Abstract:
A charging mode control circuit ( 320 ) includes a detecting circuit ( 321 ) and a charging control circuit ( 322 ). The detecting circuit detects whether a power/data interface ( 310 ) is in a communication state, and accordingly outputting a control signal. The charging control circuit receives the control signal and obtaining power from the power/data interface to charge a secondary battery ( 330 ) with a charging mode in accordance with the control signal. A portable electronic device ( 30 ) incorporates the charging mode control circuit is also provided.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to switching mode power supplies, and particularly to a switching mode power supply in which a forward circuit and a flyback circuit share one transformer.  
       RELATED ART  
       [0002]     Generally, in most traditional switching mode power supplies, only forward circuits or flyback circuits are adopted. If in situations both a forward circuit and a flyback circuit are adopted, the flyback circuit is usually configured separately from the forward circuit. That is, the forward circuit and the flyback circuit are respectively constituted by different transformers. Hence, when the forward circuit or the flyback circuit transfers electrical power to loads, an amount of electrical power is converted into magnetic power and stored in a core of a transformer constituting the forward circuit or the flyback circuit. The magnetic power is consumed in a form of heat, reducing a transfer rate of electrical power to the loads.  
         [0003]     Therefore, there is a need for providing a switching mode power supply which can solve the above-mentioned problem.  
       SUMMARY  
       [0004]     A switching mode power supply, includes: a direct current (DC) power source; a transformer connected to the DC power source and including a primary winding, a first secondary winding and a second secondary winding; an output terminal connected to the transformer; a pulse-width modulation (PWM) circuit for producing PWM wave; and a switching circuit under control of the PWM circuit by applying PWM wave thereto and connected with the primary winding of the transformer. The switching mode power supply further includes: a first rectification circuit connected to the first secondary winding and constitutes a forward circuit together with the primary winding and the first secondary winding; and a second rectification circuit connected to the second secondary winding and constitutes a flyback circuit together with the primary winding and the second secondary winding.  
         [0005]     Other advantages and novel features will be drawn from the following detailed description with reference to the attached drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  depicts an exemplary circuit diagram of a switching mode power supply in accordance with a first preferred embodiment of the present invention; and  
         [0007]      FIG. 2  depicts an exemplary circuit diagram of the switching mode power supply in accordance with a second preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0008]     Referring to  FIG. 1 , an exemplary circuit diagram of a switching mode power supply in accordance with a first preferred embodiment is shown. In this preferred embodiment, the switching mode power supply mainly includes a direct current power source  10  (simplified as “a DC source  10 ”), a transformer  20 , a pulse-width modulation (PWM) circuit  30 , a switching circuit  40 , and an output terminal  90 . The transformer  20  is interposed between the DC source  10  and the switching circuit  40 . The switching circuit  40  is controlled by the PWM circuit  30 . The PWM circuit  30  receives feedback from a feedback circuit  110  and applies PWM waves to the switching circuit  40  accordingly. The switching circuit is therefore switched on and off in accordance with the PWM waves and controls the flow of power through the transformer  20 . The feedback circuit  110  is connected with the output terminal  90  and feeds back output information to a load  100  through the output terminal  90 .  
         [0009]     The transformer includes at least one primary winding L 1  and two secondary windings L 2  and L 3 . A dotted terminal of the primary winding L 1  is connected with the DC power source  10  and an undotted terminal thereof is connected with the switching circuit  40 . A dotted terminal of the secondary winding L 2  is connected with a first rectification circuit  50  and an undotted terminal thereof is grounded together with a dotted terminal of the secondary winding L 3 . An undotted terminal of the secondary winding L 3  is connected with a second rectification circuit  80 .  
         [0010]     The primary winding L 1 , the secondary winding L 2 , and the first rectification circuit  50  constitutes a forward circuit, and the primary winding L 1 , the secondary winding L 3 , and the second rectification circuit  80  constitutes a flyback circuit. In this preferred embodiment, the forward circuit is connected to the output terminal  90  via a power storing circuit  61 , and the flyback circuit is directly connected to the output terminal  90 .  
         [0011]     When the switching circuit  40  is switched on by the PWM circuit  30 , power enters the dotted terminal of the primary winding L 1  of the transformer  20 , and meanwhile the forward circuit functions. The power is transferred to the secondary winding L 1  from the primary winding L 1 . The power is further transferred by the secondary winding L 1  to the power storing circuit  61  via the first rectification circuit  50  and stored in the power storing circuit  61 . When the switching circuit  40  is switched off by the PWM circuit  30 , the flyback circuit functions. The flyback circuit converts magnetic power stored in the core of the transformer  20  into electrical power and transfers the electrical power to the output terminal  90 . At the same time, the power storing circuit  61  also deliveries power to the output terminal  90 .  
         [0012]     Referring to  FIG. 2 , a switching mode power supply in accordance with a second embodiment is shown. In  FIG. 2 , similar components to those of  FIG. 1  have the same or a corresponding reference numeral, and explanation thereof will be omitted. In  FIG. 2 , the first rectification circuit  50  and the second rectification circuit  80  both are connected to a second power storing circuit  62 . Therefore, the power transferred by the forward circuit and the flyback circuit is stored in the second power storing circuit  62  and delivered to the output terminal  90  by the second power storing circuit  62 .  
         [0013]     In both  FIGS. 1 and 2 , the switching circuit  40  is shown as presented by a FET (field-effect transistor) transistor, and the first rectification circuit  50  and the second rectification circuit  80  are shown as presented by a Schottky diode. The power storing circuit  61  in  FIG. 1  is shown constituted by a diode and an inductor connected in series. The inductor is interposed between the first rectification circuit  50  and the output terminal  90 . An anode of the diode is grounded and a cathode thereof is connected between the first rectification circuit  50  and the inductor. The diode, the inductor, and the load  100  form a current loop. The current loop enables the power stored in the power storing circuit  61  to be delivered to the load  100  after the forward circuit finishes the transferring of power to the power storing circuit  61 . The second power storing circuit  62  in  FIG. 2  is shown as presented by an inductor.  
         [0014]     The switching mode power supply further includes a filter circuit interposed between the output terminal  90  and the ground, and used for reducing current ripples output to the load  100  and fed back to the PWM circuit  30 .  
         [0015]     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.