Abstract:
An output voltage rising slope control technique for a power converter is addressed. The output voltage rising slope control technique according to the present invention basically uses a reference voltage generator to precipitate the rising slopes of multiple output voltages of a power converter to become mutually unanimous with each other during start-up period, and thereby avoid the occurrence of system faults in the power converter due to the instantaneously rapid increase of output voltage thereof during start-up period.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is related to the field of a power converter, and more particularly for controlling the output rising slope in a power converter.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 shows a circuit configuration of a power converter using post-regulator(Mag-Amp) according to the prior art. As shown in FIG. 1, a prior art power converter using post-regulator(Mag-Amp) generally includes a switch control circuit  101 , a magnetic amplifier  102 , output rectifier  103 , an output filter  104 , a resistive-type voltage divider  105 , and a reference voltage generator  106 . The magnetic amplifier  102  is used to control the duty of an AC voltage V AC  induced at the secondary side of the transformer T 1  by way of magnetic coupling, and the resulting amplified voltage is converted into a DC voltage by the output rectifier  103 . The resulting DC voltage is then filtered and smoothed by the output filter  104  comprised of an inductor L A  and a capacitor C A , and thereby provide an output voltage Vo for a load (not shown). In addition, a fraction of the output voltage Vo (which is indicated by a reference numeral V S ) is sensed and sent to the switch control circuit  101  through a resistive-type voltage divider  105  made up of resistors R A  and R B . The switch control circuit  101  uses an internal comparator (not shown for simplicity) to compare the fractional output voltage V S  with a reference voltage Vref provided by a reference voltage generator  106 , and in response thereto generate a control signal. An internal driving circuit of the switch control circuit  101  which is normally accomplished with a transistor drives the Mag-Amp  102  to control the duty of Vac, rectified by output rectifier  103 , the output voltage Vo can be stabilized. On the other hand, a housekeeping circuit (not shown in the drawings) is able to receive a remote ON/OFF signal to the power converter to supply output voltage V o  to load, such as a motherboard of a PC or a hard disk drive.  
           [0003]    Some sort of power supplies are able to provide multiple output voltages, each of which occupies a predetermined voltage level, for example, 5V or 3.3V, and each output voltage is sufficient to enable the appropriate operations of one or more electronic appliances that can operate under such output voltage. As shown in FIG. 2 a,  a first output voltage Vout 1  of a power converter is increased up to a stable voltage level such as 5V with a fixed rising slope within its start-up period (0-t 1 ) At this time, the power converter is operating under a start-up mode. The start-up period (or set-up period) referred herein indicates a time period calculated from the moment at which an input terminal of the power converter is connected with a power source to the moment at which the output voltage becomes stable. Similarly, a second output voltage Vout 2  of a power converter is increased up to a stable voltage level such as 3.3V with a fixed rising slope within its start-up period (0-t 2 ), and the rising slope of the first output voltage Vout 1  and the rising slope of the second output voltage Vout 2  are quite close to each other. When both of the two output voltages are stabilized, the power converter can output two output voltages of different voltage levels to other loads, and then the power converter is driven to operate under a normal mode.  
           [0004]    However, the load collocated with a power converter is typically behaved as a dynamic load element. Therefore, in practical operation of a power converter, the load current of a power converter is also dynamically varying with time instead of being kept at a constant level. As a result, it is possible that the instantaneous load current variation causes the output voltage rising slope change during a start-up period of a power converter. As shown in FIG. 2( b ), the first output voltage Vout 1  outputs an excessive voltage instantaneously by the influence of a corresponding load condition, so that the rising slope of the first output voltage Vout 1  increases significantly. However, the rising slope of the second output voltage Vout 2  remains constant, which may lead to a situation that a difference between an instantaneous voltage value of the first output voltage Vout 1  and an instantaneous voltage value of the second output voltage Vout 2  at time t 3  is larger than a difference between a stabilized voltage value of the first output voltage Vout 1  and a stabilized voltage value of the second output voltage Vout 2 , for example, 5V−3.3V=1.7V. That may cause the system to conduct faulty operations. More seriously, the whole power system may halt down accordingly. Such a phenomenon in which an instantaneous voltage value of one of the output voltages is sharply increased during the start-up period and thus cause an improper operation of the power converter is generally called “rising slope out of control”.  
           [0005]    In view of the foregoing descriptions, what is need is a rising slope control technique for a power converter to obviate the occurrence of an “out of control” problem encountered during a start-up period of the power converter and ensure the proper operation of the system.  
         SUMMARY OF THE INVENTION  
         [0006]    A broader aspect of the present invention is fulfilled with a power converter, including at least two magnetic amplifiers each of which is magnetically coupled to an AC voltage induced at a winding of a transformer, at least two switch circuits each of which is coupled to one of the magnetic amplifiers for converting an amplified AC voltage from the magnetic amplifier into a DC voltage, at least two output filters each of which is coupled to one of the switch circuits for providing a predetermined output voltage to a load, at least two switch control circuits each of which compares an output voltage with a reference voltage and generates a control signal based on a comparison result to control the on/off operation of one of the switch circuits, and a reference signal generator coupled to the switch control circuits for respectively providing a reference signal to a single switch control circuit and precipitating the rising slopes of the output voltages of the power converter to become mutually unanimous during start-up period.  
           [0007]    According to a narrower sense of the present invention, the above-described reference voltage generator further includes a linear voltage generating circuit for generating a linear voltage with a rising slope, a first reference voltage generating block for limiting the linear voltage with a rising slope based on a predetermined voltage level and thereby generating a first reference voltage, and a second reference voltage generating block for imposing a time delay on the linear voltage with a rising slope and tuning a rising slop of the linear voltage with a rising slope, and thereby generating a second reference voltage.  
           [0008]    The features and advantages of the present invention will be made obvious to those skilled in the art through the following descriptions with reference to the accompanying drawings, wherein: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a circuit diagram showing a power converter according to the prior art;  
         [0010]    [0010]FIG. 2 a  is a timing diagram showing the output voltage waveforms of a power converter according to the prior art, in which an output voltage with an out-of-control rising slope is not occurred to the power converter;  
         [0011]    [0011]FIG. 2 b  is a timing diagram showing the output voltage waveforms of a power converter according to the prior art, in which an “rising slope out of control” is occurred to the power converter;  
         [0012]    [0012]FIG. 3 is a circuit diagram showing a power converter according to a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 4 is a circuit diagram showing a switch control circuit of a power converter according to a preferred embodiment of the present invention;  
         [0014]    [0014]FIG. 5 is a circuit diagram showing a reference voltage generator of a power converter according to a preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 6 is a timing diagram showing respective voltage waveforms at each circuit node in a reference voltage generator of a power converter according to a preferred embodiment of the present invention; and  
         [0016]    [0016]FIG. 7 represents the voltage waveforms of Bsense signal, remote ON/OFF signal, the first reference voltage, and the second reference voltage. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    A schematic representation illustrating the constitution of a power converter according to a preferred embodiment of the present invention is given in FIG. 3. The power converter according to a preferred embodiment of the present invention is configured to provide multiple output voltages, each of which occupies a predetermined voltage level. In this preferred embodiment, a power converter configured to provide two output voltages both occupying a different predetermined level is taken as an example to explicate the present invention. However, it can be readily understood that the principle of the present invention can be further extends to build a power converter having more than two output voltages, each of which occupies a predetermined voltage level. In this embodiment, the power converter of FIG. 3 includes two circuit sections  200  and  201 , each of which is configured to output a predetermined output voltage. The circuit section  200  includes a magnetic amplifier  204  which amplifies an AC voltage induced at a secondary winding of a transformer T 1  by way of magnetic coupling, and then the amplified AC voltage is converted into a DC voltage by the on/off operation of the switch circuits  206 . The resulting DC voltage is filtered and smoothed by an output filter  208  comprised of an inductor L 1  and a capacitor C 1 , and thereby provide a first output voltage Vout 1  for a load (not shown). Similarly, the circuit section  201  also includes a magnetic amplifier  205  which amplifies an AC voltage induced at an auxiliary winding of a transformer T 1  by way of magnetic coupling, and then the amplified AC voltage is converted into a DC voltage by the on/off operation of the switch circuits  207 . The resulting DC voltage is filtered and smoothed by an output filter  209  comprised of an inductor L 2  and a capacitor C 2 , and thereby provide a second output voltage Vout 2  for a load (not shown). In addition, a sensing voltage of the first output voltage Vout 1  which is designated with reference numeral V S1  is returned back to a switch control circuit  202  through a resistive-type voltage divider  210  comprised of resistors R 1  and R 2 . The switch control circuit  202  uses an internal comparator (for the purpose of simplification of description, it is not shown in the drawings) to compare the sensing voltage V S1  with a first reference voltage Vref 1  provided by a reference voltage generator  212  and in response thereto generate a Control signal. An internal driving circuit of the switch control circuit  202  which is normally accomplished with a transistor is biased in response to the Control signal so as to drive the switch circuits  206  to turn on or off, and thus a switching duty cycle of the switch circuits  206  can be regulated and the value of the first output voltage Vout 1  can be stabilized.  
         [0018]    In a similar manner, the sensing voltage of the second output voltage Vout 2  which is designated with reference numeral V S2  is returned back to a switch control circuit  203  through a resistive-type voltage divider  211  comprised of resistors R 3  and R 4 . The switch control circuit  203  uses an internal comparator (for the purpose of simplification of description, it is not shown in the drawings) to compare the fractional output voltage V S2  with a second reference voltage Vref 2  provided by a reference voltage generator  212 , and in response thereto generate a Control signal. An internal driving circuit of the switch control circuit  203  which is normally accomplished with a transistor is biased in response to a Control signal so as to drive the switch circuits  207  to turn on or off, and thus a switching duty cycle of the switch circuits  207  can be regulated and the value of the second output voltage Vout 1  can be stabilized. On the other hand, a housekeeping circuit (not shown) receive a remote ON/OFF signal to the power converter to make Vout 1  and Vout 2  available.  
         [0019]    [0019]FIG. 4 shows a circuit configuration of a switch control circuit  202  of FIG. 3. The circuit operation of the switch control circuit  202  will be described as follows. It is to be noted that the switch control circuit  202  and the switch control circuit  203  both have a similar configuration, and their circuit operations are alike. The switch control circuit  202  includes an error amplifier  213  which has an inverting input terminal connected with sensing voltage V S1  from the voltage divider  210  and a non-inverting input terminal for receiving a first reference voltage Vref 1  from the reference voltage generator  212 . The sensing voltage V S1  provided by the voltage divider  210  is compared with the first reference voltage Vref 1  provided by the reference voltage generator  212  by the error amplifier  213 , and thereby generate the Control signal in response to a comparison result. The Control signal is outputted to a driving circuit  214  which is preferably a bipolar junction transistor (BJT). The driving circuit  214  is used to control Mag-Amp  204  and/or switch circuits  206  to turn on or off based on control signal so as to regulate the output voltage value of the power converter.  
         [0020]    The fundamental theorem for achieving rising slope control according to the present invention is made on the basis of imposing a time delay on a reference voltage that is designated to regulate the value of an output voltage of the power converter during a start-up period of the power converter and tuning a rising slope of the reference voltage, and thereby force the rising slope of an output voltage during a start-up period of the power converter to change accordingly, that is, to precipitate the rising slopes of each output voltages of a power converter to approach an unanimity during start-up period. In this manner, the “rising slope out of control” problem can be settled efficiently.  
         [0021]    Referring to FIG. 5, the circuit configuration of a reference voltage generator  212  according to a preferred embodiment of the present invention is comprised of a comparator  301  having an inverting input terminal N 1  for receiving a sensed Bulk voltage Bsense from a secondary winding of a transformer, in which a voltage waveform of Bsense signal is demonstrated in FIG. 7, and a non-inverting input terminal for receiving a fixed voltage  307 . The comparator  301  compares the sensed Bulk voltage Bsense with the fixed voltage  307  and in response thereto provide a switch control signal Vsw for a switch  303  in a circuit for generating linear voltage within a rising slope generating circuit  315 . The circuit  315  for generating linear voltage with a rising slope includes switches  303 ,  304 , a current source  302 , a capacitor Cs and a voltage follower  305 . The switch  303  switches its on/off state in response to a state of the switch signal Vsw. When the switch  303  is OFF, the current source  302  charges the capacitor Cs and stores energy in the capacitor Cs. When the switch  303  is ON, the capacitor Cs starts to discharge the stored energy to a non-inverting input terminal of the voltage follower  305  through a circuit node N 2 . Under this condition, the voltage appearing at a non-inverting input terminal of the voltage follower  305  is a linear voltage Vcs with a rising slope, wherein its voltage waveform shown in FIG. 6.  
         [0022]    The linear voltage Vcs with a rising slope at the circuit node N 2  is then coupled to a circuit node N 3 . The linear voltage Vcs with a rising slope at the circuit node N 3  is subsequently transmitted into a first reference voltage generating block  316  and a second reference voltage generating block  317 , respectively, and applies to a circuit node N 5  located in the first reference voltage generating block  316  and a circuit node N 4  located in the second reference voltage generating block  317 .  
         [0023]    As shown in FIG. 5, the reference voltage generator  212  according to a preferred embodiment of the present invention further includes a diode D 1  having an anode to receive a remote ON/OFF signal and a cathode connecting to a circuit node N 5  located in the first reference voltage generating block  316 . A representative voltage waveform of the remote ON/OFF signal is shown in FIG. 7. As indicated in FIG. 7, it can be seen that the actual voltage level of the remote ON/OFF signal is positioned at a low level when the remote ON/OFF signal is ON, so that the diode D 1  is biased to turn off and both the first reference voltage Vref 1  and the second reference voltage Vref 2  are determined by the linear voltage Vcs with a rising slope. When the remote ON/OFF signal is OFF, the actual voltage level of the remote ON/OFF signal is positioned at a high level. If a difference between the voltage level of the remote ON/OFF signal and an instantaneous voltage value of the linear voltage Vcs with a rising slope at an instant within this OFF period of the remote ON/OFF signal is less than a threshold voltage value of the diode D 1 , the diode D 1  will remain in OFF state, and the first reference voltage Vref 1  and second reference voltage Vref 2  are also determined by the linear voltage Vcs with a rising slope. However, if a difference between the voltage level of the remote ON/OFF signal and an instantaneous voltage value of the linear voltage Vcs with a rising slope is larger than a threshold voltage value of the diode D 1  within this period, the diode D 1  will turn on. If the diode D 1  is assumed to be ideal, that is, no voltage drop will be created across the diode when the diode is ON, both the circuit node N 5  located in the first reference voltage generating block  316  and the circuit node N 4  located in the second reference voltage generating block  317  will receive a high voltage value representing the voltage value of the remote ON/OFF signal during its OFF period, an the first reference voltage Vref 1  and the second reference voltage Vref 2  are both determined by such a high voltage value.  
         [0024]    Thus, in the first reference voltage generating block  316 , the signal received at the circuit node N 5  is a linear voltage Vcs with a rising slope when the remote ON/OFF signal is ON, and then the linear voltage Vcs will be trimmed by the voltage limiter  306  so that a peak voltage thereof will be limited at a constant voltage level. Also, a resulting voltage made by the voltage limiter  306  serves as a first reference voltage Vref 1 , wherein its voltage waveform is shown in FIG. 6 and a part of FIG. 7. When the remote ON/OFF signal is OFF and a difference between the voltage level of the remote ON/OFF signal and an instantaneous voltage value of the linear voltage Vcs with a rising slope is larger than a threshold voltage value of the diode D 1  within this OFF period, the signal received at the circuit node N 5  is a high voltage representing a high voltage value of the remote ON/OFF signal, and the resulting first reference voltage Vref 1  will be a high DC voltage having a fixed voltage constant, wherein its voltage waveform is shown as the voltage waveform during the OFF period of the remote ON/OFF signal in FIG. 7.  
         [0025]    Similarly, in the second reference voltage generating block  317 , the linear voltage with rising slope Vcs is coupled to the circuit node N 4  and subtracted with a fixed voltage  308  by a voltage substractor  309  to obtain a reduced voltage Vreduced. A representative voltage waveform of the reduced voltage Vreduced is shown in FIG. 6. The reduced voltage Vreduced is further processed by a first-quadrant chopper  310  to filter out its components lying within a first quadrant, and a chopped voltage with a time delay imposed Vchopped thereon is obtained, wherein its voltage is shown in FIG. 6. The chopped voltage Vchopped is amplified by an amplifier  311  to increase its rising slope, and thus an amplified voltage V AMP  is obtained. Eventually the amplified voltage V AMP  is then processed by the voltage limiter  312  to limits its peak voltage at a fixed voltage level, and a resulting voltage serves as a second reference voltage Vref 2  for the power converter, wherein a representative voltage waveform of the second reference voltage Vref 2  is shown in FIG. 6. On the other hand, when the remote ON/OFF signal is OFF, a DC high voltage representing a high voltage of the remote ON/OFF signal is coupled to the circuit node N 4 . This high DC voltage is then subtracted with the fixed voltage  308  by the voltage substractor  309  and amplified by the amplifier  311  to generate a second reference voltage Vref 2 . The resulting voltage waveform of the second reference voltage Vref 2  during the OFF period of the remote ON/OFF signal is shown in a part of FIG. 7.  
         [0026]    It goes without saying that the output voltage rising slop control technique according to the present invention can impose the time delay on the reference voltage and tune the rising slope of the reference voltage, which is made clear for those skilled in the art from the observation to the voltage waveforms at each node in the circuit configuration of the power converter. The present invention is advantageous in that the output voltages of a power converter can be made to have their rising slopes during start-up period compulsorily approach an unanimity, while the “rising slope out of control” problem that may occur in the situation of multiple output voltages can be suppressed.  
         [0027]    Although the present 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.