Patent Publication Number: US-8116056-B2

Title: Low voltage startup timer for a microcontroller-based circuit breaker

Description:
FIELD OF THE INVENTION 
     Aspects of the present invention relates generally to circuit breakers with microcontroller-based fault detection, and in particular, to a backup tripping function for a circuit breaker with microcontroller-based fault detection. 
     BACKGROUND OF THE INVENTION 
     In a circuit breaker with microcontroller-based fault detection, a failure in the power supply regulator circuit or the microcontroller itself can lead to an inability to detect faults on the circuit being protected, leaving the load to which the circuit breaker is connected unprotected and vulnerable. It is desirable to have a circuit breaker deny power to the protected circuit if the circuit breaker does not have the ability to detect faults by tripping as soon after power is applied as possible. 
     What is needed is a backup circuit that forces a microcontroller-based circuit breaker to trip if the microcontroller does not start up correctly either due to a failure in the regulated power supply or a fault in the microcontroller itself or both. Aspects and embodiments disclosed herein are directed to addressing/solving these and other needs. 
     SUMMARY OF THE INVENTION 
     Two different backup timing circuit implementations are described. These backup timing circuits ensure that an electronic circuit breaker will trip even if certain electronics within the circuit breaker are unresponsive. The electronic circuit breaker includes a microcontroller that analyzes current or voltage signals in a circuit and trips the circuit breaker when those signals exceed certain thresholds or criteria. If the microcontroller does not work at startup, the loads being protected by the circuit breaker become vulnerable to certain types of electrical faults. In essence, the microcontroller represents a “warning system” to detect certain types of faults which are not protected by the mechanical thermal or magnetic components within the circuit breaker. The microcontroller is powered by a separate power supply within the circuit breaker, and this power supply derives its power from the current on the line. If the power supply fails, the microcontroller will become unresponsive, so one of the backup timing circuit implementations also bypasses the microcontroller if the power supply fails to operate properly. The backup timing circuits disclosed herein bypass or override the “early warning system” provided by the microcontroller if the microcontroller is unresponsive at startup or because its power supply is unresponsive or both. 
     In a first implementation, a backup timing circuit is powered by a power supply in a microcontroller-based circuit breaker. The timing circuit includes a transistor whose gate is charged by a node that is also connected to a configurable pin of the microcontroller. When the microcontroller is initially powered on, it runs through various startup and diagnostic routines. During this startup process, the pin is initially in a high impedance state. As a result, the node can build up a voltage across a capacitor that eventually becomes sufficient to energize the transistor. If the microcontroller properly completes its diagnostic and startup routines, then the microcontroller configures the pin to an output and drives it low, shorting out the capacitor of the timing circuit and preventing the transistor from turning on. 
     In a second implementation, a backup timing circuit is powered directly off of a rectified line voltage. A microcontroller is powered by a separate power supply, but because the timing circuit in this implementation is positioned upstream of the power supply, the timing circuit can react even if the timing circuit is unresponsive. A pin of the microcontroller is connected to a node of the timing circuit, which is also connected to a gate of an electronic switching device that causes the electronic circuit breaker to trip. Upon successful completion of the startup and diagnostic routines, the pin, initially in a high impedance state, is driven low by the microcontroller, shorting out a capacitor of the timing circuit thereby preventing the electronic switching device from turning on. However, if the startup and diagnostic routines fail or if the power supply fails, the pin will remain in a high impedance state, allowing a voltage to build up across the capacitor of the timing circuit, until it is sufficient to energize the electronic switching device. 
     The foregoing and additional aspects and embodiments of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1  is a functional block diagram of a microcontroller-based circuit breaker having a backing timing circuit that can trip the circuit breaker if the microcontroller is unresponsive; 
         FIG. 2  is a functional block diagram of a microprocessor-based circuit breaker having a timing circuit that can trip the circuit breaker if either the regulated power supply fails or the microcontroller is unresponsive; and 
         FIG. 3  is a functional block diagram of a circuit breaker similar to that shown in  FIG. 2  except that a drop resistor is placed downstream of a timing circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Although the invention will be described in connection with certain aspects and/or embodiments, it will be understood that the invention is not limited to those particular aspects and/or embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. 
     This disclosure describes at least two backup circuits that are used in microcontroller-based circuit breakers for tripping the circuit breaker in the event of a regulated power supply fault or a microprocessor fault.  FIG. 1  illustrates a backup circuit that is powered by a regulated power supply and is operable to trip the circuit breaker in the event that the microcontroller is unresponsive at startup.  FIGS. 2 and 3  illustrate line-powered backup circuits that are operable to trip the circuit breaker in the event that either the regulated power supply is or becomes unresponsive or the microcontroller is initially unresponsive or both. 
       FIG. 1  is a functional block diagram of a microcontroller-based circuit breaker  100  conventionally connected to line and neutral (not shown) conductors. The circuit breaker  100  includes movable contacts  108  that are forced apart by a relay  106 , solenoid, or other conventional electromechanical tripping device. The circuit breaker  100  includes a timing circuit  102 , an electronic switching device  104 , and a microcontroller  116  having a first pin  110  and a second pin  112 . It should be noted that the terms “first” pin and “second” pin are not intended to imply that these pins correspond to pins  1  and  2 , respectively, of the microcontroller  116 , but rather are used to differentiate between different pins (e.g., pin X and pin Y). The first pin  110  and the second pin  112  are configurable as high-impedance inputs at power-up of the microcontroller  116 , or as outputs where they are driven by the microcontroller  116  to logic level output low states (e.g., 0V) or logic level output high states (e.g., 3.3V), as is readily understood by those of ordinary skill in the art. An example of a suitable microcontroller  116  is the Microchip FJ32GA004 microcontroller. 
     The timing circuit  102  is coupled to the first pin  110  of the microcontroller  116 . The timing circuit  102  and the microcontroller  116  are powered by a conventional regulated power supply that produces, via a regulator circuit, a regulated direct current (DC) voltage, V DD , derived from the alternating current (AC) line voltage, which can be rectified by a conventional half- or full-wave bridge rectifier (not shown). V DD  is typically about 3.3V. 
     The first pin  110  is coupled through a resistor R 112  to a node  118  between a resistor R 113  and a capacitor C 64 , which is grounded to the regulated power supply. Two diodes, D 1  and D 2 , are connected between the resistor R 113  and a base of a bipolar junction transistor Q 8 . The emitter junction of the transistor Q 8  is connected to a gate of the electronic switching device  104 , which in the illustration is a silicon controlled rectifier (SCR). The collector of the transistor Q 8  is coupled to the regulated power supply voltage through a resistor R 114 . As used herein, the terms “base” and “gate” are not intended to denote any particular transistor and are used interchangeably to refer to any input switching terminal of a transistor. The base of the transistor Q 8  is coupled to the resistor R 113  at the node  118  through two series-connected diodes D 1  and D 2 . The combination of the voltage drop across the emitter and the two diodes D 1  and D 2  sets a minimum threshold voltage (approximately 2.1V assuming a diode drop of 0.7V across each of the three diodes) that the node  118  before a leakage current begins to flow through diodes D 1  and D 2 . When a sufficient amount of leakage current reaches the base of the transistor Q 8 , the transistor Q 8  turns on, which in turn dumps current into the gate of the SCR  104 , turning it on as well. The activation of the SCR  104  causes the circuit breaker  100  to trip. 
     When the circuit breaker  100  is connected to a live circuit and powered on, such as by urging the movable contacts  108  to the on position such that they make electrical contact with one another, the microcontroller  116  initiates its internal diagnostics and startup routines. The first and second pins  110 ,  112 , respectively, are initially in a high impedance state, and if the microcontroller  116  successfully completes its diagnostic and startup routines, the microcontroller  116  drives the first pin  110  to a low logic level state (e.g., 0V), shorting out the capacitor C 64  and preventing the node  118  from exceeding the turn-on threshold voltage for the base of the transistor Q 8 . During normal operation, if the microcontroller  116  detects an electrical fault on the circuit to which the circuit breaker  100  is connected, the microcontroller  116  drives the second pin  112  to a high voltage level (e.g., a logic level high of 3.3V) sufficient to cause the SCR  104  to turn on. The second pin  112  is sometimes referred to as a trip output, because it is the output signal by which a trip is initiated by the microcontroller  116 . 
     However, if the microcontroller  116  does not successfully complete its diagnostic and startup routines due to a fault in the microcontroller  116 , or the microcontroller  116  is otherwise initially unresponsive, the second pin  112  will remain in a high-impedance input state and the SCR  104  will not be activated by the microcontroller  116 , leaving the load being protected by the circuit breaker  100  vulnerable. Therefore, as further explained herein, the timing circuit  102  can bypass the microcontroller  116  and provide a mechanism for tripping the SCR  104  in the event that the microcontroller  116  is unresponsive at startup. 
     Those of ordinary skill in the art will appreciate that the specific components shown in  FIG. 1  are exemplary only, and certain components can be eliminated or replaced with other components or that additional components can be added without deviating from the spirit and scope of this disclosure. For example, in other implementations, only one diode D 1  or D 2  is provided between the base of the transistor Q 8  and the node  118 . The transistor Q 8  can be a field effect transistor (FET) instead of a BJT transistor as illustrated. The electronic switching transistor  104  can be an SCR as disclosed above or an FET transistor or a thyristor. The electronic switching transistor  104  can be line-powered or powered by the line-voltage derived power supply. By “line-powered,” it is understood that a component need not be directly powered from line current to which the circuit breaker is connected (this is sometimes referred to as “fault” powered), but rather can be powered from a rectified representation of the line voltage. Those of ordinary skill in the art will readily appreciate that a rectifier, such as a diode  208  shown in  FIG. 2  or a full-wave bridge rectifier  308  shown in  FIG. 3 , can be connected to the line input of the circuit breaker to supply a rectified signal to the electronic components in the circuit breaker, and this configuration is considered to be a “line-powered” configuration. By contrast, a regulated power supply typically includes a voltage regulator circuit for providing a regulated DC voltage output and a ground reference that is at least a diode drop away from the potential of the neutral input to the circuit breaker. The terms “line powered” and “powered by a power supply” are distinct. 
     In  FIG. 1 , the timing circuit  102  is powered by a regulated power supply, so if the regulated power supply fails, the timing circuit  102  will not work. The timing circuit  102  in  FIG. 1  activates the SCR only if the microcontroller is unresponsive. In  FIGS. 2 and 3 , alternative embodiments are proposed in which a circuit breaker includes a line-powered timing circuit that activates an SCR when either a power supply failure occurs or the microcontroller is unresponsive. These embodiments shall be discussed next. 
     In  FIG. 2 , a circuit breaker  200  includes the following circuits connected in parallel to one another: a line-powered timing circuit  202 , a fault detection circuit  206 , an electronic switching device  204 , and an optional protection diode  220 . A rectifier  208  rectifies the alternating current (AC) from a line input to the circuit breaker  200 , and a drop resistor R D  reduces the line voltage for the regulated power supply  214 . The fault detection circuit  206  includes a regulated power supply  214  that produces a regulated DC voltage derived from the AC line input to which the circuit breaker  200  is connected. The regulated power supply  214  powers a microcontroller  216  that detects one or more fault conditions on a circuit being protected by the circuit breaker  200  to which the circuit is connected. The microcontroller  216  includes a pin  212  that is configurable as a high-impedance input or as an output. This pin  212  is connected to a node  218  in the timing circuit  202 , which is connected to a gate of the electronic switching device  204 , which in the illustrated example is an SCR conventionally having a gate, an anode, and a cathode. The pin  212  corresponds to a trip output of the microcontroller  216 , which pulls the input pin  212  to a logic level high output state in response to the microcontroller  216  being programmed to instruct the SCR  204  to turn on and thereby cause the circuit breaker  200  to trip. 
     The timing circuit  202  includes a resistor R A  connected between the gate and the anode of the SCR  204 , and a capacitor C A  connected between the gate and the cathode of the SCR  204  as illustrated in  FIG. 2 . When the pin  212  is in its normally high-impedance input state, the voltage across the capacitor C A  in the timing circuit  202  builds up and eventually exceeds a turn-on voltage for the gate of the SCR  204 , turning the SCR  204  on, which causes a trip solenoid  210  of the circuit breaker  200  to trip a movable contact of the circuit breaker  200  and break the electrical connection of the circuit breaker  200  to the circuit to which it is connected. Thus, if the microcontroller  216  fails due to a fault in the microcontroller  216  or a fault in the regulated power supply  214 , the pin  212  will remain in a high-impedance input state, allowing the voltage across the capacitor C A  to increase until the turn-on voltage threshold of the gate of the SCR is exceeded. It is important to note that the satisfaction of either of the two conditions—a microcontroller  216  fault or a power supply  214  fault—or both will cause the pin  212  to remain in a high-impedance state. In this manner, the timing circuit  202  is responsive to a failure or fault of the regulated power supply  214  or the microcontroller  216  or both and can turn on the SCR  204 . 
     If the regulated power supply  214  operates normally and powers the microcontroller  216  and the microcontroller  216  successfully completes its diagnostic and startup routines, the microcontroller  216  will pull the pin  212  to a logic low output state, shorting out the capacitor C A , which prevents the gate of the SCR  204  from achieving a sufficient potential to cause the SCR  204  to begin conducting, and the SCR  204  remains off. If the microcontroller  216  detects a fault, the microcontroller  216  pulls the pin  212  to a logical high output state, which has a voltage that exceeds the turn-on voltage of the gate of the SCR  204 , causing the SCR  204  to conduct and thereby trip the circuit breaker  200 . 
       FIG. 3  is similar to  FIG. 2 , except that a full-wave bridge rectifier  308  is used instead of the rectifying diode  208  shown in  FIG. 2 , and the drop resistor, R D , is placed downstream of a timing circuit  302  instead of upstream of the timing circuit  202  as shown in  FIG. 2 . It is preferable to place the drop resistor R D  downstream of the timing circuit  302  as illustrated in  FIG. 3  to ensure that the timing circuit  302  will cause the SCR  304  to conduct as quickly as possible if the regulated power supply  314  or the microcontroller  316  is or becomes unresponsive due to a fault or failure and to ensure that the timing circuit  302  will operate if the drop resistor R D  fails. 
     In  FIG. 3 , a circuit breaker  300  includes the following circuits connected in parallel to one another: a line-powered timing circuit  302 , a fault detection circuit  306 , and an electronic switching device  304 . The full-wave bridge rectifier  308  fully rectifies the alternating current (AC) from a line input to the circuit breaker  300 , and the drop resistor R D , which is placed downstream of the timing circuit  302 , reduces the line voltage for a regulated power supply  314 . The fault detection circuit  306  includes the regulated power supply  314  that produces a regulated DC voltage derived from the AC line input to which the circuit breaker  300  is connected. The regulated power supply  314  powers a microcontroller  316  that detects one or more fault conditions on a circuit being protected by the circuit breaker  300  to which the circuit is connected. The microcontroller  316  includes a pin  312  that is configurable as a high-impedance input or as an output. This pin  312  is connected through the resistor R 4  to a node  318  in the timing circuit  302 , which is connected to a gate of the electronic switching device  304 , which in the illustrated example is an SCR conventionally having a gate, an anode, and a cathode. The pin  312  corresponds to a trip output of the microcontroller  316 , which pulls the input pin  312  to a logic level high output state in response to the microcontroller  316  being programmed to instruct the SCR  304  to turn on and thereby cause the circuit breaker  300  to trip. An example of a suitable microcontroller  316  is the MC68HC908QT2/4 available from Freescale, Inc. 
     The timing circuit  302  includes a resistor R 1  connected between the gate and the anode of the SCR  304 , and a capacitor C 1  connected between the gate and the cathode of the SCR  304  as illustrated in  FIG. 3 . When the pin  312  is in its normally high-impedance input state, the voltage across the capacitor C 1  in the timing circuit  302  builds up and eventually exceeds a turn-on voltage for the gate of the SCR  304 , turning the SCR  304  on, which causes a trip solenoid  310  of the circuit breaker  300  to trip a movable contact of the circuit breaker  300  and break the electrical connection of the circuit breaker  300  to the circuit to which it is connected. Thus, if the microcontroller  316  fails due to a fault in the microcontroller  316  or a fault in the regulated power supply  314 , the pin  316  will remain in a high-impedance input state, allowing the voltage across the capacitor C 1  to increase until the turn-on voltage threshold of the gate of the SCR  304  is exceeded. It is important to note that the satisfaction of either of the two conditions—a microcontroller  316  fault or a power supply  314  fault—or both will cause the pin  316  to remain in a high-impedance state. In this manner, the timing circuit  302  is responsive to a failure or fault of the regulated power supply  314  or the microcontroller  316  or both and can turn on the SCR  304 . 
     If the regulated power supply  314  operates normally and powers the microcontroller  316 , and the microcontroller  316  successfully completes its diagnostic and startup routines, the microcontroller  316  will pull the pin  312  to a logic low output state, shorting out the capacitor C 1 , which prevents the gate of the SCR  304  from achieving a sufficient potential to cause the SCR  304  to begin conducting, and the SCR  304  remains off. If the microcontroller  316  detects a fault, the microcontroller  316  pulls the pin  312  to a logical high output state, which has a voltage that exceeds the turn-on voltage of the gate of the SCR  304 , causing the SCR  304  to conduct and thereby trip the circuit breaker  300 . 
     Without limiting the scope of the present disclosure, the following Table 1 lists exemplary values for the components shown in  FIG. 3 : 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Component 
                 Exemplary Value 
               
               
                   
                   
               
             
            
               
                   
                 C 
                 0.01 uF (500 V) 
               
               
                   
                 R 1   
                 500 KΩ 
               
               
                   
                 C 1   
                 0.1 uF 
               
               
                   
                 R 4   
                 1 kΩ 
               
               
                   
                 R D1 , R D2   
                 11 kΩ 
               
               
                   
                   
               
            
           
         
       
     
     It should be understood that the electronic circuits disclosed herein can be disposed on one or more printed circuit boards (PCBs). The circuit breakers disclosed herein can be any microcontroller-based circuit breakers, including ground fault interrupter (GFI) circuit breakers, such as those based on the QO120GFI circuit breaker available from Square D Company, arc fault interrupter (AFI) circuit breakers, such as those based on the QO120AFI circuit breaker available from Square D Company, or any other industrial or residential circuit breaker that includes a microcontroller for detecting a fault condition on the circuit being protected. Although the electronic switching devices  104 ,  204 ,  304  are illustrated in the Figures as being SCRs, those of ordinary skill in the art will appreciate that other switching devices can be employed instead, such as an FET or a thyristor. The term “timing circuit” is also variously referred to as a “backup circuit” in that it is operable to trip the circuit breaker when the microcontroller is initially unresponsive or the regulated power supply is or becomes unresponsive. As mentioned above, the terms “gate” and “base” when used in conjunction with a transistor are interchangeable and are not intended to apply to any particular transistor. Rather, both terms refer to the control switch terminal of a transistor. 
     While particular aspects, embodiments, and applications of the present disclosure have been illustrated and described, it is to be understood that the disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the disclosure as defined in the appended claims.