Liquid crystal display system with lamp feedback
A liquid crystal display system and CCFL power converter circuit is provided using a high-efficiency zero-voltage-switching technique that eliminates switching losses associated with the power MOSFETs. An optimal sweeping-frequency technique is used in the CCFL ignition by accounting for the parasitic capacitance in the resonant tank circuit. Additionally, the circuit is self-learning and is adapted to determine the optimum operating frequency for the circuit with a given load. An over-voltage protection circuit can also be provided to ensure that the circuit components are protected in the case of open-lamp condition.
1. A liquid crystal display system comprising:
a liquid crystal display panel;
a cold cathode fluorescent lamp for illuminating said liquid crystal display panel;
a secondary transformer winding coupled to said cold cathode fluorescent lamp for providing current to said cold cathode fluorescent lamp;
a primary transformer winding coupled to said secondary transformer winding for providing magnetic flux to said secondary transformer winding;
a first switch coupled to said primary transformer winding for allowing a forward-biased DC current to pass through said primary transformer winding;
a second switch coupled to said primary transformer winding for allowing a reverse-biased DC current to pass through said primary transformer winding;
a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a voltage feedback signal representing current through said cold cathode fluorescent lamp;
a feedback control loop circuit coupled to said cold cathode fluorescent lamp and electrically coupled to said sense resistor for receiving a said voltage feedback signal and controlling power supplied to said cold cathode fluorescent lamp when said voltage feedback signal is above a predetermined threshold; and
an input selector pin coupled to said feedback control loop circuit for selecting between linear mode power control and pulse width modulation power control.
2. A liquid crystal display system comprising:
a liquid crystal display panel;
a cold cathode fluorescent lamp for illuminating said liquid crystal display panel;
a secondary transformer winding coupled to said cold cathode fluorescent lamp for providing current to said cold cathode fluorescent lamp;
a primary transformer winding coupled to said secondary transformer winding for providing magnetic flux to said secondary transformer winding;
a first switch coupled to said primary transformer winding for allowing a forward-biased DC current to pass through said primary transformer winding;
a second switch coupled to said primary transformer winding for allowing a reverse-biased DC current to pass through said primary transformer winding;
a capacitor divider electrically coupled to said cold cathode fluorescent lamp for providing a first voltage feedback signal representing a voltage across said cold cathode fluorescent lamp;
a first feedback signal line coupled to said capacitor divider for receiving said first voltage feedback signal from said capacitor divider representing said voltage across said cold cathode fluorescent lamp;
a voltage detector coupled to said first feedback signal line for detecting said voltage across said cold cathode fluorescent lamp;
a voltage protection circuit coupled to said voltage detector to reduce power to said cold cathode fluorescent lamp when said voltage across said cold cathode fluorescent lamp exceeds a predetermined threshold;
a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a second voltage feedback signal representing current through said cold cathode fluorescent lamp;
a feedback control loop circuit electrically coupled to said cold cathode fluorescent lamp and said sense resistor and controlling power supplied to said cold cathode fluorescent lamp based on said second voltage feedback signal; and
an input selector pin coupled to said feedback control loop circuit for selecting between linear mode power control and pulse width modulation power control.
3. A liquid crystal display system as claimed in claim 2 further comprising:
a timer coupled to said voltage protection circuit to provide a time out period of predetermined duration when said first voltage feedback signal exceeds said predetermined threshold.
4. A liquid crystal display system comprising:
a liquid crystal display panel;
a cold cathode fluorescent lamp for illuminating said liquid crystal display panel;
a secondary transformer winding coupled to said cold cathode fluorescent lamp for providing current to said cold cathode fluorescent lamp;
a primary transformer winding coupled to said secondary transformer winding for providing magnetic flux to said secondary transformer winding;
a first switch coupled to said primary transformer winding for allowing a forward-biased DC current to pass through said primary transformer winding;
a second switch coupled to said primary transformer winding for allowing a reverse-biased DC current to pass through said primary transformer winding;
a feedback control loop circuit electrically coupled to said cold cathode fluorescent lamp receiving a feedback signal from a component electrically coupled to said secondary transformer winding to reduce power supplied to said cold cathode fluorescent lamp when said feedback signal indicates an open lamp condition; and
an input selector pin coupled to said feedback control loop circuit for selecting between linear mode power control and pulse width modulation power control.
5. A liquid crystal display system as claimed in claim 1 further comprising:
a capacitor electrically coupled to said input selector pin and coupled to ground for selecting said pulse width modulation power control.
6. A liquid crystal display system as claimed in claim 5 further comprising:
an ignition circuit coupled to said feedback control loop circuit for providing lower amount of power to said cold cathode fluorescent lamp when said voltage feedback signal is below said predetermined threshold.
7. A liquid crystal display system as claimed in claim 5 further comprising:
an ignition circuit coupled to said feedback control loop circuit for providing a predetermined minimum amount of power to said cold cathode fluorescent lamp when said voltage feedback signal is below said predetermined threshold.
8. A liquid crystal display system as claimed in claim 1 further comprising:
a resistor electrically coupled to said input selector pin and coupled to an input voltage source for selecting said linear mode power control.
9. A liquid crystal display system as claimed in claim 8 further comprising:
an ignition circuit coupled to said feedback control loop circuit for providing lower amount of power to said cold cathode fluorescent lamp when said voltage feedback signal is below said predetermined threshold.
10. A liquid crystal display system as claimed in claim 8 further comprising:
an ignition circuit coupled to said feedback control loop circuit for providing a predetermined minimum amount of power to said cold cathode fluorescent lamp when said voltage feedback signal is below said predetermined threshold.
11. A liquid crystal display system as claimed in claim 2 further comprising:
a capacitor electrically coupled to said input selector pin and coupled to ground for selecting said pulse width modulation power control.
12. A liquid crystal display system as claimed in claim 2 further comprising:
a resistor electrically coupled to said input selector pin and coupled to an input voltage source for selecting said linear mode power control.
13. A liquid crystal display system as claimed in claim 2 wherein said predetermined threshold represents a value of said voltage across said cold cathode fluorescent lamp greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said secondary transformer winding.
14. A liquid crystal display system as claimed in claim 3 wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.
15. A liquid crystal display system as claimed in claim 14 further comprising:
a capacitor electrically coupled to said input selector pin and coupled to ground for selecting said pulse width modulation power control.
16. A liquid crystal display system as claimed in claim 14 further comprising:
a resistor electrically coupled to said input selector pin and coupled to an input voltage source for selecting said linear mode power control.
17. A liquid crystal display system as claimed in claim 4 further comprising:
a capacitor electrically coupled to said input selector pin and coupled to ground for selecting said pulse width modulation power control.
18. A liquid crystal display system as claimed in claim 17 wherein said feedback control loop circuit reduces power supplied to said cold cathode fluorescent lamp to a predetermined minimum amount when said feedback signal indicates said open lamp condition.
19. A liquid crystal display system as claimed in claim 4 further comprising:
a resistor electrically coupled to said input selector pin and coupled to an input voltage source for selecting said linear mode power control.
20. A liquid crystal display system as claimed in claim 19 wherein said feedback control loop circuit reduces power supplied to said cold cathode fluorescent lamp to a predetermined minimum amount when said feedback signal indicates said open lamp condition.