Patent Publication Number: US-8970122-B2

Title: LED driver and the method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and the benefit of Chinese Patent Application No. 201110201794.4, filed Jul. 19, 2011, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates generally to electronic circuits, and more particularly but not exclusively to LED drivers and the method thereof. 
     BACKGROUND 
     LEDs are applied in various solutions, for example, LCD backlight and general lighting. LED drivers are needed to supply regulated current to LED strings. Normally, a LED driver may comprise a rectifier and a transformer. The rectifier rectifies an AC signal to a DC signal. The transformer receives the DC signal and provides a converted DC signal having a voltage value decided by the turns ratio of a primary winding of the transformer to a secondary winding of the transformer. 
     The conventional primary side controlled LED driver may comprise a rectifier, a power switch, a transformer, a secondary circuit, a logic control circuit and a load, e.g. LED strings. In conventional primary side controlled LED drivers, a voltage provided to the LED strings is fixed, so that a current of a LED string is inversely proportional to the number of LEDs in the LED string. 
     The present disclosure pertains to provide a LED driver providing a constant current to a LED string despite the varying of the number of LEDs in the LED string. 
     SUMMARY 
     It is an object of the present disclosure to provide a LED driver and the method thereof. 
     In accomplishing the above and other objects, there has been provided, in accordance with an embodiment of the present disclosure, a LED driver comprising: a power switch; a transformer comprising a primary winding, a secondary winding and a third winding, wherein the primary winding is coupled to the power switch, and the transformer stores or transfers energy as the power switch is turned ON and OFF; a current sense circuit coupled to the power switch to sense a current flowing through the power switch, and to generate a current sense signal based thereupon; a zero cross detecting circuit coupled to the third winding to detect a current flowing through the secondary winding, and to generate a zero cross detecting signal based thereupon, and wherein the zero cross detecting signal indicates when the current flowing through the secondary winding crosses zero; a control circuit coupled to the current sense circuit and the zero cross detecting circuit, and based on the current sense signal and the zero cross detecting signal, the control circuit generates a control signal to control the power switch; and a compensation circuit coupled to the third winding and the control circuit, wherein the compensation circuit compensates the current sense signal based on a current flowing through the third winding. 
     Furthermore, there has been provided, in accordance with an embodiment of the present disclosure, a method for driving LED strings by a LED driver. The LED driver comprises a power switch and a transformer, wherein the transformer comprises a primary winding, a secondary winding and a third winding, wherein the primary winding is coupled to the power switch, and the transformer stores or transfers energy as the power switch is turned ON and OFF. The method comprises: detecting a current flowing through the secondary winding and generating a zero cross detecting signal based thereupon, wherein the zero cross detecting signal indicates when the current flowing through the secondary winding crosses zero; sensing a current flowing through the power switch, and generating a current sense signal based thereupon; compensating the current sense signal based on the signal from the secondary winding; and turning ON and OFF the power switch based on the compensated current sense signal and the zero cross detecting signal. 
     The current sense signal which controls the LED driver together with other signals is compensated, so that to keep the current flowing through a load, e.g. LED strings, be constant even when the number of the LEDs in the string is varying. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a prior art primary side controlled LED driver; 
         FIG. 2  shows the waveform of a percentage error of a current flowing through a LED string to an average current flowing through the LED string when the number of the LEDs in the LED string is varying of the LED driver in  FIG. 1 ; 
         FIG. 3  schematically shows a primary side controlled LED driver in accordance with an embodiment of the present disclosure; 
         FIG. 4  shows the waveform of a percentage error of a current flowing through a LED string to an average current flowing through the LED string when the number of the LEDs in the LED string is varying of the LED driver in  FIG. 3 ; 
         FIG. 5  shows a compensation circuit  510  in accordance with an embodiment of the present disclosure; 
         FIG. 6  shows a work flowchart of an LED driver in accordance with an embodiment of the present disclosure; 
     
    
    
     The use of the same reference label in different drawings indicates same or like components. 
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the disclosure. Persons of ordinary skill in the art will recognize, however, that the disclosure can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the disclosure. 
     Some embodiments are described in the present disclosure. In one embodiment, a compensation circuit compensates a current sense signal generated by a current sense circuit, wherein the compensation circuit is coupled between a third winding of a transformer and the current sense circuit. A control circuit controls the ON and OFF of a power switch based on a compensated current sense signal and a zero cross detecting signal. In one embodiment, the control circuit controls the ON and OFF of the power switch based on the compensated current sense signal, the zero cross detecting signal and a rectified signal. In the embodiments of the present disclosure, because the current sense signal is compensated, a current flowing through a LED string keeps constant even when number of the LEDs in a LED string varies. 
     In one embodiment, the compensation circuit comprises a current mirror circuit, wherein the current mirror circuit is coupled between the third winding of the transformer and the current sense circuit. 
     In one embodiment, the compensation circuit comprises a voltage-controlled current source, wherein the voltage-controlled current source is coupled between the third winding of the transformer and the current sense circuit. 
       FIG. 1  schematically shows a prior art LED driver. As shown in  FIG. 1 , the LED driver comprises: a rectifier  101 , a voltage detecting circuit  102 , a transformer  103 , a load  104 , a power switch Q 1 , a zero cross detecting circuit  106 , a current sense circuit  107  and a control circuit  108 . The transformer  103  comprises a primary winding T 1 , a secondary winding T 2  and a third winding T 3 . 
     The rectifier  101  comprises four diodes which may be replaced by other semiconductor devices, wherein the four diodes form a full-bridge. The rectifier  101  has a first terminal and a second terminal, wherein the first terminal is coupled to a first terminal of the voltage detecting circuit  102  and a first terminal of the primary winding T 1 , and the second terminal is coupled to a second terminal of the voltage detecting circuit  102  and a first terminal of the third winding T 3 . 
     The voltage detecting circuit  102  comprises a resistor R 3  and a resistor R 4  coupled in series. The voltage detecting circuit  102  detects the output voltage of the rectifier  101  and generates a rectified signal Vin-rec based thereupon. Persons of ordinary skill in the art should know that the voltage detecting circuit  102  may comprise capacitors coupled in series or may comprise other devices. 
     The power switch Q 1  is coupled to a second terminal of the primary winding T 1 , and is controlled by a control signal Cgate generated by the control circuit  108 . When the power switch Q 1  is ON, there is a current flowing through the primary winding T 1 , and thereby energy is stored in the primary winding T 1 . When the power switch Q 1  is OFF, the energy stored in the primary winding T 1  is transferred to the secondary winding T 2 . The secondary winding T 2 , a diode D 5 , a resistor R 5 , a capacitor C 1  and a load R L  are coupled as shown in  FIG. 1 . A voltage across the capacitor C 1  is an output voltage supplied to the load R L . 
     The zero cross detecting circuit  106  is coupled to a second terminal of the third winding T 3 . The zero cross detecting circuit  106  comprises a resistor R 1  and a resistor R 2  coupled in series. The zero cross detecting circuit  106  detects a current flowing through the third winding T 3  which is indicative of a current flowing through the secondary winding T 2 . When the current flowing through the secondary winding T 2  crosses zero, the zero cross detecting signal ZCD generated by the zero cross detecting circuit  106  is valid. 
     The current sense circuit  107  is coupled to the power switch Q 1  to sense a current flowing through the power switch Q 1  and to generate a current sense signal Cs based thereupon. The current sense signal Cs, the zero cross detecting signal ZCD and the rectified signal Vin-rec are provided to the control circuit  108 . The control circuit  108  generates a control signal Cgate to control the ON and OFF of the power switch Q 1 . The current sense circuit  107  comprises a resistor network formed by a resistor R 7 , a resistor R 8 , a resistor R 9  and a resistor R 10 . 
     The control circuit  108  comprises a multiplier M 1 . The multiplier M 1  receives the rectified signal Vin-rec and an error signal VCOMP, wherein the multiplier M 1  generates an expected signal Vexp by multiplying the rectified signal Vin-rec with the error signal VCOMP. A comparator COMP 2  compares the expected signal Vexp with the current sense signal Cs to provide a reset signal to a reset terminal of a RS flip-flop RS 1 . A comparator COMP 1  compares the zero cross detecting signal ZCD with a reference signal Vref, to provide a set signal to a set terminal of the RS flip-flop RS 1 . When the set signal at the set terminal “S” is logical high, the control signal Cgate provided at the output terminal “Q” of the RS flip-flop RS 1  is logical high. When the reset signal at the reset terminal “R” is logical high, the control signal Cgate is logical low. The power switch Q 1  is turned ON and OFF by the control signal Cgate, to transfer the energy stored in the primary winding T 1  to the secondary winding T 2 . 
       FIG. 2  shows a curve of an error percentage of a current flowing through a LED string to an average current flowing through the LED string under different LED numbers in the string of the LED driver in  FIG. 1 . The X axis represents the number of the LEDs in the LED string. The Y axis represents the error percentage of the current flowing through the LED string to the average current flowing through the LED string. As shown in  FIG. 2 , as the number of the LEDs increases, the difference between the current flowing through the LED string and the average current flowing through the LED string decreases no matter the input voltage is 220 Volts or 110 Volts. 
       FIG. 3  schematically shows a LED driver in accordance with an embodiment of the present disclosure. In the example of  FIG. 3 , the LED driver comprises: a rectifier  301 , a voltage detecting circuit  302 , a transformer  303 , a load  304 , a power switch Q 1 , a zero cross detecting circuit  306 , a current sense circuit  307  and a control circuit  308 . The transformer  303  comprises a primary winding T 1 , a secondary winding T 2  and a third winding T 3 . 
     The rectifier  301  comprises four diodes which may be replaced by other semiconductor devices, wherein the four diodes form a full-bridge. The rectifier  301  has a first terminal and a second terminal, wherein the first terminal is coupled to a first terminal of the voltage detecting circuit  302  and a first terminal of the primary winding T 1 , and the second terminal is coupled to a second terminal of the voltage detecting circuit  302  and a first terminal of the third winding T 3 . 
     The power switch Q 1  is coupled to the primary winding T 1 , and is controlled by a control signal Cgate generated by the control circuit  308 . The current sense circuit  307  is coupled to the power switch Q 1  to sense a current flowing through the power switch Q 1  and to generate a current sense signal Cs based thereupon. The current sense signal Cs indicates a current flowing through a LED string which is adopted as the load. In one embodiment, the current sense circuit  307  comprises resistors. Persons of ordinary skill in the art should know that any suitable current sense circuit may be adopted without detracting from the merits of the present disclosure. 
     In one embodiment, the LED driver further comprises a compensation circuit  310  coupled between a second terminal of the third winding T 3  of the transformer  303  and an output terminal of the current sense circuit  307 . The compensation circuit  310  comprises a resistor R 11  and a current mirror circuit formed by several transistors. A current flowing through the resistor R 11  is proportional to the voltage across the third winding T 3 , which is also proportional to the voltage across the secondary winding T 2 . The current flowing through the resistor R 11  could compensate the current sense signal Cs and then eliminate the variation of the current flowing through the LED string when the number of the LEDs in the LED string varies. In other words, the current sense signal Cs may be compensated by adjusting the resistance of the resistor R 11  and the resistance of the resistors of the current sense circuit  307 , and thereby generating a compensated current sense signal Cs&#39;. 
     In one embodiment, the current mirror circuit comprises a first NPN transistor and a second NPN transistor. An emitter current of the first NPN transistor and an emitter current of the second NPN transistor are almost the same. A collector of the first NPN transistor is coupled to the third winding T 3 . A collector of the second NPN transistor is coupled to an input terminal of the control circuit  308  and the output terminal of the current sense circuit  307 . 
     In one embodiment, the emitter current of the first NPN transistor is regulated by adjusting the resistance of the resistor R 11 . 
     In one embodiment, the compensation circuit  310  further comprises a diode D 6  coupled between the third winding T 3  and the resistor R 11 . 
     The zero cross detecting circuit  306  comprises a resistor R 1  and a resistor R 2  coupled in series, wherein the zero cross detecting circuit  306  is coupled to the third winding T 3 . The zero cross detecting circuit  306  detects a current flowing through the secondary winding T 2  by detecting the current flowing through the third winding T 3 , and thereby generates the zero cross detecting signal which indicates when the current flowing through the secondary winding T 2  crosses zero. 
     The voltage detecting circuit  302  comprises a resistor R 3  and a resistor R 4  coupled in series, wherein the voltage detecting circuit  302  is coupled to the rectifier  301 . The voltage detecting circuit  302  detects an output voltage of the rectifier  301  and generates a rectified signal Vin-rec based thereupon. Persons of ordinary skill in the art should know that the voltage detecting circuit  302  may comprise capacitors coupled in series or other devices. 
     In one embodiment, the control circuit  308  receives the zero cross detecting signal ZCD, the compensated current sense signal Cs&#39; and the rectified signal Vin-rec, and then generates a control signal Cgate to control the ON and OFF of the power switch Q 1 . 
     In one embodiment, the control circuit  308  comprises a multiplier Ml. The multiplier M 1  receives the rectified signal Vin-rec and an error signal VCOMP, wherein the multiplier M 1  generates an expected signal Vexp by multiplying the rectified signal Vin-rec with the error signal VCOMP. A comparator COMP 2  compares the expected signal Vexp with the compensated current sense signal Cs&#39; to provide a signal to a reset terminal of a RS flip-flop RS 1 . A comparator COMP 1  compares the zero cross detecting signal ZCD and a reference signal Vref, to provide a signal to a set terminal of the RS flip-flop RS 1 . When the signal at the set terminal S is logical high, the control signal Cgate provided at the output terminal Q of the RS flip-flop RS 1  is logical high. When the signal at the reset terminal R is logical high, the control signal Cgate is logical low. The power switch Q 1  is turned ON and OFF by the control signal Cgate, to transfer the energy stored in the primary winding T 1  to the secondary winding T 2 . 
       FIG. 4  shows the waveform of an error percentage of a current flowing through a LED string to an average current flowing through the LED string when the number of the LEDs in the LED string is varying of the LED driver in  FIG. 3 . As shown in  FIG. 4 , as the number of the LEDs increases, the difference between the current flowing through the LED string and the average current flowing through the LED string keeps constant. A current flowing through a LED string keeps constant even when the number of the LEDs in a LED string is varying no matter the rectified voltage is 220 Volts or 110 Volts. 
       FIG. 5  schematically shows a compensation circuit  510  in accordance with an embodiment of the present disclosure. The compensation circuit  510  comprises a voltage-controlled current source. The voltage-controlled current source receives the signal from the third winding  73  and then generates a compensation signal to compensate the current sense signal Cs. An adjustable resistor may be coupled to an input terminal of the voltage-controlled current source to adjust the compensation signal. 
       FIG. 6  shows a work flowchart of the control circuit  308  in accordance with an embodiment of the present disclosure. In the example of  FIG. 6 , the LED driver comprises a power switch Q 1 , a control circuit and a transformer. The transformer comprises a primary winding T 1  coupled to the power switch Q 1 , a secondary winding T 2  and a third winding T 3 . The control circuit controls the ON and OFF of the power switch so as to control the energy transferred from the primary winding T 1  to the secondary winding T 2 . 
     The flowchart of the control circuit  308  comprises steps  601 - 604 , wherein: 
     step  601 , detecting a current flowing through the power switch, and generating a current sense signal based thereupon; 
     step  602 , compensating the current sense signal based on a current from the third winding; 
     step  603 , generating a zero cross detecting signal indicating when a current flowing through the secondary winding T 2  crosses zero; 
     step  604 , generating a control signal to control the ON and OFF of the power switch at least based on the compensated current sense signal and the zero cross detecting signal by the control circuit. 
     In one embodiment, compensating the current sense signal by a current mirror circuit based on a current flowing through the third winding T 3 . 
     In one embodiment, compensating the current sense signal by a voltage-controlled current source based on a current flowing through the third winding T 3 . 
     An effective technique for sample and hold circuit has been disclosed. While specific embodiments of the present disclosure have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.