Patent Application
20110235227
The subject matter disclosed herein generally to the field of remote power controllers.
A remote power controller (RPC) is a solid state device that controls and protects a power connection to an electrical load. RPCs may be found in complex electrical systems, including but not limited to aircraft or spacecraft electrical systems. RPCs allow switching to be performed at the load, instead of at the power source, reducing the complexity of the overall electrical system. An RPC acts to control the application of power to the load, and may also act as a fuse or circuit breaker, protecting electrical equipment from fault or overload conditions. For DC power applications, an RPC may be designed to limit the current to a prescribed level to protect the power distribution system.
An RPC may comprise a metal oxide semiconductor field effect transistor (MOSFET or FET) having a relatively high power capability and a low voltage drop. The voltage drop across the RPC depends on the resistance of the FET in the ON state, a parameter called RDS(on). Low voltage drop across the RPC is important to reduce losses and increase RPC efficiency. A FET in a current limiting RPC must support the current limiting value while sustaining up to full line voltage when a fault or overload occurs. Therefore, the FET must dissipate a large amount of power during the fault. A relatively high power level may only be sustained by a FET for a limited period of time, depending on the energy capability of the FET. The energy capability of a FET is represented by a plot of voltage vs. current with defined areas for specific time durations. This specifies the safe operating area (SOA) of the FET.
Some FETs, which may be relatively small, are designed for switching applications and can achieve much lower RDS(on) values; however, the smaller FETs may have greatly reduced energy (SOA) capability. A FET having a higher RDS(on) may also be larger, and may have greatly increased SOA capability compared to a smaller FET. A type of FET with large SOA capability that is preferred for operation with both voltage and current applied is a linear FET.
According to one aspect of the invention, a remote power controller (RPC) includes a line connection; a load connection; a first field effect transistor (FET) and a second FET arranged in parallel between the line connection and the load connection, wherein the first FET has a lower safe operating area (SOA) than a SOA of the second FET, and wherein the first FET has a lower resistance at saturation (RDS(on)) than the second FET; and a voltage offset element connected between the first FET and the second FET, such that in the event a current in the RPC is above a current limiting setpoint, the voltage offset element is configured to cause the first FET to turn off.
According to another aspect of the invention, a method of operating a remote power controller (RPC), the RPC comprising a line connection, a load connection, and a first field effect transistor (FET) and a second FET arranged in parallel between the line connection and the load connection, wherein the first FET has a lower safe operating area (SOA) than a SOA of the second FET, wherein the first FET has a lower resistance at saturation (RDS(on)) than the second FET, includes in the event that a current in the RPC is below a current limiting setpoint, turning the first FET on, and turning the second FET on; and in the event that a current in the RPC is above the current limiting setpoint, turning the first FET off.
Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
Embodiments of systems and methods for an RPC with parallel FETs are provided, with exemplary embodiments being discussed below in detail. An RPC may comprise parallel FETs, each FET being of a different type. Use of parallel FETs having different characteristics in an RPC may reduce the load on each individual FET to a sustainable level. One of the FETs may be selected for use during normal operating conditions, and the other may be selected for current limiting or dissipation of overload conditions.
FET characteristics include safe operating area (SOA) and RDS(on). The SOA defines the power and energy handling capability of a FET. The SOA defines a range of drain current values and a range of drain to source voltage values that the FET is able to handle for a certain time without damage. Both the drain current and the drain to source voltage in operation must stay below their respective maximum values for safe operation of the FET, and the product of the drain current and the voltage must also stay below the maximum power dissipation for the FET device. RDS(on) gives the resistance of the FET when the FET is fully turned on (i.e., when the FET is at saturation).
The technical effects and benefits of exemplary embodiments include protection of electrical equipment from overload or fault conditions.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
