Abstract:
The present invention provides a ring choke converter, a kind of power supply, and method thereof. In addition to the transformer, the switch component, the positive feedback circuit and the regulation control circuit in a conventional RCC, the present invention provides a control circuit partially powered by terminal of the opposite polarity at the feedback winding of the transformer. With the circuit design of the control circuits disclosed in this invention, under-voltage protection (UVP), over-voltage protection, and overload protection, together with stable output voltage are all achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a switched power supply, and more particularly relates to a switched power supply using ringing choke converter (RCC) system.  
           [0003]    2. Description of the Related Art  
           [0004]    Household appliances, such as video cassette recorders (VCR) or fax machines, require a DC power supply that provides stable voltage drawing excessive amounts of power. Due to efficiency and relatively simple structure, switched power supplies using RCC system are widely utilized as for household appliances, transforming household AC power to the DC power required by the appliance circuits.  
           [0005]    An RCC system, or RCC structure, has been disclosed in many patents such as in U.S. Pat. No. 6,081,433, U.S. Pat. No. 6,101,103, and U.S. Pat. No. 6,072,702.  
           [0006]    [0006]FIG. 1 is a simple perspective view of a conventional RCC. AC represents external AC power. Through a diode bridge DB and a filtering capacitor C 1 , AC is transformed into high voltage DC power at node  1  to act as the main power for the RCC structure.  
           [0007]    RCC structure comprises a transformer T, a switch component FET Q 1 , a starting resistor RS, a positive feedback circuit  40 , a control circuit  46 , and an output rectifier  42 . The transformer T has at least three windings; a primary winding N 1 , a secondary winding N 2  with an opposite polarity to N 1 , and a feedback winding Nb with the same polarity as N 1 . The switch component FET Q 1  is connected in series to terminal  2  of the primary winding N 1 . The starting resistor RS is connected between terminal  1  of the primary winding N 1  and the gate of the switch component FET Q 1 . The positive feedback circuit  40  comprises a resistor R 4  and a capacitor C 3 , both connected in series between terminal  3  of the feedback winding Nb and the gate of the switch component FET Q 1 . The control circuit  46  is connected between terminal  3  of the feedback winding Nb and the gate of the switch component FET Q 1 . The output rectifier  42  has a serial connected diode CR 51  and a parallel connected capacitor C 51 . The anode of the diode CR 51  is connected to terminal  5  of the secondary winding N 2 .  
           [0008]    When high voltage at terminal  1  of the primary winding N 1  occurs, the resulting current through the RS gradually charges the gate of the FET Q 1 . When the voltage difference between the gate of the FET Q 1  and the source of the FET Q 1  reaches a threshold voltage Vt of the FET Q 1 , the FET Q 1  is activated to conduct current through terminal  1  and terminal  2  of the primary winding N 1 . The current change between terminal  1  and terminal  2  generates an induction voltage between terminal  4  and terminal  3  of the feedback winding Nb. Via a coupling effect of positive feedback circuit  40 , the induction voltage increases the voltage of the gate of the FET Q 1  and, as a result, increases the current value of the current between terminal  1  and terminal  2 . This positive feedback continues to increase the current value of the current between terminal  1  and terminal  2 , and stores sufficient current energy at primary winding N 1 .  
           [0009]    The resistor R 5  and the capacitor C 5  of the control circuit  46  consist of an RC delay circuit. When the capacitor C 5  is charged to a certain level, the transistor Q 3  is activated to decrease the voltage of the gate of the FET Q 1  and thereby deactivate the FET Q 1 . At switching, the current energy stored at the primary winding N 1  is transferred to the secondary winding N 2  and the feedback winding Nb. An induction current at the secondary winding N 2  charges the capacitor C 51 , and provides power to the external circuit via terminal Vo. Feedback winding Nb, resistor R 4  and capacitor C 3  construct an LC oscillator. When the voltage at terminal  3  oscillates and is converted from a negative value to a predetermined positive value, via the coupling of the capacitor C 3 , the FET Q 1  is activated again and stores the current energy at the primary winding N 1 . Through repeated cycles, the primary winding N 1  continues to transfer the current energy to the secondary winding N 2 .  
           [0010]    Though the above-described RCC structure oscillates, it does not guarantee a fixed voltage difference between the Vo and the GND. In other words, if the secondary winding N 2  continues to charge C 51 , it is possible that the resulting high voltage difference between the Vo and the GND may damage the circuit connected between the Vo and the GND.  
           [0011]    Accordingly, most of the RCC structure further includes a detect circuit  48  comprising a light emitting diode PD and a zener diode ZD, connected between two terminals of the capacitor C 51 , as shown in FIG. 1. When the voltage difference of the capacitor C 51  is higher than the breakdown voltage of the zener diode ZD, the light emitting diode Pd emits light. It follows that the photo-transistor PT of the control circuit  46  is activated by the received light from the light emitting diode PD. The process is served to decrease the time required for charging the capacitor C 5  to activate the transistor Q 3 . The process avoids overloading of current stored at the primary winding and maintains an acceptable voltage level between the Vo and the GND.  
           [0012]    However, if the AC voltage experiences a shortage, that is, if the main power voltage of the RCC structure decreases, the induction voltage generated at terminal  3  of the feedback winding Nb also decreases. Consequently, the control circuit  46  does not have sufficient voltage to activate the Q 3  and is not able to deactivate FET Q 1 . Thus, FET Q 1  may be continuously activated, wasting electrical power.  
         SUMMARY OF THE INVENTION  
         [0013]    As a result, the main object of the present invention is to provide a switched power supply using RCC system that prevents the problem of power waste when AC voltage is in limited supply.  
           [0014]    Another object of the present invention is to avoid unnecessary power output when the output load is excessive.  
           [0015]    Another object of the present invention is to efficiently avoid excessive output voltage.  
           [0016]    Still another object of the present invention is to efficiently decrease the power consumption of the switched power supply when there is no output load to meet environmental concerns.  
           [0017]    Based on the above mentioned objects, the present invention provides a switched power supply comprising a transformer, a switch component, a rectifier circuit, a positive feedback circuit, a control circuit and a regulated control circuit. The transformer includes at least a primary winding, a secondary winding and a feedback winding. The switch component includes a main control terminal and is connected in series to the primary winding. The rectifier circuit is connected to the secondary winding and used to output DC voltage. The positive feedback circuit is connected between the first terminal of the feedback winding and the main control terminal. When the switch component is switched to the On position, the positive feedback circuit provides the main control terminal with a positive feedback voltage. The control circuit includes a first control component and a delay circuit, and is connected between the feedback winding and the main control terminal. The first control component is connected between the control terminal and a first ground and has a first control terminal. The delay circuit is connected between the first terminal and the first control terminal. After the switch component is switched to the On position for a predetermined interval, the feedback winding receives a first voltage to switch the first control to the On position and thereby switch the switch component to the Off position. The regulated control circuit is connected between a second terminal of the feedback winding and the first control terminal. When the DC voltage reaches a first predetermined voltage value, the regulated control circuit provides a second voltage to switch the first control component to the On position and thereby switch the switch component to the Off position.  
           [0018]    The present invention further provides a method for controlling a switched power supply. The switched power supply comprises a transformer, a switch component and a rectifier circuit. The method comprises the following steps: (1) providing a main control terminal of the switch component with a positive feedback voltage by the positive feedback circuit when the switch component is switched to the On position; (2) providing the first control component with a first voltage by the first terminal to switch the switch component to a off state after the switch component is switched to the On position for a predetermined interval; and (3) providing the first control component with a second voltage by a second terminal of the feedback winding to switch the switch component to a off state, when the DC voltage reaches a first predetermined voltage value.  
           [0019]    The advantage of the present invention is that it provides an efficient apparatus and method for under-voltage protection (UVP), over-voltage protection, and overload protection. In addition, the present invention provides features to efficiently decrease power consumption of the RCC structure when the RCC structure is not loaded. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
         [0021]    [0021]FIG. 1 is a perspective view of a conventional RCC circuit;  
         [0022]    [0022]FIG. 2 is a perspective view of the circuit of a switched power supply using FET as the switch component in accordance with the present invention; and  
         [0023]    [0023]FIG. 3 is a perspective view of the circuit of a switched power supply using a bipolar junction transistor as the switch component in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    [0024]FIG. 2 is a perspective view of the circuit of the switched power supply using an RCC system in accordance with the present invention.  
         [0025]    The switched power supply  10  of the invention comprises an AC to DC rectifier  12 , a starting circuit  17 , a transformer  14 , a output rectifier  16 , a switch transistor FET Q 1 , a positive feedback circuit  13 , an over-current protection circuit  22 , a control circuit  20  and a detect circuit  18 .  
         [0026]    The AC to DC rectifier  12  comprises a diode bridge BD 1  and a smoothing capacitor C 1 , and provides the DC power for the operation of the RCC structure.  
         [0027]    The transformer  14  has at least three windings; primary winding N 1 , secondary winding N 2  with an opposite polarity to N 1 , and feedback winding Nb with the same polarity as N 1 . As shown in the FIG. 2, two terminals of the primary winding N 1  are called terminal  1  and terminal  2  respectively. Two terminals of the feedback winding Nb are called terminal  3  and terminal  4 . Two terminals of the secondary winding N 2  are called terminal  5  and terminal  6 .  
         [0028]    The starting circuit  17  comprises a resistor R 1  and a resistor R 22  connected in series between terminal  1  of the primary winding N 1  and the gate of the switch component FET Q 1 , and provides starting current required when the RCC system is powered up. The switch transistor FET Q 1  is connected in series to terminal  2  of the primary winding N 1  and controls the current flow of the primary winding N 1 . The over-current protection circuit  22  comprises a resistor R 12 , a resistor R 10  and a capacitor C 6 , and detects current volume through the switch transistor FET Q 1 . The positive feedback circuit  13  comprises a resistor R 4  and a capacitor C 3  connected in series between terminal  3  of the feedback winding Nb and the gate of the switch transistor FET Q 1 .  
         [0029]    The control circuit  20  is connected between terminal  3  of the feedback winding Nb and the gate of the switch component FET Q 1 . The control circuit  20  comprises a bipolar junction transistor Q 3 , an RC delay circuit  26 , a regulated control circuit  24 , and a sense circuit  28 .  
         [0030]    The RC delay circuit  26  comprises a resistor R 6  and a capacitor C 5 . The resistor R 6  is connected between terminal  3  of the feedback winding Nb and the base of the bipolar junction transistor Q 3 . The capacitor C 5  is connected between the first ground and the base of the bipolar junction transistor Q 3 .  
         [0031]    The regulated control circuit  24  comprises a diode D 1 , a resistor R 14 , a capacitor C 4  and a photo-transistor PT. The resistor R 14  and the capacitor C 4  are connected in parallel between terminal  4  of the feedback winding Nb and the first ground. The diode D 1  is connected between the first ground and terminal  3  of the feedback winding Nb. The photo-transistor PT is connected between terminal  4  of the feedback winding Nb and the base of the bipolar junction transistor Q 3 .  
         [0032]    The sense circuit  28  comprises a zener diode ZD 4  and a bipolar junction transistor Q 4 . The bipolar junction transistor Q 4  is connected between the base of the bipolar junction transistor Q 3  and the connecting point of resistor R 1  and resistor R 22 . The zener diode ZD 4  is connected to the base of the bipolar junction transistor Q 4  and terminal  4  of the feedback winding Nb.  
         [0033]    The output rectifier  16  is connected to the secondary winding N 2  and comprises a rectifier diode CR 51 , a zener diode ZD 51 , and a smoothing capacitor C 51 . The anode of the rectifier diode CR 51  is connected to terminal  5  of the secondary winding N 2 . The cathode of the zener diode ZD 51 , the cathode of the rectifier diode CR 51  and the positive terminal of the smoothing capacitor C 51  are connected to a point acting as the positive terminal of the DC voltage. The anode of the zener diode ZD 51 , the negative terminal of the smoothing capacitor C 51  and terminal  6  of the secondary winding N 2  are connected to a point acting as the second ground, a ground terminal of the output DC voltage.  
         [0034]    The detect circuit  18  is connected between the positive output terminal of the DC voltage and the second ground, and comprises a resistor R 52 , a light emitting diode PD, a resistor R 53  and a zener diode ZD 52 .  
         [0035]    The operating process of the switched power supply  10  is further described as follows.  
         [0036]    When the AC power is not connected, the FET Q 1  is in the Off position. When the AC power is connected, a starting current charges the gate of the FET Q 1  via the resistor R 1  and the R 22 . If the voltage difference between the gate of the FET Q 1  and the source of the FET Q 1  is higher than a threshold voltage Vt of the FET Q 1 , the FET Q 1  is activated and a current is consequently generated through terminal  1  to terminal  2  of the primary winding.  
         [0037]    When FET Q 1  is activated, the current through terminal  1  to terminal  2  increases and an induction current resulting from the electromagnetic induction of the feedback winding Nb is generated through terminal  4  to terminal  3 . The induction voltage generated at terminal  3  undergoes a coupling effect occurring at the resistor R 4  and the capacitor C 3  and increases the voltage at the gate of the FET Q 1 . The induction voltage also increases the current through terminal  1  to terminal  2 . It follows that positive feedback is established and continues to increase the current between terminal  1  and terminal  2 .  
         [0038]    There are at least two methods for deactivating the FET Q 1 . The first method has to go through the RC delay circuit  26 . When the voltage of terminal  3  increases, the RC delay time from going through the resistor R 6  and resistor C 5 , and the voltage of the base of the bipolar junction transistor Q 3  also increases. If the voltage difference between the base of the bipolar junction transistor Q 3  and collector reaches the turn-on voltage of the bipolar junction transistor Q 3 , the bipolar junction transistor Q 3  is activated. It follows that the voltage of the gate of the FET Q 1  drops approximately to 0 and thereby the FET Q 1  is deactivated. The second method has to go through the over-current protection circuit  22 . The voltage value crossing the resistor R 12  in the over-current protection circuit  22  is in proportion to the current value through terminal  1  and terminal  2 . When the voltage crossing the resistor R 12  reaches a certain level, via the coupling of the resistor R 10  and the capacitor C 6 , the base voltage of the bipolar junction transistor Q 3  also increases to a set level, thereby activating the bipolar junction transistor Q 3  and deactivating the FET Q 1 .  
         [0039]    When the FET Q 1  is deactivated, the current value going through terminal  1  to terminal  2  drops to 0 simultaneously. As a result, the current energy stored at the primary winding N 1  is then transferred to the secondary winding N 2  and the feedback winding Nb. The current generated at the secondary winding N 2  goes through terminal  6  to terminal  5 , charges the capacitor C 51  via diode CR 51 , and increases the voltage of the positive terminal of the capacitor C 51 . The current generated at the feedback winding Nb goes through terminal  3  to terminal  4 , charges the capacitor C 4  and at the same decreases the voltage value of terminal  3 . Due to the clamping effect of the diode D 1 , the lowest voltage at terminal  3  is 0.  
         [0040]    When the current energy at the secondary winding N 2  is finished, the voltage of terminal  3  of the feedback winding Nb begins to increase from 0 to a positive value. The current through terminal  3  to terminal  4  changes direction along with the oscillation. Via the coupling of the capacitor C 3  and the resistor R 4 , the voltage of the gate of the FET Q 1  is increased. If voltage difference between the gate of the FET Q 1  and the source of the FET Q 1  is higher than the threshold voltage (Vt), the FET Q 1  is activated and a current at the primary winding N 1  is generated. The activation of the FET Q 1  at the same time starts the above mentioned positive feedback and continues to increase the current through terminal  1  to terminal  2 .  
         [0041]    Based on the analysis above, it is known that the FET Q 1  is activated and deactivated repeatedly so as to transfer the energy to the secondary winding N 2 . The process is thus used to generate DC power.  
         [0042]    After several cycles of FET Q 1 &#39;s activation and deactivation, terminal  4  of the feedback winding Nb then forms voltage, higher than the first ground due to the combination of the diode D 1 , capacitor C 4  and resistor R 14 .  
         [0043]    The detect circuit  18  and the regulated control circuit  24  control the voltage of the output DC power. When the voltage of the Vo is higher than a predetermined level, the zener diode ZD 52  enters a breakdown state. As a consequence, the light emitting diode Pd is powered on and thereby causes the light emitting diode Pd to emit light. Based on the presence of the light, the photo-transistor PT determines the connection level between terminal  4  of the feedback winding Nb and the base of the bipolar junction transistor Q 3 . Terminal  4  of the feedback winding Nb can serve as the voltage source of the base of the bipolar junction transistor Q 3 . If the voltage of the Vo is higher than the predetermined level, voltage of the base of the bipolar junction transistor Q 3  maintains the bipolar junction transistor Q 3  in an activated state. When the FET Q 1  maintains a deactivated state, it stops the activation/deactivation cycle, and the voltage between the Vo and the GND as a result is maintained at a certain level.  
         [0044]    When terminal  4  of the feedback winding Nb is higher than other predetermined levels, zener diode ZD 4  enters a breakdown state, turning on the bipolar junction transistor Q 4 , and, as a result, a bipolar junction transistor Q 3  is activated to deactivate the FET Q 1 .  
         [0045]    The main feature of the present invention is that terminal  4  of the feedback winding Nb generates voltage for photo-transistor PT and the sense circuit  28 .  
         [0046]    The switched power supply  10  of the invention delivers the following advantages:  
         [0047]    1. The present invention provides protection against under-input voltage. When under-input AC voltage occurs, the voltage of terminal  3  of the feedback winding Nb is decreased. Accordingly, the resistor R 6  and the capacitor C 5  cannot provide sufficient voltage to activate the bipolar junction transistor Q 3 , and thereby deactivate the FET Q 1 . Under these circumstances, the activation and deactivation cycle of the FET Q 1  repeats via the over-current protection circuit  22  and continues to increase the voltage value of the Vo. Due to the fact that terminal  5  of the secondary winding N 2  and terminal  4  of the feedback winding Nb have the same polarity, the voltage of terminal  4  of the feedback winding Nb is in approximate proportion to the voltage of terminal  5  of the secondary winding N 2 . Therefore, the voltage of terminal  4  of the feedback winding Nb can be used as the voltage source of the base of the bipolar junction transistor Q 3 . Furthermore, the bipolar junction transistor Q 3  can also be activated via the effect resulting from the light emitting diode PD and the coupled photo-transistor PT, and thereby deactivate the FET Q 1  so as to prevent the voltage value of the Vo from increasing.  
         [0048]    2. The present invention provides protection against over-output voltage. If detect circuit failure occurs due to unexpected factors, the failure results in the failure of the light emitting diode PD and the phototransistor PT. Based on the analysis described in item  1 , it is understood that the voltage value of the Vo then increases and the voltage of terminal  4  of the feedback winding Nb also increases. When the voltage value of terminal  4  of the feedback winding Nb is higher than a predetermined level, via the breakdown of the zener diode ZD 4  and the activation of the bipolar junction transistor Q 4 , the base of the bipolar junction transistor Q 3  then receives voltage from the connecting point of the resistor R 1  and the resistor R 22 . It follows that the received voltage activates the bipolar junction transistor Q 3 , thereby deactivating the FET Q 1 . The deactivation and activation cycle of the FET Q 1  is thus stopped. However, the activation and deactivation cycle of the FET cannot be re-initiated until the voltage value of the Vo is decreased to a certain level and voltage of terminal  4  of the feedback winding Nb leaks through the resistor R 14  or other circuit path to a certain level.  
         [0049]    3. The present invention provides protection against overload. When output overload occurs, that is, when a low equivalent resistance load is added between the Vo and the GND, the voltage crossing the resistor R 12  of the over-current protection circuit  22  is in proportion to the current value through terminal  1  to terminal  2 . When the voltage crossing the resistor R 12  reaches a certain level, via the coupling of the resistor R 10  and the capacitor C 6 , the base voltage of the bipolar junction transistor Q 3  also increases to a level to activate the bipolar junction transistor Q 3  and thereby deactivate the FET Q 1 . The voltage value of terminal  4  of the feedback winding Nb does not decrease along with the decrease of the output voltage of the Vo. On the other hand, the voltage value of terminal  4  of the feedback winding Nb remains, providing a reverse current through the zener diode ZD 4 . In addition, the bipolar junction transistor Q 4  shares the current through the resistor R 1  via the effect of the bipolar junction transistor Q 4 . As a consequence, the current charging the gate of the FET Q 1  decreases relatively, also delaying the timing of the activation of the FET Q 1 . The activation and deactivation cycle of the FET cannot be re-initiated until the voltage value of the Vo is decreased to a certain level and voltage of terminal  4  of the feedback winding Nb leaks through the resistor R 14  or other circuit path to a certain level. Accordingly, the power consumption of the power supply is decreased due to the delayed timing of the activation of the FET Q 1 .  
         [0050]    4. The present invention lowers the power consumption of the switched power supply when the switched power supply is not loaded. When the output point between the Vo and the GND is not loaded, leakage of terminal  4  of the feedback winding Nb determines the timing of the activation of the FET Q 1 . Another factor determining the timing of the activation of the FET Q 1  is the current volume that the gate of the FET Q 1  received from the starting circuit  17 . As the advantage described in item  3 , the bipolar junction transistor Q 4  shares the current through the resistor R 1 .  
         [0051]    Accordingly, the current charging the gate of the FET Q 1  decreases relatively, which also delays the timing of the activation of the FET Q 1 . That is, when the switched power supply is not loaded, the activation of the FET Q 1  and the timing of the activation and deactivation cycle of the FET is inevitably delayed. In short, the overall power consumption average of the switched power supply  10  is decreased.  
         [0052]    In the switched power supply using RCC system of the invention, the FET Q 1  can also be substituted with a high power bipolar junction transistor as shown in the FIG. 3. In order to provide a sufficient bias current to the bipolar junction transistor Q 1  during positive feedback, a diode D 4  has to be added to the positive feedback circuit  13  in a manner shown in FIG. 3.  
         [0053]    Distinct from the conventional switched power supply using RCC system, the switched power supply of the invention forms a voltage source at terminal  4  of the feedback winding Nb. The voltage source is used as power for the photo-transistor PT and the sense circuit  28 . As a consequence, the present invention resolves problems that may be caused by under-input voltage, over-output voltage, and overload. In addition, the switched power supply of the invention decreases power consumption when the switched power supply is not loaded.  
         [0054]    Finally, while the invention has been described by way of examples 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.