Abstract:
There is provided an output voltage stabilizer for stabilizing an output voltage of a switching mode power supply by elimination power bounce appearing on the output voltage of the switching mode power supply. The output voltage stabilizer mainly includes a voltage comparator that compares a DC voltage with a reference voltage and in response thereto generates an output signal, and a control switch that is biased according to the output signal of the voltage comparator and provides the power supply a drive signal to stabilize an output voltage of the power supply according to an on/off state of the control switch.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally related to a voltage stabilizer for smoothly turning on and turning off a switching mode power supply, and more particularly, the present invention is related to a voltage stabilizer that is used to stabilize the output voltage of a switching mode power supply by the elimination of power bounce appearing on the output voltage of a power supply.  
         BACKGROUND OF THE INVENTION  
         [0002]    A power supply is generally used to convert commercially available alternating current power to direct current power for use by an ordinary electronic appliance. The conversion technique used in a power supply for a personal computer is based on the switching operation of switching devices to provide DC output voltage of multiple predetermined levels. This type of power supply is generally termed as switching regulator or switching mode power supply (SMPS).  
           [0003]    However, the implication of power quality of a switching mode power supply always depends on its capability to exclude abrupt disturbance or maintain power conversion efficiency. A common problem that would cause the instability of the power quality of a power supply is due to voltage sag or voltage dip that is caused by an accident occurring between adjacent feed lines or voltage buses. The voltage sag generally means that the root-mean-square value (rms value) of voltage is dropped below its nominal value by 10% to 90%, and it would last for a prolonged period, for example, several seconds. The generation of voltage sag in an AC power source would greatly lessen the reliability of a power supply.  
           [0004]    In a power supply system, brownout condition signifies a significant degradation of the primary power source. The danger of brownout condition is that it can repeat several times consecutively and is generally followed by a follow-on surge that can be several times of the voltage that a switching mode power supply is supposed to take in. It has been discovered in numerous brownout tests for switching mode power supply that in case the input voltage is abruptly dropped below its nominal value by about 50%, the output voltage is still under regulation. However, in case the input voltage is abruptly dropped below its nominal value by 80%, the output voltage will run out of regulation and start to bounce violently. The unstable output voltage generated by the power supply is likely to damage other electronic circuit powered by the power supply.  
           [0005]    In addition to the brownout problem that is induced as the input voltage abruptly dropped below its nominal value by a predetermined amount, when the power supply is started up, a surge current may be induced at the input terminals of the power supply. Therefore the output voltage of power supply may overshoot upon start-up and sparks may be generated. To limit the current surge from the input terminals of power supply upon start-up and provide a simple way to softly start the power supply, a device featuring with soft-start function is required to ensure a soft start-up of the power supply.  
           [0006]    In view of foregoing problems, what is needed is an output voltage stabilizer that can stabilize the output voltage of power supply during brownout stage and/or soft-start process. The present invention can satisfy these needs.  
         SUMMARY OF THE INVENTION  
         [0007]    A first object of the present invention is to provide an output voltage stabilizer for a switching mode power supply that can limit the output voltage from bounce when an abrupt voltage sag is occurred to the input AC power of the power supply.  
           [0008]    A second object of the present invention is to provide an output voltage stabilizer for a switching mode power supply that enable the switching mode power supply to turn on smoothly.  
           [0009]    Specifically, the present invention contrives an output voltage stabilizer for a switching mode power supply, comprising a voltage comparator that compares a DC voltage with a reference voltage and in response thereto generates an output signal, and a control switch that is biased according to the output signal of the voltage comparator and provides the switching mode power supply a drive signal to stabilize an output voltage of the switching mode power supply according to an on/off state of the control switch.  
           [0010]    In a boarder aspect of the present invention, a switching mode power supply is disclosed which is comprised of a rectifier for converting AC power received from an AC power source into rectified DC power, a switching transformer coupled to the rectifier and including a primary winding and a secondary winding for receiving the rectified DC power and generating AC voltage across the secondary winding through the primary winding, a switching device coupled to the primary winding of the switching transformer for converting a rectified DC power into AC power by on/off operations, an output portion coupled to the secondary winding of the switching transformer for providing an output voltage of a predetermined value, a switching control circuit which outputs switching control pulse signals to control on/off operations of the switching device, and an output voltage stabilizer coupled between the rectifier and the switching control circuit which compares the rectified direct current power generated by the rectifier with a reference voltage and in response thereto provides a drive signal to drive the switching control circuit to stabilize the output voltage of power supply.  
           [0011]    The features and advantages of the present invention will become apparent through the following descriptions with reference to the drawings presented herein, in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a circuit diagram showing a switching mode power supply in conjunction with the output voltage stabilizer incorporated therein in accordance with the present invention; and  
         [0013]    [0013]FIG. 2 shows the detailed circuitry of the output voltage stabilizer in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    The exemplary embodiments of the present invention will now be described in detail with reference to the following preferred embodiments. However, it is to be noted that various modifications can be made on the basis of the present invention, without departing from the spirit and scope of the present invention as to be encompassed within the present invention.  
         [0015]    The best mode for carrying out the present invention is depicted as follows. FIG. 1 shows the circuit topology of the switching mode power supply according to an embodiment of the present invention. As shown in FIG. 1, reference numeral  11  denotes an EMI filter that attenuates noise signals appearing between power lines extending from an AC power source, a reference numeral  12  denotes a full-wave rectifier that rectifies the input AC voltage Vac into a DC voltage Vdc, a reference numeral  13  denotes a switching device such as a power MOSFET that applies the DC voltage Vdc from the full-wave rectifier  12  to the primary winding side of a switching transformer  14  to generate a high-frequency AC power therein, a reference numeral  14  denotes a switching transformer which generates AC voltage across its secondary winding as the switch device  13  turns on and off, a reference numeral  15  denotes a switching control circuit for outputting switching control pulse signals from driving pulse pin (DRV pin) to control the on/off operations of the switch device  13 , a reference numeral  16  denotes an output portion for providing a DC output voltage Vo at output terminals. In addition, the switching transformer  14  includes an auxiliary winding Na that provides supply power for the switching control circuit  15  through the voltage supply pin VCC of the switching control circuit  15 .  
         [0016]    The switching control circuit  15  may use NCP1222 PWM current-mode controller manufactured by On Semiconductor Corporation, and a compilation of its pin function description is tabulated in Table 1 for reference.  
         [0017]    Table 1 is a compilation of pin function description of NCP1222 PWM current-mode controller manufactured by On Semiconductor Corporation.  
                                           Pin                   No.   Pin Name   Function   Description                   1   Adj   Adjust the skipping peak   This pin is used to adjust               current   the level at which the                   cycle skipping process                   takes place       2   FB   Set the peak current   By connecting an               setpoint   optocoupler to this pin,                   the peak current setpoint                   is adjusted according to                   the output power demand       3   CS   Current sense input   This in sense the primary                   current and routes it to                   the internal comparator via                   an L.E.B.       4   GND   The IC ground       5   DRV   Driving pulses   The driver&#39;s output to an                   external MOSFET       6   VCC   Supplies the IC   This pin is connected to an                   external bulk capacitor of                   typically 10 μF       7   NC   No connection   This un-connected pin                   ensures adequate creepage                   distance       8   HV   Generates the VCC from   Connected to the high-               line   voltage rail, this pin                   injects a constant current                   into the VCC bulk capacitor                  
 
         [0018]    Still referring to FIG. 1, the feature of the present invention is highlighted by an output voltage stabilizer  17 . As shown in FIG. 1, an output voltage stabilizer  17  is incorporated in the switching mode power supply and coupled between an output of the rectifier  12  and a feedback pin (FB pin) of the switching control circuit  15 . The main purpose of the incorporation of output voltage stabilizer  17  in a switching mode power supply is to drive the feedback pin of the switching control circuit  15  to a logic state so as to regulate the output voltage Vo without bounce, if the input AC voltage Vac is undergoing a voltage sag process or is experiencing an initiation process. Referring to FIG. 2, a detailed circuitry that looks inside the output voltage stabilizer  17  is shown. The output voltage stabilizer  17  according to the exemplary embodiment of the present invention principally consists of a voltage comparator  21  that compares the DC component Vdc of the input AC voltage Vac generated by the full-wave rectifier  12  with a reference signal Vref, and a control switch  22  such as a bipolar junction transistor (BJT) switch which receives the output signal from the voltage comparator  21  and responsive to the output signal of the voltage comparator  21  to output a drive signal to drive the feedback pin of the switching control circuit  15 .  
         [0019]    More preferably, the voltage comparator  21  may use LM358 single-supply dual-operational amplifier, also manufactured by On Semiconductor Corporation.  
         [0020]    The operation of the output voltage stabilizer  17  according to the present invention will be explained as follows in reference to FIGS. 1, 2 and Table 1.  
         [0021]    Assume that the input AC voltage Vac slowly increases from 0 to Vac, the DC component Vdc of input AC voltage Vac generated by the full-wave rectifier  12  will increase as well. Also, the auxiliary winding Na of the switching transformer  14  starts to release supply voltage to the switching control circuit  15  through voltage supply pin VCC. The supply voltage pin VCC for the switching control circuit  15  also applies to the eighth pin of the voltage comparator  22  of the output voltage stabilizer  17 . Moreover, the reference voltage Vref used in the voltage comparator  21  is generated by dividing the supply voltage VCC by a fraction determined by the voltage divider R 29 , R 30 .  
         [0022]    When the supply voltage VCC is higher than the voltage of the zener diode ZD 1 , the reference voltage Vref applied to the non-inverting terminal of the voltage comparator  21  will become stable. At this moment, the reference voltage Vref applied to the non-inverting terminal of the voltage comparator  21  is higher than the voltage applied to the inverting terminal of the voltage comparator  21 , so that the voltage comparator sends an output signal with a logic high state to the base terminal of the BJT switch  22 . The BJT switch  22  will then be biased to turn on, and thus send a drive signal to the feedback pin of the switching control circuit  15  to conduct the feedback pin of the switching control circuit  15  to ground. When the input AC voltage Vac increases to reach the point that voltage applied to the inverting terminal of the voltage comparator  21  is higher than that applied to the non-inverting terminal, the logic state of the output signal of the voltage comparator  21  becomes low and the BJT switch  22  is biased to turn off. At this moment the state of feedback pin of the switching control circuit  15  becomes high and the switching control circuit  15  starts to output switching control pulse signals through drive pulse pin DRV to drive switch device  13 . In the mean time, the output voltage of the power supply start to rise up and is regulated without bounce.  
         [0023]    In a second aspect of the present invention, a brownout test is performed to the switching mode power supply of the present invention. Under brownout condition, the input AC voltage Vac is dropped. When Vac is continuously dropped down to the level that the voltage applied to the inverting terminal of the voltage comparator  21  is lower than that applied to the non-inverting terminal of the voltage comparator  21 , the state of the output signal of voltage comparator  21  goes high and the BJT switch  22  is biased to turn on. The feedback pin of the switching control circuit  15  will be conducted to ground again to shutdown the switching control circuit  15 . At this moment the output voltage Vo will drop to 0V without bounce.  
         [0024]    According to the above statements, the use of the output voltage stabilizer  17  in a switching mode power supply is to clamp the state of the feedback pin of switching control circuit  15  to a low state when the input AC voltage Vac is within a certain range of voltage level, so as to ensure a complete transfer of energy from the primary side of switching transformer  14  to the output portion  16  circuit located at the secondary side of the switching transformer  14 . With the introduction of the output voltage stabilizer of the present invention into the switching mode power supply, the signal bounce occurring to the output voltage of the power supply can be eliminated, and thus a stable output voltage can be attained without effort.  
         [0025]    It is clear in virtue of the above descriptions that the basic principle of the present invention is to use an output voltage stabilizer to clamp the voltage level of feedback pin of switching control circuit to a low state when the input AC voltage Vac is within a certain range, such that a complete transfer of energy from the primary side of switching transformer  14  to the output portion  16  can be ensured and the output voltage can be stabilized. Whether the switching mode power supply is under either brownout condition or start-up condition, its output voltage can be stabilized by controlling the feedback amount that is transferred to the switching control circuit  15 .  
         [0026]    Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by the way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.