Abstract:
The present invention is to provide an LED driver circuit which includes a capacitor and at least one LED respectively connected in parallel to a power source through a switch device, and a controller having an input end connected to a line between the switch device and the power source and being configured for detecting an input voltage applied to the LED. When the controller detected that the input voltage is unable to drive the LED to emit light, the controller activates the switch device to make only the connection between the capacitor and the at least one LED, such that the capacitor discharges a voltage stored therein to the at least one LED to continuously emit light. Since the driver circuit only needs a small-capacity, low-cost capacitor for increasing the light emission time of the at least one LED, the cost of the driver circuit can be lowered effectively.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an LED driver circuit, more particularly to an LED driver circuit having a controller to detect a voltage or current applied to the LED and capable of enabling a capacitor to discharge a voltage stored therein to the LED for continuously emitting light when the controller detected that the voltage or current can&#39;t drive the LED to emit light. 
     BACKGROUND OF THE INVENTION 
     Light-emitting diodes (LEDs) are light-emitting elements made of semiconductor materials whose properties enable conversion from electric energy into light energy. LEDs have been massively used in lighting- and display-related applications due to their small physical volumes, long lifetimes, low driving voltages, short response times, and ready adaptability to various products in our daily lives. 
       FIG. 1  schematically shows the circuit structure of a conventional simple LED driver circuit  100 , which includes a power source  11  and at least one LED  13 . The power source  11  can generate a pulsed direct-current (DC) voltage for driving the LED  13 . 
     Referring to  FIG. 1  and  FIG. 2 , the input voltage V IN  generated by the power source  11  varies periodically over time. When the input voltage V IN  is higher than the forward bias voltage V F  of the LED  13 , the LED  13  can be successfully driven to emit light by the power source  11 , as in the state of S=1 in  FIG. 2 . When the input voltage V IN  is lower than the forward bias voltage V F  of the LED  13 , however, the LED  13  cannot be driven to emit light by the power source  11 , as in the state of S=0 in  FIG. 2 . 
     Referring to  FIG. 1  and  FIG. 3 , in order for the LED  13  to emit light continuously, one conventional approach is to add a capacitor  15  to the LED driver circuit  100 . The capacitor  15  is charged by the power source  11  and thus stores a stored electric energy V C  while the power source  11  is driving the LED  13 . Therefore, the power supplied from the LED driver circuit  100  to the LED  13  will be a ripple voltage, which is the sum of the input voltage V IN  and the stored electric energy V C  (i.e., V IN +V C ) and which can keep the LED  13  emitting light, as in the state of S=1 in  FIG. 3 . 
     However, despite the fact that the LED driver circuit  100  with the additional capacitor  15  can drive the LED  13  to emit light continuously, the aforesaid circuit design entails a waste of energy, for the capacitor  15  will keep discharging even when the power source  11  is at a relatively high potential level (e.g., when the input voltage V IN  is higher than the forward bias voltage V F  of the LED  13 ). 
     Moreover, in order to provide a sufficient driving power to the LED  13  incessantly, the ripple voltage (V IN +V C ) must maintain at a relatively high level for a long time, and this cannot be done without a large-capacity capacitor  15  for energy storage. Nevertheless, as a large-capacity capacitor  15  generally can only be implemented by an electrolytic capacitor, which is both costly and prone to damage, the resultant LED driver circuit  100  will have a high circuit cost and less-than-satisfactory quality. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the fact that a conventional LED driver circuit requires a large-capacity capacitor in order to enable sustained emission of light from the LED(s) to be driven, the inventor of the present invention put years of practical experience into extensive research and experiment and finally succeeded in developing an LED driver circuit in which a capacitor is controlled by a switch and will not be discharged to an LED unless the input voltage is relatively low. The present invention provides a circuit arrangement in which a small-capacity, low-cost capacitor will suffice to effectively increase the light emission time of an LED. 
     It is an object of the present invention to provide an LED driver circuit which includes a controller, a switch device, a capacitor, and at least one LED. The capacitor and the at least one LED are respectively connected in parallel to a power source through the switch device. The controller has an input end connected to a line between the switch device and the power source and is configured for detecting an input voltage of the power source. When determining that the input voltage is higher than the forward bias voltage of the at least one LED, the controller generates a first control signal and sends the first control signal to the switch device through the output end of the controller, causing the switch device to make both the connection between the power source and the capacitor and the connection between the power source and the at least one LED, so as for the input voltage to not only drive the at least one LED to emit light, but also charge the capacitor simultaneously. Conversely, when determining that the input voltage is lower than the forward bias voltage of the at least one LED, the controller generates a second control signal and sends the second control signal to the switch device through the output end of the controller, causing the switch device to make only the connection between the capacitor and the at least one LED, so as for a discharge voltage of the capacitor to drive the at least one LED to emit light. Thus, the driver circuit only needs a small-capacity, low-cost capacitor in order to increase the light emission time of the at least one LED effectively, and the circuit cost of the driver circuit can be lowered. 
     Another object of the present invention is to provide yet another LED driver circuit, which also includes at least one LED, a capacitor, a switch device, and a controller. The capacitor and the at least one LED are respectively connected in parallel to a power source through the switch device. The controller has an input end connected to the switch device and is configured for detecting a load current in the switch device. When determining that the load current is higher than a rated current value, the controller generates a first control signal and sends the first control signal to the switch device through the output end of the controller, causing the switch device to make both the connection between the power source and the capacitor and the connection between the power source and the at least one LED, so as for an input voltage provided by the power source to not only drive the at least one LED to emit light, but also charge the capacitor simultaneously. When determining that the load current is lower than the rated current value, the controller generates a second control signal and sends the second control signal to the switch device through the output end, causing the switch device to make only the connection between the capacitor and the at least one LED, so as for a discharge voltage of the capacitor to drive the at least one LED to emit light. 
     Still another object of the present invention is to provide yet another LED driver circuit, wherein the LED driver circuit includes at least one LED, a capacitor, a first switch device, a second switch device, and a controller. The positive electrode of the capacitor and an LED are respectively connected to the positive electrode of a power source through the first switch device. The first switch device is configured to prevent a discharge current of the capacitor from flowing back to the power source and to ensure that the discharge current flows to the at least one LED. The negative electrode of the capacitor and an LED are respectively connected to the negative electrode of the power source through the second switch device. The controller has an input end connected to a line between the first switch device and the positive electrode of the power source and is configured for detecting an input voltage provided by the power source. When determining that the input voltage is higher than the forward bias voltage of the at least one LED, the controller generates a first control signal and sends the first control signal to the second switch device through the output end of the controller, causing the second switch device to make both the connection between the power source and the capacitor and the connection between the power source and the at least one LED, so as for the input voltage to not only drive the at least one LED to emit light, but also charge the capacitor simultaneously. When determining that the input voltage is lower than the forward bias voltage of the at least one LED, the controller generates a second control signal and sends the second control signal to the second switch device through the output end, causing the second switch device to make only the connection between the capacitor and the at least one LED, so as for a discharge voltage of the capacitor to drive the at least one LED to emit light. 
     Yet another object of the present invention is to provide still another LED driver circuit, which also includes at least one LED, a capacitor, a first switch device, a second switch device, and a controller. The positive electrode of the capacitor and an LED are respectively connected to the positive electrode of a power source through the first switch device. The first switch device is configured to prevent a discharge current of the capacitor from flowing back to the power source and to ensure that the discharge current flows to the at least one LED. The negative electrode of the capacitor and an LED are respectively connected to the negative electrode of the power source through the second switch device. The controller has an input end connected to the second switch device and is configured for detecting a load current in the second switch device. When determining that the load current is higher than a rated current value, the controller generates a first control signal and sends the first control signal to the second switch device through the output end of the controller, causing the second switch device to make both the connection between the power source and the capacitor and the connection between the power source and the at least one LED, so as for an input voltage provided by the power source to not only drive the at least one LED to emit light, but also charge the capacitor simultaneously. When determining that the load current is lower than the rated current value, the controller generates a second control signal and sends the second control signal to the second switch device through the output end, causing the second switch device to make only the connection between the capacitor and the at least one LED, so as for a discharge voltage of the capacitor to drive the at least one LED to emit light. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The structure as well as a preferred mode of use, further objects, and advantages of the present invention will be best understood by referring to the following detailed description of some illustrative embodiments in conjunction with the accompanying drawings, in which: 
         FIG. 1  schematically shows the circuit structure of a conventional LED driver circuit; 
         FIG. 2  shows the waveform diagram of a conventional input voltage and the states of a corresponding LED in relation to time; 
         FIG. 3  shows the waveform diagram of a conventional ripple voltage and the state of a corresponding LED in relation to time; 
         FIG. 4  schematically shows the circuit structure of the LED driver circuit according to a preferred embodiment of the present invention; 
         FIG. 5  schematically shows the circuit structure of the LED driver circuit according to another embodiment of the present invention; 
         FIG. 6  schematically shows the circuit structure of the LED driver circuit according to yet another embodiment of the present invention; and 
         FIG. 7  schematically shows the circuit structure of the LED driver circuit according to still another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Please refer to  FIG. 4  for a schematic circuit diagram of the LED driver circuit according to a preferred embodiment of the present invention. As shown in the drawing, the LED driver circuit  200  in this embodiment includes a power source  21 , a controller  23 , a switch device  30 , at least one LED  27 , and a capacitor  29 . 
     The power source  21  can provide a DC input voltage. The capacitor  29  and the LED  27  are respectively connected in parallel to the power source  21  via the switch device  30 . The input end of the controller  23  is connected to a line between the switch device  30  and the power source  21  in order for the controller  23  to detect an input voltage provided by the power source  21 . Upon determining that the input voltage is higher than the forward bias voltage of the LED  27 , the controller  23  generates a first control signal and sends the first control signal to the switch device  30  via the output end of the controller  23 . Thus, the switch device  30  is driven to make the connection between the power source  21  and the capacitor  29  and the connection between the power source  21  and the LED  27 , so as for the input voltage to not only drive the LED  27  to emit light, but also charge the capacitor  29  at the same time. 
     Conversely, upon determining that the input voltage is lower than the forward bias voltage of the LED  27 , the controller  23  generates a second control signal and sends the second control signal to the switch device  30  via the output end of the controller  23 . Thus, the switch device  30  is driven to make only the connection between the capacitor  29  and the LED  27 , so as for the discharge voltage of the capacitor  29  to drive the LED  27  to emit light. Therefore, the LED driver circuit  200  only needs to be provided with a small-capacity, low-cost capacitor  29 , and the light emission time of the LED  27  can be effectively increased at a low circuit cost. 
     Referring again to  FIG. 4 , in the first preferred embodiment of the present invention, the switch device  30  includes a first diode  221 , a second diode  222 , and a transistor switch  24 . The positive electrodes of the first and the second diodes  221 ,  222  are connected to the positive electrode of the power source  21 . The input end and the output end of the transistor switch  24  are respectively connected to the negative electrodes of the two diodes  221 ,  222 . When the control end of the transistor switch  24  receives the first control signal, the transistor switch  24  cuts off the current between its input end and output end. When the control end of the transistor switch  24  receives the second control signal, the transistor switch  24  allows passage of current between its input end and output end. 
     It should be pointed out that, in the first preferred embodiment of the present invention as shown in  FIG. 4 , the transistor switch  24  is an N-type metal-oxide-semiconductor field-effect transistor (MOSFET), whose gate is the control end of the transistor switch  24  and whose drain and source are the input end and the output end of the transistor switch  24  respectively. Nevertheless, the transistor switch  24  may also be a P-type MOSFET or a bipolar junction transistor (BJT). When a P-type MOSFET is used, its gate functions as the control end, and its source and drain function as the input end and the output end respectively. When an NPN-type BJT is used, its base is the control end, and its collector and emitter are the input end and the output end respectively. In other words, the terms “input end” and “output end” merely indicate the current directions in the switch device  30  and may be adjusted in practice according to the material properties of the transistor used. 
     As the LED driver circuit  200  of the present invention is so configured that only when the input voltage generated by the power source  21  is lower than the forward bias voltage of the LED  27  will the switch device  30  make the connection between the capacitor  29  and the LED  27  under the control of the controller  23 , thus allowing the fully charged capacitor  29  to drive the LED  27  to emit light, continuous light emission by the LED  27  can be achieved with a small-capacity, low-cost capacitor (e.g., a capacitor with a solid electrolyte) serving as the capacitor  29 . Thus, the circuit cost of the driver circuit  200  will also be significantly reduced. 
     In the aforesaid embodiment, control is carried out through voltage detection. In practice, however, control may be carried out through current detection as well. Referring to  FIG. 5  for a schematic circuit diagram of the LED driver circuit  201  according to the second preferred embodiment of the present invention, the controller  23  in the LED driver circuit  201  has its input end connected to the switch device  30 ′ so as to detect a load current in the switch device  30 ′. When determining that the load current is higher than a rated current value, the controller  23  drives the switch device  30 ′ to make the connection between the power source  21  and the capacitor  29  and the connection between the power source  21  and the LED  27 ; as a result, the power source  21  charges the capacitor  29  while driving the LED  27  to emit light. When determining that the load current is lower than the rated current value, the controller  23  drives the switch device  30 ′ to make only the connection between the capacitor  29  and the LED  27 , thus allowing the capacitor  29  to drive the LED  27  to emit light. 
     Compared with its counterpart in the embodiment shown in  FIG. 4 , the switch device  30 ′ of the LED driver circuit  201  in the second preferred embodiment includes a current-limiting unit  26  (e.g., a current-limiting switch or a resistor) in addition to the first diode  221 , the second diode  222 , and the transistor switch  24 . The current-limiting unit  26  has one end connected to the negative electrode of the second diode  222 . When a current flows through the current-limiting unit  26 , the current-limiting unit  26  generates the aforesaid load current and provides the load current to the input end of the controller  23 . The input end and the output end of the transistor switch  24  are respectively connected to the negative electrode of the first diode  221  and another end of the current-limiting unit  26 , and the control end of the transistor switch  24  is connected to the output end of the controller  23 . When the controller  23  determines that the load current is higher than a rated current value, meaning the input voltage currently generated by the power source  21  is high enough to drive the LED  27  to emit light, the controller  23  generates the first control signal. When the load current is lower than the rated current value, meaning the input voltage currently generated by the power source  21  is not high enough to drive the LED  27  to emit light, the controller  23  generates the second control signal instead. 
     Continued from the above, the transistor switch  24  cuts off the current between its input end and output end upon receiving the first control signal and allows passage of current between its input end and output end upon receiving the second control signal. Thus, by means of switch control, the LED  27  can be driven to emit light continuously as in the previous embodiment. 
       FIG. 6  is a schematic circuit diagram of the LED driver circuit according to the third preferred embodiment of the present invention. As shown in the drawing, the LED driver circuit  202  in this embodiment includes a power source  21 , a controller  23 , a first switch device  31 , a second switch device  32 , at least one LED  27 , and a capacitor  29 . The positive electrode of the capacitor  29  and the positive electrode of the LED  27  are respectively connected to the positive electrode of the power source  21  via the first switch device  31 . The first switch device  31  is configured to prevent the discharge current of the capacitor  29  from flowing back to the power source  21  and to ensure that the discharge current flows to the LED  27 . The negative electrode of the capacitor  29  and the negative electrode of the LED  27  are respectively connected to the negative electrode of the power source  21  via the second switch device  32 . 
     As shown in  FIG. 6 , the input end of the controller  23  is connected to a line between the first switch device  31  and the positive electrode of the power source  21 , so as for the controller  23  to detect an input voltage provided by the power source  21 . Upon detecting that the input voltage is higher than the forward bias voltage of the LED  27 , the controller  23  generates a first control signal and sends the first control signal to the second switch device  32  through the output end of the controller  23 , causing the second switch device  32  to make the connection between the power source  21  and the capacitor  29  and the connection between the power source  21  and the LED  27 . Consequently, the input voltage not only drives the LED  27  to emit light, but also charges the capacitor  29  at the same time. 
     Conversely, upon determining that the input voltage is lower than the forward bias voltage of the LED  27 , the controller  23  generates a second control signal and sends the second control signal to the second switch device  32  through the output end of the controller  23 , causing the second switch device  32  to make only the connection between the capacitor  29  and the LED  27 . Thus, the energy stored in the capacitor  29  is discharged to the LED  27  and drives the LED  27  to emit light. 
     Referring again to  FIG. 6 , the first switch device  31  in this embodiment includes a first diode  221  and a second diode  222 . The first diode  221  has its positive electrode connected to the positive electrode of the power source  21  and its negative electrode connected to the positive electrode of the capacitor  29 . The second diode  222  has its positive electrode connected to the positive electrode of the power source  21  and its negative electrode connected to the positive electrode of the capacitor  29  and to the positive electrode of the LED  27 . The second switch device  32  includes a third diode  223 , a fourth diode  224 , and a transistor switch  25 . 
     The third diode  223  has its positive electrode connected to the negative electrode of the LED  27  and its negative electrode connected to the negative electrode of the power source  21 . The fourth diode  224  has its positive electrode connected to the negative electrode of the capacitor  29  and its negative electrode connected to the negative electrode of the power source  21 . The transistor switch  25  has its input end and output end respectively connected to the negative electrode of the LED  27  and the positive electrode of the fourth diode  224  and its control end connected to the output end of the controller  23 . When the control end of the transistor switch  25  receives the first control signal, the transistor switch  25  cuts off the current between its input end and output end. When the control end of the transistor switch  25  receives the second control signal, the transistor switch  25  allows passage of current between its input end and output end. 
     Please refer to  FIG. 7  for a schematic circuit diagram of the LED driver circuit  203  according to the fourth preferred embodiment of the present invention. The LED driver circuit  203  includes a power source  21 , a controller  23 , a first switch device  31 ′, a second switch device  32 ′, an LED  27 , and a capacitor  29 . The positive electrode of the capacitor  29  and the positive electrode of the LED  27  are respectively connected to the positive electrode of the power source  21  through the first switch device  31 ′. The first switch device  31 ′ is configured to prevent the discharge current of the capacitor  29  from flowing back to the power source  21  and to ensure that the discharge current flows to the LED  27 . 
     Continued from the above, the negative electrode of the capacitor  29  and the negative electrode of the LED  27  are respectively connected to the negative electrode of the power source  21  through the second switch device  32 ′. The input end of the controller  23  is connected to the second switch device  32 ′ in order for the controller  23  to detect a load current in the second switch device  32 ′. When the controller  23  determines that the load current is higher than a rated current value, meaning the input voltage currently generated by the power source  21  is high enough to drive the LED  27  to emit light, the controller  23  generates a first control signal and thereby drives the second switch device  32 ′ to make the connection between the power source  21  and the capacitor  29  and the connection between the power source  21  and the LED  27 . As a result, the power source  21  charges the capacitor  29  while driving the LED  27  to emit light. 
     Conversely, when the controller  23  determines that the load current is lower than the rated current value, meaning the voltage currently generated by the power source  21  is not high enough to drive the LED  27  to emit light, the controller  23  generates a second control signal and sends the second control signal to the second switch device  32 ′ through the output end of the controller  23 , causing the second switch device  32 ′ to make only the connection between the capacitor  29  and the LED  27 , such that the LED  27  is driven to emit light by the discharge voltage of the capacitor  29 . 
     Referring again to  FIG. 7 , the first switch device  31 ′ in this embodiment includes a first diode  221  and a second diode  222 . The first diode  221  has its positive electrode connected to the positive electrode of the power source  21  and its negative electrode connected to the positive electrode of the capacitor  29 . The second diode  222  has its positive electrode connected to the positive electrode of the power source  21  and its negative electrode connected to the positive electrode of the capacitor  29  and to the positive electrode of the LED  27 . The second switch device  32 ′ includes a current-limiting unit  26 , a third diode  223 , a fourth diode  224 , and a transistor switch  25 . 
     The current-limiting unit  26  has one end connected to the negative electrode of the LED  27 . When a current flows through the current-limiting unit  26 , the current-limiting unit  26  generates the load current and provides the load current to the input end of the controller  23 . The third diode  223  has its positive electrode connected to another end of the current-limiting unit  26  and its negative electrode connected to the negative electrode of the power source  21 . The fourth diode  224  has its positive electrode connected to the negative electrode of the capacitor  29  and its negative electrode connected to the negative electrode of the power source  21 . The input end and the output end of the transistor switch  25  are connected to the negative electrode of the LED  27  and the positive electrode of the fourth diode  224  respectively while the control end of the transistor switch  25  is connected to the output end of the controller  23 . When the control end receives the first control signal, the transistor switch  25  cuts off the current between its input end and output end. When the control end receives the second control signal, the transistor switch  25  allows passage of current between its input end and output end. Thus, by means of current detection, the same effect as achievable by the previous embodiment can be obtained. 
     In a nutshell, the LED driver circuits  200 ,  201 ,  202 , and  203  of the present invention are so designed that the capacitor  29  will not be discharged to drive the LED  27  unless the power source  21  is at a relatively low potential level. Hence, it is feasible to use a small-capacity, low-cost capacitor as the capacitor  29  to both effectively increase the light emission time of the LED  27  and lower the production costs of the LED driver circuits  200 ,  201 ,  202 , and  203 . 
     While the invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.