# Patent

## Title
Programmable current limit circuit for a programmable power supply

## Abstract

A control circuit of a power supply is provided. The control circuit includes a circuit and a PWM circuit. The circuit generates a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply. The PWM circuit generates a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to a level of the output voltage of the power supply. The output voltage of the power supply is programmable.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/896,243, filed on Oct. 28, 2013, the contents of which are incorporated herein by reference.


BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a programmable current limit for a power supply, and more particular to a power supply with a programmable output voltage (variable output voltage).
Description of the Related Art
Technique of current limits is utilized to meet safety requirements. The detail skill of current limits for a power supply can be found in the prior arts of U.S. Pat. No. 6,611,439 titled “PWM controller for controlling output power limit of a power supply”, U.S. Pat. No. 6,674,656 titled “PWM controller having a saw-limiter for output power limit without sensing input voltage”, U.S. Pat. No. 6,721,192 titled “PWM controller regulating output voltage and output current in primary side”, and U.S. Pat. No. 7,054,170 titled “Power-mode controlled power converter”.
BRIEF SUMMARY OF THE INVENTION
An exemplary embodiment of a control circuit of a power supply is provided. The control circuit comprises a circuit and a PWM circuit. The circuit generates a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply. The PWM circuit generates a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to a level of the output voltage of the power supply. The output voltage of the power supply is programmable.
An exemplary embodiment of a method of generating an output voltage of a power supply is provided. The method comprises the steps of generating a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply; and generating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to the level of the output voltage of the power supply. The output voltage of the power supply is programmable.
A detailed description is given in the following embodiments with reference to the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 shows an exemplary embodiment of a power supply in accordance with the present invention;
FIG. 2 shows an exemplary embodiment of a control circuit of the power supply in FIG. 1;
FIG. 3 shows an exemplary embodiment of a circuit of the control circuit in FIG. 2;
FIG. 4 shows an exemplary embodiment of a PWM circuit of the control circuit in FIG. 2; and
FIG. 5 shows a curve of a current-limit signal versus an output voltage of the power supply in FIG. 1.



DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined
FIG. 1 is an exemplary embodiment of a power supply in accordance with the present invention. A transformer 10 is coupled between an input voltage VIN of the power supply and a transistor 20. The transistor 20 is coupled to switch the transformer 10 for transforming a power. The power is related to the input voltage VIN. The transformer 10 includes a primary winding NP, a second winding Ns and an auxiliary winding NP. When the transformer 10 is switched, the power is transformed from the primary winding NP to the second winding Ns, and an output voltage VO of the power supply is generated. In other words, the secondary winding NS of the transformer 10 will generate the output voltage VO through a rectifier 40 and a capacitor 45 when the transformer 10 is switched. A control circuit 100 generates a switching signal SW coupled to drive the transistor 20 for regulating the output voltage VO of the power supply. When the transistor 20 is turned on, a switching current of the transformer 10 will generate a switching current signal CS via a resistor 25. The switching current signal CS is coupled to the control circuit 100. Resistors 31 and 32 are coupled to the auxiliary winding NA of the transformer 10 for detecting a reflected signal VS. The reflected signal VS is coupled to the control circuit 100. The reflected signal VS represents a reflected voltage of the transformer 10. The level of the reflected signal VS is related to the level of the output voltage VO during the demagnetizing period of the transformer 10. Therefore, an output-voltage signal EO (shown in FIG. 2) is produced according to the reflected signal VS. In other words, the output-voltage signal EO is correlated to the level of the output voltage VO of the power supply.
An error amplifier 50 is coupled to receive the output voltage VO via resistors 51 and 52. The error amplifier 50 has a reference voltage 70 (VR). The output of the error amplifier 50 generates a feedback signal VFB coupled to the control circuit 100 through an opto-coupler 80. Therefore, the control circuit 100 will generate the switching signal SW according to the feedback signal VFB to regulate the output voltage VO as shown in Equation (1).




  
    
      
        
          V
          O
        
        =
        
          
            
              
                R
                51
              
              +
              
                R
                52
              
            
            
              R
              52
            
          
          ×
          
            V
            R
          
        
      
    
    
      
        (
        1
        )
      
    
  




The reference voltage 70 (VR) is programmable for determining the output voltage VO of the power supply. Furthermore, the resistor 52 is adjustable for programming the output voltage VO. A resistor 56 and a capacitor 57 are coupled to the error amplifier 50 for feedback loop compensation. The capacitance of the capacitor 57 will determine the bandwidth of the feedback loop for the regulation of the output voltage VO.
FIG. 2 is an exemplary embodiment of the control circuit 100 in accordance with the present invention. In the control circuit 100, a level-shift circuit includes a transistor 120 and resistors 110, 125, and 126. The level-shift circuit is coupled to receive the feedback signal VFB for generating a signal VB. In other words, the level of the signal VB is related to the level of the feedback signal VFB. The signal VB, the switching current signal CS, and a limit signal IX are coupled to a pulse width modulation circuit (PWM) 300 for generating the switching signal SW. A circuit 200 generates the limit signal IX in accordance with the output-voltage signal EO, an input-voltage signal EIN, and an initial-timing signal ST.
A sample-hold circuit (S/H) 150 receives the reflected signal VS for generating the output-voltage signal EO and the input-voltage signal EIN. The output-voltage signal EO is correlated to the output voltage VO. The input-voltage signal EIN represents the input voltage VIN. The detail skill of sampling the reflected voltage VS of the transformer 10 and generating the output-voltage signal EO can be found in the prior arts of U.S. Pat. No. 7,349,229 titled “Causal sampling circuit for measuring reflected voltage and demagnetizing time of transformer”, and U.S. Pat. No. 7,486,528 titled “Linear-predict sampling for measuring demagnetized voltage of transformer”. The detail approach for generating the input-voltage signal EIN through the detection of the transformer's voltage can be found in the prior art of U.S. Pat. No. 7,671,578 titled “Detection circuit for sensing the input voltage of transformer”.
FIG. 3 is an exemplary embodiment of the circuit 200 in accordance with the present invention. The output-voltage signal EO generates a signal VG through a voltage divider. The voltage divider is formed by resistors 251, 252 and 253. The enabling/disabling of the resistor 253 is controlled by a transistor 265. A comparator 260 will turn on the transistor 265 once the input-voltage signal EIN is higher than a threshold VT1. In other words, when the input-voltage signal EIN is higher than the threshold VT1, the resistor 253 is enabled; and once the input-voltage signal EIN is lower than the threshold VT1, the resistor 253 is disabled. Therefore, when the input-voltage signal EIN is lower than the threshold VT1, the signal VG is shown as,




  
    
      
        
          V
          G
        
        =
        
          
            
              R
              252
            
            
              
                R
                251
              
              +
              
                R
                252
              
            
          
          ×
          
            E
            O
          
        
      
    
    
      
        (
        2
        )
      
    
  




When the input-voltage signal EIN is higher than the threshold VT1, the signal VG can be expressed as,





  
    
      
        
          V
          G
        
        =
        
          
            
              R
              P
            
            
              
                R
                251
              
              +
              
                R
                P
              
            
          
          ×
          
            E
            O
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          R
          P
        
        =
        
          
            
              R
              252
            
            ×
            
              R
              253
            
          
          
            
              R
              252
            
            +
            
              R
              253
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




An amplifier 210, a resistor 215, and a transistor 220 will generate a current IAG according to the signal VG.





  
    
      
        
          I
          AG
        
        =
        
          
            V
            G
          
          
            R
            215
          
        
      
    
    
      
        (
        5
        )
      
    
  
  
    
      
        
          I
          X
        
        =
        
          
            (
            
              K
              ×
              
                I
                AG
              
            
            )
          
          +
          
            (
            
              
                K
                0
              
              ×
              
                I
                240
              
            
            )
          
        
      
    
    
      
        (
        6
        )
      
    
  




As shown in Equation (6), the limit signal IX is generated according to the current IAG, wherein K and K0 are constant and related to a ratio of current mirrors formed by transistors 231, 232, 234, 235, 238, and 239; I240 is a current of a current source 240. The maximum value of the limit signal IX is clamped by a current source IMAX. The current source 240 is enabled by a switch 245. The switch 245 is controlled by the initial-timing signal ST. That is to say the initial-timing signal ST will enable the current source 240 when the power supply is turned on and during the output voltage VO is programmed to a higher value. Therefore, the value of the limit signal IX is determined by the output-voltage signal EO, the input-voltage signal EIN, and the initial-timing signal ST. In detailed, When the switch 245 is turned off by the initial-timing signal ST, the mirror composed of the transistors 238 and 239 mirrors only the current I235 flowing through the transistor 235 to generate the limit signal IX. When the output voltage rises (that is during the power-on period of the power supply), the switch 245 is turned on by the enabled initial-timing signal ST. At this time, the mirror composed of the transistors 238 and 239 mirrors the currents. The currents sum of the currents I235 and I240 to generate the limit signal IX. Thus, according to the above description, the level of the limit signal IX is increased during the power-on period of the power supply.

FIG. 4 is an exemplary embodiment of the pulse width modulation circuit (PWM) 300 according to the present invention. An oscillator (OSC) 400 will generate a clock signal CLK and a ramp signal RMP. The clock signal CLK is coupled to clock a flip-flop 420 via an inverter 410 for generating the switching signal SW through an AND gate 425. An adder 411 will generate a signal SP in response to the ramp signal RMP and the switching current signal CS. The signal SP is coupled to a comparator 415. The comparator 415 will reset the flip-flop 420 and turn off the switching signal SW via an AND gate 419 when the feedback signal VB is lower than the signal SP. The limit signal IX and a current source IMIN are coupled to a resistor 35 to develop the current-limit signal VLMT. According to the operation of the limit signal IX, the current source IMIN, and the resistor 35 in FIG. 4, the current-limit signal VLMT represents the limit signal IX. In other words, the current-limit signal VLMT is in the direct proportion to the limit signal IX. So that, the current-limit signal VLMT is coupled to limit the switching current of the transformer 10. A comparator 416 is coupled to reset the flip-flop 420 and turn off the switching signal SW via the AND gate 419 when the switching current signal CS is higher than the current-limit signal VLMT.
FIG. 5 shows a curve of the current-limit signal VLMT versus the output voltage VO of the power supply. When the level of the output voltage VO is programmed to a higher value (e.g. 20V), the current-limit signal VLMT will be set to a higher value (e.g. 1V). If the level of the output voltage VO is programmed to be a lower voltage (e.g. 5V), then the current-limit signal VLMT will be automatically set as be lower level (e.g. 0.5V) to meet the safety requirement. As described in above, the current-limit signal VLMT represents the limit signal IX. In other words, the level of the limit signal IX is decreased in response to the decrease of the level of the output voltage VO.
Besides, when the level of the input voltage VIN is higher, the current-limit signal VLMT will be automatically adjusted to a lower value. In other words, the level of the limit signal IX is decreased in response to the increase of the level of the input voltage VIN.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. A control circuit of a power supply comprising:
a sample-and-hold circuit coupled to the transformer for generating an input-voltage signal, wherein the input-voltage signal represents an input voltage of the power supply;
a circuit receiving the input-voltage signal and generating a limit signal in response to an output voltage of the power supply and the input-voltage signal for limiting a switching current of a transformer of the power supply; and
a PWM circuit generating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply,
wherein a level of the feedback signal is related to a level of the output voltage of the power supply, and the output voltage of the power supply is programmable.

2. The control circuit as claimed in claim 1 further comprising:
a sample-and-hold circuit to sample a reflected voltage of the transformer for generating an output-voltage signal,
wherein the output-voltage signal represents the output voltage of the power supply.

3. The control circuit as claimed in claim 1, wherein a level of the limit signal is decreased in response to decrease a level of the output voltage of the power supply.

4. The control circuit as claimed in claim 1, wherein the control circuit decreases a level of the limit signal when the input voltage is increased.

5. The control circuit as claimed in claim 1, wherein a level of the limit signal is increased during a power-on period of the power supply.

6. The control circuit as claimed in claim 1, wherein a level of the limit signal is increased during a period when the output voltage is programmed to a higher value.

7. A method of generating an output voltage of a power supply, comprising:
generating an input-voltage signal by sampling the reflected voltage of the transformer by a sample-and-hold circuit, wherein the input-voltage signal represents an input voltage of the power supply,
generating a limit signal in response to an output voltage of the power supply and the input-voltage signal for limiting a switching current of a transformer of the power supply; and
generating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply,
wherein a level of the feedback signal is related to the level of the output voltage of the power supply, and the output voltage of the power supply is programmable.

8. The method as claimed in claim 7 further comprising:
generating an output-voltage signal by sampling a reflected voltage of the transformer, wherein the output-voltage signal represents the output voltage of the power supply.

9. The method as claimed in claim 7, wherein a level of the limit signal is decreased in response to decrease a level of the output voltage of the power supply.

10. The method as claimed in claim 7, wherein a level of the limit signal is decreased when the input voltage is increased.

11. The method as claimed in claim 7, wherein a level of the limit signal is increased during a power-on period of the power supply.

12. The method as claimed in claim 7, wherein a level of the limit signal is increased during a period when the output voltage is programmed to a higher value.