Abstract:
A power supply has a normal operation mode and a standby operation mode. The power supply includes a main circuit, a first control circuit, a second control circuit, and a switching controller. The main circuit has at least one output port for converting an input voltage into an output voltage at the output port. The first control circuit is used for controlling the main circuit under the normal operation mode. The second control circuit is used for controlling the main circuit under the standby operation mode. And, the switching controller processes a control signal to control the first control circuit and the second control circuit under either of the normal operation mode and the standby operation mode in response to a load status of the output port.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a power supply and more particularly to a power supply having an efficient low power standby mode. 
   BACKGROUND OF THE INVENTION 
   Low standby loss is becoming a basic requirement for power supplies. Generally, there are two methods to achieve this requirement. One method is to reduce the operating frequency of a power converter, and the other one is to operate a power converter in a burst mode. When in the burst mode, the converter operates in several continuous switching cycles and then follows a no switching time period. With this means, the equivalent switching cycles in a certain time is low and thus the standby loss of the converter, mainly including the switching cycle related power loss, is low. 
   Please refer to  FIG. 1 .  FIG. 1  is a block diagram of a conventional power converter having a normal operation mode and a standby mode according to the prior art. The power converter circuit includes a main circuit  10 , a main control circuit  12 , and a standby controller  14   a . The main control circuit  12  includes a normal operation controller  12   a  and an error amplifier  12   b . Accepting the output voltage of the power converter as its input, the error amplifier  12   b  outputs an error-amplified voltage V E/A . 
   In the normal operation mode, the normal operation controller  12   a  provides a driving signal S normal  to the main circuit  10 . And in the standby operation mode, the standby controller  14   a  provides a driving signal S standby  to the main circuit  10 . 
   As for a PWM converter with diode rectifier at its output side, the error-amplified feedback voltage V E/A  in the standby operation mode is very different from that in the normal operation mode. Meanwhile, V E/A  changes obviously with load in the standby operation mode. So V E/A  can be utilized to identify the loading condition of a PWM converter, and can be applied to the light load conditions for selecting the standby controller and for achieving the low standby loss of a power supply. 
   But in some kinds of converters and even in case of the PWM converter with synchronous rectifier, V E/A  changes little with the load and it becomes an impractical approach to running the converter in the burst mode under light load conditions by sensing the V E/A  signal. 
   Because of the technical defects described above, the applicant keeps on carving unflaggingly to develop a general approach to achieve a power supply having efficient low power standby mode through wholehearted experience and research. 
   SUMMARY OF THE INVENTION 
   The present invention provides a general control approach to realize low standby loss for power converters. 
   In the converter of the invention, only one of the normal operation controller and the standby controller is selected to control the main circuit in one time. Wherein, a loading hysteretic comparator, by sensing the load status, selects one controller to control the converter. When the load decreases to a low threshold of the loading hysteretic comparator, the loading hysteretic comparator generates a signal and then this signal disables the normal operation controller while enables the standby controller. When the load increases to a high threshold of the loading hysteretic comparator, the loading hysteretic comparator generates a signal, and then this signal enables the normal operation controller while disables the standby controller. Because the output current directly reflects the load status, it is generally sensed as the input signal of the loading hysteretic comparator. 
   With this means, moreover, in standby operation mode, a voltage hysteretic comparator functions as delta control of the output voltage. When the output voltage decreases to a low threshold of the voltage hysteretic comparator, the main circuit of the power converter operates and the output voltage increases. When the output voltage increases to a high threshold of the voltage hysteretic comparator, the main circuit of the power converter stops switching and the output voltage decreases. Therefore, in the standby operation mode, the power converter operates in the burst mode which results in a low standby loss. 
   With the above and the additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawing, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a conventional power converter having a normal operation mode and a standby operation mode according to the prior art; 
       FIG. 2  is a block diagram of a power converter having a voltage hysteretic comparator and a loading hysteretic comparator according to the present invention; 
       FIG. 3  shows a graph of the loading hysteretic comparator based on the load of  FIG. 2 ; 
       FIG. 4  shows a graph of the voltage hysteretic comparator based on the output voltage of  FIG. 2 ; and 
       FIG. 5  shows a schematic diagram of a power supply implemented by a switching mode converter according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Please refer to  FIG. 2 .  FIG. 2  is a block diagram of a power converter having a voltage hysteretic comparator and a loading hysteretic comparator according to the present invention. The power converter according to the present invention includes a main circuit  10  of a power converter with an input port V in  and an output port V o , a main control circuit  12 , a standby control circuit  14 , a current sensor  18  and a loading hysteretic comparator  16 . The main control circuit  12  includes a series arrangement of an error amplifier  12   b  and a normal operation controller  12   a . The error amplifier  12   b  accepts the voltage Vo of the output port of the main circuit  10  and outputs a V E/A  signal which is input to the normal operation controller  12   a , whose output S normal  is used to control the main circuit  10 . The standby control circuit  14  includes a series arrangement of a voltage hysteretic comparator  14   b  and a standby controller  14   a . The voltage hysteretic comparator  14   b  accepts the voltage Vo of the output port of the main circuit  10  and outputs a V 4  signal which is input to the standby controller  14   a , whose output S standby  is used to control the main circuit  10 . The output load current Io is sensed by the current sensor  18  and a load signal is generated. This load signal is input to the loading hysteretic comparator  16 . The output of the loading hysteretic comparator  16 , V 3 , is coupled to the normal operation controller  12   a  and the standby controller  14   a.    
   The operation of the power supply of the present invention will now be described with reference to  FIG. 3  and  FIG. 4 . 
   Referring to  FIG. 3 , when the load of the output port, sensed by the current sensor  18 , decreases from a normal load to a low threshold Load-L of the loading hysteretic comparator  16 , the loading hysteretic comparator  16  generates a high level signal V 3  and then the high level V 3  disables the normal operation controller  12   a  while enables the standby controller  14   a . As a result, the main circuit is controlled by the standby controller  14   a . When the load of the output port, sensed by the current sensor  18 , increases to a high threshold Load-H of the loading hysteretic comparator  16 , the loading hysteretic comparator  16  generates a low level signal V 3  and then the signal enables the normal operation controller  12   a  and disables the standby controller  14   a . Then the main circuit is controlled by the main control circuit  12 . 
   In the normal operation mode, the load is high enough and the loading hysteretic comparator  16  will always have a low-level signal V 3  output. Thus the standby controller  14   a  is always disabled while the main control circuit  12 , comprising the error amplifier  12   b  and the normal operation controller  12   a , controls the output voltage Vo. 
   In the standby operation mode, the load is low enough and the loading hysteretic comparator  16  will always have a high level V 3  signal output. Thus the normal operation controller  12   a  is disabled while the standby control circuit  14 , including the standby controller  14   a  and the voltage hysteretic comparator  14   b , controls the output voltage Vo. 
   Moreover, in the standby operation mode, the voltage hysteretic comparator  14   b  functions as a delta control of the output voltage V o . 
   Referring to  FIG. 4 , when the output voltage V o  of the main circuit  10  decreases to a low threshold V L  of the voltage hysteretic comparator  14   b . The output of  14   b , V 4 , is a high level signal and then the standby controller  14   a  produces S standby  which has a driving signal for the main circuit, therefore, the output voltage V o  increases. When the output voltage V o  increases to a high threshold V H  of the voltage hysteretic comparator  14   b , the V 4  signal is at a low level, and then the standby controller  14   a  produces S standby  which has no driving signal for the main circuit, therefore, the output voltage V o  decreases. 
   The normal operation controller  12   a  and the standby controller  14   a  operate independently each with an independent feedback. The error amplifier  12   b  functions as the feedback loop of the normal operation controller  12   a , and the voltage hysteretic comparator  14   b  functions as the feedback loop of the standby controller  14   a.    
   The loading hysteretic comparator  16  processes a control signal V 3  by sensing the load status of the output port to switch the normal operation controller  12   a  and the standby controller  14   a  respectively to either of the normal operation mode and the standby operation mode in response to the load status of the output port. In the illustrated embodiment, the loading hysteretic comparator  16 , by sensing the load status, selects one controller from the normal operation controller  12   a  and the standby controller  14   a  to control the main circuit  10 . 
   In particular, for simple design, a single controller can be used to function as both the normal operation controller  12   a  and the standby controller  14   a.    
   It is understood that the present invention may be implemented in different power supplies, which will be described in detail hereinafter. 
   Please refer to  FIG. 5 .  FIG. 5  shows a diagram of a power supply implemented by a switching mode converter according to an embodiment of the present invention. One DC voltage Vin is applied to the power converter main circuit  310  which outputs a voltage Vo and has a load  312 . At normal operation, Vo is input to a feedback loop  313  including an error amplifier. A signal V 5 , the output of the feedback loop  313 , is then coupled to a control circuit  311  to provide a frequency or PWM signal V 1  for a driver  315 . The driver  315  drives the power converter main circuit  310 . 
   A current sensor  316  generates a signal V 2  corresponding to the load status. This signal is then coupled to a loading hysteretic comparator  211  and operated as shown in  FIG. 3 . In the standby operation mode of this embodiment, the loading hysteretic comparator  211  outputs a high level signal V 3  to the feedback loop  313  which then regulates its reference voltage Vr to a higher level compared with that of the normal operation mode. With this means, the output voltage Vo will rise higher than that of the normal operation mode. Based on this higher Vo, a voltage hysteretic comparator  210  is used to control the operation of the main circuit. 
   In the standby operation mode, because the reference voltage of the feedback loop  313  is regulated higher as aforementioned, the control circuit  311  will generate a corresponding signal V 1  and with which the driver  315  drives the power converter main circuit  310 . The output voltage Vo will rise higher. Once the Vo is higher than a setting value V H , the voltage hysteretic comparator  210 , which functions as shown in  FIG. 4 , will produce a low level V 4  signal and thus the driver  315  is latched. So there is no driving for the power converter main circuit  310  and then Vo decreases. 
   Once the Vo is decreased to a setting value V L , the voltage hysteretic comparator  210  will produce a high level V 4  signal and the driver  315  is unlatched. Thereby, the power converter main circuit  310  is driven by the driver  315  and the output voltage Vo increases. Thus the power converter main circuit  310  operates with burst mode in standby mode. 
   Herein, the main control circuit is composed of the feedback loop  313 , the control circuit  311 , and the driver  315  as shown in  FIG. 5  for the normal operation mode. While the standby control circuit is composed of the voltage hysteretic comparator  210 , the control circuit  311  and the driver  315  as shown in  FIG. 2 . The corresponding normal operation controller  12   a  and the standby controller  14   a  in  FIG. 5  are the same as those shown in  FIG. 2 , including the control circuit  311  and the driver  315 , which benefits to the simple and low cost circuit design. 
   It is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.