Abstract:
Electrical equipment is safeguarded from damage due to faults by a circuit that provides several levels of protection. A semiconductor switch and a current sensor are placed in series with the electrical equipment. When the current to the equipment exceeds a first threshold for a predefined period of time, the semiconductor switch is rendered non-conductive until the circuit is specifically reset. When the current to the equipment exceeds a greater second threshold, a pulsed signal alternately places the semiconductor switch in conductive and non-conductive states so that the average current applied to the equipment is within an acceptable level. Should the current exceed an even greater third threshold, the semiconductor switch is immediately rendered non-conductive until the circuit is manually reset.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to mechanisms that protect equipment from damage due to electrical faults and short circuits; and particularly to such devices which electronically monitor performance of the electric equipment and take protective action in the event of a fault, short or overload. 
     It is important that electrical apparatus be protected from damage when electrical failures occur. For example, conventional fuses and electromechanical circuit breakers are commonly employed to disconnect equipment from an electrical supply upon detection of excessive current when a short circuit occurs. Nevertheless, these conventional protection devices are relatively slow in disconnecting the current flow to the apparatus being protected. As a consequence, enough excessive electrical current can flow into the equipment to cause damage during a fault. 
     Furthermore, various electrical apparatus require different response characteristics for the protection device. For example, electronic equipment may draw a substantially constant current level from initial start-up through a normal shut-down and be very intolerant of excessive current levels of even short duration. The protection device for such equipment has to respond very quickly to even relatively small over current conditions. Other types of electrical equipment draw large instantaneous current levels at certain times, such as upon start-up, in comparison to the current level drawn during remainder of their operation. Thus, a circuit protection device that responds too rapidly to an high current condition may inadvertently shut-off current to the equipment during normally occurring events. As a consequence, the protection device for this type of equipment must respond in a manner that tolerates brief high currents. The manner in which a protection device responds to over currents is referred to as the trip response characteristic or trip curve, and has to be matched to the particular type of electrical apparatus being protected. 
     This usually means that a manufacturer of protection devices must design, manufacture and stock in inventory, a large variety of protection devices that have different trip response characteristics in terms of current level and duration. Thus it is desirable to provide a basic configuration of a protection device which can be customized easily with different trip response characteristics. 
     SUMMARY OF THE INVENTION 
     An apparatus for protecting an electrical load from excessive current employs a semiconductor switch to connect the electrical load to a source of current. A current sensor is coupled in series with the semiconductor switch and produces a sensor signal that indicates the magnitude of current flowing to the electrical load. 
     A control circuit is connected to the current sensor and the semiconductor switch. The control circuit responds to the sensor signal by producing a control signal that is applied to a control input of the semiconductor switch. In a first mode of operation when the magnitude of current is less than a first threshold, the control circuit maintains the semiconductor switch in a continuous conductive state. When the magnitude of current is greater than the first threshold and less than a second threshold, the control circuit in a second mode of operation renders the semiconductor switch non-conductive after a predefined period of time. In a third mode of operation when the magnitude of current is greater than the second threshold, the semiconductor switch is alternately pulsed conductive and non-conductive by the control circuit to apply an average current through the load that is within an acceptable level wherein damage does not occur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a solid state circuit protector according to the present invention; 
     FIG. 2 illustrates the details of an instant trip circuit in the protector; and 
     FIG. 3 is a graph of an exemplary trip response characteristic of the solid state circuit protector. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 1, a solid state circuit protector  10  controls application of a direct current to an electrical load  14 , depicted as capacitance in parallel with a resistance. The solid state circuit protector  10  has a positive voltage terminal  12  which is connected to the electrical source for powering a load  14 . Current flows from the positive voltage terminal to the load through a back-up fuse  16 , a semiconductor switch  18  and an inductor  20  to a load terminal  22 . The load is connected between the load terminal  22  and the negative side of the voltage supply, represented as ground. 
     The back-up fuse  16  is a conventional device with a conductor which heats-up and ultimately breaks when excessive current flows for a given period of time. Standard devices, such as glass tube encased fuses or an appropriate trace on printed circuit board, can be employed for the back-up fuse  16 . The back-up fuse  16  provides redundant protection in case the semiconductor switch  18  fails in the conductive state or upon failure of electronic circuits controlling the semiconductor switch. As will be understood, the trip response time of the back-up fuse is considerably slower than the trip response characteristic of the electronic circuit protection. 
     The semiconductor switch  18  must be able to interrupt the load current and handle transient currents, over currents and in-rush at a specified operational voltage range as dictated by the particular load  14  to be controlled. An n-channel field effect transistor (FET), such as model IRF1404 from International Rectifier of El Segundo, Calif. 90245 USA may be used as the semiconductor switch  18 . The channel resistance in the conductive state has to be relatively low to minimize the voltage drop across the FET and the heat dissipation. Although the preferred embodiment employs the semiconductor switch  18  between the positive voltage terminal  12  and the load  14 , alternatively the switch could be placed on the ground side of the load. However, this alternative approach has the disadvantage that a fault from load to ground would be unprotected. 
     A voltage sensor  28  produces an analog signal which indicates the voltage level at the load terminal. That analog signal is applied to an analog input of a microcontroller  26 . As will be described, the microcontroller  26  responds to an indication from the sensor  28  that the voltage across the load  14  is too low by turning off the semiconductor switch  18 . 
     A current sensor  24  is provided to detect the level of current flowing between the positive voltage terminal  12  and the load  14 . This sensor must have a dynamic range which is large enough to cover the current extremes for the desired trip response characteristic of the protection device and have a transient response that is sufficiently fast to implement the desired trip response characteristic. The current sensor  24  may be a Hall effect sensor that produces an output voltage indicative of the DC current magnitude and which output voltage can be applied via line  31  directly to an analog input of a microcontroller  26 . Other types of conventional current sensors, such as a shunt resistor, may be used to provide a current magnitude indication to the microcontroller  26 . 
     The microcontroller  26  is microprocessor based and includes an internal analog-to-digital converter with a multiplexed input for signals from the current and voltage sensors. Digital input/output circuits of the microcontroller handle signals for other components of the solid state circuit protector  10 . For example, a user control panel  25  has a keypad  27  and light emitters  29 , such as LED&#39;s. The keypad  27  has separate momentary contact switches that supply input signals to the microcontroller  26  to manually turn the solid state circuit protector  10  on and off, as well as reset a trip condition. The light emitters  29  are powered by signals from the microcontroller to indicate the operational states of the circuit protector. One of those light emitters  29  indicates when the circuit protector  10  is tripped. The microcontroller  26  also has an internal non-volatile memory which stores a software program defining the protection function and which stores data, such as the trip response characteristic, for use by that software program. The microcontroller  26  and the control panel  25  optionally can control additional poles of a circuit protector as indicated by a second pole  11  drawn in phantom lines. 
     The microcontroller  26  operates the semiconductor switch  18  through a trip circuit  36  that generates a drive voltage which is adequate to control the FET  19  in the preferred embodiment of the semiconductor switch  18 . Because the voltage driving the gate of an N-channel FET  19  has to be approximately ten volts greater than the voltage at the source electrode of the FET, the trip circuit  36  includes a charge pump or similar circuit to generate voltage greater than that found on the positive input terminal  12 . 
     FIG. 2 illustrates the details of the trip circuit  36  wherein the output signal I SENSE  on line  31  from the current sensor  24  is applied to a first voltage comparator  40 . The sensed current level I SENSE  is compared to a second threshold I TH2  which is produced on an analog output line  37  of the microcontroller  26 . A fixed value for the second threshold I TH2  is programmed into the microcontroller  26  depending upon the over current tolerance of the specific load  14 . The result of that comparison at the output of the first comparator  40  is applied to the RESET input of a flip-flop  42 . The reset input also is connected to a positive supply voltage V +  by a pull-up resistor  44 . 
     The SET input of the flip-flop  42  is connected to the output of a dual input NAND gate  46 , having both inputs tied together to function as an inverter. The inputs of first NAND gate  46  are connected to a digital output line  33  from the microcontroller  26  which carries a pulsed signal at a fixed frequency in excess of 15 kHz., specifically in the range of 20-30 kHz. and preferably at 25 kHz. The pulsed signal has a fixed duty cycle thereby forming a train of constant-width pulses. As will be described, the pulse train periodically sets the flip-flop output which is tied to one input of a second NAND gate  48  having three inputs. Another input of the second NAND gate  48  receives an ON signal on another digital output line  33  from the microcontroller  26 . Whether the ON signal is active or inactive is determined by manual operation of switches on keypad  27  of the control panel  25 . 
     The third input of the second NAND gate  48  receives an output signal from an instant trip mechanism formed by a second voltage comparator  50  and a second flip-flop  51 . Specifically, the second comparator  50  compares the current sensor output signal I SENSE  to a third threshold I TH3 . The third current threshold I TH3  is generated on another analog output line  38  by the microcontroller  26  and is defined by a fixed value programmed into the solid state protection circuit  10 . The third current threshold I TH3  is greater than the second current threshold I TH2 . The precise relationship between those two current thresholds will become apparent from a subsequent description of the operation of the solid state current protection circuit. The second and third current thresholds I TH2  and I TH3 , instead of being programmable, may be set by conventional voltage dividers at the inputs to the respective comparators  40  and  50 . The output of the second comparator  50  is latched by the second flip-flop  51  with an output connected to another input of the second NAND gate  48 . The set input of the second comparator  50  is connected to RESET output line  35  from the microcontroller  26 . 
     The components of the trip circuit  36  described thus far, provide input signals to the second NAND gate  48 . The output of that gate is fed through a third NAND gate  52  which is connected as an inverter. The signal emanating from the third NAND gate  52  is coupled by a resistor  56  to an isolation circuit  54 , such as a standard opto-isolator. The isolation circuit  54  produces an output on line  58  that is applied to an input of a conventional FET gate driver circuit  60 . A charge pump  62  provides a voltage level that the FET gate driver  60  uses to bias the gate of the FET  19  via line  39 . 
     The operation of the solid state circuit protector  10 , in FIG. 1, commences with the operator pressing an appropriate switch on keypad  27 . The microcontroller  26  responds to this switch activation by applying a high level, or active ON signal, via line  34  to the second NAND gate  48 . At this time, the microcontroller  26  also begins producing a pulse train on digital output line  33  connected to the first NAND gate  46 . A high logic level of that pulse train causes the output of the first flip-flop  42  to go high, applying another high level to another input of the second NAND gate  48 . 
     During normal operation of the load  14 , the output signal I SENSE  from the current sensor  24  is less than the third threshold I TH3 . As a consequence, the second voltage comparator  50  produces a high logic level at the third input of the second NAND gate  48 . Thus the second NAND gate  48  produces a low level output signal that, upon inversion by the third NAND gate  52  and conduction through isolator  54 , activates the FET gate driver  60 . This causes the gate driver  60  to bias the gate of the FET  19  into a conductive state, thereby applying current from the positive voltage terminal  12  through the inductor  20  to the load  14 . 
     The level of current through the semiconductor switch  18  rises rapidly and soon exceeds the second threshold I TH2 . At that time, the output of the first comparator  40  goes low resetting the flip-flop  42  and causing the second NAND gate  48  to change output states. This results in the FET gate driver  60  rendering the semiconductor switch  18  non-conductive. The energy stored in the inductor  20  produces a decaying current that flows through the load  14  and the fly back diode  21 . 
     When the next positive pulse occurs in the pulse train on line  33  to the first NAND gate  46 , the flip-flop  42  will be SET to produce another high logic output level which once again turns on the FET gate driver  60  and the semiconductor switch  18 . This on-off cycling of the semiconductor switch continues chopping the current at the rate of the signal on line  33  until the capacitance in the load  14  adequately charges, at which time the load current becomes substantially constant at a level less than the second threshold I TH2 . Thus the load current during start-up is limited to being less than the second threshold I TH2  while still applying current to initialize the load operation. Once the excursions of current through the semiconductor switch  18  fall below this threshold, the flip-flop  42  no longer is reset and the FET gate driver  60  maintains the semiconductor switch  18  in a conductive state. That conductive state continues as long as the load  14  functions normally. 
     If there is a fault with the load during start-up, the load current does not drop below the second threshold I TH2 . The current chopping could continue indefinitely in this case. To prevent that, the duration of the current chopping is limited by counting the current pulses applied to the load and terminating the chopping upon the occurrence of given number of pulses that normally is sufficient to charge a typical load capacitance. Specifically, the microcontroller  26  monitors the input line  31  from the current sensor  24  which indicates alternate high current and zero current conditions and counts the number of high current pulses. That count is compared to a reference number and the chopping mode is terminated when that reference number of current pulses has occurred. At that time, the microcontroller  26  sends a low logic level signal on line  34  to the trip circuit  36 , which renders the semiconductor switch non-conductive until a person presses the RESET switch on the control panel  25  and resets the microcontroller. 
     Alternatively the voltage sensor  28  can be employed to safeguard against operating in the current chopping mode for too long a time period. During a short circuit condition when the load  14  is drawing excessive current, the voltage across the load will be significantly lower than during normal operation. The voltage across the load  14  is detected by the voltage sensor  28  which applies an analog voltage level indication to the microcontroller  26 . If that sensed load voltage remains below a given threshold for greater than a predefined time interval during the current chopping mode, the microcontroller  26  turns off the trip circuit  36  by applying a low logic level, (an inactive ON signal) to the ON/OFF line  34 . 
     The operation of the solid state protection circuit  10  during an over current condition after a normal start-up may best be understood with respect to an exemplary trip response characteristic, such as the one depicted in FIG. 3. A load current which is below a first threshold I TH1  can be tolerated indefinitely by the load  14  and thus will be conducted continuously by the semiconductor switch  18 . The first threshold I TH1  is set between 100% and 125% of the current rating for the load  14  being protected. Load currents between levels I TH1  and I 2  can be tolerated by the load for an amount of time which is inversely proportional to the current magnitude. In other words, small deviations above the first threshold I TH1  can be tolerated for a longer period of time than over currents which approach level I 2 . This produces a linear trip response characteristic in portion  70  of the response curve. This portion of the trip response characteristic is programed into the microcontroller  26  and stored in its memory either as a linear equation or as a data table. That data table has pairs of values with one value being a current magnitude and the other value defining a time interval during which that current magnitude can be tolerated before the solid state circuit protector  10  must trip. 
     Current between level I 2  and the third threshold I TH3  can be tolerated by the load for a period designated T 1 . Current above that higher level I TH3  cannot be tolerated by the load  14 , even momentarily, and thus the current protection device will trip immediately. It should be noted, that load current within the cross-hatched region  72  between second threshold I TH2  and a third threshold I TH3  while tolerated by the load  14 , can damage the FET  19 . Thus, when operation within this region is determined to occur, the solid state circuit protector  10  enters a current chopping mode of operation. In this mode the semiconductor switch is pulsed on and off at a rate which produces an average current that is less than the second threshold I TH2 . Thus the load remains powered so that the load capacitance remains charged but the current applied to the load is limited to that second threshold level. 
     When the sensed current I SENSE  is between the first threshold I TH1  and a second threshold I TH2 , the trip circuit  36  initially maintains semiconductor switch  18  in a conductive state because that current is below the two comparator thresholds I TH2  and I TH3 . However, the microcontroller  26  receives the output signal I SENSE  from the current sensor  24  on line  31  utilizes the programed trip response characteristic for section  70  to determine whether to turn-off the semiconductor switch  18 . Specifically, the microcontroller  26  determines whether the over current magnitude has occurred for the time period defined by the trip response characteristic. Once that has occurred, the microcontroller  26  turns off the trip circuit  36  by applying a low logic level, inactive ON signal, to the digital line  34 . This constant low logic level toggles the output level from the second NAND gate  48  which turns-off the FET gate driver  60  and thus the semiconductor switch  18 . The microcontroller  26  also illuminates the light emitter  29  on the control panel  25  which indicates the tripped condition. The OFF signal continues to be applied by the microcontroller  26  to the trip circuit  36  until a manual reset switch on the control panel  25  pressed. 
     When the sensed load current signal I SENSE  is between the current thresholds I TH2  and I TH3 , the microcontroller  26  does not utilize the trip response characteristic data to determine whether to turn-off the trip circuit  36 . Instead, the solid state protection circuit  10  enters a current chopping mode in which the FET  19  is pulsed on and off at the rate of the pulsed signal on line  33 . 
     Specifically with reference to FIG. 2, when the current sensor  24  produces an output signal I SENSE  on line  31  which is greater than the second threshold I TH2  on line  37 , the output of first comparator  40  goes low. That low output resets the flip-flop  42 , thereby applying a low logic level to an input of the second NAND gate  48 . This produces a high logic level at the output of the second NAND gate  52  which is inverted by the third NAND gate  52  thus applying a low logic level to the opto-isolator  54 . This in turn deactivates the FET gate driver  60  which renders the semiconductor switch  18  nonconductive. At that time current from the inductor  20  flows through the load  14  and a fly back diode  21 . 
     The semiconductor switch  18  remains off until the next high logic level pulse in the pulse train from the microcontroller  26  that is applied to the trip circuit  36  on digital line  33 . That pulse upon inversion by the first NAND gate  46  sets the flip-flop  42  which produces a high output level that is applied to the second NAND gate  48 . This high logic level activates the FET gate driver  60 , once again rendering the semiconductor switch  18  conductive. 
     When the FET  19  turns on again, the inductor  20  limits the rate at which the current rises so that the current level does not immediately exceed the second threshold I TH2 . Thus, a small amount of current will be applied to the load  14  and charging its capacitance. However, the current through the semiconductor switch  18  eventually rises above the second current threshold I TH2  which will be detected by the first comparator  40 . When this occurs the first comparator  40  changes output states and resets the flip-flop  42 , which in turn applies a signal to the second NAND gate  48  that ultimately results in the FET gate driver  60  shutting off the semiconductor switch  18 . This cycling of the semiconductor switch  18  off and on continues which results in an average load current that is below the first threshold I TH1 . 
     Although the semiconductor switch  18  is not exposed to as great a degree of thermal stress as with linear current limiting, damage to the FET  19  or the load still may occur if the current chopping mode continues for too long a time period. As discussed previously with respect the start-up operation of the solid state circuit protector  10 , the duration of the current chopping can be limited by the microcontroller  26  counting the number of current pulses applied to the load and sending a low logic level OFF signal on line  34  to the trip circuit  36  when a given number of pulses has occurred. Alternatively the voltage sensor  28  can be employed to detect a short circuit and inform the microcontroller  26  to turn-off the trip circuit  36 . 
     The current chopping should ensure that the load current never exceeds the third threshold level I TH3 . However, in the event that a malfunction occurs, the second comparator  50  detects a load current above that third threshold level I TH3  and produces an output that renders the semiconductor switch  18  continuously non-conductive. Specifically, the output of the second comparator  50  goes low which resets the second flip-flop  51  thereby applying a low logic level to the second NAND gate  48 . This results in the FET turning off.