Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-04-21
2001-12-18
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06331748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a driving circuit of a piezo-ceramic transformer and a dimming control method thereof, more particularly, to a driving circuit of a piezo-ceramic transformer capable of controlling an input voltage and a dimming control method thereof without a DC—DC converter.
2. Description of the Prior Art
Technology for making a relatively thin and small notebook computer with a LCD (Liquid Crystal Display) is being developed. It is therefore desirable to develop a DC-AC inverter that is relatively small and has a relatively high efficiency to use as an LCD backlight.
However, a magnetic transformer used with a conventional DC-AC inverter for a LCD backlight has limits for making it small in size and obtaining a high efficiency.
Although an inverter using a PCT (Piezo-Ceramic Transformer) for improving the above conventional problems has been studied for several years, it also has several problems in that the driving circuit is very complicated and another DC—DC converter is needed in order to constantly maintain an applied voltage to the PCT driving circuit because in the application circuit, an input voltage has a wide variable range.
That is, general types of the PCT being used practically are as follows:
1. a buck or boost DC—DC converter+a push-pull type inverter,
2. a DC—DC converter+a single ended quasi resonant type inverter,
3. a DC—DC converter+a half bridge type inverter,
4. a DC—DC converter+a flyback single ended quasi resonant type inverter.
Since the above types are all operated by two stage inverter, a switching device and a control circuit in every singe stage inverter must be used, thereby making the circuit very complicated.
FIG. 7
is a circuit diagram of a conventional a half bridge type inverter with a piezo-ceramic transformer.
As shown in
FIG. 7
, when an input voltage is changed from Vmin to Vmax, a constant voltage is made at the point A′ of the circuit by the DC—DC converter and provided to transistor Q
1
and Q
2
for driving a PCT, as a switching device. In the circuit, the duty ratio of the transistor Q
1
and Q
2
are continuously controlled so that a constant voltage is applied to point C′ at the input terminal of the PCT.
In the case of resonant type inverter using the magnetic transformer, a single stage inverter is adopted to drive the CCFL lamp. A wide range input line regulation may be controlled using frequency modulation. But the method of controlling frequency modulation has a critical problem in that the brightness efficiency of the lamp fluctuates by varying the driving frequency of the magnetic transformer or the PCT. As a result, even thought the electrical efficiency of the lamp is maintained at a constant value, the brightness efficiency of the lamp rapidly fluctuates with the variation of driving frequency. Because the Q(Quality factor) value of the PCT is much higher than the magnetic transformer, the frequency modulation method to drive the PCT results in higher brightness fluctuation than to drive the magnetic transformer.
SUMMARY OF THE INVENTION
The present invention avoids the complexity of conventional control circuits. Although several semiconductor companies use control circuits which are integrated into a single chip as a single stage DC-AC inverter such devices disadvantageously require the use of several switching devices.
To solve the aforementioned problems, an object of the present invention,
FIG. 1
, is to provide a fixed frequency driving circuit of a piezo-ceramic transformer which is capable of wide range input line regulation using a Pulse Width Modulator (PWM) so as to avoid the problems in using the controlling method by frequency modulation described above.
Another object of the present invention is to provide a temperature compensated Voltage Controlled Oscillator (VCO) to adjust the temperature characteristics of VCO and PCT so that they are substantially the same and ensure that the efficiency of the PCT is not deteriorated.
Another object of the invention is to provide a phase detector
5
which compares the input and output signals of the PCT so that the circuit is operated at the driving frequency(f) by the VCO's oscillation frequency, f>fr, although the resonant frequency(fr) of the PCT and driving frequency(f) by the VCO are changed by a temperature, impact, etc., in the neighborhood.
To accomplish the above objects, this invention relates to a fixed frequency driving circuit of a piezo-ceramic transformer including a lamp
12
; a PCT(Piezo-Ceramic transformer)
11
for driving the lamp
12
; a charge pump circuit
9
, a circuit for driving the PCT; a voltage doubler rectifier
1
for rectifying an output voltage; a summing & error amplifying circuit
2
for summing a dimming voltage and the output voltage of the voltage doubler; a phase detector
5
for comparing the phase of the input and output signals of the PCT; a PWM modulator
8
for continuously maintaining a constant amplitude of a first harmonic component when the input voltage is changed from Vmin(Minimum input voltage) to Vmax(Maximum input voltage).
The present invention is also directed to a dimming control method for a driving circuit of a piezo-ceramic transformer by determining a duty ratio Dx at an input voltage Vx for producing the maximum output current from the input voltage Vx, and providing a current having a desired brightness by reducing the value of Dx for dimming.
REFERENCES:
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patent: 5739622 (1998-04-01), Zaitsu
patent: 5739679 (1998-04-01), Takehara et al.
patent: 5866969 (1999-02-01), Shimada et al.
patent: 5886477 (1999-03-01), Honbo et al.
patent: 5886514 (1999-03-01), Iguchi et al.
patent: 5923542 (1999-07-01), Sasaki et al.
patent: 5942835 (1999-08-01), Furuhashi et al.
patent: 6016052 (2000-01-01), Yaughn
patent: 61-152165 (1986-07-01), None
patent: 10-75575 (1998-03-01), None
Darby & Darby
Dongil Technology Ltd.
Dougherty Thomas M.
LandOfFree
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