# Patent

## Title
Under-voltage protection circuit for programmable power supplies

## Abstract

A control circuit of a power supply is provided. The power supply includes a current-sense circuit, a voltage-sense circuit, an output voltage regulation circuit, and a current regulation circuit. The current-sense circuit generates a current-sense signal in response to an output current of the power supply. The voltage-sense circuit generates a voltage-sense signal in response to an output voltage of the power supply. The output voltage regulation circuit is coupled to regulate the output voltage of the power supply according to a voltage reference signal and the voltage-sense signal. The output current regulation circuit is coupled to regulate the output current of the power supply according to a current reference signal and the current-sense signal. The output voltage of said power supply is programmable.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/905,386, filed on Nov. 18, 2013, the contents of which are incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a programmable power converter, and, more particularly, the present invention relates to an under-voltage protection circuit for programmable power supplies.
2. Description of the Related Art
Programmable power supplies are developed to provide different constant output voltage levels, such as 5 V, 9 V, 12 V, and 19 V, to power different equipments. Some of them having constant output current characteristic are called chargers which are used to charge batteries. An output current usually has a current limit to ensure that the batteries can be charged under or equal to a safe current that won't damage the batteries.
FIG. 2 shows a characteristic curve of an output voltage VO versus an output current IO for a traditional power supply with constant output current and constant output voltage characteristic. When the output current IO demanded by the output load of the power supply reaches a threshold current IT, the output voltage VO will start to decrease automatically to keep the output current IO as a constant. In a case, the threshold current IT is the maximum value of the characteristic curve. However, as the output voltage VO drops to a too low level, the batteries, especially to lithium batteries, would be damaged permanently due to its over discharging.
BRIEF SUMMARY OF THE INVENTION
Thus, an alert mechanism (under-voltage protection circuit) is needed for programmable power supplies to protect their output loads, especially for batteries.
An exemplary embodiment of control circuit of a power supply is provided. The control circuit comprises a current-sense circuit, a voltage-sense circuit, an output voltage regulation circuit, a current regulation circuit, and a time-delay circuit. The current-sense circuit generates a current-sense signal in response to an output current of the power supply. The voltage-sense circuit generates a voltage-sense signal in response to an output voltage of the power supply. The output voltage regulation circuit is coupled to regulate the output voltage of the power supply according to a voltage reference signal and the voltage-sense signal. The output current regulation circuit is coupled to regulate the output current of the power supply according to a current reference signal and the current-sense signal. The current reference signal is changed in response to a level of the output voltage of the power supply. The output current of the power supply is decreased in response to a decrement of the current reference signal. The time-delay circuit is coupled to the output current regulation circuit for changing the current reference signal. A level of the current reference signal is decreased in response to a decrement of the level of the output voltage of the power supply. The output voltage of the power supply is programmable. A level of the current reference signal is decreased once the output voltage is lower than a protection threshold. A level of the protection threshold is decreased in response to a decrement of the voltage reference signal.
An exemplary embodiment of a method of regulating a power supply. The method comprises: generating a current-sense signal in response to an output current of the power supply; generating a voltage-sense signal in response to an output voltage of the power supply;
regulating the output voltage in response to a voltage reference signal and the voltage-sense signal; regulating the output current in response to a current reference signal and the current-sense signal; and generating a delay time to change the current reference signal. The current reference signal is changed in response to the output voltage of the power supply. The output current is decreased in response to a decrement of the current reference signal. A level of the current reference signal is decreased in response to a decrement of the output voltage of the power supply. The output voltage of the power supply is programmable. A level of the current reference signal is decreased once the output voltage is lower than a protection threshold. A level of the protection threshold is decreased in response to a decrement of the voltage reference signal.
An exemplary embodiment a method of under-voltage protection for a power supply. The method comprises: regulating an output voltage of the power supply at a regulated level; regulating an output current of the power supply approximately at a first current level whenever the output voltage is below the regulated level and higher than a lower limit of a protection threshold; and regulating the output current of the power supply approximately at a second current level whenever the output voltage is below an upper limit of the protection threshold. The output voltage of the power supply is programmable. The first current level is higher than the second current level. The upper limit of the protection threshold is higher than the lower limit of the protection threshold.
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 programmable power supply according to the present invention;
FIG. 2 shows a characteristic curve of an output voltage versus an output current for a traditional power supply with constant output current;
FIG. 3 shows an exemplary embodiment of a control circuit of a programmable power supply according to the present invention;
FIG. 4 shows one characteristic curve of an output voltage versus an output current according to the present invention; and
FIG. 5 shows another characteristic curve of the output voltage versus the output current according to the present invention.



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 by reference to the appended claims.
FIG. 1 shows an embodiment of a programmable power supply according to the present invention. A transformer 10 is coupled to receive an input voltage VIN of a power supply. A transistor 25 is coupled to switch a primary winding NP of the transformer 10. A pulse width modulation (PWM) circuit 20 generates a switching signal SW coupled to drive the transistor 25 for regulating an output voltage VO and an output current IO of the power supply. A secondary winding NS of the transformer 10 will generate the output voltage VO and the output current IO through a rectifier 30 and capacitors 31 and 32. A voltage-sense circuit formed by resistors 71 and 72 generate a voltage-sense signal VS in response to the output voltage VO. A resistor 50 generates a differential current signal pair CS+ and CS− in response to the output current IO. A control circuit 100 is coupled to receive the voltage-sense signal VS and the differential current signal pair CS+ and CS−. The control circuit 100 has a voltage reference signal VRV and a current reference signal VRI (shown in FIG. 3) for generating a feedback signal VFB in response to the voltage-sense signal VS and the differential current signal pair CS+ and CS−. The feedback signal VFB is further coupled to the PWM circuit 20 through an opto-coupler 40 to form a feedback loop. Therefore, the PWM circuit 20 generates the switching signal SW in response to the feedback signal VFB to regulate the output voltage VO and the output current IO as shown in equation (1) and equation (2).




  
    
      
        
          V
          O
        
        =
        
          
            
              
                R
                71
              
              +
              
                R
                72
              
            
            
              R
              72
            
          
          ×
          
            V
            RV
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        
          I
          O
        
        =
        
          
            K
            
              R
              50
            
          
          ×
          
            V
            RI
          
        
      
    
    
      
        (
        2
        )
      
    
  




where R50, R71, and R72 are the resistance of the resistors 50, 71, and 72 respectively. K is a constant determined by the gain of a differential amplifier in the control circuit 100.
FIG. 3 shows an embodiment of the control circuit 100 according to the present invention. An amplifier 150 and resistors 110, 115, 120, and 125 develop a differential amplifier (current-sense circuit) coupled to receive the differential current signal pair CS+ and CS− for generating an current-sense signal VCS. The ratio of the resistors 110 and 115 and the resistors 120 and 125 determine the value of the constant K (the gain of the differential amplifier). An error amplifier (output current regulation circuit) 200 receives the current-sense signal VCS to generate a current-feedback signal COMI according to the current reference signal VRI and the current-sense signal VCS. The current reference signal VRI is coupled to the error amplifier 200 via a switch 175. An error amplifier (output voltage regulation circuit) 300 receives the voltage-sense signal VS to generate a voltage-feedback signal COMV according to the voltage reference signal VRV and voltage-sense signal VS. The voltage-feedback signal COMV is coupled to generate the feedback signal VFB through an open-drain buffer (OD) 350. The current-feedback signal COMI is further coupled to generate the feedback signal VFB through an open-drain buffer (OD) 250. Therefore, the level of the feedback signal VFB is determined by the level of the voltage-feedback signal COMV and the level of the current-feedback signal COMI.
The voltage reference signal VRV is programmed by a programmable current source 400 and resistors 410 and 420. The value of the programmable current source 400 determines the level of the voltage reference signal VRV. The voltage reference signal VRV therefore determines the output voltage VO according to equation (1) as aforementioned. The programmable current source 400 and the resistor 420 further generate a threshold VT. The level of the threshold VT is lower than the level of the voltage reference signal VRV. A comparator 500 having a hysteresis will generate a control signal SX through a time-delay circuit (DLY) 510 when the voltage-sense signal VS is higher than the threshold VT. The control signal SX is coupled to control the switch 175. The control signal SX is further coupled to control a switch 185 via an inverter 520. Once the voltage-sense signal VS is lower than the threshold VT, the switch 185 will be turned on and another current reference signal VRIL will be coupled to the error amplifier 200 to regulate the output current IO. Thus, the current reference signals VRI or VRIL is changed actively in response to the level of the output voltage VO. In detailed, the level of the active current reference signal VRI or VRIL is decreased in response to the decrement of the level of the output voltage VO. The level of the current reference signal VRIL is lower than the level of the current reference signal VRI. Therefore, the output current IO will be regulated as a lower value when the voltage-sense signal VS is lower than the threshold VT. In other words, the output current IO is decreased in response to a decreased current reference signal, that is the lower current reference signal VRIL.
FIG. 4 shows one characteristic curve of the output voltage VO having a first level VO1 versus the output current IO according to the present invention. When the output voltage VO is programmed to regulate at the first level VO1, a first protection threshold VTH1 is correspondingly set by the threshold VT. An upper limit 4b and a lower limit 4a forming a hysteresis range ΔV of the first protection threshold VTH1 is determined by the hysteresis of the comparator 500. A first current level IOH of the output current IO is determined by the current reference signal VRI. A second current level IOL of the output current IO is determined by the current reference signal VRIL. The first level IOH is higher than the second level IOL. As the output voltage VO falls below the lower limit 4a of the first protection threshold VTH1, the output current IO being regulated will instantly fall to the second current level IOL from the first current level IOH. As the output voltage VO raises above the upper limit 4b of the first protection threshold VTH1, the output current IO being regulated will instantly raise to the first current level IOH from the second current level IOL.
FIG. 5 shows another characteristic curve of the output voltage VO having a second level VO2 versus the output current IO according to the present invention. The whole characteristic curve in FIG. 5 is shifted downward corresponding to the characteristic curve shown in FIG. 4. When the output voltage VO is programmed to regulate at the second level VO2, a second protection threshold VTH2 having the hysteresis range ΔV formed between an upper limit 5b and a lower limit 5a will be correspondingly set.
Referring to both FIG. 4 and FIG. 5, the first level VO1 is higher than the second level VO2. The first protection threshold VTH1 is higher than the second protection threshold VTH2.
Referring to FIG. 4, in another perspective of the present invention, a method of under-voltage protection for a programmable power supply is also proposed. The method comprises: regulating an output voltage VO of the programmable power supply at a regulated level VO1; regulating an output current IO of the programmable power supply approximately at a first current level IOH whenever the output voltage VO is below the regulated level VO1 and higher than a lower limit 4a of a protection threshold VTH1; and regulating the output current IO of the programmable power supply approximately at a second current level IOL whenever the output voltage VO is below an upper limit 4b of the protection threshold VTH1. The first current level IOH is higher than the second current level IOL. The upper limit 4b of the protection threshold VTH1 is higher than the lower limit 4a of the protection threshold VTH1.
Referring to FIG. 5, in another perspective of the present invention, the method described in last paragraph also comprises: regulating the output voltage VO of the programmable power supply at a regulated level VO2; regulating the output current IO of the programmable power supply approximately at the first current level IOH whenever the output voltage VO is below the regulated level VO2 and higher than a lower limit 5a of a protection threshold VTH2; and regulating the output current IO of the programmable power supply approximately at the second current level IOL whenever the output voltage VO is below an upper limit 5b of the protection threshold VTH2. The upper limit 5b of the protection threshold VTH2 is higher than the lower limit 5a of the protection threshold VTH2.
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 programmable power supply, comprising:
a current-sense circuit generating a current-sense signal in response to an output current of said programmable power supply;
a voltage-sense circuit generating a voltage-sense signal in response to an output voltage of said programmable power supply;
an output voltage regulation circuit coupled to regulate said output voltage of said programmable power supply according to a voltage reference signal and said voltage-sense signal; and
an output current regulation circuit, coupled to regulate said output current of said programmable power supply according to a current reference signal and said current-sense signal,
wherein said current reference signal is changed in response to a level of said output voltage of the programmable power supply, and said output current of said power supply is decreased in response to a decrement of said current reference signal, and
wherein said output voltage of said programmable power supply is programmable.

2. The control circuit as claimed in claim 1 further comprising a time-delay circuit coupled to said output current regulation circuit for changing said current reference signal.

3. The control circuit as claimed in claim 1, wherein a level of said current reference signal is decreased in response to a decrement of said level of said output voltage of said programmable power supply.

4. The control circuit as claimed in claim 1, wherein a level of said current reference signal is decreased once said output voltage is lower than a protection threshold.

5. The control circuit as claimed in claim 4, wherein a level of said protection threshold is decreased in response to a decrement of said voltage reference signal.

6. A method of regulating a programmable power supply, comprising:
generating a current-sense signal in response to an output current of said programmable power supply;
generating a voltage-sense signal in response to an output voltage of said programmable power supply;
regulating said output voltage in response to a voltage reference signal and said voltage-sense signal; and
regulating said output current in response to a current reference signal and said current-sense signal,
wherein said current reference signal is changed in response to said output voltage of said programmable power supply, and said output current is decreased in response to a decrement of said current reference signal, and
wherein said output voltage of said programmable power supply is programmable.

7. The method as claimed in claim 6 further comprising generating a delay time to change said current reference signal.

8. The method as claimed in claim 6, wherein a level of said current reference signal is decreased in response to a decrement of said output voltage of said programmable power supply.

9. The method as claimed in claim 6, wherein a level of said current reference signal is decreased once said output voltage is lower than a protection threshold.

10. The method as claimed in claim 9, wherein a level of said protection threshold is decreased in response to a decrement of said voltage reference signal.

11. A method of under-voltage protection for a programmable power supply, comprises:
regulating an output voltage of said power supply at a regulated level;
regulating an output current of said programmable power supply approximately at a first current level whenever said output voltage is below said regulated level and higher than a lower limit of a protection threshold; and
regulating said output current of said programmable power supply approximately at a second current level whenever said output voltage is below an upper limit of said protection threshold,
wherein said output voltage of said programmable power supply is programmable.

12. The method as claimed in claim 11, said first current level is higher than said second current level.

13. The method as claimed in claim 11, said upper limit of said protection threshold is higher than said lower limit of said protection threshold.