Patent Publication Number: US-2023136026-A1

Title: Rectification by battery protection system

Description:
BACKGROUND 
     An electric power system that transfers electric power from a battery to a load may include a protection system to protect the load from a reverse battery connection, where the load may receive a negative input voltage from the battery. The protection system can isolate the load from the negative input voltage to prevent the load from being damaged by the negative input voltage. Some examples of the protection system can also block a reverse current from flowing from the load to the battery, to allow the load side additional time to operate before turning off. While the battery may output a direct current (DC) signal, the battery outputs may also include alternating current (AC) signals superimposed on the DC signal. It is also desirable that the protection system can have a short response time, so that the protection system can properly and promptly respond to transient signals output by the battery to protect the load. 
     SUMMARY 
     A controller circuit includes a voltage subtractor circuit, an internal voltage generator circuit, and a gate control circuit. The voltage subtractor circuit has a subtractor output and first and second subtractor inputs. The first subtractor input is adapted to be coupled to a first current terminal of a transistor. The second subtractor input is adapted to be coupled to a second current terminal of the transistor. The internal voltage generator circuit has a generator input and a generator output. The generator input is adapted to be coupled to the first current terminal. The gate control circuit has a first gate control input, a second gate control input, and a gate control output. The first gate control input is coupled to the subtractor output. The second gate control input is coupled to the generator output. The gate control output is adapted to be coupled to a gate of the transistor. The gate control circuit also includes a switch coupled between the gate control output and the second gate control input. 
     In a method, a first voltage is received via a first terminal of a controller circuit, the first terminal being coupled to a first current terminal of a transistor. A second voltage is received via a second terminal of the controller circuit, the second terminal being coupled to a second diffusion of the transistor. Based on the first voltage and the second voltage, a switch between a voltage reference and a third terminal of the controller circuit coupled to a gate of the transistor is closed. A voltage of the gate is set by the voltage reference to form a conduction channel between the first current terminal and the second current terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an electric power transfer system in accordance with various examples. 
         FIGS.  2 A through  2 F  illustrate examples of internal components of a protection system and their operations. 
         FIGS.  3 A,  3 B, and  3 C  illustrate examples of rectification operations performed by the protection system of  FIG.  2 A - FIG.  2 F . 
         FIGS.  4 A through  4 F  illustrate examples of a protection system including internal components to perform a rectification operation in accordance with various examples. 
         FIG.  5    illustrates examples of voltage graphs depicting a rectification operation by the protection system of  FIGS.  4 A through  4 F . 
         FIG.  6    is a flowchart of an example method for performing a rectification operation in accordance with various examples. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, an electric power system may include a protection system to protect the load from a reverse battery connection. In a case where the electric power system is part of a vehicle, reverse battery connection may occur during maintenance of the vehicle&#39;s battery or jump start of the vehicle. Without the protection system, the load may receive a negative voltage from the battery when the battery is reversely connected. The negative voltage can cause huge current to flow from various electronic components of the load, such as electrostatic discharge (ESD) circuits, voltage regulators, etc., which can cause severe damage to these components. 
     The protection system can include a controller circuit and a transistor. The transistor can include a body diode, of which the anode can be coupled to the battery and the cathode can be coupled to the load. In a reverse battery connection, the battery may output a negative voltage, and the controller circuit can disable the transistor and rely on the reversed-bias body diode to isolate the load from the negative voltage, and to prevent a reverse current from flowing from the load back to the battery. If the battery is connected in the correct polarities, the controller circuit can enable the transistor to transmit a positive voltage and a forward current from the battery to the load. In a case where the anode receives a voltage including both DC and AC components, the protection system can perform a rectification operation to transmit positive AC components (e.g., AC components that adds to the DC component) and not to transmit negative AC components (e.g., AC components that subtracts from the DC component) to the load. For reasons to be described below, the controller circuit may incur substantial delay to enable the transistor, which makes it difficult to perform rectification operations for high frequency AC components. Example techniques described herein speed up the rectification response of the controller circuit, which allows the protection system to perform rectification operations for AC components at a high frequency (e.g., 200 kilo-Hertz (kHz) or above). 
       FIG.  1    illustrates an example of a system  100 . System  100  may include a battery  102 , an electric power system  104 , and a load  106 . Load  106  may include an internal power supply  108 , which can include a linear regulator (e.g., a low dropout regulator) and/or a switch mode regulator (e.g., a buck converter, a boost converter, a buck-boost converter, etc.) to provide a supply voltage, and a holdup capacitor to supply a current. Load  106  may further include subsystems  110  that draw power from internal power supply  108 . Both internal power supply  108  and subsystems  110  can include various electronic components. 
     Electric power system  104  is configured to transfer electric power from battery  102  to load  106 . Electric power system  104  may receive a voltage V in  and a current I in , and provide a voltage V out  and a current I out  to load  106 . Internal power supply  108  can receive voltage V out  and current I out  from electric power system  104  and provide a voltage V out  internal and a current I out_internal  to subsystems  110 . Voltage V out  and current I out  provided by electric power system  104  can be based on, respectively, voltage V in  and current I in  provided by battery  102 . Also, voltage V out_internal  and current I out_internal  can be based on a configuration of internal power supply  108  and subsystems  110 . For example, V out_internal  can be a fraction of V out  to provide a reduced supply voltage required by subsystems  110 , and I out_internal  can be reduced from I in  due to power consumption by electric power system  104  and internal power supply  108 . In  FIG.  1   , the positive terminal of battery  102  can be coupled to electric power system  104  to supply a positive voltage V in , while the negative terminal of battery  102  can be coupled to ground. With such configuration, voltages V out  and V out_internal  can be positive, and currents I in , I out , and I out_internal  can be part of a forward current that flows from battery  102  to load  106 . 
     In some examples, electric power system  104  can include a reverse battery protection system  112  to protect load  106  from a reverse battery connection, where the positive terminal of battery  102  is coupled to ground and the negative terminal of battery  102  is coupled to electric power system  104 . As a result, battery  102  may transmit a negative voltage, such as −V in , to electric power system  104 . Without reverse battery protection system  112 , electric power system may transmit the negative voltage to load  106 . The negative voltage can cause a huge current to flow from various electronic components of load  106 , such as electrostatic discharge (ESD) circuits, voltage regulators of internal power supply  108 , etc., which can cause severe damage to these components. Moreover, a reverse current may also flow from load  106  back to battery  102 . The reverse current may discharge the holdup capacitor of internal power supply  108  and reduces the holdup capacitor&#39;s capability of supplying power to subsystems  110 . Reverse battery protection system  112  can isolate load  106  from the negative voltage −V in . In some examples, reverse battery protection system  112  can also block the reverse current from flowing from load  106  to battery  102 , to allow subsystems  110  additional time to operate before turning off. 
       FIGS.  2 A through  2 F  illustrate examples of internal components of reverse battery protection system  112  of system  100  and their operations. Referring to the left side of  FIG.  2 A , reverse battery protection system  112  can include a controller circuit  200  and a transistor  202 . Transistor  202  can be an n-channel FET (NFET) or a p-channel FET (PFET). Transistor  202  can have a gate  204 , a first current terminal  206 , and a second current terminal  206 . A body diode  210  can be formed at a p-n junction between first current terminal  206  and second current terminal  208 , with first current terminal  206  being an anode (denoted “A” in the figures) and second current terminal  208  being a cathode (denoted “C” in the figures). In a case where transistor  202  is an NFET, first current terminal  206  can be a source whereas second current terminal  208  can be a drain. In a case where transistor  202  is a PFET, first current terminal  206  can be a drain whereas second current terminal  208  can be a source. In system  100 , first current terminal  206  can be coupled to battery  102  at a node  220 , and second current terminal  208  can be coupled to load  106  at a node  222 . 
     In  FIG.  2 A , system  100  may include a capacitor  224  and a capacitor  226 . Capacitor  224  can model a combination of parasitic capacitances at node  220 , such as capacitances of wires and electrical connectors between battery  102  and transistor  202 , the junction capacitance at first current terminal  206 , etc. Moreover, capacitor  226  can model a combination of parasitic capacitances at node  222 , such as capacitances of wires and electrical connectors between load  106  and transistor  202 , the junction capacitance at second current terminal  208 , etc. Capacitor  226  can also include a physical hold up capacitor to provide a temporary power supply to load  106  when transistor  202  is disabled. 
     Transistor  202  can be coupled to and controlled by controller circuit  200  to emulate an ideal diode having the same anode and cathode as body diode  210 . In some examples, controller circuit  200  can include a terminal  230  adapted to be coupled to first current terminal  206 . First current terminal  206  can be the anode of the ideal diode. Controller circuit  200  can also include a terminal  232  adapted to be coupled to gate  204  of transistor  202 , and a terminal  234  adapted to be coupled to second current terminal  208  of transistor  202 . Second current terminal  208  can be the cathode of the ideal diode. Terminals  230 ,  232 , and  234  can include interconnects (e.g., chip-chip interconnects, traces on printed circuit board (PCB), etc.) that allow signals (e.g., current, voltage, etc.) to flow between controller circuit  200  and transistor  202 . Controller circuit  200  can monitor the anode voltage V A  at first current terminal  206  and the cathode voltage V C  at second current terminal  208 , and adjust the voltage of gate  204  of transistor  202  via terminal  232  responsive to changes of the anode-cathode voltage V AC  to emulate an ideal diode coupled between battery  102  and load  106 . 
     The right side of  FIG.  2 A  illustrates an example transfer function graph  212  of an ideal diode to be emulated by transistor  202 . Transfer function graph  212  illustrates a relationship between the amount of a forward current I F  conducted by the diode, from anode to cathode, with respect a difference voltage between the anode and cathode V AC . If V AC  is below a forward voltage V F , the diode can be reverse-biased, and no forward current (or a minimum amount of forward current) flows through the diode. If V AC  is above the forward voltage V F , the diode is forward bias and can conduct a forward current I F . When the diode is forward-biased, the anode-cathode voltage V AC  can remain constant at V F  independent of the amount of forward current I F  being conducted, so that the cathode voltage V C  can be equal to the anode voltage V A  minus the forward voltage V F . 
     To emulate the ideal diode, in a case where V AC  is above a forward voltage threshold representing the forward voltage of the ideal diode, controller circuit  200  can increase the gate-source voltage (V GS ) of transistor  202  (if transistor  202  is an NFET), or the source-gate voltage (V SG ) of transistor  202  (if transistor  202  is a PFET), to be above a threshold voltage V th  of the transistor. Raising V GS  (V SG ) to above V th  can turn on/enable transistor  202  by forming a conduction channel between first current terminal  206  and second current terminal  208  under gate  204 . The conduction channel can transmit a positive voltage and a forward/positive current from battery  102  to load  106 . However, in a case where V AC  is below the forward voltage threshold, controller circuit  200  can reduce the gate-source voltage V GS  (if transistor  202  is NFET) or source-gate voltage V SG  (if transistor  202  is PFET) to be below the threshold voltage V th . Dropping V GS  (or V SG ) below V th  can turn off/disable transistor  202  by removing (or at least reducing) the conduction channel. Body diode  210  is reverse-biased due to V AC  being below the forward voltage threshold, and the reverse-biased body diode can block a negative voltage and a reverse/negative current from reaching load  106  from battery  102 . 
     Although transfer function graph  212  shows that an ideal diode has a single forward voltage V F , in some examples controller circuit  200  can enable a conduction channel of transistor  202  (between first current terminal  206  and second current terminal  208 ) in response to V AC  exceeding multiple thresholds, which can indicate that the battery is connected with the correct polarity. Controller circuit  200  can also disable/remove the conduction channel of transistor  202  to block a reverse current/negative voltage in response to V AC  being below a reverse bias threshold, which can indicate a reverse battery connection. Such arrangements can improve the robustness of system  100  in light of transient noises. 
       FIG.  2 B  illustrates a flowchart of an example method  240  performed by controller circuit  200  in controlling transistor  202 . Method  240  can be performed after controller circuit  200  starts up and has not yet started enabled transistor  202 . 
     In step  241 , controller circuit  200  can determine an anode-cathode voltage (V ac ) across transistor  202 . Controller circuit  200  can monitor the anode voltage (V A ) at terminal  230  and the cathode voltage (V C ) at terminal  234 . Controller circuit  200  can include a subtraction circuit (e.g., implemented using a differential amplifier) to subtract V C  from V A  to obtain V AC . 
     Controller circuit  200  can then proceed to compare V AC  with a forward conduction threshold voltage V F-on , in step  242 . If V AC  exceeds V F-on , controller circuit  200  can start a regulation loop to raise the gate-source voltage V GS  (or V SG  if transistor  202  is PFET) to enable a conduction channel of transistor  202 , and to regulate V AC  at a target forward voltage V F-reg , in step  243 . V F-reg  can represent V F  of an ideal diode in transfer function graph  212  of  FIG.  2 A , and transistor  202  can be controlled to emulate a forward-biased diode. In step  244 , V AC  reaches (and can be regulated) at V F-reg . 
     The forward conduction threshold voltage V F-on  can be made higher than V F-reg . By having V AC  to be higher than V F-on  (and to be much higher than V F-reg ) to start the forward conduction, the likelihood of mistaking a transient noise at node  220  as a positive voltage supplied by battery  102 , and falsely enabling transistor  202  as a result, can be reduced. The target forward voltage V F-reg  can be regulated at a lower voltage than V F-on  to reduce voltage drop and power loss across transistor  202  when emulating the forward-biased diode. 
     Also, controller circuit  200  can compare V AC  with a reverse bias threshold voltage V R , in step  246 , to detect a reverse battery connection. The reverse bias threshold voltage V R  can be a negative voltage that can be received from the negative terminal of battery  102  when the polarity of battery  102  is reversed. Therefore, comparing V AC  against a negative voltage to detect a reverse battery connection can reduce the likelihood of false detection of reverse battery connection, such as caused by a transient voltage at node  220 . If V AC  is below V R , which can indicate a reverse battery connection, or if V AC  is above V R  but below V F-on , which can indicate a small transient voltage rather than a large positive voltage supplied by battery  102 , controller circuit  200  can maintain transistor  202  in a disabled state, in step  247 . In a case where transistor  202  is disabled and the conduction channel is removed, the reverse-biased body diode  210  can block a negative voltage/a reverse current. 
       FIG.  2 C  illustrates examples of internal components of controller circuit  200 . Referring to  FIG.  2 B , controller circuit  200  can include a gate control circuit  250 . Gate control circuit  250  can include an input  251 , an input  252 , and an output  253 . Input  251  can be adapted to be coupled to first current terminal  206  of transistor  202 , which can be the anode of the diode to be emulated, via terminal  230 . Output  253  can be adapted to be coupled to gate  204  via terminal  232 . Controller circuit  200  further includes a voltage subtractor circuit  254 , which can include an op-amp subtractor or other suitable circuits, to receive an anode voltage V A  via terminal  230  and a cathode voltage V C  via terminal  234 , generate an anode-cathode voltage V AC  representing a difference between V A  and V C , and provide V AC  to input  252  of gate control circuit  250 . Gate control circuit  250  can generate a gate voltage signal V G  in response to V AC , based on the techniques described in method  240  of  FIG.  2 B , and provide gate voltage signal V G  via output  253  and terminal  232  to enable transistor  202  to conduct a forward current from the anode to the cathode (and from battery  102  to load  106 ), or to disable transistor  202  to block the flow of a reverse current from the cathode back to the anode (and from load  106  back to battery  102 ). 
     Also, controller circuit  200  can include a local voltage generator circuit  256  to generate local voltages. Local voltage generator circuit  256  can receive the anode voltage, which can be a positive voltage provided by battery  102 , via terminal  230  as an input (V in ). Local voltage generator circuit  256  can provide a high supply voltage (V h ) to a high power supply terminal (labelled “PWRH” in  FIG.  2 C ) of gate control circuit  250 , and a low supply voltage (V l ) to a low power supply terminal (labelled “PWRL” in  FIG.  2 C ) of gate control circuit  250 . The high supply voltage and the low supply voltage can be generated from the anode voltage V A  and supplied to gate control circuit  250  to reduce the drain-source voltages (VDS) of devices of gate control circuit  250  and the resulting voltage stress. Local voltage generator circuit  256  can include a charge pump to generate the high supply voltage V h  by adding an offset voltage to the anode voltage V A . Local voltage generator circuit  256  can also include a linear regulator, such as a floating-rail low drop out (LDO) regulator, to generate the low supply voltage Vi by subtracting an offset voltage from the anode voltage V A . 
       FIG.  2 D  illustrates examples of internal components of gate control circuit  250 . Referring to  FIG.  2 D , gate control circuit  250  can include a reverse current blocking (RCB) circuit  260 , which can include a network of comparators including comparators  262   a  and  262   b , an RCB logic circuit  264 , and a switch  266 . Switch  266  is coupled between input  251  (which can be coupled to first current terminal  206 /anode of transistor  202 ) and output  253  (which can be coupled to gate  204 ). Gate control circuit  250  can also include a forward conduction control circuit  270 , which can include an amplifier  272 , such as an operational transconductance amplifier (OTA), an op-amp, etc., and a switch  274 . Switch  274  can be coupled between the output of amplifier  272  and output  253 . In some examples, switch  274  can be part of a switchable output stage of amplifier  272 . RCB logic  264  can control switches  266  and  274  via a pair of complimentary control signals  280   a  and  280   b . Accordingly, when switch  266  is closed, switch  274  can be opened, and vice versa. Gate control circuit  250  can include an inverter  282  to generate control signal  280   b  from control signal  280   a.    
     RCB circuit  260  and forward conduction control circuit  270 , through switches  266  and  274 , can set the gate-source voltage V GS  of transistor  202  in response to the anode-cathode voltage V AC , to enable the flow of a forward current from the anode to the cathode (and from battery  102  to load  106 ), and to block the flow of a reverse current from the cathode back to the anode (and from load  106  back to battery  102 ), based on techniques described in  FIG.  2 B . 
     Specifically, referring to  FIG.  2 E , each of comparators  262   a  and  262   b  can receive V AC  from voltage subtractor circuit  254 . Comparator  262   a  can compare V AC  against forward conduction threshold voltage V F-on  to generate a first decision, and comparator  262   b  can compare V AC  against reverse bias threshold voltage V R  to generate a second decision. If V AC  is below V F-on  (which can indicate V AC  is raised by a small transient voltage), or if V AC  is below V R  (which can indicate a reverse battery connection), RCB logic  264  can provide a control signal  282   a  to disable transistor  202 . Controller circuit  200  can generate control signal  280   a  to close switch  266  to connect first current terminal  206  with gate  204 . By connecting first current terminal  206  with gate  204 , the gate voltage V G  can be set to be equal to the source voltage V s , and the gate-source voltage (V GS ) for transistor  202  can be reduced to zero. With the V GS  voltage below a threshold voltage V th  for forming a channel below gate  204 , transistor  202  can be disabled, and the flow of current between first current terminal  206  and second current terminal  208  of transistor  202  can also be disabled. Moreover, inverter  282  can generate control signal  280   b  as a complimentary version of control signal  280   a  to open switch  274 , and the output of amplifier  272  can be disconnected from output  253  (and terminal  232 ) to avoid interfering with the setting of the gate-source voltage (V GS ) for transistor  202  by RCB circuit  260 . 
     In some examples, RCB logic circuit  264  can include a timing circuit, such as a timer. RCB logic circuit  264  can start the timer after disabling switch  266 . The timer can define an RCB timing window in which transistor  202  is to be continuously disabled regardless of whether V AC  is below or above the forward conduction threshold voltage V F-on , and switch  266  is to be continuously enabled. Within the RCB timing window, RCB logic circuit  264  can ignore decisions from comparators  262   a  and  262   b  to continue closing switch  266  to disable transistor  202 , and continue opening switch  274  to disconnect the output of amplifier  272  from gate  204 . Such arrangements can reduce the likelihood of controller circuit  200  falsely starting a forward conduction due to transient signals at the anode/cathode. The duration of the RCB timing window can be fixed (e.g., built into RCB logic circuit  264 ) or can be programmable via a register coupled to RCB logic circuit  264  (not shown in the figures). 
       FIG.  2 F  illustrates examples of operations of gate control circuit  250  to enable forward conduction. Referring to  FIG.  2 F , if the decisions of comparators  262   a  and  262   b  indicate that V AC  is higher than V F-on , which can indicate that the battery is connected in the correct polarities (e.g., positive terminal being coupled to the anode of transistor  202 ), RCB logic  264  can generate control signal  280   a  to open switch  266 , while control signal  280   b , being a complimentary version of control signal  280   b , can close switch  274  to connect the output of amplifier  272  with output  253  (and gate  204  via terminal  232 ). Amplifier  272  is then allowed to adjust the gate voltage V G  (or decrease the gate voltage V G  if transistor  202  is PFET) via output  253  and terminal  232 . With the anode voltage V A  largely fixed by battery  102 , if the gate-source voltage V GS  (or V SG  if transistor  202  is PFET) becomes higher than the threshold voltage V th  of transistor  202 , a conduction channel can be created between first current terminal  206  and second current terminal  208  of transistor  202 . The conduction channel can then enable the flow of forward current I F  from first current terminal  206  to second current terminal  208  of transistor  202  (and from battery  102  to load  106 ). 
     Also, amplifier  272  can implement a feedback loop to set the gate voltage of transistor  202  to regulate the voltage V AC  across transistor  202  at a value equal to V F-reg  across different forward currents IF, to emulate a forward-biased diode as shown in  FIG.  2 A . Amplifier  272  can generate an output (e.g., a current, a voltage, etc.) that is linearly related to a difference between the anode-cathode voltage V AC  and a target forward voltage V F-reg  to adjust the on-resistance of the conduction channel of transistor  202 . The current provided by amplifier  272  can be converted to a voltage to set the gate voltage of transistor  202 , which in turn can set the on-resistance of transistor  202 . The on-resistance of transistor  202  can be adjusted, so the voltage V AC  across transistor  202  (which can be equal to a product between the on-resistance and the forward current) is maintained at the target forward voltage V F-reg . For example, if load  106  sinks more current, the voltage V AC  across transistor  202  can become larger than V F . In response, amplifier  272  can increase the gate-source voltage V GS  of transistor  202  (or the source-gate voltage V SG  if transistor  202  is PFET) to reduce the on-resistance of transistor  202 , to reduce the voltage V AC  back to V F-reg . However, if load  106  sinks less current, the voltage V AC  can decrease. In response, amplifier  272  can reduce the gate-source voltage V GS  of transistor  202  (or V SG  if transistor  202  is PFET) to increase its on-resistance, to increase the voltage V AC  back to V F-reg . 
     With such arrangements, a voltage V AC  across transistor  202  can be maintained to emulate a forward-biased diode. The voltage provided by transistor  202  to load  106  can be maintained constant (or within a narrow range) and can be independent of forward current I F . This also allows the internal power supply (e.g., internal power supply  108 ) of load  106  to provide a stable supply voltage. Moreover, V AC  can be maintained at a low value to reduce power loss incurred by transistor  202 , especially when transistor  202  conducts a huge forward current I F  to load  106 . 
     Referring again to  FIG.  2 D , RCB circuit  260  can receive low supply voltage Vi from local voltage generator circuit  256 , and forward conduction circuit  270  and inverter  282  can receive both low supply voltage V l  and high supply voltage V h  from local voltage generator circuit  256 . Specifically, RCB circuit  260  can operate within a voltage range below the anode voltage V A  to either disable transistor  202  by shorting the gate and source of transistor  202 , or releasing the gate of transistor  202 , therefore RCB circuit  260  can operate on low supply voltage Vi to reduce voltage stress and to improve reliability of the internal devices of RCB circuit  260 . Moreover, forward conduction control circuit  270  and inverter  282  can operate within a voltage range above the anode voltage V A . Such arrangements can increase gate overdrive voltage to enable transistor  202  while limiting the gate-drain voltage (V GD ) and gate-source voltage (V GS ) to reduce voltage stress across transistor  202 , which can improve the reliability of transistor  202 . Also, by operating forward conduction control circuit  270  and inverter  282  between the high supply voltage and the low supply voltage, the voltage swing in the devices of forward control circuit  270  and inverter  282  can be reduced, which can also reduce voltage stress and improve reliability of the internal devices of forward control circuit  270  and inverter  282 . 
     Referring back to  FIG.  1   , while battery  102  outputs a DC voltage signal, reverse battery protection system  112  may receive AC voltage signals that are superimposed on the DC voltage output by battery  102 . The AC voltage signals can originate from various sources, such as electromagnetic interference, electrical noises, etc., from other electrical systems. For example, in a case where the system  100  is part of a vehicle, various electrical components, such as motors, switching power converters, etc., can generate periodic AC voltage signals that can be coupled into the wires that couple between battery  102  and electric power system  104 . The AC voltage signals can appear as part of the output voltage of battery  102  and can be superimposed with the DC voltage signal. The AC voltage signals can have a positive half-cycle and a negative half-cycle. In the positive half-cycle, the AC voltage signals add to the DC voltage signal to provide an increased anode voltage. In a negative half-cycle, in which the AC signals have negative voltages that subtract from the DC voltage signal to provide a reduced anode voltage. Body diode  210  of transistor  202  can switch between a forward-biased condition during the positive half-cycle and a reverse-biased condition periodically during the negative half-cycle. To protect load  106 , controller circuit can control the transistor to operate as a rectifier where the transistor is enabled under the forward-biased condition and is disabled under the reverse-biased condition. 
       FIG.  3 A  illustrates an example of a DC voltage signal of battery  102  superimposed with AC voltage signals, and the rectification operation of reverse battery protection system  112 . Referring to the left side of  FIG.  3 A , graph  302  is an example graph of the anode voltage V A  with respect to time. The output voltage of battery  102  provides a DC input voltage (labelled “V in_DC ” in  FIG.  3 A ) to reverse battery protection system  112 . As shown in graph  302 , the anode voltage V A  can include the DC component (V in_DC ) as well as an AC component having an amplitude labelled “V in_AC ” superimposed with the DC component. The AC component can be attributed to the AC voltage signals. The AC component can be periodic and can have a positive half-cycle in which the AC component has a positive voltage that adds to the DC voltage and a negative half-cycle in which the AC component has a negative voltage that subtracts from the DC voltage. For example, the durations between times T 0  and T 1  and between times T 2  and T 3  can represent positive half-cycles, in which the AC component can add to the DC component, and output voltage of battery  102  (and anode voltage V A ) exceeds the DC component V in_DC  and has a maximum voltage of V in_DC +V in_AC . Moreover, the duration between times T 1  and T 2  can represent a negative half-cycle, in which the AC components can be subtracted from the DC component, and output voltage of battery  102  (and anode voltage V A ) falls below the DC component V in_DC  and has a minimum voltage of V in_DC −V in_AC . 
     Transistor  202  can switch between a forward-biased condition and a reverse-biased condition periodically between each half cycle of the AC signals superimposed with the DC voltage of battery  102 . During the positive half-cycles, the anode voltage (e.g., between V in_DC  and V in_DC +V in_AC ) can be higher than the cathode voltage (e.g., V in_DC ) and put transistor  202  in the forward-biased condition, and capacitor  226  can be charged up to V in_DC . During the negative half-cycles, the anode voltage (e.g., between V in_DC −V in_AC  and V in_DC ) can be lower than the cathode voltage (e.g., V in_DC ) and put transistor  202  in the reverse-biased condition. As part of the rectification operation to prevent a reverse current from flowing back from load  106  to battery  102  during the negative half-cycles, controller circuit  200  can enable transistor  202  during the positive half-cycles and disable transistor  202  during the negative half-cycles. 
     Graph  304  of  FIG.  3 A  shows an example graph of the cathode voltage V C  with respect to time as a result of a rectification operation performed by controller circuit  200  with transistor  202 . Specifically, during a positive half-cycle, such as between times T 0  and T 1 , the anode-cathode voltage V AC  can exceed the forward conduction threshold voltage V F-on , and controller circuit  200  can enable transistor  202  to form a conduction channel between first current terminal  206  and second current terminal  208  to connect battery  102  to load  106 . Accordingly, the cathode voltage V C  across load  106  can track the anode voltage V A . The voltage V AC  can also be maintained at the target forward voltage V F-reg  by amplifier  272 . Accordingly, the cathode voltage V C  can have a DC component of V out _DC which equals V in_DC -V F-reg , as well as an AC component having the same (or similar) amplitude V in_AC  as the AC signals at the anode. Within the positive half-cycle, the cathode voltage V C  can first increase from V out_DC , reaching a peak of V out_DC +V in_AC , and then decrease back to V out _DC. 
     Moreover, during a negative half-cycle (e.g., between times T 1  and T 2 ), the anode voltage V A  is reduced by the AC component, while the cathode voltage V C  can be held at V out_DC  by holdup capacitor  226 , and the anode-cathode voltage V AC  can be below the reverse bias threshold voltage V R . This can cause controller circuit  200  to disable transistor  202  by removing/disabling the conduction channel between first current terminal  206  and second current terminal. Accordingly, the cathode voltage V C  can stop tracking the anode voltage V A  during the negative half-cycle. The negative half-cycle is followed by a subsequent positive half-cycle (e.g., between times T 2  and T 3 ), in which controller circuit  200  can enable transistor  202  again to allow a forward conduction from battery  102  to load  106 , and the cathode voltage V C  can track the anode voltage V A  again. 
     The AC voltage signals that superimpose with the DC voltage signal output by battery  102  can have a high frequency, which can lead to high frequency changes in the input voltage to protection system  112 .  FIG.  3 B  illustrates examples of AC voltage signals that may be received by protection system  112  in a vehicle application. For example, according to LV  124  and LV  148 , which define testing of electronic components for vehicles, the electric systems of a vehicle can have AC voltage signals of 2-6 peak-to-peak voltage (Vpp) at a frequency of 15 Hz to 200 kHz superimposed on the DC voltage signal output by a battery. Assuming the AC voltage signals have a frequency of 200 kHz and a cycle time of 5 micro-seconds (us), to perform rectification, controller circuit  200  may need to be able to repeatedly enable transistor  202  within a positive half-cycle of 2.5 us and disable transistor  202  within a negative half-cycle of 2.5 us. Accordingly, it is desirable that the controller circuit  200  can have a short rectification response time, in order to enable and disable the transistor promptly in response to high-frequency AC voltage signals present at the battery output. 
     But controller circuit  200  may have a long rectification response time, especially in enabling transistor  202  to start forward conduction, which makes it challenging to handle AC ripples up at a frequency of 200 kHz and beyond. Specifically, as described above, to emulate a forward-biased diode having a constant forward voltage, controller circuit  200  (and forward conduction control circuit  270 ) may include amplifier  272 , which can be linear amplifier such as an OTA, to implement a feedback loop to regulate the anode-cathode voltage V AC  across the transistor at the target forward voltage V F-reg . The output of the amplifier can be linearly related to V AC . But the amplifier may have a low gain (e.g., a low transconductance (gm) for OTA) to improve loop stability, especially for low forward current/load current. The low gain can reduce the speed by which the amplifier can raise the gate voltage of the transistor, which in turn increases the time it takes for controller circuit  200  to enable transistor  202  (by forming a conduction channel between first current terminal  206  and second current terminal  208 ) in the positive half-cycle of an AC ripple. 
       FIG.  3 C  illustrates a graph  310  and a graph  320 . Graph  310  provides an example of variations of V AC  with respect to time, and graph  320  provides an example of variations of gate-source voltage V GS  of transistor  202  with respect to time, where transistor  202  is under the control of controller circuit  200 . Referring to graph  310 , V AC  can increase from a first voltage below V R  to a second voltage above V F-on  at time T 0 . In response to detecting that V AC  increases from below V R  to above V F-on , gate control circuit  250  to switch from disabling transistor  202  to enabling transistor  202  by changing the gate voltage. 
     Referring to graph  320 , the gate-source voltage V GS  is initially at 0V and transistor  202  is disabled prior to time T 0 . In response to detecting the transition of V AC  at time T 0 , gate control circuit  250  may maintain V GS  at 0V, and transistor  202  can remain in the disabled state, until time T 1 . At time T 1 , gate control circuit  250  can start increasing V GS . At time T 2 , V GS  reaches the threshold voltage V th  of transistor  202 . In response to V GS  reaching V th , transistor  202  can form a conduction channel between first current terminal  206  and second terminal  208 , and transistor  202  is enabled. Accordingly, a total delay T D  has elapsed from the time T 0  when the anode-cathode voltage V AC  transitions from lower than V R  to higher than V F-on  to the time T 2  when transistor  202  is enabled. 
     The delay T D  can be attributed to various sources. For example, a first part of the delay T D , between time T 0  and T 1 , can be attributed to the delay incurred by RCB  260  in detecting the changes in V AC  and opening switch  266  (and closing switch  274 ) to allow amplifier  272  to start increasing gate voltage V G . Moreover, a second part of the delay T D , between times T 1  and T 2 , can be incurred by amplifier  272  in raising the gate voltage V G  as part of the feedback loop to regulate V AC  at V F-reg . The rate at which amplifier  272  increases V G  can be based on, for example, the gain (e.g., transconductance) of amplifier  272 , the capacitance of gate  204 , etc. 
     The total delay T D  between time T 0  and time T 2  can represent a rectification response time of protection system  112 . Depending on various factors, such as comparator delay, the transconductance of amplifier  272 , the capacitance of gate  204 , etc., the rectification response time T D  can be near 2 us. In a case where the AC ripples have a frequency of 200 kHz and beyond, the half-cycle period will be less than 2.5 us. With a rectification response time that spans most of the positive half-cycle period, transistor  202  can remain disabled for most of the positive half-cycle period, which can prevent load  106  from receiving electric power from battery  102  during most of the positive half-cycle period. Accordingly, protection system  112  may be unable to perform the rectification operation in response to high-frequency AC signals present at the output of battery  102 . 
       FIG.  4 A  through  FIG.  4 F  illustrate examples of a controller circuit  400  that can address at least some of the issues. As shown in  FIG.  4 A , controller circuit  400  can be part of system  100  of  FIG.  1    and can include RCB circuit  260  of  FIGS.  2 A through  2 F . In addition, controller circuit  400  can include a forward conduction (FC) control circuit  402 , which can include an FC acceleration circuit  404 , as well as amplifier  272  and switch  274  as described in  FIGS.  2 A through  2 F . RCB circuit  260  and FC control circuit  402  can be part of a gate control circuit  410 , and each of RCB circuit  260  and FC control circuit  402  is coupled with terminal  232  to set the gate voltage of transistor  202 . 
     FC control circuit  402  can set a gate voltage to form a conduction channel between first terminal  206  and second current terminal  208 , to connect battery  102  to load  106 . FC acceleration circuit  404  can first set the gate voltage to form the conduction channel, followed by amplifier  272  adjusting the gate voltage to regulate the V AC  voltage. Specifically, FC acceleration circuit  404  can include a switch  406  coupled between terminal  232  (and gate  204  of transistor  202 ) and a voltage reference  408  (labelled “VREF” in  FIG.  4 A ). Switch  406  can be an NFET, a PFET, or a parallel combination of both. FC acceleration circuit  404  can receive an indication of a forward-biased condition. The indication can be based on control signal  280   a . For example, based on detecting a transition of control signal  280   a  to a state to open switch  266 , FC acceleration circuit  404  can determine that the forward-biased condition is present (with V AC  above V F-on ), and close switch  406 . The closing of switch  406  can connect voltage reference  408  with gate  204  of transistor  202  via terminal  232 , which can raise the gate voltage (or reduce the gate voltage if transistor  202  is PFET) to enable transistor  202 . Voltage reference  408  can provide a target voltage to enable transistor  202 . After transistor  202  is enabled, if protection system  112  is still within the positive half-cycle, forward conduction circuit  402  can open switch  406 , and provide a control signal  416  to close switch  274 , which connects the output of amplifier  272  to terminal  232  and allows amplifier  272  to start a regulation loop to further adjust the gate voltage. 
     Components of FC control circuit  402 , including amplifier  472  and FC acceleration circuit  404 , can operate on a high supply voltage V h  and a low supply voltage V l  generated by local voltage generator circuit  256  from anode voltage V A . For example, voltage reference  408  can be provided by the high supply voltage V h  if transistor  202  is an NFET. Voltage reference  408  can also be provided by low supply voltage V l  if transistor  202  is a PFET. The high supply voltage V h  and low supply voltage v l  can be configured based on, for example, a margin above (or below) a threshold voltage V th  of transistor  202  for forming the conduction channel to enable the transistor. In some examples, the high supply voltage V h  and low supply voltage vi can also be configured based on a voltage stress threshold for the V GS /V SG  voltage, to improve reliability of transistor  202 . Further, switch  406  can be controlled by a control signal that swings between V h  and V l  to reduce the on-resistance as well as the voltage stress on switch  406 , which can improve both the bandwidth and reliability of switch  406 . 
     With the arrangements of  FIG.  4 A , forward conduction control circuit  402  need not rely on amplifier  272  to both start the enabling of transistor  202  and regulate the V AC  of transistor  202 , and the reduced gain/transconductance of amplifier  272  (e.g., due to the constraint of loop stability) can have less impact on the delay in enabling transistor  202 . Moreover, the speed of enabling transistor  202  can be further improved by increasing the bandwidth (e.g., by reducing the on-resistance) of switch  406 . All these can reduce the rectification response time of controller circuit  400  and improve the handling of high-frequency AC signals at the output of battery  102 . 
       FIG.  4 B  illustrates a flowchart of an example method  420  performed by controller circuit  400  in controlling transistor  202 . Method  400  can be performed after controller circuit  400  starts up and has not yet enabled transistor  202 , and with battery  102  supplying a DC voltage to transistor  202 . AC ripples may be coupled into and superimposed with the DC voltage, and appear as an AC component of the anode voltage V A  of transistor  202 . 
     In step  422 , controller circuit  400  can determine an anode-cathode voltage (V AC ) across transistor  202 . Controller circuit  400  can monitor the anode voltage (V A ) at terminal  230  and the cathode voltage (V C ) at terminal  234 . Controller circuit  400  can include a subtraction circuit (e.g., implemented using a differential amplifier) to subtract V C  from V A  to provide V AC . 
     Controller circuit  200  can compare V AC  with forward conduction threshold voltage V F-on  using comparator  262   a , in step  424 . In addition, controller circuit  200  can also compare V AC  with reverse conduction threshold voltage V R , in step  426 . V AC  can exceed V F-on  (and V R ) during a positive half-cycle of the AC signals/ripples, and can be below V F-on  or V R  during a negative half-cycle of the AC signals/ripples, or when the battery is reverse connected. 
     In step  428 , if V AC  exceeds V F-on , RCB blocking logic  264  can output control signal  280   a  to open switch  266  to disconnect RCB circuit  260  from the gate of transistor  202 . The opening of switch  266  can also enable FC control circuit  402  to set the gate voltage of transistor  202 . Moreover, based on control signal  280   a  is in a state to open switch  266 , FC acceleration circuit  404  can close switch  406  to raise the gate voltage of transistor  202  (or reduce the gate voltage if transistor  202  is PFET). As to be described below, FC acceleration circuit  404  can close switch  406  for a pre-determined duration, or until the gate-source voltage exceeds the threshold voltage V th  of transistor  202  (or falls below V th  for PFET). Moreover, if V AC  is below V F-on  or V R  (in steps  424  and  426 ), RCB logic  264  can close switch  266  to bring the gate-source voltage V GS  of transistor  202  to zero to disable the transistor, and switch  406  can be open, in step  430 . 
     Referring back to step  428 , after transistor  202  is enabled by FC acceleration circuit  404  and a conduction channel is formed between first current terminal  206  and second current terminal  208 , controller circuit  400  can determine whether the positive half-cycle ends, in step  432 . The determination can be based on comparing a new V AC  (obtained after transistor  202  is enabled) against V F-on  and V R . If the new V AC  is below either V F-on  or V R , controller circuit  400  can determine that the positive half-cycle has ended, and can proceed to step  430  to disable transistor  202 . 
     Moreover, if the new V AC  is above V F-on  in step  432 , which can indicate that the positive half-cycle has not yet ended, controller circuit  400  can start a regulation loop to regulate V AC  at a target forward voltage V F-reg , in step  434 . As part of step  434 , FC acceleration circuit  404  can transmit control signal  416  to close switch  274 , which allows amplifier  272  to set the gate voltage of transistor  202  via terminal  232 . Amplifier  272  can also implement a feedback loop in which the output of amplifier  272  is linearly related to a difference between V AC  and V F-reg , as described above. In step  444 , V AC  reaches V F-reg , and can be regulated at V F-reg  by amplifier  272 . 
       FIGS.  4 C through  4 F  illustrate examples of additional internal components of FC acceleration circuit  404 . Referring to  FIG.  4 C , in addition to switch  406  and voltage reference  408 , FC acceleration circuit  404  can include a control circuit  450  and a driver circuit  452 . Control circuit  450  can receive control signal  280   a . In response to receiving control signal  280   a , control circuit  450  can provide control signal  416  to switch  274 , and a control signal  454  to switch  406 . Control circuit  450  can operate with the low supply voltage Vi. Driver circuit  452  can receive control signal  454  and convert it to a control signal  456  that swings between a pre-determined minimum voltage v min  and high supply voltage V h , and provide control signal  456  to switch  406 . 
       FIG.  4 D  and  FIG.  4 E  illustrate examples of internal components of control circuit  450 . Control circuit  450  can include a start pulse circuit  460 , an end pulse circuit  462 , and a pulse generator  464 . Pulse generator  464  can generate control signal  454 , which can be in the form a pulse signal, to close switch  406  for a finite duration, with the start time and end time of the pulse signal being defined by, respectively, start pulse circuit  460  and end pulse circuit  462 . Start pulse circuit  460  can include an edge detection circuit  470  to detect a transition in control signal  280   a  that indicates the start of a forward-biased condition. For example, the transition of control signal  280   a  can be to a state to open switch  266 . Responsive to detecting the transition, start pulse circuit  460  can provide a start signal  472  that reflects the timing of the transition to pulse generator circuit  464 . In response to receiving start signal  472 , pulse generator circuit  464  can start the pulse of control signal  454 . 
     In addition, end pulse circuit  462  can provide an end signal  474  to pulse generator circuit  464  to end the closing of switch  406 . End pulse circuit  462  can generate end signal  474  based on various techniques. Referring to  FIG.  4 D , end pulse circuit  462  can include a delay circuit  476  to provide end signal  474  as a delayed version of start signal  472 , with the delay setting a width of the pulse of control signal  454 . Delay circuit  476  can include delay elements (e.g., buffers) to delay pull-up start signal  470 . In some examples, delay circuit  476  can include programmable delay elements such that the delay introduced to start signal  472  in generating end signal  474  is programmable. In such examples, delay circuit  476  can be coupled with a programming register to receive a setting for the delay. 
     The delay of delay circuit  476  (and the pulse width) can be set based on various criteria. For example, the delay can be set based on the minimum half-cycle period of the AC ripples, such that the enabling of the pull-up path can start and end within a positive half-cycle period for the highest-frequency AC ripples to be rectified by controller circuit  400 . The delay can also be set based on other factors, such as the gate capacitance of transistor  202 , which can vary based on the dimension of the transistor. In some examples, the delay can also be set based on a target on-resistance of transistor  202  for the forward conduction operation. For example, based on the threshold voltage of transistor V th , a target gate voltage of transistor  202  can be determined, which can be a certain percentage (e.g., 90%) of the voltage provided by voltage reference  408 . Based on the gate capacitance of transistor  202  as well as the on-resistance of switch  406 , the time need to bring the gate voltage of transistor  202  to the target voltage can be determined, and the pulse width (and delay of delay circuit  476 ) can be pre-configured accordingly. 
       FIG.  4 E  illustrates another example of pulse end circuit  462 . Specifically, pulse end circuit  462  can end the pulse and open switch  406  when the gate-source voltage V GS  goes above a threshold, such as threshold V th  of transistor  202  for forming a conduction channel between first current terminal  206  and second current terminal  208 . Referring to  FIG.  4 E , pulse end circuit  462  can include a comparator  478  to compare the gate-source voltage V GS  (or V SG  for PFET) which can be obtained from a subtraction circuit that subtracts the source voltage V s  (obtained via terminal  230 ) from the gate voltage V G  (obtained via terminal  232 ). If V GS  rises above V th , comparator  478  can trip to generate pull-up end signal  472 , which can signal the end of the pull-up operation to pulse generator  466 . 
       FIG.  4 F  illustrates examples of internal components of driver circuit  450 , as well as switch  406  and voltage reference  408 , in a case where transistor  202  is an NFET. Referring to  FIG.  4 F , switch  406  can be implemented using a PFET, coupled between voltage reference  408  and terminal  232 . Voltage reference  408  can be provided as the high supply voltage V h  from V high  terminal of local voltage generator circuit  256 . In addition, driver circuit  450  includes a level-shifter  480 , a load network  482 , and an NFET  484  coupled between load network  482  and terminal  230 . Level-shifter  480  can have a positive voltage supply terminal (labelled “Vp”) coupled with high supply voltage V h  (and voltage reference  408 ), and a negative voltage supply terminal (labelled “Vn”) coupled with terminal  230 , which provides access to the anode voltage V A . Level-shifter  480  can receive, from pulse generator circuit  464 , control signal  454  that can swing between 0 to V l , and perform a level-shifting of control signal  454  to generate a control signal  486  with a voltage swing between V h  and V A , and drive NFET  484  with control signal  486 . Both control signals  452  and  486  can be pulse signals. 
     In the example of  FIG.  4 F , control signals  454  and  486  can be active-high pulse signals, with the high pulse defining the duration when switch  406  is closed. When NFET  484  is enabled by control signal  486 , NFET  484  can form a voltage divider with load network  482 , and the voltage divider can set the minimum voltage V in  of control signal  456  to PFET/switch  406 . Responsive to control signal  456  being at V min , PFET/switch  406  can be enabled/closed to connect gate  204  of transistor  202  to voltage reference  408 . When NFET  484  is disabled by control signal  486 , load network  482  can pull control signal  456  to the high supply voltage V h , and PFET/switch  406  can be disabled/opened. Load network  482  can include a network of passive elements, such as resistors, Zener diodes, etc., to limit the voltage swing of the gate of pull-up transistor  480  to reduce voltage stress and to improve reliability. 
     With the arrangements of  FIG.  4 F , the gate voltage V G  can be set at a value higher than the anode voltage V A  (e.g., high supply voltage V h ) to enable transistor  202 . As the anode voltage V A  is also the source voltage V S  of transistor  202 , the gate-source voltage V GS  can exceed the threshold voltage V th  to form the conduction channel between first current terminal  206  and second current terminal  208  of transistor  202 . Moreover, by using level-shifter  480  to level-shift a pulse signal that swings between low supply voltage V l  and ground to another pulse signal that swings between high supply voltage V h  and the anode voltage V A , the maximum voltages experienced by the transistor devices of pulse generator circuit  464 , as well as NFET  484  and PFET/switch  406 , can be reduced, which can reduce voltage stress and improve reliability of control circuit  400 . Further, as the anode voltage V A  (and source voltage V s ) may vary based on the DC voltage output by battery  102 , generating the control signals and internal voltages based on the anode voltage V A  can ensure that the control signals and internal voltages have a deterministic relationship with the anode voltage V A  and source voltage V S , which can improve the predictability of control circuit  400 . 
       FIG.  5    illustrates examples of a graph  510  of V AC  across transistor  202  with respect to time, and a graph  520  of the gate-source voltage V GS  of transistor  202  with respect to time, under the control of controller circuit  400 . Referring to graph  510 , V AC  increases from a first voltage below V R  to a second voltage above V F-on  at time T 0 . Referring to graph  520 , the gate-source voltage V GS  is initially at 0V and transistor  202  is disabled. It takes until time T 1  when RCB circuit  260  generates control signal  280   a  to disable switch  266 . 
     Between times T 1  and T 2 ′, responsive to detecting that control signal  280   a  is at a state to open switch  266 , FC acceleration circuit  404  can close switch  406  to raise (or reduce) the gate voltage of transistor  202 . The rate of change of the gate voltage can be impacted more by, for example, the on-resistance of switch  406 , the capacitance of gate  204 , etc., and less by the gain/transconductance of amplifier  472 . Accordingly, in  FIG.  5   , the V GS  voltage can change at a higher rate and can reach the threshold V th  at time T 2 ′, which is earlier than time T 2  in  FIG.  3 C . Accordingly, the total delay T D ′ between times T 1  and T 2 ′ can also be reduced compared with T D  in  FIG.  3 C . Switch  406  can be opened at time T 2 ′. Between times T 2 ′ and T 3 , amplifier  272  can operate a regulation loop to set the V GS  voltage in order to regulate the anode-cathode voltage V AC  at V F-REG . 
       FIG.  6    is a flowchart of an example method  600  for performing a rectification operation by an electric power system coupled between a battery and a load, in a case where the DC voltage signal output by the battery is superimposed with AC signals. Method  600  can be performed by, for example, controller circuit  400  of  FIGS.  4 A through  4 F . Although the method steps are described in conjunction with  FIGS.  4 A through  4 F , any system configured to perform the method steps, in any suitable order, is within the scope of this description. 
     At step  602 , controller circuit  400  can receive, via a first terminal (e.g., terminal  230 ), a first voltage. The first terminal can be adapted to be coupled to a transistor&#39;s first current terminal (e.g., transistor  202 &#39;s first current terminal  206 ). The first current terminal can be an anode of a diode. The first current terminal can be a source if the transistor is NFET, and can be a drain if the transistor is PFET. The first current terminal can be adapted to be coupled to a battery (e.g., battery  102 ). The first voltage can be an anode voltage (V A ). 
     At step  604 , controller circuit  400  can receive, via a second terminal (e.g., terminal  234 ), a second voltage. The second terminal can be adapted to be coupled to the transistor&#39;s second current terminal. The second current terminal can be a cathode of the diode. The second current terminal can be a drain if the transistor is NFET, and can be a source if the transistor is PFET. The second current terminal can be adapted to be coupled to a load (e.g., load  106 ). The second voltage can be cathode voltage (V C ). 
     At step  606 , based on the first voltage and the second voltage, controller circuit  400  can close a switch (e.g., switch  406 ) between a voltage reference (e.g., voltage reference  408 ) and a third terminal (e.g., terminal  232 ) of the controller circuit coupled to a gate of the transistor, in which a voltage of the gate is set by the voltage reference to form a conduction channel between the first current terminal and the second current terminal. 
     Specifically, controller circuit  400  can include switch  406  coupled between terminal  232  (and gate  204  of transistor  202 ) and voltage reference  408 . Voltage reference  408  can provide a target gate voltage to turn on/enable transistor  202 . The target gate voltage can be higher than the source voltage of transistor  202  if transistor  202  is NFET. The target gate voltage can also be lower than the source voltage of transistor  202  is transistor  202  is PFET. Voltage reference  408  can be provided by a local voltage generator circuit, such as local voltage generator circuit  256 , based on the anode voltage. 
     If the anode-cathode voltage V AC  exceeds reverse bias threshold voltage V R  and forward conduction threshold voltage V F-on , RCB logic  264  can generate control signal  280   a  to open switch  266 . Based on control signal  280   a , control circuit  400  can close switch  406  to connect the gate of transistor  202  to voltage reference  408  to raise (or reduce) the gate voltage. In some examples, controller circuit  400  can generate a pulse having a pre-determined pulse width to close switch  406 , with the start of the pulse triggered by control signal  280   a . In some examples, control circuit  400  can end the pulse (and open switch  406 ) based on the gate-source voltage V GS  (or source-gate voltage V SG  for PFET) exceeding the threshold voltage V th  of transistor  202  for forming the conduction channel between first current terminal  206  and second current terminal  208 . 
     Step  606  can be performed as part of a rectification operation during the positive half-cycle of the AC signals. In some examples, if the positive half-cycle has not yet ended when the pulse ends and switch  406  is opened, controller circuit  400  can close switch  274  to enable amplifier  272  to further adjust the gate voltage to regulate V AC  at the target forward voltage V F-reg . If the positive half-cycle has ended and the system is in the negative half-cycle of the AC signals, RCB logic  264  can close switch  266  to set the V GS  voltage (or V SG  voltage for PFET) to zero to disable transistor  202 . 
     Accordingly, in some examples as described above, a protection system can include a controller circuit and a transistor, which can be a power transistor. Examples of protection system  100  are shown in  FIG.  1    through  FIG.  4 B , whereas examples of controller circuit  400  are shown in  FIGS.  4 A through  4 D . Referring to  FIG.  4 A , controller circuit  400  can include a first terminal (e.g., terminal  230 ), a second terminal (e.g., terminal  234 ), and a third terminal (e.g., terminal  232 ). The first terminal is adapted to be coupled to a first current terminal of a transistor (e.g., first current terminal  206  of transistor  202 ). The second terminal is adapted to be coupled to a second current terminal of the transistor (e.g., second current terminal  206  of transistor  202 ). The third terminal is adapted to be coupled to a gate of the transistor (e.g., gate  204 ). Controller circuit  400  can control the transistor to emulate an ideal diode, of which the first current terminal is the anode and the second current terminal is the cathode. The anode can be adapted to be coupled to a battery (e.g., battery  102 ), and the cathode can be adapted to be coupled to a load (e.g., load  106 ). In a case where the transistor is an NFET, the first current terminal can be a source and the second current terminal can be a drain. In a case where the transistor is a PFET, the first current terminal can be a drain and the second current terminal can be a source. 
     Referring to  FIG.  4 A , controller circuit  400  can include gate control circuit  410 , which can include reverse current blocking (RCB) circuit  260  and forward conduction (FC) control circuit  402 . FC control circuit  402  can include a FC acceleration circuit  404  as well as amplifier  272 . Each of RCB circuit  260 , FC acceleration circuit  404 , and amplifier  272  can be coupled to terminal  232  (and gate  204  of transistor  202 ) to set a gate voltage. RCB circuit  260  can set the gate-source voltage V GS  (or source-gate voltage V SG  for PFET) to zero to disable transistor  202  if the anode-cathode voltage V AC  is below a reverse bias threshold voltage V R  or a forward conduction threshold voltage V F-on . RCB circuit  260  may include switch  266  coupled between terminals  230  and  232 , and may generate control signal  280   a  to close switch  266  to set V GS /V SG  to zero to disable transistor  202 , or to open switch  266  to enable FC control circuit  402  to set the gate voltage. In some examples, RCB circuit  260  can disable transistor  202  in the negative half-cycles of the AC signals that superimpose with the DC signal output by the battery, as part of a rectification operation performed by controller circuit  400  with transistor  202 . 
     In addition, FC control circuit  402  can enable transistor  202  by raising (or reducing) the gate voltage to increase V GS /V SG  to enable transistor  202 , if V AC  is above V F-on . FC acceleration circuit  404  can include switch  406  coupled between voltage reference  408  and terminal  232 . If V AC  is above V F-on , FC acceleration circuit  404  can close switch  406  to connect terminal  232  (and gate  204 ) to voltage reference  408  to change the gate voltage. The rate at which the gate voltage changes can be based on, for example, the on-resistance of switch  406 , the capacitance of gate  204 , etc., and can be independent from the gain/transconductance of amplifier  272 . FC acceleration circuit  404  can generate a pulse to close switch  406  for a finite duration. The start time of the pulse can be based on detection of V AC  being above V F-on  (e.g., based on control signal  280   a ). The end time of the pulse can be based on a pre-determined delay from the start time, or based on the V GS  voltage (or V SG  voltage for PFET) exceeding a threshold voltage (e.g., V th  of transistor for forming the conduction channel). In some examples, FC control circuit  402  can enable transistor  202  in the positive half-cycles of the AC signals, as part of a rectification operation performed by controller circuit  400  with transistor  202 . 
     Moreover, the output of amplifier  272  can be coupled to terminal  232  via switch  274 . After the pulse that closes switch  406  ends, if V AC  is still above V F-on  (e.g., the positive half-cycle has not yet ended), FC control circuit  402  can close switch  274  to allow amplifier  272  to further adjust the gate voltage by regulating V AC  at the target forward voltage V F-reg . 
     In this description, the term “couple” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action, then: (a) in a first example, device A is coupled to device B; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A. Also, in this description, a device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. Furthermore, in this description, a circuit or device that includes certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture, such as by an end-user and/or a third party. 
     While certain components may be described herein as being of a particular process technology, these components may be exchanged for components of other process technologies. Circuits described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available before the component replacement. Components illustrated as resistors, unless otherwise stated, are generally representative of any one or more elements coupled in series and/or parallel to provide an amount of impedance represented by the illustrated resistor. For example, a resistor or capacitor illustrated and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in series or in parallel between the same two nodes as the single resistor or capacitor. Also, uses of the phrase “ground voltage potential” in this description include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. Unless otherwise stated, “about,” “approximately,” “nearly,” or “substantially” preceding a value means+/−10 percent of the stated value. 
     Modifications are possible in the described examples, and other examples are possible, within the scope of the claims.