Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2000-03-16
2001-02-06
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S2090PZ, C315S291000, C310S318000, C310S311000
Reexamination Certificate
active
06184631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric inverter for driving a load using a piezoelectric transformer, and, more particularly, to a piezoelectric inverter that is preferably used as a lighting circuit for a discharge tube, such as a cold-cathode tube for use in a liquid-crystal backlight.
2. Description of the Related Art
Small cold-cathode tubes are conventionally used as an backlight illumination source for a liquid-crystal display apparatus. To drive a cold-cathode tube, a piezoelectric transformer, rather than a magnetic transformer, is used because of the compact design and low cost thereof.
Japanese Unexamined Patent Publication No. 7-220888 discloses a driver of a backlight cold-cathode tube employing a piezoelectric transformer. According to this disclosure, a chopper circuit is connected between a direct-current power source and an inverter driving the piezoelectric transformer. The piezoelectric transformer is connected to the cold-cathode tube, and a current flowing through the cold-cathode tube is detected by a tube current detector circuit. The luminance of the cold-cathode tube is maintained constant by controlling a duty factor of the chopper circuit to maintain the tube current constant.
Japanese Unexamined Patent Publication No. 9-107684 discloses a piezoelectric transformer drive circuit which controls a tube current to a desired value by making use of frequency-versus-gain characteristics of the piezoelectric transformer. Connected between an input terminal and the piezoelectric transformer are a drive voltage control circuit having no parts for rectifying and smoothing, and a voltage multiplication circuit. The drive voltage control circuit maintains constant a mean input voltage applied to the voltage multiplication circuit. A cold-cathode tube is connected to the piezoelectric transformer. Also provided is a frequency control circuit which detects a current flowing through the cold-cathode tube, and controls the tube current to a desired value taking advantage of the frequency-versus-gain characteristics of the piezoelectric transformer.
When the input voltage to the voltage multiplication circuit increases with no drive voltage control circuit employed in the control method using the frequency-versus-gain characteristics of the piezoelectric transformer, a drive voltage frequency of the piezoelectric transformer shifts to a high frequency side where the voltage multiplication ratio or gain of the piezoelectric transformer is small, thereby cancelling the increase in the input voltage. The conversion efficiency of the piezoelectric transformer drops in a frequency region where the voltage multiplication ratio is small. In this conventional art, the drive voltage control circuit maintains constant the mean voltage to the voltage multiplication circuit, thereby maintaining the drive voltage frequency of the piezoelectric transformer to a frequency at which efficiency is high. It is therefore believed that the conventional art keeps a relatively high efficiency within a wide input voltage range.
In the conventional art disclosed in Japanese Unexamined Patent Publication No. 7-220888, the output of the chopper circuit is a direct current, and the chopper circuit is thought to be a DC-DC converter. To construct the chopper circuit of a DC-DC converter, inductors and capacitors, for rectifying and smoothing, are required. The component count of the circuit increases, and loss attributed thereto is also increased.
The piezoelectric transformer drive circuit, disclosed in Japanese Unexamined Patent Publication No. 9-107684, needs no rectifier circuit, thereby avoiding the loss attributed thereto.
The conventional art, disclosed in Japanese unexamined Patent Publication No. 9-107684, however, needs two types of feedback control: 1) frequency control for maintaining constant the tube current through a frequency control circuit, and 2) pulse width duty factor control through the drive voltage control circuit for maintaining constant the voltage input to the voltage multiplication circuit. The control circuit therefore becomes complicated, increasing costs involved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a piezoelectric inverter which is low cost, has a simplified control circuit, free from the above problem, needs no rectifying and smoothing circuits, and reliably drives a load using a piezoelectric transformer.
According to one broad aspect of the present invention, a piezoelectric inverter for driving a load using a piezoelectric transformer, includes an input voltage controller, having a switching transistor and a current circulating element, for converting a direct-current input voltage into a rectangular alternating-current voltage, a piezoelectric transformer driver, connected between the input voltage controller and the piezoelectric transformer, and including an inductive element, for outputting, to the piezoelectric transformer, an alternating-current voltage having a substantially constant frequency that is lower than the frequency of an output alternating-current voltage of the input voltage controller, a first oscillator for determining an operating frequency of the input voltage controller, a second oscillator for determining an operating frequency of the piezoelectric transformer driver, the piezoelectric transformer having an input electrode and an output electrode with the input electrode thereof connected to the piezoelectric transformer driver and the output electrode thereof connected to the load, a load current detector, connected to the load, for detecting a load current, and a duty factor controller, connected to the load current detector, for controlling a duty factor of a rectangular pulse of the input voltage controller in response to the output of the load current detector so that the load current is maintained to a substantially constant target current value, wherein an oscillation frequency of the second oscillator is not higher than a frequency at which the voltage multiplication ratio of the piezoelectric transformer becomes maximized with no load applied to the output of the piezoelectric transformer, and the oscillation frequency of the second oscillator is not lower than a frequency at which the voltage multiplication ratio of the piezoelectric transformer becomes maximized with the piezoelectric transformer driving the load connected to the output thereof.
Preferably, the second oscillator includes a frequency divider that frequency-divides the frequency of the first oscillator, and a signal into which the frequency of the first oscillator is divided is the output of the second oscillator, and a single oscillator is shared by the first oscillator and the second oscillator.
Preferably, a piezoelectric inverter of the present invention further includes a temperature-compensating circuit which controls the temperature dependence of required mean output voltage of the input voltage controller, thereby compensating for the dependence of the oscillation frequency of the second oscillator on ambient temperature.
The temperature-compensating circuit preferably includes one of a thermistor or a temperature-compensating capacitor.
The target current value is preferably changed in response to an externally applied, first dimmer signal.
Preferably, a piezoelectric inverter of the present invention further includes a variable oscillation-frequency circuit that varies the oscillation frequency of one of the first and second oscillators in response to the first dimmer signal without using feedback control. The oscillation frequency of the second oscillator may be varied by varying the output frequency of the first oscillator, and then by frequency-dividing the output frequency of the first oscillator. Preferably, a piezoelectric inverter of the present invention further includes a load drive time controller which varies an on time ratio of the load in response to an externally applied, second dimmer signal by intermittently switching
Morishima Yasuyuki
Noma Takashi
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Philogene Haissa
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