Patent Application
20120235571
1. Technical Field
The disclosure relates to backlight driving systems, and particularly to a multi-lamp driving system.
2. Description of Related Art
Cold cathode fluorescent lamps (CCFL) are usually used for a backlight of a liquid crystal display. An inverter converts direct current power into alternating current power to provide proper driving power to light the CCFLs. The inverter usually utilizes a single sided printed circuit board without a high voltage capacitor in order to reduce costs. In this structure, the inverter does not have voltage feedback. In order to solve the above problem, the inverter first uses frequency hopping and soft starting, and then uses a fixed duty cycle driving mode to light the CCFLs.
The inverter using the fixed duty cycle driving mode uses a decreased duty cycle because of concern of tolerance voltage of transformers and lighting voltage. In poor environment, such as, environments with low temperature, low lamp current, or darkroom, lamp current fluctuates when the CCFLs are driven by the inverter, which results in the CCFLs flickering, mis-acting to protect, or the CCFLs lighting failure.
The switch circuit 120 converts the DC power signals from the filter circuit 100 into first AC power signals. In one embodiment, the first AC power signals are square wave signals. The switch circuit 120 may be one of a full bridge circuit, a half bridge circuit, a push-pull circuit, and other related circuits. The PWM controller 110 controls output of the switch circuit 120. The PWM controller 110 generates PWM signals to control on-off of the switch circuit 120, thus, the switch circuit 120 converts the DC power signals from the filter circuit 100 into the first AC power signals.
Each of the plurality of transformers T includes a primary winding and a secondary winding. The primary windings of the plurality of transformers T are connected to the switch circuit 120 in parallel, and high voltage terminals of the secondary windings of the plurality of transformers T are connected to the plurality of lamps L, respectively. That is, the high voltage terminal of the secondary winding of the first transformer T is connected to the first lamp L, and the high voltage terminal of the secondary winding of the second transformer T is connected to the second lamp L, and so on. The plurality of transformers T convert the first AC power signals from the switch circuit 120 into second AC power signals. In one embodiment, the second AC power signals are sine wave signals. In one embodiment, positive voltage inputs of the primary windings of the first and the second transformers T are connected to a positive voltage output of the switch circuit 120, and positive voltage inputs of the primary windings of the third and the fourth transformers T are connected to a negative voltage output of the switch circuit 120. Thus, the positive voltage inputs of the primary windings of the first and the second transformers T have opposite phases to the positive voltage inputs of the primary windings of the third and the fourth transformers T.
The current difference detection circuit 130 is connected to the secondary windings of the plurality of transformers T, and detects difference among current flowing through the plurality of lamps L to determine if the current flowing through the plurality of lamps L fluctuates. In a normal condition, the current flowing through the plurality of lamps L is nearly the same. If there is a big difference among the current flowing through the plurality of lamps L, it indicates that the current flowing through the plurality of lamps L fluctuates. In this embodiment, the current difference detection circuit 130 determines if the difference among the current flowing through the plurality of lamps L exceeds a first predetermined value, to determine if the current flowing through the plurality of lamps L fluctuates. If the difference among the current flowing through the plurality of lamps L exceeds the first predetermined value, the current difference detection circuit 130 determines the current flowing through the plurality of lamps L fluctuates. If the difference among the current flowing through the plurality of lamps L does not exceed the first predetermined value, the current difference detection circuit 130 determines the current flowing through the plurality of lamps L does not fluctuate.
The lighting detection circuit 140 is connected to the current difference detection circuit 130, and determines if the plurality of lamps L are lit according to the current flowing through the plurality of lamps L and generates a lighting indication signal Vo after the plurality of lamps L are lit. In one embodiment, if the current flowing through the plurality of lamps L are greater than a second predetermined value, which indicates the plurality of lamps L are lit, the lighting detection circuit 140 generates the lighting indication signal Vo.
The frequency regulating circuit 170 is connected to the lighting detection circuit 140 and the PWM controller 110, and regulates frequency of the PWM signals generated by the PWM controller 110 according to the lighting indication signal Vo. When the plurality of lamps L are lit, the frequency regulating circuit 170 regulates the frequency of the PWM signals low, and if the plurality of lamps L are being lit, the frequency regulating circuit 170 regulates the frequency of the PWM signals high.
The frequency scanning detection circuit 150 is connected to the lighting detection circuit 140, and determines if the multi-lamp driving system 10 is in a frequency scanning process according to the lighting indication signal Vo. The frequency scanning detection circuit 150 further generates a frequency scanning indication signal Vs if the multi-lamp driving system 10 is in the frequency scanning process. In one embodiment, when the multi-lamp driving system 10 is in a lighting transient, and the frequency of the PWM signals changes from high to low, the multi-lamp driving system 10 is in the frequency scanning process. Because the lighting indication signal Vo indicates if the plurality of lamps L are lit, the frequency scanning detection circuit 150 determines if the multi-lamp driving system 10 is in the frequency scanning process according to voltage level of the lighting detection circuit 140
The duty cycle regulating circuit 160 is connected to the current difference detection circuit 130, the lighting detection circuit 140, and the frequency scanning detection circuit 150, and regulates duty cycles of the PWM signals generated by the PWM controller 110 upon the condition that the current flowing through the plurality of lamps L fluctuates ad the multi-lamp driving system 10 is in the frequency scanning process.
The comparison circuit 1330 is connected to the highest current retrieving circuit 1310 and the lowest current retrieving circuit 1320, and determines if difference between the highest voltage signal Vmax and the lowest voltage signal Vmin exceeds the first predetermined value. If the difference between the highest voltage signal Vmax and the lowest voltage signal Vmin exceeds the first predetermined value, the comparison circuit 1330 generates a current fluctuating indication signal Vd, which indicates the current flowing through the plurality of lamps L fluctuates. In one embodiment, the first predetermined value can be set according to actual needs, such as, to be 0.7V. The multi-lamp driving system 10 dynamically retrieves the highest current signal and the lowest current signal to compare to determine if there is abnormity, replacing with comparison with a fixed reference voltage. Thus, when environment temperature and parameters of the plurality of lamps L change, the highest current signal and the lowest current signal change correspondingly, which avoids mis-determination and misact.
The lighting detection circuit 140 is connected to the lowest current retrieving circuit 1320, and generates the lighting indication signal Vo when the lowest voltage signal Vmin exceeds the second predetermined value. The duty cycle regulating circuit 160 regulates the duty cycles of the PWM signals higher when receiving the current fluctuating indication signal Vd and the frequency scanning indication signal Vs at the same time. The duty cycle regulating circuit 160 regulates the duty cycle of the PWM signal lower when not receiving the current fluctuating indication signal Vd and the frequency scanning indication signal Vs at the same time. In one embodiment, the second predetermined value can also be set according to actual needs, such as, to be 0.7V.
An anode of the second diode D2 is connected to the first pole of the first switch Q1, and a cathode of the second diode D2 acts as an output of the lighting detection circuit 140 to output the lighting indication signal Vo. The fourth resistor R4 is connected between the cathode of the second diode D2 and the ground, and the second capacitor C2 is connected to the fourth resistor R4 in parallel. In one embodiment, the second diode D2 is used to rectify to output DC signals, that is, the lighting indication signal Vo is a DC signal. The fourth resistor R4 and the second capacitor C2 is used to charge and discharge.
In this embodiment, the multi-lamp driving system 10 just lights the plurality of lamps L, the current flowing through the plurality of lamps L is low at this time. After the plurality of lamps L are lit, the current flowing through the plurality of lamps L increases. Therefore, the lighting detection circuit 140 determines if the plurality of lamps L are lit according to if the lowest current flowing through the plurality of lamps L exceeds the second predetermined value.
In one embodiment, the first switch Q1 is a N-type metal oxide semiconductor field effect transistor (NMOSFET), the control pole is a gate of the NMOSFET, the first pole is a drain of the NMOSFET, and the second pole is a source of the NMOSFET.
The comparator 1500 compares the lighting indication signal Vo with the second reference power supply Vcc2 as well as the second switch Q2 compares the lighting indication signal Vo with a threshold of the second switch Q2, to commonly determine if the multi-lamp driving system 10 is in the frequency scanning process. If the lighting indication signal Vo is less than the second reference power supply Vcc2 and greater than the threshold of the second switch Q2, the multi-lamp driving system 10 is in the frequency scanning process, and the frequency scanning detection circuit 150 generates the frequency scanning indication signal Vs.
The duty cycle regulating circuit 160 includes third to sixth switches Q3 to Q6 and fifth to ninth resistors R5 to R9. Each of the third to sixth switches Q3 to Q6 respectively includes a control pole, a first pole, and a second pole. The control pole of the third switch Q3 receives one of the current fluctuating indication signal Vd and the frequency scanning indication signal Vs, the first pole of the third switch Q3 is connected to a third reference power supply Vcc3 via the fifth resistor R5, and the second pole of the third switch Q3 is connected to the first pole of the fourth switch Q4. The control pole of the fourth switch Q4 receives the other one of the current fluctuating indication signal Vd and the frequency scanning indication signal Vs, and the second pole of the fourth switch Q4 is connected to the first pole of the second switch Q2 of the frequency scanning detection circuit 150.
The control pole of the fifth switch Q5 is connected to the first pole of third switch Q3 via the sixth resistor R6, the first pole of the fifth switch Q5 is connected to the PWM controller 110 via the seventh resistor R7, and the second pole of the fifth switch Q5 is connected to the first pole of the sixth switch Q6. The control pole of the sixth switch Q6 receives the lighting detection signal Vo via the eighth resistor R8, the first pole of the sixth switch Q6 is connected to the PWM controller 110 via the ninth resistor R9 commonly with the first pole of the fifth switch Q5 via the seventh resistor R7, and the second pole of the sixth switch Q6 is grounded. In this embodiment, the duty cycle regulating circuit 160 is connected to a cout pin of the PWM controller 110. That is, the first pole of the fifth switch Q5 and the first pole of the sixth switch Q6 are commonly connected to the cout pin of the PWM controller 110, respectively via the seventh resistor R7 and the ninth resistor R9.
In one embodiment, the second to the sixth switches Q2 to Q6 are N-type metal oxide semiconductor field effect transistors (NMOSFETs). The control poles of the second to the sixth switches Q2 to Q6 are gates of the NMOSFETs, the first poles of the second to the sixth switches Q2 to Q6 are drains of the NMOSFET, and the second poles of the second to the sixth switches Q2 to Q6 are sources of the NMOSFET. The current fluctuating indication signal Vd and the frequency scanning indication Vs are both high level signals.
The highest current retrieving circuit 1310 includes a plurality of third diodes D3, and the plurality of third diodes D3 are correspondingly connected to the plurality of signal converting units and the low voltage terminals a of the secondary windings of the plurality of transformers T. Anodes of the plurality of third diodes D3 are correspondingly connected to the low voltage terminals a of the secondary windings of the plurality of transformers T, and cathodes of the plurality of third diodes D3 are connected together to output the highest voltage signal Vmax. The plurality of third diodes D3 selects the highest voltage signal Vmax from the voltage signals from the signal converting units, and outputs the highest voltage signal Vmax to the comparison circuit 1330.
Because the low voltage terminals a of the secondary windings of the plurality of transformers T have opposite phase, the lowest current retrieving circuit 1320 includes a plurality of fourth diodes D4, a plurality of fifth diodes D5, and a plurality of sixth diodes D6. For example, supposing that there are four lamps L and four transformers T and the low voltage terminals a of the secondary windings of the first and the second transformers T have opposite phase to the low voltage terminals a of the secondary windings of the third and the fourth transformers T, the lowest current retrieving circuit 1320 includes four fourth diodes D4, two fifth diodes D5, and two sixth diodes D6.
The plurality of fourth diodes D4 are correspondingly connected to the plurality of signal converting units and the low voltage terminals a of the secondary windings of the plurality of transformers T. Cathodes of the plurality of fourth diodes D4 are correspondingly connected to the low voltage terminals a of the secondary windings of the plurality of transformers T, and anodes of two of the plurality of fourth diodes D4 are connected together respectively. That is, the anodes of the first and the second fourth diodes D4 are connected together and connected to a first fifth reference power supply Vcc5 via a first thirteenth resistor R13, and the anodes of the third and the fourth fourth diodes D4 are connected together and connected to a second fifth reference power supply Vcc5 via a second thirteenth resistor R13.
An anode of a first fifth diode D5 is connected to the anodes of the first and the second fourth diodes D4, an anode of a second fifth diode D5 is connected to the anodes of the third and the fourth fourth diodes D4, and cathodes of the first and the second fifth diodes D5 are connected together to output the lowest voltage signal Vmin to the comparison circuit 1330. A cathode of a first sixth diode D6 is connected to the anode of the first fifth diode D5, and an anode of the first sixth diode D6 is grounded. A cathode of a second sixth diode D6 is connected to the anode of the second fifth diode D5, and an anode of the second sixth diode D6 is grounded. The sixth diodes D6 are used to convert negative voltage signals into positive voltage signals. In one embodiment, the highest voltage signal Vmax and the lowest voltage signal Vmin are both AC signals. In alternative embodiment, if the low voltage terminals a of the secondary windings of the plurality of transformers T have the same phases, the lowest current retrieving circuit 1320 only includes a plurality of fourth diodes D4 and one fifth diode D5, which has similar structures to that of the above, therefore, descriptions are omitted here.
The comparison circuit 1330 includes a comparator 1331 including an inverting input, a non-inverting input, and an output. The non-inverting input of the comparator 1331 receives the highest voltage signal Vmax, the inverting input of the comparator 1331 receives the lowest voltage signal Vmin, and the output of the comparator 1331 output the current fluctuating indication signal Vd. In this embodiment, the current fluctuating indication signal Vd is a DC signal. The inverting input, the non-inverting input, and the output of the comparator 1331 are connected to necessary resistors, which are omitted here for brevity. In one embodiment, the lighting detection circuit 140 can have the same structure to that of the comparison circuit 1330, and the comparison circuit 1330 can have the same structure to that of the lighting detection circuit 140 of
When the multi-lamp driving system 10 is in the lighting transient, the current flowing through the plurality of lamps L are low, that is, the lowest voltage signal Vmin is low. Therefore, the first switch Q1 is off, and the lighting detection circuit 140 outputs a high level signal. Thus, voltage of the inverting input of the comparator 1500 of the frequency scanning circuit 150 is greater voltage of the non-input of the comparator 1500, which makes the comparator 1500 outputs a low level signal to the fourth switch Q4 of the duty cycle regulating circuit 160. Thus, the fourth switch Q4 is off, and the fifth and the sixth switches Q5 and Q6 are turned on. Therefore, the seventh resistor R7 and the ninth resistor R9 are connected in parallel, and the duty cycle of the PWM signal is low. At this time, the seventh switch Q7 of the frequency regulating circuit 170 is turned on, which means that the tenth resistor R10 and eleventh resistor R11 are connected in parallel. Therefore, the frequency of the PWM signal is high.
If the multi-lamp driving system 10 cannot drive the plurality of lamps L normally, there is high current flowing through the plurality of lamps L, but the current fluctuates. That is, there is great difference among the current flowing through the plurality of lamps L. Thus, there is great difference among the voltage signals converted by the signal converting units corresponding to the current flowing through the plurality of lamps L. Because the plurality of third diodes D3 have the same parameters and the cathodes of the plurality of third diodes D3 are connected together, the third diode D3 corresponding to the lamp L with the highest current is turned on. Thus, the plurality of third diodes D3 retrieves the highest voltage signal Vmax corresponding to the highest current, and outputs the highest voltage signal Vmax to the non-inverting input of the comparator 1331.
Because the plurality of fourth diodes D4 have the same parameters and the anodes of two of the plurality of fourth diodes D4 are connected together respectively, one of per two fourth diodes D4 corresponding to the lamp L with lower current is turned on. Thus, per two fourth diodes D4 retrieve the lower voltage signal corresponding to the lower current. That is, the first and second fourth diodes D4 retrieve the lower voltage signal corresponding to the lower current flowing through the first or the second lamp L, and the third and fourth fourth diodes D4 retrieve the lower voltage signal corresponding to the lower current flowing through the third or the fourth lamp L. Similarly, the fifth resistors D5 compares the retrieved lower voltage signal to retrieve the lowest voltage signal Vmin corresponding to the lowest current flowing through the plurality of lamps L.
Because the highest voltage signal Vmax is greater than the lowest voltage signal Vmin, the comparator 1331 of the current difference detection circuit 130 outputs the current fluctuating indication signal Vo with the high level. Because the current flowing through the plurality of lamps L is higher than the second predetermined value, the first switch Q1 is turned on, resulting in the second diode D2 off. At this time, the second capacitor C2 discharges via the fourth resistor R4, so the lighting indication signal Vo drops off.
When the lighting indication signal Vo is lower than the voltage of the non-inverting input of the comparator 1500 of the frequency scanning circuit 150 and higher than the threshold of the second switch Q2, the multi-lamp driving system 10 may be in the frequency scanning process. Therefore, the frequency scanning detection circuit 150 determines if the multi-lamp driving system 10 is in the frequency scanning process according to the voltage of the lighting indication signal Vo, and output the frequency scanning indication signal Vs with the high level. At this time, although the lighting indication signal Vo drops off, the lighting indication signal Vo also turns on the second switch Q2 and the sixth switch Q6.
Because the frequency scanning indication signal Vs and the current fluctuating indication signal Vd are high level signal, the third switch Q3 and the fourth switch Q4 are turned on. Therefore, the fifth switch Q5 is turned off. The ninth resistor R9 is connected to the PWM controller 110 alone, which resulting in the higher duty cycle of the PWM signal. Thus, when the plurality of lamps L fluctuate, the duty cycle of the PWM signal is regulated higher to improve the current fluctuating.
The multi-lamp driving system 10 detects if the current flowing the plurality of lamps L fluctuates when being lit by the current difference detection circuit 130, and regulates the duty cycle of the PWM signal if the current fluctuates and the multi-lamp driving system 10 is in the frequency scanning process. Thus, due to the change of the duty cycle of the PWM signal, the current flowing through the plurality of lamps L is stable, which improves the condition that a screen flicks or cannot be driven normally when the plurality of lamps L are being driven and avoids misact.
The foregoing disclosure of various embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201110062586.0 | Mar 2011 | CN | national |
