Patent Publication Number: US-2015070954-A1

Title: Controller having adjustable frequency-reduction function and system using same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a controller for power conversion, especially to a controller having an adjustable frequency-reduction function to provide appropriate switching frequencies for different power conversion applications. 
     2. Description of the Related Art 
     As energy efficiency is more and more demanded globally, regulations for power supplies are becoming more and more stringent. Taking the California Energy Commission as an example, it proposed an efficiency level 4 regulation on Jul. 1, 2006, an efficiency level 5 regulation on Nov. 1, 2008, and an efficiency level 6 regulation—a DOE (Department of Energy) regulation—on Jul. 1, 2013. The required efficiency of a power supply has been continually pushed upward step by step. For example, a 12 W power supply is requested to have an efficiency of not less than 72.4% for the efficiency level 4, not less than 74.74% for the efficiency level 5, and not less than 79.94% for the efficiency level 6. 
     The change from the efficiency level 5 to the efficiency level 6 involves an efficiency improvement of 5.2%. As the components of general power supplies have been designed to have nearly approached their best efficiencies, it is very difficult for a power supply to meet the requirement of the efficiency level 6. 
     The power consumption of a power supply depends mostly on the switching frequency the power supply is operating at—the higher the switching frequency is, the larger the power consumption will be. To further improve the efficiency of a power supply, one solution utilizing a frequency-reduction mechanism has been proposed. The principle of the frequency-reduction mechanism is that—a controller of the power supply will reduce the switching frequency when a feedback voltage from a load falls below a preset threshold voltage, wherein the feedback voltage reflects the loading condition of the power supply in a way as follows: the feedback voltage will become lower when the loading becomes lighter, and will become higher when the loading becomes heavier. 
     However, as the preset threshold voltage is a fixed voltage and is generated inside the controller, different power conversion applications using the same controller will have different energy efficiency, and the efficiency level 6 is therefore still not attained. 
     To solve the foregoing problem, a novel controller is needed. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to disclose a controller having an adjustable frequency-reduction function for power conversion applications to reduce their power consumption. 
     Another objective of the present invention is to disclose a controller having an adjustable frequency-reduction function for power conversion applications to meet the requirement of the efficiency level 6. 
     Another objective of the present invention is to disclose a controller having an adjustable frequency-reduction function and being compatible with the existing structures of general power conversion systems. 
     Still another objective of the present invention is to disclose a controller having an adjustable frequency-reduction function to reduce the power consumption of general power conversion systems without interfering with the designs for safety regulation thereof. 
     To attain the foregoing objectives, a controller having an adjustable frequency-reduction function for a power conversion application is proposed, including:
         a threshold voltage sampling unit, having a first node coupled to an external resistor network for generating a threshold voltage at the first node, which is then sampled by the threshold voltage sampling unit;   a PWM unit having a second node for receiving a feedback voltage from a load, and a third node for providing a PWM signal of a switching frequency; and   a driver unit for driving an external power transistor according to the PWM signal;   wherein the switching frequency starts to decrease from a first frequency when the feedback voltage falls below a first threshold voltage, the first threshold voltage being a first function of the threshold voltage.       

     To make it easier for our examiner to understand the objective of the invention, its structure, innovative features, and performance, we use preferred embodiments together with the accompanying drawings for the detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a circuit diagram of a power converter using a controller of the present invention according to a preferred embodiment of the present invention. 
         FIG. 2  illustrates an arrangement of an operation frequency in response to a feedback voltage of the power converter of  FIG. 1 . 
         FIG. 3  illustrates a circuit diagram of a power converter using a controller of the present invention according to another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in more detail hereinafter with reference to the accompanying drawings that show the preferred embodiments of the invention. 
     Please refer to  FIG. 1 , which illustrates a circuit diagram of a power converter using a controller of the present invention according to a preferred embodiment of the present invention. As illustrated in  FIG. 1 , a power converter includes a controller  100 , a power transistor  110 , a power transmission unit  120 , a current sensing resistor  130 , a feedback circuit  140 , and a resistor network  150  for converting an input voltage V IN  to an output voltage V O  for a load  160 . 
     The controller  100  includes a threshold voltage sampling unit  101 , a PWM (pulse width modulation) unit  102 , and a driver unit  103 . 
     The threshold voltage sampling unit  101  has a first node coupled to the resistor network  150  for generating a threshold voltage V X  at the first node, which is then sampled by the threshold voltage sampling unit  101 . The threshold voltage sampling unit  101  provides a constant current I S  when the power transistor  110  is on to generate the threshold voltage V X , which is equal to −V IN *(N a /N p )*R 2 /(R 1 +R 2 )+I S *R 2 , wherein N a  is a turn number of an auxiliary coil  121  of the power transmission unit  120 , N p  is a turn number of a primary coil  122  of the power transmission unit  120 , R 1  is the resistance of a resistor  151  of the resistor network  150 , and R 2  is the resistance of a resistor  152  of the resistor network  150 . The threshold voltage sampling unit  101  generates a first threshold voltage V a  according to a first function of the threshold voltage V X , a second threshold voltage V b  according to a second function of the threshold voltage V X , and a third threshold voltage V c  according to a third function of the threshold voltage V X , wherein V a &gt;V b &gt;V c . The first function, the second function, and the third function are preferably but not limited to first order polynomial functions of V X . For example, V a  can be equal to K 1 *V X +V dc1 , V b  can be equal to K 2 *V X +V dc2 , and V c  can be equal to K 3 *V X +V dc3 , wherein K 1 , K 2 , K 3 , V dc1 , V dc2 , and V dc3  are constants. 
     The PWM unit  102  has a second node for receiving a feedback voltage V FB  from the load  160  via the feedback circuit  140 , a third node for providing a PWM signal V PWM  of a switching frequency, and a fourth node for receiving a current sensing signal V CS  from the current sensing resistor  130 . The PWM unit  102  adjusts the duty of the PWM signal V PWM  in response to V FB  and V CS , so as to generate the output voltage V O . To reduce power consumption of the power converter, the PWM unit  102  adjusts the switching frequency of the PWM signal V PWM  in response to V FB  in a way as illustrated in  FIG. 2  that: the switching frequency starts to decrease from a first frequency f 1  when the feedback voltage V FB  falls below the first threshold voltage V a ; the switching frequency stops decreasing and then remains at a second frequency f 2  when the feedback voltage V FB  falls below the second threshold voltage V b ; and when the feedback voltage V FB  falls below the third threshold voltage V c , the PWM signal V PWM  enters a green mode. The green mode can be a burst mode or a skipping mode, in which the PWM signal V PWM  is active only once in a while. 
     The driver unit  103  is used for generating a driving signal V G  to drive the power transistor  110  according to the PWM signal V PWM . 
     The power transistor  110 , illustrated as an NMOS transistor in the figure though, can also be implemented with a bipolar transistor. The power transistor  110  is used to control a power transmission of the power transmission unit  120  from V IN  to the load  160 . 
     The power transmission unit  120  includes the auxiliary coil  121 , the primary coil  122 , a secondary coil  123 , a diode  124 , and a capacitor  125  to transmit power from V IN  to the load  160  under the control of the power transistor  110 . As the principle of the power transmission unit  120  is well known, it is not addressed here. 
     The current sensing resistor  130  is used to convert a primary current I P  to the current sensing signal V CS . 
     The feedback circuit  140  is used to generate the feedback voltage V FB  according to a difference between the output voltage V O  and a reference voltage (not shown in the figure). 
     When the controller  100  is used in different power conversion applications, the resistance of the resistor  151  and the resistance of the resistor  152  can be adjusted to generate appropriate values of the threshold voltage V X  for the different power conversion applications, so as to meet the requirement of the efficiency level 6. 
     Based on the principle elaborated above, the present invention proposes another embodiment. Please refer to  FIG. 3 , which illustrates another embodiment of the controller of the present invention used in a power conversion application. As illustrated in  FIG. 3 , a power converter includes a controller  300 , a power transistor  310 , a power transmission unit  320 , a first resistor  330 , a second resistor  331 , and a feedback circuit  340  for converting an input voltage V IN  to an output voltage V O  for a load  350 . 
     The controller  300  includes a threshold voltage sampling unit  301 , a PWM unit  302 , and a driver unit  303 . 
     The threshold voltage sampling unit  301  has a first node coupled to a resistor network consisting of the first resistor  330  and the second resistor  331  for generating a threshold voltage V X  at the first node, which is then sampled by the threshold voltage sampling unit  301 . The threshold voltage sampling unit  301  provides a constant current I S  when the power transistor  310  is off to generate the threshold voltage V X , which is equal to I S *(R 1 +R 2 ), wherein R 1  is the resistance of the first resistor  330 , and R 2  is the resistance of the second resistor  331 . The threshold voltage sampling unit  301  generates a first threshold voltage V a  according to a first function of the threshold voltage V X , a second threshold voltage V b  according to a second function of the threshold voltage V X , and a third threshold voltage V c  according to a third function of the threshold voltage V X , wherein V a &gt;V b &gt;V c . The first function, the second function, and the third function are preferably but not limited to first order polynomial functions of V X . For example, V a  can be equal to K 1 *V X +V dc1 , V b  can be equal to K 2 *V X +V dc2 , and V c  can be equal to K 3 *V X +V dc3 , wherein K 1 , K 2 , K 3 , V dc1 , V dc2 , and V dc3  are constants. 
     As the principles of the remaining parts have been described above with reference to  FIG. 1 , they will not be readdressed here. 
     With the designs elaborated above, the present invention possesses the following advantages:
         1. The controller of the present invention is capable of providing an adjustable frequency-reduction function for power conversion applications to reduce power consumption.   2. The controller of the present invention is capable of making power conversion applications meet the requirement of the efficiency level 6.   3. The controller of the present invention is capable of providing an adjustable frequency-reduction function without changing the existing structures of general power conversion systems.   4. The controller of the present invention is capable of providing an adjustable frequency-reduction function to reduce the power consumption of general power conversion systems without interfering with the designs for safety regulation thereof.       

     While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 
     In summation of the above description, the present invention herein enhances the performance over the conventional structure and further complies with the patent application requirements and is submitted to the Patent and Trademark Office for review and granting of the commensurate patent rights.