Patent Publication Number: US-11646581-B2

Title: Power adapters with multiple charging ports

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     BACKGROUND 
     Portable electronic devices have internal batteries that periodically need to be recharged. In many cases, each portable electronic device comes with a power adapter designed and constructed to modify alternating current (AC) power available on wall sockets (e.g., 120V, 230V), to a suitable direct current (DC) charging voltage. For portable devices that charge by way of a Universal Serial Bus (USB) architecture, the portable electronic device may communicate with the power adapter to select a charging voltage based on the state of the portable electronic device. In the example USB architecture, the charging voltage may be selectable in a range from 3.3 Volts to 20 Volts or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of example embodiments, reference will now be made to the accompanying drawings in which: 
         FIG.  1    shows a block diagram of a power adapter in accordance with at least some embodiments; 
         FIG.  2    shows a partial block diagram, partial electrical schematic, of a power adapter in accordance with at least some embodiments; 
         FIG.  3    shows a partial block diagram, partial electrical schematic, of a power adapter in accordance with at least some embodiments; 
         FIG.  4    shows a partial schematic, partial block diagram, of a shunt regulator in accordance with at least some embodiment; 
         FIG.  5    shows a conceptual block diagram a controller in accordance with at least some embodiments; and 
         FIG.  6    shows a method in accordance with at least some embodiments. 
     
    
    
     DEFINITIONS 
     Various terms are used to refer to particular system components. Different companies may refer to a component by different names—this document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. 
     “AC” shall mean alternating current as that term is understood in electrical engineering. 
     “DC” shall mean direct current as that term is understood in electrical engineering. 
     The terms “input” and “output” when used as nouns refer to connections (e.g., electrical, software), and shall not be read as verbs requiring action. For example, an operational amplifier may define a non-inverting input, an inverting input, and a driven output. The example operational amplifier may drive a signal on the driven output responsive to the state of the signals applied to the non-inverting input and the inverting input. In systems implemented directly in hardware (e.g., on a semiconductor substrate), these “inputs” and “outputs” define electrical connections. In systems implemented in software, these “inputs” and “outputs” define parameters read by or written by, respectively, the instructions implementing the function. 
     “Assert” shall mean changing the state of a Boolean signal. Boolean signals may be asserted high or with a higher voltage, and Boolean signals may be asserted low or with a lower voltage, at the discretion of the circuit designer. Similarly, “de-assert” shall mean changing the state of the Boolean signal to a voltage level opposite the asserted state. 
     “About” shall mean the recited value plus or minus five percent (+/−5%) of the recited value. 
     “Controller” shall mean, alone or in combination, individual circuit components, an application specific integrated circuit (ASIC), a microcontroller with controlling software, a digital signal processor (DSP), a processor with controlling software, a programmable logic device (PLD), or a field programmable gate array (FPGA), configured to read inputs and drive outputs responsive to the inputs. 
     DETAILED DESCRIPTION 
     The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
     Various example embodiments are directed to power adapters with multiple charging ports. More particularly, various example embodiments are directed to power adapters (sometimes referred to as travel adapters) that have multiple charging ports that simultaneously charge multiple portable electronic devices. More particularly still, example embodiments are directed to power adapters with multiple charging ports (e.g., two or four) where each charging port may provide power different than other charging port(s), and where the power adapter may select an internal DC link voltage taking into account efficiency. More particularly still, at least some example embodiments are directed to shunt regulators that select a link voltage for a DC link within the power adapter taking into account voltages and/or currents provided by the various charging ports. The specification first turns to an overall system to orient the reader. 
       FIG.  1    shows a block diagram of an example power adapter. In particular,  FIG.  1    shows an example power adapter  100  comprising an AC-DC converter  102 , a DC-DC converter  104 , a DC-DC converter  106 , a first charging port  108 , and a second charging port  110 . The example AC-DC converter  102  is designed and constructed to couple to AC power, such as may be available in a wall socket. The AC power from the wall socket may be fed to a rectifier (not shown), such as a half-bridge rectifier or a full-bridge rectifier, to create a DC supply voltage having a magnitude about the same as the peak voltage of the AC power. The AC-DC converter  102  lowers the magnitude of the DC supply voltage to create a link voltage on the DC link  112 . In example cases, the link voltage is controllable or selectable in a range of voltages (e.g., 3.3V to 20V) to implement a strategy of increasing the efficiency of the power adapter  100  taking into account parameters such as the efficiency of the AC-DC converter  102 , the number of devices being charged by way of the charging ports  108  and  110 , and the bus voltages on the charging ports  108  and  110  (all discussed more below). 
     In example cases, the AC-DC converter  102  is a flyback power converter comprising a primary side  114  and a secondary side  116 . In the example case of a flyback power converter, the primary side  114  and secondary side  116  are separated or delineated by a transformer (not specifically shown), with a primary winding within the primary side  114  and a secondary winding within the secondary side  116 . However, other types of converters may be used as part of the AC-DC converter  102 , and thus the description that follows based on a flyback power converter shall not be considered a limitation. 
     The example secondary side  116  comprises a shunt regulator  118 . The shunt regulator  118  is designed and constructed to sense a bus voltage of a charging bus associated with the charging port  108 , and also to sense a bus voltage of a charging bus associated with the charging port  110 . There are several variations regarding sensing the bus voltages, and those variations are discussed more below. For now, however,  FIG.  1    shows the shunt regulator  118  coupled to the charging buses of the charging ports  108  and  110  by way of dashed lines. The shunt regulator  118  is electrically coupled within a feedback path (discussed more below) from the secondary side  116  to the primary side  114 , the feedback path provided such that primary side  114  can regulate the link voltage supplied on the DC link  112 . The example shunt regulator  118  selects a setpoint for the link voltage of the DC link  112 , and to which setpoint the primary side  114  controls. In particular, by creating a signal indicative of a setpoint voltage within the feedback path, the example shunt regulator  118  selects the setpoint for the link voltage of the DC link  112 . 
     Still referring to  FIG.  1   . The example DC-DC converter  104  defines a link input  120  coupled to the DC link  112 , a charging bus  122  coupled to the charging port  108 , an enable input  124 , and a communicational channel  126 . The DC-DC converter  104  changes the voltage on the DC link  112  to a voltage selected by a portable electronic device (not shown) coupled to the charging port  108 . In some cases, the DC-DC converter  104  is a buck-boost converter, meaning that the DC-DC converter  104  can raise or increase the link voltage supplied to the link input  120  and apply the increased voltage to the charging bus  122 , or lower or decrease the voltage supplied on the link input  120  and apply the decreased voltage to the charging bus  122 , as needed. In other cases, the DC-DC converter  104  may be buck-only converter. 
     The example DC-DC converter  106  defines a link input  128  coupled to the DC link  112 , a charging bus  130  coupled to the charging port  110 , an enable input  132 , and a communicational channel  134 . The DC-DC converter  106  changes the voltage on the DC link  112  to a voltage selected by a portable electronic device (not shown) coupled to the charging port  110 . In some cases, the DC-DC converter  106  is a buck-boost converter, and in other cases, the DC-DC converter  106  may be buck-only converter. 
     The example power adapter  100  further comprises bus controller  136 . In example cases, the charging ports  108  and  110  are operated under the USB power delivery (PD) specification. The bus controller  136  is labeled as USB-PD controller in the figure, and hereafter is referred to as the USB-PD controller  136 , though other types of busses may be implemented. The example USB-PD controller  136  defines an enable output  138  coupled to the enable input  124  of the DC-DC converter  104 , an enable output  140  coupled to the enable input  132  of the DC-DC converter  106 , a communication channel  142  coupled to the communication channels  126  and  134 , a sense input  144  coupled to the charging bus  122 , a sense input  146  coupled to the charging bus  130 , a plurality of data lines  148  coupled to the charging port  108 , and a plurality of data lines  150  coupled to the charging port  110 . 
     Consider, for purposes of explanation, that the AC-DC converter  102  is operational and producing a link voltage on the DC link  112 , and that no portable electronic devices are coupled to the charging ports  108  and  110 . In such a situation, the USB-PD controller  136  may de-assert both the enable output  138  and enable output  140  such that both DC-DC converters  104  and  106  are disabled and no voltages are provided to the charging buses  122  and  130 . 
     Now consider that a portable electronic device is coupled to the charging port  108 . When a portable electronic device is coupled to the charging port  108 , the USB-PD controller  136  and the portable electronic device communicate over the plurality of data lines  148  to establish a charging voltage, which for the USB-PD specification may range from 3.3V to 20V. Once a charging voltage is established, the USB-PD controller  136  communicates a required bus voltage to the DC-DC converter  104  by way of the communication channels  142  and  126 , and enables the DC-DC converter  104  by asserting the enable output  138 . The DC-DC converter  104 , in turn, supplies the selected bus voltage to the charging bus  122  to charge the portable electronic device coupled to the charging port  108 . 
     Now consider that another portable electronic device is coupled to the charging port  110 . As before, when a portable electronic device is coupled to the charging port  110 , the USB-PD controller  136  and the portable electronic device communicate over the plurality of data lines  150  to establish a charging voltage. Once a charging voltage is established, the USB-PD controller  136  communicates a required bus voltage to the DC-DC converter  106  by way of the communication channels  142  and  134 , and enables the DC-DC converter  106  by asserting the enable output  140 . The DC-DC converter  106 , in turn, supplies the selected bus voltage to the charging bus  130  to charge the portable electronic device coupled to the charging port  110 . 
     As alluded to above, there could be a wide range of bus voltages applied to the charging buses  122  and  130  (and possibly others). In the example case of systems operated under the USB-PD specification, the low end of the voltage range may be 3.3V, while the high end of the voltage range may be 20V or more. Various example systems and methods are directed to selecting and implementing a link voltage for the DC link  112  that provides increased efficiency. In some cases, the selection of the link voltage may consider the efficiency of conversion of the DC-DC converters  104  and  106  without regard to the efficiency of conversion of the AC-DC converter  102 . In other cases, the selection of the link voltage may consider both the efficiency of the AC-DC converter  102  and the DC-DC converters  104  and  106 . That is, a link voltage may be selected that provides less than peak performance for the one or both of the DC-DC converters  104  and  106 , but taking into account the efficiency of the AC-DC conversion process the link voltage selected may provide better overall efficiency for the power adapter  100 . In example systems, the shunt regulator  118  selects a setpoint for the link voltage of the DC link  112  based on the bus voltage of the charging bus  122  and the bus voltage of the charging bus  130 . With a setpoint for link voltage of the DC link  112  selected, the AC-DC converter  102  then regulates the link voltage to the setpoint while the DC-DC converters supply their respective bus voltages on their respective charging buses. 
     Before proceeding it is noted that the power adapter  100  of  FIG.  1    comprises two charging ports  108  and  110 . However, a power adapter having two charging ports is merely an example, and the various embodiments of selecting a setpoint for the link voltage of the DC link  112  may be extended to any power adapter having two or more charging ports (e.g., three, four, five, six, or eight). The specification now turns to an example system in greater detail. 
       FIG.  2    shows a partial block diagram, partial electrical schematic, of an example power adapter  100 . In particular, on the left side of  FIG.  2    is an example secondary side  116 , and with the primary side  114  omitted so as not to further complicate the figure. On the right side of  FIG.  2    are the example DC-DC converters  104  and  106  and related components associated with the example charging ports  108  and  110 . The various components on the right side of  FIG.  2    are the same as in  FIG.  1   , carry the same reference numbers, and thus will not be introduced again so as not to unduly lengthen the description. 
     The example secondary side  116  includes the shunt regulator  118  and various additional components. In particular, the example secondary side  116  comprise a secondary winding  200  defining a first lead  202  and a second lead  204 . The first lead  202  couples to a first lead of an inductor  206 , and the second lead of the inductor  206  defines the DC link  112 . The example secondary side  116  further comprises an electrically-controlled switch  208  operating as a synchronous rectifier. In the example of  FIG.  2   , the electrically-controlled switch  208  is shown as a field effect transistor (FET), and will hereafter be referred to as SR FET  208 . The SR FET  208  defines a first connection or drain coupled to the second lead  204  of the secondary winding  200 , a second connection or source coupled a return or common on the secondary side  116 , and a control input or gate  210 . An output capacitor  212  has first lead coupled to the first lead  202  of the secondary winding  200 , and a second lead coupled to the common the secondary side  116 . Another output capacitor  214  has a first lead coupled to the DC link  112 , and a second lead coupled to the common on the secondary side  116 . 
     In order to control the SR FET  208 , the example secondary side  116  comprises an SR controller  216  in the form of a packaged integrated circuit device. The example SR controller  216  defines a gate terminal coupled to the gate  210  of the SR FET  208 , a drain terminal coupled to the drain of the SR FET  208 , and various additional terminals to enable operation (e.g., an input voltage terminal coupled to the output capacitor  212 , a ground terminal coupled to common on the secondary side  116 ). 
     In operation, the transformer, of which secondary winding  200  is a part, is operated as a flyback transformer. That is, during a charge mode a primary FET (not shown) on primary side  114  ( FIG.  1   ) is made conductive. Current flows through the primary FET and a primary winding (not shown), storing energy in the field surrounding the transformer. During the charge mode, the voltage on the secondary winding  200  reverse biases the body diode  207  of the SR FET  208 , the SR FET  208  is non-conductive, and thus no current flows. During the charge mode, current and voltage are supplied to the DC link  112  and downstream components by way of the output capacitors  212  and  214  and the field associated with current flow through the inductor  206 . 
     Still referring to  FIG.  2   , when the charge mode ends the primary FET (not shown) on the primary side  114  ( FIG.  1   ) is made non-conductive and the discharge mode begins. During the discharge mode, the voltage on the secondary winding  200  reverses, which forward biases the body diode  207  of the SR FET  208 , and thus current begins to flow from the first lead  202  to the output capacitor  212  and other downstream components. The example SR controller  216  senses the current flow, and makes the SR FET  208  conductive to reduce conduction losses. As the field surrounding the transformer collapse, the secondary winding  200  provides voltage and current to the downstream components. In some case, the field around the transformer fully discharges before the next charge mode begins, and in other cases the secondary winding  200  may still be providing current when the next charge mode begins. Regardless, when SR controller  216  senses that the positive current flow through the secondary winding  200  has stopped, the SR controller  216  makes the SR FET  208  non-conductive, and the process starts anew. 
     In example systems, the energy transferred across the transformer, and thus the voltage developed and maintained on the DC link  112 , is controlled by the primary side  114  ( FIG.  1   ). For example, if the DC link  112  voltage droops below a setpoint (e.g., because of increased power drawn by portable electronic devices coupled to the charging ports), the AC-DC converter  102  increases the amount of energy transferred across the transformer in each charge-to-discharge mode transition to maintain the setpoint for the link voltage of the DC link. Increasing the energy transferred may include increasing the switching frequency on the primary side (e.g., increasing the frequency of the charge modes), increasing the pulse width of the signals applied to the primary FET (e.g., making each charge mode longer), or both. Oppositely, if the DC link  112  voltage rises above a setpoint (e.g., because of decreased power drawn by portable electronic devices coupled to the charging ports), the AC-DC converter  102  decreases the amount of energy transferred across the transformer to maintain the setpoint for the link voltage of the DC link. Decreasing the energy transferred may include decreasing the switching frequency on the primary side (e.g., decreasing the frequency of the charge modes), decreasing the pulse width of the signals applied the primary FET (e.g., making each charge mode shorter), or both. 
     In order for the primary side  114  ( FIG.  1   ) to know the state of the voltage on the secondary side  116 , the secondary side  116  implements a feedback path to the primary side  114 . In the example system, and taking into account the galvanic isolation implemented by the transformer, the feedback path includes an opto-coupler. In particular, the example secondary side  116  comprises a light emitting diode (LED)  218  having an anode coupled to the upper lead of the output capacitor  212 , and a cathode coupled to the common the secondary side  116  by way of the shunt regulator  118 . Ignoring for a moment the shunt regulator  118 , current flows through the LED  218  producing photons, with the rate of photon production directly proportional to the voltage on the output capacitor  212 . The second half of the opto-coupler is on the primary side  114  and comprises a transistor with base optically coupled to (though electrically isolated from) the LED  218 . The conductance of the transistor is based on the rate of photon production by the LED  218 . It follows that a primary side controller (not shown) is provided a feedback signal (created by the transistor of the opto-coupler) based on the voltage on the secondary side  116 . The primary-side controller will have an internal setpoint to which it controls; however, in accordance with example systems the setpoint for the link voltage of the DC link  112  is manipulated by current flow through the LED  218 , the manipulation implemented by the shunt regulator  118 . 
     Still referring to  FIG.  2   , the example shunt regulator  118  defines a cathode terminal  220  (labeled K), an anode terminal  222  (labeled A), a reference terminal  224  (labeled R), a first sense terminal  226  (labeled R 1 ), and a second sense terminal  228  (labeled R 2 ). In example cases, the shunt regulator  118  is a packaged semiconductor device, and an example set of internal components is discussed in greater detail below. The cathode terminal  220  is coupled to the cathode of the LED  218 . The anode terminal  222  is coupled to the common on the secondary side  116 . The reference terminal  224  is coupled to the DC link  112  by way of a voltage divider circuit comprising resistor  230  and resistor  232 . In particular, resistors  230  and  232  are coupled in series between the DC link  112  and the common on the secondary side  116 , and the reference terminal  224  is coupled to the node between the resistors  230  and  232 . The sense terminal  226  is coupled to the charging bus  122  by way of a voltage divider circuit comprising resistor  234  and resistor  236 . In particular, resistors  234  and  236  are coupled in series between the charging bus  122  and the common on the secondary side  116 , and the sense terminal  226  is coupled to the node between the resistors  234  and  236 . The sense terminal  228  is coupled to the charging bus  130  by way of a voltage divider circuit comprising resistor  238  and resistor  240 . In particular, resistors  238  and  240  are coupled in series between the charging bus  130  and the common on the secondary side  116 , and the sense terminal  228  is coupled to the node between the resistors  238  and  240 . 
     In accordance with example embodiments, the shunt regulator  118  creates a signal indicative of a setpoint voltage across the cathode terminal  220  and the anode terminal  222 . That is, the shunt regulator  118  is designed and constructed to control current flow into the cathode terminal  220 , and out of the anode terminal  222 , to control the voltage developed at the cathode terminal  220  with respect to the anode terminal  222  (e.g., common on the secondary side  116 ). Considered from the standpoint of current flow, by controlling or limiting current flow through the shunt regulator  118 , the shunt regulator  118  controls current flow through the LED  218  and thus controls the rate of photon production. Considered from the standpoint of voltage developed across the cathode terminal  220  and the anode terminal  222 , by controlling the voltage across the cathode terminal  220  and the anode terminal  222  the shunt regulator  118  controls the rate of photon production by the LED  218 . Because the shunt regulator  118  is disposed within the feedback path to the primary side  114  ( FIG.  1   ), the signal indicative of setpoint voltage across the cathode terminal  220  and the anode terminal  222  sets or controls the link voltage of the DC link  112 . 
     Consider, for purposes of explanation, that the AC-DC converter  102  is operational and producing a link voltage on the DC link  112 , and that no portable electronic devices are coupled to the charging ports  108  and  110 . In such a situation, the USB-PD controller  136  may de-assert both the enable output  138  and the enable output  140  such that both DC-DC converters  104  and  106  are disabled and no voltages are provided to the charging buses  122  and  130 . In such a situation, the voltage provided to the reference terminal  224  controls the setpoint for the link voltage of the DC link  112 . In one example situation, the voltage created by the voltage divider comprising resistors  230  and  232  creates a reference voltage of about 2.5V on the reference terminal  224 . The reference voltage of 2.5V results in a certain current flow through the shunt regulator  118  from the cathode terminal  220  to the anode terminal  222 . Further consider that having 2.5V applied to the reference terminal  224  results in a link voltage on the DC link  112  of 20V. The example of the 2.5V reference voltage resulting in a link voltage of 20V is just that, an example. The system can be designed and constructed such that 2.5V results in different link voltages, and different link voltages may be implemented when none of the charging ports are charging portable electronic devices. 
     Now consider that a portable electronic device is coupled to the charging port  108 , and that no portable electronic device is coupled to the charging port  110 . As before, when a portable electronic device is coupled to the charging port  108 , the USB-PD controller  136  and the portable electronic device communicate over the plurality of data lines  148  to establish a charging voltage, which for USB-PD may range from 3.3V to 20V. Once a charging voltage is established, the USB-PD controller  136  communicates a bus voltage to the DC-DC converter  104  by way of the communication channels  142  and  126 , and enables the DC-DC converter  104  by asserting the enable output  138 . The DC-DC converter  104 , in turn, supplies the selected charging voltage to the charging bus  122  to charge the portable electronic device coupled to the charging port  108 . 
     In this example situation where only one portable electronic device is coupled to the power adapter  100  by way of charging port  108 , further consider that the bus voltage on the charging bus  122  is selected to be 3.3V. While possible for the DC-DC converter  104  to buck the link voltage at the link input  120  down to 3.3V, better efficiency may be achieved by the DC-DC converter  104  if the link voltage is closer to bus voltage of the example charging bus  122  (e.g., the link voltage reduced to 3.3V). In example situations the shunt regulator  118  senses, by way of the sense terminal  226 , the bus voltage of the charging bus  122 , and the shunt regulator  118  senses, by way of the sense terminal  228 , the bus voltage of the charging bus  130 . In the example situation of reducing the link voltage from 20V to 3.3V, and responsive to the changing internal reference voltage VREF, the current/voltage across the shunt regulator  118  is changed to increase the rate of photon production as a signal to decrease the amount of energy transferred from the primary side  114  to the secondary side  116 . 
     In the example situation with a portable electronic device coupled the charging port  108  and no portable electronic device coupled to the charging port  110 , if the portable electronic device and the USB-PD controller  136  negotiate a different charging voltage for the charging bus (e.g., 5V, 10V, 15V), the shunt regulator  118  may sense the selected bus voltage on the charging bus  122  and set the link voltage for the DC link  112  accordingly. In the situation where the bus voltage on the charging bus  122  is 20V, and the bus voltage for the charging bus  122  is selected to be 20V, the shunt regulator  118  may make no change to the link voltage for the DC link  112 . 
     Still considering the example situation with a portable electronic device coupled to the charging port  108  and no portable electronic device coupled to the charging port  110 . In the description to this point the shunt regulator  118  selected a setpoint for the link voltage considering only efficiency of the active DC-DC converter, in this case DC-DC converter  104 . However, the AC-DC converter  102  likewise has operational states in which better efficiency may be achieved. Thus, there may be situations in which particular link voltages result in a loss of efficiency by the AC-DC converter  102  that is greater than a gain in efficiency of the DC-DC converter  104  a particular link voltage may provide. Thus, in yet still other examples, the shunt regulator  118  may select a setpoint for the link voltage that takes into account overall efficiency of the power adapter  100  (e.g., both the AC-DC converter  102  and the DC-DC converter  104 ). 
     An AC-DC converter  102  operated as a flyback power converter has lower efficiency at lower link voltages, and higher efficiency at higher link voltages, assuming the same power rating. Thus, the shunt regulator  118  may implement a lower boundary for link voltages such that the AC-DC converter  102  operates at better efficiency, and the DC-DC converter  104  operates at a lower efficiency, but where the overall efficiency of the power adapter  100  is better than if efficiency of the AC-DC converter  102  is not taken into account. For example, while an AC-DC converter  102  operated in the form of a flyback converter coupled to AC wall voltages (e.g., 120V, 230V) may be able to create a link voltage of 3.3V or 5V on the DC link  112 , the overall efficiency of the power adapter  100  may be low. Thus, in example cases the shunt regulator  118  may be designed and constructed to implement a lower boundary of the link voltage for the DC link  112  higher than charging voltage of the charging bus  122 . In particular, for an example AC-DC converter  102  implemented as a flyback power converter, the inventors of the specification have found that better overall efficiency may be achieved by implementing a link voltage of about 10V for the DC link  112  in spite of the fact the selected bus voltage for the charging bus  122  is below 10V (e.g., 3.3V or 5V). 
     Thus, as before the shunt regulator  118  senses, by way of the sense terminal  226 , the bus voltage of the charging bus  122 , and the shunt regulator  118  senses, by way of the sense terminal  228 , the bus voltage of the charging bus  130 . As before, the shunt regulator  118  creates a signal indicative of a setpoint voltage by changing the current flow through the cathode terminal  220  and anode terminal  222 . However, in this case signal indicative of setpoint voltage implements a predetermined lower boundary for the link voltage for the DC link  112 . In the example situation of a charging voltage for the charging bus  122  below 10V (e.g., 3.3V or 5V), and reducing the link voltage from 20V, the current/voltage across the shunt regulator  118  may be lowered to the lower boundary for the link voltage—in this example case about 10V. 
     Couple of points before proceeding. First, the “no charging” link voltage of 20V on the DC link  112  is merely an example. The “no charging” voltage may be any voltage selected by the circuit designer, and it follows that in certain situations the shunt regulator  118  may raise the link voltage of the DC link  112  responsive to a selected charging voltage for the charging bus  122 . Moreover, a reference voltage of 2.5V resulting in a link voltage of 20V on the DC link  112  is also merely an example, and the relationship of the link voltage to the reference voltage may be selected by adjusting the voltage divider resistors  230  and  232  at the discretion of the circuit designer. The description of operation of the system in the example situation of a portable electronic device coupled to the charging port  108  and no portable electronic device coupled to the charging port  110  is equally applicable to the reverse situation—no portable electronic device coupled to charging port  108  and a portable electronic device coupled to the charging port  110 —and thus a description of the reverse situation will not be presented so as not to unduly lengthen the specification. Finally, the lower boundary of the link voltage of about 10V for the DC link  112  is a specific example for the case of a specific flyback power converter. Other flyback converters, and other AC-DC converters generally (e.g., a forwarding converter, or resonant-primary (LLC) converter) may have different efficiency considerations, and thus different lower boundary link voltages for the DC link  112  are possible, including cases where no lower boundary is implemented. 
     Now consider that a portable electronic device is coupled to the charging port  108  and another portable electronic device is coupled to the charging port  110 . As before, when a portable electronic device is coupled to the charging port  108 , the USB-PD controller  136  and the portable electronic device communicate over the plurality of data lines  148  to establish a charging voltage. Once a charging voltage is established, the USB-PD controller  136  communicates a bus voltage to the DC-DC converter  106  by way of the communication channels  142  and  126 , and enables the DC-DC converter  104  by asserting the enable output  138 . The DC-DC converter  104 , in turn, supplies the selected bus voltage to the charging bus  122  to charge the portable electronic device coupled to the charging port  108 . Moreover, when a portable electronic device is coupled to the charging port  110 , the USB-PD controller  136  and the portable electronic device communicate over the plurality of data lines  150  to establish a charging voltage. Once a charging voltage is established, the USB-PD controller  136  communicates a bus voltage to the DC-DC converter  106  by way of the communication channels  142  and  134 , and enables the DC-DC converter  106  by asserting the enable output  140 . The DC-DC converter  106 , in turn, supplies the selected bus voltage to the charging bus  130  to charge the portable electronic device coupled to the charging port  110 . 
     The shunt regulator  118  senses, by way of the sense terminal  226 , the bus voltage of the charging bus  122 , and the shunt regulator  118  senses, by way of the sense terminal  228 , the bus voltage of the charging bus  130 . As before, the shunt regulator  118  creates a signal indicative of a setpoint voltage by changing the current flow through the cathode terminal  220  and anode terminal  222 . However, in the situation in which two (or more) charging ports are charging portable electronic devices, there may be a large range of bus voltages on the charging buses. For example, one charging bus may implement a bus voltage on the low end of the voltage range (e.g., 3.3V), while another charging bus may implement a bus voltage on the high end of the voltage range (e.g., 20V). There are several variations regarding how the shunt regulator  118  may select a setpoint for the link voltage of the DC link  112  when differing bus voltage on the charging buses are used, and each variation is discussed below. 
     In one example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is about the same as the higher of the bus voltages. In the example case of one selected bus voltage of 3.3V on a charging bus and another selected bus voltage of 20V on a charging bus, the shunt regulator  118  may select the setpoint link voltage to be 20V. Selecting the setpoint for the link voltage that is the same as the higher than the highest bus voltage may be used in situations in which the DC-DC converters  104  and  106  are both buck-only converters, but such an implementation is not limited to situations of buck-only converters. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is between the bus voltages of the charging buses (e.g., average of the bus voltages). In the example case of one selected bus voltage of 3.3V and another selected bus voltage of 20V, the shunt regulator  118  may select the setpoint link voltage to be 11.65V (e.g., the average value). Selecting the setpoint for the link voltage that is between the bus voltages of the charging buses may be used in situations in which the DC-DC converters  104  and  106  are both buck-boost converters. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is about the same as the lower of the bus voltages. In the example case of one selected bus voltage of 3.3V and another selected bus voltage of 20V, the shunt regulator  118  may select the setpoint link voltage to be 3.3V. Selecting the setpoint for the link voltage that is between the charging voltages may be used in situations in which the DC-DC converters  104  and  106  are both buck-boost converters. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is higher than both charging voltages. In the example case of one selected bus voltage of 3.3V and another selected bus voltage of 20V, the shunt regulator  118  may select the setpoint link voltage to be 21V or more. Selecting the setpoint for the link voltage that is between the charging voltages may be used in situations in which the DC-DC converters  104  and  106  are both buck-only converters, but such an implementation is not limited to situations of buck-only converters. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage based on a predetermined mathematical relationship that results in better efficiency taking into consideration the bus voltages of each charging bus, and the efficiency of each of the DC-DC converter  104  and  106 . 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage according to any of the previous examples, but also implementing the lower boundary of the link voltage that takes into account efficiency of the AC-DC converter  102 . In the example case of one selected bus voltage of 3.3V, another selected bus voltage of 20V, and the lower boundary of the link voltage being 10V, the shunt regulator  118  may select the setpoint link voltage to be 10V (i.e., the lower boundary), 11.65V (e.g., the average value), 20V, or above 20V, all at the discretion of the circuit designer. 
     In the examples discussed with respect to  FIG.  2   , the shunt regulator  118  senses the charging voltages in an analog sense. That is, the shunt regulator  118  senses a signal indicative of the bus voltage of the charging bus  122 , and senses a signal indicative of the bus voltage of the charging bus  130 , and selects the setpoint for the link voltage based on the signals. However, in other embodiments the sensing of the bus voltage of the charging buses may take place in other ways. 
       FIG.  3    shows a partial block diagram, partial electrical schematic, of an example power adapter. In particular, on the left side of  FIG.  3    is the example secondary side  116 , and on the right side of  FIG.  3    are the example DC-DC converters  104  and  106  and related components associated with the example charging ports  108  and  110 . Many of the components of  FIG.  3    are the same as  FIG.  2   , and carry the same reference numbers, and thus will not be introduced again so as not to unduly lengthen the description. In  FIG.  3   , however, the shunt regulator  118 , though possibly the same shunt regulator  118  of  FIG.  2   , senses the charging voltages differently than in  FIG.  2   . Moreover, the USB-PD controller  136 , though possibly the same USB-PD controller  136  of  FIG.  2   , directly communicates with the shunt regulator  118 . Operation of these two devices will be addressed in turn, starting with the USB-PD controller  136 . 
     The example USB-PD controller  136  further comprises additional communication channels  300  and  302 . The example communication channel  300  is coupled to the sense terminal  226 . The example communication channel  302  is coupled to the sense terminal  228 . In one example, the communication channels  300  and  302  are distinct communication channels. In another example, however, the communication channels  300  and  302  implement two conduction pathways of a serial communication channel (e.g., the SCL and SDA lines of an Inter-Integrated Circuit (I 2 C) serial communication bus). Further, while the communication channel  142  is shown as separate from the communication channels  300  and  302 , in yet still further case a single serial communication bus may be coupled to and communicate among the USB-PD controller  136 , the DC-DC converters  104  and  106 , and the shunt regulator  118 . 
     In examples in which the shunt regulator  118  is communicatively coupled to the USB-PD controller  136 , the shunt regulator  118  may sense the bus voltages of the charging buses  122  and  130  through communication with the USB-PD controller  136 . In particular, in example cases the shunt regulator  118  may receive a value (e.g. a digital value) indicative of the bus voltage of the charging bus  122  by way of the communication channels  300  and  302 . Further, the shunt regulator  118  may receive a value (e.g. a digital value) indicative of the bus voltage of the charging bus  130  by way of the communication channels  300  and  302 . Using the values, the shunt regulator  118  may select and implement a setpoint for the link voltage of the DC link  112  as discussed above. In another example case, a setpoint for the link voltage of the DC link  112  is determined in the USB-PD controller  136  and the determined setpoint is directly communicated from the USB-PD controller  136  to the shunt regulator  118  through the communication channels  300  and  302 . Moreover, the magnitude of the selected setpoint for the link voltage of the DC link  112  may be consistent with any of the examples above (e.g., link voltage equals highest bus voltage, link voltage equals lowest bus voltage, link voltage equals average of the bus voltages, link voltage higher than highest bus voltage, and variants that implement the lower boundary link voltage). 
     The example power adapter  100  of  FIG.  3    may also implement additional cases based on additional information provided from the USB-PD controller  136 . In particular, in addition to receiving values indicative of the bus voltage for each charging bus, the USB-PD controller  136  may send, and the shunt regulator  118  may receive, values indicative of an amount of power provided to the portable electronic device coupled to each respective charging port. For example, the USB-PD controller  136  may send values indicative of the current being provided to each charging port. Using the additional information regarding power, the shunt regulator  118  may select a setpoint link voltage for the DC link  112  that takes into account values indicative of power provided, where the setpoint for the link voltage may be different than selections that take into account only bus voltage of each charging bus. 
     In one example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is about the same as the charging voltage being provided by the charging port providing the most power. Consider, as an example, one selected bus voltage of 3.3V providing 3A (i.e., 9.9 Watts), and another selected bus voltage of 20V providing 0.1A (i.e., 2 Watts). The example shunt regulator  118  may select the setpoint link voltage to be 3.3V to be the same as the charging port providing more power. That is, better overall efficiency may be achieved by improving the efficiency of the highest loaded DC-DC converter. Consider, as another example, one selected bus voltage of 3.3V providing 0.1A (i.e., 0.33 Watts), and another selected bus voltage of 20V providing 1A (i.e., 20 Watts). The example shunt regulator  118  may select the setpoint link voltage to be 20V to be the same as the charging port providing more power. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage that is higher than the bus voltage providing the most power, yet still possibly below the highest bus voltage. Consider, as an example, one selected charging voltage of 3.3V providing 3A (i.e., 9.9 Watts), and another selected charging voltage of 20V providing 0.1A (i.e., 2 Watts). The example shunt regulator  118  may select the setpoint link voltage to be 5.0V to be higher than the bus voltage of the charging port providing more power, but still less than the highest charging voltage. Consider, as another example, one selected bus voltage of 3.3V providing 1A (i.e., 3.3 Watts), and another selected charging voltage of 20V providing 2A (i.e., 40 Watts). The example shunt regulator  118  may select the setpoint link voltage to be higher than 20V. 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage based on mathematical relationship that results in better efficiency taking into consideration the bus voltages, power provided by each charging bus, and the efficiency of each DC-DC converter  104  and  106 . 
     In another example case, the shunt regulator  118  may be designed and constructed to select a setpoint for the link voltage according to any of the previous examples, but also implementing the lower boundary of the link voltage that takes into account efficiency of the AC-DC converter  102 . In the example case of one selected charging voltage of 3.3V at 3A, another selected charging voltage of 20V at 0.1A, and the lower boundary of the link voltage being 10V, the shunt regulator  118  may select the setpoint link voltage to be 10V (i.e., the lower boundary) or higher. 
       FIG.  4    shows a partial schematic, partial block diagram, of an example shunt regulator  118 . In particular,  FIG.  4    shows that the shunt regulator  118  may comprise one or more substrates of semiconductor material (e.g., silicon), such as substrate  400 , encapsulated within packaging to create a packaged semiconductor product. Bond pads or other connection points of the substrate  400  couple to terminals of the shunt regulator  118  (e.g., cathode terminal  220 , anode terminal  222 , etc.). While a single substrate  400  is shown, in other cases multiple substrates may be combined to form the shunt regulator  118  (e.g., a multi-chip module). The example shunt regulator  118  has all the previous terminals introduced, and further including an offset terminal  402  (labeled OS). The purpose of the offset terminal  402  is discussed in greater detail below. 
     Internally, the example shunt regulator  118  comprises a semiconductor circuit  404 . In example cases, the semiconductor circuit  404  is coupled between the cathode terminal  220  and the anode terminal  222  and operates as a controllable Zener diode with its anode coupled to the anode terminal  222 , and its cathode coupled to the cathode terminal  220 —hence the naming convention. In many cases, the functionality is implemented by transistors, diodes, and an operational amplifier, not an actual Zener diode. In particular, the example semiconductor circuit  404  includes transistor  406 . The example transistor  406  is shown as an NPN-type junction transistor, but other junction transistor types, and other types of transistors (e.g. FETs) may be used. The example transistor  406  defines a first connection or collector coupled to the cathode terminal  220 , a second connection or emitter coupled to the anode terminal  222 , and a base. The semiconductor circuit  404  further includes a diode  408  having an anode coupled to the anode terminal  222  and a cathode coupled to the cathode terminal  220 . Another diode  410  defines a cathode coupled to the cathode terminal  220  and an anode. 
     The example semiconductor circuit  404  further comprises an operational amplifier  412  defining a non-inverting input  414  coupled to the anode of the diode  410 , an inverting input  416 , and a control output  418  coupled to the base of the transistor  406 . The non-inverting input  414  is also coupled to the reference terminal  224 . 
     The example shunt regulator  118  further defines the controller  420 . The controller  420  is coupled to the first sense terminal  226 , the second sense terminal  228 , and the offset terminal  402 . In example systems, the controller  420  is designed and constructed to: sense, by way of first sense terminal  226 , the first bus voltage supplied to the first charging port; and sense, by way of the second sense terminal  228 , the second bus voltage supplied to a second charging port. The example controller  420  creates a signal indicative of a setpoint voltage across the cathode terminal  220  and anode terminal  222 , the creation by selecting an internal reference voltage VREF  422  driven to the inverting input  416  of the operational amplifier  412  referenced to the anode terminal  222 . The internal reference voltage VREF  422  is selected based on the first bus voltage and the second bus voltage as discussed above. 
     Still referring to  FIG.  4   , in example cases in which the shunt regulator  118  senses the bus voltages in analog form, the controller  420  is designed and constructed to: sense, by way of the sense terminal  226 , an analog signal indicative of the first bus voltage; and sense, by way of the sense terminal  228 , an analog signal indicative of the second bus voltage. If additional charging ports are implemented, the shunt regulator  118  may include additional sense terminals. Based on the analog signals received on the example sense terminals  226  and  228 , the controller  420  generates the internal reference voltage VREF  422 . Stated slightly differently, the controller  420  creates the signal indicative of the setpoint voltage across the cathode terminal  220  and anode terminal  222 , in all the example variants above, by selecting and generating the internal reference voltage VREF  422 . 
     Referring simultaneously to  FIGS.  2  and  4   . Consider, as an example, that the node between the voltage divider comprising resistors  230  and  232  creates a voltage ratio of 10:1 (voltage on the reference terminal  224  is one-tenth of the link voltage of the DC link  112 ). Further consider that the node between the voltage divider comprising resistors  234  and  236  creates a voltage ratio of 10:1 (voltage on the sense terminal  226  is one-tenth of the bus voltage of the charging bus  122 ). Further consider that the node between the voltage divider comprising resistors  238  and  240  creates a voltage ratio of 10:1 (voltage on the sense terminal  228  is one-tenth of the bus voltage of the charging bus  130 ). Further consider that the voltage on the sense terminal  226  is 1.5V (which means the bus voltage of the charging bus  122  is 15V in the example), and the voltage on the sense terminal  228  is 0.5V (which means bus voltage on the charging bus  130  is 5V in the example). In example systems in which the link voltage of the DC link  112  is selected to be the higher of the bus voltages of the charging buses, the controller  420  generates the internal reference voltage VREF  422  to be 1.5V, resulting in the link voltage of the DC link  112  of 15V. In example systems in which the link voltage of the DC link  112  is selected to be the lower of the bus voltages of the charging buses, the controller  420  generates the internal reference voltage VREF  422  to be 0.5V, resulting in the link voltage of the DC link  112  of 5V. As yet another example, in example systems in which the link voltage of the DC link  112  is selected to be between the bus voltages of the charging buses (e.g., the average), the controller  420  generates the internal reference voltage VREF  422  to be 1.0V, resulting in the link voltage of the DC link  112  of 10V. 
     Returning to  FIG.  4   , in example cases in which the shunt regulator  118  receives voltages in digital form from the USB-PD controller  136 , the controller  420  is designed and constructed to: receive a value indicative of the first bus voltage (and possibly a value indicative of a first bus power) by way of a communication channel associated with the sense terminal  226 ; and receive a value indicative of the second bus voltage (and possibly a value indicative of a second bus power) by way of a communication channel associated with the sense terminal  228 . As discussed above, the communication channels may be dedicated channels, or the sense terminals  226  and  228  together may form a shared communication channel in the example form of a serial communication channel (e.g., I 2 C). Based on the values received on the sense terminals  226  and  228 , the controller  420  generates the internal reference voltage VREF  422 . Stated slightly differently, the controller  420  creates the signal indicative of the setpoint voltage across the cathode terminal  220  and anode terminal  222 , in all the example variants above (including, additionally, a possible selection based on power provided), by selecting and generating the internal reference voltage VREF  422 . 
     Referring simultaneously to  FIGS.  3  and  4   . Consider again that the node between the voltage divider comprising resistors  230  and  232  creates a voltage ratio of 10:1 (voltage on the reference terminal  224  is one-tenth of the link voltage of the DC link  112 ). Further consider that the controller  420  receives a value indicating the bus voltage of the charging bus  122  is 15V, and a value indicating the bus voltage on the charging bus  130  is 5V. In example systems in which the link voltage of the DC link  112  is selected to be the higher of the bus voltages of the charging buses, the controller  420  generates the internal reference voltage VREF  422  to be 1.5V, resulting in the link voltage of the DC link  112  of 15V. In example systems in which the link voltage of the DC link  112  is selected to be the lower of the bus voltages of the charging buses, the controller  420  generates the internal reference voltage VREF  422  to be 0.5V, resulting in the link voltage of the DC link  112  of 5V. As yet another example, in example systems in which the link voltage of the DC link  112  is selected to be between the bus voltages of the charging buses (e.g., the average), the controller  420  generates the internal reference voltage VREF  422  to be 1.0V, resulting in the link voltage of the DC link  112  of 10V. 
     Still referring simultaneously to  FIGS.  3  and  4   . Consider again that the node between the voltage divider comprising resistors  230  and  232  creates a voltage ratio of 10:1 (voltage on the reference terminal  224  is one-tenth of the link voltage of the DC link  112 ). Further consider that the controller  420 : receives a value indicating the bus voltage of the charging bus  122  is 15 V, and receives a value indicating the bus current on the charging bus  122  is 0.1A; and receives a value indicating the bus voltage on the charging bus  130  is 5 V, and receives a value indicating the bus current on the charging bus  130  is 3A. In example systems in which the link voltage of the DC link  112  is selected to be about the same or higher than the charging bus delivery higher power, the controller  420  generates the internal reference voltage VREF  422  to be about 0.5V, resulting in the link voltage of the DC link  112  of 5V. 
     Returning to  FIG.  4   . In some example shunt regulators the sense terminal  226  and sense terminal  228  perform dual functions. That is, if the shunt regulator  118  is placed in a power adapter  100  ( FIG.  1   ) in which the bus voltages of the charging buses are read in analog form (e.g.,  FIG.  2   ), then the controller  420  is designed and constructed to sense the bus voltages in analog form. However, if the shunt regulator  118  is placed in a power adapter  100  ( FIG.  1   ) in which the bus voltages of the charging buses are read or received in digital form (e.g.,  FIG.  3   ), then the controller  420  is designed and constructed to sense the serial communication across the sense terminals  226  and  228  (e.g., sense the header packets at a clock frequency), and thus operate using digital values. In other cases, the shunt regulator  118  may be designed and constructed to operate only by sensing analog representations of the bus voltages of the charging buses, or designed and constructed to operate only by receiving digital representations of the bus voltages (and possibly power delivery) of the charging buses. 
     The optional offset terminal  402  may be used to bias or increase the setpoint voltage developed across the cathode terminal  220  and the anode terminal  222  by a predetermined amount. For example, the designer of the power adapter  100  ( FIG.  1   ) may want to implement a system in which the link voltage of the DC link  112  ( FIG.  1   ) is slightly higher than the setpoint voltage otherwise selected by controller  420 . The use of the bias or increase may be beneficial, for example, in cases in which the DC-DC converters are buck-only converters, to ensure the link voltage is always greater than the bus voltage of the charging buses. 
     Creation of the internal reference voltage VREF  422  may take any suitable form. For example, the controller  420  may use a digital-to-analog converter to create the internal reference voltage VREF  422 . In other cases, the controller  420  may use a controlled-current source feeding a fixed resistor to create the internal reference voltage VREF  422 , with the control input to the controlled current source set by the controller  420 . One having ordinary skill in the art, with the benefit of this disclosure, could formulate multiple ways to generate the internal reference voltage VREF  422  applied to the operational amplifier  412 . 
       FIG.  5    shows a conceptual block diagram an example controller  420 . In particular,  FIG.  5    shows the controller  420 , the sense terminal  226 , the sense terminal  228 , and the offset terminal  402 .  FIG.  5    further shows a conceptual block diagram of operation the controller  420 . In particular,  FIG.  5    shows a multiplexer  500  comprising three input ports: one input port coupled to the sense terminal  226 ; one input port coupled to the sense terminal  228 ; and a third input port coupled to a fixed reference voltage, illustratively shown as 1V. The multiplexer comprises a control input coupled to a trim option logic block  502 . The operation of the multiplexer  500  is dependent upon the control philosophy implemented by the controller  420 , which may be selectable at the time of manufacture based on the trim option logic block  502 . For example, if the control philosophy is to set the link voltage of the DC link  112  ( FIG.  1   ) to be the same as the higher of the bus voltages of the charging buses, then the trim option logic block  502  is designed, constructed, or modified (e.g., laser scribing) to select the inputs ports (e.g., A or B) having a voltage with the greater magnitude. The selected voltage is conceptually passed to the internal reference voltage VREF  422  coupled to the operation amplifier  412  ( FIG.  4   ). Selecting the voltage with the greater magnitude to be the internal reference voltage VREF  422  is merely an example—the example controller  420  of  FIG.  5    may implement any of the variants above based on the original design, the construction, and/or the modification to the trim option logic block  502 . 
       FIG.  5    further shows an example conceptual implementation of a lower boundary for the link voltage of the DC link  112  ( FIG.  1   ). In particular, the example multiplexer  500  has a fixed reference voltage, in this case 1V, coupled to the third input port (labelled C). In example situations, the trim option logic block  502  may implement any of the variants above, modified in that the lower value of any selected internal reference voltage VREF  422  is bounded or capped at the lower voltage indicated by the fixed reference voltage applied to the third input of the multiplexer. For example, if the sense terminal  226  and the sense terminal  228  indicated the bus voltage of the charging busses is below 10V (e.g., one bus voltage being 3.3V, and the second being 5.0V), the example controller  420  may select the fixed reference voltage to pass through to the internal reference voltage VREF  422 . Considering the voltage divider ratios above (e.g., 10:1), an internal reference voltage VREF  422  of 1.0V may result in a link voltage of the DC link  112  to be 10V in spite of both bus voltages of the charging buses being below 10V. Other lower boundary reference voltages may be used, possibly selected based on the efficacy of the of the AC-DC converter  102 . 
       FIG.  5    also shows an example implementation of the bias or increase to the setpoint voltage developed across the cathode terminal  220  and the anode terminal  222  associate with the offset terminal  402 . In particular, in the example implementation a current source  504  has first connection coupled to a rail voltage, and a second connection coupled to the offset terminal  402 . The second connection is also coupled to a summation block  506 . The summation block  506  sums an initial internal reference voltage selected by the multiplexer  500  with the voltage at the offset terminal  402 , and the resultant becomes the internal reference voltage VREF  422 . Referring simultaneously to  FIGS.  2  and  5   , the designer of the power adapter  100  may optionally bias the internal reference voltage VREF  422  (and thus bias the setpoint voltage) by selection of the resistor  250  coupled between the offset terminal  402  and the common on the secondary side  116 . 
       FIG.  5    implies an analog implementation for the controller  420 ; however, the functionality may be implemented in whole or in part in instructions executed by a processor, microprocessor, microcontroller, or any other programmable circuit that can be implemented within the controller  420 . 
     Returning briefly to  FIG.  3   . As discussed above, in example cases the DC-DC converters may be buck-boost converters, but in other cases the DC-DC converters may be buck-only converters. In the case of buck-only DC-DC converters, and when the shunt regulator  118  receives values indicative of bus voltages of the charging buses by way of one or more communication channels, the USB-PD controller  136  is designed and constructed to facility the use of buck-only DC-DC converts by timing of communication among the shunt regulator  118  and the DC-DC converters  104  and  106 . For example, in the buck-only case, once the USB-PD controller  136  determines bus voltages for the charging buses, the USB-PD controller first communicates the values indicative of the bus voltages to the shunt regulator  118 . The shunt regulator  118 , in turn, makes its selection for the link voltage of the DC link  112 , and implements the selection. Either by expiration of the predetermined amount of time, or by return communication from the USB-PD controller  136 , the USB-PD controller  136  waits to ensure a link voltage rises above the highest negotiated bus voltage for the charging buses to ensure buck-only operation of the DC-DC converters  104 . 
       FIG.  6    shows a method in accordance with at least some embodiments. In particular, the method starts (block  600 ) and comprises: supplying a first bus voltage to a first device by a first DC-DC converter coupled to a link voltage (block  602 ); supplying a second bus voltage to a second device by a second DC-DC converter coupled to the link voltage (block  604 ); converting an AC voltage to the link voltage by way of an AC-DC converter (block  606 ); selecting, by a shunt regulator, a setpoint for the link voltage based on the first bus voltage and the second bus voltage (block  608 ); and regulating the link voltage to the setpoint by the AC-DC converter (block  610 ). Thereafter the method ends (block  612 ). The order the steps of  FIG.  6    shall not necessarily imply an order of implementation. For example, in the analog cases (and with buck-boost DC-DC converters) the bus voltages to the charging buses may be supplied before the shunt regulator makes a selection for the setpoint for the link voltage. Before the selection, the link voltage may be at a default voltage, or may the voltage from a previous selection. In the digital cases, the selecting of the link voltage and regulating the link voltage may take place before supplying (or changing) the bus voltages of the charging buses. 
     Many of the electrical connections in the drawings are shown as direct couplings having no intervening devices, but not expressly stated as such in the description above. Nevertheless, this paragraph shall serve as antecedent basis in the claims for referencing any electrical connection as “directly coupled” for electrical connections shown in the drawing with no intervening device(s). 
     The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.