Abstract:
A fail open circuit that includes a controllable switch coupling a power source to a load. Control circuitry is provided that determines if the switch is in the proper conduction state based on a switch control signal and a signal indicative of power delivered to the load. If the switch is determined as improperly closed (conducting), the control circuitry diverts energy delivered to the load through fuse circuitry, thereby blowing a fuse and decoupling the load from the power source. In preferred embodiments, logic circuitry determines the relative states of the control switch and the load and generates a control signal to divert energy away from the load and blow a fuse.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a fail open circuit for use with a power control device to detect a short circuit in the power control device. Particularly, the fail open circuit detects a shorted power control device by comparing the power input current and a power output current when the power control device is intended to be off. Further, the present invention uses the output of the power control device itself to open the respective circuit if that power control device is shorted. The present invention is applicable to DC or AC power switching applications. Also, a typical power control device is a solid state power switch, although the present invention is not limited to such as power control device.  
           [0003]    2. Description of Related Art  
           [0004]    Solid state power switches used as power control devices tend to fail in shorted states, which is often unacceptable in power switching applications. Various fail open circuits for detecting such short circuits in power switching applications are known. In one such technique, a second solid state power switch device is used to turn off the current when a primary solid state power switch fails to respond. This method is not failsafe, since both devices may be shorted by one large power surge. In another technique, a mechanical fuse or circuit breaker is added in series with the solid state power switch device. This technique is based on the assumption that when the solid state power switch device is shorted, so is the load. This is not always the case. Therefore, this technique fails to detect a shorted solid state power switch where the power switch is shorted but the load is not.  
           [0005]    In yet another technique, a thermal/current fuse is placed in close proximity to a power control device such that the fuse opens the circuit when the solid state power switch carries the full load current, thereby overheating the fuse, or in the event that the temperature of the power switch exceeds the fuse&#39;s temperature rating. This technique is effective even where the power switch is shorted but the load is not. However, this technique is not failsafe in that it requires the power control device itself to exceed the fuse&#39;s temperature or current rating in order for the circuit to be opened.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention solves the aforementioned drawbacks of the prior art by providing a fail open circuit that compares the conduction state of a switch coupling power to a load, and if the switch is improperly closed the present invention utilizes the energy delivered to the load to blow a fuse and decouple the load from a power source. In one embodiment, the present invention provides a fail open circuit comprising an input voltage coupled to an output load through a control switch; a control signal regulating the conduction state of said control switch; and fail open circuitry receiving said control signal and a signal indicative of the energy delivered to said load, and adapted to determine if said control switch is in the proper conduction state based on said control signal, said fail open circuit further adapted to decouple said input voltage from said load using said energy delivered to said output load if said control switch is in an improper conduction state.  
           [0007]    In method form, the present invention provides a method for fail-open circuit operation, comprising the steps of:  
           [0008]    coupling an input voltage to a load through a control switch;  
           [0009]    regulating the conduction state of said control switch with a control signal;  
           [0010]    determining if said control switch is in a proper conduction state based on said control signal and the energy delivered to said load;  
           [0011]    decoupling said input voltage from said load using said energy delivered to said load if said control switch is in an improper conduction state.  
           [0012]    It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited only as set forth in the accompanying claims.  
           [0013]    Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a circuit diagram of an exemplary power switching circuit of the present invention including an exemplary fail open circuit and solid-state power switch;  
         [0015]    [0015]FIG. 2 is a chart of inputs and outputs of the components of heater control circuitry in a preferred embodiment of the present invention.  
         [0016]    [0016]FIG. 3 provides a chart in summary of the conditions and results of the preferred embodiment of fail open circuit  100 , illustrating the conditions for opening the circuit. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0017]    [0017]FIG. 1 is circuit diagram of an exemplary power switching circuit  10 , including a power input  24 , control voltage  22 , power control device (switch)  28 , an exemplary fail open circuit  100 , and power output  40 . The power input  24  supplies power to the circuit, and comprises one or more AC or DC sources. The control voltage  22  turns on or off switch  28  either allowing or disallowing, respectively, current from the power input  24  to be transmitted to the power output  40 .  
         [0018]    In order to ensure fail-open operation of the circuit  10 , a fail open circuit  100  with control circuitry  35  is provided to determine when to open the circuit  10  based on the condition of the control voltage  22 , and the conduction state of the switch  28 . In the example shown in FIG. 1, the fail open circuitry  100  includes fuse circuitry  30  for opening the circuit  10 . Control circuitry  35  monitors both the control voltage  22  and the output condition to determine if the switch  28  is incorrectly conducting (i.e., failed shorted), thereby commanding fuse  30  to blow. The exemplary circuits  35  and  100  are described in greater detail below.  
         [0019]    Fuse circuitry  30  comprises a heating resistor  31  and a thermal fuse  32  for opening the circuit  10 . To determine the output condition an output sense resistor R o    37 , a current shunt and amplifier  38  are provided. The signal developed across resistor  37  is fed into the control circuitry  35  to control the conduction state of the fuse control switch  20 , as described below. Thus, the present invention uses the energy delivered to the load to effectuate blowing the fuse.  
         [0020]    It should be noted that the following description assumes that switch  28  is active (conducting) high, but it will be apparent that the present invention can likewise be adapted to operate with active low switches. Switch  28  is in an improper conduction state if it conducts when control voltage  22  commands that switch  28  be open. That is, switch  28  is in an improper condition when it is shorted. In the preferred embodiment, the control voltage  22  delivers a high/on signal to the switch  28  to command the switch to close, and a low/off signal to command the switch to open. In this embodiment, note that, if the control voltage  22  is off, irrespective of the status of the power input, the switch  28  should be open, and therefore, conduction through the switch indicates an improper conduction state. Fail open circuit  100  opens the circuit  10  when the control voltage  22  is off and the switch  28  conducts.  
         [0021]    Control circuitry  35  determines whether the above condition is satisfied, and generates a signal that causes the opening of the circuit  10 . In the preferred embodiment, circuitry  35  receives as inputs, signals determinative of whether the control voltage  22  is on, and determinative of whether power is being supplied to the output  40  of the circuit  10 . The latter input is supplied by the output of the aforementioned current shunt and amplifier  38 . In the preferred embodiment, current shunt and amplifier  38  respectively diverts and amplifies current through output sense resistor, R o    37 , in series with the power output  40 . Each of the inputs is received by circuitry  35  as a high or low, i.e., binary 1 or 0, signal determining whether that signal is on or off respectively. This is discussed below.  
         [0022]    Preferably, control circuitry  35  comprises logic devices including an inverter  36  and an AND gate  34  in the configuration illustrated by FIG. 1. Inverter  36  generates the compliment of the input signal from control voltage  22 . That is, inverter  36  generates a high signal when control voltage  22  commands that switch  28  be open. Gate  34  performs an AND operation on the above result of the inverter  36  and on the output of current shunt and amplifier  38 . That is, the result of gate  34  is the result of output of control circuitry  35 . In the preferred this output is a high signal when control voltage  22  is low/off, and output power is still on.  
         [0023]    [0023]FIG. 2 provides a table of binary inputs and outputs of inverter  36  and AND gate  34 , and of the preferred embodiment of circuitry  35 . As seen from the illustration, the output of the circuitry  35 , i.e., output of AND gate  34 , is high when both inputs of the AND gate  34  are high. This condition is satisfied if power is delivered to the output  40  of the circuit  10  and the output of the inverter  36  is high. The output of the inverter  36  is high when control voltage  22  to the switch  28  is low/off. In alternative embodiments and based on alternative control signal configurations for switch  22 , control circuitry  35  may comprise other components to generate a commanding signal causing the circuit  10  to open when switch  28  is in an improper conduction state.  
         [0024]    Returning to FIG. 1, in the preferred embodiment, when power switch  28  is shorted and the output of the control circuitry  35  is high as discussed above, the output of power switch  28  is used to open the circuit  10 . As seen in FIG. 1, the fuse control switch  20  receives the output of control circuitry  35 . Generally, fuse control switch  20  functions as a current sink upon being commanded on by a signal from circuitry  35 . Preferably, device  20  is a PNP transistor, with the output of circuitry  35  received at its base, or gate, and allowing current to sink to ground upon receiving a high signal from circuitry  35 . Alternatively, device  20  may be an NPN transistor triggered by a low input at its base or gate. Of course, such an alternative embodiment requires an alternative configuration of logic components in the control circuitry  35  to deliver a low signal when the power switch  28  is in an improper conduction state. In the preferred embodiment, when device  20  receives a high signal from circuitry  35 , current is sunk from the output of switch  28 , thereby powering heating resistor  31  in series with the fuse control switch  20 . Resistor  31  heats a thermal fuse  32 , in series with switch  28  and power output  40 , and placed in thermal proximity to the resistor  31 . In turn, in the preferred embodiment, thermal fuse  32  exceeds its temperature rating and blows, opening circuit  10  and preventing unwanted power from being delivered at the power output  40 . It will be apparent to one skilled in the art that alternatively, any device, which generates heat upon receiving power, may replace resistor  31 .  
         [0025]    [0025]FIG. 3 provides a flowchart  200  in summary of the conditions and results of the preferred embodiment of fail open circuit, illustrating the conditions for opening the circuit. For clarity reference will be made to the components of FIG. 1 without reference numerals. Initially, fail open circuit checks the status of control signal  202 , where, in the preferred embodiment, control signal is on when it commands control switch to be closed, or off when it commands control switch to be open. If the control signal is off, then power to the output load is checked  204 . If power exists at the load, then the thermal fuse is blown  206 , decoupling the power input from the load. That is, in the above condition, switch  28  is determined to be in an improper conduction state, i.e., shorted, and the circuit is opened. Additionally, the fuse blows when the current output of the control switch exceeds the current rating of the fuse  208 . Of course, power input is on when this condition occurs. Further, fuse is placed in proximity to switch such that fuse blows when the switch itself exceeds the fuse&#39;s temperature rating  210 . These are secondary measures provided in addition to the primary function of fail open circuit in the preferred embodiment of the present invention.  
         [0026]    Alternative embodiments of the present invention allow for circuitry  35  to determine further conditions of the circuit  10  prior to commanding to decouple the power input  24  from the power output  40 . In an example, circuitry  35  may include additional logic components to determine if the power input  24  is on or off, and, further, give weight to that determination in calculating the appropriate signal to generate as its output.