Abstract:
A short-circuit detection method provides a dimmer signal for driving a light source and a feedback signal which varies according to the voltage drop across the light source. When the voltage level of the feedback signal is below a reference voltage, a high-level compare signal is provided. When the voltage level of the feedback signal is above the reference voltage, a low-level compare signal is provided. When the dimmer signal is at high level and the compare signal is at low level, a high-level count signal is provided. When the count signal has switched to high level more than a predetermined number of times, a short-circuit signal is outputted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to a short-circuit detection method and related circuit. 
         [0003]    2. Description of the Prior Art 
         [0004]    Light-emitting diodes (LEDs) are widely used as display devices If a single failure occurs to one LED of the LED string (for example, one short-circuited LED), other LEDS may be over-driven by a larger current, thereby deviating from normal brightness or causing permanent device damage. Therefore, the dimmer circuit normally provides short-circuit detection. 
         [0005]      FIG. 1  shows a diagram illustrating a dimmer circuit  100  with short-circuit detection. The dimmer circuit  100  includes a current source  10 , a switch  12 , a signal generator  14 , and a voltage comparator  16 . The current source drives a semiconductor light-emitting device  11 . The voltage levels at both ends of the semiconductor light-emitting device  11  are represented by V OUT  and V FB , while the voltage drop across each LED is represented by V D . The signal generator  14 , coupled to the switch  12  and the voltage comparator  16  generates a dimmer signal V DIMMER  and a control signal V ON  for selectively turning on/off the switch  12  and the voltage comparator  16 , respectively. The switch  12  controls the current path between the current source  10  and the semiconductor light-emitting device  11  according to the dimmer signal V DIMMER . Therefore, the voltage level of the feedback signal V FB  also varies with the dimmer signal V DIMMER . When turned on by the control signal V ON , the voltage comparator  16  outputs a corresponding short-circuit signal V SH  by comparing the voltage levels of the feedback signal V FB  and a reference voltage V REF . 
         [0006]      FIG. 2  shows a timing diagram illustrating the operation of the dimmer circuit  100 .  FIG. 2  shows the waveforms of the dimmer signal V DIMMER , the feedback signal V FB , the control signal V ON , and the short-circuit signal V SH . At T 2  and T 4 , the control signal V ON  switches from low level to high level, thereby turning on the voltage comparator  16 : if the feedback signal V FB  is smaller than the reference voltage V REF , the voltage comparator  16  outputs a low-level short-circuit signal V SH ; if the feedback signal V FB  is larger than the reference voltage V REF , the voltage comparator  16  outputs a high-level short-circuit signal V SH . For example, if all devices (such as N LEDs coupled in series) function normally when the dimmer signal V DIMMER  switches from low level to high level at T 1 , the dimmer signal V DIMMER  turns on the switch  12 . At this moment, a voltage drop V D  is established across each LED due the current flowing through the semiconductor light-emitting device  11 , thereby causing the feedback signal V FB  to fall from a high level V H  (about V OUT ) to a low level V L  (about V OUT −N*V D ). In the ideal case, the feedback signal V FB  immediately switches from high level to low level when the dimmer signal V DIMMER  switches from low level to high level. Under this circumstance, the voltage level V L  of the feedback signal V FB  is lower than reference voltage V REF  at T 2 , and the voltage comparator  16  thus outputs the low-level short-circuit signal V SH . On the other hand, if (N−n) LEDs in the semiconductor light-emitting device  11  become short-circuited at T 5 , voltage drops V D  are only established across n normal LEDs in the semiconductor light-emitting device  11  when the dimmer signal V DIMMER  switches from low level to high level at T 6 . At this moment, the feedback signal V FB  can only be lowered to a level V L ′ (about V OUT −n*V D ) instead of to the ideal level V L  (about V OUT −N*V D ). As the number of short-circuited LEDs increases, the voltage level V L ′ eventually exceeds the reference voltage V REF . The voltage comparator  16  can thus notify short-circuit by outputting the high-level short-circuit signal V SH . 
         [0007]    The dimmer circuit  100  provide analog short circuit detection. In the ideal case, the feedback signal V FB  immediately switches from high level to low level when the dimmer signal V DIMMER  switches from low level to high level at T 1 . However in reality, a delay time T DELAY  is required before the semiconductor light-emitting device  11  becomes stable. For example, when the dimmer signal V DIMMER  switches from low level to high level at T 3 , the level of the feedback signal V FB  gradually decreases and eventually reaches the stable level V L  at T 5 . If the voltage comparator  16  is turned on before the feedback signal V FB  becomes stable, the detected level V L ′ may be higher than the reference voltage V REF . Even if all devices in the semiconductor light-emitting device  11  (such as N LEDs coupled in series) function normally, the voltage comparator  16  may output the high-level short-circuit signal at T 4 , which causes false short-circuit alarm. 
       SUMMARY OF THE INVENTION 
       [0008]    A dimmer circuit with short-circuit detection is disclosed. The dimmer circuit comprises a signal generator configured to generate a dimmer signal for driving a light source; a voltage comparator configured to provide a compare signal and switches a level of the compare signal according to voltage levels of a feedback signal and a reference voltage, wherein the feedback signal is varies according to a voltage drop across the light source; a short-circuit signal generator configured to provide a count signal and switch the count signal to a first level when the dimmer signal is at the first level and the compare signal is at a second level; and a count circuit configured to generate a short-circuit signal according to how many times the count signal switches to the first level. 
         [0009]    A dimmer circuit with short-circuit detection for driving a light source is also disclosed. The dimmer circuit comprises a current source; a switch for controlling a signal transmission path between the current source and the light source according to a dimmer signal; a signal generator configured to generate the dimmer signal; and a short-circuit judging circuit configured to determine whether a short-circuit occurs in the light source according to the dimmer signal and how many times a voltage level of a feedback signal drops below a voltage level of a reference voltage, wherein the feedback signal varies according to a voltage drop across the light source. 
         [0010]    A method for providing short-circuit detection is also disclosed. The method comprises providing a dimmer signal for driving a light source; providing a feedback signal which varies according to a voltage drop across the light source; switching a compare signal from a second level to a first level when the feedback signal is below a reference voltage; providing the compare signal having the second level when the feedback signal is above the reference voltage; switching a count signal from the second level to the first level when the dimmer signal is at the first level and the compare signal is at the second level; and outputting a short-circuit signal when the count signal has switched from the second level to the first level over a predetermined number of times. 
         [0011]    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 
         [0012]      FIG. 1  is a diagram illustrating a dimmer circuit with short-circuit detection. 
           [0013]      FIG. 2  is a timing diagram illustrating the operation of the dimmer circuit in  FIG. 1 . 
           [0014]      FIG. 3  is a diagram illustrating a dimmer circuit with short-circuit detection according to the present invention. 
           [0015]      FIG. 4  is a circuit diagram of a dimmer circuit according an embodiment of the present invention. 
           [0016]      FIG. 5  is a timing diagram illustrating the operation of the dimmer circuit in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but in function. In the following discussion and in the claims, the terms “include”, “including”, “comprise”, and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” is intended to mean either a direct or an indirect electrical connection. Accordingly, if one device is coupled to another device, the electrical connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
         [0018]      FIG. 3  illustrates a diagram illustrating a dimmer circuit  200  with short-circuit detection. The dimmer circuit  200  includes a current source  20 , a switch  22 , a signal generator  24 , a voltage comparator  26 , a reset circuit  28 , a short-circuit signal generator  32 , and a counter  34 . The current source  20  drives a semiconductor light-emitting device  21 . The voltage levels at both ends of the semiconductor light-emitting device  21  are represented by V OUT  and V FB , while the voltage drop across each LED is represented by V D . The signal generator  24 , coupled to the switch  22 , the reset circuit  28  and the short-circuit signal generator  32 , generates a dimmer signal V DIMMER  for selectively turning on/off the switch  22 . The reset circuit  28 , coupled to the signal generator  24  and the short-circuit signal generator  32 , generates a corresponding reset signal V RESET  according to the dimmer signal V DIMMER . The voltage comparator  26 , the short-circuit signal generator  32 , and a counter  34  together form a short-circuit judging circuit which provides a short-circuit signal V SH  according to how many times the voltage level of the feedback signal V FB  drops below that of a reference voltage V REF . The voltage comparator  26 , coupled between the semiconductor light-emitting device  21  and the short-circuit signal generator  32 , generates a corresponding compare signal V COMP  by comparing the feedback signal V FB  with the reference voltage V REF . The voltage comparator  26  is always on during operation. The short-circuit signal generator  32 , coupled to the voltage comparator  26 , the reset circuit  28  and the signal generator  24 , generates a corresponding count signal V CT  according to the compare signal V COMP  and the reset signal V RESET . The short-circuit signal generator  32  outputs a high-level count signal V CT  only when receiving a high-level dimmer signal V DIMMER  and a low-level compare signal V COMP ; otherwise, the short-circuit signal generator  32  outputs a low-level count signal V CT . The counter  34 , coupled to the short-circuit signal generator  32 , generates the corresponding short-circuit signal V SH  based on how many times the count signal V CT  switches levels. 
         [0019]      FIG. 4  is a circuit diagram of a dimmer circuit  200  according an embodiment of the present invention. In this illustrated embodiment, the reset circuit  28  includes a delay unit  36 , an inverter  42  and NAND gates  44  and  45 . The short-circuit signal generator  32  includes an RS latch  38 , an inverter  43 , and a NAND gate  46 . According to the dimmer signal V DIMMER , the reset circuit  28  transmits the corresponding reset signal V RESET  to a RESET terminal of the RS latch  38  so as to reset the status of the RS latch  38  in the previous period. When the dimmer signal V DIMMER  is at high level and the feedback signal V FB  is smaller than the reference voltage V REF , the compare signal V COMP  received at a SET terminal of the RS latch  38  switches to high level. The RS latch  38  thus outputs a low-level signal at a  Q  terminal, thereby generating a low-level count signal V CT  using the NAND gate  46  and the inverter  43 . When the dimmer signal V DIMMER  is at high level and the feedback signal V FB  is larger than the reference voltage V REF , the compare signal V COMP  received at the SET terminal of the RS latch  38  is at low level. The RS latch  38  thus outputs a high-level signal at the  Q  terminal, thereby outputting the dimmer signal V DIMMER  as the count signal V CT  using the NAND gate  46  and the inverter  43 . 
         [0020]      FIG. 5  is a timing diagram illustrating the operation of the dimmer circuit  200  according to the present invention.  FIG. 5  shows the waveforms of the dimmer signal V DIMMER , the feedback signal V FB , the compare signal V COMP , the count signal V CT  and the short-circuit signal V SH . Assuming that all devices of the semiconductor device  21  (such as N LEDs coupled in series) function normally, the switch  22  is turned on when the dimmer signal V DIMMER  switches to high level at T 1 . The current flowing through the semiconductor light-emitting device  21  establishes a voltage drop across each LED, and the feedback signal V FB  thus gradually decreases from high level V H  (about V OUT ) to low level V L  (about V OUT −N*V D ). At T 2  when the feedback signal V FB  becomes smaller than the reference voltage V REF , the voltage comparator  26  outputs a high-level compare signal V COMP , the short-circuit signal generator  32  outputs a low-level count signal V CT , and the counter  34  outputs a low-level short-circuit signal V SH . At T 3  when the dimmer signal V DIMMER  switches from high level to low level, the switch  22  is turned off and no current flows through the semiconductor light-emitting device  21 . The feedback signal V FB  thus gradually rises from low level V L  to high level V H . The voltage comparator  26  outputs a low-level compare signal V COMP , while the count signal V CT  and the short-circuit signal V SH  remain at low level. 
         [0021]    If (N−n) LEDs in the semiconductor light-emitting device  21  become short-circuited after T 3 , voltage drops are only established across the n normal LEDs due the current flowing through the semiconductor light-emitting device  21  when the dimmer signal V DIMMER  switches from low level to high level at T 4 . At this moment, the feedback signal V FB  can only be lowered to a level V L ′ (about V OUT −n*V D ) instead of the ideal level V L  (about V OUT −N*V D ). If the number of short-circuited LEDs exceeds a certain number, the voltage level V L ′ becomes larger than the reference voltage V REF  and the voltage comparator  26  continues to output the low-level short-circuit signal V SH . The short-circuit signal generator  32  thus outputs the dimmer signal V DIMMER  as the count signal V CT , and the counter  34  increases its count value by 1 upon receiving the high-level count signal V CT . If the count value of the counter  34  does not exceed a predetermined value, the counter  34  continue to output the low-level short-circuit signal V SH . If the feedback signal V FB  is still larger than the reference voltage V REF  at T 6 , the counter  34  continue to receive the high-level count signal V CT  and its count value is again increased by 1. If the count value of the counter  34  exceeds the predetermined value, the counter  34  outputs the high-level short-circuit signal V SH . 
         [0022]    The proposed dimmer circuit  200  provides short circuit detection. The short-circuit signal generator  32  outputs a high-level count signal V CT  only when the dimmer signal V DIMMER  is at high level and the compare signal V COMP  is at low level. The counter  34  then measures how many times the count signal V CT  switches levels, based on which the occurrence of a short-circuit failure can be detected. The voltage comparator  26  is always on during operation, thereby capable of providing accurate short-circuit detection without the influence of the delay time. 
         [0023]    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.