# Patent

## Title
Current control method and apparatus

## Abstract

Current control method and apparatus are disclosed. A current limiter is coupled to a switch connected in series with an energy transfer element of a power supply. The current limiter detects a current flowing through the switch and, when the current exceeds a current limit signal, turns off the switch. A limit signal generator provides the current limit signal, detects the maximum current value of the current, and updates the current limit signal according to the maximum current value and an ideal current limit value.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply, and more particularly, to a current control apparatus and method employed in a power supply for limiting a maximum current.
2. Description of the Prior Art
The technology of pulse width modulation has been widely applied to a variety of switching power supplies for controlling or regulating output power. In order to avoid permanent damage occurring to a power supply, the power supply is normally embedded with protection circuits such as an over-voltage protection circuit, an over-current protection circuit, and so forth. In general, the power supply is also installed with a protection mechanism for limiting output power regarding overloading or output shorting situations.
Please refer to FIG. 1, which is a schematic diagram showing a prior-art pulse width modulation (PWM) power supply 100. Controller 106 functions to generate a PWM signal for controlling on/off states of a power switch 102. When power switch 102 is turned on, a power voltage VIN will charge the primary winding of transformer 104 making the current increase gradually flowing through the primary winding. When power switch 102 is turned off, the energy stored in transformer 104 can be released for charging an output capacitor via the secondary winding. Resistor RCS is connected with power switch 102 in series, so that voltage drop VCS across resistor RCS is corresponding to the current flowing through power switch 102 and/or the primary winding. When voltage drop VCS is greater than or equal to a predetermined value such as the value of current limit signal VLIMIT, the current, flowing through power switch 102 and/or the primary winding, is then estimated to be an over current. Under such over-current situation, controller 106 will turn off power switch 102 to cease the current flowing through the primary winding. In other words, current limit signal VLIMIT can be utilized to put a limit of maximum power output to operation of the PWM power supply 100.
However, if current limit signal VLIMIT is set as a constant, the maximum output power may change in response to a variation of power voltage VIN due to an occurrence of signal propagation delay. When voltage drop VCS is greater than or equal to the value of current limit signal VLIMIT, a signal delay time tDELAY is required for the controller 106 to complete turning off the power switch 102. In the process during the signal delay time tDELAY, the current flowing through the primary winding is still increasing, and the growth amount of the current is approximately proportional to the contemporary voltage level of power voltage VIN. That is, the maximum power output is actually increased following the increase of power voltage VIN.
A solution of the aforementioned problem is provided by Yang et al. in U.S. Pat. No. 6,674,656 filed on Oct. 28, 2002, entitled “PWM controller having a saw-limiter for output power limit without sensing input voltage”, which is referred to as '656 patent hereinafter. FIG. 2 presents a schematic diagram briefing a methodological construct regarding the '656 patent. In the methodological construct provided by the '656 patent, current limit signal VLIMIT is not a constant. A saw-tooth signal generated by oscillator 204 is furnished to waveform converter 202. Waveform converter 202 then performs slope-adjusting, clamping, and level-shifting operations on the saw-tooth signal for generating current limit signal VLIMIT as shown in FIG. 2. The value of current limit signal VLIMIT is changing with time during each period. As shown in FIG. 2, during each period, the value of current limit signal VLIMIT is rising from a lowest voltage and is eventually clamped at a highest voltage. FIG. 3 illustrates the waveforms regarding current limit signal VLIMIT and two different voltage drops VCS generated in accordance with an embodiment of the '656 patent. Referring to FIG. 3, the waveform of VCS(VINHIGH) represents the waveform of voltage drop VCS corresponding to a higher power voltage VIN, and the waveform of VCS(VINLOW) represents the waveform of voltage drop VCS corresponding to a lower power voltage VIN. Based on the waveforms shown in FIG. 3, it is obvious that the slope of voltage drop VCS(VINHIGH) is higher as the corresponding power voltage VIN is higher. Accordingly, when the power voltage VIN is higher, voltage drop VCS(VINHIGH) is rising quickly so as to reach a lower voltage of current limit signal VLIMIT, and the problem of unstable maximum output power, resulting from the occurrence of signal propagation delay, can be roughly solved.
SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention, a current control method is provided. The current control method comprises turning on a switch connected in series with an energy transfer element of a power supply; providing a current limit signal and an ideal current limit value; detecting a current flowing through the switch; turning off the switch when the current exceeds the current limit signal; detecting a maximum current value of the current; and updating the current limit signal according to the maximum current value and the ideal current limit value.
An embodiment of the present invention provides a current control apparatus comprising a current limiter and a limit signal generator. The current limiter is electrically coupled to a switch which is connected in series with an energy transfer element of a power supply. The current limiter functions to detect a current flowing through the switch and to turn off the switch when the current exceeds a current limit signal. The limit signal generator is utilized for providing the current limit signal, detecting a maximum current value of the current, and updating the current limit signal according to the maximum current value and an ideal current limit value.
An embodiment of the present invention provides a current control method. The current control method comprises turning on a switch during a period, the switch being connected in series with an energy transfer element of a power supply; detecting a current flowing through the switch when the switch is turned on; providing a current limit signal; and turning off the switch when the current exceeds the current limit signal. The current limit signal is substantially retained to be a fixed value during the period. The current limit signal is updated according to a current detection result during the period; in turn, the current limit signal updated is put in use for a subsequent period.
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
FIG. 1 is a schematic diagram showing a prior-art pulse width modulation (PWM) power supply.
FIG. 2 presents a schematic diagram briefing a methodological construct regarding the '656 patent.
FIG. 3 illustrates the waveforms regarding the current limit signal VLIMIT and two different voltage drops VCS generated in accordance with an embodiment of the '656 patent.
FIG. 4 is a circuit diagram schematically showing a power supply in accordance with an embodiment of the present invention.
FIG. 5 is a signal schematic diagram showing the voltage drop VCS and the current limit signal VLIMIT generated by the limit signal generator shown in FIG. 4 during nth and (n+1)th periods, having time along the abscissa.
FIG. 6 is a schematic diagram showing an embodiment of the limit signal generator in FIG. 4 for generating the reference value VLIMIT(n+1).
FIG. 7 is a schematic diagram showing an embodiment of the limit signal update unit in FIG. 6 for updating the current limit signal VLIMIT base on the correction signal VLIMIT-next and the control signal VG.
FIG. 8 is a schematic diagram showing a mechanism for generating the ideal current limit value VCS-IDEAL in accordance with an embodiment of the present invention.
FIG. 9 is a signal schematic diagram showing the waveform of ideal current limit value VCS-IDEAL corresponding to the embodiment shown in FIG. 8 during nth and (n+1)th periods, having time along the abscissa.
FIG. 10 is a schematic diagram showing a mechanism for generating the ideal current limit value VCS-IDEAL in accordance with another embodiment of the present invention.



DETAILED DESCRIPTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Here, it is to be noted that the present invention is not limited thereto.
FIG. 4 is a circuit diagram schematically showing a power supply in accordance with an embodiment of the present invention. Power supply 400 is a flyback power converter comprising a power switch 402, a transformer 404, a limit signal generator 500, a comparator 410, a controller 412, a resistor RCS, a diode 414, and a rectification load capacitor CO. Controller 412 provides a control signal VG so as to control on/off states of power switch 402 for enabling charging or discharging operation of transformer 404. Resistor RCS is utilized for detecting the current flowing through the primary winding of transformer 404 so as to control the output power of power supply 400. Limit signal generator 500 functions to generate a current limit signal VLIMIT. The detailed explanation on limit signal generator 500 will be set forth later on. Comparator 410 generates a compare signal VP through comparing current limit signal VLIMIT with voltage drop VCS across resistor RCS. Controller 412 controls the operation of power switch 402 according to compare signal VP generated by comparator 410.
Limit signal generator 500 is employed to detect a maximum value of VCS, corresponding to a maximum value of the current flowing through power switch 402, in the present period so as to generate current limit signal VLIMIT for using in a subsequent period. In other words, limit signal generator 500 is able to update the current limit signal VLIMIT period by period, and the current limit signal VLIMIT is substantially retained to be a fixed value while power switch 402 is turned on.
FIG. 5 is a signal schematic diagram showing the voltage drop VCS and the current limit signal VLIMIT generated by the limit signal generator shown in FIG. 4 during nth and (n+1)th periods, having time along the abscissa. Referring to FIG. 5, VCS-MAX(n) represents a current peak value of the current flowing through power switch 402 during the nth period, VLIMIT(n) represents a reference value used for limiting the current flowing through power switch 402 during the nth period, VCS-IDEAL represents an ideal current limit value of the current flowing through power switch 402, dVCS(n) represents the resulted difference between the current peak value VCS-MAX(n) and the ideal current limit value VCS-IDEAL, i.e. dVCS(n)=VCS-MAX(n)−VCS-IDEAL; and dVLIMIT(n) represents an update difference between the reference value VLIMIT(n) and the reference value VLIMIT(n+1), i.e. dVLIMIT(n)=VLIMIT(n)−VLIMIT(n+1).
As shown in FIG. 5, after reaching the reference value VLIMIT(n) in the nth period, voltage drop VCS keeps rising, rather than immediately falling off, until reaches the current peak value VCS-MAX(n) due to signal propagation delay. The current peak value VCS-MAX(n) is greater than the ideal current limit value VCS-IDEAL, implying that the reference value VLIMIT(n) used in the nth period is too high for limiting the current flowing through power switch 402. Accordingly, if the reference value VLIMIT(n+1) for use in the (n+1)th period is reduced, the current peak value VCS-MAX(n+1) in the (n+1)th period will be reduced closer to the ideal current limit value VCS-IDEAL. For instance, if the resulted difference dVCS(n) (=VCS-MAX(n)−VCS-IDEAL) is directly put in use as the update difference dVLIMIT(n) for generating the reference value VLIMIT(n+1), the current peak value VCS-MAX(n+1) is then expected to be approximately equal to the ideal current limit value VCS-IDEAL as illustrated in the (n+1)th period shown in FIG. 5. [Para 27] In view of that, the reference value VLIMIT is updated periodically in accordance with an embodiment of the present invention, and the update difference dVLIMIT(n) for using in each updating process is proportional to the resulted difference dVCS(n). The adjusted reference value VLIMIT(n+1) for use in the (n+1)th period can be expressed as Formula (1) listed below.




  
    
      
        
          
            
              
                
                  V
                  LIMIT
                
                ⁡
                
                  (
                  
                    n
                    +
                    1
                  
                  )
                
              
              =
              
              ⁢
              
                
                  
                    V
                    LIMIT
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                -
                
                  k
                  ×
                  
                    
                      dV
                      CS
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  
                    V
                    LIMIT
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                -
                
                  k
                  ×
                
              
            
          
        
        
          
            
              
              ⁢
              
                (
                
                  
                    
                      V
                      
                        CS
                        -
                        MAX
                      
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                  -
                  
                    V
                    
                      CS
                      -
                      IDEAL
                    
                  
                
                )
              
            
          
        
      
    
    
      
        Formula
        ⁢
        
          
        
        ⁢
        
          (
          1
          )
        
      
    
  




In Formula (1), k represents a proportion. Accordingly, the difference between the current peak value VCS-MAX(n+1) and the reference value VLIMIT(n+1) is expected to be the same as the difference between the current peak value VCS-MAX(n) and the reference value VLIMIT(n). For that reason, the current peak value VCS-MAX(n+1) can be expressed as Formula (2) listed below.

VCS-MAX(n+1)=VLIMIT(n+1)+(VCS-MAX(n)=VLIMIT(n))  Formula (2)

Substitute the expression of Formula (1) for the reference value VLIMIT(n+1) in Formula (2), and the current peak value VCS-MAX(n+1) can be expressed as Formula (3) listed below.




  
    
      
        
          
            
              
                
                  V
                  
                    CS
                    -
                    MAX
                  
                
                ⁡
                
                  (
                  
                    n
                    +
                    1
                  
                  )
                
              
              =
              
              ⁢
              
                
                  
                    V
                    
                      CS
                      -
                      MAX
                    
                  
                  ⁡
                  
                    (
                    n
                    )
                  
                
                -
                
                  k
                  ×
                
              
            
          
        
        
          
            
              
              ⁢
              
                (
                
                  
                    
                      V
                      
                        CS
                        -
                        MAX
                      
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                  -
                  
                    V
                    
                      CS
                      -
                      IDEAL
                    
                  
                
                )
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  
                    (
                    
                      1
                      -
                      k
                    
                    )
                  
                  ×
                  
                    
                      V
                      
                        CS
                        -
                        MAX
                      
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                
                +
                
                  k
                  ×
                  
                    V
                    
                      CS
                      -
                      IDEAL
                    
                  
                
              
            
          
        
      
    
    
      
        Formula
        ⁢
        
          
        
        ⁢
        
          (
          3
          )
        
      
    
  




Based on Formula (3), a plurality of corresponding expressions can be deduced as the followings.



  
    
      
        V
        
          CS
          -
          MAX
        
      
      ⁡
      
        (
        1
        )
      
    
    =
    
      
        
          (
          
            1
            -
            k
          
          )
        
        ⁢
        
          
            V
            
              CS
              -
              MAX
            
          
          ⁡
          
            (
            0
            )
          
        
      
      +
      
        k
        ⁢
        
          
        
        ⁢
        
          V
          
            CS
            -
            IDEAL
          
        
      
    
  




  
    
      
        V
        
          CS
          -
          MAX
        
      
      ⁡
      
        (
        2
        )
      
    
    =
    
      
        
          
            (
            
              1
              -
              k
            
            )
          
          ⁢
          
            
              V
              
                CS
                -
                MAX
              
            
            ⁡
            
              (
              1
              )
            
          
        
        +
        
          k
          ⁢
          
            
          
          ⁢
          
            V
            
              CS
              -
              IDEAL
            
          
        
      
      ⁢
      
        

      
      ⁢
      
        
      
      =
      
        
          
            
              (
              
                1
                -
                k
              
              )
            
            2
          
          ⁢
          
            
              V
              
                CS
                -
                MAX
              
            
            ⁡
            
              (
              0
              )
            
          
        
        +
        
          
            (
            
              
                
                  (
                  
                    1
                    -
                    k
                  
                  )
                
                ⁢
                k
              
              +
              k
            
            )
          
          ×
          
            V
            
              CS
              -
              IDEAL
            
          
        
      
    
  




  ...




  
    
      
        V
        
          CS
          -
          MAX
        
      
      ⁡
      
        (
        n
        )
      
    
    =
    
      
        
          
            (
            
              1
              -
              k
            
            )
          
          n
        
        ⁢
        
          
            V
            
              CS
              -
              MAX
            
          
          ⁡
          
            (
            0
            )
          
        
      
      +
      
        
          [
          
            1
            -
            
              
                (
                
                  1
                  -
                  k
                
                )
              
              n
            
          
          ]
        
        ×
        
          V
          
            CS
            -
            IDEAL
          
        
      
    
  



When |1−k|<1, the current peak value VCS-MAX(n) will approach the ideal current limit value VCS-IDEAL following an increase of n. That is, the final stable value of current peak value VCS-MAX(n) is ensured to be locked to the ideal current limit value VCS-IDEAL regardless of any initial value such as VLIMIT(0) or VCS-MAX(0). Accordingly, as long as the circuit of the embodiment is able to generate the relationship based on Formula (1) and the proportion k is set to be within a range between 0 and 2, the current peak value VCS-MAX(n) will be eventually locked to the ideal current limit value VCS-IDEAL regardless of any preset initial value VLIMIT(0). However, with the aim of speeding a converging process for generating the final stable value of current peak value VCS-MAX(n), the proportion k is better set to be within a range between 0.5 and 1.5. In a preferred embodiment, the proportion k is assigned to be 1 so that the current peak value VCS-MAX(1) becomes the ideal current limit value VCS-IDEAL immediately after a starting period.
FIG. 6 is a schematic diagram showing an embodiment of the limit signal generator in FIG. 4 for generating the relationship based on Formula (1). As shown in FIG. 6, limit signal generator 500 comprises a correction signal generator 502 and a limit signal update unit 504. Correction signal generator 502 detects the voltage drop VCS as well as the current limit signal VLIMIT and functions to generate a correction signal VLIMIT-next based on the current limit signal VLIMIT before updated, the detected peak value of voltage drop VCS, and the ideal current limit value VCS-IDEAL. Limit signal update unit 504 is used to update the current limit signal VLIMIT based on the correction signal VLIMIT-next.
In the operation of correction signal generator 502, the currents I1 and I2 have substantially the same current value, and the currents I3 and I4 also have substantially the same current value. MOS transistors M30 and M20 are coupled to form a current mirror so that the current ratio of currents flowing through MOS transistors M30 and M20, a current ratio m, is substantially equal to that of currents I3 and I2. Also, the MOS transistors M40 and M10 are coupled to form another current mirror, thus the current ratio of currents flowing through MOS transistors M40 and M10 is substantially equal to that of currents I4 and I1 and is expected to be the current ratio m, too.
Correction signal generator 502 comprises a peak value sampler Psample. When limit signal generator 500 is working during an nth period and power switch 402 is turned off by control signal VG, the voltage retained in the capacitor of peak value sampler Psample will be equal to the current peak value VCS-MAX(n).
MOS transistor M1 is configured to be part of a source follower. Therefore, when the current peak value VCS-MAX(n) is retained in the capacitor of peak value sampler Psample, the left end of resistor R1 is expected to have the voltage equal to the summation of the current peak value VCS-MAX(n) and the threshold voltage Vth-M1 of MOS transistor M1.
Similarly, the voltage at the right end of resistor R1 is expected to be the summation of the ideal current limit value VCS-IDEAL and the threshold voltage Vth-M2 of MOS transistor M2. The threshold voltage Vth-M2 is about the same as the threshold voltage Vth-M1. Under such situation, the current flowing through resistor R1 becomes I1x=(VCS-MAX(n)−VCS-IDEAL)/R10. R10 is the resistance of resistor R1.
The current I1x also represents the current difference between the currents flowing through MOS transistors M1 and M2, i.e. the current difference between the currents flowing through MOS transistors M10 and M20. Based on the circuit operation regarding the aforementioned current mirrors, the current I2x flowing through resistor R2 is expected to be proportional to the current I1x, i.e. I2x=m×I1x and m is the current ratio.
The right end of resistor R2 will have the summation voltage of the reference value VLIMIT(n) and the threshold voltage Vth-M4 of MOS transistor M4. Suppose that the resistance of resistor R2 is R20 and the threshold voltage of MOS transistor M3 is Vth-M3. Based on the above description, the correction signal VLIMIT-next can be expressed as Formula (4) listed below.




  
    
      
        
          V
          
            LIMIT
            -
            next
          
        
        =
        
          
            
              
                V
                LIMIT
              
              ⁡
              
                (
                n
                )
              
            
            +
            
              V
              
                th
                -
                
                  M
                  ⁢
                  
                    
                  
                  ⁢
                  4
                
              
            
            -
            
              
                I
                
                  2
                  ⁢
                  x
                
              
              ×
              
                R
                20
              
            
            -
            
              V
              
                th
                -
                
                  M
                  ⁢
                  
                    
                  
                  ⁢
                  3
                
              
            
          
          ⁢
          
            

          
          ≅
          
            
              
                V
                LIMIT
              
              ⁡
              
                (
                n
                )
              
            
            -
            
              
                
                  R
                  20
                
                /
                
                  R
                  10
                
              
              ×
              m
              ×
              
                (
                
                  
                    
                      V
                      
                        CS
                        -
                        MAX
                      
                    
                    ⁡
                    
                      (
                      n
                      )
                    
                  
                  -
                  
                    V
                    
                      CS
                      -
                      IDEAL
                    
                  
                
                )
              
            
          
        
      
    
    
      
        Formula
        ⁢
        
          
        
        ⁢
        
          (
          4
          )
        
      
    
  




FIG. 7 is a schematic diagram showing an embodiment of the limit signal update unit in FIG. 6. Referring to FIG. 7, Limit signal update unit 504 employs a delay unit 602 for generating a signal VS by delaying the control signal VG with a delay time dt. After power switch 402 has been turned on by the control signal VG for a while, the signal VS turns on MOS transistor MS1 and turns off MOS transistor MS2. Then after the power switch 402 is turned off by the control signal VG, MOS transistor MS1 will keep turned on and MOS transistor MS2 will keep turned off by the signal VS for a while. Accordingly, the correction signal VLIMIT-next is retained at a memory end of capacitor C1, and the reference value VLIMIT(n) of current limit signal VLIMIT regarding the present period is retained at a memory end of capacitor C2. The delay time dt is required to be long enough so that the current peak value VCS-MAX(n) is capable of affecting the correction signal VLIMIT-next sufficiently to satisfy Formula (4). Accordingly, at the delay time dt after power switch 402 is turned off by the control signal VG, the signal VS will turn off the MOS transistor MS1 and turn on the MOS transistor MS2 to update the current limit signal VLIMIT based on the correction signal VLIMIT-next so that the reference value VLIMIT(n+1) for using in the next period can be generated and retained at the memory end of capacitor C2. Suppose that the capacitance of capacitor C1 is C10 and the capacitance of capacitor C2 is C20. Based on a capacitor charge sharing rule and Formula (4), the reference value VLIMIT(n+1) can be expressed as Formula (5) listed below.

VLIMIT(n+1)=VLIMIT(n)−R20/R10×m×C10/(C10+C20)×(CCS-MAX(n)−VCS-IDEAL)  Formula (5)

Compare Formula (5) with Formula (1), it is obvious that the peak value of voltage drop VCS can be definitely converged to the ideal current limit value VCS-IDEAL after experiencing several switching periods as long as the value of R20/R10×m×C10/(C10+C20) falls into the range between 0 and 2, preferable of 1, in the embodiment, achieving the purpose of limiting the current flowing through power switch 402. Besides, the resistors and capacitors used in the circuit of the embodiment are not required to have accurate values and the required aforementioned resistor/capacitor-related ratios have a relatively wide acceptable range. Accordingly, it is quite easy to implement the circuit layout of the embodiment.
Please continue referring to FIG. 7, limit signal update unit 504 further employs two ideal diodes for clamping the value of current limit signal VLIMIT to be within a range between the ideal current limit value VCS-IDEAL and a preset minimum value VCS-MIN so as to solve any potential problem caused by the peak value of voltage drop VCS going beyond a predetermined range before the convergence of current limit signal VLIMIT.
In summary, the embodiment of the present invention is able to update the current limit signal VLIMIT period by period, and the peak value of voltage drop VCS can be accurately locked to the ideal current limit value VCS-IDEAL. Furthermore, the circuit embodiment of the present invention can be easily implemented while the resistors and capacitors used are not required to have accurate resistances and capacitances.
In the embodiment shown in FIG. 6, the updating process for updating the current limit signal VLIMIT is performed once during each turn-on and turn-off operation cycle of power switch 402, regardless of whether the current flowing through the power switch is an over current. For that reason, the present invention further provides another embodiment for performing an updating process once only when an over current flowing through the power switch occurs during one turn-on and turn-off operation cycle of power switch 402. For instance, in one embodiment, the control signal VG in FIG. 6 and FIG. 7 can be replaced with a signal VG′ generated by performing an AND operation on the control signal VG and the compare signal VP shown in FIG. 4.
Accordingly, when the ideal current limit value VCS-IDEAL is a fixed value, an embodiment of the present invention is able to limit the maximum current flowing through the power switch to a corresponding fixed value for solving the aforementioned problem caused by signal propagation delay.
However, based on the embodiment shown in FIG. 4, an output current IO, forwarded from the rectification load capacitor CO to a load, may change in accordance with the change of power voltage VIN even though the maximum current flowing through power switch 402 is limited to a corresponding fixed value. For instance, when power supply 400 is operated in a continuous conduction mode (CCM) and the output voltage VO is a preset value, the average output current IO is likely to change following a change of power voltage VIN.
In other embodiments of the present invention, the ideal current limit value VCS-IDEAL may not be a fixed value. That is, the ideal current limit value VCS-IDEAL may change with time or period by period.
FIG. 8 is a schematic diagram showing a mechanism for generating the ideal current limit value VCS-IDEAL in accordance with an embodiment of the present invention. Referring to FIG. 8, oscillator 804 is employed to provide an oscillation signal VOSC, such as a saw-tooth signal or a triangular signal, forwarded to waveform converter 802 for generating the ideal current limit value VCS-IDEAL forwarded to the gate of MOS transistor M2 shown in FIG. 6. Waveform converter 802 can be implemented based on but not limited to the disclosed embodiment of the '656 patent. FIG. 9 is a signal schematic diagram showing the waveform of ideal current limit value VCS-IDEAL corresponding to the embodiment shown in FIG. 8. As shown in FIG. 9, the ideal current limit value VCS-IDEAL increases with time during at least part of an interval when the power switch is turned on in each period regardless of the nth period or the (n+1)th period.
FIG. 10 is a schematic diagram showing a mechanism for generating the ideal current limit value VCS-IDEAL in accordance with another embodiment of the present invention. Referring to FIG. 10, duty cycle detector 1002 is employed to generate a duty cycle Dturn-on of the current period based on the control signal VG fetched from the control end of power switch 402. Converter 1004 functions to convert the duty cycle Dturn-on into the ideal current limit value VCS-IDEAL for use in the subsequent period. For instance, when the duty cycle Dturn-on is greater than 0.4, converter 1004 sets the ideal current limit value VCS-IDEAL equal to 0.85V; when the duty cycle Dturn-on is less than 0.2, converter 1004 sets the ideal current limit value VCS-IDEAL equal to 0.75V; and when the duty cycle Dturn-on falls into a range between 0.2 and 0.4, converter 1004 adjusts the ideal current limit value VCS-IDEAL linearly between 0.75V and 0.85V. As illustrated in FIG. 10, the ideal current limit value VCS-IDEAL is substantially unchanged while the power switch is turned on, and the ideal current limit value VCS-IDEAL is updated period by period.
It is noted that the embodiments of the present invention are not limited to be put in use for a flyback power converter. That is, the embodiments of the present invention can be applied to any functional circuit requiring a current limit signal provided for accurately limiting some circuit current to an ideal expected value.
The present invention is by no means limited to the embodiments as described above by referring to the accompanying drawings, which may be modified and altered in a variety of different ways without departing from the scope of the present invention. Thus, it should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alternations might occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.

## Claims

1. A current control apparatus, comprising:
a current limiter electrically coupled to a switch, the switch being connected in series with an energy transfer element of a power supply, the current limiter detecting a current flowing through the switch and turning off the switch when the current exceeds a current limit signal; and
a limit signal generator for providing the current limit signal, detecting a maximum current value of the current, and updating the current limit signal according to the maximum current value and an ideal current limit value, the limit signal generator comprising:
a correction signal generator for generating a correction signal according to the current limit signal before updated, the maximum current value, and the ideal current limit value; and
a limit signal update unit for updating the current limit signal according to the correction signal, the limit signal update unit comprising:
a first capacitor having a first memory end for retaining the correction signal; and
a second capacitor having a second memory end for retaining the current limit signal;
wherein the limit signal update unit controls an electrical connection between the first memory end and the second memory end for updating the current limit signal;


wherein when the maximum current value of the current is higher than the ideal current limit value, the updated current limit signal is reduced.

2. The current control apparatus of claim 1, wherein the limit signal generator updates the current limit signal according to a resulted difference between the maximum current value and the ideal current limit value.

3. The current control apparatus of claim 1, wherein an update difference between the current limit signal before updated and an updated current limit signal is substantially proportional to a resulted difference between the maximum current value and the ideal current limit value with a fixed proportion k, and the fixed proportion k is within a range between 0 and 2.

4. The current control apparatus of claim 1, wherein the correction signal generator comprises:
a peak value sampler for detecting the maximum current value of the current.