Abstract:
An LED driving circuit with open-circuit protection is disclosed. The LED driving circuit pulls down an operation voltage supplied to a converting controller of the LED driving circuit when a voltage across an LED module of the LED driving circuit is too high, so as to stop the converting controller operating and further latch the converting controller at the protection state until that the converting controller is restarted or the condition of the voltage supplied to the LED module being too high is solved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Taiwan application serial no. 100123717, filed on Jul. 5, 2011. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an LED driving circuit, and more particularly relates to an LED driving circuit with open-circuit protection. 
         [0004]    2. Description of Related Art 
         [0005]      FIG. 1  is a schematic diagram of an LED driving circuit. The LED driving circuit comprises a converting controller  10 , an inductance L, a diode D, an output capacitance C, a transistor switch SW, a current detecting resistance Rse to convert a voltage source VIN into an equivalent current source to drive an LED module  30  to light. Mutual connection between the elements and operation thereof in the LED driving circuit are described as following. 
         [0006]    The positive terminal of the LED module  30  is coupled to the voltage source VIN, the negative terminal thereof is coupled to a terminal of the inductance L and the other terminal of the inductance L is coupled to a first terminal of the transistor switch SW. The output capacitance C is connected parallel with the LED module  30 . The positive terminal of the diode D is coupled to the connection node of the inductance L and the transistor switch SW, and the negative terminal thereof is coupled to the voltage source VIN to freewheel the current from the inductance L when the transistor switch SW is turned off. A second terminal of the transistor switch SW is grounded through the current detecting resistance Rse. The converting controller  10  is coupled to a control terminal of the transistor switch SW and adjusts the power for the LED module  30  by controlling the ratio of the turn-on and turn-off periods of the transistor switch SW. 
         [0007]    When the transistor switch SW is turned on, the current from the voltage source VIN flows through the LED module  30 , the inductance L, the transistor switch SW and the current detecting resistance Rse to ground. A current detecting signal FB is generated by the current detecting resistance Rse when the current flows therethrough. The converting controller  10  adjusts an amount of the current flowing through the LED module  30  according to the current detecting resistance Rse. When the current flowing through the LED module  30  reaches a preset current value, the converting controller  10  turns the transistor switch SW off for a constant time period. Then, the current from the inductance L freewheels through the diode D and the energies saved on the inductance L and the output capacitance C are released to light the LED module  30 . After the constant time period, the converting controller  10  turns on the transistor switch SW again to store energies on the inductance L and the output capacitance C. By the foregoing process, the current flowing through the LED module  30  is kept around an average current value so as to reach the object of stably lighting. 
         [0008]    However, when there is anyone LED damaged or abnormal condition in the circuit is happened, no current flows through the LED module  30  and so the LED module is open-circuited and not lighting. If the converting controller  10  keeps operating at this time, it still consumes power. Due to the LED module  30  is open-circuited and do not light, the user may get an electric shock if the user touches the LED module  30  for making sure the condition of the circuit or replacing the LED module with a new one. 
       SUMMARY OF THE INVENTION 
       [0009]    The conventional LED driving circuit cannot be protected the LEDs from overvoltage. The present invention disclosed an LED driving circuit with open-circuit protection. The LED driving circuit pulls down an operation voltage for a converting controller of the LED driving circuit when a voltage across an LED module of the LED driving circuit is too high, so as to stop the converting controller operating and further latch the converting controller at the protection state until that the converting controller is restart or the voltage across to the LED module being too high is solved. 
         [0010]    Furthermore, the invention provides an LED driving circuit with open-circuit protection which comprises an LED module, a converting circuit, a converting controller, an operation voltage generating circuit, an overvoltage detecting circuit and a protecting circuit. The converting circuit is coupled to a voltage source and the LED module. The converting controller controls the converting circuit to provide an electric power to light the LED module according to a current detecting signal representing a current of the LED module. The operation voltage generating circuit is coupled to the voltage source to supply an operation voltage to the converting controller operating. The overvoltage detecting circuit is coupled to the LED module, and generates a open-circuit protecting signal when a voltage across the LED module is higher than a preset voltage protecting value. The protecting circuit is coupled to the operation voltage generating circuit, and decreases the operation voltage to stop the converting controller operating responsive to the open-circuit protecting signal. 
         [0011]    It is to be understood that both the foregoing general description and the following detail description are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
           [0013]      FIG. 1  is a schematic diagram of an LED driving circuit; 
           [0014]      FIG. 2  is a schematic diagram of an LED driving circuit according to a first embodiment of the invention; 
           [0015]      FIG. 3  is a schematic diagram of an LED driving circuit according to a second embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0016]    In the following detail description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
         [0017]      FIG. 2  is a schematic diagram of an LED driving circuit according to a first embodiment of the invention. The LED driving circuit comprises a converting controller  100 , an operation voltage generating circuit  110 , an LED module  130 , an overvoltage detecting circuit  140 , a protecting circuit  150  and a converting circuit. In the present embodiment, the converting circuit is a buck converting circuit, comprising an inductance L, a diode D, an output capacitance C and a transistor switch SW. The converting circuit is coupled to a voltage source VIN and converts the voltage source VIN into a suitable equivalent current source to drive the LED module  130  to stably light according to the controlling of the converting controller  100 . The output capacitance C is connected with the LED module  130  in parallel to filter high frequency noises out to supply a stable power to the LED module  130 . The positive terminal of the LED module  130  is coupled to the voltage source VIN, the negative terminal of the LED module  130  is coupled to a terminal of the inductance L. The other terminal of the inductance L is coupled to a first terminal of the transistor switch SW and a positive terminal of the diode D. A negative terminal of the diode D is coupled to the voltage source VIN and is used to be the current freewheeling path for the inductance L when the transistor switch is turned off. A second terminal of the transistor switch SW is grounded and a control terminal thereof is coupled to the converting controller  100  to be turned on or off according to the controlling of the converting controller  100 . The current detecting resistance Rse is coupled between the inductance L and the transistor switch SW to generate current detecting signals FB+ and FB−. The converting controller  100  adjusts an amount of the current of the LED module  130  according to the current detecting signal FB+ and FB−. The converting controller  100  turns off the transistor switch SW when the current of the LED module  130  increases to a preset maximal current value and turns on the transistor switch SW when it decreases to the preset minimal current value, wherein the preset maximal current value is higher than the preset minimal current value. Therefore, the LED module  130  lights stably with the current of the LED being controlled between the preset maximal and the preset minimal current values. 
         [0018]    The operation voltage generating circuit  110  is coupled to the voltage source VIN to generate an operation voltage VDD to supply the converting controller  100 . The operation voltage generating circuit  110  also can be coupled to other voltage source to replace the voltage source Vin to supply the operation voltage VDD. The operation voltage generating circuit  110  comprises two resistances R 1  and R 2 , an input capacitance C 1  and a zener diode ZD. The resistance R 2 , the input capacitance C 1  and the zener diode ZD are connected in parallel and coupled to the voltage source VIN through the resistance R 1 . The input capacitance C 1  stores the power electric energy from the voltage source VIN to supply the operation voltage VDD that is determined by a breakdown voltage of the zener diode ZD. The resistance R 2  is used to be a releasing path to release the energy stored by the input capacitance C 1  when the voltage source VIN is not supplied to the LED driving circuit. 
         [0019]    The overvoltage detecting circuit  140  is coupled to the LED module  130  to detect a voltage across the LED module  130  and generates an open-circuit protecting signal Top when determining that the voltage across the LED module  130  is higher than a preset voltage protecting value. The overvoltage detecting circuit  140  comprises three resistances R 11 , R 12 , R 14  and a bipolar junction transistor BT. The resistances R 11  and R 12  serves as a voltage divider and are connected with the LED module  130  in parallel to generate a voltage detecting signal VD representing the voltage across the LED module  130 . A base of the bipolar junction transistor BT is coupled to a connection node of the resistances R 11  and R 12  to receive the voltage detecting signal VD. An emitter of the bipolar junction transistor BT is coupled to the voltage source VIN via the resistance R 14  and a collector thereof is coupled to the protecting circuit  150 . The bipolar junction transistor BT is a PNP-type bipolar junction transistor. The bipolar junction transistor BT is turned on and generates the open-circuit protecting signal Iop when a voltage level of the base is a threshold voltage lower than a voltage level of the emitter thereof. 
         [0020]    The protecting circuit  150  is coupled to the overvoltage detecting circuit  140  and the operation voltage generating circuit  110  and makes the operation voltage VDD generated by the operation voltage generating circuit  110  decreasing to put the operation voltage VDD down to be lower than a minimal voltage of the operation voltage of the converting controller  100  and then the converting controller  100  is stopped operating. The protecting circuit  150  comprises a resistance R 15  and the transistor M 1 . A terminal of the resistance R 15  is coupled to a control terminal of the transistor M 1  and the other terminal thereof is grounded. A first terminal of the transistor M 1  is coupled to the operation voltage generating circuit  110  and a second terminal is grounded. When the resistance R 15  receives the open-circuit protecting signal Iop to make a voltage level of the control terminal of the transistor M 1  to be higher than a threshold voltage of the transistor M 1 , the transistor M 1  is turned on. At this time, the input capacitance C 1  in the operation voltage generating circuit  110  is discharged by the transistor M 1  and so the operation voltage VDD supplied by the input capacitance C 1  is decreased to stop the converting controller  100  operating. Therefore, the converting circuit does not convert the power from the voltage source VIN to drive the LED module  130  anymore, and so the voltage across the LED module  130  is stopped increasing to execute the open-circuit protecting function. 
         [0021]      FIG. 3  is a schematic diagram of an LED driving circuit according to a second embodiment of the invention. The LED driving circuit comprises a converting controller  200 , an operation voltage generating circuit  210 , a power delivering circuit  220 , an LED module  230 , an overvoltage detecting circuit  240 , a protecting circuit  250 , a latch circuit  260  and a converting circuit. The converting circuit is coupled to the voltage source VIN and comprises an inductance L, a diode D, an output capacitance C and a transistor switch SW. The converting controller  200  controls the operation of the converting circuit to supply the power to drive the LED module  230  to light stably according to a current detecting signal FB representing an amount of a current of the LED module  230 . In the present embodiment, the overvoltage detecting circuit  240  replaces the bipolar junction transistor BT with the transistor M 2  of the embodiment shown in the  FIG. 2 . 
         [0022]    In the present embodiment, the main differences between the LED driving circuit in the embodiment is and that in  FIG. 2  are the power delivering circuit  220  and the latch circuit  260 . The following will describe the main differences. 
         [0023]    The latch circuit  260  is coupled between the overvoltage detecting circuit  240  and a common potential (it is the ground potential here) to supply a return path for an overvoltage detecting circuit  240 . In the present embodiment, the latch circuit  260  comprises a resistance R 13 . A terminal of the resistance R 13  is coupled to the resistance R 12  in the overvoltage detecting circuit  240  and the other terminal is grounded. When the voltage across the LED module  230  is over high, the overvoltage detecting circuit  240  generates an open-circuit protecting signal Iop to decrease the operation voltage VDD through the protecting circuit  250  and so the operation of converting controller  200  is stopped because the operation voltage VDD is insufficient. If there is not the latch circuit  260  in the converting circuit, the converting circuit is stopped to convert the power from the voltage source VIN, and so the voltage of the output capacitance C is gradually decreased because of current leakage path in the converting circuit. Therefore, the overvoltage detecting circuit  240  may incorrectly determines that the overvoltage issue is removed and so turns the transistor M 1  in the protecting circuit  250  off. It results that the operation voltage VDD supplied by the operation voltage generating circuit  210  increases again and so the converting controller  200  re-operates and the overvoltage issue occurs again. In the present embodiment, the latch circuit  260  is also coupled to the output capacitance C in the converting circuit and maintains a voltage across the output capacitance C to be higher than the preset voltage protecting value once the voltage across the LED module is higher than the preset voltage protecting value. The latch circuit  260  supplies a current from the voltage source Vin into the overvoltage detecting circuit  240  to make the overvoltage detecting circuit  240  keep generating open-circuit protecting signal Top. At this time, the voltage across the output capacitance C is determined by a voltage divider ratio of a voltage divider composed of the resistances R 11  and R 12  in the overvoltage detecting circuit  240  and the resistance R 13  in the latch circuit  260 , i.e.; vin*(r 11 +r 12 )/(r 11 +r 12 +r 13 ), wherein the vin is the voltage of the voltage source VIN and r 11 , r 12  and r 13  are the resistance values of the resistance R 11 , R 12  and R 13  separately. The vin*(r 11 +r 12 )/(r 11 +r 12 +r 13 ) must be higher than the preset voltage protecting value to make the overvoltage detecting circuit  240  keep generating the open-circuit protecting signal Iop after the voltage across the LED module is higher than the preset voltage protecting value. Therefore, the LED driving circuit is latched at a protecting state before the cause of the voltage across the LED module higher than the preset voltage protecting value, such as, open-circuit, is solved for avoiding the personnel safety for the user. When the voltage across the LED module higher than the preset voltage protecting value is solved and the power supplied by the latch circuit  260  cannot supplies the LED module  230  lighting without decreasing the current voltage of the output capacitance C. Therefore, the voltage across the output capacitance C is decreased to make the overvoltage detecting circuit  240  stop generating open-circuit protecting signal Iop and so the transistor M 1  in the protecting circuit  250  is turned off. Then, the operation voltage VDD rises again to remove the protecting state of the LED driving circuit and the converting controller  200  re-operates. 
         [0024]    The power delivering circuit  220  is coupled between the converting circuit and the operation voltage generating circuit  210 . When the converting circuit converts the voltage source into the current source to drive the LED module  230  to light, the power delivering circuit  220  delivers energy into the input capacitance C 1  in the operation voltage generating circuit  210 . Therefore, the resistance value of the resistance R 1  in the operation voltage generating circuit  210  may be setup larger to decrease the power consumption of the operation voltage generating circuit  210  when the LED driving circuit is not operating. The insufficient power supplied by the operation voltage generating circuit  210  may be complemented by the power delivering circuit  220 . The diodes D 1 , D 2  and the resistance R 3  are connected in series between the ground potential and the negative terminal of the zener diode ZD in the operation voltage generating circuit  210 . The capacitance C 2  is coupled to the positive terminal of the diode D in the converting circuit and the connection point of the diodes D 1  and D 2 . When the transistor switch SW in the converting circuit is turned on, the capacitance C 2  stores energy from the diode D 1 . When the transistor switch SW in the converting circuit is turned off, a voltage level of the positive terminal of the diode D is increased and slightly higher the voltage source VIN. At this time, the capacitance C 2  releases the energy stored therein to the operation voltage generating circuit  210  through the diode D 2 . 
         [0025]    All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.