JP2672692B2 - EL lighting circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a lighting circuit such as an organic dispersion type EL used for a backlight of a liquid crystal display, etc., and particularly to stabilization of driving frequency, small size, high efficiency, and long EL length. The present invention relates to an EL lighting circuit that can realize a life.

2. Description of the Related Art Organic dispersed EL is widely used because of its flexibility, light weight, low cost, and thinness. Especially, it is a backlight for liquid crystal displays (hereinafter referred to as LCD) that display characters and figures for various measuring instruments. Is used as.

To turn on the above EL, EL as a capacitive load
This is done by applying an AC voltage of 120V, 600Hz, etc. to the EL and applying an AC current to the EL.However, when using it for the backlight of an LCD such as a personal computer, the personal computer itself usually has only a DC voltage of about 12V. First, to light up EL, the DC voltage of about 12V
A DC-AC inverter circuit that converts to 20V AC voltage is required.

Conventionally, a dedicated inverter for EL has been used as this inverter circuit.
There is an inverter circuit proposed in 2. The configuration and operating principle of this inverter circuit will be described in detail with reference to FIG.

The inverter circuit in Fig. 5 is a transformer (T
1), the capacitor (C3) connected in parallel with EL on the secondary side of the transformer (T1), the input voltage terminal (Vin) and the transformer (T
1) and a switching circuit (S1) provided between the two. The switching circuit (S1) includes a transistor (Q1) having an emitter and a collector connected between a transformer (T1) and an input terminal (Vin), and a transistor (Q1).
A resistor (R1, R2) and a capacitor (C1) connected between the base and the collector of and a coupling capacitor (a capacitor (C1) connected between the base of the transistor (Q1) and the transformer (T1) via the resistor (R2) ( C2) and.

In the above configuration, the DC voltage Vin is applied to the transistor (Q1)
Is turned on and off to convert to AC voltage and the transformer (T
1) is applied to the primary side and the transformer (T
1) The secondary side output voltage obtained by multiplying the ratio of the primary and secondary windings N2 / N1 is applied to EL. This operation will be described below.

First, Vin-resistor (R1) -resistor (R2) -transistor (Q1) base-emitter-transformer (T1) coil (N1) -GND and transistor (Q1)
1) turns on and Vin is applied to the coil (N1) of the transformer (T1). Therefore, the current flowing through the coil (N1) of the transformer (T1) increases with time, so the transformer (T1)
The voltage to the capacitor (C2) connected to is also increased. This becomes the base current of the transistor (Q1) and the transistor (Q1) can maintain the ON state.

On the other hand, the magnetic flux passing through the core of the transformer (T1) also increases with time, but eventually reaches the saturation magnetic flux density, and the increase of the magnetic flux stops. Then the voltage to the capacitor (C2) drops,
This acts so as to reverse the base bias of the transistor (Q1), so that the transistor (Q1) is rapidly turned off, the current flowing in the coil (N1) of the transformer (T1) is cut off, and this state is accelerated.

The electric charge of the capacitor (C2) charged when the transistor (Q1) is ON is V during the OFF period of the transistor (Q1).
in-Resistance (R1) -Capacitor (C2) -Transformer (T1)-
Discharging continues through the GND path, the base voltage of the transistor (Q1) becomes higher than that of the emitter, and the transistor (Q1) turns on again.

Here, the capacitor (C1) turns on the transistor (Q1).
-Responsiveness during OFF operation is suppressed low, the output voltage waveform of the transformer (T1) is made closer to a sine wave from a square wave, and it also serves to prevent abnormal oscillation of the circuit. The capacitor (C3) is also used for the same purpose and for stabilizing the operation of the EL, and the original operation is possible without the capacitors (C1) and (C3).

As described above, alternating voltage is applied to EL by repeating ON-OFF of the transistor (Q1), and it lights up.
This inverter is a self-excited inverter in which the voltage value and frequency of the AC voltage applied to EL are determined by Vin, R1, C2, C3, T1, and EL impedance, and the transistor (Q1) turns ON-OFF. Because it repeats, it is usually called a blocking oscillation inverter.

Also, the switch is turned on by an external signal instead of the self-excited type.
The separately-excited inverter that turns off and determines the frequency of the AC voltage will be described in detail with reference to FIG.

The circuit of FIG. 6 has a transformer (T2), an inductor (hereinafter referred to as a choke coil for convenience) (L1) and EL connected in series on the secondary side of the transformer (T2), and the transformer (T2).
The switching transistors (Q1) and (Q
2) is connected, and ON-OFF is repeated by the drive signal corresponding to the EL lighting frequency given from the outside, but the transistor (Q1) and transistor (Q2) are turned on at the same time.
The external drive signal is designed so as not to do so.
The resistors (R1) and (R2) are transistors (Q1) and (Q
It is for applying an external drive signal to 2), and capacitors (C1) and (C2) are used to improve the switching characteristics of transistors (Q1) and (Q2).

In this configuration, turn on transistors (Q1) and (Q2)
-Transforms to AC voltage by OFF operation and transformer (T1)
Is applied to the primary side of the transformer, and the output voltage on the secondary side obtained by multiplying the primary side voltage by the ratio N2 / N1 of the primary winding and secondary winding of the transformer (T1) is applied to EL. , EL has a power factor of about 0.25 so that the power factor is improved by inserting a choke coil (L1) in series, and the reactance component also compensates for the decrease in EL brightness.

In the self-excited blocking oscillator circuit according to the above, the frequency of the AC voltage applied to the EL is determined by the circuit constant and the impedance of the EL, but the capacitance of the EL changes with lighting time. Therefore, the driving frequency was fluctuating. When used as a backlight of an LCD, for example, the EL drive frequency and the LCD drive frequency are highly likely to be synchronized with each other during a certain lighting time, which causes flicker. In addition, since a transformer is used up to the magnetic saturation point due to its operating principle, a beat occurs due to magnetostriction, and since one transformer combines oscillation and boosting, loss is large and conversion efficiency is generally less than 60%. Further, the inductance on the secondary side of the transformer has a large value such that the series resonance frequency of the inductance and the capacitance of EL is almost equal to the drive frequency, so that it is difficult to downsize the transformer.

On the other hand, in the separately excited inverter described above, the EL lighting frequency can be prevented from fluctuating due to the change in EL impedance over time, but the transformer (T2) and choke coil (L
Since 1) is required, miniaturization is difficult, and there are drawbacks such as high loss during conversion and high conversion efficiency.

Means for Solving the Problems (1) DC power supply, first and second switches connected in series to the DC power supply, and an EL connected as a load between the connection point of both switches and the DC power supply And a inductor, a control circuit that outputs a signal that controls each of the first and second switches, and an oscillator circuit that generates a signal of a frequency that drives EL and outputs the signal to the control circuit. In an EL lighting circuit that charges and discharges EL by switching the first and second switches in the control circuit, the inductor frequency is set so that the resonance frequency of the series circuit including EL and the inductor becomes higher than the drive frequency of EL. The inductance is set and the first switch is closed to charge EL and subsequently discharged via a first diode connected in parallel outside or inside the first switch, After the first switch is opened, the second switch is closed to charge EL with a reverse polarity and then discharged through a second diode connected in parallel outside or inside the second switch. , The above operation is performed at a constant frequency, and (2) The DC power supply detects the current flowing in the inductor of the ringing choke type DC-DC converter and controls the peak value of this current to be constant. Therefore, it is a constant power output type DC-DC converter that outputs a constant power.

Operation According to the above configuration, the frequency at which the EL lights up is stable because it is determined by the frequency of the external control circuit regardless of the change in the EL characteristics.

In addition, the choke coil can be reduced in size because the resonance frequency is set to about twice as high as the drive frequency as compared with the transformer used in the blocking oscillator circuit or the separately excited inverter described in the section of the prior art. Since it is used in a region where magnetic saturation does not occur, beat does not occur due to magnetostriction.

In addition, due to the series resonance characteristic, the EL voltage is higher than the power supply voltage.
Since it can be charged, and it is discharged immediately after being charged through the diode connected in parallel, the charge discharged through the diode is recovered to the power supply and used for recharging the EL, so there is little energy loss and good efficiency. Inverter can be realized.

Further, the DC power supply is a ringing choke type DC-DC converter having a transformer, and the current flowing through the primary coil of the transformer is detected, and its peak value is controlled to be constant, so that an easy and low-cost constant power supply can be obtained. In addition to being able to provide the constant power output, the EL has a characteristic that the driving voltage of the EL increases and the decrease in brightness is reduced against the high impedance of the EL that accompanies the life of the EL. Therefore, the lighting life of the EL can be extended. .

Embodiment One embodiment of the present invention will be described below with reference to FIG.

Figure 1 shows that EL and choke coil (L1) are connected in series, and the resonance frequency of this series circuit is about 2 times the lighting frequency of EL.
Double the EL side of this series circuit with capacitors (C1, C2),
And a transistor (Q11, Q1) connected to the middle point of the resistors (R1, R2) and flowing a current from the DC high voltage (HV) to the choke coil side.
Q12) and a diode (D
1) and a transistor (Q2) that allows current to flow to GND, and an EL switching circuit (10) that connects a diode (D2) that allows current to flow from GND, and these transistors (Q11, Q12, Q
2) timing control circuit (11), EL drive frequency fEL generating oscillator circuit (12), and DC high voltage (H)
DC-DC that generates V) from a DC power supply (Vin) of about 12v
It is composed of a converter (13).

Here, the transistors (Q11, Q12) that flow current from the DC high voltage (HV) to the series circuit of the choke coil and EL are PNP
It has a thyristor configuration in which a transistor (Q11) and an NPN transistor (Q12) are combined, and the transistor (Q5) and the resistor (R6) are used for the time determined by the signal (a) output from the timing control circuit (11). By turning on (Q13) and triggering the transistor (Q11), a transistor (Q11, Q12) with a thyristor configuration
It is a circuit that turns on. The resistors (R3, R4) and the capacitors (C3, C4) are filter circuits for preventing the transistors (Q11, Q12) from malfunctioning due to surge voltage or the like.

Next, the operating principle of this circuit will be described in detail with reference to the timing charts of FIGS.

In FIG. 2, a and b are transistors (Q11,
Q12, Q2) control signal, signal (a) is from L level to H
When the level is reached, a thyristor transistor (Q11,
Q12) is triggered and becomes conductive, and the signal (b) shows that the transistor (Q2) becomes conductive at the H level.

If the timing is t0 shown in FIG. 2, EL has a voltage of HV / 2 at the C terminal and a voltage of V2 'at the B terminal in FIG. 1 and the B terminal is charged with a negative polarity. When the transistor (Q11, Q12) turns on at, HV-Q11, Q12-L1-
A current flows through the path of EL-C2-GND, and EL has the opposite polarity, that is, the B terminal becomes positive polarity and is charged. However, due to the series resonance characteristics of EL and the choke coil (L1), the B terminal of EL is It is raised to V1 which is higher than the DC high voltage (HV) (timing t1). At this time, the current flowing through EL becomes zero,
The transistors (Q11, Q12) of the thyristor structure are turned off, and at the same time the A terminal in Fig. 1 tries to rise to V1.
The diode (D1) becomes conductive in the forward direction, and a current flows from the A terminal in the direction of direct current high voltage (HV). This current is
It is a discharge current for discharging the charged electric charge of EL, and at the timing of t2, the voltage of the B terminal of EL becomes V1 '.

Next, when the transistor (Q2) is turned on at the timing of t2, a current flows through the path of EL-L1-Q2-GND, and EL is charged with the opposite polarity again. Similarly, EL and choke coil ( Due to the series resonance characteristic of (L1), the B terminal of EL becomes -V2, which is lower than GND (timing t3). At this time, the current flowing through EL becomes zero, but at the same time, the A terminal tries to fall to -V2, but the diode (D2) becomes conductive in the forward direction, and the current flows from GND to the A terminal. . Also,
At this time, since the collector-emitter of the transistor (Q2) is reverse biased, no current flows through the transistor (Q2) even when the control of the signal (b) is turned on. The current flowing through the diode (D2) is a discharge current for discharging the charged electric charge of EL, and the voltage at the B terminal of EL becomes V2 'at the timing of t4.

Next, turn on the transistors (Q11, Q12) at the timing of t4.
And one cycle is completed, and by repeating t0 to t4
EL continues to light.

On the other hand, the DC high voltage (HV) is a constant power output D from Vin.
This DC is generated by the C-DC converter (13).
-The DC converter is a ringing choke type circuit that achieves constant power by controlling the peak value of the current flowing through the primary side of the transformer to a constant value. The oscillator circuit (12) can be composed of a timer IC such as a μPC1555 using a CR constant, and the timing control circuit can be composed of a logic IC such as a flip-flop or an AND gate.

Next, as another embodiment, transistors (Q11, Q12) and NPN transistors (Q2) of thyristor structure and diodes (D1, D2) were used as switching elements in FIG. 1, but instead of these transistors, There is a circuit that uses FET.

Since the control current of FET is smaller than that of transistor and a diode is usually built in between the drain and source, using the FET eliminates the need for the aforementioned diodes (D1, D2). This circuit diagram and timing chart are shown in Fig. 3,
As shown in FIG.

FIG. 3 shows a circuit in which the transistors (Q11, Q12) of the thyristor structure shown in FIG. 1 are replaced by PchFETs (Q3) and the NPN transistors (Q2) of FIG. 1 are replaced by NchFETs (Q4). Here, the FET (Q3) is connected to a circuit of a capacitor (C7), a resistor (R7), and a zener diode (ZD7) so that it can be driven by a logic level signal of a timing control circuit.

The control signal of this FET (Q3, Q4) is basically the same as the control signal (a, b) shown in FIG. 2, but the FET (Q3) turns ON when the control signal is at L level. Therefore, the signal (a ') is the inverted signal (a') of FIG. Further, the drive waveform and operation principle of the AC voltage applied to the EL under the control of these FETs (Q3, Q4) are exactly the same as those described in FIG. 1 and FIG.

Next, in the above-described embodiment, since positive and negative symmetrical voltages are applied between the terminals of EL, one terminal (C terminal) of EL is connected to the capacitors (C1, C2) and the resistors (R1, R2) from high DC voltage. , And the terminal voltage was set to HV / 2, this C terminal may be any DC voltage, and the same operation can be realized by connecting it to GND, for example. C terminal is G
Even when set to ND, since the voltage shown in Fig. 2 is applied to the B terminal of EL, asymmetrical voltages of V1 for positive polarity and V2 for negative polarity are applied, but one side terminal of EL is GND. Because there is L
It is convenient when incorporating it into a CD.

As described above, the EL lighting circuit according to the present invention turns on the EL at the frequency of the external oscillator circuit.
When used as an LCD backlight, it can be set to a frequency that is not synchronized with the LCD drive frequency, and
Flickering of the LCD does not occur because it can be lit at the same frequency regardless of changes in the impedance of.

Also, the resonance frequency between the inductance of the choke coil and the capacitance of the EL can be set to be about twice as high as the EL lighting frequency, so the choke coil can be made smaller, and because this choke coil is used in the area where magnetic saturation does not occur, magnetostriction occurs. There is no growl caused by.

On the other hand, in terms of efficiency, in principle the LC of the choke coil and EL
Generates high DC voltage because it uses series resonance
Even if it includes a DC-DC converter, a highly efficient inverter of 70-80% can be realized.

Furthermore, since a constant power output type is used for the DC-DC converter that generates the DC voltage of the inverter, it is possible to correct the decrease in brightness due to the lighting of the EL, and it is possible to achieve a lifespan that is approximately 1.5 times longer than that of the blocking oscillation inverter. , Constant power output,
The constant power input makes it an ideal inverter for battery-powered laptop personal computers.

[Brief description of the drawings]

1 and 2 are a circuit diagram and a timing chart of the EL lighting circuit of the present invention, and FIGS. 3 and 4 are a circuit diagram and a timing chart of another EL lighting circuit of the present invention, and FIG. FIG. 6 shows a conventional EL lighting circuit. The reference numerals of the main parts in FIGS. 1 to 6 are 10 ... EL switching circuit, 11 ... Timing control circuit, 12 ... Oscillation circuit, 13 ... DC-DC converter, L1 ... Inductor. (Eg choke coil), T1, T2 ... Transformer, Q11, Q12, Q2, Q3, Q4 ... Switching elements.

Claims (2)

(57) [Claims] 1. A series consisting of a DC power supply, first and second switches connected in series to the DC power supply, and an EL and an inductor connected as a load between the connection point of both switches and the DC power supply. A control circuit that outputs a signal that controls each of the first and second switches; and an oscillator circuit that generates a signal of a frequency driving EL and outputs the signal to the control circuit. In an EL lighting circuit that charges and discharges EL by switching the first and second switches,
The inductance of the inductor is set so that the resonance frequency of the series circuit composed of EL and the inductor is higher than the drive frequency of EL, and the first switch is closed.
Charging EL and then discharging through a first diode connected in parallel outside or inside the first switch,
Next, the first switch is opened and then the second switch is closed to charge EL with a reverse polarity, and subsequently through a second diode connected in parallel outside or inside the second switch. An EL lighting circuit characterized by discharging and performing the above operations at a constant frequency. 2. The DC power supply is a ringing choke type DC-DC.
The EL lighting circuit according to claim 1, which is a constant power output type DC-DC converter that outputs a constant power by detecting a current flowing through the inductor of the converter and controlling the peak value of the current to be constant.