Patent Publication Number: US-2021194266-A1

Title: Adaptive Multi-Mode Charging

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 62/951,876, filed 20 Dec. 2019, the disclosure of which is hereby incorporated by reference in its entirety herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to battery charging and, more specifically, to a charger that can operate in multiple modes. 
     BACKGROUND 
     Batteries are reliable, portable energy sources that are used by a wide range of electronic devices including mobile phones, laptops, toys, power tools, medical device implants, electronic vehicles, and satellites. A battery, however, stores a fixed amount of charge that is depleted during mobile operation of the electronic device. Instead of requiring the purchase of a replacement, many batteries are rechargeable via another power source. The same battery can therefore be used multiple times. 
     An electronic device can include a charger to recharge the battery. The charger is designed to provide a particular voltage or current that is appropriate for charging the battery. Thus, the charger enables a transfer of power between, for instance, an adaptor that is plugged into a wall socket and the battery. By including the charger in the device, it is easier for a user to recharge the battery during the day as the user moves around. Unfortunately, incorporating into an electronic device a charger that can handle different charging scenarios is challenging. 
     SUMMARY 
     Apparatuses and techniques are disclosed that implement adaptive multi-mode charging. In particular, an example single charger can selectively operate as a charge pump (e.g., a voltage divider-type charge pump or a voltage multiplier-type charge pump), as a direct charger (e.g., a pass-through charger or a bypass charger), or another type of charger with a different conversion ratio. The charger can also selectively provide forward charging or reverse charging. With the ability to operate in different modes, the charger can support both wired and wireless charging. The charger can also be used to charge single-cell or multi-cell batteries. 
     In some situations, the multi-mode charger dynamically switches between different modes to optimize efficiency for different operating temperatures and loads. The charger can also be implemented to support different types of adaptors. Use of the example multi-mode charger obviates the need for implementing additional chargers within the apparatus, which can conserve space and reduce cost of the apparatus. Furthermore, any protection functions or features can be active for the different modes of the charger. Some apparatuses can include multiple multi-mode chargers to support multi-phase charging or multi-cell battery charging. 
     In an example aspect, an apparatus is disclosed. The apparatus includes at least one charger having a first node and a second node. The at least one charger is configured to accept an input voltage at the first node. The at least one charger is also configured to selectively operate in a first mode to generate a first output voltage at the second node that is greater than or less than the input voltage or operate in a second mode to generate a second output voltage at the second node that is substantially equal to the input voltage. 
     In an example aspect, an apparatus is disclosed. The apparatus includes supply means for providing an input voltage and load means for accepting an output voltage. The apparatus also includes charging means for transferring power from the supply means to the load means by selectively providing a first voltage as the output voltage in accordance with a first mode or a second voltage as the output voltage in accordance with a second mode. The first voltage is greater than or less than the input voltage and the second voltage is substantially equal to the input voltage. 
     In an example aspect, a method for adaptive multi-mode charging is disclosed. The method includes operating a charger as a voltage-divider-type charge pump or a voltage-multiplier-type charge pump during a first time interval. The operating the charger during the first time interval comprises accepting a first input voltage at a first node of the charger and generating, based on the first input voltage, a first output voltage at a second node of the charger. The first output voltage is less than or greater than the input voltage based on the charger operating as the voltage-divider-type charge pump or the voltage-multiplier-type charge pump, respectively. The method also includes operating the charger as a direct charger during a second time interval. The operating the charger during the second time interval comprises accepting a second input voltage at the first node of the charger and generating, based on the second input voltage, a second output voltage at the second node of the charger. The second output voltage is substantially equal to the second input voltage based on the charger operating as the direct charger. 
     In an example aspect, an apparatus is disclosed. The apparatus includes at least one power supply circuit, at least one load, at least one battery, a switching circuit coupled to the at least one power supply circuit and the at least one load, and at least one charger. The at least one charger comprises a first node coupled to the switching circuit and a second node coupled to the at least one battery. The at least one charger is configured to selectively transfer power from the at least one power supply circuit to the at least one battery based on the switching circuit connecting the at least one power supply circuit to the first node or transfer power from the at least one battery to the at least one load based on the switching circuit connecting the at least one load to the first node. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example environment for adaptive multi-mode charging. 
         FIG. 2  illustrates example power transfer circuitry for adaptive multi-mode charging. 
         FIG. 3  illustrates an example charger for adaptive multi-mode charging. 
         FIG. 4-1  illustrates an example voltage-divider forward-charging mode of a charger for adaptive multi-mode charging. 
         FIG. 4-2  illustrates an example direct forward-charging mode of a charger for adaptive multi-mode charging. 
         FIG. 4-3  illustrates an example voltage-multiplier reverse-charging mode of a charger for adaptive multi-mode charging. 
         FIG. 4-4  illustrates an example direct reverse-charging mode of a charger for adaptive multi-mode charging. 
         FIG. 5  illustrates example implementations of a switching circuit and a charger for adaptive multi-mode charging. 
         FIG. 6  illustrates example power transfer circuitry with multiple chargers coupled together in parallel for adaptive multi-mode charging. 
         FIG. 7  illustrates example power transfer circuitry with multiple chargers to provide adaptive multi-mode charging for a multi-cell battery. 
         FIG. 8  illustrates another example charger for adaptive multi-mode charging. 
         FIG. 9  illustrates an example protection circuit for adaptive multi-mode charging. 
         FIG. 10  is a flow diagram illustrating an example process for performing adaptive multi-mode charging. 
     
    
    
     DETAILED DESCRIPTION 
     An electronic device can include a charger to recharge the battery. The charger is designed to provide a particular voltage or current that is appropriate for charging the battery. Thus, the charger enables a transfer of power between, for instance, an adaptor that is plugged into a wall socket and the battery. By including the charger in the device, it is easier for a user to recharge the battery during the day as the user moves around. Unfortunately, incorporating into an electronic device a charger that can handle different charging scenarios is challenging. 
     Different types of chargers can be designed to perform under different operating conditions. For example, some chargers operate at high efficiency while providing a large charging current to the battery, and others operate at high efficiency while providing a small charging current to the battery. Additionally, some chargers can be used with different types of adaptors or can accept a wide range of input voltages. 
     Each of these different types of chargers are designed for a specific operating condition. Consequently, each individual charger type is unable to dynamically adapt to changes in the operating conditions. To address this, some techniques may implement multiple chargers within the electronic device and then enable an appropriate charger according to a current operating condition. Including multiple chargers can, however, increase a size and cost of the electronic device. 
     To address this, an apparatus is disclosed that implements adaptive multi-mode charging. In particular, the apparatus includes a multi-mode charger that can selectively operate as a charge pump (e.g., a voltage divider-type charge pump or a voltage multiplier-type charge pump), as a direct charger (e.g., a pass-through charger or a bypass charger), or another type of charger with a different conversion ratio. The charger can also selectively provide forward charging or reverse charging. With the ability to operate in different modes, the charger can support both wired and wireless charging. The charger can also be used to charge single-cell or multi-cell batteries. 
     In some situations, the multi-mode charger dynamically switches between different modes to optimize efficiency for different operating temperatures and loads. The charger can also be implemented to support different types of adaptors. Use of the charger obviates the need for implementing additional chargers within the apparatus, which can conserve space and reduce cost of the apparatus. Furthermore, any protection functions or features can be active for the different modes of the charger. Some apparatuses can include multiple multi-mode chargers to support multi-phase charging or multi-cell battery charging. 
       FIG. 1  illustrates an example environment  100  for adaptive multi-mode charging. In the example environment  100 , an example computing device  102  receives power from a power source  104  or provides power to an external load  105 . The power source  104  can represent any type of power source, including a power outlet, a solar charger, a portable charging station, a wireless charger, another battery, and so forth. The external load  105  can represent an external peripheral, such as a headset or another computing device (e.g., another smartphone). In this example, the computing device  102  is depicted as a smartphone. However, the computing device  102  can be implemented as any suitable computing or electronic device, such as a modem, a cellular base station, a broadband router, an access point, a cellular phone, a gaming device, a navigation device, a media device, a laptop computer, a desktop computer, a tablet computer, a wearable computer, a server, a network-attached storage (NAS) device, a smart appliance or other internet of things (IoT) device, a medical device, a vehicle-based communication system, a radar, a radio apparatus, and so forth. 
     As illustrated, the computing device  102  can includes at least one power supply circuit  106 , at least one load  108 , and power transfer circuitry  110 . Example types of power supply circuits  106  include a wireless power receiver  112 , a power adaptor  114 , or a battery  116 . As an example, the power adaptor  114  can include a universal serial bus (USB) adaptor. Depending on the type of computing device  102 , the battery  116  may comprise a lithium-ion battery, a lithium polymer battery, a nickel-metal hydride battery, a nickel-cadmium battery, a lead acid battery, and so forth. The battery  116  can also include a single-cell battery, a multi-cell battery (e.g., a two-cell battery), or multiple batteries, such as a main battery and a supplemental battery. 
     In some cases, the power supply circuit  106  jointly operates with the external power source  104  to provide power to the computing device  102 . For example, the wireless power receiver  112  provides wireless charging using the external power source  104 , which can include a wireless power transmitter of another device. As another example, the power adaptor  114  provides wired charging using the external power source  104 , which can include the power outlet. 
     The load  108  is internal to the computing device  102 . Example types of loads include the power adaptor  114 , the battery  116 , or a wireless power transmitter  118 . Other example loads  108  include a fixed load, a variable load, or a load associated with a component of the computing device  102 , such as an application processor, an amplifier within a wireless transceiver, or a display (not shown in  FIG. 1 ). In some cases, the load  108  provides power to the external load  105 . For example, the wireless power transmitter  118  provides wireless charging for the external load  105 , which can include a wireless power receiver of another device. As another example, the power adaptor  114  provides wired charging to the external load  105 , which can include a battery of another device. 
     The power transfer circuitry  110  of the computing device  102  includes one or more power paths  120 - 1  to  120 -N, at least one switching circuit  122 , and at least one charger  124 . The variable N represents a positive integer. The power transfer circuitry  110  can transfer power from one or more power sources (e.g., the external power source  104  or the power supply circuit  106 ) to one or more loads (e.g., the external load  105  or the load  108 ). This power is transferred along one or more power paths  120 - 1  to  120 -N, which couple the one or more power sources or one or more loads to the switching circuit  122 . 
     The switching circuit  122  can provide isolation for individual power paths  120 - 1  to  120 -N. For example, the switching circuit  122  can isolate one of the power paths  120 - 1  to  120 -N from the battery  116  to prevent leakage current from flowing from the battery  116  to one of the power paths  120 - 1  to  120 -N. For implementations that include multiple power paths  120 - 1  to  120 -N, the switching circuit  122  can enable individual power paths  120  to be connected to the charger  124  and provide isolation between the power paths  120 - 1  to  120 -N. 
     The charger  124  implements, at least partially, adaptive multi-mode charging. The charger  124  includes at least one flying capacitor  126  and switches  128 - 1  to  128 -S, where S represents a positive integer. The flying capacitor  126  and the switches  128 - 1  to  128 -S are further described with respect to  FIGS. 3 and 8 . The charger  124  can be implemented on a stand-alone integrated circuit or as part of a power-management integrated circuit (PMIC), which implements additional functions. 
     The charger  124  can operate in different modes, which enables the charger  124  to operate as a charge pump (e.g., a voltage divider-type charge pump or a voltage multiplier-type charge pump) or a direct charger. In some cases, a conversion ratio of the charger  124  can vary for different modes. For example, the charge pump can implement a divide-by-two charge pump that provides a 2:1 conversion ratio, a multiply-by-two charge pump that provides a 1:2 conversion ratio, or a direct charger that provides a 1:1 conversion ratio. 
     Generally, the charger  124  can implement a divide-by-N charge pump or a multiply-by-N charge pump, where N represents a positive integer (e.g., 1, 2, 3, or 4). Some types of chargers  124  can operate with additional conversion ratios, such as a 1:3 conversion ratio, a 3:1 conversion ratio, a 2:3 conversion ratio, a 3:2 conversion ratio, a 1:4 conversion ratio, a 4:1 conversion ratio, a 2:4 conversion ratio, a 4:2 conversion ratio, and so forth. Some modes can enable the charger  124  to perform forward charging, and other modes can enable the charger  124  to perform reverse charging. These modes are further described below. 
     The power transfer circuitry  110  also includes at least one mode-control circuit  130  and at least one protection circuit  132 . The mode-control circuit  130  can include a bias voltage generator (not shown in  FIG. 1 ), which generates different bias voltages based on a software or hardware command. These bias voltages, which can establish different switch states, control a mode of the charger  124  and a configuration of the switching circuit  122 . By providing different bias voltages, the mode-control circuit  130  can dynamically change the mode of the charger  124  as the operating conditions change. 
     The protection circuit  132  can provide a variety of protections, including input under-voltage lock-out, input over-voltage lock-out, surge protection, input current limit regulation, input peak current limit, battery overvoltage, battery overcurrent, programmable die and/or skin thermal regulation, die thermal shutdown, reverse current protection, input short protection, output short protection, input-to-output voltage ratio monitoring, or some combination thereof. In some implementations, thresholds associated with the protection circuit  132  can be adjusted based on an operational mode of the charger  124 . For example, the input-to-output voltage ratio monitoring can have an expected voltage ratio adjusted based on whether the charger  124  operates as a voltage divider-type charge pump or a voltage multiplier-type charge pump. The expected voltage ratio can also be adjusted as the charger  124  operates in different modes that provide different conversion ratios. Some protection circuits  132  can be designed to provide protection during both forward charging and reverse charging. In this case, the protection circuits  132  can be designed to sense currents that flow in a forward direction from the power path  120  to the battery  116  and currents that flow in a reverse direction from the battery  116  to the power path  120 . Various types of protection circuits  132  are further described with respect to  FIG. 9 . 
     In some implementations, the power transfer circuitry  110  includes a main charger (not shown), which can be implemented in parallel with the charger  124 . In this case, the charger  124  can operate as a slave charger while the main charger operates as a master charger. The power transfer circuitry  110  is further described with respect to  FIG. 2 . 
       FIG. 2  illustrates example power transfer circuitry  110  for adaptive multi-mode charging. In the depicted configuration, the power transfer circuitry  110  is coupled to the wireless power receiver  112 , the power adaptor  114 , the wireless power transmitter  118 , and the battery  116 . Although not explicitly shown, the power transfer circuitry  110  can also include an output capacitor coupled in parallel with the battery  116 . 
     The power transfer circuitry  110  includes a first power path  120 - 1 , a second power path  120 - 2 , and a third power path  120 - 3 . The first power path  120 - 1  couples the wireless power receiver  112  to the switching circuit  122 . The second power path  120 - 2  couples the power adaptor  114  to the switching circuit  122 . The third power path  120 - 3  couples the wireless power transmitter  118  to the switching circuit  122 . 
     The switching circuit  122  is coupled between the power paths  120 - 1  to  120 - 3  and the charger  124 . The switching circuit  122  includes a first switch  202 - 1 , a second switch  202 - 2 , and a third switch  202 - 3 . The first switch  202 - 1  selectively connects or disconnects the wireless power receiver  112  to the charger  124 . Likewise, the second switch  202 - 2  selectively connects or disconnects the power adaptor  114  to the charger  124 . The third switch  202 - 3  selectively connects or disconnects the wireless power transmitter  118  to the charger  124 . The charger  124  is coupled between the switching circuit  122  and the loads  108 - 1  and  108 - 2 . 
     The mode-control circuit  130  is coupled to the switching circuit  122  and the charger  124 . During operation, the mode-control circuit  130  generates a power-path control signal  204 , which controls states of the switches  202 - 1  to  202 - 3 . With the power-path control signal  204 , the mode-control circuit  130  can enable power to be transferred between the charger  124  and any one of the power paths  120 - 1  to  120 - 3 . The mode-control circuit  130  also generates a mode-control signal  206 , which controls a mode of the charger  124 . 
     As described above, each mode can be associated with a particular conversion ratio and charging direction. A mode that supports forward charging  208  enables power to transfer from one of the power paths  120 - 1  or  120 - 2  to the charger  124 . Another mode that supports reverse charging  210  enables power to transfer from the charger  124  to one of the power paths  120 - 2  or  120 - 3 . For example, power can be transferred from the wireless power receiver  112  or the power adaptor  114  to the battery  116  during forward charging  208 . In contrast, power can be transferred from the battery  116  to the power adaptor  114  or the wireless power transmitter  118  during reverse charging  210 . As described above, the battery  116  can act as the load  108  during forward charging  208  or can act as the power supply circuit  106  during reverse charging  210 . Likewise, the power adaptor  114  can act as the power supply circuit  106  during forward charging  208  or can act as the load  108  during reverse charging  210 . Both the power-path control signal  204  and the mode-control signal  206  can include multiple bias voltages, which bias the gate voltage of transistors that implement the switches  202 - 1  to  202 - 3  and the switches  128 - 1  to  128 -S of the charger  124  (shown in  FIG. 1 ). The charger  124  is further described with respect to  FIG. 3 . 
       FIG. 3  illustrates an example charger  124  for adaptive multi-mode charging. In the depicted configuration, the charger  124  is implemented as a divide-by-two charge pump or a multiply-by-two charge pump, which can provide a conversion ratio of 2:1 or 1:2, respectively. Other implementations can include a divide-by-three charge pump, a divide-by-four charge pump, or a divide-by-N charge pump. 
     The charger  124  includes a node  302 , another node  304 , and a ground node  306 . The node  302  is coupled to the switching circuit  122 . The other node  304  is coupled to the battery  116 . For forward charging  208 , the node  302  operates as an input node and the node  304  operates as an output node. Alternatively, for reverse charging  210 , the node  304  operates as the input node and the node  302  operates as the output node. 
     The charger  124  includes the flying capacitor  126  (C Fly    126 ), which is coupled to a positive node  308  (CP  308 ) and a negative node  310  (CN  310 ). The charger  124  also includes four switches  128 - 1  to  128 - 4 . The switches  128 - 1  to  128 - 4  can be implemented using transistors, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), junction field-effect transistors (JFETs), bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), diodes, and so forth. An example implementation of the switches  128 - 1  to  128 - 4  is further described with respect to  FIG. 5 . 
     The first switch  128 - 1  (S 1   128 - 1 ) is coupled between the positive node  308  and the node  302 . The second switch  128 - 2  (S 2   128 - 2 ) is coupled between the negative node  310  and the node  304 . The first switch  128 - 1  and the second switch  128 - 2  can operate together to form a charging circuit, which charges the flying capacitor  126 . 
     The third switch  128 - 3  (S 3   128 - 3 ) is coupled between the ground node  306  and the negative node  310 . The fourth switch  128 - 4  (S 4   128 - 4 ) is coupled between the positive node  308  and the node  304 . The third switch  128 - 3  and the fourth switch  128 - 4  can operate together to form a discharging circuit, which discharges the flying capacitor  126 . The charger  124  of  FIG. 3  can operate as a divide-by-two charge pump, a multiply-by-two charge pump, or a direct charger to support forward charging  208  or reverse charging  210 , as further described with respect to  FIGS. 4-1 to 4-4 . 
       FIG. 4-1  illustrates an example voltage-divider forward-charging mode  400 - 1  of the charger  124  for adaptive multi-mode charging. During the voltage-divider forward-charging mode  400 - 1 , the charger  124  operates as a voltage-divider-type charge pump to support forward charging  208 . In the depicted configuration, the switches  128 - 1  to  128 - 4  are active. In particular, the switches  128 - 1  and  128 - 2  open and close according to a charging phase signal while the switches  128 - 3  and  128 - 4  open and close according to a discharging phase signal. During this mode, the charger  124  provides a conversion ratio of 2:1. In other words, the charger  124  generates an output voltage  404  at the node  304  that is half an input voltage  402  at the node  302 . In this mode, an output current that flows to the battery  116  is twice an input current that flows into the node  302 . By increasing the output current relative to the input current, the charger  124  can reduce a time it takes to charge the battery  116 . 
     During operation, the switching circuit  122  can connect the wireless power receiver  112  or the power adaptor  114  to the node  302 . In particular, the switch  202 - 1  (of  FIG. 2 ) can be closed and the switches  202 - 2  and  202 - 3  (of  FIG. 2 ) can be open to connect the wireless power receiver  112  to the node  302  and isolate both the power adaptor  114  and the wireless power transmitter  118  from the node  302 . Alternatively, the switches  202 - 1  and  202 - 3  can be opened and the switch  202 - 2  can be closed to connect the power adaptor  114  to the node  302  and isolate both the wireless power receiver  112  and the wireless power transmitter  118  from the node  302 . 
     The voltage-divider forward-charging mode  400 - 1  enables the charger  124  to operate at a high efficiency while providing a large current to the battery  116 . The voltage-divider forward-charging mode  400 - 1  can be used while the power transfer circuitry  110  operates within a particular thermal or current threshold. To manage the temperature, the charger  124  can dynamically switch to a direct forward-charging mode  400 - 2 , as further described in  FIG. 4-2 . 
       FIG. 4-2  illustrates an example direct forward-charging mode  400 - 2  of the charger  124  for adaptive multi-mode charging. During the direct forward-charging mode  400 - 2 , the charger  124  operates as a direct charger to support forward charging  208 . In the depicted configuration, the first switch  128 - 1  and the fourth switch  128 - 4  are closed (e.g., in a closed state) while the second switch  128 - 2  and the third switch  128 - 3  are opened (e.g., in an open state). During this mode, the charger  124  provides a conversion ratio of 1:1. In other words, the charger  124  generates an output voltage  404  at the node  304  that is substantially equal to (e.g., within approximately 90% of) an input voltage  402  at the node  302 . In this mode, the output current that flows to the battery  116  is substantially equal to an input current that flows into the node  302 . During operation, the switching circuit  122  can connect the wireless power receiver  112  or the power adaptor  114  to the node  302 . 
     The direct forward-charging mode  400 - 2  enables the charger  124  to operate at a high efficiency while providing a small current to the battery  116 . Although this can increase the time it takes to charge the battery  116 , the temperature within the power transfer circuitry  110  can decrease. In general, the charger  124  can dynamically switch between the direct forward-charging mode  400 - 2  and the voltage-divider forward-charging mode  400 - 1  of  FIG. 4-1  to manage temperature of the power transfer circuitry  110  while decreasing charging times. 
     For example, the mode-control circuit  130  can monitor a temperature associated with the computing device  102 , such as a temperature associated with the power supply circuit  106 , the load  108 , or the power transfer circuitry  110 . If the monitored temperature exceeds a first threshold, the mode-control circuit  130  causes the charger  124  to transition from the voltage-divider forward-charging mode  400 - 1  to the direct forward-charging mode  400 - 2 , to enable the temperature to decrease. If the monitored temperature drops below a second threshold, the mode-control circuit  130  causes the charger  124  to transition the direct forward-charging mode  400 - 2  to the voltage-divider forward-charging mode  400 - 1 . 
       FIG. 4-3  illustrates an example voltage-multiplier reverse-charging mode  400 - 3  of the charger  124  for adaptive multi-mode charging. During the voltage-multiplier reverse-charging mode  400 - 3 , the charger  124  operates as a voltage-multiplier-type charge pump to support reverse charging  210 . In the depicted configuration, the switches  128 - 1  to  128 - 4  are active. In particular, the switches  128 - 1  and  128 - 2  open and close according to a charging phase signal while the switches  128 - 3  and  128 - 4  open and close according to a discharging phase signal. During this mode, the charger  124  provides a conversion ratio of 1:2. In other words, the charger  124  generates an output voltage  404  at the node  302  that is twice an input voltage  402  at the node  304 . In this mode, an output current that flows into the switching circuit  122  is half an input current that flows into the node  304  from the battery  116 . 
     This mode enables power to be transferred from the battery  116  to the external load  105  using the power adaptor  114  of the power path  120 - 2  or the wireless power transmitter  118  of the power path  120 - 3 . During operation, the switching circuit  122  can connect the power adaptor  114  or the wireless power transmitter  118  to the node  302 . The voltage-multiplier reverse-charging mode  400 - 3  enables the charger  124  to support high-power reverse wireless or wired charging without relying on additional components or chargers. 
       FIG. 4-4  illustrates an example direct reverse-charging mode  400 - 4  of the charger  124  for adaptive multi-mode charging. During the direct reverse-charging mode  400 - 4 , the charger  124  operates as a direct charger to support reverse charging  210 . In the depicted configuration, the first switch  128 - 1  and the fourth switch  128 - 4  are closed while the second switch  128 - 2  and the third switch  128 - 3  are opened. During this mode, the charger  124  provides a conversion ratio of 1:1. In other words, the charger  124  generates an output voltage  404  at the node  302  that is substantially equal to an input voltage  402  at the node  304 . In this mode, an output current that flows to the switching circuit  122  is substantially equal to an input current that flows into the node  304  from the battery  116 . During operation, the switching circuit  122  can connect the power adaptor  114  of the power path  120 - 2  or the wireless power transmitter  118  of the power path  120 - 3  to the node  302 . The direct reverse-charging mode  400 - 4  enables the charger  124  to support low-power reverse wireless or wired charging without relying on additional components or chargers. 
     In some cases, the computing device  102  can send a command to the power transfer circuitry  110  or the mode-control circuit  130  to enable one of the reverse-charging modes  400 - 3  or  400 - 4 . In other cases, the power transfer circuitry  110  (or the mode-control circuit  130 ) can automatically activate reverse charging. As an example, the power transfer circuitry  110  can activate one of the reverse-charging modes  400 - 3  or  400 - 4  responsive to determining that no input power is present and determining that the battery voltage is sufficient for reverse charging. For reverse wireless charging, the power transfer circuitry  110  can activate one of the reverse-charging modes  400 - 3  or  400 - 4  responsive to receiving a wireless signal from the other device&#39;s wireless receiver. 
     In general, the charger  124  can dynamically switch between the direct reverse-charging mode  400 - 4  and the voltage-multiplier reverse-charging mode  400 - 3  of  FIG. 4-3  to manage temperature of the power transfer circuitry  110  while decreasing charging times. In order to switch between different modes  400 - 1  to  400 - 4 , the power transfer circuitry  110  may implement a soft-start process that gradually adjusts a voltage at one of the power paths  120 - 1  to  120 - 3  to avoid providing a large initial current. 
       FIG. 5  illustrates example implementations of the switching circuit  122  and the charger  124  for adaptive multi-mode charging. In the depicted configuration, the switches  202 - 1  and  202 - 3  (of  FIG. 2 ) and the switches  128 - 1  to  128 - 4  (of  FIG. 3 ) are implemented using MOSFETs. The MOSFETs are in a common-gate configuration, which enables power to transfer in either direction across the other terminals (e.g., across the source terminal and the drain terminal). The switches  202 - 1  to  202 - 3  and  128 - 1  to  128 - 4  also include respective diodes coupled between the source and drain terminals. The mode-control circuit  130  is coupled to the gates of these MOSFETs and provides respective bias voltages to the gates. The bias voltages cause the switches  202 - 1  to  202 - 3  to respectively connect the power paths  120 - 1  to  120 - 3  to the charger  124 . Other bias voltages cause the switches  128 - 1  to  128 - 4  to open or close according to one of the modes  400 - 1  to  400 - 4  described above. 
       FIG. 6  illustrates example power transfer circuitry  110  with multiple chargers  124 - 1  and  124 - 2  for adaptive multi-mode charging. In the depicted configuration, the chargers  124 - 1  and  124 - 2  are coupled together in parallel. The mode-control circuit  130  provides a first mode-control signal  206 - 1  to the charger  124 - 1  and a second mode-control signal  206 - 2  to the charger  124 - 2 . The chargers  124 - 1  and  124 - 2  can operate in any of the modes  400 - 1  to  400 - 4  described above. In some cases, the chargers  124 - 1  and  124 - 2  operate with different phases, in order to provide dual-phase charging. Other implementations can include more than two chargers  124  to support multi-phase charging. 
       FIG. 7  illustrates example power transfer circuitry  110  with multiple chargers  124 - 1  to  124 - 2  to provide adaptive multi-mode charging for a multi-cell battery  116 . In the depicted configuration, the power transfer circuitry  110  includes a mater charger  702 , which can be coupled to the wireless power receiver  112  and/or the power adaptor  114 . 
     In  FIG. 7 , the chargers  124 - 1  and  124 - 2  are implemented in different directions. For example, the node  302  of the charger  124 - 1  (shown as  302 - 1 ) is coupled to the switching circuit  122  and the node  304  of the charger  124 - 1  (shown as  304 - 1 ) is coupled to the battery  116 . In contrast, the node  302  of the charger  124 - 2  (shown as  302 - 2 ) is coupled to the battery  116  and the node  304  of the charger  124 - 2  (shown as  304 - 2 ) is coupled to the master charger  702  and the load  108 . By having opposite nodes  304 - 1  and  302 - 2  coupled to the battery  116 , the charger  124 - 1  can operate as a voltage divider-type charge pump for forward charging  208  and the charger  124 - 2  can operate as a voltage divider-type charge pump for reverse charging  210 . Additionally, the charger  124 - 1  can operates as a voltage-multiplier-type charge pump for reverse charging  210  and the charger  124 - 2  can operate as a voltage-multiplier-type charge pump for forward charging  208 . The charger  124 - 2  can also operate in the direct forward-charging mode  400 - 3  or the direct reverse-charging mode  400 - 4  (of  FIGS. 4-3 and 4-4 ). 
     In some implementations, the charger  124 - 1  implements a different type of charge pump than the charger  124 - 2 . This enables the chargers  124 - 1  and  124 - 2  to provide different conversion ratios. Although not shown, the battery  116  can include two or more cells that are connected together in series. 
     During operation, power can be transferred between either one of the power paths  120 - 1  and  120 - 2  and the battery  116  using the charger  124 - 1  or the charger  124 - 2 . The master charger  702  can provide another conversion ratio to enable the charger  124 - 2  to support different types of power adaptors  114  or different types of wireless power receivers  112 . The charger  124 - 2  can also transfer power from the battery  116  to the load  108 , which can include the wireless power transmitter  118 . 
       FIG. 8  illustrates another example charger  124  for adaptive multi-mode charging. In the depicted configuration, the charger  124  can operate as a divide-by-four charge pump, a divide-by-two charge pump, a multiply-by-four charge pump, a multiply-by-two charge pump, or a direct charger to provide a conversion ratio of 4:1, 2:1 (or 4:2), 1:4, 1:2 (or 2:4), or 1:1, respectively. 
     The charger  124  includes the node  302 , the node  304 , and the ground node  306  (of  FIG. 3 ). As described above, the node  302  operates as an input node and the node  304  operates as an output node for forward charging  208 . Alternatively, for reverse charging  210 , the node  304  operates as the input node and the node  302  operates as the output node. 
     The charger  124  includes multiple flying capacitors  126 - 1  to  126 - 5  (C Fly    126 - 1  to  126 - 5 ) and multiple switches  128 - 1  to  128 - 8 . Nodes  802 - 1  to  802 - 7  exist between respective pairs of the switches  128 - 1  to  128 - 8 . In particular, the node  802 - 1  is coupled between the first switch  128 - 1  (S 1   128 - 1 ) and the second switch  128 - 2  (S 2   128 - 2 ), the node  802 - 2  is coupled between the second switch  128 - 2  and the third switch  128 - 3  (S 3   128 - 3 ), the node  802 - 3  is coupled between the third switch  128 - 3  and the fourth switch  128 - 4  (S 4   128 - 4 ), the node  802 - 4  is coupled between the fourth switch  128 - 4  and the fifth switch  128 - 5  (S 5   128 - 5 ), the node  802 - 5  is coupled between the fifth switch  128 - 5  and the sixth switch  128 - 6  (S 6   128 - 6 ), the node  802 - 6  is coupled between the sixth switch  128 - 6  and the seventh switch  128 - 7  (S 7   128 - 7 ), and the node  802 - 7  is coupled between the seventh switch  128 - 7  and the eight switch  128 - 8  (S 8   128 - 8 ). The node  802 - 6  is the same as the node  304 . 
     The flying capacitors  126 - 1  to  126 - 5  are coupled between different pairs of the nodes  802 - 1  to  802 - 7 . In particular, the first flying capacitor  126 - 1  is coupled between the node  802 - 1  and the node  802 - 3 . The flying capacitor  126 - 2  is coupled between the node  802 - 3  and the node  802 - 5 . The flying capacitor  126 - 3  is coupled between the node  802 - 5  and the node  802 - 7 . The flying capacitor  126 - 4  is coupled between the node  802 - 2  and the node  802 - 4 . The flying capacitor  126 - 5  is coupled between the node  802 - 4  and the node  802 - 6 . 
     The charger  124  of  FIG. 8  can operate according to the voltage-divider forward-charging mode  400 - 1 . To provide a 4:1 conversion ratio between the node  302  and the node  304  during the voltage-divider forward-charging mode  400 - 1 , the switches  128 - 1  to  128 - 8  alternate between open and closed states. In this mode, the charger  124  can also provide a 2:1 conversion ratio between the node  302  and the node  802 - 4 . In this case, the node  802 - 4  can be coupled to a load  108  within the computing device  102 . Alternatively, the charger  124  can provide a 2:1 conversion ratio between the node  302  and the node  802 - 4  by operating the switches  128 - 3  and  128 - 4  in the closed state and having the switches  128 - 1 ,  128 - 2 ,  128 - 7 , and  128 - 8  alternate between the open and closed states. 
     Additionally or alternatively, the charger  124  can operate according to the direct forward-charging mode  400 - 2  or the direct reverse-charging mode  400 - 3  to provide a 1:1 conversion ratio between the node  302  and the node  304 . During either of these modes, the switches  128 - 1  to  128 - 6  are in the closed state and the switches  128 - 7  and  128 - 8  are in the open state. 
     Additionally or alternatively, the charger  124  can operate according to the voltage-multiplier reverse-charging mode  400 - 4  to provide a 1:4 conversion ratio between the node  304  and the node  302 . During this mode, the switches  128 - 1  to  128 - 8  alternate between the open state and the closed state. In this mode, the charger  124  can also provide a 1:2 conversion ratio between the node  802 - 4  and the node  302 . In this case, the node  802 - 4  can be coupled to a power supply circuit  106  within the computing device  102 . Alternatively, the charger  124  can provide a 1:2 conversion ratio between the node  304  and the node  302  by operating the switches  128 - 3  and  128 - 4  in the closed state and having the switches  128 - 1 ,  128 - 2 ,  128 - 7 , and  128 - 8  alternate between the open and closed states. 
       FIG. 9  illustrates an example protection circuit  132  for adaptive multi-mode charging. In the depicted configuration, the protection circuit  132  can include an input under-voltage lock-out circuit  902 , an input over-voltage lock-out circuit  904 , a surge protection circuit  906 , an input current limit regulation circuit  908 , an input peak current limit circuit  910 , a batter over-voltage circuit  912 , a battery over-current circuit  914 , a thermal regulation circuit  916 , a thermal shutdown circuit  918 , a reverse current protection circuit  920 , an input short protection circuit  922 , an output short protection circuit  922 , or some combination thereof. 
     The input under-voltage lock-out circuit  902  and the input over-voltage lock-out circuit  904  are coupled to the node  302  of the charger  124  and the mode-control circuit  130 . Each of these circuits  902  and  904  can be implemented using a comparator (e.g., an operational amplifier). The input under-voltage lock-out circuit  902  and the input over-voltage lock-out circuit  904  jointly control operation of the mode-control circuit  130  based on an input voltage at the node  302 . For example, the input under-voltage lock-out circuit  902  compares the input voltage at the node  302  to an under-voltage lock-out threshold, and the input over-voltage lock-out circuit  904  compares the input voltage to an over-voltage lock-out threshold. If the input voltage is between the under-voltage lock-out threshold and the over-voltage lock-out threshold, the input under-voltage lock-out circuit  902  and the input over-voltage lock-out circuit  904  allow the mode-control circuit  130  to operate the charger  124  (e.g., enable the charger  124  to charge and discharge the flying capacitor  126 ). Alternatively, if the input voltage is less than the under-voltage lock-out threshold or greater than the over-voltage lock-out threshold, the associated input under-voltage lock-out circuit  902  or the input over-voltage lock-out circuit  904  prevents the mode-control circuit  130  from enabling the charger  124  (e.g., prevents the mode-control circuit  130  from operating the switches  128 - 1  to  128 -S of the charger  124 ). 
     The surge protection circuit  906  is coupled to one of the power paths  120 - 1  to  120 -N. For example, the surge protection circuit  906  is coupled to the power path  120 - 2  of  FIG. 2 . The surge protection circuit  906  can include a diode, such as a transient-voltage-suppression (TVS) diode. Using the diode, the surge protection circuit  906  absorbs energy during a surge event. This provides additional time for the input over-voltage lock-out circuit  904  to detect an over-voltage event. 
     The input current limit regulation circuit  908  is coupled to the node  302  of the charger  124  and includes a current sensor and a comparator. Using the current sensor, the input current limit regulation circuit  908  monitors the input current and compares an average of the input current to an average current threshold. If the average of the input current is greater than or equal to the average current threshold, the input current limit regulation circuit  908  limits the input current to the charger  124  to protect the power adaptor  114 . 
     The input peak current limit circuit  910  is coupled to the node  302  of the charger  124  and the mode-control circuit  130 . In an example implementation, the input peak current limit circuit  910  includes a current sensor and a comparator. Using the current sensor, the input peak current limit circuit  910  monitors the input current and compares a peak of the input current to a peak current threshold. If the peak of the input current is greater than or equal to the peak current threshold, the input peak current limit circuit  910  directs the mode-control circuit  130  to power down the charger  124 . In some cases, the input peak current limit circuit  910  can delay powering down the charger  124  until the peak current threshold has been exceeded a predetermined number of times. 
     The battery over-voltage circuit  912  and the battery over-current circuit  914  are each coupled to the battery  116  and the mode-control circuit  130 . The battery over-voltage circuit  912  includes a voltage sensor and a comparator to monitor a voltage across the battery  116 . During operation, the battery over-voltage circuit  912  can direct the mode-control circuit  130  to stop charging the flying capacitor  126  of the charger  124  if the voltage across the battery  116  is greater than or equal to an over-voltage threshold. By disabling the charging cycle, the battery over-voltage circuit  912  can prevent the battery  116  from being over charged. 
     The battery over-current circuit  914  includes a current sensor and a comparator to monitor an input current to the battery  116 . The battery over-current circuit  914  directs the mode-control circuit  130  to limit the current provided to the battery  116  responsive to the current being greater than or equal to an over-current threshold. This ensures safe charging of the battery  116 . 
     The thermal regulation circuit  916  is coupled to the power adaptor  114 . During operation, the thermal regulation circuit  916  monitors a skin thermal of the power adaptor  114 . If the skin thermal becomes greater than a thermal window, the thermal regulation circuit  916  directs the power adaptor  114  to reduce the current provided to the charger  124  to enable the temperature to decrease. Alternatively, if the skin thermal drops below the thermal window, the thermal regulation circuit  916  directs the power adaptor  114  to increase the current to increase charging efficiency. 
     The thermal shutdown circuit  918  is coupled to the charger  124  and the mode-control circuit  130 . The thermal shutdown circuit  918  monitors a temperature of the die associated with the charger  124 . If the die temperature becomes greater than or equal to a threshold, the thermal shutdown circuit  918  directs the mode-control circuit  130  to power down the charger  124  until the die temperature drops below a predetermined level. 
     The reverse current protection circuit  920  includes the switch  202 - 2  of the switching circuit  122 , which is implemented between the power adaptor  114  and the charger  124 . The reverse current protection circuit  920  detects when the power adaptor  114  is disconnected from the external power source  104  or the external load  105  and causes the switch  202 - 2  to be in the open state to disconnect the power adaptor  114  from the charger  124 . In this way, the reverse current protection circuit  920  can prevent power from being transferred from the battery  116  to the power adaptor  114 . 
     The input short protection circuit  922  can include the switch  202 - 2 . During operation, the input short protection circuit  922  detects a short event in which the power adaptor  114  or the power path  120 - 2  is shorted to ground. In this situation, the input short protection circuit  922  causes the switch  202 - 2  to be in the open state to prevent the battery  116  from discharging. 
     The output short protection circuit  924  detects a short event in which the node  304  is shorted to ground. The output short protection circuit  924  includes a comparator to monitor the voltage at the node  304 . If the voltage at the node  304  is less than a threshold, such as two volts, the output short protection circuit  924  directs the mode-control circuit  130  to power down the charger  124 . This prevents the charger  124  from delivering a large current that can damage the battery  116 . 
       FIG. 10  is a flow diagram illustrating an example process  1000  for adaptive multi-mode charging. The process  1000  is described in the form of a set of blocks  1002 - 1004  that specify operations that can be performed. However, operations are not necessarily limited to the order shown in  FIG. 10  or described herein, for the operations may be implemented in alternative orders or in fully or partially overlapping manners. Operations represented by the illustrated blocks of the process  1000  may be performed by the power transfer circuitry  110  (e.g., of  FIG. 1 or 2 ). More specifically, the operations of the process  1000  may be performed by the charger  124  as shown in  FIG. 3, 5 , or  8 . 
     At block  1002 , a charger operates as a voltage-divider-type charge pump or a voltage-multiplier-type charge pump during a first time interval. For example, the charger  124  operates as the voltage-divider-type charge pump or a voltage multiplier-type charge pump during a first time interval to support forward charging  208 , as shown in  FIG. 2 . 
     At block  1004 , a first input voltage is accepted at a first node of the charger. For example, the charger  124  accepts the input voltage  402  at the node  302 , as shown in  FIG. 4-1 . 
     At block  1006 , a first output voltage is generated at a second node of the charger. The first output voltage is based on the first input voltage. The first output voltage is less than or greater than the input voltage based on the charger operating as the voltage-divider-type charge pump or the voltage-multiplier-type charge pump, respectively. For example, the charger  124  generates an output voltage  404  at the node  304  that is less than or greater than the input voltage  402 . As an example, the charger  124  can operate according to the voltage-divider forward-charging mode  400 - 1  of  FIG. 4-1 . 
     At block  1008 , the charger operates as a direct charger during a second time interval. For example, the charger  124  operates as the direct charger during the second time interval to support forward charging  208 , as shown in  FIG. 2 . 
     At block  1010 , a second input voltage is accepted at the first node of the charger. For example, the charger  124  accepts the input voltage  402  at the node  302 , as shown in  FIG. 4-2 . 
     At block  1012 , a second output voltage is generated at the second node of the charger. The second output voltage is based on the second input voltage and is substantially equal to the second input voltage based on the charger operating as the direct charger. For example, the charger  124  generates the output voltage  404  at the node  304  that is substantially equal to (e.g., within 90% of) the input voltage  402  at the node  302 . As an example, the charger  1002  can operate according to the direct forward-charging mode  400 - 2  of  FIG. 4-2 . 
     Additionally or alternatively, the charger can selectively operate as the voltage-divider-type charge pump, the voltage-multiplier-type charge pump, or the direct charger to support reverse charging  210 , as shown in  FIG. 2, 4-3 , or  4 - 4 . In this case, the charger  124  can generate an output voltage  404  at the node  302  that is less than, greater than, or substantially equal to an input voltage  402  at the node  304 . As an example, the charger  124  can operate according to the voltage-multiplier reverse-charging mode  400 - 3  or the direct reverse-charging mode  400 - 4  of  FIGS. 4-3 and 4-4 , respectively. 
     Unless context dictates otherwise, use herein of the word “or” may be considered use of an “inclusive or,” or a term that permits inclusion or application of one or more items that are linked by the word “or” (e.g., a phrase “A or B” may be interpreted as permitting just “A,” as permitting just “B,” or as permitting both “A” and “B”). Further, items represented in the accompanying figures and terms discussed herein may be indicative of one or more items or terms, and thus reference may be made interchangeably to single or plural forms of the items and terms in this written description. Finally, although subject matter has been described in language specific to structural features or methodological operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or operations described above, including not necessarily being limited to the organizations in which features are arranged or the orders in which operations are performed.