs the first comparison signal. A voltage detection voltage is compared with a load voltage reference value obtained by superimposing a predetermined voltage on the output signal of the switch, and a mutual deviation signal is output to the switching element as a negative feedback control signal. Therefore, the voltage detection voltage is controlled to match the load voltage reference value, and the load current detection voltage is controlled to match the load current reference value.

[0013]

FIG. 1 is a circuit diagram showing an example of the configuration of a cold-cathode tube lighting device according to the present invention. In this figure, the same parts as those in FIG. 2 are denoted by the same reference numerals, and different parts will be described below.

Reference numeral 6 denotes a DC-DC conversion circuit, which includes a transistor 21, a diode 22, a choke coil 23, and a dual comparator 25 which is commercially available under the model number MB47082 and controls the switching of the transistor 21 as in FIG. .

The dual comparator 25 includes a divided voltage Vi1 of a load current detection voltage VS1 to be described later and a dimming variable resistor V
A first comparator CP1 for comparing the load current reference value VR adjusted by R1 and outputting an analog signal due to a mutual deviation, and a first comparator CP1 which generates a signal by superimposing a predetermined voltage on the output signal of the first comparator CP1. Load voltage reference value Vv1 and DC
A voltage detection voltage V proportional to the output voltage of the DC conversion circuit 2
F2 and a second comparator CP2 which outputs a digital signal based on the mutual deviation to the transistor 21 as a negative feedback control signal. FIG. 3 shows a dual comparator 25.
3 is an operation waveform diagram of each part of FIG.

Numeral 7 denotes a current detecting circuit, a load current detecting resistor RS similar to that shown in FIG. 2, and a diode D0 for rectifying the load current detecting voltage VS1 detected by the resistor RS.
A smoothing capacitor Co, voltage dividing resistors R1 and R2 for dividing the rectified voltage Va1 to generate a load current detection voltage Vi1 for comparison control, and a capacitor C2 for cutting noise of the voltage Vi1. , The voltage Vi1 to the first comparator C
It is given to the-terminal of P1.

Reference numeral 8 denotes a dimming circuit, which is a Zener diode ZD.
Resistors R3 and R4 for dividing the constant voltage of the comparator CP1 and a dimming variable resistor VR1 for adjusting the voltage dividing ratio. The load current reference value (voltage dividing VR) based on the adjusted voltage dividing ratio is calculated by the comparator CP1. It is given to the + terminal. The divided voltage VR is set as a comparison reference voltage with respect to the load current detection voltage Vi1 in order to make the current Io of the cold cathode tube 5 correspond to the dimming designation. R5 and C3 are resistors and capacitors for converting the output of the comparator CP1 into an analog signal, and C4 is a capacitor for smoothing the output.

R5 and R6 are voltage-dividing resistors for detecting the output voltage of the DC-DC conversion circuit 6, and provide a voltage detection voltage VF proportional to the output voltage of the circuit 6 to the + terminal of the second comparator CP2. . C5 is a capacitor for cutting noise of the voltage VF. The output voltage and the voltage VF of the circuit 6 form a resonant full-wave rectified waveform determined by the capacitors 36 and 37 and the inductance of the transformer 31.

The resistor R7 is a resistor for superimposing the output signal of the first comparator CP1 and the constant voltage of the Zener diode ZD at a predetermined ratio. The resistor R7 compares the superimposed load voltage reference value (voltage Vv1) with the second comparator. This is applied to the-terminal of CP2. The voltage Vv1 is set as a comparison reference voltage with respect to the voltage detection voltage VF so that the voltage applied to the inverter circuit 3 and the current Io of the cold cathode tube 5 correspond to the dimming designation. The second comparator CP2 outputs the voltage VF and the voltage V
Transistor 2 converts digital signal due to mutual deviation from v1 to transistor 2.
1 as a negative feedback control signal.

The operation of the cold cathode tube lighting device shown in FIG. 1 will be described. When the DC power supply 1 is turned on, the cold cathode tube 5 is turned on as in the configuration of FIG. Cold cathode tube 5
Is detected as a load current detection voltage Vs1 and divided, and the load current detection voltage V
It is output to the first comparator CP1 as i1. The voltage Vi1
Is compared with the voltage VR by the comparator CP1, and the output of the comparator CP1 is adjusted so that the voltages become equal to each other. The output is superimposed on the constant voltage by the Zener diode ZD to become the voltage Vv1, and the negative voltage of the second comparator CP2 is
Output to terminal. The voltage Vv1 is compared with the voltage detection voltage VF by the comparator CP2, and the output of the comparator CP1 is adjusted so that the voltages become equal to each other, and is output to the transistor 21. The transistor 21 is connected to the comparator C
When the output of P2 is at the L level, it conducts, and when it is at the H level, it becomes non-conductive. The current detection voltage Vi1 and the voltage detection voltage VF are controlled by negative feedback. Here, the dimming variable resistor VR
By adjusting 1, the voltage VR is adjusted and the transistor 2
1 is adjusted, and thus the brightness of the cold cathode tube 5 is adjusted.

[0021]

As described above, according to the present invention, a DC-DC converter for chopping and controlling a DC power supply voltage by a switching element is provided, and the output voltage of the DC-DC converter is converted to DC-AC. In a cold cathode tube lighting device for driving a cold cathode tube, a first comparator compares a load current detection voltage of the cold cathode tube with a load current reference value and outputs a mutual deviation signal; A voltage detection voltage of an output voltage of the DC-DC conversion circuit,
The output signal of the first comparator is compared with a load voltage reference value in which a predetermined voltage is superimposed, and a mutual deviation signal is output as a negative feedback control signal to the switching element. It is possible to provide an arc tube lighting device capable of performing stable luminance control at a low cost.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of a cold-cathode tube lighting device of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a conventional cold-cathode tube lighting device.

3 is an operation waveform diagram of each part of the dual comparator of FIG.

[Explanation of symbols]

1. DC power supply 3. Inverter circuit 5. Cold cathode tube 6.
... DC-DC conversion circuit, 7 ... current detection circuit, 8 ... dimming circuit, 21 ... transistor, 25 ... dual comparator, CP1 ... first comparator, CP2 ... 21st comparator,
RS: load current detecting resistor, R1, R2, R3, R4,
R5, R6, R7: resistor, VR1: variable resistor for dimming, Z
D: Zener diode.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) H05B 41/38-41/42 H02M 3/155 H05B 41/24-41/29

Claims (1)

(57) [Scope of request for utility model registration] 1. A cold-cathode tube lighting device comprising: a DC-DC conversion circuit for chopping control of a DC power supply voltage by a switching element; A load current detecting means for detecting a load current of the cold-cathode tube and outputting a load current detection voltage proportional to the load current; and adjusting as a dimming means and setting as a reference voltage for comparison with the load current detection voltage. A first comparator that compares the obtained load current reference value with the load current detection voltage and outputs a mutual deviation signal; and detects an output voltage of the DC-DC conversion circuit and is proportional to the output voltage. Voltage detection means for outputting a detected voltage detection voltage, and a predetermined voltage superimposed on an output signal of the first comparator, which is generated as a comparison reference voltage for the voltage detection voltage. A second comparator for comparing a load voltage reference value with the voltage detection voltage and outputting a mutual deviation signal as a negative feedback control signal to the switching element. apparatus.