JP3082259B2 - EL lighting circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for lighting an EL used for a backlight of a liquid crystal display or the like, and more particularly to a lighting circuit having improved luminance stability with respect to an ambient temperature.

[0002]

2. Description of the Related Art EL is mainly used as a backlight of a liquid crystal display (hereinafter, referred to as LCD) for displaying characters and figures of various measuring instruments and the like. A relatively high AC voltage, such as 600 Hz, must be applied.

Since such an AC voltage generally differs in voltage and frequency from a commercial power supply, a dedicated lighting circuit for generating a predetermined AC voltage from a DC voltage is generally used.

The lighting circuit includes, for example, Japanese Patent Publication No. Sho 62-15
There is a self-excited lighting circuit as proposed in, for example, U.S. Pat. This lighting circuit utilizes the fact that EL is a capacitive element, and switches at a lighting frequency determined mainly by the inductance of the transformer (T1) and the capacitance of EL (10) as shown in FIG. (Q1) is turned ON / OFF and the secondary side (N
2) Apply the AC voltage generated in (2) to EL (10) to light it.

As shown in FIG. 8, a separately-excited lighting circuit whose driving frequency can be arbitrarily determined includes a primary side (N1) of a transformer (T2) by a control circuit (201).
Is turned on / off by switches (Q2, Q3) and the transformer (T2
) Is applied to the secondary side (N2).
There is a lighting circuit of a method of applying voltage to

These lighting circuits have the advantage that they are simple circuits and have a small number of parts and are low in cost, but they have poor efficiency and poor EL life compensation characteristics. There is a problem such as the occurrence of interference fringes between the LCD and the LCD.
There has been a demand for a lighting circuit having an excellent life compensating ability of L.

Therefore, the present inventors have proposed a separately-excited lighting circuit as described in Japanese Patent Application No. 1-138568.

As shown in FIG. 9, this separately-excited lighting circuit is a power control type DC-DC converter (30) which outputs a DC high voltage (VD) with a DC voltage of about 5 to 18 Vdc as an input voltage (Vin). ) And a switch (Q5, Q6) that connects the EL and the inductor (L1) in series, and switches the inductor side of this series circuit between DC high voltage (VD) and GND. The switch (Q5, Q6) is turned on / off by the control circuit (201) of the DC-AC inverter (20), and the EL (10)
Is applied with an AC voltage.

[0009]

As described above, the separately-excited lighting circuit shown in FIG. 9 provides a high-efficiency and stable driving frequency, and makes the power control type DC-DC converter a constant power characteristic. Although the lighting circuit has an ideal EL compensation capability, as shown in FIG. 10, EL has a characteristic that the efficiency increases as the ambient temperature increases, and in a lighting circuit that outputs constant power, Since the EL brightness increases as the ambient temperature increases, it is necessary to adjust the luminance in accordance with the ambient temperature, for example, when used in an LCD backlight. Further, when the ambient temperature increases and the luminance of the EL increases, there is also a problem that the lifetime of the EL (the time when the luminance of the EL becomes 1/2 of the initial luminance) is shortened.

[0010]

SUMMARY OF THE INVENTION The present invention provides a lighting circuit for converting a DC voltage to an AC voltage, applying the converted voltage to an EL, and lighting the EL so that the output power of the lighting circuit has a negative temperature characteristic. Features.

[0011]

According to the above method, even if the ambient temperature changes, E
The luminance variation of L can be reduced, and the life characteristics of the EL at high temperatures can be improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. The lighting circuit in FIG.
DC-DC converter (30) that outputs DC high voltage (VD) with DC voltage of about 5 to 18 Vdc as input voltage (Vin)
, EL (10) and inductor (L1) are connected in series, and the EL side of this series circuit is connected to capacitors (C5, C6)
A switch (Q5, Q6) that connects the DC high voltage (VD) to the midpoint (VD / 2) obtained by dividing the DC high voltage (VD) with resistors (R5, R6) and switches the inductor side between the DC high voltage (VD) and GND.
), Diodes (D5, D6) and switches (Q5
, Q6) and a control circuit (201) for controlling ON / OFF of the DC-AC inverter (20).

Here, the operation principle of the DC-AC inverter (20) will be described with reference to FIG. The DC-AC inverter (20) is controlled by a switch (Q5,
This circuit turns on / off Q6) and applies an AC voltage to EL (10). As shown in FIG. 2, when the switch (Q5) is turned on at time t1, a current flows from the DC high voltage (VD) and the terminal voltage of the EL (10) rises to Vp1 as shown in FIG. . Then, when the switch (Q6) is turned on at time t2, the electric charge stored in the EL (10) starts discharging and a current flows. Energy is stored in the inductor (L1) by the discharge current, and this energy causes After the electric charge is discharged, EL has a further opposite voltage (−V
Charged at p2). At time t1 ', the switch (Q
5) turns ON and one cycle is completed.

Next, the DC-DC converter (30) which is a main circuit of the present invention will be described. DC-DC converter (3
0) is the transformer (T3) by the control IC (301).
ON / OFF the primary side (N1) of switch with switch (Q4).
The voltage generated on the secondary side (N2) is rectified by a diode (D1) and capacitors (C5, C6) to produce a DC high voltage (VD).
Is a circuit that generates

Here, the current flowing on the primary side (N1) of the transformer (T3) when the switch (Q4) is turned on is:
The power (Pout) which becomes a sawtooth wave as shown in FIG. 3 and is output to the secondary side (N2) of the transformer (T3) is a current immediately before the switch (Q4) is turned off, that is, a peak current (Ip). The primary side (N1) of the transformer (T3)
The inductance LN1, ON / frequencies OFF fD, the efficiency and ηD, Pout = 1/2 · LN1 · Ip 2 · f
D · ηD (1) Since LN1 and ηD are considered to be almost constant, it can be seen that the output power (Pout) is proportional to the square of the peak current (Ip) if fD is constant.

The power input to the EL (10) is obtained by multiplying the output power (Pout) by the conversion efficiency (ηA) of the DC-AC inverter (20), and ηA is considered to be almost constant. Therefore, the power input to the EL (10) is proportional to the output power (Pout).

As described above, by controlling the peak current (Ip) flowing through the switch (Q4), a power control type DC-DC converter can be realized.
The power input to (10) can be controlled. Conventionally, a constant power type lighting circuit in which the input power of the EL (10) is constant is configured by keeping the peak current (Ip) constant in consideration of the luminance compensation capability of the EL. In the circuit of the present invention, the peak current (Ip) is controlled by the ambient temperature.

As a method of realizing this, as shown in FIG. 1, a current detection resistor (Rp1) is connected to a switch (Q4) of a DC-DC converter (30), and a connection point (point a) is used for a filter. It is connected to the Is input point (point b) of the control IC (301) through a resistor (Rp2) and a capacitor (Cp). This Is input controls the output (Out terminal) of the control IC (301). When the Is input point becomes higher than a reference voltage inside the control IC (301), the output (Out terminal) becomes L.
The sequence inside the control IC is set such that

In this circuit configuration, the switch (Q4)
Is turned on and a current (Id) flows, a voltage (Rp1 × Id) is generated at a point a of the current detecting resistor (Rp1), and a control IC (Rp2) and a capacitor (Cp) are passed through the control IC (Cp). A voltage is also generated at the Is input point (point b) of 301). Then, as the current (Id) increases, the voltage at the Is input point (point b) also increases. However, when the voltage exceeds the reference voltage value inside the IC, the output (Out terminal) becomes L and the switch (Q4) turns off. Become. At this time, a current flows through the diode (D1), and the capacitors (C5, C6) are charged.

Here, the filter resistor (Rp2) and the capacitor (Cp) are used to prevent malfunction due to switching noise of the switch (Q4), and are usually about 1 kΩ and about 1000 pF. The output power (Pout) increases as the values of Rp2 and Cp increase. If a thermistor is used for the filter resistor (Rp2), the thermistor has a negative temperature characteristic as shown in FIG.
Becomes larger, and conversely, at high temperatures, the resistance becomes lower and the output power (Pout) becomes smaller. Here, in order to make the EL luminance constant with respect to the temperature, the output power (P
out)), it is necessary to match the rate of change of the efficiency of the EL, etc., but by inserting a series or parallel resistor in the thermistor as shown in FIG. In contrast, the luminance of EL can be made substantially constant.

Similarly, the same effect can be obtained by using a temperature compensating capacitor having a negative temperature characteristic as shown in FIG. 5 for the filter capacitor (Cp).

As described above, the output power (Pout) can be controlled by forming the resistor (Rp2) and the capacitor (Cp) with elements having temperature characteristics.
The rate of change of the filter resistor (Rp2) and the capacitor (Cp) with respect to the temperature must be selected within a range that prioritizes the original purpose of preventing the malfunction of (1).

On the other hand, since the output power (Pout) of the current detection resistor (Rp1) decreases as the resistance value increases, the current detection resistor (Rp1) has a positive temperature characteristic as shown in FIG. By using the resistor having the resistance, a stable luminance with respect to the temperature can be obtained.

As described above, according to the present invention, the output power can be controlled by configuring at least one of the resistor (Rp1), the resistor (Rp2) and the capacitor (Cp) with an element having a temperature characteristic. E stable against temperature change
L luminance can be realized.

[0025]

As described above, the EL lighting circuit according to the present invention can keep the EL brightness almost constant even if the ambient temperature changes depending on the environment in which it is used. Even when used for lights, there is no need to adjust the brightness according to the ambient temperature.

In the prior art, there was a problem that the life of the EL was shortened due to the high luminance at high temperatures. However, in the EL lighting circuit according to the present invention, the increase in luminance at high temperatures was suppressed. Service life is improved.

Furthermore, in order to realize the EL lighting circuit of the present invention, only a thermistor and a capacitor for temperature compensation are required, so that there is almost no adverse effect on the size and cost of the lighting circuit.

Further, by inserting a resistor or a capacitor in series or in parallel with these thermistors or temperature compensating capacitors, the rate of change in EL luminance with respect to temperature can be freely set. A wide range of applications is conceivable.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the DC-AC inverter of the embodiment.

FIG. 3 is a diagram for explaining the operation of the DC-DC converter of the embodiment.

FIG. 4 is a temperature characteristic diagram of a resistor used in the embodiment of the present invention.

FIG. 5 is a temperature characteristic diagram of a capacitor used in an embodiment of the present invention.

FIG. 6 is a temperature characteristic diagram of a resistor used in the embodiment of the present invention.

FIG. 7 is a circuit diagram of a conventional self-excited EL lighting circuit.

FIG. 8 is a circuit diagram of a conventional separately excited EL lighting circuit.

FIG. 9 is a circuit diagram of another example of a conventional separately excited EL lighting circuit.

FIG. 10 is a graph showing temperature characteristics of EL.

[Description of Signs] 10 EL element 20 DC-AC inverter 30 DC-DC converter

Claims (1)

(57) [Claims] At least a switching element and its control circuit
DC-A converter on the secondary side of a DC-DC converter
C inverter is connected, and this DC-AC inverter
A series circuit of an inductor and an EL element is connected to the secondary side
In the EL lighting circuit, the DC-DC converter is of a power control type, and
An element having temperature characteristics is provided.
Peak current flowing through the switching element in response to the
By controlling the flow, E L point <br/> lamp circuit, characterized in that which gave a negative temperature characteristic to the DC-DC converter output power.