Patent Publication Number: US-2013229832-A1

Title: Controlling a flyback converter for use with a computer system

Description:
RELATED APPLICATION 
     This application hereby claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/606,237, entitled “Controlling a Flyback Converter for Use with a Computer System,” by Bharatkumar K. Patel, Abby Cherian, Manisha P. Pandya and Prasad S. Joshi, filed 2 Mar. 2012 (Atty. Docket No.: APL-P13505USP1). 
    
    
     BACKGROUND 
     1. Field 
     The present embodiments relate to techniques for controlling a flyback converter. More specifically, the present embodiments relate to techniques for controlling a flyback converter for use with a computer system. 
     2. Related Art 
     Adapters for powering portable computer systems such as laptop computers often use flyback converters due to their low cost and smaller package size. However, computer systems, including laptop computers, are increasingly being manufactured with chips capable of substantially raising their power demands for short periods of time (e.g., by entering a “turbo” mode). When these chips enter a high power demand state, the required power may temporarily exceed the output power capabilities of a flyback converter, resulting in saturation of the transformer core, or causing power limiting circuits to limit the output power of the adapter. This may impact the performance of the computer system and result in a poor user experience. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a flyback converter in accordance with an embodiment. 
         FIG. 2  shows a flowchart depicting the process for controlling a flyback converter in accordance with an embodiment. 
     
    
    
     In the figures, like reference numerals refer to the same figure elements 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing code and/or data now known or later developed. 
     The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. 
     Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
       FIG. 1  depicts a flyback converter in accordance with an embodiment. Input supply  102  is coupled to flyback converter  104  which is coupled through current sensor  114  and across voltage sensor  118  to computer system  120 . Flyback converter  104  includes primary side coil  106 , switch  108 , and secondary side coil  110  coupled to secondary rectifier  112  and across capacitor  116 . Feedback controller  122  is coupled to current sensor  114 , voltage sensor  118 , and switch  108  through electrical isolation  128  and switch controller  130 . Feedback controller  122  includes discontinuous mode controller  124  and continuous mode controller  126 . 
     Input supply  102  can be any supply used to input electrical power into a flyback converter, including but not limited to a full-bridge rectifier or a half-bridge rectifier connected to household alternating current (AC) electricity. Primary side coil  106  and secondary side coil  110  can be any flyback transformer coils with the appropriate specifications (e.g., peak and average power) that are generally used in a flyback converter. Switch  108  can be any switch for use in a flyback converter and can include but is not limited to a field effect transistor (FET). Capacitor  116  can be any appropriate capacitor generally used for a flyback converter. 
     Voltage sensor  118  can be any voltage sensor that can sense the output voltage of a flyback converter and transmit a signal to feedback controller  122 . Voltage sensor  118  may be implemented in any technology including but not limited to analog or digital technology, or a combination thereof, and in some embodiments includes a voltage divider containing two or more resistors. 
     Current sensor  114  can be any current sensor that can sense the output current of a flyback converter and transmit a signal to feedback controller  122 . Current sensor  114  can be implemented in any technology, including but not limited to analog or digital technology, or a combination thereof, and in some embodiments includes a current sense resistor which is coupled at each end to feedback controller  122 . 
     Computer system  120  can be any computing system that uses electrical power that can be supplied by a flyback converter. Computer system  120  may include but is not limited to a laptop computer, a tablet computer, a smartphone, or a desktop computer. 
     Electrical isolation  128  electrically isolates voltages and signals on the secondary side of flyback converter  104  from those on the primary side. Electrical isolation  128  may be implemented in any technology including but not limited to one or more optocouplers. 
     Switch controller  130  may be implemented in any technology, including any combination of analog and digital circuitry, and hardware and/or software. In some embodiments, switch controller  130  is implemented as an integrated circuit chip and may also include one or more discrete components such as capacitors or resistors. In other embodiments, switch controller  130  and electrical isolation  128  are implemented in feedback controller  122 . As will be discussed below, switch controller  130  controls switch  108  based on one or more analog and/or digital signals through electrical isolation  128  from feedback controller  122 . 
     Feedback controller  122  can be implemented in any technology and may include but is not limited to any combination of hardware, software, and analog and/or digital components, and may include one or more processors, and volatile and/or non-volatile memory. Feedback controller  122  receives input from current sensor  114  and voltage sensor  118 , and includes discontinuous mode controller  124  and continuous mode controller  126 . Note that both discontinuous mode controller  124  and continuous mode controller  126  can receive input from current sensor  114  and voltage sensor  118 . 
     In some embodiments, discontinuous mode controller  124  and continuous mode controller  126  are each separate controllers implemented in feedback controller  122 . Discontinuous mode controller  124  and continuous mode controller  126  may each be implemented in any combination of analog and digital components and may include any combination of hardware and software. In some embodiments, discontinuous mode controller  124  and continuous mode controller  126  each implement separate digital proportional integral derivative (PID) controllers. Note that discontinuous mode controller  124  and continuous mode controller  126  may share one or more resources in feedback controller  122  such as a processor and/or memory. 
     During operation, feedback controller  122  uses the output from either discontinuous mode controller  124  or continuous mode controller  126  to control flyback converter  104  using switch controller  130  to control switch  108 . When feedback controller  122  uses the output of discontinuous mode controller  124  to control flyback converter  104 , flyback converter  104  is controlled in a discontinuous mode and feedback controller  122  sends a voltage control feedback signal generated by discontinuous mode controller  124  through electrical isolation  128  to switch controller  130  to control switch  108 . This controls flyback converter  104  in a voltage mode control that controls for the voltage output of flyback converter  104 . Discontinuous mode controller  124  may also control switch  108  through switch controller  130  to be in a quasi-resonant, zero-voltage and zero-current switching mode. 
     When feedback controller  122  uses the output of continuous mode controller  126  to control flyback converter  104 , flyback converter  104  is controlled in a continuous mode. Feedback controller  122  sends a current control feedback signal generated by continuous mode controller  126  through electrical isolation  128  to switch controller  130  to control switch  108 . This controls flyback converter  104  in a current mode control that controls for the current output of flyback converter  104 . Feedback controller  122  may also send a signal to switch controller  130  through electrical isolation  128  that controls switch controller  130  to operate switch  108  at a predetermined higher frequency than switch  108  operates at when flyback converter  104  is in the discontinuous mode. The predetermined high frequency may be determined based on information including but not limited to the output voltage and current desired from flyback converter  104 , and the specifications of input supply  102 . For example in some embodiments, when flyback converter  104  is in the discontinuous mode switch  108  may operated at a frequency in the range from 60 kHz to 100 kHz, while when flyback converter  104  is in the continuous mode the frequency of operation of switch  108  may be 120 kHz. Note that the power output from flyback converter  104  may be larger when it is controlled by continuous mode controller  126  than when it is controlled by discontinuous mode controller  124 . Additionally, note that in some embodiments, when feedback controller  122  switches from using continuous mode controller  126  to using discontinuous mode controller  124  to control switch controller  130 , feedback controller  122  may also send a signal to switch controller  130  through electrical isolation  128  to stop operating at the predetermined higher frequency and to resume operating in a quasi-resonant mode as described above. 
     Secondary rectifier  112  may be any rectifier generally used as a secondary rectifier in a flyback converter. In some embodiments, secondary rectifier  112  is a synchronous rectifier and is coupled through a connection (not shown) to feedback controller  122 . In these embodiments, feedback controller  122  can send a signal to the synchronous rectifier of secondary rectifier  112  to turn off and thus act like a passive rectifier when feedback controller  122  switches from using discontinuous mode controller  124  to using continuous mode controller  126  to control flyback converter  104 . Then, when feedback controller  122  switches from using continuous mode controller  126  to using discontinuous mode controller  124  to control flyback converter  104 , feedback controller  122  can send a signal to secondary rectifier  112  to turn on the synchronous rectifier in secondary rectifier  112  so that it again works as synchronous rectifier. 
     An embodiment operates as follows. Electrical power is supplied by input supply  102  to flyback converter  104  and converted to a voltage level for use by computer system  120 . During normal operation of computer system  120  (e.g., when it is not in a high power usage state such as a “turbo” mode) feedback controller  122  controls flyback converter  104  using the output from discontinuous mode controller  124 . 
     When computer system  120  increases its demand for power, such as by entering a “turbo” mode, computer system  120  will start to draw more current from flyback converter  104 . As more current is drawn from flyback converter  104 , eventually the voltage from flyback converter  104  will start to fall as the power demanded by computer system  120  starts to exceed the power that can be delivered from flyback converter  104  while regulating the output voltage of flyback converter  104  at the desired level. When feedback controller  122  senses that the current from flyback converter  104  has exceeded a predetermined current and that the voltage from flyback converter  104  has fallen below a predetermined voltage, then feedback controller  122  will switch from using discontinuous mode controller  124  to control flyback converter  104  to using continuous mode controller  126  to control flyback converter  104 . 
     The predetermined voltage and predetermined current may be selected as follows. In some embodiments, when flyback converter  104  is controlled by feedback controller  122  using discontinuous mode controller  124  in a quasi-resonant voltage mode control, the output of flyback converter  104  is regulated to be within a predetermined value or percentage (e.g., 5%) of its nominal output. The predetermined voltage may then be selected to be equal to or less than a voltage that falls outside the nominal regulated output voltage of flyback converter  104  (e.g., 95% or less of the nominal voltage). 
     The predetermined current value may be selected to be the current drawn from flyback converter  104  that would result in the voltage output of flyback converter  104  falling below the nominal voltage regulation of flyback converter  104  (e.g., 95%), or it may be a fixed percentage higher than the maximum output current that can be supplied at the nominal output voltage by flyback converter  104  when being controlled by discontinuous mode controller  124 . For example, for a 60 watt flyback converter, with a nominal output voltage of 16.5 volts and a nominal steady-state peak current of 3.6 amps, the predetermined voltage may be set to be at or less than 15.675 volts (e.g., 95% of 16.5 volts), and the predetermined current may be set to be in the range of 5.4 to 7.2 amps (e.g., 50% to 100% above the nominal steady-state peak current). 
     Note that in some embodiments the predetermined voltage and predetermined current may be determined based on measurements of current and voltage during operational or other testing of flyback converter  104  while powering a computer system performing a series of operations to mimic power usage profiles of a computer system during use in the field. The predetermined voltage and predetermined current may then be selected based on factors including but not limited to thermal or other operation characteristics of flyback converter  104  and/or input supply  102  and the user experience while operating the computer system. 
     When the voltage sensed by feedback controller  122  using voltage sensor  118  is less than the predetermined voltage, and the current sensed by feedback controller  122  using current sensor  114  is above the predetermined current, then feedback controller  122  switches from using discontinuous mode controller  124  to control flyback converter  104  to using continuous mode controller  126  to control flyback converter  104 . 
     Then, while feedback controller  122  is using continuous mode controller  126  to control flyback converter  104 , if the voltage sensed by feedback controller  122  using voltage sensor  118  is greater than the predetermined voltage or the current sensed by feedback controller  122  using current sensor  114  is below the predetermined current, then feedback controller  122  switches from using continuous mode controller  126  to control flyback converter  104  to using discontinuous mode controller  124  to control flyback converter  104 . Note that in some embodiments one set of values for the predetermined voltage and predetermined current may be used by feedback controller  122  to switch from using discontinuous mode controller  124  to continuous mode controller  126  to control flyback converter  104 , and another set of values may be used for switching from using continuous mode controller  126  to discontinuous mode controller  124  to control flyback converter  104 . 
     In some embodiments feedback controller  122  limits the amount of time that feedback controller  122  can use continuous mode controller  126  to control flyback converter  104 . The duration of this time period may be determined by one or more of the thermal and electrical characteristics of flyback converter  104  and/or input supply  102 , and in some embodiments may be  10  milliseconds. When this time period expires, feedback controller  122  may switch to using discontinuous mode controller  124  to control flyback converter  104 . Additionally, in some embodiments when this time period expires, a second time period begins during which feedback controller  122  prevents continuous mode controller  126  from controlling flyback converter  104 . This second time period may be determined based on thermal characteristics of flyback converter  104  and/or input supply  102 , and may be selected to be long enough to prevent flyback converter  104  and/or input supply  102  from overheating due to operating in a continuous mode controlled by continuous mode controller  126 . In some embodiments the second time period may be 300 milliseconds. Additionally, in some embodiments, if the current from flyback converter  104  sensed by feedback controller  122  using current sensor  114  exceeds a predetermined threshold, then feedback controller  122  may shut off flyback converter  104  and/or input supply  102  using a connection not shown in  FIG. 1 . 
       FIG. 2  shows a flowchart depicting the process for controlling a flyback converter in an adapter for use with a computer system in accordance with an embodiment. In step  202 , the power adapter is in a vampire mode. At step  204 , if no load is present (e.g. if the adapter is not plugged in to a computer system), then the process returns to step  202 , while if a load is present the process continues to step  206  and enters a default mode. In the default mode (step  206 ), the flyback converter is controlled by a feedback controller in the adapter in a voltage-mode controlled quasi-resonant discontinuous mode. 
     The output voltage and output current of the flyback converter are then measured (step  208 ). If the output voltage is not less than a predetermined voltage or the output current is not greater than a predetermined current (step  210 ), the process continues to step  212 . If the computer system is disconnected (step  212 ), then the process returns to step  202 . If the computer system is not disconnected (step  212 ), then the feedback controller is put into voltage mode control (step  214 ), and quasi-resonant mode and discontinuous mode ( 216 ) if it is not already in these modes. The process then returns to step  208 . 
     At step  210 , if the output voltage is less than the predetermined voltage and the output current is greater than the predetermined current, then the process continues to step  218 . At step  218 , if timer 2  is still running then the process continues to step  220 . At step  220 , if an over-current protection fault is present, then the process continues to step  222  and turns the adapter off (e.g., latches it), and the process stops. At step  220 , if an over-current protection fault is not present, then the process continues to step  206 . 
     At step  218 , if timer 2  is not still running (e.g., timer 2  has expired), then the feedback controller is put into current mode control (step  224 ) and controlled in a continuous mode and at a higher frequency relative to the quasi-resonant, discontinuous mode (step  226 ). Then, timer 1  is started for a first predetermined time period if it has not already been started; if timer 1  has already been started, it is incremented (step  228 ). Then, if timer 1  has not expired, the process returns to step  208 . If timer 1  has expired (step  230 ), then timer 2  is started for a second predetermined time period (step  232 ) and the process returns to step  206 . Note that in some embodiments the first predetermined time period for timer 1  is 10 milliseconds and the second predetermined time period for timer 2  is 300 milliseconds. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.