Abstract:
A power supply circuit for a trip unit of a circuit interrupter includes a current transformer, a startup circuit receiving a regulated voltage and a DC/DC converter. The startup circuit is structured to: (i) burden the current transformer with an impedance approximating the trip unit and cause the DC/DC converter to enter the shutdown mode when the regulated voltage is below a predetermined value, (ii) remove the burden and cause the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to the regulated voltage reaching the predetermined value, and (iii) remove the burden and cause the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to a rate of change of the regulated voltage being at least a predetermined level.

Description:
BACKGROUND 
     Field 
     This invention pertains generally to circuit interrupters and, more particularly, to circuit interrupters including a trip unit and a power supply. The invention also relates to a power supply start-up circuit for a circuit interrupter trip unit. 
     Background Information 
     Circuit breakers and circuit breaker trip units are well known in the art. See, for example, U.S. Pat. Nos. 5,910,760; 6,144,271; and 6,850,135. 
     Circuit breaker trip units require power for both energizing and tripping the trip actuator of the trip unit and energizing the signal processing circuitry of the trip unit Power for trip units is typically provided by an iron core current transformer (CT), which may or may not also provide primary current indication (i.e., the current indication used for monitoring for overcurrent conditions). Generally, this CT is regulated to provide a relatively large output voltage to a capacitor which stores energy that is needed to energize and trip the trip actuator of the trip unit. Because this CT is capable of supplying only a certain amount of power, a relatively efficient switching power supply is preferred over a relatively less efficient linear regulator to convert the capacitor voltage to a relatively smaller voltage supply (at a relatively higher current) for the signal processing circuitry of the trip unit. Preferably, current-powered trip units run at the lowest possible CT primary current, or equivalently at the lowest possible load current flowing through the circuit breaker. This is desirable for both display/metering purposes and for protection purposes. 
     A conventional switching regulator integrated circuit may be electrically connected to receive the capacitor voltage. However, this configuration does not provide the lowest possible current power-up of the trip unit. 
     Given a fixed load requirement and being approximately constant power devices, all switching regulators draw more input current from their input power supply at a relatively lower input voltage than at a relatively higher input voltage. Therefore, for a current-limited source, such as a CT secondary when operated at relatively low primary current, the switching regulator starting at its minimum operating voltage will require greater input supply current than a regulator starting at a higher voltage. At relatively low primary current, the current output of CT secondary is limited by the CT primary current divided by the number of secondary turns even though its voltage output in an unloaded state can be quite high. In other words the CT is a current source providing large voltage and limited current. The switching regulator as a load requires decreasing current with increasing input voltage. There is a need to balance these two facts. 
     Accordingly, there is room for improvement in circuit interrupters and in power supplies for trip units. 
     SUMMARY 
     These needs and others are met by embodiments of the disclosed concept, which are, in one implementation, directed to a power supply circuit for a trip unit of a circuit interrupter that includes a current transformer structured to provide a voltage indicative of a primary current flowing through the circuit interrupter, a rectifier and regulator structured to rectify and regulate the voltage from the current transformer and output a regulated voltage, a startup circuit structured to be powered by the regulated voltage, and a DC/DC converter coupled to an output of the startup circuit for receiving the regulated voltage, wherein the DC/DC converter has a shutdown mode. The startup circuit includes a burden impedance, a switch electrically connected in series with the burden impedance. The startup circuit is structured to: (i) burden the current transformer through the series combination of the switch and the burden impedance and cause the DC/DC converter to enter the shutdown mode when the regulated voltage is below a predetermined value, (ii) remove the burden and cause the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to the regulated voltage reaching the predetermined value, and (iii) remove the burden and cause the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to a rate of change of the regulated voltage being at least a predetermined level. 
     In another implementation, a method of powering a trip unit of a circuit interrupter is provided. The method includes providing a voltage indicative of a primary current flowing through the circuit interrupter from a secondary of a current transformer, rectifying and regulating the voltage from the secondary of the current transformer and outputting a regulated voltage, providing the regulated voltage to a DC/DC converter, the DC/DC converter having a shutdown mode, burdening the current transformer with a burden impedance and causing the DC/DC converter to enter the shutdown mode when the regulated voltage is below a predetermined value, removing the burden and causing the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to the regulated voltage reaching the predetermined value, and removing the burden and causing the DC/DC converter to exit the shutdown mode and provide power to the trip unit responsive to a rate of change of the regulated voltage being at least a predetermined level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a circuit interrupter according to an exemplary embodiment of the disclosed concept; and 
         FIG. 2  is a circuit diagram of a startup circuit forming part of the circuit interrupter of  FIG. 1  according to one exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Directional phrases used herein, such as, for example, left, right, front, back, top, bottom and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
     As employed herein, the statement that two or more parts are “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. 
     As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
     As employed herein, the statement that a part is “electrically interconnected with” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors or generally electrically conductive intermediate parts. Further, as employed herein, the statement that a part is “electrically connected to” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors. 
     The invention is described in association with a three-pole circuit breaker, although the invention is applicable to a wide range of circuit interrupters having any number of poles. 
     Referring to  FIG. 1 , a circuit interrupter, such as three-pole circuit breaker  2 , includes separable contacts  4 , an operating mechanism  6  structured to open and close the separable contacts  4 , a sensor  8  structured to sense current flowing through the separable contacts  4 , a trip unit  10  cooperating with the sensor  8  and the operating mechanism  6  to trip open the separable contacts  4 , and a power supply  12  for the trip unit  10 . In this example, the three-pole circuit breaker  2  includes three separable contacts  4  and three Rogowski coil or di/dt sensors  8  for sensing the three-phase current flowing through the separable contacts  4 , although any suitable current sensor may be employed. 
     The power supply  12  includes a current transformer (CT)  14  for each pole having a single turn primary coil and a plural turn secondary coil including a secondary voltage  20 . The secondary voltage is generated by switching the current output of the CT between a diode/capacitor combination and a shorting FET. A rectifier (e.g., a full-wave rectifier (FWR))  22  is structured to rectify the CT secondary voltage  20 . The rectifier  22  includes an input electrically interconnected with the CT secondary and an output coupled to a voltage regulator  23 . The voltage regulator  23  outputs a regulated voltage  26 . 
     The power supply  12  further includes a startup circuit  24  that is electrically interconnected with a DC/DC converter  32 , such as a switching regulator. Startup circuit  24  is structured to monitor the power available from CT  14  and only allow trip unit  10  to start when sufficient power is available. In particular, as described in greater detail herein, CT  14  is burdened by a load that is electrically equivalent to that of the operating trip unit  10 . When a predetermined voltage is reached, it is known that sufficient power is available to run trip unit  10 . No shutdown of trip unit  10  will occur as sufficient available power has already been confirmed. Thus, the disclosed concept provides a startup circuit  24  that is structured to allow the higher voltages needed for operation of trip unit  24  to develop so that DC/DC converter  32  and therefore trip unit  10  are able to start up and run continuously at lower primary currents. DC/DC converter  32  includes an input  34  powered from the regulated voltage  26 , an enable pin  36 , and an output  38  (e.g., ±5 V) structured to power the signal processing electronics of trip unit  10 . 
       FIG. 2  is a circuit diagram of startup circuit  24  according to an exemplary embodiment of the disclosed concept. As seen in  FIG. 2 , startup circuit  24  is powered by the regulated voltage  26  and includes a capacitor  40 , which in the exemplary embodiment is a 47 uF capacitor, that is provided between the line carrying the regulated voltage  26  and ground. Among other functions (described herein), capacitor  40  supplies the energy (in the form of regulated voltage  26 ) needed by the trip actuator of circuit breaker  2 . Startup circuit  24  further includes a switch, such as a field effect transistor (FET)  28 , that is electrically connected in series with a burden impedance, such as a resistor  30 . As seen in  FIG. 2 , the series combination of FET  28  and resistor  30  is connected in parallel with capacitor  40 . The collector of an NPN bipolar junction transistor (BJT)  42  is connected to the gate of FET  28 . The base of BJT  42  is coupled to the anode of a zener diode  44  through a current limiting resistor  46 , and the collector of BJT  42  is connected to the line carrying the regulated voltage  26  through current limiting resistor  47 . In the exemplary embodiment, zener diode  44  is a 10 V zener diode. The anode of zener diode  44  is also connected to ground through a resistor  48 . The cathode of zener diode  44  is connected to the line carrying the regulated voltage  26 . A zener diode  50  is provided between the gate of FET  30  and ground and acts as a clamp to make sure the gate of FET  30  does not see an overvoltage. A voltage divider circuit  52  is connected to the drain of FET  30 . An enable line  54 , which provides an enable signal to the enable pin  36  of DC/DC converter  32 , is coupled to voltage divider circuit  52  as shown. Voltage divider circuit  52  makes sure that enable signal  54  is at the right level for input  36  of the DC/DC converter  32 . Startup circuit  24  also includes a capacitor  56  (which in the exemplary embodiment is a 0.01 uF capacitor) and capacitor  58 , which are provided in series between the line carrying regulated voltage  26  and ground. A series connection of a resistor  60 , a diode  62  (which may be a standard silicon diode or a zener diode), and a zener diode  64  is connected between the point of interconnection of capacitors  56  and  58  and the voltage divider circuit  52 . As seen in  FIG. 2 , the anodes of zener diodes  62 ,  64  are directly connected to one another. The function and operation of all of these components is described elsewhere herein. 
     It is important to note that the current requirement from the signal provided to input  34  decreases as the voltage of the signal provided to input  34  increases because of the DC/DC converter  32  providing the +5 V. Without the startup circuit  24 , as primary current increases, the voltage at the signal provided to input  34  will rise rapidly at the CT is unburdened. With inadequate current available, the DC/DC converter  34  will be unable to support its output voltage and will rapidly shut back down. This cycle will continue (start-up/shutdown) until the primary current reaches high enough current to support operation. Referring again to  FIG. 1 , given a predetermined primary current for the CT  14  that supplies power to the trip unit  10 , enough secondary current may be available at relatively higher CT secondary voltages if those voltages are given time to develop. The disclosed power supply  12  allows those relatively higher CT secondary voltages to develop, in order that the switching regulator  32  and, therefore, the trip unit  10  are both able to “startup” at a relatively lower CT primary current. 
     This is accomplished by initially (at relatively very low primary current) burdening the secondary of CT  14  with the resistive load of burden resistor  30  rather than with the DC/DC converter  32  and the trip unit  10 . As described above, this resistive load is electrically interconnected with the secondary of CT  14  by the FET  28  tied to circuit ground. The resistance of the burden resistor  30  is selected such that its power dissipation at minimum operating conditions is equal to or slightly greater than that of the trip unit  10  operating under the same conditions. In the exemplary embodiment, zener diode  44  is used to sense when the regulated voltage  26  reaches a predetermined level which is sufficient to power the trip unit  10 . At that point, the FET  28  is turned off, which removes the resistive burden of resistor  30  and takes DC/DC converter  32  out of its shutdown mode. As a result, the trip unit  10  “starts-up” cleanly at a relatively lower primary current than without such a circuit and without any “false starts” as described above. 
     More specifically, referring to  FIG. 2 , as the voltage on capacitor  40  rises from zero, FET  28  turns on very early (around 2 V on capacitor  40 ). FET  28  being on makes the signal on enable line  54  low, which holds DC/DC converter  32  in shutdown mode. Also, as described elsewhere herein, current through resistor  30  controls the voltage of the secondary of CT  14 . When the voltage on capacitor  40  rises high enough, the 10 V zener diode  44  breaks over. This turns on BJT  42 , which turns off FET  28 . FET  28  being off makes the signal on enable line  54  high, which turns on the DC/DC converter  32  (takes is out of shutdown mode). Choosing resistor  30  properly will guarantee that when FET  28  turns off, CT  14  will be able to supply enough energy to keep trip unit  10  running. Without this proper choice, capacitor  40  will quickly discharge and trip unit  10  will shut down. Zener diode  64  provides hysteretic feedback to keep DC/DC converter  32  on at lower voltages on capacitor  40 , and zener diode  62  is a blocking diode that makes sure the hysteresis works in only one direction. This hysteresis ensures that the converter will not shut down as soon as a trip signal is issued and the voltage on capacitor  40  gets pulled down. 
     In high fault situations, there is sufficient current available to turn on trip unit  10 . However, waiting for capacitor  40  to charge to a point above the break over threshold of zener diode  44  (thereby enabling DC/DC converter  32  and starting trip unit  10  as described) may compromise trip time performance. This situation is addressed in the disclosed concept by providing capacitor  56  described above, which is coupled to the line/node that carries the regulated voltage  26 . Capacitor  56  is responsive to the change in voltage on the line/node that carries the regulated voltage  26 . In high fault situations, this voltage will rise rapidly. The current flowing through capacitor  56  will increase in direct proportion to this rapidly rising voltage. This proportional current will be injected into the base of BJT  42 , which in turn will turn FET  30  off, thereby enabling DC/DC converter  32 . By choosing the right value for capacitor  56 , startup circuit  24  can be triggered earlier in such high fault conditions, i.e., at a lower voltage on the line/node that carries the regulated voltage  26 . In other words, at high currents, DC/DC converter  32  will be allowed to start if the voltage is increasing quickly enough. This earlier triggering is acceptable and desirable because the rapid dv/dt on the line/node that carries the regulated voltage  26  is an indication that sufficient current is available to start and that rapid trip times may be required. 
     While specific embodiments of the disclosed concept have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in tight of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.