# Patent

## Title
Short circuit detection circuit and control method thereof

## Abstract

A short circuit detection circuit for detecting short circuit of a series of light-emitting diodes includes a constant current source, a rectifier, and a detector. The constant circuit source provides a setting current. The rectifier is coupled between a feedback node and the constant current source. The feedback node is coupled to a terminal of the series of light-emitting diodes. When a feedback voltage of the feedback node exceeds a short reference voltage, the rectifier allows the setting current sinks the feedback node. The detector detects the feedback voltage of the feedback node. When the feedback voltage exceeds a predetermined value, short-circuit protection is triggered to make the series of light-emitting diodes be turned off.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a short circuit detection circuit and a control method, and particularly to a short circuit detection circuit and a control method that can be applied for detecting short circuit of a series of light-emitting diodes.
2. Description of the Prior Art
In an energy saving and carbon reduction era, a light-emitting diode can be used for a popular light source due to higher luminous efficiency and an smaller component size of the light-emitting diode. For example, liquid crystal panel manufacturers are gradually utilizing light-emitting diode light sources as a substitute for traditional cold-cathode fluorescent lamp light sources as backlight sources of liquid crystal panels.
FIG. 1 is a diagram illustrating a light-emitting diode power supply 8 applied as a backlight module of a liquid crystal panel. The light-emitting diode power supply 8 is mainly used for controlling turning-on of a plurality of series of light-emitting diodes L1-LN, where each series of light-emitting diodes of the plurality of series of light-emitting diodes L1-LN has a plurality of light-emitting diodes. In a booster 4, a pulse width controller 23 can control a power switch to make an inductor component sink power from an input terminal IN and release power to an output terminal OUT. Thus, the booster 4 can build a proper output voltage VOUT at the output terminal OUT to drive the plurality of series of light-emitting diodes L1-LN. A current balance controller 13 controls current flowing through each series of light-emitting diodes to be the same to make luminance of each series of light-emitting diodes the same.
A circuit 12 has a plurality of diodes all coupled to a minimum voltage terminal MIN for detecting a minimum voltage of cathode terminals D1-DN. A voltage VMIN of the minimum voltage terminal MIN roughly corresponds to the minimum voltage of the cathode terminals D1-DN. In addition, the current balance controller 13 can also influence transmission efficiency of the burst circuit 4 through a control terminal CTRL to make the voltage VMIN maintain a predetermined voltage value. Thus, NMOS transistors N1-NN can be ensured to operate in a more efficient state to reduce unnecessary power consumption.
Some light-emitting diodes of the plurality of series of light-emitting diodes L1-LN may be aging, short circuit, or open circuit with increase in using time. When the plurality of series of light-emitting diodes L1-LN have abnormal operational states, the current balance controller 13 should execute a corresponding operation to prevent the plurality of series of light-emitting diodes L1-LN from being damaged.
A circuit 14 can provide short-circuit detection. The circuit 14 has a plurality of diodes all coupled to a maximum voltage terminal MAX. A voltage VMAX of the maximum voltage terminal MAX roughly corresponds to the maximum voltage of cathode terminals D1-DN. For example, if the series of light-emitting diodes Ln has a plurality of light-emitting diodes that are short circuited, a cathode voltage VDn of a cathode terminal Dn is much higher than other cathode voltages VD1-VDn-1 when the plurality of series of light-emitting diodes L1-LN are turned on, resulting in the voltage VMAX of the maximum voltage terminal MAX being increased. The current balance controller 13 can detect the voltage VMAX through a detection terminal SD. Therefore, when the voltage VMAX exceeds a threshold value, the current balance controller 13 can determine that a light-emitting diode short circuit event occurs, and further turn off the plurality of series of light-emitting diodes L1-LN.
However, the light-emitting diode power supply 8 in FIG. 1 has disadvantages as follows:
First, the current balance controller 13 can not determine which one of the plurality of series of light-emitting diodes L1-LN has a light-emitting diode short circuit event. That is to say, the current balance controller 13 still turns off all series of light-emitting diodes when only one series of light-emitting diodes Ln has a light-emitting diode short circuit event. Second, cost of the light-emitting diode power supply 8 is very expensive. The light-emitting diode power supply 8 needs a high voltage switch coupled between the maximum voltage terminal MAX and the detection terminal SD to prevent high cathode voltages VD1-VDN from being inputted to the current balance controller 13 when the plurality of series of light-emitting diodes L1-LN are turned off (dimming off). Thus, the current balance controller 13 needs an additional pin DT to control the high voltage switch.
SUMMARY OF THE INVENTION
An embodiment provides a control method for detecting short circuit of a series of light-emitting diodes. The control method includes providing a feedback terminal coupled to a terminal of the series of light-emitting diodes; sinking reference current from the feedback terminal to make a feedback voltage be not higher than a short circuit reference voltage, wherein the reference current has a predetermined maximum value; comparing feedback current with the reference current when the series of light-emitting diodes are turned on, wherein the feedback current flows from the terminal of the series of light-emitting diodes to the feedback terminal; and triggering short-circuit protection to make the series of light-emitting diodes be turned off when the feedback current is higher than the reference current.
Another embodiment provides a short circuit detection circuit for detecting short circuit of a series of light-emitting diodes. The short circuit detection circuit includes a constant current source, a rectifier, and a detector. The constant current source provides setting current. The rectifier is coupled between a feedback terminal and a constant current source. The feedback terminal is coupled to a terminal of the series of light-emitting diodes. When a feedback voltage of the feedback terminal exceeds a short circuit reference voltage, the rectifier make the setting current sink the feedback terminal. The detector detects the feedback voltage of the feedback terminal, and triggers a short-circuit protection to make the series of light-emitting diodes be turned off when the feedback voltage is higher than a predetermined value.
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.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a light-emitting diode power supply applied for a backlight module of a liquid crystal panel.
FIG. 2 is a diagram illustrating a power control circuit for controlling turning-on of a plurality of series of light-emitting diodes according to an embodiment.
FIG. 3 is a diagram illustrating the backlight controller in FIG. 2.
FIG. 4 is a diagram illustrating the driving module and corresponding circuits thereof.
FIG. 5 is a diagram illustrating a driving module instead of the driving module in FIG. 3.
FIG. 6A and FIG. 6B are diagrams illustrating light-emitting diode short circuit detectors according to another embodiment.



DETAILED DESCRIPTION
FIG. 2 is a diagram illustrating a power control circuit 18 for controlling turning-on of a plurality of series of light-emitting diodes L1-LN according to an embodiment. The plurality of series of light-emitting diodes L1-LN have cathode terminals D1-DN coupled to feedback terminals FB1-FBN of a backlight controller 20 through resistors R1-RN, respectively. The backlight controller 20 controls NMOS transistors N1-NN through gate terminals G1-GN, and roughly detects currents flowing through the NMOS transistors N1-NN through current detection terminals CS1-CSN. The backlight controller 20 also controls a power switch of a booster 4 through a driving terminal DRV to store energy/release energy in/from an inductor of the booster 4. In another embodiment of the present invention, the backlight controller 20 is a monolithic integrated circuit.
FIG. 3 is a diagram illustrating the backlight controller 20 in FIG. 2. The backlight controller 20 includes a pulse width controller 30, a minimum voltage controller 26, and a plurality of driving modules 281-28N.
The minimum voltage controller 26 generates a minimum feedback voltage VFB-MIN at a minimum feedback terminal FB-MIN according to a minimum value of feedback voltages VFB1-VFBN of the feedback terminals FB1-FBN. The pulse width controller 30 controls the power switch of the booster 4 through the driving terminal DRV to make a voltage VOUT of an output terminal OUT be increased or decreased to make the minimum feedback voltage VFB-MIN roughly maintain a predetermined value. Thus, the NMOS transistors N1-NN can operate more efficiently. For example, the pulse width controller 30 can maintain the minimum feedback voltage VFB-MIN at about 1V, and similarly the minimum value of the feedback voltages VFB1-VFBN may be about 1V.
The driving modules 281-28N correspond to the plurality of series of light-emitting diodes L1-LN, respectively. Circuits, structures and functions of the driving modules 281-28N are the same or similar. Taking the driving modules 281 for example as follows, those skilled in the art can simply infer or implement inner structures, coupling relationships, and functions of the driving modules 282-28N according to description of the driving module 281.
FIG. 4 is a diagram illustrating the driving module 281 and corresponding circuits thereof, which includes a light-emitting diode short circuit detector 221 and a light-emitting diode driver 241.
The light-emitting diode driver 241 roughly makes current flowing through the series of light-emitting diodes L1 be a predetermined current value through the gate terminal G1 and the current detection terminal CS1 when the series of light-emitting diodes L1 is turned on. Due to a negative feedback mechanism provided by an operational amplifier 641, the current flowing through the series of light-emitting diodes L1 can be roughly equal to a setting voltage VISET divided by a resistance of the detection resistor RS1. Because the setting voltages VISET in the driving modules 281-28N are the same, the currents flowing through the plurality of series of light-emitting diodes L1-LN are the same if the detection resistors RS1-RSN are the same. Thus, luminances of the plurality of series of light-emitting diodes L1-LN are roughly the same.
The light-emitting diode short circuit detector 221 coupled to the feedback terminal FB1 can determine whether the series of light-emitting diodes L1 has a light-emitting diode short circuit event when the series of light-emitting diodes L1 is turned on. If the light-emitting diode short circuit detector 221 determines that the series of light-emitting diodes L1 has the light-emitting diode short circuit event, the light-emitting diode short circuit detector 221 forces disabling of the light-emitting diode driver 241 through a signal SH1. Meanwhile, a register 25 can disable the operational amplifier 641 to turn off the NMOS transistors N1 to make the series of light-emitting diodes L1 be turned off.
In the light-emitting diode short circuit detector 221, a comparator C1 compares the feedback voltage VFB1 of the feedback terminal FB1 with a short circuit reference voltage VREF-SH. A constant current source CC1 is coupled to a switch SW1 and a buffer BU through a detection terminal SE1. If the feedback voltage VFB1 is higher than the short circuit reference voltage VREF-SH, the switch SW1 is turned on. Thus, a part of a setting current ISET provided by the constant current source CC1 becomes a reference current IREF which sinks current flowing from the feedback terminal FB1 to the switch SW1 to decrease the feedback voltage VFB1. Therefore, a maximum of the reference current IREF is the setting current ISET. That is to say, if a current IFB1 flowing through the resistor R1 to the feedback terminal FB1 is not greater than the setting current ISET, the feedback voltage VFB1 can be roughly maintained in a range lower than the short circuit reference voltage VREF-SH; if the current IFB1 is smaller than the setting current ISET, a voltage of the detection terminal SE1 can be maintained at a low voltage level (such as 0V) which can be regarded as a logic “0”. Meanwhile, the buffer BU maintains the signal SH1 at the logic “0”, so the light-emitting diode driver 241 can drive the series of light-emitting diodes L1 to be turned on.
On the other hand, when the feedback voltage VFB1 is lower than the short circuit reference voltage VREF-SH, the current IFB1 is about equal to 0, so the feedback voltage VFB1 is roughly equal to a cathode voltage VD1. Thus, pulse width controller 30 and the minimum voltage controller 26 in FIG. 3 can operate normally.
When the current IFB1 is greater than the setting current ISET, the feedback voltage VFB1 exceeds the short circuit reference voltage VREF-SH. Meanwhile, the switch SW1 is turned on completely (fully-ON), and the voltage of the detection terminal SE1 is roughly equal to the feedback voltage VFB1. Therefore, the buffer BU acts as a detector to detect the voltage of the detection terminal SE1 (that is, to detect the feedback voltage VFB1). If the voltage of the detection terminal SE1 is higher than a threshold value VTH (such as 3V), the buffer BU may regard the voltage of the detection terminal SE1 as a logic “1”, and the buffer BU can output the signal SH1 with the logic “1”. Thus, it is considered that the series of light-emitting diodes L1 has the light-emitting diode short circuit event. Meanwhile, the light-emitting diode driver 241 can be forced to be disabled to roughly make the series of light-emitting diodes L1 be turned off.
In one embodiment of the present invention, the short circuit reference voltage VREF-SH is higher than the threshold value VTH which is utilized to determine the logic “1” or the logic “0” by the buffer BU. Thus, when the series of light-emitting diodes L1 is regarded to have the light-emitting diode short circuit event, relationships among the setting current ISET, the short circuit reference voltage VRES-SH, a trigger cathode voltage VD1-TH, and the resistor R1 can be determined according to equation I:

ISET=IFB1=(VD1-TH−VREF-SH)/R1  I

Rearrangement of equation I generates equation II as:

R1=(VD1-TH−VREF-SH)/ISET  II

As shown in equation II, the trigger cathode voltage VD1-TH can be set when the series of light-emitting diodes L1 has the light-emitting diode short circuit event through selection of the proper resistor R1. For example, when the pulse width controller 30 operates normally, the minimum value of the feedback voltage VFB1-VFBN (equal to the minimum value of the cathode voltage VD1-VDN) is roughly maintained at 1V; number of light-emitting diodes of each series of light-emitting diodes is the same; a voltage drop of each light-emitting diode is roughly equal to 3.5V when each light-emitting diode operates normally; the setting current ISET is equal to 20 uA; the short circuit reference voltage VRES-SH is 4V; the threshold value VTH is 3V; and the series of light-emitting diodes L1 is regarded as having the light-emitting diode short circuit event when at least three light-emitting diodes are shorted. Thus, the trigger cathode voltage VD1-TH should be equal to 11.5 (=3.5*3+1) V, the resistor R1 should be determined to be (11.5−4)/20 MΩ.
In another embodiment of the present invention, the short circuit reference voltage VREF-SH is lower than the threshold value VTH which is utilized to determine the logic “1” or the logic “0” by the buffer BU. Thus, only the short circuit reference voltage VREF-SH is replaced with the threshold value VTH in equation I and equation II, and equation for selecting the proper resistor R1 can be obtained.
The driving module 281 in FIG. 4 can determine whether the series of light-emitting diodes L1 has the light-emitting diode short circuit event. And, operation of light-emitting diode short circuit detectors of other driving modules is not influenced after the light-emitting diode short circuit detector 221 disables the light-emitting diode driver 241.
FIG. 5 is a diagram illustrating a driving module 28A1 instead of the driving module 281 in FIG. 3. A difference between the driving module 281 and the driving module 28A1 is that a short circuit reference voltage VREF-SH of the driving module 28A1 corresponds to the minimum feedback voltage VFB-MIN which corresponds to a minimum value of the cathode voltage VD1-VDN, where the short circuit reference voltage VREF-SH of the driving modules 281 is fixed. In FIG. 5, a constant voltage source VSF adds a predetermined voltage to the minimum feedback voltage VFB-MIN of the feedback terminal FB-MIN to act as the short circuit reference voltage VREF-SH.
In FIG. 4 and FIG. 5, the comparator C1 and the switch SW1 can be regarded as a rectifier. When the feedback voltage VFB1 of the feedback terminal FB1 exceeds the short circuit reference voltage VREF-SH, the rectifier is turned on, and the setting current ISET sinks the feedback terminal FB1. When the feedback voltage VFB1 of the feedback terminal FB1 is lower than the short circuit reference voltage VREF-SH, the rectifier is turned off, and the setting current ISET can not sink the feedback terminal FB1. Therefore, the threshold voltage of the rectifier is the short circuit reference voltage VREF-SH.
FIG. 6A and FIG. 6B are diagrams illustrating light-emitting diode short circuit detectors 22A1 and 22B1 according to another embodiment, where each of the light-emitting diode short circuit detectors 22A1 and 22B1 can substitute for light-emitting diode short circuit detector 221 in FIG. 4. In FIG. 6A, a plurality of diodes are electrically connected in series between the feedback terminal FB1 and the detection terminal SE1. An equivalent threshold voltage of the plurality of diodes electrically connected in series is equal to a sum of a threshold voltage of each diode of the plurality of diodes. Therefore, the equivalent threshold voltage can act as a short circuit reference voltage. For example, in FIG. 6A, the equivalent threshold voltage of the plurality of diodes is 3V, so the setting current ISET can sink the feedback terminal FB1 when the feedback voltage VFB1 of the feedback terminal FB1 exceeds 3V. Further, subsequent operational principles of the light-emitting diode short circuit detector 22A1 in FIG. 6A are the same as those of the light-emitting diode short circuit detector 221 in FIG. 4A, so further description thereof is omitted for simplicity. The light-emitting diode short circuit detector 22B1 in FIG. 6B utilizes a zener diode to substitute for the plurality of diodes in FIG. 6A. A breakdown voltage of the zener diode can also act as a short circuit reference voltage. Those skilled in the art can realize operational principles of the light-emitting diode short circuit detector 22B1 in FIG. 6B according to the above mentioned description, so further description thereof is omitted for simplicity.
In another embodiment of the present invention, a zener diode is coupled between the feedback terminal FB1 in FIG. 4 and ground for clamping a maximum voltage of the feedback terminal FB1 to prevent other components from being damaged by a high voltage stress. Thus, the backlight controller 20 can be formed on a monolithic integrated circuit through a low voltage process to reduce cost of the backlight controller 20.
To sum up, compared to the light-emitting diode power supply 8 in FIG. 1, the power control circuit 18 in FIG. 2 does not need high voltage components, so cost of the power control circuit 18 not only cheaper, but the backlight controller 20 in FIG. 2 can also independently detect short circuit events of different series of light-emitting diodes.
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.

## Claims

1. A short circuit detection circuit for detecting short circuit of a series of light-emitting diodes, the short circuit detection circuit comprising:
a constant current source for providing setting current;
a rectifier coupled between a feedback terminal and the constant current source, wherein the feedback terminal is coupled to a terminal of the series of light-emitting diodes for making the setting current sink the feedback terminal when a feedback voltage of the feedback terminal exceeds a short circuit reference voltage; and
a detector for detecting the feedback voltage of the feedback terminal, and triggering short-circuit protection to make the series of light-emitting diodes be turned off when the feedback voltage is higher than a predetermined value.

2. The short circuit detection circuit of claim 1, wherein the rectifier comprises:
a comparator having two input terminals coupled to the short circuit reference voltage and the feedback terminal, respectively; and
a switch coupled between the feedback terminal and a monitor terminal, wherein the switch has a control terminal coupled to an output terminal of the comparator;
wherein the constant current source is coupled to the monitor terminal.

3. The short circuit detection circuit of claim 2, wherein the detector coupled to the monitor terminal detects the feedback voltage through the switch.

4. The short circuit detection circuit of claim 1, wherein the rectifier comprises:
a series of diodes having a threshold voltage which is about equal to the short circuit reference voltage.

5. The short circuit detection circuit of claim 1, wherein the rectifier comprises:
a zener diode having a threshold voltage which is about equal to the short circuit reference voltage.

6. A power control circuit for controlling turning-on of a plurality of series of light-emitting diodes, the power control circuit comprising:
a plurality of short circuit detection circuits coupled to a plurality of series of light-emitting diodes through a plurality of feedback terminals, wherein each of the plurality of short circuit detection circuits comprises:
a constant current source for providing setting current;
a rectifier coupled between a feedback terminal of the plurality of feedback terminals and the constant current source, wherein the feedback terminal is coupled to a terminal of a series of light-emitting diodes of the plurality of series of light-emitting diodes for making the setting current sink the feedback terminal when a feedback voltage of the feedback terminal exceeds a short circuit reference voltage; and
a detector for detecting the feedback voltage of the feedback terminal, and triggering short-circuit protection to make the series of light-emitting diodes be turned off when the feedback voltage is higher than a predetermined value;

a minimum value detector coupled to the plurality of feedback terminals to provide a lowest feedback voltage; and
a conversion controller controlling power conversion of a power converter according to the lowest feedback voltage.

7. The power control circuit of claim 6, further comprising:
a reference voltage generator for providing the short circuit reference voltage to each short circuit detection circuit of the plurality of short circuit detection circuits according to the lowest feedback voltage.