Lamp current control using profile synthesizer
View Patent ↗An efficient and flexible current-mode driver delivers power to one or more light sources in a backlight system. In one application, the current-mode driver is configured as an inverter with an input current regulator, a non-resonant polarity-switching network, and a closely-coupled output transformer. The input current regulator can output a regulated current source in a variety of programmable wave shapes. The current-mode driver may further include a rectifier circuit and a second polarity-switching network between the closely-coupled output transformer and a lamp load. In another application, the current-mode driver delivers power to a plurality of light sources in substantially one polarity by providing a regulated current to a network of time-sharing semiconductor switches coupled in series with different light sources coupled across each semiconductor switch.
1. A controller for a current-mode inverter comprising:
a clock generator configured to output a periodic timing waveform;
a current profile generator configured to produce a profile signal in a programmable waveform shape defined by at least a capacitor and two input control signals, wherein the profile signal is provided to a current regulator to generate a regulated current with the programmed waveform shape; and
a register state machine configured to control the operations of the current profile generator with reference to the periodic timing waveform.
2. The controller of claim 1 , wherein the frequency of the periodic timing waveform is determined by a resistor and a capacitor.
3. The controller of claim 1 , further comprising an attenuation circuit coupled between an output of the current profile generator and an input of the current regulator to reduce the amplitude range of the profile signal for comparison with a feedback signal.
4. The controller of claim 1 , further comprising a hysteretic pulse width modulation circuit configured to generate a control signal to the current regulator based on a comparison of the profile signal to a feedback signal representative of a load current.
5. The controller of claim 1 , wherein the regulated current is coupled to a fluorescent lamp using a non-resonant inverter, and the two input control signals to the current profile generator are independently adjustable to vary the brightness of the fluorescent lamp.
6. The controller of claim 5 , further comprising a pulse width modulation circuit configured to generate polarity switching signals for semiconductor switches in the non-resonant inverter with reference to the periodic timing waveform.
7. The controller of claim 6 , further comprising a fault processor configured to output a fault signal to shut down the pulse width modulation circuit upon detecting faulty conditions in the fluorescent lamp or in the non-resonant inverter, wherein the pulse width modulation circuit configures the polarity switching signals keep the semiconductor switches on continuously in response to the fault signal.
8. The controller of claim 1 , wherein the register state machine cycles through four states corresponding to zero, rising edges, plateaus, and falling edges of the profile signal.
9. A method to generate a current profile for a current regulator in a current-mode inverter, the method comprising the acts of:
periodically setting the current profile to a reference level, wherein the period is determined by a periodic triangular waveform;
comparing a first control input signal to the periodic triangular waveform to determine an active duration of the current profile in each period, wherein the current profile increases at a programmable rising rate and a capacitor starts charging at about the beginning of the active duration;
holding the current profile at a substantially constant level when the voltage across the capacitor is about equal to or above a predefined holding threshold, wherein the voltage of the periodic triangular waveform is noted at about the time the voltage across the capacitor crosses the predefined holding threshold and the capacitor stops charging;
reducing the level of the current profile at a programmable falling rate and discharging the capacitor when the voltage of the periodic triangular waveform is approximately less than the noted voltage, wherein the current profile returns to the reference level by the end of the active duration.
10. The method of claim 9 , wherein the reference level is zero.
11. The method of claim 9 , wherein the current-mode inverter drives at least one fluorescent lamp and two substantially identical cycles of the current profile are generated for every cycle of current flowing through the fluorescent lamp.
12. The method of claim 9 , wherein the current profile pulse-width is determined by the first control input signal, and the current profile amplitude is determined by the predefined holding threshold.
13. The method of claim 9 , wherein the current profile is a periodic square waveform with rise and fall times of less than one microsecond.
14. The method of claim 9 , wherein the current profile is a periodic trapezoidal waveform with rise and fall times that are greater than one microsecond.
15. The method of claim 9 , wherein the current profile is a periodic trapezoidal waveform that is substantially identical to the periodic triangular waveform with its amplitude clipped at substantially ⅔ of the peak amplitude of the periodic triangular waveform.
16. The method of claim 15 , further comprising filtering the current profile to generate a sinusoidal waveform.
17. A controller for a current-mode inverter comprising:
means for generating a periodic timing signal;
means for defining a rising slope, a plateau, and a falling slope of a current profile signal with a capacitor and two input control signals, wherein the current profile signal determines the wave shape of a regulated current source; and
means for generating a pair of substantially identical wave shapes for every two cycles of the periodic timing signal.
18. The inverter with a current-mode controller of claim 17 , further comprising a switching network that operates synchronously with the periodic timing signal to periodically alternate conduction paths for the regulated current source to produce an AC driving current.
19. The inverter with a current-mode controller of claim 17 , further comprising means for adding pre-emphasis to the current profile signal by providing an upslope to the plateau to compensate for magnetizing current in a transformer of the inverter.
20. The inverter with current-mode controller of claim 17 , further comprising means for adjusting amplitude range of the current profile signal to match amplitude range of a feedback signal.