Abstract:
Power controllers and related primary-side control methods are disclosed. A disclosed power controller has a comparator and an ON-triggering controller. The comparator compares a feedback voltage with an over-shot reference voltage. Based on an inductance-coupling effect, the feedback voltage represents a secondary-side voltage of a secondary winding. Coupled to the comparator, the ON-triggering controller operates a power switch at about a first switching frequency when the feedback voltage is lower than the over-shot reference voltage. The ON-triggering controller operates the power switch at about a second switching frequency when the feedback voltage exceeds the over-shot reference voltage. The second switching frequency is less than the first switching frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. patent application Ser. No. 13/650,098, filed on Oct. 11, 2012, and all benefits of such earlier application are hereby claimed for this new continuation application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a primary side control (PSC) switching-mode power supply (SMPS), and particularly to a PSC SMPS that has reduced output voltage jitter. 
         [0004]    2. Description of the Prior Art 
         [0005]    Power supplies are a necessary electronic device in most electronic products, and are used for converting battery or grid power to power required by the electronic product and having specific characteristics. In most power supplies, switching-mode power supplies have superior electrical energy conversion efficiency and smaller product dimensions, making them popular in the power supply market. 
         [0006]    Two different control schemes are used in current switching-mode power supplies: primary side control (PSC) and secondary side control (SSC). SSC directly couples a detection circuit to an output node of a secondary winding of a power supply, then through a photo coupler, transmits a detection result to a power supply controller located on the primary side to control energy of the power supply that is to be stored and converted on the primary winding. Compared to SSC, PSC indirectly detects voltage outputted by the secondary winding through directly detecting reflected voltage on an auxiliary winding, and indirectly completes detection of output voltage on an output node of the power supply. PSC completes detection and energy conversion control on the primary side. Compared to SSC, PSC is able to lower cost, as PSC does not require the photo coupler having both greater size and cost. PSC may also have higher conversion efficiency, because PSC does not require the detection circuit on the secondary side that constantly drains energy. 
         [0007]      FIG. 1  is a diagram of a switching-mode power supply that uses PSC. Bridge rectifier  20  rectifies alternating current from grid node AC to establish direct current input power at input node IN. Voltage V IN  of output power may have an M-shaped waveform, but may also be filtered into a fixed level that roughly does not vary over time. Transformer has three windings: primary winding PRM, secondary winding SEC, and auxiliary winding AUX. Power supply controller  26  periodically controls power switch  34  through gate node GATE. When power switch  34  is ON, primary winding PRM performs energy storage. When power switch  34  is OFF, secondary winding SEC and auxiliary winding AUX discharge to establish output voltage VOUT on output node OUT for supply to load  24 , and control voltage VCC for supply to power supply controller  26 . 
         [0008]    Voltage divider resistors  28 ,  30  detect voltage V AUX  of auxiliary winding AUX to provide feedback voltage V FB  to feedback node FB of power supply controller  26 . According to feedback voltage V FB , power supply controller  26  establishes compensation voltage V COM  on compensation capacitor  32 , and controls power switch  34  according thereto. 
         [0009]      FIG. 2  shows the power supply controller  26  of  FIG. 1  and some external components. Power supply controller  26  comprises sampler  12 , pulse generator  14 , transconductor  15 , and pulse width controller  16 . During discharging of secondary winding SEC and auxiliary winding AUX, pulse generator  14  provides a short pulse to sampler  12 , so that sampler  12  samples feedback voltage V FB  to generate feedback voltage V IFB  at intermediate node IFB. Through feedback node FB, voltage divider resistors  28  and  30 , and auxiliary winding AUX, feedback voltage V IFB  equivalently represents voltage level of secondary winding voltage V SEC  of secondary winding SEC during discharging, and roughly represents output voltage V OUT . Transconductor  15  controls compensation voltage V COM  on compensation node COMP according to a comparison result of feedback voltage V IFB  and target voltage V REF . Pulse width controller  16  controls power switch  34  according to compensation voltage V COM . Overall, power supply controller  26  provides a feedback mechanism that roughly stabilizes feedback voltage V IFB  to target voltage V REF , and is thus able to stabilize output voltage V OUT . 
       SUMMARY OF THE INVENTION 
       [0010]    According to an embodiment, a primary-side control method comprises providing a feedback voltage, the feedback voltage representing a secondary-side voltage of a secondary winding through an inductance-coupling effect; controlling a power switch by a first switching frequency; comparing the feedback voltage and an over-shot reference voltage; and controlling the power switch by a second switching frequency when the feedback voltage is greater than the over-shot reference voltage. The second switching frequency is lower than the first switching frequency. 
         [0011]    According to an embodiment, a power supply controller for performing primary-side control comprises a comparator and an ON triggering controller. The comparator is for comparing a feedback voltage and an over-shot reference voltage. The feedback voltage represents a secondary-side voltage of a secondary winding through an inductance-coupling effect. The ON-triggering controller is coupled to the comparator. When the feedback voltage is lower than the over-shot reference voltage, the ON-triggering controller causes a power switch to operate at approximately a first switching frequency. When the feedback voltage is higher than the over-shot reference voltage, the ON-triggering controller causes the power switch to operate at approximately a second switching frequency. The second switching frequency is lower than the first switching frequency. 
         [0012]    According to an embodiment, a power management system comprises a transformer, a power switch, and a power supply controller. The transformer has a primary winding, an auxiliary winding, and a secondary winding. The power switch is coupled to the primary winding for controlling an inductance current flowing through the primary winding. The power supply controller is for controlling the power switch, and comprises a feedback node, a comparator, and an ON-triggering controller. The feedback node is coupled to the auxiliary winding. The comparator is for comparing a feedback voltage and an over-shot reference voltage. The feedback voltage represents a secondary-side voltage of the secondary winding through the feedback node and the auxiliary winding. The ON-triggering controller is coupled to the comparator. The ON-triggering controller causes the power switch to operate approximately at a first switching frequency when the feedback voltage is lower than the over-shot reference voltage, and the ON-triggering controller causes the power switch to operate approximately at a second switching frequency when the feedback voltage is higher than the over-shot reference voltage. The second switching frequency is lower than the first switching frequency. 
         [0013]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a diagram of a switching-mode power supply that uses PSC. 
           [0015]      FIG. 2  shows the power supply controller of  FIG. 1  and some external components. 
           [0016]      FIG. 3  is a diagram of a power supply controller according to an embodiment. 
           [0017]      FIG. 4  is a diagram of a power supply controller according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following examples, components sharing the same reference numerals have similar or the same function, structure, and operation. Persons of ordinary skill in the art may arrive at simple alterations or modifications of the embodiments of the detailed description according to the teachings and disclosure herein without leaving the spirit of the present invention. 
         [0019]    The power supply controller  26  of  FIG. 2  may exhibit excessive output voltage VOUT jitter during light-heavy load switching. 
         [0020]    For example, when load  24  suddenly transitions from a heavy load to a light load or no load, output voltage V OUT  will suddenly rise. And, power supply controller  26  must wait for a period of time, in which transconductor  15  pulls compensation voltage V COM  down to a certain level, such that energy converted by transformer is lower than energy consumed by load  24 , before output voltage V OUT  can begin to fall. However, at this time, output voltage V OUT  is very likely to already have exceeded the required specification of the power supply management system. 
         [0021]      FIG. 3  is a diagram of a power supply controller  26   a  according to an embodiment. Power supply controller  26   a  replaces power supply controller  26  of  FIG. 1 . 
         [0022]    Power supply controller  26   a  comprises sampler  12 , pulse generator  14 , transconductor  15 , comparator  60 , oscillator  62 , and pulse width controller  64 . 
         [0023]    After pulse width controller  64  turns power switch  34  off, secondary winding SEC and auxiliary winding AUX begin to release energy stored previously by primary winding PRM while power switch  34  was turned on. The time for secondary winding SEC and auxiliary winding AUX to release electrical energy is called discharge time T DIS . During discharge time T DIS , pulse generator  14  provides a short pulse to cause sampler  12  to sample feedback voltage V FB  on feedback node FB. A sample result is then stored on intermediate node IFB as feedback voltage V IFB . Thus, feedback voltage V IFB  approximately represents output voltage V OUT  through voltage division and inductive coupling through feedback node FB, voltage divider resistors  28  and  30 , auxiliary winding AUX, and secondary winding SEC. 
         [0024]    Transconductor  15  controls compensation voltage V COM  according to feedback voltage V IFB  and target voltage V REF . In some embodiments, pulse width controller  64  determines ON time T ON  of power switch  34  per one switching period according to compensation voltage V COM  on compensation node COMP, which is time in which power switch  34  is short circuited. 
         [0025]    Oscillator  62  provides set signal S SET  through set node SET, which periodically triggers turning on of power switch  34 . Thus, switching frequency of power switch  34  is approximately equal to frequency of set signal S SET . In some embodiments, frequency of set signal S SET  can be determined from compensation voltage V COM . For example, frequency of set signal S SET  can decrease with decreasing compensation voltage V COM . 
         [0026]    Comparator  60  compares feedback voltage V IFB  and over-shot reference voltage V OS-REF . Comparison result S OV  of comparator  60  affects frequency of set signal S SET  provided by oscillator  62 . For example, when feedback voltage V IFB  is lower than over-shot reference voltage V OS-REF , comparison result S OV  is logic 0, and frequency of set signal S SET  may be determined solely by compensation voltage V COM  to be, for example, 60 KHz. As soon as feedback voltage V IFB  exceeds over-shot reference voltage V OS-REF /comparison result S OV  becomes logic 1, and frequency of set signal S SET  immediately drops to be fixed at, for example, 25 KHz. 
         [0027]    Power supply controller  26   a  of  FIG. 3  can suppress output voltage V OUT  jitter when transitioning from a heavy load to a light load. The following description is made with reference to  FIG. 1 , with power supply controller  26   a  replacing power supply controller  26  thereof, and target voltage V REF  and over-shot reference voltage V OS-REF  assumed to be 2.5V and 2.6V, respectively. As soon as load  24  suddenly transitions from heavy loading to light loading or no loading, because energy output of the transformer exceeds energy consumption of load  24 , output voltage V OUT  suddenly rises, causing feedback voltage V IFB  to start rising in turn. As soon as feedback voltage V IFB  exceeds over-shot reference voltage V OS-REF  of 2.6V, frequency of set signal S SET  immediately drops to a low value, so that electrical power outputted by transformer immediately drops. Compared to the prior art, which must wait for compensation voltage V COM  to be pulled down to a certain level before transmitted energy can drop noticeably, as soon as power supply controller  26   a  discovers that feedback voltage V IFB  has exceeded over-shot reference voltage V OS-REF  of 2.6V, frequency of set signal S SET  is dropped immediately, which also lowers electrical power output of the transformer, thus rapidly prohibiting output voltage V OUT  from increasing. 
         [0028]    Feedback voltage V IFB  is periodically updated as set signal S SET  periodically turns on power switch  34 , so as to track current output voltage V OUT . As long as feedback voltage V IFB  is lower than over-shot reference voltage V OS-REF  of 2.6V, power supply controller  26   a  will return to normal operation, e.g. frequency of set signal S SET  being determined only on by compensation voltage V COM . So, for normal operation, power supply controller  26   a  and power supply controller  26  are the same, each causing feedback voltage V IFB  to converge to target voltage V REF  of 2.5V. 
         [0029]      FIG. 4  is a diagram of a power supply controller  26   b  according to an embodiment. In the following description, power supply controller  26   b  replaces power supply controller  26  of  FIG. 1  as another embodiment. 
         [0030]    Compared to the power supply controller  26   a  of  FIG. 2 , power supply controller  26   b  has OFF time controller  66  coupled to feedback node FB. OFF time controller  66  may employ valley switching. For example, after discharge time T DIS , auxiliary winding voltage V AUX  of auxiliary winding AUX starts oscillating, and gradually converges to 0V. So-called “valley switching” may mean that, after power switch  34  is turned off, power switch  34  is turned on when a 1 st  valley, a 2 nd  valley, a 3 rd  valley, and so on of auxiliary winding voltage V AUX  occurs. This type of operating scheme is typically called quasi-resonance (QR) mode. 
         [0031]    Through feedback node FB, OFF time controller  66  can determine when auxiliary winding voltage V AUX  drops across 0V, so-called zero crossing. OFF time controller  66  may be designed to trigger pulse width controller  64  to turn on power switch  34  through set node SET a predetermined period after auxiliary winding voltage V AUX  drops across 0V. Thus, valley switching can be approximately realized. In order to avoid zero-crossing never being detected, OFF time controller  66  can be designed to forcefully trigger pulse width controller  64  to turn on power switch  34  if no zero-crossing has been detected after a maximum OFF time. 
         [0032]    In the embodiment of  FIG. 4 , when feedback voltage V IFB  is lower than over-shot reference voltage V OS-REF , comparison result S OV  is logic 0. At this time, timing of set signal S SET  triggering turning on of power switch  34  may be determined according to compensation voltage V COM  and zero-crossing detected by OFF time controller  66  through feedback node FB. Simply speaking, when feedback voltage V IFB  is lower than over-shot reference voltage V OS-REF , power supply controller  26   b  approximately operates in QR mode, and may trigger turning on of power switch  34  at any valley appearing in auxiliary winding voltage V AUX . 
         [0033]    When feedback voltage V IFB  is greater than over-shot reference voltage V OS-REF , comparison result S OV  is logic 1, and OFF time controller  66  only triggers pulse width controller  64  to turn on power switch  34  after maximum OFF time. At this time, switching frequency of power switch  34  is necessarily lower than when operating in QR mode. 
         [0034]    Similar to power supply controller  26   a  of  FIG. 3 , when output voltage V OUT  is on the high side, causing feedback voltage V IFB  to exceed over-shot reference voltage V OS-REF , power supply controller  26   b  of  FIG. 4  causes OFF time of power switch  34  to be maximum OFF time, so that switching frequency immediately drops. Electrical power transmitted by the transformer can be lowered rapidly, which can rapidly prevent output voltage V OUT  from rising further. 
         [0035]    It is predictable that the power supply controllers of  FIG. 3  and  FIG. 4  can both rapidly prevent feedback voltage V IFB  from rising further, which can reduce output voltage V OUT  jitter, and cause output voltage V OUT  to converge more rapidly. 
         [0036]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.