Driving circuit of a piezo-ceramic transformer capable of controlling an input voltage and a dimming control method thereof

[0002] This Application is a Continuation-in-Part of 09/268,983 filed Mar. 16, 1999 now abandoned.

[0003] 1. Field of the Invention

[0004] 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.

[0005] 2. Description of the Prior Art

[0006] 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.

[0007] 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.

[0008] 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.

[0009] That is, general types of the PCT being used practically are as follows:

[0010] 1. a buck or boost DC—DC converter+a push-pull type inverter,

[0011] 2. a DC—DC converter+a single ended quasi resonant type inverter,

[0012] 3. a DC—DC converter+a half bridge type inverter,

[0013] 4. a DC—DC converter+a flyback single ended quasi resonant type inverter.

[0014] 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.

[0015] FIG. 7 is a circuit diagram of a conventional a half bridge type inverter with a piezo-ceramic transformer.

[0016] 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 Q1 and Q2 for driving a PCT, as a switching device. In the circuit, the duty ratio of the transistor Q1 and Q2 are continuously controlled so that a constant voltage is applied to point C′ at the input terminal of the PCT.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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).

[0023] 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.

[0024] FIG. 1 is a block diagram according to the present invention;

[0025] FIG. 2 is a circuit diagram of the PCT driving circuit in FIG. 1;

[0026] FIG. 3 is a waveform at point of B according to the input voltage in FIG. 1;

[0027] FIG. 4 is an outline view for expanding the waveform in FIG. 3 by Fourier Series;

[0028] FIG. 5 is a characteristic curve according to an input and output boosting ratio to the frequency, temperature of a PCT;

[0029] FIG. 6 is a temperature characteristic of the VCO; and

[0030] FIG. 7 is a prior art circuit diagram for driving a piezo-ceramic transformer.

[0031] The above and other objects, features and other advantage of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. In the drawings the same reference numerals depict the same parts.

[0032] FIG. 1 is a block diagram according to the present invention.

[0033] As shown in FIG. 1, the driving circuit includes a voltage doubler rectifier 1 for rectifying an output voltage, a summing & error amplifier 2 for producing an output signal by summing a dimming voltage and an output voltage, a duty ratio varying circuit 3 for dimming and duty ratio control, and a phase detector 5 for detecting a relative phase in the input and output signal of the PCT.

[0034] The driving circuit also includes a temperature compensated Voltage Controlled Oscillator (VCO) 6 for providing the driving frequency from an output signal of the phase detector 5, a saw-tooth wave generator 7, a pulse width modulator (PWM) 8 for varying the duty ratio according to the output of the duty ratio varying circuit 3 and the saw-tooth wave generator 7. A charge pump circuit 9, a piezo-ceramic transformer 11, and a lamp 12.

[0035] Hereinafter, the PCT driving circuit will be described in detail with reference to FIG. 2.

[0036] The PCT driving circuit comprises a charge pump circuit, three transistors, Q1, Q2 and Q3 and an LC filter. Although the basic construction of FIG. 2 is similar to that of FIG. 7, the ON/OFF control method of transistors Q2 and Q3 are different. Furthermore, in FIG. 7 a driving transformer must be used, whereas the driving transformer must be used, whereas the driving transformer has been eliminated in FIG. 2.

[0037] Also, in FIG. 7, a constant voltage must be provided at point A′ by controlling the input voltage. However, according to the construction of the control circuit of the present invention, although a variable input voltage at point A is provided, a constant voltage is always provided at point C.

[0038] In FIG. 2, the ON/OFF operation of transistors Q2 and Q3 will be described in detail. When transistor Q2 is turned ON at each one period, the transistor Q3 is necessarily turned ON at another one period, so that the duty ratio of the transistor Q2 and Q3 due to the variation of the input voltage are different. Furthermore, in this circuit, when transistors Q1 and Q3 are ON, capacitor C1 is charged to a voltage (V_R-V_D) through diode D. When transistors Q1 and Q3 are turned OFF, the gate voltage of transistor Q2 is pulled to the above voltage, and transistor Q2 is turned ON. Since the input impedance of transistor Q2 is very high, the charged value across C1 is maintained long enough to turn transistor Q2 completely ON. Also, capacitor C1 is selected to be large enough to sustain this charge. In general, when the transistor Q2 is improperly driven or the duty ratio is very narrow, the zero-voltage switching is lost and there is an increased switching loss of transistor Q2. The charge pump circuit maintains the zero-voltage switching and reduces the switching loss of transistor Q2 even though the duty ratio is narrower than 5% in combination with the phase-locked loop operation.

[0039] The waveform at point B is shown in FIG. 3 and FIG. 4.

[0040] The circuit is controlled so that when the input voltage is high, the duty ratio of transistor Q2 is small, and when the input voltage is low, the duty ratio of transistor Q2 is large. Accordingly, the following theory is inferred.

[0041] The waveform of FIG. 3 can be expanded as a Fourier series by referring to FIG. 4, as follows. $\begin{array}{cc}\begin{array}{c}𝔉\left[f\left(t\right)\right]=\frac{1}{2}{a}_0+\sum _{k=0}^{\infty }\left(a_k\cos k{\omega }_{0}t+b_k\sin k{\omega }_{0}t\right)\left({\omega }_0=\frac{2\pi }{T}\right)\\ a_k=\frac{2}{T}{\int }_{-\frac{T}{2}}^{\frac{T}{2}}f\left(t\right)\cos k{\omega }_{0}tdt\\ =\frac{2}{T}{\int }_{-\frac{T}{2}}^{\frac{T}{2}}V\cos k{\omega }_{0}tdt\\ ={\frac{2}{T}\left[\frac{V}{k{\omega }_O}\sin k{\omega }_{o}t\right]}_{-\frac{T_o}{2}}^{\frac{T_o}{2}}\\ =\frac{4V}{\mathrm{Tk}{\omega }_0}\sin k{\omega }_0\frac{T_0}{2}\\ =\frac{2V}{k\pi }\sin k\frac{T_0}{T}\pi \left(\sin \mathrm{cx}=\frac{\sin \pi x}{\pi x}\right)\\ =\frac{2V}{k\pi }\frac{\frac{T_0}{T}}{\frac{T_0}{T}}\sin k\frac{T_0}{T}\pi \\ =2V\frac{T_0}{T}\sin \mathrm{cn}\frac{T_0}{T}\pi \\ b_k=0\left[\because f\left(t\right)\mathrm{is}\mathrm{an}\mathrm{even}\mathrm{function}\right]\end{array}& \left(1\right)\end{array}$

[0042] In formula (1), $\begin{array}{cc}\begin{array}{c}\begin{array}{c}\begin{array}{c}{a}_0=\frac{2V}{T}{T}_0\\ a_1=\frac{2V}{\pi }\sin \frac{T_0}{T}\pi \end{array}\\ a_2=\frac{2V}{2\pi }\sin 2\frac{T_0}{T}\pi \end{array}\\ a_k=\frac{2V}{k\pi }\sin k\frac{T_0}{T}\pi \end{array}& \left(2\right)\end{array}$

[0043] The input voltage V is changed from Vmin to Vmax. When the input voltage is changed to Vmin, the duty ratio is controlled to be Dmin(Dmin: duty ratio at input voltage Vmin which is max. 50%), and when the input voltage is changed Vmax, the duty ratio is controlled in order to be the same magnitude of the first harmonic component i.e. a1 (min)=a1 (max), regardless of the input voltage.

[0044] That is, when the input voltage varies between Vmin and Vmax, a first harmonic component having a constant amplitude can always be obtained, so that input line regulation is possible. That is expanded as the following formula (2). $a_1\left(\min \right)=\frac{2V_{\min }}{\pi }\sin \mathrm{Dmin}\pi \left(\mathrm{Dmin}\text{:}\mathrm{Duty}\mathrm{ratio},\mathrm{when}V=\mathrm{Vmin}\right)$ $a_1\left(\max \right)=\frac{2V_{\max }}{\pi }\sin \mathrm{Dmax}\pi \left(\mathrm{Dmax}\text{:}\mathrm{Duty}\mathrm{ratio},\mathrm{when}V=\mathrm{Vmax}\right)$

[0045] The equation may be generally represented as $a_{1x}=\frac{2V_x}{\pi }\sin D_x\pi$

[0046] Here, Dx at α₁(min)=α_1xis $\begin{array}{cc}\frac{2V_{\min }}{\pi }\sin \mathrm{Dmin}\pi =\frac{2V_x}{\pi }\sin D_x\pi \therefore D_x=\left[{\sin }^{-1}\frac{V_{\min }}{V_x}\sin \mathrm{Dmin}\pi \right]/\pi & \left(3\right)\end{array}$

[0047] Equation (3) above is a general formula for computing a duty ratio in which the amplitude is the same amplitude of the first harmonic frequency component obtained by the voltage of Vmin and Dmin at an input voltage Vx.

[0048] Table 1 below is the duty ratio at several input voltages.

[0049] In FIG. 2, a duty ratio of the PWM modulator 8 is controlled so that the waveform at point B is 8V: 50%, 10V: 29.5%, . . . 20V: 13% and when the first harmonic frequency component is passed by a LC filter 10, a sine wave is always obtained as al(x)=3.6Vrms at point C.

[0050] According to the present invention, without the use of a DC—DC converter, a single stage DC-AC inverter can be designed capable of controlling a wide range input line regulation.

[0051] A dimming method in accordance with the present invention will now be described.

[0052] As mentioned above, in the circuit of the present invention, the duty ratio is controlled to obtain a maximum output current due to the variation of the input voltage. First, the duty ratio Dx is determined to obtain the maximum output current at a voltage Vx and then the value of Dx is reduced for dimming to the desired brightness.

[0053] In FIG. 1, the voltage doubler rectifier 1 and the summing & error amplifier 2 are designed so that the Dx of the PWM modulator 8 is reduced by the signal of the duty ratio varying circuit 3.

[0054] In FIG. 5 fr is the resonant frequency of the PCT, and f is the driving frequency by the VCO's oscillating frequency. If the PCT is operated at the driving frequency by the VCO, which is slightly low compared to the resonant frequency(fr) of the PCT by the temperature characteristic of the VCO and PCT, impact, etc., in the neighborhood, the efficiency of the PCT is deteriorated. So, the driving frequency by the VCO must always be a little higher than the resonant frequency of the PCT.

[0055] For this reason, in FIG. 1, a phase detector 4 is provided for comparing the input and output signals of the PCT. Although the VCO is controlled by the output of phase detector, if the temperature characteristic of VCO and PCT is not substantially the same, the PCT is not operated in the driving frequency by the VCO to be able to get the maximum efficiency. To solve this problem, the driving circuit in accordance with the present invention includes a temperature compensated VCO 6, a phase detector 5, and a low-pass filter 4. This circuit is operated in phase locked loop to tract and search a driving frequency by the VCO to be able to realize a maximum efficiency. As a result, although the resonant frequency(fr) of the PCT and driving frequency(f) by the VCO change due to variations of temperature, impact, etc., in the neighborhood, the PCT is always operated by a driving frequency that is greater than the resonant frequency(fr).

[0056] Whereas this invention is here illustrated and described with reference to an embodiment thereof presently contemplated as the best mode of carrying out such invention in actual practice, it is to be understood that various changes may be made in adapting the invention to different embodiments without departing from the broader inventive concepts disclosed herein and comprehended by the claims that follow.