Patent Application
20120049741
This application claims priority to and the benefit of Chinese Patent Application No. 201010269938.5, filed Aug. 24, 2010, which is incorporated herein by reference in its entirety.
The present disclosure relates generally to electrical circuits, and more particularly but not exclusively to LED drivers.
Light emitting diode (LED) has become increasingly popular as a lighting choice, and for many applications, has begun to replace conventional lamps having a filament. For example, LED is now widely used in traffic signal lights and for the back lighting of liquid crystal display (LCD) panels.
The LEDs are often arranged in parallel “strings” driven by a shared voltage source, and each LED string has a plurality of LEDs connected in series. To provide consistent light output between the LED strings, each LED string is typically driven at a regulated current that is substantially equal among all of the LED strings.
Although driven by currents of equal magnitude, there is often a considerable variation in the driving voltages due to variations in the forward-voltage drop of individual LED of the LED strings resulting from process variations in the fabrication and manufacturing of the LED. Dynamic variations due to changes in temperature when the LEDs are enabled and disabled may also contribute to the variation in the driving voltages with a fixed current. In view of this variation, conventional LED drivers typically provide a driving voltage that is sufficiently higher than an expected worst-case bias drop, so as to ensure proper operation of each LED string. However, as the power consumed by the LED driver and the LED strings is approximately a product of the output voltage of the LED driver and the sum of the currents of the individual LED strings, the use of an excessively high driving voltage unnecessarily increases the power consumption. Accordingly, an improved technique for driving LED strings would be advantageous.
The present disclosure provides a current balance scheme for driving a plurality of LED strings which is free from the shorted voltage damage and huge power dissipation, and the method thereof.
It is an object of the present disclosure to provide a current balance scheme for driving a plurality of LED strings and the method thereof.
In accomplishing the above and other objects, there has been provided, in accordance with an embodiment of the present disclosure, a LED driver, comprising: a power supply unit having an input terminal, a feedback terminal, and an output terminal, wherein the input terminal is configured to receive an input signal, the feedback terminal is configured to receive a minimum feedback signal, and wherein based on the input signal and the minimum feedback signal, the power supply unit generates a driving voltage at the output terminal to drive a plurality of LED strings, wherein the plurality of LED strings comprises N LED strings coupled in parallel, and wherein N is a natural number, and each LED string comprises an upper end and a lower end; a current balancing circuit having an input terminal and N output terminals, wherein N is a natural number, and wherein the input terminal is configured to receive a reference voltage, and each output terminal is respectively coupled to the lower end of each LED string to provide a regulated current to each LED string; and a feedback selector having N input terminals and an output terminal, wherein N is a natural number, and wherein each input terminal is respectively coupled to the lower end of each LED string to receive a feedback signal indicative of a voltage drop of each LED string, and based on the feedback signals, the feedback selector generates the minimum feedback signal at the output terminal.
In addition, there has been provided, in accordance with an embodiment of the present disclosure, A LED driver, comprising: a power supply unit circuit for generating a driving signal to drive a plurality of LED strings in response to an input signal and a minimum feedback signal; a current balancing circuit for providing a regulated current signal to the plurality of LED strings in response to a reference voltage; and a feedback selector circuit for generating the minimum feedback signal in response to a plurality of feedback signals.
Furthermore, there has been provided, in accordance with an embodiment of the present invention, a method of driving a plurality of LED strings, comprising: receiving an input signal; deriving feedback signals indicative of voltage drops of each LED string; generating a minimum feedback signal by selecting a feedback signal with minimum magnitude from the feedback signals; providing a regulated current signal to each LED string, wherein the current of each LED string is substantially similar; and generating a driving voltage in response to the input signal and the minimum feedback signal, the driving voltage being used to drive the plurality of LED strings.
These and other features of the present disclosure will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.
The use of the same reference label in different drawings indicates the same of like components.
In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the disclosure. Persons of ordinary skill in the art will recognize, however, that the disclosure may be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the disclosure.
In
In one embodiment, the current balancing circuit 301 comprises: a plurality of current sources, wherein each single current source CS is coupled to the lower end of each LED string, to supply and regulate the current of each LED string, and wherein the voltage across each current source CS is coupled to the feedback selector 303 as the feedback signal; and a current setting resistor RSET having a first terminal and a second terminal, wherein the first terminal is coupled to every current source CS and the second terminal is connected to ground, and wherein the current supplied by the current source CS is adjusted by changing the value of the current setting resistor RSET.
The feedback signals indicative of the driving voltage are all coupled to the feedback selector 303. In the example of
In one embodiment, the power supply unit 302 further comprises a dimming output terminal configured to provide a dimming signal to each current source. The dimming signal may be a pulse width modulation (PWM) signal. The function of the dimming signal is described with referring to
Due to the “short” characteristic of the amplifier EA, the voltage across the adjusting resistor Rs is clamped to the reference voltage VREF by the amplifier EA, so that the current flowing through the adjusting resistor Rs is fixed to VREF/Rs. The current supplied by each current source may be regulated by changing the adjusting resistor Rs if the reference voltage VREF is fixed. The reference voltage VREF of all current sources is supplied by one voltage source to eliminate the variance. In one embodiment, the reference voltage VREF is the voltage across the setting resistor RSET in
In one embodiment, the protection circuit 304 comprises N transistors, wherein each transistor T has a first terminal, a second terminal and a control terminal, and wherein the control terminal of the transistor is coupled to a predetermined voltage VG, the first terminal of each transistor is respectively coupled to the lower end of each LED string, and the second terminal of the transistor T is coupled to the current balancing circuit 301, and further wherein each transistor T has a breakdown voltage higher than the driving voltage. Persons of ordinary skill in the art should know that the transistor T may be realized by MOSFET, triode, JFET and other semiconductor devices.
The operation of the LED driver 50 is similar to the operation of the LED driver 30. The protection circuit protects the current balancing circuit from high voltage which maybe the driving voltage when the LED string coupled to this current balancing circuit is shorted. The breakdown voltage of the transistor used in the protection circuit is chosen to be higher than the driving voltage so as to suffer the most of the driving voltage when the LED string is shorted. Thus the current source CS is protected.
Furthermore, the present disclosure discloses a method of controlling a LED driver. Referring to
An effective technique for getting the feedback signal from the primary side of the switching mode power supply has been disclosed. While specific embodiments of the present disclosure have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201010269938.5 | Aug 2010 | CN | national |
