Abstract:
A method of controlling a solid state power controller includes selectively allowing a transient current through a solid state power control switch in response to the transient current exceeding at least one threshold.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to vehicle power systems and, more particularly, to solid state power controls. 
     Vehicles, such as aircraft, typically utilize one or more power distribution units to distribute power from a primary power source to various vehicle systems. The solid state power controls in a power distribution unit typically include an electronic switch, such as a FET, and electronic circuitry that provides wiring protection. The FET and circuitry are often referred to as a solid state power controller (“SSPC”). The SSPC has found widespread use because of its desirable status capability, reliability, and packaging density. A typical power distribution unit may include hundreds or thousands of SSPCs. 
     SSPCs also must operate in the presence of lightning, which can adversely impact electronic devices. Traditionally, aircraft had an aluminum skin that attenuated the lightning current induced on the wires. Some aircraft now use composite materials instead of aluminum for weight and strength benefits. However, composite materials do not provide the same level of attenuation to lightning as aluminum. When lightning occurs, hundreds of volts may surge between a load in the vehicle system and the aircraft chassis. As such, the lightning requirements of SSPCs have increased. 
     The increase in lightning levels poses a significant additional burden because the SSPC does not provide galvanic isolation in the off state, as would a typical electro-mechanical circuit for example. Instead, the SSPC uses the FET for switching and the electronic circuit to provide the circuit breaker function. If the SSPC is in an off state when lightning strikes, the large voltage potential will undesirably increase the voltage across the FET. An over-voltage clamp can be used to protect the FET from exceeding its maximum voltage capability by placing the FET into a linear region. However, the increased power dissipation of the FET in the linear region limits the amount of lightning energy that can be dissipated. Alternatively, high voltage FETs may be used to block the voltage in the off state, transient suppression devices can be placed across the FETs or more parallel FETs may be added, but these solutions are expensive, require larger packaging, and reduce reliability. Additionally, to protect the loads, a transient voltage suppression is often used by the load to shunt or divert the lightning current. This further increases the lightning energy that the SSPC must survive. 
     There is a need for a simple, relatively inexpensive SSPC with improved lightning protection. This invention addresses those needs while avoiding the shortcomings and drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     An example method of controlling a solid state power controller includes selectively allowing a transient current through a solid state power control switch in response to the transient current exceeding at least one threshold. 
     An example solid state power controller system includes a switch having a closed state and an open state and a microcontroller that controls the switch. A module having an associated threshold also controls the switch and selectively allows a current through the switch in response to the current exceeding the threshold. 
     Another example method of controlling a solid state power controller includes turning a solid state power controller switch to an OFF state in response to a current through the switch exceeding a threshold, and automatically resetting the switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates selected portions of an example solid state power controller with lightning protection. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates selected portions of an example solid state power controller (SSPC)  18  for use in a vehicle, such as an aircraft. Under some conditions, such as a lightning strike, a transient current may surge through the vehicle. The transient current may be, for example, an induced current, other known type of transient current, or a transient current from another source besides lightning. In the disclosed example, the SSPC  18  provides lightning protection to reduce the risk that the SSPC  18  becomes damaged from the transient current. As will be appreciated from the illustration and the following description, the SSPC  18  of the disclosed examples provides lightning protection without significant packaging density, reliability, or cost penalty. Although the SSPC  18  is a direct current type in the disclosed examples, one of ordinary skill in the art who has the benefit of this disclosure will recognize that the disclosed examples are also applicable to alternating current type SSPCs. 
     In this example, the SSPC  18  includes a logic section  32  and a power section  34 . The logic section  32  includes a power supply  20 , which provides power to a microcontroller  38  that controls operation of the SSPC  18 . The microcontroller  38  interfaces with a gate drive  40 , a switch  42 , an instantaneous trip module  44 , and a lightning module  46 . Although only one switch  42  is shown in this example, multiple switches  42  may be used in the same manner as described. The instantaneous trip module  44  and lightning module  46  sense the electrical current flow through the SSPC  18 . 
     In the disclosed example, the microcontroller  38  is in serial communication with a vehicle control  54 . In this example, the vehicle control  54  includes an active microprocessor  56  and a standby microprocessor  58 . The active microprocessor  56  communicates with the microcontroller  38  to control the operation of the SSPC  18 . The standby microprocessor  58  communicates with the microcontroller  38  to establish or maintain a status of the SSPC  18 . Each of the active microprocessor  56  and the standby microprocessor  58  send periodic signals to the microcontroller  38  to confirm that communication between the vehicle control  54  and the SSPC  18  has not been interrupted. 
     The instantaneous trip module  44  includes associated instantaneous trip logic and the lightning module  46  includes an associated lightning logic. The SSPC  18  responds in a variety of ways to a transient current, depending on the magnitude of the current flowing through the SSPC  18 , a preset instantaneous trip threshold associated with the instantaneous trip module  44 , a preset lightning threshold associated with the lightning module  46 , and whether the SSPC  18  is an ON state (e.g., switch  42  is closed/ON) or an OFF state (e.g., switch  42  is open/OFF). 
     Currents larger than the instantaneous trip threshold are possible with loads that have transient voltage suppression (e.g., shunts) or loads that are highly capacitive in nature. In the disclosed example, the switch  42  includes a voltage clamp circuitry that protects the switch  42  in a known manner from certain voltage transients. The voltage clamp activates in a linear region to dissipate energy to thereby absorb transient currents up to the instantaneous trip threshold. Beyond the instantaneous trip threshold, the instantaneous trip module  44 , the lightning module  46 , and their associated logic protect the SSPC  18  from the transient current, as will be described below. 
     The following examples illustrate the operation of the instantaneous trip module  44  and the lightning module  46  under various conditions for transient currents. In the disclosed examples, turning ON the SSPC  18  passes the transient current to the load and thereby protects against formation of a fusible link and destruction of the switch  42 . The below examples are intended only to illustrate concepts of the instantaneous trip module  44  and the lightning module  46 , and one of ordinary skill who is given this description will recognize application of the concepts to other examples. 
     In one example, the SSPC  18  is OFF when the transient current occurs. Under this condition (i.e., SSPC  18  OFF), the only time that the current is expected to increase above the instantaneous trip threshold is if the SSPC  18  line V line  or load output connection is shorted to a source voltage that exceeds the switch  42  over-voltage clamp protection, or if the SSPC  18  line V line  or load connection is coupled to a transient voltage that exceeds the switch  42  over-voltage clamp protection. The switch  42  over-voltage clamp protects the switch  42  by turning on in a linear region to dissipate energy in a known manner. This provides the benefit of absorbing transient currents up to the instantaneous trip threshold. If, however, the transient current exceeds the instantaneous trip threshold, the instantaneous trip module, the lightning trip module and their associated logic function to protect the switch  42 . 
     In the disclosed example, if the transient current increases above the instantaneous trip threshold, the instantaneous trip module  44  turns on the gate drive  40  to turn ON the switch  42 . In this example, the switch  42  can handle more transient current when ON because the voltage across the switch  42  will be lower, which reduces the transient energy that switch  42  must absorb. The transient current flows to the load during this time to thereby protect the SSPC  18  from damage. As the transient current decreases below the instantaneous trip threshold, the instantaneous trip module  44  removes the gate drive  40  command to force the switch  42  OFF. Optionally, a time delay is used before turning off gate drive  40  to allow the SSPC  18  to cool. 
     The direct communication between the instantaneous trip module  44  and the gate drive  40  provides the benefit of allowing transient current protection even when the microcontroller  38  is in an inactive mode (e.g., a sleep mode). In a sleep mode for example, the microcontroller  38  software will not be awake to control the SSPC  18  to respond to a lightning threat. By directly linking the instantaneous trip module  44  and the gate drive  40 , the instantaneous trip module  44  can directly control the gate drive  40  without the microcontroller  38  to quickly turn ON the switch  42 . 
     In another example, the SSPC  18  is ON when the transient current occurs. The transient current increases the current through the switch  42  until the instantaneous trip threshold is exceeded. At this point, the microcontroller  38  begins to turn OFF the switch  42  and set an auto-recover function. The process of turning OFF the switch  42  takes some time, typically fractions of a second. During this time, the current may increase, decrease, or remain steady. 
     If the current increases, but not fast enough to exceed the lightning threshold before the switch turns OFF, the switch  42  is turned back ON upon exceeding the lightning threshold. If the current increases quickly enough to exceed the lightning threshold before the switch  42  turns OFF, the microcontroller  38  cancels the command to turn OFF the switch  42  such that the switch remains ON. In the ON state, the transient current passes to the load to thereby protect the SSPC  18  from damage. 
     If the transient current decreases or remains steady and does not reach the lightning threshold, the switch  42  turns OFF and the auto-recover logic functions to turn the SSPC  18  back ON after a time delay time to allow cooling. 
     After the switch  42  is turned back ON, if the current remains above the instantaneous trip threshold but below the lightning threshold, the switch  42  turns OFF and the auto-recover logic again functions to turn the SSPC  18  back ON after another time delay time to allow cooling. It is assumed that current above the instantaneous trip threshold is from a shorted load and not lightning if multiple attempts to auto-recover continue to produce current above the instantaneous trip threshold but below the lightning threshold. In response, a protective trip is set in the microprocessor  38 , and the SSPC  18  turns OFF. 
     The auto-recover logic can also be used in SSPCs that do not include the lightning module  46 . In such an embodiment, the instantaneous trip module  44  and its associated instantaneous trip threshold function to turn the SSPC  18  OFF if the current through the switch  42  exceeds the instantaneous trip threshold. The auto-recover logic turns the switch  42  back ON after a time delay time to allow cooling. If the current remains above the instantaneous trip threshold, the switch  42  turns OFF. The auto-recover logic again functions to turn the SSPC  18  back ON after another time delay time to allow cooling. If multiple attempts to auto-recover continue to produce current above the instantaneous trip threshold, a protective trip is set in the microprocessor  38 , and the SSPC  18  turns OFF. The auto-recover logic provides the benefit of automatically resetting the switch  42  without the need for manual resetting (e.g., from an aircraft pilot), which is a drawback of prior SSPCs. 
     Thus, the disclosed examples provide for turning the SSPC  18  ON in response to the transient current exceeding the instantaneous trip threshold or lightning threshold, depending on the initial ON or OFF state of the SSPC  18 . This provides the benefit of passing the transient current on to the load to protect the SSPC  18  from damage. Furthermore, the lightning module  46  requires little additional hardware in the SSPC  18 , which helps keep costs and packaging density low. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.