Abstract:
A power converter determines a feedback signal according to a voltage signal related to an output voltage of the power converter and a reference voltage, thereby regulating the output voltage. A control circuit and method for programming the output voltage of the power converter utilize an offset current generator to inject a current or sink a current for changing the voltage signal or the reference signal, thereby adjusting the output voltage. As a result, it gets rid of complicated circuitry but provides more steps adjustment, which reduces related costs.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to programmable power converters. More particularly, the present invention is related to a control circuit and method for programming an output voltage of a power converter. 
       BACKGROUND OF THE INVENTION 
       [0002]    One of the charging methods for the portable electronic devices, for example the smart phones and the tablet personal computers, is to utilize a power converter to convert the power into the output voltage. The output voltage is provided to the electronic devices for charging through the transmission line connected to the power converter.  FIG. 1  shows a conventional power converter  2 , which includes a transformer TX generated an output voltage V OUT  at a secondary side of the transformer TX. Resistors R 1  and R 2  form a voltage divider which is coupled to an output terminal of the power converter  2  to detect the output voltage V OUT  and to divide the output voltage V OUT  to generate a voltage signal V DIV . A shunt regulator  4  is coupled to the resistors R 1  and R 2  as well as an optical coupler  6 . The shunt regulator  4  will compare the voltage signal V DIV  with a reference voltage to determine the current Is flowing through the optical coupler  6 . Accordingly, the optical coupler  6  generates a feedback signal Ifb related to the current Is for a control integrated circuit (not shown in  FIG. 1 ) to regulate the output voltage V OUT . There are more and more systems that need to change the output voltage V OUT  according to different needs, for example the rapid charging mode or the sleep mode, in order to improve their performance. There are two existing methods for changing the feedback signal Ifb in order to change the output voltage V OUT : changing a divider ratio of the voltage divider to adjust the voltage signal V DIV , or choosing a different reference voltage. 
         [0003]    As disclosed by U.S. Pat. No. 5,773,963, one of the methods for changing a divider ratio to adjust the output voltage of a power converter is illustrated in  FIG. 2 , in which a power circuit  8  converts a power input V AC  into an output voltage V OUT  for charging a battery  10 . The output voltage V OUT  is divided by a divider ratio to generate a voltage signal V DIV  that is compared with a reference voltage Vref to generate a feedback signal S FB  for the power circuit  8  to adjust the output voltage V OUT . A microcontroller  12  controls the switching of a transistor Q 1  so as to determine whether a resistor R 3  and the resistor R 2  should be in a parallel connection. Accordingly, the divider ratio will be controlled and thereby adjust the output voltage V OUT . 
         [0004]    On the other hand, U.S. Pat. No. 7,242,339 discloses a method for choosing a reference voltage to adjust the output voltage of a power converter, as shown in  FIG. 3 , in which a resistor switch circuit  14  is utilized to choose a needed reference Vref in a way that switches Q 2  to Q 7  in the resistor switch circuit  14  are selectively turned on to set either one of different voltages as the voltage Vp to be divided by several serially-connected resistors, and channel gates Gate  1  to Gate  5  are controlled to choose one of the divided voltage as the reference voltage Vref. 
         [0005]    The existing methods for changing the output voltage mainly utilize the switch and the resistor to change the output voltage V OUT . If the adjustable steps of the output voltage V OUT  are to be raised, the numbers of the resistor and the switch have to be increased. Adversely, related costs are also increased. Moreover, these switches and resistors can be merely installed out of the integrated circuit (IC), which results in a complicate circuitry. 
       SUMMARY OF THE INVENTION 
       [0006]    An objective of the present invention is to provide a control circuit and method for programming an output voltage of a power converter. 
         [0007]    According to the present invention, a control circuit for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage for regulating the output voltage, comprises a voltage divider and an offset current generator. The voltage divider divides the output voltage to generate a voltage signal for controlling the feedback signal. The offset current generator is coupled to the voltage divider for injecting a first current to the voltage divider or sinking a second current from the voltage divider, thereby adjusting the voltage signal and the output voltage. 
         [0008]    According to the present invention, a control circuit for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage for regulating the output voltage, comprises a voltage divider, an offset resistor, and an offset current generator. The voltage divider divides the output voltage to generate a first voltage signal. The offset resistor has a first terminal coupled to the voltage divider to receive the first voltage signal, and a second terminal to provide a second voltage signal for controlling the feedback signal. The offset current generator is coupled to the second terminal of the offset resistor for injecting a first current to the offset resistor or sinking a second current from the offset resistor, thereby offsetting the first voltage signal to generate the second voltage signal. The offset current generator adjusts the first current or the second current to adjust the second voltage so that the feedback signal will be changed so as to adjust the output voltage. 
         [0009]    According to the present invention, a control circuit for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage for regulating the output voltage at a target level, comprises a first digital to analog converter, an offset resistor, and an offset current generator. The first digital to analog converter provides a first reference voltage. The offset resistor has a first terminal coupled to the first digital to analog converter to receive the first reference voltage, and a second terminal to provide a second reference voltage for determining the target level. The offset current generator is coupled to the second terminal of the offset resistor for injecting a first current to the offset resistor or sinking a second current from the offset resistor, thereby offsetting the first reference voltage to generate the second reference voltage. The offset current generator adjusts the first current or the second current to adjust the second reference voltage, thereby adjusting the target level. 
         [0010]    According to the present invention, a control circuit for programming an output voltage of a power converter, which includes a transformer, an auxiliary coil in a primary side of the transformer and configured to operably detect the output voltage to generate a first voltage signal, and a voltage divider coupled to the auxiliary coil to divide the first voltage signal to generate a second voltage signal, comprises an offset resistor and an offset current generator. The offset resistor has a first terminal coupled to the voltage divider to receive the second voltage signal, and a second terminal to provide a third voltage signal. The offset current generator is coupled to the second terminal of the offset resistor for injecting a first current to the offset resistor or sinking a second current from the offset resistor, thereby offsetting the second voltage signal to generate the third voltage signal so as to regulate the output voltage. The offset current generator adjusts the first current or the second current to adjust the third voltage signal, thereby adjusting the output voltage. 
         [0011]    According to the present invention, a control method for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage so as to regulate the output voltage, comprises the steps of: dividing the output voltage by a voltage divider to generate a voltage signal so as to control the feedback signal, and injecting a first current to the voltage divider or sinking a second current from the voltage divider so as to change the voltage signal to adjust the output voltage. 
         [0012]    According to the present invention, a control method for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage so as to regulate the output voltage, comprises the steps of: dividing the output voltage to generate a first voltage signal applied to a first terminal of an offset resistor, injecting a first current to a second terminal of the offset resistor or sinking a second current from the second terminal of the offset resistor, thereby offsetting the first voltage signal to generate a second voltage signal so as to control the feedback signal, and adjusting the first current or the second current so as to adjust the second voltage signal, thereby changing the feedback signal to adjust the output voltage. 
         [0013]    According to the present invention, a control method for programming an output voltage of a power converter, which includes a feedback loop configured to operably provide a feedback signal related to the output voltage so as to regulate the output voltage at a target level, comprises the steps of: providing a first reference voltage to a first terminal of an offset resistor, injecting a first current to a second terminal of the offset resistor or sinking a second current from the second terminal of the offset resistor, thereby offsetting the first reference voltage to generate a second reference voltage so as to determine the target level, and adjusting the first current or the second current so as to adjust the second reference voltage, thereby adjusting the target level. 
         [0014]    According to the present invention, a control method for programming an output voltage of a power converter, which includes a transformer, and an auxiliary coil in a primary side of the transformer and configured to operably detect the output voltage to generate a first voltage signal, comprises the steps of: dividing the first voltage signal to generate a second voltage signal applied to a first terminal of an offset resistor, injecting a first current to a second terminal of the offset resistor or sinking a second current from the second terminal of the offset resistor, thereby offsetting the second voltage signal to generate a third reference voltage so as to regulate the output voltage, and adjusting the first current or the second current so as to regulate the third voltage signal to adjust the output voltage. 
         [0015]    In contrast with the conventional methods for changing the divider ratio and choosing the reference voltage, control circuits and methods according to the present invention, which adjust the output voltage of a power converter via controlling the first and the second currents, are simpler. Particularly, the present invention needs no extra switches or resistors to increase the adjustable steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments according to the present invention taken in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  shows a conventional power converter; 
           [0018]      FIG. 2  shows a conventional method for changing a divider ratio of a voltage divider; 
           [0019]      FIG. 3  shows a conventional method for choosing a reference voltage; 
           [0020]      FIG. 4  shows a first embodiment of the present invention; 
           [0021]      FIG. 5  is a simplified circuitry of  FIG. 4 ; 
           [0022]      FIG. 6  shows a second embodiment of the present invention; 
           [0023]      FIG. 7  shows a simplified circuitry of  FIG. 6 ; 
           [0024]      FIG. 8  shows a third embodiment of the present invention; and 
           [0025]      FIG. 9  shows a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 4  shows a first embodiment of the present invention, and  FIG. 5  shows a simplified circuitry of  FIG. 4 . Referring to  FIGS. 4 and 5 , at a secondary side of a transformer TX of a power converter  2 , an optical coupler  6  is served as a feedback loop for providing a feedback signal Ifb related to the output voltage V OUT  to regulate the output voltage V OUT . A control circuit  16  is coupled to the optical coupler  6  for controlling the feedback signal Ifb so as to regulate the output voltage V OUT . Alternatively, the optical coupler  6  can be replaced by other feedback loop. 
         [0027]    The control circuit  16  includes a voltage divider  18  that is formed by the series resistors R 1  and R 2  for dividing the output voltage V OUT  to generate a voltage signal V DIV  for controlling the feedback signal Ifb. An offset current generator  20  has a terminal  22  coupled to the voltage divider  18  and a terminal  24  coupled to a microcontroller  26 . The offset current generator  20  injects a current I 1  to the voltage divider  18  or sinks a current I 2  from the voltage divider  18 , thereby adjusting the voltage signal V DIV  so as to adjust the output voltage V OUT . In the offset current generator  20 , a decoder  28  decodes a digital signal S D1  to generate control signals S C1  and S C2  as well as switch signals S S1  and S S2 . A digital to analog converter  30  is coupled to the decoder  28  and the voltage divider  18 . The digital to analog converter (DAC)  30  includes switches SW 1  and SW 2  and variable current sources  32  and  34 . The switch SW 1  is coupled between the variable current source  32  and the voltage divider  18 . The switch SW 2  is coupled between the voltage divider  18  and the variable current source  34 . The variable current source  32  adjusts the amount of the current I 1  according to the control signal S C1 , and the variable current source  34  adjusts the amount of the current I 2  according to the control signal S C2 . The switches SW 1  and SW 2  will be turned on or turned off according to the switch signals S S1  and S S2 . When the switch signal S S1  turns on the switch SW 1 , the variable current source  32  injects the current I 1  into the resistor R 2  in the voltage divider  18 . When the switch signal S S2  turns on the switch SW 2 , the variable current source  34  sinks the current I 2  from the resistor R 2  in the voltage divider  18 . 
         [0028]    The control circuit  16  shown in  FIG. 4  further includes a microcontroller  26  that includes a program memory  46  and a data memory  48  for saving information. The microcontroller  26  generates the digital signals S D1  and S D2  according to the saved information to the decoder  28  and a digital to analog converter  36 , respectively. According to the digital signal S D2 , the digital to analog converter  36  generates a reference voltage Vref at an output terminal  38  and generates an overvoltage threshold Vref_ov at an output terminal  40 . An error amplifier  42  has a positive input terminal coupled to the voltage divider  18  and the digital to analog converter  30 , a negative input terminal coupled to the output terminal  38  of the digital to analog converter  36 , and an output terminal connected to a control terminal  50  of a transistor Q 8 . The transistor Q 8  is coupled between the optical coupler  6  and a ground terminal. The error amplifier  42  controls the current Is flowing through the transistor Q 8  according to a difference between the voltage signal V DIV  and the reference voltage Vref. The optical coupler  6  generates the feedback signal Ifb according to the current Is. A shunt regulator  4  in  FIG. 5  is formed by the transistor Q 8  and the error amplifier  42 . In this embodiment, a comparator  44  is utilized for achieving an overvoltage protection, in which a positive input terminal of the comparator  44  is coupled to the voltage divider  18 , and a negative input terminal of the comparator  44  is coupled to the output terminal  40  of the digital to analog converter  36 . When the voltage signal V DIV  is higher than the overvoltage threshold Vref_ov, the comparator  44  generates a protecting signal Sov to turn off the power converter  2  so as to achieve the overvoltage protection. In other embodiments, the comparator  44  can be also utilized for achieving an under voltage protection. In such circumstances, the digital to analog converter  36  will provide an under voltage threshold supplied to the positive terminal of the comparator  44 , and the negative input terminal of the comparator  44  will receive the voltage signal V DIV . When the voltage signal V DIV  is lower than the under voltage threshold, the comparator  44  will generate a protecting signal to turn off the power converter  2 , thereby achieving the under voltage protection. 
         [0029]    In  FIG. 4 , when the offset current generator  20  neither injects the current I 1  nor sinks the current I 2 , the output voltage is 
         [0000]        V   OUT   =V ref×( R 1+ R 2)/ R 2.  (EQ-1)
 
         [0000]    When the offset current generator  20  injects the current I 1  into the voltage divider, the output voltage is 
         [0000]        V   OUT   =V ref×( R 1+ R 2)/ R 2− I 1× R 1.  (EQ-2)
 
         [0000]    When the offset current generator  20  sinks the current I 2   f  from the voltage divider, the output voltage is 
         [0000]        V   OUT   =V ref×( R 1+ R 2)/ R 2+ I 2× R 1.  (EQ-3)
 
         [0000]    As illustrated by the equations EQ-2 and EQ-3, the offset current generator  20  can adjust the output voltage V OUT  by adjusting the current I 1  or I 2 . The adjustable steps of the output voltage V OUT  can be increased as long as the decoder  28  with a higher resolution (i.e. bit numbers) and the digital to analog converter  30  with a higher resolution are selected. No extra components (resistors or switches) are needed. Moreover, as shown in  FIG. 5 , the offset current generator  20 , the microcontroller  26 , and the digital to analog converter  36  can be integrated in an IC in order to simplify the circuitry of the power converter  2 . The resolutions of the digital to analog converters  30  and  36  can be determined according to requirements. The resolution of the digital to analog converter  30  can be lower than that of the digital to analog converter  36 . 
         [0030]      FIG. 6  shows a second embodiment of the present invention, and  FIG. 7  shows a simplified circuitry of  FIG. 6 . In  FIGS. 6 and 7 , the optical coupler  6  serves as a feedback loop for providing the feedback signal Ifb related to the output voltage V OUT  for regulating the output voltage V OUT . The control circuit  16  is coupled to the optical coupler  6  for controlling the feedback signal Ifb and adjusting the output voltage V OUT . Alternatively, the optical coupler  6  can be replaced by other suitable feedback loop. The control circuit  16  in  FIG. 6  has the same offset current generator  20 , microcontroller  26 , and digital to analog converter  36  as those in  FIG. 4 . In the control circuit  16  of  FIG. 6 , the voltage divider  18  formed by the series resistors R 1  and R 2  divides the output voltage V OUT  to generate the voltage signal V DIV1 . A first terminal of the offset resistor Ros is coupled to the voltage divider  18 , and a second terminal of the offset resistor Ros is coupled to the terminal  22  of the offset current generator  20 . When the switch signal S S1  turns on the switch SW 1  that is coupled between the variable current source  32  and the second terminal of the offset resistor Ros in the offset current generator  20 , the variable current source  32  thence injects the current I 1  to the offset resistor Ros. When the switch signal S S2  turns on the switch SW 2  coupled between the second terminal of the offset resistor Ros and the variable current source  34  in the offset current generator  20 , the variable current source  34  sinks current I 2  from the offset resistor Ros. The offset resistor Ros generates an offset voltage in accordance with the current I 1  or I 2 , thereby offsetting the voltage signal V DIV1  so as to generate the voltage signal V DIV2  at the second terminal of the offset resistor Ros. 
         [0031]    An error amplifier  42  of the control circuit  16  in  FIG. 6  has the positive input terminal coupled to the offset resistor Ros and the digital to analog converter  30 , the negative input terminal coupled to the output terminal  38  of the digital to analog converter  36 , and the output terminal coupled to the control terminal  50  of the transistor Q 8 . The transistor Q 8  is coupled between the optical coupler  6  and the ground terminal. The error amplifier  40  controls the current Is that is passing through the transistor Q 8  according to the difference between the voltage signal V DIV2  and the reference voltage Vref. The optical coupler  6  generates the feedback signal Ifb according to the current Is. The variable current sources  32  and  34  of the offset current generator  20  respectively control the amounts of the currents I 1  and I 2  according to the control signals Sc 1  and Sc 2 , thereby controlling the voltage signal V DIV2  so as to adjust the feedback signal Ifb to adjust the output voltage V OUT . In the control circuit  16  in  FIG. 6 , the positive terminal of the comparator  44  is coupled to the offset resistor Ros, the negative terminal thereof is coupled to the output terminal  40  of the digital to analog converter  36 . The comparator  44  generates the protecting signal Sov when the voltage signal V DIV2  is higher than the overvoltage threshold Vref_ov for turning off the power converter  2  so as to achieve the overvoltage protection. In other embodiments, the comparator  44  can be applied to the under voltage protection. In such circumstances, the positive terminal of the comparator receives the under voltage threshold provided by the digital to analog converter  36 , and the negative terminal of the comparator receives the voltage signal V DIV2 . When the voltage signal V DIV2  is lower than the under voltage threshold, a protecting signal will be generated, thereby turning off the power converter  2  to achieve the under voltage protection. 
         [0032]    In  FIG. 6 , supposed that the resistance value of the offset resistor Ros is much higher than the resistors R 1  and R 2 , when the current I 1  is injected to the offset resistor Ros, the output voltage is 
         [0000]        V   OUT =( V ref− I 1× Ros )×( R 1+ R 2)/ R 2.  (EQ-4)
 
         [0000]    When the current I 2  is sunken from the offset resistor Ros, the output voltage is 
         [0000]        V   OUT =( V ref+ I 2× Ros )×( R 1+ R 2)/ R 2.  (EQ-5)
 
         [0000]    As illustrated by the equations EQ-4 and EQ-5, the control circuit  16  in  FIG. 6  is able to adjust the output voltage V OUT  via adjusting the currents I 1  or I 2 . The adjustable steps of the output voltage V OUT  can be increased as long as the decoder  28  with a higher resolution (i.e. bit numbers) and the digital to analog converter  30  with a higher resolution are selected. No extra components (resistors or switches) are needed. Moreover, as shown in  FIG. 7 , the offset current generator  20 , the microcontroller  26 , and the digital to analog converter  36  can be integrated in the IC  52  in order to simplify the circuitry of the power converter  2 . The resolutions of the digital to analog converters  30  and  36  can be determined according to requirements. The resolution of the digital to analog converter  30  can be lower than that of the digital to analog converter  36 . 
         [0033]      FIG. 8  shows a third embodiment of the present invention, which comprises the same voltage divider  18 , offset current generator  20 , microcontroller  26 , and digital to analog converter  36  as those in  FIG. 4 . In the circuitry of  FIG. 8 , the optical coupler  6  serving as the feedback loop is also set at the secondary side of the transformer TX (not shown in  FIG. 8 ) of the power converter  2  for providing the feedback signal Ifb related to the output voltage V OUT  so as to regulating the output voltage V OUT  at a target level. In this embodiment, the voltage divider  18  divides the output voltage V OUT  to generate the voltage signal V DIV  related to the output voltage, the output terminal  38  of the digital to analog converter  36  provides the reference voltage Vref 1 , the first terminal of the offset resistor Ros is coupled to the output terminal  38  of the digital to analog converter  36  for receiving the reference voltage Vref 1 , the second terminal of the offset resistor Ros is coupled to the terminal  22  of the offset current generator  20  and outputs the reference voltage Vref 2  for determining the target level of the output voltage V OUT , the offset current generator  20  injects the current I 1  to the offset resistor Ros or sinks the current I 2  from the offset resistor Ros, and the error amplifier  42  has a positive input terminal coupled to the voltage divider  18 , a negative input terminal coupled to the digital to analog converter  30  and the offset resistor Ros, and an output terminal coupled to the control terminal  50  of the transistor Q 8  that is coupled between the optical coupler  6  and a ground terminal. The error amplifier  42  controls the current Is flowing through the transistor Q 8  according to a difference between the voltage signal V DIV  and the reference voltage Vref 2 . The optical coupler  6  determines the feedback signal Ifb according to the current Is. When the switch signal S S1  turns on the switch SW 1  of the offset current generator  20 , the variable current source  32  determines the current I 1  that is injected into the offset resistor Ros according to the control signal S C1 . When the switch signal S S2  turns on the switch SW 2  of the offset current generator  20 , the variable current source  34  determines the current I 2  that is sunken from the offset resistor Ros according to the control signal S C2 . Accordingly, the offset current generator  20  can change the offset voltage of the offset resistor Ros by adjusting the currents I 1  and I 2 , thereby determining the reference voltage Vref 2  to change the target level of the output voltage V OUT . The conventional complicated circuitry for choosing the reference voltage Vref 2  is not required. The adjustable steps of the output voltage V OUT  can be increased as long as the decoder  28  with a higher resolution (i.e. bit numbers) and the digital to analog converter  30  with a higher resolution are selected. No extra components (resistors or switches) are needed. Moreover, the offset current generator  20 , the microcontroller  26 , and the digital to analog converter  36  can be integrated in the IC in order to simplify the circuitry of the power converter  2 . The resolutions of the digital to analog converters  30  and  36  can be determined according to requirements. The resolution of the digital to analog converter  30  can be lower than that of the digital to analog converter  36 . 
         [0034]    The control circuit  16  in  FIG. 8  also includes the comparator  44 . The positive terminal of the comparator  44  is coupled to the voltage divider  18  for receiving the voltage signal V DIV . The negative terminal the comparator  44  is coupled to the output terminal  40  of the digital to analog converter  36 . The comparator  44  generates the protecting signal Sov when the voltage signal V DIV  is higher than the overvoltage threshold Vref_ov provided by the digital to analog converter  36 , thereby turning off the power converter  2  to achieve the overvoltage protection. In other embodiments, the comparator  44  can be applied for an under voltage protection. In such circumstances, the positive terminal of the comparator  44  receives the under voltage threshold provided by the digital to analog converter  36 , and the negative terminal thereof receives the voltage signal V DIV . When the voltage signal V DIV  is lower than the under voltage threshold, the protecting signal will be generated, thereby turning off the power converter  2  to achieve the under voltage protection. 
         [0035]    The offset current generator  20  in  FIGS. 4 to 8  is configured at the secondary side of the transformer TX of the power converter  2 . However, the offset current generator  20  can be configured at the primary side of the transformer TX, as illustrated in a fourth embodiment of the present invention as shown in  FIG. 9 . The power converter  2  includes a transformer TX converting an input voltage V IN  into the output voltage V OUT  by a primary side coil Np and a secondary side coil Ns. An auxiliary coil N FB  of the transformer TX is installed at the primary side of the transformer TX for detecting the output voltage V OUT  to generate the voltage signal V FB . The serially-connected resistors R 1  and R 2  form the voltage divider  18  coupled to the auxiliary coil N FB  for dividing the voltage signal V FB  to generate the voltage signal V DIV1 . In the embodiment of  FIG. 9 , the control circuit of the present invention includes the offset resistor Ros and the offset current generator  20 . The first terminal of the offset resistor Ros is coupled to the voltage divider  18  for receiving the voltage signal V DIV1 . The second terminal of the offset Ros is coupled to a feedback terminal FB of the control IC  54  and provides the voltage signal V DIV2 . The control IC  54  controls the switching of the transistor Q 9  according to the voltage signal V DIV2 , thereby regulating the output voltage V OUT . The offset current generator  20  is integrated in the IC so as to simplify the circuitry of the power converter  2 . The offset current generator  20  is coupled to the offset resistor Ros via the terminal  22  and receives the digital signal S D1  via the terminal  24 . The digital signal S D1  is provided by the microcontroller  26  as shown in  FIG. 4 . In the offset current generator  20 , the decoder  28  decodes the digital signals S D1  to generate control signals S C1  and S C2  as well as switch signals S S1  and S S2 . The digital to analog converter  30  of the offset current generator  20  includes variable current sources  32  and  34  and switches SW 1  and SW 2 . The switch SW 1  is coupled between the variable current source  32  and the second terminal of the offset resistor Ros. The switch SW 2  is coupled between the second terminal of the offset resistor Ros and the variable current source  34 . The variable current source  32  adjusts the current I 1  that is injected into the offset resistor Ros according to the control signal S C1  when the switch signal S S1  turns on the switch SW 1  of the offset current generator  20 . The variable current source  34  adjusts the current I 2  that is sunken from the offset resistor Ros according to the control signal S C2  when the switch signal S S2  turns on the switch SW 2  of the offset current generator  20 . The offset current generator  20  changes the offset voltage of the offset resistor Ros by adjusting the currents I 1  and I 2 , thereby adjusting the voltage signal V DIV2  so as to the output voltage V OUT . The adjustable steps of the output voltage V OUT  can be increased as long as the decoder  28  with a higher resolution (i.e. bit numbers) and the digital to analog converter  30  with a higher resolution are selected. No extra components (resistors or switches) are needed. 
         [0036]    In contrast with the conventional method for changing the divider ratio by changing the resistances and choosing the reference voltage, control circuits and methods according to the present invention, which control the current I 1  or I 2  to adjust the output voltage V OUT , are simpler. Moreover, the adjustable steps can be easily increased. The present invention can be applied to not only the AC to DC power converters but also the DC to DC power converters, or other type of power converters. 
         [0037]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.