Abstract:
A flyback switching power supply capable of regulating an operation frequency based on a current regulation mechanism is disclosed. The flyback switching power supply includes a transformer, a switch, a switch control circuit, and a regulation circuit. The transformer includes a primary winding for receiving an input voltage, a secondary winding for generating an output voltage, and an auxiliary winding. The switch is serially connected to the primary winding for controlling a current flowing through the primary winding. The switch control circuit has a frequency control port and functions to work around an operation frequency for controlling the switch. The operation frequency is under control by a frequency setting current flowing through the frequency control port. The regulation circuit is electrically coupled between the auxiliary winding and the frequency control port. The regulation circuit adjusts the frequency setting current based on an induced current generated by the auxiliary winding.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flyback switching power supply and control method thereof, and more particularly, to a flyback switching power supply and related control method capable of regulating an operation frequency based on a current regulation mechanism. 
     2. Description of the Prior Art 
     Along with well-known advantages of high efficiency, low power consumption, small size and light weight, the flyback switching power supply has been widely employed as a power converter in various electronic products. Please refer to  FIG. 1 , which is a schematic diagram showing a prior-art flyback switching power supply  100 . Rectifier  102  and filter capacitor  105  are put in use for performing rectification and filter operations on an alternating input voltage Vac, provided by alternating power supply  101 , so as to generate an input voltage Vin furnished to transformer  120 . Transformer  120  comprises a primary winding  121  for receiving input voltage Vin, a secondary winding  122  for generating a preliminary output voltage, and an auxiliary winding  123 . Rectify/filter circuit  170  is utilized for performing rectification and filter operations on the preliminary output voltage for generating an output voltage forwarded to load  195  and feedback circuit  140 . Feedback circuit  140  functions to convert the output signal into a feedback signal furnished backwards to switch control circuit  130 . 
     Power generation circuit  190  is used to generate a power voltage Vcc for powering switch control circuit  130  based on an induced current generated by auxiliary winding  123 . In general, the operation frequency of control signal Sc is mainly adjusted by switch control circuit  130  based on a frequency setting current If flowing through the current setting resistor Rx externally connected. However, the operation frequency is normally preset to be a fixed value in that the current setting resistor Rx is mostly set to be a fixed resistor as shown in  FIG. 1 . 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a flyback switching power supply is provided. The flyback switching power supply comprises a transformer, a switch, a switch control circuit, and a regulation circuit. The transformer comprises a primary winding for receiving an input voltage, a secondary winding for generating an output voltage, and an auxiliary winding. The switch is electrically coupled to the primary winding in series and functions to control a current flowing through the primary winding. The switch control circuit comprises a frequency control port. The switch control circuit is working around an operation frequency for controlling the switch. The operation frequency is controlled by a frequency setting current flowing through the frequency control port. The regulation circuit is electrically coupled between the auxiliary winding and the frequency control port and functions to adjust the frequency setting current based on an induced current generated by the auxiliary winding. 
     The present invention further provides a control method adaptive for use in a flyback switching power supply. The flyback switching power supply comprises a transformer and a switch control circuit. The transformer comprises a primary winding for receiving an input voltage, a secondary winding for generating an output voltage, and an auxiliary winding for generating an induced voltage. The switch control circuit comprises a frequency control port. The switch control circuit is working around an operation frequency for controlling a current flowing through the primary winding. The operation frequency is controlled by a frequency setting current flowing through the frequency control port. The control method comprises firstly adjusting the frequency setting current according to an induced current generated by the auxiliary winding and then regulating the operation frequency based on the frequency setting current adjusted. 
     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 flyback switching power supply. 
         FIG. 2  is a flyback switching power supply in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic diagram showing the related signal waveforms regarding the continuous conduction operation of the flyback switching power supply in  FIG. 2 , having time along the abscissa. 
         FIG. 4  is a schematic diagram showing the related signal waveforms regarding the discontinuous conduction operation of the flyback switching power supply in  FIG. 2 , having time along the abscissa. 
         FIG. 5  is a schematic diagram showing the related signal waveforms regarding the discontinuous conduction operation of the flyback switching power supply in  FIG. 1 , having time along the abscissa. 
     
    
    
     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. 2  is a flyback switching power supply in accordance with an embodiment of the present invention. As shown in  FIG. 2 , flyback switching power supply  200  comprises a rectifier  202 , a filter capacitor  205 , a transformer  220 , a switch  225 , a rectify/filter circuit  270 , a feedback circuit  240 , a regulation circuit  250 , a power generation circuit  290 , and a switch control circuit  230 . Transformer  220  comprises a primary winding  221 , a secondary winding  222  and an auxiliary winding  223 . Compared with the prior-art flyback switching power supply  100 , flyback switching power supply  200  further comprises regulation unit  250  electrically coupled between auxiliary winding  223  and switch control circuit  230 . As is well known to those skilled in the art, the functionalities and/or structures of the rectifier  202 , the filter capacitor  205 , the transformer  220 , the switch  225 , the rectify/filter circuit  270 , the feedback circuit  240 , the power generation circuit  290  and the switch control circuit  230  in  FIG. 2  can be similar, equivalent, or identical to those of the rectifier  102 , the filter capacitor  105 , the transformer  120 , the switch  125 , the rectify/filter circuit  170 , the feedback circuit  140 , the power generation circuit  190  and the switch control circuit  130  in  FIG. 1 . 
     In the embodiment shown in  FIG. 2 , feedback circuit  240  comprises an optocoupler  245 . Accordingly, the feedback signal Sfb is sent backwards to switch control circuit  230  by means of an optical couple interface for providing an electrical isolation between the input side and the output side of flyback switching power supply  200 . 
     Switch control circuit  230  in  FIG. 2  comprises a frequency control port  231 . The operation frequency of control signal Sc is under control by a frequency setting current If flowing through frequency control port  231 . Current setting resistor Rx is electrically coupled between frequency control port  231  and a ground for providing a default current Ix almost fixed. 
     Regulation circuit  250  is electrically coupled between auxiliary winding  223  and frequency control port  231  and functions to provide an adjustment current Iad based on an induced current Isa generated by auxiliary winding  223 . As shown in  FIG. 2 , frequency setting current If is actually a sum current of default current Ix and adjustment current Iad. In other words, adjustment current Iad is employed to adjust frequency setting current If; in turn, frequency setting current If is used to regulate the operation frequency of control signal Sc. Regulation circuit  250  comprises a diode  251 , an adjustment resistor Rad 1 , a zener diode  253 , a capacitor  254 , and an adjustment resistor Rad 2 . As shown in  FIG. 2 , diode  251 , adjustment resistor Rad 1  and zener diode  253  are electrically coupled to form a serial circuit, i.e. the positions of components disposed in the serial circuit are interchangeable without affecting related circuit operations. Low-pass filter, such as capacitor  254 , is electrically coupled between the serial circuit and the ground for performing a low-pass filter operation. Adjustment resistor Rad 2  is electrically coupled between frequency control port  231  and capacitor  254  for controlling the magnitude of adjustment current Iad. Consequently, the resistance ratio of adjustment resistor Rad 2  to current setting resistor Rx can be assigned so that regulation circuit  250  is capable of regulating the operation frequency in a desirable range. 
     When switch  225  is turned on, due to the rectification operation of diode  251 , frequency setting current If can be adjusted only in a process during which induced voltage Vsa is a negative voltage. The zener breakdown voltage of zener diode  253  is employed to set a negative threshold voltage Vth. As induced voltage Vsa generated by auxiliary winding  223  is less than the negative threshold voltage Vth, diode  251  is forward-biased and zener diode  253  is operating in the reverse breakdown region, and adjustment current Iad having a desired magnitude can be generated according to the filter and current regulation operations of capacitor  254 , adjustment resistor Rad 1  and adjustment resistor Rad 2 . In turn, frequency setting current If is adjusted by adjustment current Iad for regulating the operation frequency of control signal Sc. Besides, an occurrence of iron core saturation regarding transformer  220 , caused by an unacceptable low operation frequency, can be avoided in that the operation frequency of control signal Sc is set to be higher than a lowest acceptable frequency based on the impedance of current setting resistor Rx. On the other hand, the highest operation frequency of control signal Sc might be determined by the parallel impedance of adjustment resistor Rad 2  and current setting resistor Rx. 
       FIG. 3  is a schematic diagram showing the related signal waveforms regarding the continuous conduction operation of flyback switching power supply  200  in  FIG. 2 , having time along the abscissa. The signal waveforms in  FIG. 3 , from top to bottom, are control signal Sc, the secondary current Is of secondary winding  222 , the induced voltage Vsa of auxiliary winding  223 , the switch voltage drop V DS  of switch  225 , and the switch current I DS  of switch  225 . Please refer to  FIG. 3  together with  FIG. 2 , when switch  225  is turned off by control signal Sc having a low-level voltage, switch current I DS  is substantially equal to zero; meanwhile, induced voltage Vsa is retained to be around a first positive voltage, switch voltage drop V DS  is retained to be around a second positive voltage, and secondary current Is decreases gradually over time. When induced voltage Vsa is around the first positive voltage, capacitor  292  is charged for powering switch control circuit  230 . Alternatively, when switch  225  is turned on by control signal Sc having a high-level voltage, the second positive voltage of switch voltage drop V DS  will first cause a surge current of switch current I DS ; thereafter, switch voltage drop V DS  shifts down to almost zero, and switch current I DS  drops off to a first current and then increases gradually from the first current to a second current following an increase of primary current Ip flowing through primary winding  221 ; meanwhile, induced voltage Vsa is retained to be around a negative voltage, and secondary current Is is almost zero due to the reverse-biased operation of diode  271 . As shown in  FIG. 3 , in the continuous conduction operation of flyback switching power supply  200 , the switching power consumption caused by the surge current is not significant compared with the power consumption caused in a process during which switch current I DS  flowing through switch  225  increases from the first current to the second current. 
       FIG. 4  is a schematic diagram showing the related signal waveforms regarding the discontinuous conduction operation of the flyback switching power supply  200  in  FIG. 2 , having time along the abscissa. Please refer to  FIG. 4  together with  FIG. 2 , after switch  225  is turned off by control signal Sc having a low-level voltage, switch current I DS  is almost zero; meanwhile, induced voltage Vsa is firstly retained to be around a voltage Vsa 1 , switch voltage drop V DS  is firstly retained to be around a voltage V DS   1 , and secondary current Is decreases gradually from a high current to zero. After secondary current Is decreases to zero, secondary current Is holds zero current during an interval ΔT 1 . During interval ΔT 1 , induced voltage Vsa and switch voltage drop V DS  are oscillating due to an occurrence of resonance. In a first oscillating period, both induced voltage Vsa and switch voltage drop V DS  will first shift down with a sharp slope. When induced voltage Vsa shifts down to a voltage lower than the negative threshold voltage Vth, diode  251  is forward-biased and zener diode  253  is operating in the reverse breakdown region so that regulation circuit  250  is able to increase adjustment current Iad to some extent; in turn, frequency setting current If is raised to some extent following an increase of adjustment current Iad. As induced voltage Vsa is shifting lower, the operation frequency is adjusted to be higher, and the timing of switching control signal Sc from the low-level voltage to the high-level voltage is advanced more. In general, the device parameters such as the resistances of the resistors in regulation circuit  250  can be properly devised so that the timing of turning on switch  225  is advanced to a moment at which switch voltage drop V DS  is oscillating to around a wave trough of the first oscillating period as shown in  FIG. 4 . That is, the switching of switch  225  from off-state to on-state occurs while switch voltage drop V DS  is oscillating to about the lowest voltage. Accordingly, the power consumption regarding the switching operation of switch  225  can be reduced significantly in that the surge current is lower as switch voltage drop V DS  is lower at a moment prior to turning on switch  225 . 
       FIG. 5  is a schematic diagram showing the related signal waveforms regarding the discontinuous conduction operation of the flyback switching power supply  100  in  FIG. 1 , having time along the abscissa. The signal waveforms in  FIG. 5 , from top to bottom, are control signal Sc, the secondary current Is of secondary winding  122 , the induced voltage Vsa of auxiliary winding  123 , the switch voltage drop V DS  of switch  125 , and the switch current I DS  of switch  125 . 
     Please refer to  FIG. 5  together with  FIG. 1 , after switch  125  is turned off by control signal Sc having a low-level voltage, switch current I DS  is almost zero; meanwhile, induced voltage Vsa is firstly retained to be around a voltage Vsa 2 , switch voltage drop V DS  is firstly retained to be around a voltage V DS   2 , and secondary current Is decreases gradually from a high current to zero. After secondary current Is decreases to zero, secondary current Is holds zero current during an interval ΔT 2 . During interval ΔT 2 , induced voltage Vsa and switch voltage drop V DS  are oscillating due to an occurrence of resonance. In the operation of flyback switching power supply  100 , the operation frequency of control signal Sc provided by switch control circuit  130  is almost fixed. For that reason, as shown in  FIG. 5 , the switching of control signal Sc from the low-level voltage to the high-level voltage is likely to occur while switch voltage drop V DS  is oscillating to around a wave crest. That is, the switching of switch  125  from off-state to on-state occurs while switch voltage drop V DS  is oscillating to a high-level voltage. Since the surge current is higher as switch voltage drop V DS  is higher at a moment prior to turning on switch  125 , the power consumption regarding the switching operation of switch  125  is then higher accordingly. 
     In summary, compared with the prior-art flyback switching power supply  100 , the flyback switching power supply  200  of the present invention is able to dynamically adjust the timing of switching the switch  225  from off-state to on-state so that the timing of switching the switch  225  from off-state to on-state can be set to a moment at which the switch voltage drop V DS  is oscillating to around a wave trough, for reducing the switching power consumption. 
     In the embodiment shown in  FIG. 2 , if induced voltage Vsa is oscillating and the magnitude of input voltage Vin is high enough for incurring a high oscillating amplitude of induced voltage Vsa, regulation circuit  250  is then enabled to generate adjustment current Iad for reducing the switching power consumption as induced voltage Vsa is oscillating to be a negative voltage less than the negative threshold voltage Vth determined by the zener breakdown voltage of zener diode  253 . Alternatively, if input voltage Vin is not high enough, regulation circuit  250  may not be enabled to generate adjustment current Iad for reducing the switching power consumption in that the switching power consumption is not significant under such situation. Zener diode  253  plays a role determining whether regulation circuit  250  reacts in respect to the magnitude of input voltage Vin. In another embodiment, the zener diode  253  of flyback switching power supply  200  is omitted, and regulation circuit  250  is enabled for reducing the switching power consumption as long as the flyback switching power supply  200  is operating in the discontinuous conduction mode regardless of the magnitude of input voltage Vin. 
     Referring to the waveforms shown in  FIG. 4 , when switch  225  is turned on, induced voltage Vsa is also less than the negative threshold voltage Vth; therefore, regulation circuit  250  is enabled as well to generate adjustment current Iad for performing a frequency compensation operation by adjusting frequency setting current If. Capacitor  254  together with adjustment resistor Rad 1  functions as a low-pass filter. Once the turn-on interval of switch  225  is too short or the duty cycle of control signal Sc is too small, the amount of adjustment current Iad turns out to be smaller because of the circuit operation of the low-pass filter. On the other hand, if the output is almost at a full-load condition or the duty cycle of control signal Sc is higher, the amount of adjustment current Iad becomes larger for performing a desired significant frequency compensation operation with the aid of capacitor  254  and adjustment resistor Rad 1  having device values properly designed. In other words, capacitor  254  and adjustment resistor Rad 1  make regulation circuit  250  to react during heavy load and not to react during light load or no load. Please refer to  FIG. 2 , in another embodiment, capacitor  254  can be removed and adjustment resistor Rad 1  can be shorted; therefore the frequency compensation operation, similar or identical to the aforementioned, is performed regardless of the length of the duty cycle of control signal Sc. Also, please continue referring to  FIG. 2 , in an alternative embodiment, capacitor  254  can be removed and zener diode  253  and adjustment resistor Rad 1  can be shorted. 
     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.