The present invention claims priority from Japanese Patent Application No.9-137180 filed May 27, 1997, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a device for driving a cold cathod fluorescent lamp (CCFL) used as a back light of a liquid crystal display.
2. Description of Related Art
A piezoelectric transformer which utilizes piezoelectric effect has been known as a device for generating a high voltage for lightening a discharge tube such as a cold-cathode tube. Japanese Patent Application Laid-open No. Hei 8-107678 discloses an example of such driver for driving a cold-cathode tube utilizing a piezoelectric transformer. A construction of the disclosed driver is shown in FIG. 1.
In FIG. 1, a drive circuit 19 is connected to a primary side of a piezoelectric transformer 110 and a signal having a frequency close to a resonance frequency of the piezoelectric transformer 110 and generated by a frequency sweeping oscillator 113 is supplied to the drive circuit 19. In the drive circuit 19, a D.C. voltage supplied from a power source 11 is converted into an A.C. voltage having sinusoidal waveform with which the piezoelectric transformer 110 is driven. A secondary side of the piezoelectric transformer 110 is connected to one of terminals of the cold-cathode tube 111. The other terminal of the cold-cathode tube 111 is connected to a load current comparator circuit 112 and a current flowing from the piezoelectric transformer 110 through the cold-cathode tube 111 is input to the load current comparator circuit 112. In the load current comparator circuit 112, a current-voltage conversion is performed and a resultant voltage is compared with a reference voltage V.sub.refA corresponding to a desired load current value. An output of the load current comparator circuit 112 is supplied to the frequency sweeping oscillator 113 and a sweeping direction of driving frequency of the piezoelectric transformer 110 is determined by the result of comparison.
The piezoelectric transformer 110 has a boosting characteristics in which the boosting ratio becomes maximum at the resonance frequency thereof and rapidly reduced in a lower and higher frequency range with respect to the resonance frequency. The output frequency of the frequency sweeping oscillator 113 is changed toward the high frequency side when the current value of the cold-cathode tube 111 reaches a desired value to lower the boosting ratio of the piezoelectric transformer 110 to thereby reduce the current supplied to the CCFL 111, by utilizing this characteristics of the piezoelectric transformer. When the load current is smaller than the desired value, the output frequency of the frequency sweeping oscillator 113 is changed toward the low frequency side to increase the value of current supplied to the cold-cathode tube. Therefore, the frequency sweeping oscillator 113 is controlled such that it outputs a frequency in a range with which the desired load current is generated by the piezoelectric transformer 110.
By using the construction disclosed in Japanese Patent Application Laid-open No. Hei 8-107678, an inverter capable of flowing a constant A.C. current through the cold-cathode tube can be realized.
In the construction disclosed in Japanese Patent Application Laid open No. Hei 8-107678, however, there are some technical problems when a cold-cathode tube is lit as a load.
A first problem is that it is necessary to use a power source having large current capacity. That is, when a control is performed such that the value of current flowing through the cold-cathode tube is kept constant, a D.C. current I.sub.DD flowing from the power source through the drive circuit of the piezoelectric transformer increases rapidly up to a peak value within several minutes immediately after the cold-cathode tube is lit, decreases gradually thereafter and becomes constant, as shown in FIG. 2. This characteristics is caused by a temperature characteristics of the cold-cathode tube. That is, the voltage of the cold-cathode tube tends to increase when temperature of the cold-cathode tube is low in such a time immediately after the cold-cathode tube is lit. When the cold-cathode tube is lit continuously for a while, its temperature is increased by self-heat generation and then becomes in an equilibrium state at a constant temperature. In this state, when the drive circuit performs a control such that a constant current is flown through the cold-cathode tube, a power consumption of the tube increases immediately after the tube is lit. Therefore, the constant D.C. current supplied from the power source to the drive circuit is increased. Similarly, when ambient temperature is low, the tube voltage becomes high. Therefore, a current required in the drive circuit is also increased when compared with a case of a normal temperature. For these reasons, the current capacity of the power source of the drive circuit must have a margin large enough to supply the peak current immediately after the cold-cathode tube is lit and large current at practically minimum ambient temperature of the cold-cathode tube, resulting in an increase of cost of the power source.
A second problem is that it is impossible to easily set the maximum current of the power source. The reason for this is that, since an increase of power is caused by the temperature characteristics of the cold-cathode tube, it is necessary to know the power increase for every kind of cold-cathode tube and it is impossible to calculate the maximum output current value of the power source without evaluation of the temperature characteristics of the cold-cathode tube.
A third problem is that, when the cold-cathode tube is driven by using the piezoelectric transformer, it is impossible to use an over current protection circuit for limiting an output current by performing a pulse width modulation (PWM) at a drive frequency, which is well known system for limiting an output current. That is, when the circuit current of the cold-cathode tube by using such over-current protection circuit, luminance of the cold-cathode tube becomes unstable.
The over-current protection circuit for limiting the output current by using the pulse width modulation will be described. An example of the over-current protection circuit of this kind is disclosed in Japanese Patent Application Laid-open No. Sho 63-35171. A construction of the over-current protection circuit disclosed therein is shown in FIG. 3. In FIG. 3, a D.C. power source V.sub.IN is connected to one terminal of a primary side of a boosting electromagnetic transformer T.sub.1 and a switching element Q.sub.1 is connected to the other terminal of the electromagnetic transformer T.sub.1. A resistor R.sub.2 is connected to a source of the switching element Q.sub.1 to detect an over-current and the source is connected to an oscillator circuit OSC and a pulse width modulator circuit PWM through a resistor R.sub.1. An output of the pulse width modulation circuit PWM is supplied through an amplifier AMP to a gate of the switching element Q.sub.1 to form a feedback loop A. An output of the oscillator circuit OSC is supplied to the pulse width modulator circuit PWM to form a feedback loop B. A capacitor C.sub.1 for removing spike noise current caused by a switching operation of the switching element Q.sub.1 is connected to the resistor R.sub.1. A circuit composed of a rectifying diode D.sub.1, a fly-wheel diode D.sub.2, a smoothing inductor L.sub.1, a smoothing capacitor C.sub.2 and a load L.sub.o is connected to a secondary side of the electromagnetic transformer, as shown.
When an output current I.sub.o flowing from the electromagnetic transformer T.sub.1 through the load L.sub.o becomes a predetermined value or larger, a current i flowing through the overcurrent detecting resistor R.sub.2 increases proportionally to the current of the load. A voltage drop iR.sub.2 across the resistor R.sub.2 due to the current i is fedback to the pulse width modulator circuit PWM to shorten an on period of the switching element Q.sub.1 when the current i becomes larger than a reference value. Further, an over-current detection signal is fedback to the oscillator circuit OSC. In this manner, it is possible to limit the current supplied from the electromagnetic transformer T.sub.1 to the load L.sub.o.
Another example of the over-current protection circuit is disclosed in Japanese Patent Application Laid-open No. Hei 6-311734, a construction of which is shown in FIG. 4. In FIG. 4, a MOS-FET Q.sub.2 is connected between an input terminal V.sub.i and an output terminal V.sub.out and a rectifying/smoothing circuit composed of a diode D.sub.a a coil L.sub.a and a capacitor C.sub.a is connected between the MOS-FET Q.sub.2 and the output terminal V.sub.out. A series circuit of a resistor R.sub.c and a Zener diode ZD is connected between an electrode of the MOS-FET Q.sub.2 on the side of the input terminal V.sub.i and a common potential point and a detector portion composed of a sync switch SW and voltage dividing resistors R.sub.a and R.sub.b is connected between an electrode of the MOS-FET Q.sub.2 on the side of the output terminal V.sub.out and the common potential point. A comparator CMP is provided for comparing a potential of a junction point between a resistor R.sub.c and the Zener diode ZD with a potential of a junction between the voltage dividing resistors R.sub.a and R.sub.b and an output of the comparator is fedback through a pulse width control circuit PWMC and a drive circuit DRV to the MOS-FET Q.sub.2. A saturation voltage when the MOS FET Q.sub.2 is on is proportional to a current flowing through the MOS-FET Q.sub.2 due to the presence of an onresistance of the switching element Q.sub.1. When an over-current flows in such case that the output is short circuited in a state where the MOS-FET Q.sub.2 is on, a drain current is detected as a voltage drop V.sub.ds due to the on-resistance of the MOS-FET Q.sub.2. That is, a voltage divided by the voltage dividing resistors R.sub.a and R.sub.b is compared with the reference voltage given by the Zener diode ZD by the comparator CMP and the comparison result output thereof is input to a time ratio control terminal of the PWM control circuit. When the voltage obtained by the voltage dividing resistors R.sub.a and R.sub.b exceeds the reference voltage, the over-current protection is performed by shortening the on-time of given by the Zener diode ZD by the MOS-FET Q.sub.2.
In each of the above mentioned two examples of the over-current protection circuit, the switching time for which the current is supplied to the electromagnetic transformer having a voltage boosting function or the coil at their driving frequency is controlled by the pulse width modulator PWM to limit current input to the electromagnetic transformer or the coil. However, these methods can not be applied to the drive circuit of the cold-cathode tube using the piezoelectric transformer. The reason for this will be described below.
In the previously mentioned construction disclosed in Japanese Patent Application Laid-open No. Hei 8-107678, the boosting ratio of the piezoelectric transformer 110 is changed by controlling the drive frequency of the piezoelectric transformer 110 such that the current supplied to the cold-cathode tube 111 becomes constant. Since the tube voltage of the cold-cathode tube 111 is not controlled, it is impossible, when the tube voltage is changed by the temperature characteristics of the cold-cathode tube, to avoid the increase of the power consumed by the tube, as mentioned previously.
Further, since the boosting capability of the piezoelectric transformer 110 is effective in only the vicinity of the resonance frequency thereof and has no such wide transmission frequency band as that of the electromagnetic transformer, the piezoelectric transformer 110 must be driven by a signal having sinusoidal waveform or other waveforms close to the sinusoidal waveform, otherwise the efficiency of the piezoelectric transformer is lowered. Assuming a case where a method for controlling the value of current supplied from the power source 11 within a predetermined value by driving the piezoelectric transformer 110 with a pulse width modulated waveform while sacrificing the efficiency of the piezoelectric transformer is employed, it becomes impossible to supply a predetermined tube current to the cold-cathode tube 111 since the boosting ratio is made variable by controlling the drive frequency of the piezoelectric transformer 110 as mentioned previously. Therefore, the frequency sweeping oscillator can not be locked to the resonance frequency of the piezoelectric transformer 110 and continues to sweep through the oscillation frequency range, so that the cold-cathode tube can not be lit stably. Thus, there may be a sudden change of luminance of the cold-cathode tube and the latter operates unsuitably as a light source.
That is, when the cold-cathode tube is used as a back light source of the liquid crystal display, the operation of the cold-cathode tube by which the light source becomes unstable is not allowed and it is necessary to maintain a stable amount of light even if the increase of current consumed is allowed. Therefore, it is impossible to limit the output current by using the PWM control at the drive frequency as in the case of the electromagnetic transformer.