Patent Publication Number: US-7710095-B2

Title: Power converter having PWM controller for maximum output power compensation

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/939,552 filed May 22, 2007, and the subject matter thereof is hereby incorporated herein by reference thereto. 

   FIELD OF THE INVENTION 
   The present invention relates to power converters, and more particularly, to a control circuit having a power limiter used for compensating a maximum output power of a switching power converter. 
   BACKGROUND OF THE INVENTION 
   Power converters are generally used to power many of electronic devices. The pulse-width modulation (PWM) technique is a conventional technology used in a power converter to control and regulate the output power. Various protection functions are built-in in the power converter to protect the power converter from permanent damage. The function of compensating the maximum output power is commonly used for overload and short-circuit protections. 
     FIG. 1  shows a traditional power converter. The power converter includes a power transformer T 1  having a primary winding N P  and a secondary winding N S . The power transformer T 1  is to provide galvanic isolation between AC line input and an output of the power converter for safety. The primary winding N P  is supplied with an input voltage V IN  Of the power converter. In order to regulate an output voltage V O  of the power converter, a control circuit coupled in series with the primary winding N P  of the power transformer T 1  generates a PWM signal V PWM  in response to a feedback signal V FB . The control circuit comprises an oscillator  10 , a first comparator  31 , a second comparator  32 , a logic circuit  33 , and a flip-flop  20 . The PWM signal V PWM  controls a power switch Q 1  to switch the power transformer T 1 . A sense resistor R S  is connected in series with the power switch Q 1  to determine the maximum output power of the power converter. The sense resistor R S  turns the switching current of the transformer T 1  to a current signal V CS . The current signal V CS  is coupled to the control circuit. If the current signal V CS  is greater than a maximum threshold V M  through the first comparator  31 , the control circuit is coupled to disable the PWM signal V PWM , and it also restricts the maximum output power of the power converter. 
     FIG. 2  shows the signal waveforms of the PWM signal V PWM  and the current signal V CS  of the power converter in  FIG. 1 . As the PWM signal V PWM  becomes logic-high, a primary-side switching current I P  will be generated accordingly. A peak value I P1  of the primary-side switching current I P  can be given by, 
                   I     P   ⁢           ⁢   1       =         V   IN       L   P       ×     T   ON               (   1   )               
The maximum output power P O  can be expressed by,
 
   
     
       
         
           
             
               
                 
                   P 
                   O 
                 
                 = 
                 
                   
                     
                       
                         L 
                         P 
                       
                       
                         2 
                         × 
                         
                           T 
                           S 
                         
                       
                     
                     × 
                     
                       I 
                       
                         P 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       2 
                     
                   
                   = 
                   
                     
                       
                         V 
                         IN 
                         2 
                       
                       × 
                       
                         T 
                         ON 
                         2 
                       
                     
                     
                       2 
                       × 
                       
                         L 
                         P 
                       
                       × 
                       
                         T 
                         S 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   In Equations (1) and (2), L P  is the inductance of the primary winding N P  of the transformer T 1 , and T ON  is an on-time of the PWM signal V PWM  while the power switch Q 1  is switched on, and T S  is the switching period of the PWM signal V PWM . 
   From Equation (2), we find that the output power varies as the input voltage V IN  varies. The input voltage ranges between 90V AC  and 264V AC  when the safety regulations are taken into consideration, and wherein the power limit in high line voltage is many times higher than the power limit in low line voltage. There is a delay time T D  from the moment the voltage in current signal V CS  is higher than the maximum threshold V M  to the moment the PWM signal V PWM  is actually turned off. The maximum output power is also affected by the delay time T D  of the control circuit. In the period of the delay time T D , the power switch Q 1  is still turned on, and it keeps on-state for delivering the output power. Therefore, the actual on-time of the PWM signal V PWM  is equal to T ON +T D , and the actual maximum output power P O  becomes as follows: 
   
     
       
         
           
             
               
                 
                   P 
                   O 
                 
                 = 
                 
                   
                     
                       V 
                       IN 
                       2 
                     
                     × 
                     
                       
                         ( 
                         
                           
                             T 
                             ON 
                           
                           + 
                           
                             T 
                             D 
                           
                         
                         ) 
                       
                       2 
                     
                   
                   
                     2 
                     × 
                     
                       L 
                       P 
                     
                     × 
                     
                       T 
                       S 
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Although the delay time T D  is short, generally within the range of 200 nsec˜350 nsec, the higher the operating frequency and smaller the switching period T S , the more influential impact is caused by the delay time T D . Therefore, the input voltage V IN  should be compensated properly, such that the input voltage V IN  does not affect the maximum output power. 
   SUMMARY OF THE INVENTION 
   An objective of the present invention is to provide a control circuit for compensating the maximum output power of a power converter. A power limiter of the control circuit can compensate the difference caused by the input voltage and the delay time, and an identical output power limit for the low line and high line voltage input can be achieved. 
   Another objective of the present invention is to develop a delay counter to flatten a limit signal of the power limiter before an output voltage is generated. By properly selecting a delay period and disabling the limit signal, an unable start-up problem for the low-line voltage and heavy-load condition can be solved. 
   Another objective of the present invention is to develop a feedback detector to sense a feedback signal of the power converter. The feedback detector generates a detecting signal to determine the limit signal in response to the feedback signal. 
   In order to achieve the above and other objections, a PWM controller is provided according to the present invention. The PWM controller compensates a maximum output power of a power converter having a power switch. The PWM controller includes an oscillator for generating a saw signal and a pulse signal, a power limiter coupled to the oscillator for generating a saw-limited signal in response to the saw signal, and a PWM unit coupled to the power limiter and the oscillator to generate a PWM signal for controlling the power switch in response to the saw-limited signal and the pulse signal. The saw-limited signal has a level being flattened during a period of time before an output voltage is generated, and is then transformed to a saw-limited waveform after the period of time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a traditional power converter; 
       FIG. 2  shows the signal waveforms of the PWM signal and the current signal of the traditional power converter shown in  FIG. 1 ; 
       FIG. 3  shows a power converter having a control circuit in accordance with the present invention; 
       FIG. 4  illustrates one embodiment of the power limit of the control circuit in accordance with the present invention; 
       FIG. 5  shows the waveforms of the control circuit in accordance with the present invention; 
       FIG. 6  illustrates another embodiment of the power limit of the control circuit in accordance with the present invention; and 
       FIG. 7  illustrates another embodiment of the power limit of the control circuit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparently understood by those in the art after reading the disclosure of this specification. The present invention can also be performed or applied by other different embodiments. The details of the specification may be on the basis of different points and applications, and numerous modifications and variations can be devised without departing from the spirit of the present invention. 
   Referring to  FIG. 3 , which is a circuit diagram of a power converter according to an embodiment of the present invention. The power converter includes a power transformer T 1  having a primary winding N P  and a secondary winding N S . The power transformer T 1  transfers the stored energy from the primary winding N P  to the secondary winding N S . The primary winding N P  is supplied with an input voltage V IN  of the power converter. In order to regulate an output voltage V O  of the power converter, a PWM controller  41  is coupled in series with the primary winding N P  of the power transformer T 1  to generate a PWM signal V PWM  in response to a feedback signal V FB . The PWM signal V PWM  controls a power switch Q 1  to switch the power transformer T 1 . A sense resistor R S  is connected in series with the power switch Q 1  to determine the maximum output power of the power converter. The sense resistor R S  transforms the switching current of the power transformer T 1  to a current signal V CS . The current signal V CS  is coupled to the PWM controller  41 . 
   In one embodiment, the PWM controller  41  includes an oscillator  10 , a power-limiter  60  and a PWM unit  42 . The oscillator  10  generates a saw signal V SAW  and a pulse signal PLS. The power limiter  60  is coupled to the oscillator  10  for generating a saw-limited signal V LMT  in response the saw signal V SAW . The PWM unit  42  is coupled to the power limiter  60  and the oscillator  10  to generate the PWM signal V PWM  for controlling the power switch Q 1  in response to the saw signal V SAW  and the pulse signal PLS. The PWM unit  42  comprises a flip-flop  20 , comparators  31 ,  32  and an AND gate  33 . 
   As shown in  FIG. 4 , in the first embodiment, the power-limiter  60  includes a delay unit  64  and a saw-limited circuit. The delay unit  64  comprises a delay counter  641  and an inverter  642 . The delay counter  641  is coupled to a voltage source V CC  for generating a delay signal S 1  after a delay time in response to the enabling of the voltage source V CC . The inverter  642  is coupled to an output of the delay counter  641  for inverting the delay signal S 1  to become a control signal V D . 
   Referring to  FIG. 4  again, the saw-limited circuit further comprises a first reference circuit, a second reference circuit, and a current-limited circuit. A voltage-to-current circuit (V-I)  621  and a current mirror composed of transistors  622 ,  623  develop the first reference circuit. The voltage-to-current circuit (V-I)  621 , a current mirror composed of transistors  622 ,  624  and a switch S W2  develop the second reference circuit. The voltage-to-current circuit (V-I)  621  of the first reference circuit is coupled to receive a reference voltage V REF  for converting to a corresponding reference current I REF , and then mirrors to an output of the transistor  623  for generating a first reference signal I 1 . The reference current I REF  mirrors to an output of the transistor  624  for generating a second reference signal I 2  when the switch S W2  is turned on. The switch S W2  is controlled by the control signal V D  of the delay unit. 
   A voltage-to-current circuit (V-I)  611 , a switch S W1 , two current mirrors composed of transistors  612 ,  613 , and  631 ,  632 , respectively, develop the current-limited circuit. The voltage-to-current circuit (V-I)  611  is coupled to receive and convert the saw signal V SAW  of the oscillator  10  (as shown in  FIG. 3 ) to a corresponding current signal I SAW . The switch S W1 , coupled between an output of the voltage-to-current circuit (V-I)  611  and a ground, is controlled by the control signal V D  of the delay unit. Therefore, the current signal I SAW  is passed to the ground when the switch S W1  is turned on. The current signal I SAW  mirrors to an output of the transistor  613  for generating a third reference signal I 3  when the switch S W1  is turned off. A source terminal of the transistors  612  and  613  is further coupled to a current source I T , so that a peak value of the saw signal V SAW  will be limited by the current source I T . Another current mirror comprises transistors  631  and  632 , wherein a gate terminal of the transistor  631  is coupled to a drain terminal of the transistor  631 . A drain terminal of the transistor  623  generates the first reference signal I 1  in accordance with the reference current I REF . A drain terminal of the transistor  624  through the switch S W2  generates the second reference signal I 2  in accordance with the reference current I REF . A current-limited signal I LMT  is generated by a drain terminal of the transistor  632  in response to the first reference signal I 1 , the second reference signal I 2 , and the third reference signal I 3 . A saw-limited signal V LMT  is therefore generated in accordance with the current-limited signal I LMT  flowing through a resistor R LMT , i.e., V LMT =I LMT *R LMT . 
   Referring to  FIG. 4  and  FIG. 5 , the waveform of the saw-limited circuit is shown in  FIG. 5 . When the voltage source V CC  is logic-high before an output voltage is generated, the delay signal S 1  is still logic-low, and the control signal V D  is logic-high through the inverter  642 . Therefore, the switches S W1  and S W2  are turned on, and the current signal I SAW  is bypassed to the ground through the switch S W1  and the current-limited signal I LMT =I 1 +I 2 . During this moment, the waveform of the saw-limited signal V LMT  is a flatten level. The saw-limited signal V LMT  is therefore generated in accordance with the current-limited signal I LMT  flowing through the resistor R LMT , wherein V LMT =I LMT *R LMT . After a delayed time t delay , the delay signal S 1  turns from logic-low to logic-high, and the control signal V D  is low through the inverter  642 . The switches S W1  and S W2  are turned off, and the current-limited signal I LMT =I 1 +I 3  and therefore the waveform of the saw-limited signal V LMT  is a saw-limited waveform. From the above description, the waveform of the saw-limited signal V LMT  is generated in response to the states of the control signal V D . The saw-limited signal V LMT  is at the flatten level when the control signal V D  is enabled (ex: logic-high), and the waveform is the saw-limited waveform when the control signal V D  is disabled (ex: logic-low). 
     FIG. 6  shows a second embodiment of a power limiter  60   a  of the invention, the power-limiter  60   a  includes a delay unit  65  and a saw-limited circuit. The saw-limited circuit is the same as the schematic of the first embodiment so the description is omitted. The delay unit  65  in the second embodiment comprises a delay counter  651 , an inverter  652 , a comparison circuit  654  and a logic unit  653 . The delay counter  651  is coupled to a voltage source V CC  for generating a delay signal S 1  after a delay time T delay  in response to the enabling of the voltage source V CC . The inverter  652  is coupled to an output of the delayed counter  651  for inverting the delay signal S 1  to an inverted delay signal  S   1 . The comparison circuit  654  is coupled to a feedback voltage V FB  and a reference voltage V A  for outputting a comparison signal S C . The logic unit  653  (ex: an AND gate) is coupled to an output of the comparison circuit  654  and an output of the inverter  652  for generating a control signal V D  in response to the inverted delay signal  S   1  and the comparison signal S C . 
   When the voltage source V CC  is logic-high before an output voltage is generated, the output of the delay counter  651  is still logic-low, and the output of the inverter  652  is logic-high. Since the output capacitor C O  (as shown in  FIG. 3 ) is short-circuit to the ground when AC line input is start-up before an output voltage is generated, the feedback voltage V FB  will be pulled high by coupling to a voltage source V CC  through a resistor R P . The comparison signal S C  at the output of the comparison circuit  654  is logic-high when the feedback signal V FB  is higher than the reference voltage V A . The control signal V D  at the output of the logic unit  653  is also logic high because an output of the inverter  652  and the comparison signal S C  are both logic-high. Thus, the switches S W1  and S W2  are turned on, a current signal I SAW  is flowed to the ground through the switch S W1 , and the current-limited signal I LMT =I 1 +I 2 . During this moment, the waveform of the saw-limited signal V LMT  is a flatten level. A saw-limited signal V LMT  is therefore generated in accordance with the current-limited signal I LMT  flowed through a resistor R LMT , wherein V LMT =I LMT *R LMT . 
   The control signal V D  will be turned to logic-low when the inverted delay signal  S   1  or the comparison signal S C  is logic-low. After a period of the delay time T delay  or the output voltage at the secondary winding N S  of the power transformer T 1  generated, the feedback voltage V FB  will be stabilized and decreased. The control signal V D  is logic-low when the feedback signal V FB  is smaller than the reference voltage V A . The switches S W1  and S W2  are turned off, and the current-limited signal I LMT =I 1 +I 3  and therefore the waveform of the saw-limited signal V LMT  is a saw-limited waveform. 
     FIG. 7  shows a third embodiment of a power limiter  60   b  of the present invention, the power-limiter  60   b  includes a delay unit  66  and a saw-limited circuit. The saw-limit circuit is the same as the schematic of the first embodiment. The delay unit  66  of the third embodiment comprises a negative delta V counter  661 , an inverter  662 , and a pull-high resistor R P . The negative delta V counter  661  is coupled to a feedback voltage V FB  and a voltage source V CC  through the pull-high resistor R P . Since the output capacitor C O  (as shown in  FIG. 3 ) is short-circuit to the ground when AC line input is start-up before an output voltage is generated, the feedback voltage V FB  will be pulled high by coupling to a voltage source V CC  through the resistor R P . At the mean time, the negative delta V counter  661  does not detect a negative delta V voltage, so the output of the negative delta V counter  661  is logic-low, and the output of the inverter  662  is logic-high. Therefore, the switches S W1  and S W2  are turned on, current signal I SAW  is flowed to the ground through the switch S W1  and the current I LMT =I 1 +I 2 . During this moment, the waveform of the saw-limited signal V LMT  is a flatten level. A saw-limited signal V LMT  is therefore generated in accordance with the current-limited signal I LMT  flowed through a resistor R LMT , wherein V LMT =I LMT *R LMT . Since the output capacitor C O  (as shown in  FIG. 3 ) is short-circuit to the ground when AC line input is start-up before an output voltage is generated, the feedback voltage V FB  will be pulled high by coupling to a voltage source V CC  through a resistor R P . 
   After the output voltage at the secondary winding N S  is established, the feedback voltage V FB  will be stabilized and decreased. The negative delta V voltage is detected by the negative delta V counter  661 , so the output terminal of the negative delta V counter  661  is logic-high (enable), and the control signal V D  is logic-low through the inverter  662 . When the control signal V D  is logic-low, the switches S W1  and S W2  are turned off, and the current-limited signal I LMT =I 1 +I 3  and therefore the waveform of the saw-limited signal V LMT  is a saw-limited waveform. 
   In contrast to the prior art, the power limiters  60 ,  60   a  and  60   b  of the present invention further employ the delay units  64 ,  65 , and  66 , respectively, to control the waveform of the saw-limited signal. By properly selecting a delay period and disabling the saw-limited signal, an unable start-up problem for the low-line voltage and heavy-load condition can be solved. 
   The foregoing descriptions of the detailed embodiments are only illustrated to disclose the features and functions of the present invention and not restrictive of the scope of the present invention. It should be understood to those in the art that all modifications and variations according to the spirit and principle in the disclosure of the present invention should fall within the scope of the appended claims.