Patent Publication Number: US-9907128-B2

Title: Driving circuit for driving a LED array

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 15/047,154, filed Feb. 18, 2016, which claims priority to Chinese Patent Application No. 201510124204.0, filed Mar. 20, 2015, and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which in its entirety are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to a driving circuit for driving a light emitting diode (LED) array and, more particularly, to a LED driving circuit that does no need an additional winding. 
     BACKGROUND OF THE INVENTION 
     Generally, a driving circuit of a LED array usually utilizes a critical conduction mode (CRM) or quasi-resonant (QR) buck-boost converter. Such buck-boost converter needs an additional winding for providing a current to a control chip, thereby implementing a zero-current switching function.  FIG. 1  shows a conventional driving circuit of the LED array. An alternating current (AC) voltage VACIN is rectified by a rectifier  15  so as to generate a direct current (DC) voltage Vin. A control chip  13  controls a transistor Q 1  to be turned on or turned off so as to generate a stable output voltage Vo or a stable output current Io, so that the LED array  11  can be driven. Referring to  FIG. 1 , a ground terminal of the LED array  11  is different from that of the control chip  13 . Accordingly, the control chip  13  is unable to detect the voltage and the current on the LED array  11  directly. Thus, an additional winding N 2  is required in order to implement the following functions: 
     1.) providing a current Ivdd to charge a capacitor CVDD so as to provide a supply voltage to the control chip  13 ; 
     2.) zero-current switching function; and 
     3.) over-voltage protection function of the output voltage. 
       FIG. 2  is a waveform diagram of the voltages and the currents in the circuitry shown in  FIG. 1 , in which the waveform  17  represents a voltage Vds of a drain terminal of the transistor Q 1 , the waveform  19  represents a voltage VAUX on the winding N 2 , and the waveform  21  represents a current Idout on a diode Dout. The current Ivdd provided by the winding N 2  in the circuitry of  FIG. 1  will charge the capacitor CVDD, thereby maintaining the supply voltage VDD of the control chip  13 . When the transistor Q 1  is turned off, the voltage VAUX on the winding N 2  is proportional to the voltage Vds of the drain terminal of the transistor Q 1 , as shown by waveforms  17  and  19 . Simultaneously, a voltage on a winding N 1  is almost the same as the output voltage Vo. Thus, the voltage VAUX is also proportional to the output voltage Vo. Resistors Rzcd 1  and Rzcd 2  divide the voltage VAUX to generate the voltage Vd to a pin ZCD of the control chip  13 . The control chip  13  is able to judge the value of the output voltage Vo via the voltage Vd, thereby achieving the over-voltage protection of the output voltage Vo. Referring to waveform  21  in  FIG. 2 , when the current Idout on the diode Dout decreases to zero, the voltage VAUX on the winding N 2  generates a resonance, which can be utilized to implement the zero-current switching of the current Idout. However, the additional winding N 2  will cause a higher cost and increase the size of the driving circuit board. 
     Therefore, it is desired a LED driving circuit that needs no additional winding. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a driving circuit that is able to drive a LED array without an additional winding. 
     According to the present invention, a driving circuit for driving a LED array comprises a positive voltage output terminal and a negative voltage output terminal to be respectively coupled to an anode and a cathode of the LED array, a rectifier for rectifying an AC voltage to generate a DC voltage, a transistor coupled to the rectifier and a circuit ground terminal, a capacitor coupled between the positive voltage output terminal and the negative voltage output terminal, an inductor coupled to the capacitor via a diode, and a control chip that includes a first pin coupled to the transistor, a second pin coupled to the circuit ground terminal, and a third pin configured to operably detect a voltage of the inductor. The first pin of the control chip provides a switching signal for controlling switching of the transistor, so that a stable output voltage or a stable output current can be generated between the positive voltage output terminal and the negative voltage output terminal. When the transistor is turned off, a current is provided by the inductor so as to charge the capacitor. The third pin of the control chip directly detects the voltage of the inductor to acquire a discharging time of the inductor and the voltage between the positive voltage output terminal and the negative voltage output terminal for implementing a zero-current switching function and an over-voltage protection function respectively. Additionally, when the transistor is turned off, the LED array can provide a current to charge a power supply capacitor, thereby providing the supply voltage to the control chip. 
     The driving circuit of the present invention requires no additional winding but still provides the supply voltage, and the zero-current switching function and the over-voltage protection function can be also implemented. Therefore, the related costs can be decreased, and the size of the driving circuit board is smaller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments according to the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a conventional driving circuit for driving a LED array; 
         FIG. 2  is a waveform diagram of voltages and currents in the circuitry of  FIG. 1 ; 
         FIG. 3  shows a circuitry of a first embodiment of a driving circuit for driving a LED array according to the present invention; 
         FIG. 4  is a waveform diagram of voltages and currents in the circuitry of  FIG. 3 ; and 
         FIG. 5  shows a circuitry of a second embodiment of a driving circuit for driving a LED array according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows a first embodiment of a driving circuit  23  according to the present invention, which includes a positive voltage output terminal  25  and a negative voltage output terminal  27  coupled to an anode  311  and a cathode  313  of a LED array  31 , respectively. In the driving circuit  23 , a rectifier  15  rectifies an AC voltage VACIN for generating a DC voltage Vin. A drain of a transistor Q 1  is coupled to a capacitor Cin and the rectifier  15  for receiving the DC voltage Vin. A current sensing resistor RCS is coupled between a source of the transistor Q 1  and a circuit ground terminal  35 . An inductor L has a first terminal  37  coupled to the anode  311  of the LED array  31 , and a second terminal  39  coupled to the circuit ground terminal  35 . The cathode  313  of the LED array  31  is coupled to the circuit ground terminal  35  via a diode Dout. The diode Dout avoids reverse current from the positive voltage output terminal  25  to the first terminal  37  of the inductor L when the output voltage Vo is higher than a voltage VL of the inductor L. A capacitor Cout is coupled between the positive voltage output terminal  25  and the negative voltage output terminal  27 . A first pin GD of the control chip  33  is coupled to a gate of the transistor Q 1  and provides a switching signal Vg to control switching of the transistor Q 1 . Accordingly, a stable output voltage Vo or a stable output current Io can be generated between the positive voltage output terminal  25  and the negative voltage output terminal  27 . Since the brightness of a LED is proportional to the current flowing through the LED, a constant current is generally utilized to drive a LED. When the transistor Q 1  is turned on, a current Iq 1  flows through the resistor Rcs from the capacitor Cin to the inductor L to charge the inductor L. When the transistor Q 1  is turned off, the inductor L provides a current Idout to charge the capacitor Cout. A second pin GND of the control chip  33  is coupled to the circuit ground terminal  35 . A voltage of the second pin GND determines a ground potential of the control chip  33 . Resistors Rzcd 1  and Rzcd 2  divide the voltage VL on the inductor L to generate a voltage Vd applied to a third pin ZCD of the control chip  33 . A fourth pin VDD of the control chip  33  is coupled to the power supply capacitor CVDD. The LED array  31  provides a current Ivdd to charge the power supply capacitor CVDD via the diode Daux. Accordingly, a supply voltage can be provided to the control chip  33 . Wherein, the diode Daux avoids reverse current from the capacitor CVDD to the LED array  31 . In the embodiment shown in  FIG. 3 , the current Ivdd will be drawn from an anode of one of the LEDs in the LED array  31 . Alternatively, the diode Dout in the embodiment shown in  FIG. 3  can be coupled between the first terminal  37  of the inductor L and the positive voltage output terminal  25 . 
       FIG. 4  is a waveform diagram of the voltages and currents in the circuitry shown in  FIG. 3 , in which the waveform  41  represents the voltage VL and the waveform  43  represents the current Idout. When the transistor Q 1  becomes off from the on state as shown at time t 1 , the inductor L starts discharging to provide the current Idout for charging the capacitor Cout. In the same time, the LED array  31  also provides the current Ivdd to charge the power supply capacitor CVDD. When the inductor provides the current Idout as shown from time t 1  to time t 2 , the voltage VL on the inductor L equals the output voltage Vo, and the voltage VL is almost maintained at a fixed value. The resistors Rzcd 1  and Rzcd 2  divide the voltage VL to generate the voltage Vd, which is related to the output voltage Vo, to apply to the third pin ZCD of the control chip  33 . Herein, the diode Dout is on, so the second pin GND of the control chip  33  and the negative voltage output terminal  27  (or a negative terminal of the capacitor Cout) are coupled to the same circuit ground terminal  35 . Thereby, the control chip  33  is able to judge the value of the output voltage Vo according to the voltage Vd directly, so as to implement the over-voltage protection when the output voltage Vo is too high. When the current Idout decreases to zero as shown at time t 2 , the voltage VL of the inductor L generates a resonance and starts decreasing. As a result, the voltage Vd also decreases. When the voltage Vd becomes lower than a preset value, the current Idout is regarded as decreasing to zero by the control chip  33 . That is to say, the control chip  33  can directly detect the discharging time of the inductor L from the voltage VL of the inductor L, thereby implementing the zero-current switching function. Generally speaking, the zero-current switching will turn on the transistor Q 1  immediately when the current Idout decreases to zero. In some applications, the transistor will not be turned on until the current Idout has decreased to zero for a while. For example, as shown by the waveform  41  in  FIG. 4 , the transistor Q 1  will not be turned on until the voltage VL decrease to a valley value as shown at time t 3 , thereby achieving better performance. 
       FIG. 5  shows a second embodiment of the driving circuit  23  according to the present invention. The circuitry shown in  FIG. 5  is almost the same as that in  FIG. 3 , while the diode Dout in this embodiment is coupled between the first terminal  37  of the inductor L and the positive voltage output terminal. The locations of the diodes Dout in the embodiments of  FIG. 5  and  FIG. 3  are different, but both diodes Dout can prevent reverse current. Another difference between the circuitries in the embodiments of  FIG. 5  and  FIG. 3  is that the current Ivdd in  FIG. 5  is drawn from the anode  311  of the LED array  31 . The current Ivdd in  FIG. 5  also charges the capacitor CVDD to generate the supply voltage to the control chip  33 . In order to prevent the supply voltage of the control chip  33  over a permissible range due to the output voltage Vo, a Zener diode DZ can be coupled between the anode  311  of the LED array  31  and the capacitor CVDD so as to clamp the supply voltage of the control chip  33 . Wherein, the diode Dout in  FIG. 5  can be also coupled between the second terminal  39  of the inductor L and the negative voltage output terminal  27 . 
     The driving circuit  23  of the present invention doesn&#39;t need an additional winding and can implement the over-voltage protection function of the output voltage, implement the zero-current switching function, and provide the current Ivdd to the power supply capacitor CVDD. Accordingly, the related costs and the size of the driving circuit board can be decreased. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.