1. Field of the Invention
The present invention relates to a scheme for controlling light-emitting diodes (LED), and more particularly, to a controlling apparatus and related lighting module for controlling a plurality of LED strings.
2. Description of the Prior Art
Recently, light sources implemented with light-emitting diodes (LED) have become more popular. For example, light sources in a backlight module in a conventional liquid crystal display (LCD) panel are usually implemented with cold cathode fluorescent lamps (CCFL). However, as the optical efficiency of an LED increases repeatedly, and cost of the LED decreases continuously, the cold cathode fluorescent lamps are replaced by light-emitting diodes gradually as light sources in a backlight module due to their being more economical.
In the prior art schemes, multiple light-emitting diodes are connected in sequence for reducing the number of required driving circuits, and for decreasing a total driving current utilized for driving the light-emitting diodes. However, because of fabrication process variations of different light-emitting diodes, it is very difficult to ensure that parameters of the light-emitting diodes in different LED strings are identical. Additionally, the parameters of the light-emitting diodes may usually be affected by some environmental factors (e.g. temperature). For instance, the forward voltages (VF) of different light-emitting diodes are usually a little different due to the above-mentioned factors. Therefore, the scheme utilizing multiple light-emitting diodes connected in sequence to be an LED string will accumulate forward voltage differences caused by different light-emitting diodes in the LED string. Usually, the total voltage differences accumulated by the forward voltage differences in different LED strings are also different.
In this situation, even though an identical operating voltage is applied for driving all LED strings, currents passing through different LED strings are also different since the accumulated values of the forward voltage differences in different LED strings are not identical. Therefore, the brightness of different LED strings will be different because currents passing through different LED strings are not identical. If the above-mentioned LED strings are used as light sources in a backlight module in an LCD panel, inconsistencies will be introduced on the display area of the LCD panel, since the brightness of the lighting source in the backlight module is uneven.
Therefore one of the objectives of the present invention is to provide a controlling apparatus and related lighting module for controlling the brightness of a plurality of LED strings, to solve the above-mentioned problems.
According to the claimed invention, a controlling apparatus for controlling a plurality of LED strings is disclosed. The first terminals of the plurality of LED strings are electronically connected to an operating voltage. The controlling apparatus comprises a plurality of transistors and a transistor controller. Each of the plurality of transistors has a control terminal, a first terminal being electronically connected to a second terminal of a corresponding LED string in the plurality of LED strings, and a second terminal. The second terminals of the plurality of transistors are respectively grounded through a plurality of impedance elements. The transistor controller is electronically connected to the second terminals of the plurality of transistors and is utilized for adjusting an input signal of the control terminal of each transistor to control a current passing through the first terminal of the transistor according to a voltage at the second terminal of the transistor.
According to the claimed invention, a lighting module is disclosed. The lighting module comprises a plurality of LED strings, a plurality of transistors, an error calculating circuit, and a transistor controller. Each of the LED strings has a first terminal being electronically connected to an operating voltage and a second terminal. Each of the transistors has a control terminal, a first terminal, and a second terminal. The first terminal of each transistor is electronically connected to a second terminal of a corresponding LED string in the plurality of the LED strings. The second terminals of the plurality of transistors are grounded through a plurality of impedance elements respectively. The error calculating circuit is electronically connected to the second terminals of the plurality of transistors and is utilized for calculating a difference between a voltage at the second terminal of each transistor and a corresponding reference voltage. The transistor controller is electronically connected to the error calculating circuit and the control terminals of the plurality of transistors, and is utilized for controlling a current passing through the first terminal of each transistor according to a calculation result generated by the error calculating circuit.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
Please refer to
In this embodiment, the transistors 130a˜130n in the controlling apparatus 120 are bipolar junction transistors (BJT), and each of the transistors 130a˜130n has a control terminal (base), a first terminal (collector), and a second terminal (emitter). As shown in
As mentioned above, because of fabrication process variations of different light-emitting diodes, or environmental factors (e.g. temperature), the total voltage difference accumulated by the forward voltage differences in each of the LED strings 110a˜110n may be different than that of others in the LED strings 110a˜110n. The currents Ic1˜Icn passing through the LED strings 110a˜110n are therefore different. The controlling apparatus 120 is utilized for controlling the currents Ic1˜Icn passing through the LED strings 110a˜110n by respectively utilizing the transistors 130a˜130n so that the brightness of the LED strings 110a˜110n is substantially identical.
More specifically, in the controlling apparatus 120, the error calculating circuit 140 is utilized for calculating a difference between the voltage VF1 at an emitter of each of the transistors 130a˜130n and a corresponding reference voltage Vref. In a preferred embodiment, the error calculating circuit 140 can amplify a difference between the voltage VF1 at an emitter of each in the transistors 130a˜130n and the corresponding reference voltage Vref for boosting the difference. In practice, the error calculating circuit 140 can be implemented with an operational amplifier or multiple operational amplifiers. For instance, the error calculating circuit 140 can be implemented by only one operational amplifier for calculating differences between voltages at the emitters of the transistors 130a˜130n and the corresponding reference voltage Vref respectively. Otherwise, the error calculating circuit 140 can also be implemented by a plurality of operational amplifiers calculating differences between voltages at the emitters of the transistors 130a˜130n and the corresponding reference voltage Vref simultaneously. For example, a first operational amplifier (it is not shown in
Please refer to
When the transistor 130a is operating in the active region, the following equation relates to the collector current Ic1 and base current Ib1 established:
Ic1=β×Ib1 Equation (1)
wherein the parameter β is meant to be the common-emitter current gain of the transistor 130a. In addition, a calculation result Ver1, calculated by the error calculating circuit 140, corresponding to the difference between the voltage VF1 at the emitter of the transistor 130a and the corresponding reference voltage Vref can be represented as follows:
Ver1=A×(Vref−VF1) Equation (2)
wherein the parameter A is meant to be the gain of the error calculating circuit 140. It is assumed that the resistance of the variable resistor 220 equals to a resistance R1. The relation between the resistance R1 and the base current Ib1 passing through the transistor 130a can be represented as Equation (3):
Ib1×R1=Ver1−(VF1+Vbe) Equation (3)
wherein the parameter Vbe is meant to a voltage drop between the base and emitter of the transistor 130a. By Equation (3), the base current Ib1 passing through the transistor 130a can be represented as Equation (4):
Similarly, the base current Ib1 passing through the transistor 130a can be represented as Equation (5) by Equation (2):
By a relation between Equation (1) and Equation (5), the collector current Ic1 can be illustrated by the following equation:
Suppose that equations K1=β×A and K2=β×(A+1) are established. Since the gain the A of the error calculating circuit 140 is usually larger than unity, the parameters K1 and K2 are approximate. Equation (6) can be rewritten as follows:
Since the parameters K1, Vbe, and β in Equation (7) are fixed, the decision unit 230 in the transistor controller 150 can control the resistance R1 of the variable resistor 220 for adjusting the base current Ib1 passing through the transistor 130a according to the calculation result Ver1 generated by the error calculating circuit 140. Therefore, the decision unit 230 can control the collector current Ic1 passing through the transistor 130a by adjusting the base current Ib1 passing through the transistor 130a. In this embodiment, the decision unit 230 keeps the collector current Ic1 passing through the transistor 130a at a predetermined value or within a predetermined range by adjusting the resistance R1 of the variable resistor 220. The above-mentioned scheme for controlling the collector current Ic1 passing through the transistor 130a can also be applied in controlling the collector currents Ic2˜Icn passing through the other transistors 130b˜130n. Therefore, the currents Ic1˜Icn passing through the LED strings 110a˜110n will be substantially identical, and the brightness of the LED strings 110a˜110n, which are not identical in the prior art, are avoided. If a lighting source in a backlight module corresponding to the LCD panel is implemented with the lighting module 100, the lighting inconsistency will be not introduced on a display area of an LCD panel.
In the above-mentioned embodiment, the light-emitting diodes included within the LED strings 110a˜110n in the lighting module 100 all correspond to the same color. However, this is not meant to be a limitation of the present invention. The light-emitting diodes included within different LED strings can also correspond to different colors. For example, the light-emitting diodes in at least an LED string corresponding to a first color and the light-emitting diodes in at least an LED string corresponding to a second color can be included in the LED strings 110a˜110n. In practice, the total voltage values accumulated by the forward voltages in the LED strings corresponding to different colors may not be identical. One of solutions is to utilize different corresponding reference voltages corresponding to different colors. Therefore, the error calculating circuit 140 can generate a calculation result to the transistor controller 150 according to a specific corresponding reference voltage of a specific color and a voltage at an emitter of a transistor in the LED string corresponding to the specific color. Additionally, the decision unit 230 in the transistor controller 150 can set different target currents in accordance with different colors respectively and adjust the brightness of each of the LED strings by the above-mentioned scheme for controlling the collector currents Ic1˜Icn.
In addition, in other embodiments, part or all of the transistors 130a˜130n (i.e. bipolar junction transistors) in the controlling apparatus 120 can be replaced with insulated-gate bipolar transistors (IGBT). It is preferred that the insulated-gate bipolar transistors have the substantially the same transconductance. It should also be noted that the control terminal of an insulated-gate bipolar transistor is a gate of the insulated-gate bipolar transistor, and the first and second terminals of the insulated-gate bipolar transistor are a collector and emitter of the insulated-gate bipolar transistor respectively. The error calculating circuit 140 calculates a calculation result according to a voltage at the emitter of each insulated-gate bipolar transistor. The transistor controller 150 adjusts an input voltage at the gate of the insulated-gate bipolar transistor to control a collector current passing through the insulated-gate bipolar transistor according to the calculation result generated by the error calculating circuit 140. This also obeys the spirit of the present invention.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
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