Inverter with adjustable resonance gain

The present invention includes a PWM unit, a switch unit, a resonance unit, a transformer, a feedback unit and a frequency control unit, wherein the switch unit obtains a DC power from a power source, the PWM unit produces a working cycle signal to drive the switch unit to convert the DC power into a pulse power and the resonance unit converts the pulse power into a driving power for providing to the transformer to convert thereof into an output power, characterized in that when the resonance unit is under a starting frequency and a working frequency higher than the starting voltage, a starting voltage gain and a working voltage gain respectively corresponding thereto are produced, wherein the starting voltage gain is larger than the working voltage gain, so that the larger starting voltage gain can produce the output power with higher voltage to smoothly initiate the lamp tube set.

FIELD OF THE INVENTION

The present invention is related to an inverter with adjustable resonance gain, and more particularly to an inverter applied to drive lamp tube which utilizes resonance change to adjust driving voltage.

BACKGROUND OF THE INVENTION

The main elements for LCD (Liquid Crystal Display) are polarizer and backlight module. The backlight module produces uniform light, and then, the light is polarized by polarizers with different colors so as to generate multi-color picture. For generating uniform light, the backlight module has to equip with multiple long life lamp tubes and an inverter for providing power thereto, wherein the brightness of the lamp tube can be altered through adjusting the magnitude of the output power from the inverter. The conventional inverter, as shown inFIG. 1, includes a power source1, a dimming signal source6, a PWM (Pulse Width Modulation) unit3, a switch unit2, a resonance unit4, a transformer5and a feedback unit7. The power source1provides DC power, the PWM unit3produces a working cycle signal to drive the switch unit2, and the DC power is switched by the switch unit2to form a pulse power. The resonance unit4obtains the pulse power and converts thereof into a driving power through resonance, and then, the transformer5converts the driving power into an output power to drive a lamp tube set9. The dimming signal source6produces a dimming signal to the PWM unit3for adjusting the duty cycle of the working cycle signal so as to control the magnitude of the output power. The feedback unit7draws a feedback signal from the secondary side of the transformer5and transmits the feedback signal to the PWM unit3for achieving feedback regulation. In the conventional inverter, the duty cycle of the working cycle signal is utilized to control the magnitude of the output power. Mostly, the resonance circuit used to drive the inverter of the backlight module is an LC resonance circuit. The equivalent circuit of basic LC resonance circuit is shown inFIG. 2A, wherein Rlamprepresents equivalent resistor of a lamp tube, Vd(t) represents input voltage, Ls represents resonance inductor, Cp represents resonance capacitor and the input voltage is a pulse power of constant voltage, and Vλrepresents potential difference of lamp. The input voltage and the output voltage are calculated as followed:

After transposition, it obtains

For simplifying the formula described above, it further defines that:

wherein Q is defined as serial resonance quality factor, and

wherein ZOis the property impedance of the resonance circuit.

Through formulas (1-3) and (1-4), formula (1-2) can be simplified as followed:

Please refer toFIG. 2B, which shows the gain curve of transfer function of LC resonance circuit. The conventional LC resonance circuit is operated under fixed frequency so as to form fixed voltage gain. Through the PWM unit3, the duty cycle of the working cycle signal can be altered for adjusting the brightness of the lamp tube set9, as disclosed in R.O.C. Patent No. 1290707, entitled “Parallel driving circuit of multiple lamps in LCD and a uniformity control method thereof”. InFIG. 3of this patent, a resonance inductor (Lr) and a resonance capacitor (Cr) are disposed at the primary side of transformer T1. Through the sine wave produced by the resonance inductor (Lr) and the resonance capacitor (Cr), the transformer T1can convert the power. However, the inverter described above which controls the brightness through altering the working cycle signal is disadvantageous that:

1. The adjustment of the duty cycle of the working cycle signal will cause the switch unit of the inverter to have an unstable zero voltage switching, so as to cause extra loss.

2. The adjusting range of the duty cycle is limited since the elements of the switch unit have limited voltage withstand.

Therefore, there is the need to improve the inverter used for driving backlight module.

SUMMARY OF THE INVENTION

Consequently, the object of the present invention is to provide an inverter with improved resonance, thereby enlarging the range of dimming and zero voltage switching.

The present invention is to provide an inverter with adjustable resonance gain including a PWM unit, a switch unit, a resonance unit, a transformer, a feedback unit and a frequency control unit, wherein the switch unit obtains a DC power from a power source, the PWM unit produces a working cycle signal to drive the switch unit to convert the DC power into a pulse power and the resonance unit converts the pulse power into a driving power for providing to the transformer to convert thereof into an output power, so as to drive a lamp tube set linked to the inverter, characterized in that when the resonance unit is under a starting frequency and a working frequency higher than the starting voltage, a starting voltage gain and a working voltage gain respectively corresponding thereto are produced, wherein the starting voltage gain is larger than the working voltage gain, so that the larger starting voltage gain can produce the output power with higher voltage so as to smoothly initiate the lamp tube set, and thus, through controlling the frequency of the pulse power, the voltage gain of the resonance unit can be altered.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Please refer toFIG. 3, which is a schematic view showing an inverter with adjustable resonance gain. The inverter includes a PWM (Pulse Width Modulation) unit3, a switch unit2, a resonance unit4, a transformer5, a feedback unit7and a frequency control unit8. The PWM unit3produces a working cycle signal to drive the switch unit2, and through the driving by the working cycle signal, a DC power obtained from a power, source1by the switch unit2is converted into a pulse power for outputting to the resonance unit4. The resonance unit4converts the pulse power into a driving power, and then, the transformer5converts the driving power into an output power for driving a lamp tube set9. Moreover, the frequency control unit8is connected to a dimming signal source6and the feedback unit7, wherein the feedback unit7draws a feedback signal from the secondary side of the transformer5, and the dimming signal source6provides a dimming signal. Then, the frequency control unit8, in accordance with the dimming signal and the feedback signal, produces a reference frequency signal for providing to the PWM unit3, so that the PWM unit3can adjust the frequency of the working cycle signal according to the reference frequency signal, thereby adjusting the frequency of the pulse power. The resonance unit4has a resonant property, so that the frequency of the pulse power can influence the resonance unit4to convert the voltage gain of the driving power. When the pulse power is under a starting frequency and a working frequency higher than the starting frequency, the resonance unit4may form a starting voltage gain and a working voltage gain respectively corresponding to the starting frequency and the working frequency, wherein the starting voltage gain is larger than the working voltage gain, so that it can provide higher voltage to initiate the lamp tube set9. The circuit for achieving this property is shown as the resonance unit3inFIG. 3. The resonance unit4includes a first resonance inductor (Lr)42, a second resonance inductor (Lm)43and a resonance capacitor (Cs)41, wherein the first resonance inductor42and the resonance capacitor41are serially connected with the primary side coil of the transformer5, and the second resonance inductor43is connected to the two ends of the primary side coil, so that the first resonance inductor42and the second resonance inductor43are connected in parallel as view from the primary side coil. The transfer function of the resonance unit4described above is as followed:

which is the specific value of the first resonance inductor (Lr)42and the second resonance inductor (Lm)43, wherein
L=Lr+Lm(2-2)

which is the serial equivalent inductors of the resonance unit4,

which is the resonance frequency,

which is the property impedance of the resonance unit4, and

which is the serial resonance quality factor. Therefore, the transfer function of the resonance unit4is

And, the gain of formula 2-6 is

When the inverter wants to initiate the lamp tube set9, the lamp tube set9has a higher equivalent resistor (Rlamp) since it is identical to an open circuit, and since the inverter does not start to work, there is no current passing through the resonance unit4, and thus, at this time, the PWM unit3can produce a working cycle signal at the preset starting frequency, such that the resonance unit4can be initiated at the starting frequency, so as to produce the starting voltage gain corresponding to the starting frequency. Therefore, the resonance unit4can have a higher voltage gain for smoothly initiating the lamp tube set9. Then, after the lamp tube set9is initiated, the current passes through the secondary side of the transformer5, so that the feedback unit7can obtain the feedback signal for transmitting to the PWM unit3, so as to force the PWM unit3to work at the preset working frequency, and then, the resonance unit4can produce a working voltage gain corresponding to the working frequency of the pulse power. As shown inFIG. 4, when the lamp tube set9is initiated, the resonance unit4works at the starting voltage gain P1, and after the lamp tube set9works normally, the resonance unit4works at the working voltage gain P2with higher frequency. Therefore, the resonance unit4can provide different voltage gains as the lamp tube set9is initiated and is normally working, so as to facilitate the initiation of the lamp tube set9, and after the lamp tube set9works normally, the resonance unit4can provide the corresponding voltage gain in accordance with the frequency of the pulse power, thereby achieving the brightness control of the lamp tube set9.FIG. 5shows the test result of the circuit described above, which controls the output power through adjusting the resonance unit4, wherein Pin represents the power inputted to the inverter, and Fs represents the working frequency of the resonance unit4. As shown in Table inFIG. 5, from row1to row6, when the resonance unit4has a lower working frequency, a higher lamp tube voltage (Vlamp) can be produced which facilitates the initiation of the lamp tube set9, and after the lamp tube set9works normally, the equivalent resistor (Rlamp) thereof reduces and the frequency rises to a higher working frequency, and further, owing to the frequency variation, the voltage gain of the resonance unit4reduces so as to lower down the lamp tube voltage (Vlamp). According to the test result shown in FIG.5, it confirms that the circuit architecture of the first resonance inductor42, the second resonance inductor43and the resonance capacitor41used in the resonance unit4can alter the voltage of the output power through controlling the working frequency, thereby achieving a brightness adjustment of the lamp tube set9.

In addition, the inductances of the first resonance inductor42and the second resonance inductor43are different, and the ratio thereof should be lower than 10:1, so as to ensure that the resonance unit4possesses the resonant property described above.