Patent Publication Number: US-2023138627-A1

Title: Handling of battery loss event

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. It is also desirable that the protection system can handle other events, such as a battery loss event in which the battery is disconnected from or otherwise cannot transfer electric power to the electric power system. 
     SUMMARY 
     A controller circuit includes a voltage subtractor circuit, a gate control circuit, and a discharge 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 gate control circuit has a gate control input and a gate control output. The gate control input is coupled to the subtractor output. The gate control output is adapted to be coupled to a gate of the transistor. The discharge circuit has a discharge circuit input and a discharge circuit output. The discharge circuit input is coupled to the gate control circuit. The discharge circuit output is adapted to be coupled to the first current terminal of the transistor. 
     In a method, a first voltage is received via a first terminal of a controller circuit. The first terminal is coupled to a transistor&#39;s first current terminal. A second voltage is received via a second terminal of the controller circuit. The second terminal is coupled to the transistor&#39;s second current terminal. Based on the first voltage and the second voltage, a gate voltage of the transistor is provided via a third terminal of the controller circuit. The third terminal of the controller circuit is coupled to a gate of the transistor. Based on an indication of whether the gate voltage has been changed, a charge is removed from the first diffusion of the transistor via the first 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 operations performed by the protection system of  FIGS.  2 A through  2 F  in a battery loss event. 
         FIGS.  4 A through  4 D  illustrate examples of a protection system including internal components to handle a battery loss event in accordance with various examples. 
         FIG.  5 A  and  FIG.  5 B  illustrate examples of voltage and current graphs depicting the operations of the protection system of  FIGS.  4 A through  4 D  in a battery loss event. 
         FIG.  6    is a flowchart of an example method for handling a battery loss event 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 turn off/disable the transistor and rely on the reverse-biased 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 turn on/enable the transistor to transmit a positive voltage and a forward current from the battery to the load. For reasons to be described below, the controller circuit may repeatedly enable and disable the transistor in a battery loss event where the battery is disconnected from (or otherwise does not drive) the anode. Example techniques described herein reduce or even eliminate the repeated enabling and disabling of the transistor by the controller circuit in a battery loss event, which can improve the predictability of the protection system&#39;s behavior in the battery loss event. 
       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 1 ) 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 (V DS ) 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 V l  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 Vs, 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 I F , 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 V l  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 V l  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 . 
     While controller circuit  200  and transistor  202  of  FIGS.  2 A through  2 F  can protect load  106  from a reverse battery connection, issues may arise in a battery loss event. A battery loss event can occur, such as when battery  102  is disconnected from the anode of transistor  202 , or battery  102  no longer supplies charge to the anode, etc., so the anode becomes floating and not driven by the battery. In such a battery loss event, controller circuit  200  of  FIGS.  2 A through  2 F  may cause transistor  202  to transition between an enabled state and a disabled state repeatedly due to charge coupling between the gate and the anode of the transistor, and cannot properly disable the transistor as a result. 
       FIG.  3 A  illustrates examples of operations of controller circuit  200  and transistor  202  in a battery loss event, while  FIG.  3 B  and  FIG.  3 C  illustrate examples of voltage and current graphs of controller circuit  200  and transistor  202  in a battery loss event. The description is based on transistor  202  being an NFET for brevity, but can also apply to a case where transistor  202  is a PFET. As shown in  FIG.  3 A , transistor  202  can include a parasitic capacitance  302  between gate  204  and first current terminal  206 , which can be gate-source capacitance C GS . Transistor  202  can also include a parasitic capacitance  304  between gate  204  and second current terminal  208 , which can be gate-drain capacitance C GD . Parasitic capacitance  302  can store a charge that reflects the gate-source voltage V GS , and parasitic capacitance  204  can store a charge that reflects the gate-drain voltage V GD .  FIG.  3 B  illustrates a voltage graph  310  of anode voltage V A  and cathode voltage V C , a voltage graph  320  of control signal  265  provided by RCB logic circuit  264 , and a voltage graph  330  of gate-source voltage (V GS ) of transistor  202 . 
     Referring to  FIG.  3 A  and  FIG.  3 B , between times T0 and T1, a battery loss event occurs where battery  102  is disconnected from node  220 , which is coupled to capacitor  224 , switch  266 , and first current terminal  206  of transistor  202  that forms the anode. As a result, node  220  (and the anode of the diode) is no longer driven by battery  102 , and the anode-cathode voltage V AC  may become lower than the forward voltage V R , which can be detected by comparator  262   b . Referring to graph  310  of  FIG.  3 A , this can be represented by V A  being below V C  by more than V R  at time T0. Referring to graph  330 , RCB circuit  260  can enable switch  266  to electrically short gate  204  with first current terminal  206  to bring gate-source voltage (V GS ) to near zero at time T1, thereby disabling transistor  202 . Moreover, referring to graph  320 , the enabling of switch  266  can also start a RCB timing window  340  at time T0. 
     The change of the gate-source voltage V GS  can cause charge previously stored in parasitic capacitance  304  (C GD ) of transistor  202 , represented by charge  306  in  FIG.  3 A , to flow from gate  304  via switch  266  into node  220 /first current terminal  206  of transistor  202  at time T1. As the anode is not being driven by battery  102 , the injection of the charge can cause the anode voltage V A  to go up. The resulting anode voltage V A  can be based on the gate-drain voltage V GD  prior to switch  266  being switched off, and a ratio between parasitic capacitance  304  (C GD ) and the parasitic capacitance at node  220  (and the anode of the diode) represented by capacitor  224 , as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     A 
                   
                   = 
                   
                     
                       
                         C 
                         GD 
                       
                       × 
                       
                         V 
                         GD 
                       
                     
                     
                       C 
                       224 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     In Equation 1, the product C GD ×V GD  can represent a quantity of charge being injected into capacitor  224 , and C 224  can represent the capacitance of capacitor  224 . As shown in graph  310  of  FIG.  3 B , due to the charge injection, the anode voltage V A  can rise above V C +V F-on , which causes the anode-cathode voltage V AC  to become higher than forward conduction threshold voltage V F-on  as well. 
     Between times T1-T2, as the gate-source voltage V GS  settles at a low value (e.g., zero) and transistor  202  is disabled, the charge injection stops, but the anode-cathode voltage V AC  settles at a value above V F-on . Since the duration between times T1-T2 is still within RCB timing window  340 , RCB circuit  260  can continue to disable transistor  202  (by closing switch  266 ) and disregard the output of comparator  262   a  indicating that V AC  is higher than V F-on . Switch  266  can continue to be closed and switch  274  can continue to be opened, which can prevent forward conduction control circuit  270  from increasing the gate voltage of transistor  202 . 
     RCB timing window  340  expires at time T2. Between times T2-T3, RCB circuit  260  determines that the anode-cathode voltage V AC  exceeds V F-on , and de-assert control signal  280   a  to open switch  266  and to close switch  274 . RCB circuit  260  may mistake the rise of V AC  due to the charge injection in the battery loss event as indicating that battery  102  is connected with the proper polarities, and start the forward conduction of transistor  202  as a result. Amplifier  272  is allowed to raise the gate voltage of transistor  202  based on a difference between V AC  and V F-reg . The change in the gate voltage of transistor  202  can also inject charge into first current terminal  206  and the anode via parasitic capacitance  302  (C GS ), which can further increase the anode voltage V A . 
     Referring to graph  330 , at time T3, the V GS  voltage of transistor  202  can become high enough to enable transistor  202  and create a conduction channel between first current terminal  206  and second current terminal  208  and under gate  204 . Via the conduction channel, the anode can be discharged, and the anode voltage V A  drops. If the anode-cathode voltage V AC  falls below the reverse bias threshold voltage V R  or the forward conduction threshold V F-on , RCB circuit  260  can reduce the gate voltage to disable transistor  202  again, and the change in the gate voltage can again inject charge into the anode and increase the anode voltage V A . The charging and discharging of the anode can repeat, which causes transistor  202  to transition between the enabled state and the disabled state repeatedly. The repeated transitions can continue until all the electric energy previously stored in capacitor  226  on the load side is dissipated in transistor  202  as power loss when the transistor conducts current from first current terminal  206  to second current terminal  208 . 
       FIG.  3 C  illustrates additional examples of voltages and currents of transistor  202  in a battery loss event when controlled by controller circuit  200 .  FIG.  3 C  illustrates a graph  350  of the anode voltage V A , a graph  360  of the gate-source voltage (V GS ) of transistor  202 , and a graph  370  of a current (labelled “I_FET”) that flows from first current terminal  206  to second current terminal  208  of transistor  202  (and load  106 ). As shown in graph  350 , starting from time T0, the anode voltage V G  drops as it is not driven by battery  102 , but spikes occur repeatedly due to repeated injection of charge from C GD . Moreover, as shown in graph  360 , gate-source voltage V GS  repeatedly fluctuates between a zero value and a non-zero value, which causes transistor  202  to transition between the enabled state and the disabled state repeatedly. Further, as shown in graph  370 , within each cycle in which V GS  is non-zero and transistor  202  is enabled, a negative I_FET current can flow away from load  106  when V AC  is below V R  initially, which triggers RCB circuit  260  to disable transistor  202 . But then as V GS  increases to start forward conduction by transistor  202 , a positive I_FET current flows to load  106  from the anode, which causes V A  to drop. After V AC  drops below V F_on , transistor  202  is disabled, bringing V GS  and the I_FET current to zero towards the end of a cycle. 
     The repeated enabling and disabling of the transistor in a battery loss event is undesirable, because it can create unexpected and unpredictable operations in the load. Specifically, in a battery loss event, the load is no longer supplied with power from the battery. The electronic systems on the load side, such as subsystems  100 , are specified to be disabled, so they do not draw current via the transistor. However, repeatedly enabling and disabling of the transistor in a battery loss event may allow the electronic systems on the load side to draw current and operate intermittently, contrary to the specification. 
     Moreover, whether the repeated enabling and disabling of a particular transistor in a battery loss event may also become unpredictable. Specifically, as shown in Equation 1, whether the repeated enabling and disabling of the transistor occurs in a battery loss event may depend on the quantity of charge injected into the anode by the gate, which in turn depends on various factors, such as the change in the gate voltage, and the dimensions of the transistor (which can determine C GD ), the parasitic capacitances at the anode (e.g., capacitor  224 ), etc. Therefore, power systems having a certain model of transistor as transistor  202  may experience repeated enabling and disabling under some operation conditions, while power systems having other models of transistor as transistor  202  may not experience such repeated enabling and disabling at all. As a result, the power system&#39;s handling of a battery loss event can become unpredictable, which in turn can lead to unpredictable operations in the load. While controller circuit  200  can use lowpass filtered anode voltage to reduce false detection of a forward-biased condition, the lowpass filtering can increase the response time of the controller circuit, which can degrade the controller circuit&#39;s performance for other applications, such as AC superimposed conditions. 
       FIGS.  4 A through  4 D  illustrate examples of a controller circuit  400  that that can address at least some of the issues described above. As shown in  FIG.  4 A , controller circuit  400  can be coupled to transistor  202 , which can be an NFET or a PFET, via terminals  230 ,  232 , and  234 . Controller circuit  400  can include gate control circuit  250  (which can include RCB circuit  260  and forward conduction control circuit  270 ) of controller circuit  200  of  FIGS.  2 A through  2 F . Some of the components of controller circuit  200 , such as voltage subtractor circuit  254  and local power supply  256 , can be part of controller circuit  400  but are not shown in  FIGS.  4 A through  4 D  for brevity. 
     Also, controller circuit  400  can include a discharge circuit  402  having inputs  404  and an output  406 . Discharge circuit  402  may also include a switchable discharge path coupled to output  406 , which can be adapted to be coupled to first current terminal  206  of transistor  202 . Discharge circuit  402  can receive, via inputs  404 , an indication that the gate voltage V G  has been changed (e.g., by RCB circuit  260 ) to disable transistor  202 . Responsive to receiving the indication, discharge circuit  402  can connect the discharge path to output  406  to discharge first current terminal  206 . The discharge operation can be configured to, in a battery loss event, remove charge injected (or will be injected) from the C GD  parasitic capacitance. Such arrangements can bring down the anode voltage, which can reduce the anode-cathode voltage V AC  to below the forward conduction threshold voltage V F-on . The reduction of V AC  can reduce the likelihood of a false detection of a forward-biased condition, as well as the repeated transition between the enabled state and the disabled state of transistor  202  in a battery loss event, as described in  FIG.  3 A — FIG.  3 C . 
     Discharge circuit  402  can start the discharge operation responsive to receiving the indication that gate voltage V G  has been changed to disable transistor  202 . The indication can come from various sources. In some examples, the indication can be based on a transition of control signal  280   a  to a state to close switch  266  to bring V GS  to zero. The transition of control signal  280   a  to a state to close switch  266  (or other state to reduce or otherwise bring V GS  to zero) can indicate that transistor  202  is disabled. Gate control circuit  250  may include an output  408  that can be coupled directly to input  404  of discharge circuit  402 , such as to provide control signal  280   a.    
     In some examples, the indication can be based on the voltages of transistor  202 . For example, discharge circuit  402  can receive the anode-cathode voltage V AC  from voltage subtractor circuit  254 , and compare V AC  with a threshold (e.g., a threshold based on V F-on ) to generate a decision. If V AC  is above the threshold, this can indicate that charge is injected from the C GD  parasitic capacitance to the anode. As another example, discharge circuit  402  can also monitor gate voltage V G  at terminal  232 , and a source voltage Vs (which can the anode voltage monitored at terminal  230 , or the cathode voltage monitored via terminal  234 ). Discharge circuit  402  can include a voltage subtractor circuit (not shown in the figures) to obtain the gate-source difference voltage V GS  (or V SG  for a PFET), and determine whether the gate-source voltage V GS  (or V SG ) falls below the threshold voltage V th  for forming the conduction channel. If V GS  or V SG  falls below V th , it can also indicate that transistor  202  is disabled. In some examples, discharge circuit  402  can combine control signal  280   a , anode-cathode voltage V AC , and/or gate-source voltage V GS  to determine the indication of whether transistor  202  is disabled to improve accuracy. Responsive to receiving the indication that transistor  202  is disabled, discharge circuit  402  can connect the discharge path to output  406  to discharge first current terminal  206 . In these examples, inputs  404  of discharge circuit  402  can be coupled to inputs  251  and  252  and output  253  of gate control circuit  250  to receive V GS  and V AC . 
       FIGS.  4 B through  4 D  illustrate examples of internal components of discharge circuit  402  and their operations. Referring to  FIG.  4 B , discharge circuit  402  can include a switchable current/discharge path, which includes a current source  414  and a switch  416 , coupled between output  406  and a voltage reference (e.g., a ground) configured as a charge sink. The switchable current path, when enabled with switch  416  closed, can remove charge from first current terminal  206  via output  406  and terminal  230 . Discharge circuit  402  can also include a discharge control circuit  418  and a pulse generator  420 . Discharge control circuit  418  can determine the start time of the discharge operation based on the timing of the indication that RCB circuit  260  disables transistor  202 . Discharge control circuit  418  can also determine the end time of the discharge operation based on various techniques. Discharge control circuit  418  can then provide the start time and end time information to control pulse generator  420  to generate a pulse signal  422  for switch  416 . The closing of switch  416  by pulse signal  422  can start the discharge operation, and the opening of switch  416  by pulse signal  422  can end the discharge operation. 
     Depending on the operation voltage levels, switch  416  can be a PFET, an NFET, or a parallel combination of both. Pulse generator circuit  420  can generate pulse signal  422  as an active low signal (for PFET), or as an active high signal (for NFET). In some examples, pulse generator circuit  420  can generate pulse signal  422  to include both an active low signal and an active high signal as a pair of complimentary signals, in a case where switch  406  includes a parallel combination of an NFET or PFET (e.g., a CMOS switch). The active high and active low voltage levels can be defined by the high supply voltage (V h ) and the low supply voltage (V l ) supplied by local power supply  256  to pulse generator circuit  420 . 
       FIG.  4 C  and  FIG.  4 D  illustrate examples of internal components of discharge timing circuit  418 . As shown in  FIG.  4 C  and  FIG.  4 D , discharge control circuit  418  can include a discharge start circuit  430  and a discharge end circuit  432 . Discharge start circuit  420  can include an edge detection circuit  434 . Comparator  434  can include an analog comparator circuit, a digital edge detection circuit (e.g., an edge-trigged flip-flop circuit), etc., to determine the state of an input signal, which can include control signal  280   a , anode-cathode voltage V AC , gate voltage V G , etc. Comparator  434  can detect a transition of control signal  280   a , or compare a voltage against a threshold (e.g., comparing V AC  against V F-on , comparing V GS /V SG  against V th , etc.) to generate a decision. The transition and the decision can indicate whether transistor  202  is disabled and charge is injected into node  220 /first current terminal  206  from parasitic capacitance  304  (C GD ) of transistor  202 . Responsive to the transition/decision, discharge start circuit  430  can generate a discharge start signal  450  based on the timing of the transition/decision, and transmit discharge start signal  450  to pulse generator circuit  420  to control the start of pulse signal  422 . 
     Also, discharge end circuit  432  can generate a discharge end signal  452  and transmit discharge end signal  452  to pulse generator circuit  420  to control the end of pulse signal  422 . Discharge end circuit  432  can determine the timing of discharge end signal  452  based on various techniques. Referring to  FIG.  4 C , in some examples, discharge end circuit  432  can include a delay circuit  460  to generate discharge end signal  452  as a delayed version of discharge start signal  450 , with the delay setting a width of pulse signal  412 . Delay circuit  440  can include delay elements (e.g., buffers) to delay discharge start signal  450 . In some examples, delay circuit  460  can include programmable delay elements, so the delay introduced to discharge start signal  450  in generating discharge end signal  452  is programmable. In such examples, delay circuit  460  can be coupled to a programming register to receive a setting for the delay. 
     In some examples, the delay introduced by delay circuit  460  which sets the pulse width of pulse signal  422 , and the amount of current sunk by current source  414 , can be pre-configured to match the total charge injected by parasitic capacitance  304  (C GD ) of transistor  202 , as follows: 
         T   Discharge   ×I   Discharge   =C   GD   ×V   GD   (Equation 2)
 
     In Equation 2, T Discharge  represents the pulse width of pulse signal  422  set by delay circuit  460 , and I Discharge  represents the discharge current sunk by current source  414 . The total charge injected can be given by the capacitance of parasitic capacitance  304  (C GD ) of transistor  202 , and the gate-drain voltage V GD  of transistor  202  prior to transistor  202  being disabled. Delay circuit  460  can be programmed with different delay settings, such as according to the dimension of transistor  202  (which can determine C GD ), the gate-drain voltage of transistor  202 , etc., to remove an amount of charge that commensurate with transistor  202  and the operation condition. 
     Also, the pulse width T Discharge  can be determined based on other information, such as the expiration time of RCB timing window  340 . As described above, in a case where RCB circuit  260  continues to disable transistor  202  within the RCB timing window, the discharge operation can end before the RCB timing window expires, to prevent a false detection of a forward-biased condition by RCB circuit  260 . Accordingly, the pulse width T Discharge , and the start time of pulse signal  422  and the discharge current I Discharge , can be configured according to the expiration time of RCB timing window  340 , to ensure that a target amount of the injected charge can be removed before RCB timing window  340  expires. 
       FIG.  4 D  illustrates additional examples of discharge end circuit  432 . Specifically, discharge end circuit  432  can generate discharge end signal  452  based on monitoring a change in the anode-cathode voltage V AC , which can drop due to the discharge operation. If V AC  falls below another threshold, discharge end circuit  432  can determine that the charge injected by the capacitance of parasitic capacitance  304  (C GD ) of transistor  202  has been removed, and the discharge operation can stop. In some examples, threshold V R ′ can be equal to reverse bias threshold voltage V R , or can be set lower than V R  to provide a noise margin. Referring to  FIG.  4 D , discharge end circuit  432  can include a comparator  470  to compare V AC  against threshold V R ′ to generate a decision, and discharge end circuit  432  can generate discharge end signal  452  in response to the decision, and provide discharge end signal  452  to pulse generator circuit  420 . In some examples, comparator  470  can also compare the anode voltage V A  against the cathode voltage V C  generate discharge end signal  452 . 
     The examples of controller circuit  400  of  FIGS.  4 A through  4 D  can also handle reverse battery connection. Specifically, in a reverse battery connection, the anode voltage is largely set by the negative terminal of the battery, and the anode-cathode voltage V AC  can remain below the reverse threshold voltage V R  with or without the discharge of the anode by the discharge circuit. In such a case, a false detection of a forward-biased condition also may not occur, and the discharge circuit is unlikely to interfere with the operation of RCB circuit  260  in disabling transistor  202 . 
       FIG.  5 A  and  FIG.  5 B  illustrate examples of voltage and current graphs of controller circuit  400  and transistor  202  in a battery loss event.  FIG.  5 A  illustrates a voltage graph  502  of anode voltage V A  and cathode voltage V C , a voltage graph  504  of control signal  280   a  provided by RCB logic circuit  264 , a voltage graph  506  of gate-source voltage (V GS ) of transistor  202 , and a voltage graph  508  of pulse signal  422  generated by pulse generation circuit  410 . 
     Referring to  FIG.  5 A , between times T0-T1, a battery loss event occurs where battery  102  is disconnected from node  220 . As shown in graph  502 , node  220  (and the anode of the diode) is no longer driven by battery  102 , and the anode-cathode voltage V AC  may become lower than the reverse bias threshold voltage V R . RCB circuit  260  may change the state of control signal  280   a  to close switch  266 , which can then electrically short gate  204  with first current terminal  206  to bring gate-source voltage (V GS ) to zero to disable transistor  202 . The change of the gate-source voltage V GS  can cause charge previously stored in parasitic capacitance  304  (C GD ) of transistor  202  to be injected into node  220 /first current terminal  206  of transistor  202 , which causes the anode voltage V A  to go up above the forward voltage threshold at time T1. 
     Meanwhile, the enabling of switch  266  can also start an RCB timing window  510  at time TO, as shown in graph  506 . Further, the transition of control signal  265  can be detected by discharge start circuit  420  to trigger the start of pulse signal  412  to discharge node  220 /first current terminal  206 , as shown in graph  508   
     Between times T1-T2, discharge circuit  402  can discharge node  220 /first current terminal  206  to bring down the anode voltage V A , as shown in graph  504 . The timing of the discharge operation can be based on pulse  422 . The start of the discharge operation (and pulse  422 ) by discharge circuit  402  can be triggered by various sources, such as by the transition of control signal  280   a  to a state to close switch  266  at time T0, the falling of gate-source voltage V GS  of transistor  202  to below the threshold voltage V th  between times T0-T1, the rise of anode-cathode voltage V AC  to above V F_on  at time T1, etc. The duration between times T1-T2 can also be within RCB timing window  510 , where RCB circuit  260  continues to disable transistor  202 , switch  266  continues to be enabled and switch  274  continues to be disabled. At time T2, the anode voltage V A  is well below the cathode voltage V C . 
     Referring to graph  508 , pulse signal  422  stops at time T2, which also stops the discharge operation. Anode voltage V A  settles at a voltage well below the cathode voltage V C , and the anode-cathode voltage V AC  is well below V F-on , as shown in graph  502 . RCB timing window  510  expires at or after time T2, as shown in graph  506 . Between times T2 and T3, RCB circuit  260  can obtain the latest anode voltage V A  and the latest cathode voltage V C  and determine that the latest anode-cathode voltage V AC  remains below the forward voltage V F-on . RCB circuit  260  can continue closing switch  266  to keep gate-source voltage V GS  of transistor  202  at zero to maintain transistor  202  in the disabled state. This is also reflected in graph  506  where V GS  settles to and remains at a low value near zero to disable transistor  202  after time T0. 
       FIG.  5 B  illustrates additional examples of voltage and current graphs of transistor  202  in a battery loss event when controlled by controller circuit  400 .  FIG.  5 B  illustrates a graph  520  of the anode voltage V A , a graph  530  of the gate-source voltage V GS  of transistor  202 , and a graph  540  of a current (labelled “I_FET”) that flows from first current terminal  206  to second current terminal  208  of transistor  202  (and load  106 ). As shown in graph  520 , starting from time T0, anode voltage V G  drops as it is no longer driven by battery  102  while being discharged by discharge circuit  402 . Accordingly, the repeated pattern of spikes at the anode voltage V A  caused by the repeated injection of charge from C GD  can be eliminated or at least reduced. Moreover, as shown in graph  530 , V GS  also drops to a low value near zero after time T0. Further, as shown in graph  540 , as transistor  202  is disabled after time T0, current that flows to load  106  drops to zero after time T0, which allows the circuits in load  106  to be completely turned off/disabled. 
       FIG.  6    is a flowchart of an example method  600  for handling a battery loss event. Method  600  can be performed, such as by controller circuit  400  of  FIGS.  4 A through  4 D . Although the method steps are described in conjunction with  FIGS.  4 A through  4 D , 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 ), which 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, which 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 , controller circuit  400  can provide, via a third terminal (e.g., terminal  232 ) adapted to be coupled to a gate of the transistor, a third voltage based on the first voltage and the second voltage. 
     Specifically, RCB circuit  260  of controller circuit  400  can include comparators  262   a  and  262   b  to compare a difference between the first voltage (the anode voltage V A ) and the second voltage (the cathode voltage V C ) against a first threshold voltage (e.g., reverse bias threshold voltage V R ) to determine whether there is a reverse battery connection. RCB circuit  260  can also compare the difference against a second threshold voltage (e.g., forward conduction threshold voltage V F-on ) to determine whether to start forward conduction by transistor  202 . If V AC  is below V R  and below V F-on , RCB circuit  260  can proceed to disable transistor  202 . For example, the disabling can be based on transmitting a control signal  280   a  to enable switch  266  coupled between the first terminal and the second terminal to bring the gate-source voltage (V GS ) of transistor  202  to zero. The disabling of transistor  202  can also trigger the start of an RCB timing window (e.g., RCB timing window  340  shown in  FIG.  3 B , RCB timing window  510  shown in  FIG.  5 A , etc.), within which RCB circuit  260  maintains transistor  202  in a disabled state and does not open switch  266  regardless of whether the anode-cathode voltage V AC  exceeds the forward conduction threshold voltage V F-on  or reverse bias threshold voltage V R . 
     At step  608 , based on an indication of whether the gate voltage has been changed, controller circuit  400  can remove a charge from the first current terminal of the transistor via the first terminal. 
     As described above, when RCB circuit  260  changes the gate-source voltage (V GS ) to disable the transistor, charge can be injected from the gate-drain parasitic capacitance (C GD ) into the anode/source of the transistor and increase the anode voltage V A . If the anode-cathode voltage V AC  goes above the forward conduction threshold voltage V F-on , controller circuit  400  may enable the transistor to start a forward conduction from the battery to the load when in fact the battery is not connected to the transistor, and can lead to subsequent repeated enabling and disabling of the transistor as descried in  FIG.  3 A — FIG.  3 C . Discharge circuit  402  can enable the discharge path to remove the charge that has been or will be injected by the gate-drain parasitic capacitance to reduce the likelihood of V AC  rising above the forward conduction threshold voltage V F-on . Discharge circuit  402  can include a pulse generator  420  to generate a pulse to enable the discharge path for a duration to remove the charge. 
     Discharge circuit  402  can start the pulse (and the discharge operation) based on receiving an indication that the gate voltage has been changed. The indication can be based on, for example, detecting a transition of control signal  280   a  that closes switch  266  to set gate-source voltage V GS  (or source-gate voltage V SG ) to zero, a transition of the anode-cathode voltage V AC  across a threshold (e.g., reverse bias threshold voltage V R ) due to injection of charge by parasitic capacitance C GD  as the gate voltage is changed, a transition of the gate-source voltage V GS  or V SG  across the transistor&#39;s threshold voltage V th , etc., all of which can indicate that the gate voltage has been changed to disable the transistor. Discharge circuit  402  can end the pulse, such as based on a predetermined delay (which can be fixed or programmable) has elapsed from the start of the pulse, the anode-cathode voltage V AC  falling below a threshold V R ′ based on the reverse bias threshold voltage V R , etc. In some examples, discharge circuit  402  can also end the discharge operation before the RCB timing window expires, after which RCB circuit  260  can determine whether to enable or disable transistor  202  based on new anode voltage V A  and new cathode voltage V C . 
     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 , in some examples, controller circuit  400  can include a gate control circuit  250  and a discharge circuit  402 . Internal components and operations of gate control circuit  250  are described in  FIGS.  2 A through  2 F  and  FIGS.  4 A through  4 D . Gate control circuit  250  can be adapted to be coupled to the transistor via the first, second, and third terminals, and the discharge circuit can be adapted to be coupled to the source of the transistor via the first terminal. RCB circuit  260  can set a gate voltage of the transistor via terminal  232  to disable the transistor, and forward conduction control circuit  270  can set the gate voltage of the transistor via terminal  232  to enable the transistor, based on the techniques described in  FIG.  2 B . RCB circuit  260  can disable/remove the conduction channel of the transistor between the source and the drain, if the anode-cathode voltage V AC  is below a reverse bias threshold voltage V R . The body diode between the source and the drain can be reverse-biased, and the load can be isolated from the battery. RCB circuit  260  can also start an RCB timing window (e.g., RCB timing window  340  of  FIG.  3 B , RCB timing window  510  of  FIG.  5 A , etc.) after disabling transistor  202 . RCB circuit  260  can maintain transistor  202  in the disabled state during the RCB timing window and ignore changes in the anode-cathode voltage V AC , to avoid incorrectly enabling transistor  202  in response to transients in the anode-cathode voltage V AC . 
     Also, referring to  FIGS.  2 A through  2 F  and  FIGS.  4 A through  4 D , forward conduction control circuit  270  can set a gate voltage of the transistor to enable the transistor to transmit a positive voltage and a forward current from the battery to the load. Forward conduction control circuit  270  can enable transistor  202  if the anode-cathode voltage V AC  exceeds a forward conduction threshold voltage V F-on , which can be a positive threshold voltage, to start forward conduction by the transistor. Forward conduction control circuit  270  can also include a linear amplifier such as an OTA, an op-amp, etc., to implement a feedback loop to regulate the anode-cathode voltage V AC  at a target forward voltage V F-reg  across different forward currents I F . The feedback loop can regulate V AC  by adjusting the gate voltage via terminal  232 . 
     Referring to  FIGS.  2 A through  2 F  and  FIGS.  4 A through  4 D , controller circuit  400  includes a switch  266  coupled between terminals  230  and  232 , and a switch  274  coupled between terminal  232  and forward conduction control circuit  270 . RCB circuit  260  can close switch  266  to connect terminals  230  and  232  to set gate-source voltage (V GS ) to zero to disable transistor  202 . When switch  266  is closed, RCB circuit  260  can open switch  274  to disconnect forward conduction circuit  270  from terminal  232 . Moreover, when switch  266  is opened, RCB circuit  260  can close switch  274  to enable forward conduction circuit  270  to set the gate voltage. 
     Referring to  FIGS.  4 A through  4 D , discharge circuit  402  can discharge the anode via terminal  230  in a battery loss event. In some examples, discharge circuit  402  can discharge a predetermined quantity of charge from the anode based on detecting that the reverse current blocking circuit disables the transistor. The quantity of charge can be based on Equation 2. Discharge circuit  402  can include a current source  414  and a switch  416  coupled between terminal  230  and a voltage reference (e.g., a ground) that can provide a charge sink. Discharge circuit  402  can also include a discharge control circuit  418  and a pulse generator  420 . Discharge control circuit  418  can determine the start time and end time of the discharge operation. Pulse generator  420  can generate a pulse signal having the start time and the end time, and provide the pulse signal to close the switch  416  between the start time and the end time to perform the discharge operation. 
     Discharge control circuit  418  can determine the start time and the end time of the discharge operation based on various techniques. Referring to  FIGS.  4 A through  4 D , discharge control circuit  418  can receive an indication that RCB circuit  260  disables transistor  202 . The indication can be based on, for example, a control signal from RCB circuit  260  that disables transistor  202  (e.g., control signal  280   a ), the gate-source voltage V GS  (or source-gate voltage V SG  for PFET) falling below a threshold of transistor  202 , the anode-cathode voltage V AC  rising above the forward conduction threshold voltage V F-on , etc. The start time can be based on the indication. In some examples, the end time can be by adding a predetermined delay to the start time, with the delay configured based on the total quantity of charge to be removed according to Equation 2. In some examples, the end time can be set when the anode-cathode voltage V AC  falls below a threshold V R , as the discharge operation can stop when V AC  is no longer in the forward-bias regime. In some examples, the end time can also be set based on the RCB timing window. 
     With the described examples, a controller circuit operating a transistor to provide a protection system between a battery and a load can discharge the anode in a battery loss event. The discharging can be configured to prevent false detection of a forward-biased condition in the battery loss event. As a result, repeated transition between an enabled state and a disabled state of the transistor in a battery loss event can be avoided. All these can improve the predictability of the power system and the behaviors of the load in a battery loss event. 
     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, so 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.