TWI394488B - A control method of a cold cathode lamp converter and a commutation device using the method - Google Patents

Description

Control method of cold cathode lamp tube converter and commutation device using same

The invention relates to a control method of an inverter, in particular to a control method of a cold cathode lamp inverter.

Nowadays, the backlight of the liquid crystal display is roughly divided into a Cold Cathod Fluorescent Lamp (CCFL) and a Light Emitting Diode (LED). The liquid crystal display whose backlight is a cold cathode lamp needs to be installed with a current from DC. A high-voltage inverter (also known as a lighter) that turns to an alternating current, used to illuminate a cold cathode lamp of a panel. At present, the converter used in the prior art uses a fixed frequency to control the power switch in the inverter after the cold cathode lamp is turned on, and adjusts the output power by controlling the conduction duty cycle of the power switch.

Referring to FIG. 1 , a common push-pull (Push-Pull) parallel resonant converter, wherein transistors Q1 and Q2 are power switches, and controller 94 controls the opening and closing of transistors Q1 and Q2 to pass a DC voltage through a transformer 91. It is converted into an AC voltage, which in turn generates a driving voltage by the resonant circuit 92 to illuminate the cold cathode lamp 93. However, at the moment when the cold cathode lamp is lit, high voltage will generate a plurality of stray capacitances to the ground, and the number of these stray capacitances will vary depending on the types of cold cathode lamps and panels used in the liquid crystal display. The equivalent circuit is represented by the stray capacitance 94 of FIG. And when the cold cathode lamp is lit, the stray capacitance 94 is added in parallel with the capacitance in the resonant circuit, resulting in a characteristic curve that destroys the operating frequency of the original power switch and the gain ratio of the resonant circuit 92, so that the resonant circuit 92 Output/input gain ratio It will fall, causing the brightness of the cold cathode lamp 93 to drop.

At present, the conventional solution is to increase the duty cycle of the power switch after the cold cathode lamp is turned on, so that the effective voltage flowing into the transformer 91 is increased to compensate for the output/input gain ratio lost due to the stray capacitance 94. However, when the duty cycle of the power switch is increased to a preset maximum value, for example, 45%, this method cannot be used to increase a larger output current, and if the liquid crystal display has an overpower protection device installed, When the duty cycle of the power switch reaches the maximum value, the over-power protection device will shut down the entire cold cathode lamp, that is, not long after the cold cathode lamp is turned on, the power protection device will have a duty cycle due to the power switch. Turning off the cold cathode lamp at the maximum value is not a designer's expectation. Therefore, in order to solve the above problem, it is conventional practice to install the power protection device regardless of the display, because the duty cycle of the control signal can no longer be adjusted. Therefore, the conventional method is to manually adjust the frequency of the control signal to increase the output current, so as to restore the original output/input gain ratio, but the manual adjustment method is not only time-consuming but also costly, so the conventional method There are still improvements.

Accordingly, it is an object of the present invention to provide a cold cathode lamp tube commutation device that can automatically adjust the frequency.

Therefore, the cold cathode lamp tube commutation device of the present invention comprises a driving circuit, a controller, a detector and a frequency reducing circuit. The driving circuit is used to drive a cold cathode lamp, and the controller is used to generate a control signal to control the driving circuit actuation, wherein the frequency of the control signal is inversely proportional to the output/input gain ratio of the driving circuit, and when the output/input When the gain ratio exceeds a preset value, The duty cycle of the control signal will be reduced accordingly. The detector of the converter device detects the duty cycle of the control signal. If the duty cycle of detecting the control signal is greater than an upper limit, the detector sends a trigger signal to the detector and the controller. The frequency reduction circuit receives the trigger signal and gradually reduces the frequency of the control signal according to the trigger signal until the duty cycle of the control signal is less than the upper limit.

In addition, the frequency down circuit of the present invention further includes a computing unit and a frequency adjusting circuit coupled to the controller. The calculation unit starts counting when the trigger signal is received for the first time, and sends a down-converted signal to the frequency adjustment circuit according to the calculation result, so that the frequency of the control signal is gradually decreased according to the number of received down-converted signals.

Preferably, the computing unit of the present invention can be a timer that starts counting until the predetermined time when the trigger signal is received for the first time, and sends a frequency reduction when it finds that the trigger signal is still received at the end of the predetermined time. Signal. Of course, the computing unit can also be a counter, and starts counting the number of times the trigger signal is received when the trigger signal is received for the first time, and sends a down-converted signal when it determines that the count value reaches a preset value.

Preferably, the frequency adjustment circuit of the present invention comprises a multiplexer, a first capacitor for adjusting the control signal frequency, and a second capacitor having a larger capacitance ratio than the first capacitor and sequentially increasing. The first capacitor and all the second capacitors are respectively coupled to the multiplexer and the controller, and the down-converted signal controls the multiplexer to switch between selecting the first capacitor and the second capacitor to change the frequency of the control signal.

Preferably, the frequency adjustment circuit of the present invention includes a carry counter and a plurality of passive components that adjust the control signal frequency. Passive components are coupled to the carry The counter and the controller, the down-converting signal can control the carry counter to perform carryover switching to accumulate these passive components, thereby changing the frequency of the control signal. These passive components can be one of a capacitor and a resistor.

Preferably, the frequency adjustment circuit of the present invention comprises a digital analog converter, a non-inverting amplifier coupled to the digital analog converter, and a switch coupled between the non-inverting amplifier and the controller. The digital analog converter converts the down-converted signal into an analog signal, and then amplifies it through a non-inverting amplifier. The amplified analog signal will control the opening or closing of the switch, and use the switch to open and close to change the decision of the controller. The charge and discharge current of the frequency of the signal, which in turn changes the frequency of the control signal.

The effect of the invention is that the commutation device can detect the duty cycle of the control signal sent by the controller, and automatically reduce the frequency of the control signal step by step to achieve the effect of reducing manpower and time cost.

The above and other technical contents, features and advantages of the present invention will be apparent from the following detailed description of FIG.

Referring to FIG. 3 and FIG. 4, a first preferred embodiment of the cold cathode lamp tube commutating device (hereinafter referred to as a commutating device) of the present invention is applied to a cold cathode fluorescent lamp (Cold Cathod Fluorescent). Lamp, CCFL) 2 is a liquid crystal display as a backlight. The converter device 1 includes an inverter 3 and a control module 4. As shown in FIG. 4, the inverter 3 includes a controller 31 and a drive circuit 32. The driving circuit 32 includes a push-pull parallel resonant circuit for illuminating the cold cathode lamp, which mainly utilizes two The power transistors 33, 34 controlled by the controller 31 act as switches, convert an input DC voltage into an AC voltage through a transformer 35, and then drive a cold cathode lamp through a resonant circuit 36 composed of an inductor and a capacitor. Tube 2 to illuminate the cold cathode tube 2.

In general, the switching frequency of the power transistors 33, 34 may be different when the cold cathode lamp is turned on and after lighting. Referring to FIG. 5, in an ideal case, L1 is a power transistor 33, A characteristic curve of the switching frequency of 34 and the gain ratio of the driving circuit 32. When the cold cathode lamp 2 is lit, the switching frequency of the power transistors 33, 34 is f1, so that the gain ratio generated by the driving circuit 32 is in the characteristic curve. At point A of L1, and after the cold cathode lamp 2 is turned on, its frequency will drop to f2, so that the gain ratio generated by the drive circuit 32 is slightly lowered to the point B of the characteristic curve L1, but the cold cathode lamp 2 can still be maintained. At a certain brightness. In addition, the duty cycle of the power transistors 33, 34 is turned on to affect the DC effective voltage of the input transformer 35. In other words, if the duty cycle of the power transistors 33, 34 is turned on, the power transistors 33, 34 are indicated. The longer the time that is turned on, the more the effective voltage value that can be converted into the AC voltage, and the electric power that illuminates the cold cathode lamp 2 can be increased.

Referring to FIG. 4, FIG. 5 and FIG. 6, however, in fact, at the moment when the cold cathode lamp 2 is lit, the high voltage end of the cold cathode lamp 2 generates a plurality of stray capacitances 21 to the ground, and these stray capacitances The number of 21 will vary depending on the type of cold cathode lamp 2 and the panel used in the liquid crystal display, and when the cold cathode lamp 2 is lit, these stray capacitances 21 and the lamp 2 itself are illuminated. The lamp capacitor 22 is added in parallel with the capacitor 361 in the resonant circuit 36. , causing the characteristic curve to change from the original characteristic curve L1 to the characteristic curve L2, so that at the switching frequency after the same lighting, the gain ratio falls from the point B of the characteristic curve L1 to the point C of the characteristic curve L2, that is, at the same power. At the switching frequency of the transistors 33, 34, the output/input gain ratio of the driving circuit 32 is lowered from the original Gain1 to Gain2, resulting in the amplification of the AC signal being insufficient to supply the power required for the cold cathode lamp 2 to make the cold cathode The brightness of the lamp 2 will decrease after it is turned on.

Therefore, it can be seen from the characteristic curve L2 which is changed by the stray capacitance 21 in FIG. 5 that if the switching frequency of the power transistors 33 and 34 is lowered, a higher output/input gain ratio can be obtained, and FIG. 4 is also referred to. It can be seen that the opening and closing of the power transistors 33 and 34 is controlled by a control signal (which is a pulse width modulation signal) sent by the controller 31, that is, the switching frequency of the power transistors 33 and 34 is the frequency of the control signal. Therefore, with reference to FIG. 3, the control module 4 of this embodiment is used to adjust the frequency of the control signal output by the controller 31 to change the switching frequency of the power transistors 33, 34, thereby adjusting the output of the driving circuit 32. Input gain ratio.

The control module 4 includes a detector 5 coupled to the controller 31 and a frequency reduction circuit 600 coupled to the detector 5. The frequency reduction circuit 600 has a calculation unit 6 and a frequency adjustment circuit 7 for cooperation. Referring to FIG. 7, when the cold cathode lamp 2 is lit, the output/input gain ratio is reduced to the point C in L2 due to the influence of the stray capacitance 21, resulting in a corresponding output/input gain ratio insufficient to maintain the cold cathode lamp. The tube 2 is at a normal brightness. Therefore, in order to compensate for the brightness of the cold cathode tube 2, the controller 31 increases the duty cycle of the control signal in exchange for a larger input effective voltage, but is driven by the embodiment. The circuit 32 uses a push-pull parallel resonant circuit, so in order to avoid the occurrence of a large current short circuit, the maximum duty cycle of the preset control signal is 45%. Therefore, if the duty cycle of the control signal is at a maximum of 45% for a long time, it means that the output power of the drive circuit 32 is still insufficient to maintain the cold cathode lamp 2 at normal brightness.

Therefore, after the cold cathode lamp 2 is illuminated, the detector 5 will start detecting whether the duty cycle of the control signal sent by the controller 31 has reached (or exceeded) the preset maximum value, and if so, will issue A trigger signal is coupled to the computing unit 6 of the detector 5.

Moreover, in order to avoid arbitrarily changing the frequency of the control signal due to misjudgment or malfunction, the calculation unit 6 of the embodiment is a counter, starts counting when the trigger signal is received for the first time, and calculates the number of times the trigger signal is received. That is, when the detector 5 always detects that the duty cycle of the control signal is 45%, the detector 5 continuously sends a trigger signal to the counter, so when the counter count value exceeds a preset value (in this embodiment, five times) After that, the counter will send a down-converted signal to the frequency adjustment circuit 7.

In addition, the calculating unit 6 of this embodiment may also be a timer, which starts counting after a first time receiving the trigger signal until a predetermined time, and determines whether the trigger signal is still received at the end of the predetermined time, and if so, That is, the down-converted signal is sent to the frequency adjustment circuit 7. Of course, the value or time preset in the calculation unit 6 can be adjusted by the designer, and is not limited to this embodiment.

Referring to FIG. 8 , a detailed circuit of the frequency adjustment circuit 7 of the embodiment includes a multiplexer 71 coupled to the computing unit 6 for adjusting A first capacitor C1 of the frequency of the entire control signal and four second capacitors (C2, C3, C4, and C5 having different capacitance relationships with the capacitance values of the first capacitor, wherein the relationship of the capacitance values is C1<C2<C3 <C4<C5), wherein the first capacitor C1 and the four second capacitors are respectively coupled to the first to fifth output ends of the multiplexer 71, and one of the multiplexers switches to select one of the controllers 31. connection. Initially, the output of the multiplexer 71 is first switched to the first output terminal, so that the CT end of the controller 31 is connected to the first capacitor C1, that is, after the cold cathode lamp 2 is turned on, the controller 31 The frequency of the generated control signal is determined by the charge and discharge time constant (RC1) of the RC1 circuit formed by the first capacitor C and a resistor R externally connected to the RT terminal of the controller 31, and the first capacitor C1 and the resistor R The frequency of the determined control signal is f2 in Fig. 7 (i.e., point C of the characteristic curve). Therefore, when the multiplexer 71 of the frequency adjustment circuit 7 receives the variable frequency signal, it indicates that the duty cycle of the control signal is maintained at the maximum value of 45% for a certain period of time, so the output of the multiplexer 71 is switched to the second output terminal. The original RC time constant of the control signal of the controller 31 is determined by the RC2 time constant formed by the second capacitor C2 and the resistor R. Since the charging and discharging time constant becomes larger, the frequency of the control signal will be from FIG. The f2 falls to f3, so that the gain of the driving circuit 32 is shifted from the C point of the characteristic curve to the left point D, that is, the resonant circuit 36 in the driving circuit 32 is also larger due to the decrease of the frequency of the control signal. Output / input gain ratio.

Referring again to FIG. 3, after the frequency of the control signal is changed, the driving circuit 32 can drive the cold cathode lamp 2 at a higher output/input gain ratio than before, and the controller 31 still maintains the duty cycle of the control signal. At a maximum of 45%, and to determine the output produced by the new output / input gain ratio Is the power sufficient to supply the cold cathode lamp 2 to operate at a normal brightness. If the output power of the control signal is still maintained at the maximum value, the control module 4 repeats the above process and reduces the frequency of the control signal again, that is, with reference to Figure 8, multiplex The device 71 will once again receive the frequency conversion signal sent by the calculation unit 6, and switch the output to the third output terminal, so that the charge and discharge time constant of the control signal will be RC3, so that the frequency of the control signal will be as shown in FIG. The f3 in the lower limit is further reduced to f4, so that the gain ratio of the driving circuit 32 is shifted from the D point of the characteristic curve to the E point, so that the resonant circuit 36 can have a larger output/input gain ratio; similarly, the multiplexer The charge and discharge time constants generated by the fourth and fifth output terminals are RC4 and RC5, respectively, and the frequency corresponding to the control signal can be f5 in Fig. 7 (point F corresponding to the characteristic curve) and f6 (corresponding characteristic curve G) point). Therefore, in the case where the duty cycle of the control signal is still maintained at 45%, the control module 4 of the present invention continuously repeats the above steps to gradually reduce the frequency of the control signal until the duty cycle of the control signal is less than 45% and is driven. Circuit 32 can have sufficient output power to drive cold cathode lamp 2 to operate at a normal brightness.

If the frequency of the control signal is changed, the output/input gain ratio of the driving circuit 32 exceeds the current that drives the cold cathode lamp 2 to operate at a normal brightness. At this time, the controller will control the signal. The duty cycle starts from a maximum of 45% (when the output/input gain ratio is fixed) to reduce the input current obtained by the drive circuit 32, and then adjust the output current to just drive the cold cathode lamp 2 at a normal brightness. The current under work.

In addition, in the present embodiment, the charging and discharging time constant of the control signal is adjusted by increasing the capacitance, and the frequency of the control signal is changed. Of course, the frequency of the control signal can be adjusted by increasing the resistance, which is not limited to this embodiment.

It should be noted that, after the controller 31 of the embodiment reduces the frequency of the control signal to f6 in FIG. 7, if the driving circuit 32 is still unable to drive the cold cathode lamp to operate normally, the control module 4 determines the liquid crystal display. There are some problems, such as overload or short circuit, etc., that is, the frequency of the control signal will not be lowered again, and when the multiplexer 71 receives the down-converted signal again, the output is switched to the sixth output, and the controller 31 is The CT terminal is short-circuited to ground, so that the controller 31 cannot generate a control signal because there is no charge/discharge current, so that the drive circuit 32 does not receive the input voltage and the cold cathode lamp 2 is turned off.

Referring to FIG. 3, FIG. 4 and FIG. 9, a second preferred embodiment of the cold cathode lamp tube commutation device of the present invention is substantially the same as the first preferred embodiment, except that the frequency adjustment circuit 7' It can be composed of a carry counter 72 coupled to the computing unit 6, a capacitor 731 for generating a fundamental frequency, four capacitors 732-735 having the same capacitance value, and four switches 741-745, wherein one ends of the switches 741-745 are connected. The CT end of the controller 31 is connected to a capacitor at the other end, and the carry counter 72 controls the opening or closing of all the switches 741-745 to determine whether the capacitor is coupled to the controller 31. In this embodiment, a capacitor 731 is fixedly coupled to the CT end of the controller 31, that is, the frequency of the start control signal is the capacitor 731 and a resistor R externally connected to the RT terminal of the controller 31. The charge and discharge time constant RC of the configuration is determined, which corresponds to f2 in Fig. 7, that is, point C of the characteristic curve. Similarly, when the cold cathode lamp is turned on, if the controller 31 determines that the output power of the driving circuit 32 is insufficient, the duty cycle of the control signal is increased. However, when the detector 5 detects the control signal, the duty cycle is When the maximum value is 45%, the calculation unit 6 starts counting or accumulates the number of times the trigger signal is received, and sends a down-converted signal to the frequency adjustment circuit 7' after exceeding a predetermined value (or time).

At this time, the carry counter 72 performs a carry after receiving the variable frequency signal, so that the first output terminal is at a high level and the switch 741 is turned on, so that the capacitor 732 is added in parallel with the capacitor 731, so that the frequency of the control signal is charged. The discharge time constant increases as R(C+C1) and decreases, and its frequency decreases from f2 in Fig. 7 to f3, and rises from point C of the characteristic curve to point D. Therefore, the control module 4 of the present invention continuously repeats the above steps to gradually reduce the frequency of the control signal until the resonant circuit 36 can obtain a sufficient output/input gain ratio so that the output power can make the cold cathode lamp 2 Work at a normal brightness. However, although this embodiment provides another method of increasing the capacitance to reduce the frequency of the control signal, it is of course possible to adjust the frequency of the control signal by increasing the resistance, and thus it is not limited to this embodiment.

It is worth mentioning that, after the controller 31 of the embodiment reduces the frequency of the control signal to f6 in FIG. 7, if the driving circuit 32 is still unable to drive the cold cathode lamp 2 to operate normally, the control module 4 is no longer The frequency of the control signal is lowered, and the frequency conversion signal sent by the calculation unit 6 when the frequency of the control signal is f6 is used to cause the carry counter 72 to generate an overflow (ie, beyond the range that the carry counter 72 can carry), and The fifth output sends an overflow signal, which causes the switch 745 to be turned on and shorts the CT end of the controller to ground, so that the control The controller 31 cannot generate a control signal because there is no charge/discharge current, so that the drive circuit 32 does not receive the input voltage and the cold cathode lamp 2 is turned off.

Referring to FIG. 3, FIG. 4 and FIG. 10, a third preferred embodiment of the cold cathode lamp tube commutation device of the present invention is substantially the same as the first preferred embodiment, except that the frequency adjustment circuit 7" The device includes a digital/analog converter 75 (D/A Converter) coupled to the computing unit 6, a non-inverting amplifier 76 coupled to the digital/analog converter 75, and a non-inverting amplifier 76 coupled to the The switch 77 between the controllers 31, wherein the non-inverting amplifier 76 has a first resistor R1, a second resistor R2 and an operational amplifier 761. One end of the first resistor R1 is grounded, and the other end thereof is connected to the operational amplifier 761. An inverting terminal, and one end of the second resistor R2 is connected to the inverting terminal of the operational amplifier 761, and the other end thereof is connected to the output terminal of the operational amplifier 761, and the non-inverting terminal of the operational amplifier 761 is connected to the digital/analog converter 75, And the magnification is determined by the resistance ratio of the first resistor R1 and the second resistor R2.

In this embodiment, the charge and discharge time constant of the control signal is fixed by a capacitor C coupled to the CT terminal of the controller 31 and a resistor R coupled to the RT terminal, and the switch 77 is turned off at the beginning. That is to say, the current I _CT outputted by the CT terminal will all flow into the capacitor C, so that the frequency of the control signal will correspond to f2 in Fig. 7 (corresponding to the characteristic curve C point). Similarly, when the cold cathode lamp 2 is turned on, if the controller 31 determines that the output power of the driving circuit 32 is insufficient, the duty cycle of the control signal is increased. However, when the detector 5 detects the duty cycle of the control signal. When the maximum value is 45%, the calculation unit 6 starts counting or accumulates the number of times the trigger signal is received, and sends a down signal after exceeding a predetermined value (or time).

At this time, the digital/analog converter 75 converts the received down-converted signal into a corresponding analog signal, and then amplifies through the non-inverting amplifier 76, and turns on the switch 77. When the switch 77 is turned on, the current I _CT is shunted, so that the current I _CT does not completely flow into the capacitor C, causing the control signal to decrease in frequency due to the charge and discharge current of the capacitor C, and the control signal is The frequency is decreased from f2 (point C of the corresponding characteristic curve) in Fig. 7 to f3 (corresponding to point D of the characteristic curve), and the resonance circuit 36 of the drive circuit 32 is made to have a higher output/input gain ratio. Similarly, when the duty cycle of the detection signal detected by the detector 5 is still 45%, the control module 4 of the embodiment continuously repeats the above steps to gradually reduce the frequency of the control signal until the resonant circuit 36 A sufficient output/input gain ratio can be obtained so that the output power can operate the cold cathode lamp 2 at a normal brightness, and the controller 31 can reduce the duty cycle of the control signal to less than 45%.

It should be noted that, when the controller 31 of the embodiment reduces the frequency of the control signal to f6 in FIG. 7, if the driving circuit 32 cannot be driven to drive the cold cathode lamp 2 to operate normally, the control module 4 does not. The frequency of the control signal is further decreased, and the calculation unit 6 increases the frequency of the down-converted signal sent after the frequency of the control signal is reduced to f6, so that the analog signal converted correspondingly is amplified, and the switch 77 can be enabled. When the maximum is turned on, all the current I _CT will flow through the switch 77 without flowing into the capacitor C, that is, the controller 31 will not be able to generate the control signal due to the absence of the charging and discharging current, so that the driving circuit 32 cannot obtain the input. The voltage causes the cold cathode lamp 2 to be turned off.

In addition, the frequency adjustment circuit 7" of the embodiment further includes a diode 78 and a limit connected in series between the output end of the operational amplifier 761 and the switch 77. The resistor current Rg, wherein the diode 78 prevents the current of the switch 77 from flowing back to the non-inverting amplifier 76 to affect the operation of the operational amplifier 761, and the limiting resistor current Rg limits the current amplified by the non-inverting amplifier 76. In order to prevent the switch 77 from being burnt due to an instantaneous large current.

In addition, the number and amplitude of the control signal that can be gradually reduced by the frequency can be changed by the designer according to different requirements, and is not limited to the five frequencies (f2~f6) in the above embodiment, and if the liquid crystal display has been installed In the case of the power protection device, the overcurrent protection device turns off the cold cathode lamp 2 when the duty cycle of the control signal is 45%, so in order to avoid this phenomenon, the preset maximum value of the detector 5 can be reduced to 43. %, that is to say, the control module 4 will start to reduce the frequency when the duty cycle of the control signal is maintained at 43%, so that the overcurrent protection device will not cause the cold cathode lamp when the control module 4 is still adjusting the frequency. Tube 2 is turned off. Of course, the maximum duty cycle of the control signal can be changed by the designer's needs, not limited to 43% or 45%.

In summary, the cold cathode lamp tube commutation device of the present invention compensates the driving circuit by automatically detecting whether the duty cycle of the control signal of the driving circuit reaches a preset maximum value to gradually change the frequency of the control signal. The output/input gain ratio lost due to the stray capacitance of the liquid crystal display, and the cold cathode lamp tube commutation device of the present invention can also be integrated into the same wafer in the semiconductor process, and the automatic stepwise frequency change can save manpower and time. cost.

The above is only the preferred embodiment of the present invention, and the scope of the invention is not limited thereto, that is, the simple equivalent changes and modifications made by the scope of the invention and the description of the invention are Still It is within the scope of the patent of the present invention.

1‧‧‧Commutation device

2‧‧‧Cold cathode tube

21‧‧‧Stray capacitance

22‧‧‧ Lamp Capacitance

3‧‧‧Inverter

31‧‧‧ Controller

32‧‧‧Drive circuit

33, 34‧‧‧ Power transistors

35‧‧‧Transformers

36‧‧‧Resonance circuit

361‧‧‧ Capacitance

4‧‧‧Control Module

5‧‧‧Detector

600‧‧‧down frequency circuit

6‧‧‧Computation unit

7, 7', 7" ‧ ‧ frequency adjustment circuit

71‧‧‧Multiplexer

72‧‧‧ Carry counter

731, 732, 733, 734, 735‧‧‧ capacitors

741, 742, 743, 744, 745‧‧ ‧ switch

75‧‧‧Digital Analog Converter

76‧‧‧Non-inverting amplifier

761‧‧‧Operational Amplifier

77‧‧‧ switch

78‧‧‧ diode

1 is a circuit diagram illustrating the relationship of components in a conventional converter; FIG. 2 is an equivalent circuit diagram illustrating the influence of a conventional inverter on stray capacitance; FIG. 3 is a circuit block diagram illustrating the cold of the present invention FIG. 4 is a circuit diagram illustrating the internal component relationship of the inverter of the present invention; FIG. 5 is a waveform diagram illustrating the switching frequency and driving circuit of the ideal and actual power transistors. FIG. 6 is an equivalent circuit diagram illustrating the influence of the stray capacitance of the inverter of the present invention; FIG. 7 is a waveform diagram illustrating the corresponding output/input gain when the frequency is adjusted according to the present invention. Figure 8 is a circuit diagram illustrating a first preferred embodiment of the cold cathode lamp tube commutation device of the present invention; and Figure 9 is a circuit diagram illustrating the second comparison of the cold cathode lamp tube commutation device of the present invention. A preferred embodiment; and FIG. 10 is a circuit diagram illustrating a third preferred embodiment of the cold cathode lamp tube commutation device of the present invention.

1‧‧‧Cold cathode tube tube converter

2‧‧‧Cold cathode tube

3‧‧‧Inverter

4‧‧‧Control Module

5‧‧‧Detector

600‧‧‧down frequency circuit

6‧‧‧Computation unit

7‧‧‧frequency adjustment circuit

Claims (28)

A control method for a cold cathode lamp inverter, the cold cathode lamp converter comprising a driving circuit of a cold cathode lamp and a controller for generating a control signal to control the driving circuit, wherein the frequency of the control signal And an output/input gain ratio of the driving circuit is inversely proportional, and when the output/input gain ratio exceeds a preset value, the duty cycle of the control signal is correspondingly reduced, and the control method includes: (a) detecting When the duty cycle of the control signal is greater than an upper limit value, a trigger signal is sent; (b) the frequency of the control signal is adjusted according to the trigger signal; and (c) steps (a) and (b) are repeated until The duty cycle of the control signal is less than the upper limit. According to the control method of the cold cathode lamp inverter according to claim 1, in step (b), it is further determined whether the trigger signal is continuously received within a predetermined time, and if so, the control is lowered. The frequency of the signal. According to the control method of the cold cathode lamp inverter according to claim 1, in step (b), it is further determined whether the trigger signal is received within a predetermined time to reach a predetermined number of times, and if so, Decrease the frequency of the control signal. A control module for controlling a cold cathode lamp inverter, the cold cathode lamp converter comprising a drive circuit of a cold cathode lamp and a controller for generating a control signal to control actuation of the drive circuit, wherein The frequency of the control signal is inversely proportional to an output/input gain ratio of the driving circuit, and when the output/input gain ratio exceeds a predetermined value, the control signal is responsible The control module includes: a detector for detecting a duty cycle of the control signal, and if the duty cycle of detecting the control signal is greater than an upper limit, the detector will Sending a trigger signal; and a frequency down circuit coupled to the detector and the controller, and receiving the trigger signal, and gradually reducing the frequency of the control signal until the duty cycle of the control signal according to the trigger signal Less than the upper limit. The control module of claim 4, wherein the frequency reduction circuit further comprises a computing unit and a frequency adjustment circuit coupled to the controller, the computing unit receiving the trigger for the first time When the signal is started, a frequency reduction signal is sent to the frequency adjustment circuit according to the calculation result, so that the frequency of the control signal is gradually decreased according to the received frequency of the down-converted signal. The control module of claim 5, wherein the calculation unit is a timer, and the calculation result is a predetermined time, when the first time the trigger signal is received, the time is started until the predetermined time. And when the trigger signal is still received at the end of the predetermined time, the down signal is sent. According to the control module of claim 5, wherein the calculation unit is a counter, and the counter starts counting the number of times the trigger signal is received when the trigger signal is received for the first time, and determines the count When the value reaches a preset value, the down-converted signal is sent. The control module of claim 5, wherein the frequency adjustment circuit comprises a multiplexer, and a frequency for adjusting the frequency of the control signal a second capacitor having a capacitance and a plurality of capacitance values greater than the first capacitance value and sequentially increasing, the first capacitor and the second capacitor being coupled to the multiplexer and the controller, wherein the down-converting signal controls the The multiplexer switches one of the first capacitor and the second capacitor to change the frequency of the control signal. The control module of claim 5, wherein the frequency adjustment circuit comprises a multiplexer, a first resistor for adjusting the frequency of the control signal, and a plurality of resistor values greater than the first resistor value and sequentially An incremental second resistor, the first resistor and the second resistor are coupled to the multiplexer and the controller, and the down-converting signal controls the multiplexer to switch between the first resistor and the second resistor. One to change the frequency of the control signal. The control module of claim 5, wherein the frequency adjustment circuit comprises a carry counter and a plurality of passive components for adjusting the frequency of the control signal, the passive components being coupled to the carry counter and the controller, The down-converted signal can control the carry counter to perform carryover switching to accumulate the passive components, thereby changing the frequency of the control signal. The control module of claim 10, wherein the passive components are one of a resistor and a capacitor. The control module of claim 5, wherein the frequency adjustment circuit comprises a digital analog converter, a non-inverting amplifier coupled to the digital analog converter, and a non-inverting coupled to the non-inverting a switch between the amplifier and the controller, the digital analog converter converts the down-converted signal into an analog signal, and then amplified by the non-inverting amplifier, and the amplified analog signal controls the switch to be turned on or off. Using the switch to open and close to change the controller to determine the control signal The frequency of the charge and discharge current of the capacitor, which in turn changes the frequency of the control signal. The control module of claim 12, wherein the non-inverting amplifier has a first resistor, a second resistor, and an operational amplifier, wherein the first resistor is grounded at one end, and the other end is connected to the operation An inverting terminal of the amplifier, one end of the second resistor is connected to the inverting terminal of the operational amplifier, and the other end is connected to the output end of the operational amplifier, and the non-inverting terminal of the operational amplifier is connected to the digital analog converter. According to the control module of claim 4, wherein the frequency reduction circuit sequentially lowers the frequency of the control signal according to the trigger signal received successively. The control module of claim 4, wherein when the number of times the control signal frequency is lowered reaches a preset value, the controller is stopped. A cold cathode lamp tube commutation device comprises: a driving circuit for driving a cold cathode lamp; a controller for generating a control signal for controlling the driving circuit to act, wherein the frequency of the control signal and one of the driving circuit The output/input gain ratio is inversely proportional, and when the output/input gain ratio exceeds a preset value, the duty cycle of the control signal is correspondingly reduced; a detector is used to detect the duty cycle of the control signal, if When the duty cycle of detecting the control signal is greater than an upper limit value, the detector sends a trigger signal; A frequency down circuit is coupled to the detector and the controller, and receives the trigger signal, and according to the trigger signal, gradually reduces the frequency of the control signal until the duty cycle of the control signal is less than the upper limit. The cold cathode lamp tube commutation device according to claim 16, wherein the frequency down circuit further comprises a calculating unit and a frequency adjusting circuit coupled to the controller, the calculating unit being the first time When the trigger signal is received, the calculation starts, and a frequency reduction signal is sent to the frequency adjustment circuit according to the calculation result, so that the frequency of the control signal is gradually decreased according to the received frequency of the down signal. The cold cathode lamp tube commutation device according to claim 17, wherein the calculation unit is a timer, and the calculation result is a predetermined time, which starts when the trigger signal is received for the first time. The down-converted signal is sent until the predetermined time is reached, and when the trigger signal is still received at the end of the predetermined time. The cold cathode lamp tube commutation device according to claim 17, wherein the calculating unit is a counter, and the counter starts counting the number of times the trigger signal is received when the trigger signal is received for the first time, and The down signal is sent when it is judged that the count value reaches a preset value. The cold cathode lamp tube commutation device according to claim 17, wherein the frequency adjustment circuit comprises a multiplexer, a first capacitor and a complex capacitance ratio adjusting the frequency of the control signal, and the first capacitor a second capacitor that is sequentially incremented, the first capacitor and the second capacitor are coupled to the multiplexer and the controller, and the down-converting signal controls the multiplexer to switch to select the first capacitor and the One of the second capacitors to change the control signal The frequency of the number. The cold cathode lamp tube commutation device according to claim 17, wherein the frequency adjustment circuit comprises a multiplexer, a first resistor and a plurality of resistor ratios for adjusting the frequency of the control signal, and the first resistor a second resistor that is sequentially incremented, the first resistor and the second resistor are coupled to the multiplexer and the controller, and the down-converting signal controls the multiplexer to switch to select the first resistor and the One of the second resistors changes the frequency of the control signal. The cold cathode lamp tube commutation device according to claim 17, wherein the frequency adjustment circuit comprises a carry counter and a plurality of passive components for adjusting the frequency of the control signal, wherein the passive components are coupled to the carry counter and The controller, the down-converting signal can control the carry counter to perform carryover switching to accumulate the passive components, thereby changing the frequency of the control signal. The cold cathode lamp tube commutation device according to claim 22, wherein the passive components are one of a resistor and a capacitor. The cold cathode lamp tube commutation device of claim 17, wherein the frequency adjustment circuit comprises a digital analog converter, a non-inverting amplifier coupled to the digital analog converter, and a coupling a switch between the non-inverting amplifier and the controller, the digital analog converter converts the down-converted signal into an analog signal, and then amplified by the non-inverting amplifier, and the amplified analog signal controls the switch Turning on or off, using the switch to open and close to change the charge and discharge current of a capacitor of the controller that determines the frequency of the control signal, thereby changing the The frequency of the control signal. The cold cathode lamp tube commutation device according to claim 24, wherein the non-inverting amplifier has a first resistor, a second resistor and an operational amplifier, wherein the first resistor is grounded at one end, and the other end thereof Connected to the inverting terminal of the operational amplifier, one end of the second resistor is connected to the inverting terminal of the operational amplifier, and the other end is connected to the output end of the operational amplifier, and the non-inverting terminal of the operational amplifier is connected to the digit Analog converter. The cold cathode lamp tube commutation device according to claim 16, wherein the frequency down circuit sequentially lowers the frequency of the control signal according to the trigger signal received successively. The cold cathode lamp tube commutation device according to claim 16, wherein when the number of times the control signal frequency is lowered reaches a preset value, the controller is stopped. The cold cathode lamp tube commutation device according to claim 16, wherein the driving circuit is a push-pull parallel resonant circuit.