Current limiting circuit for a control circuit for controlling a semiconductor switch system

A power distribution system and method has a controller and at least one semiconductor switch. The power distribution system additionally has an on status detector which detects the status of the semiconductor switches, and an overcurrent status circuit which checks for overcurrent conditions.

BACKGROUND OF THE INVENTION

This application relates generally to power distribution systems, and more specifically to overcurrent protection in DC power distribution systems.

In power distribution systems where DC power is distributed to multiple different loads, it is known to use a set of switches/relays in conjunction with a power system controller to control the power flowing to each load. In order to facilitate operation and power distribution to all of the loads, a controller, which is capable of controlling the state of each of the switches/relays in the system, is typically utilized.

Initially, systems designed in this way used mechanical relay switches. However, mechanical relay systems suffered from reliability problems. In order to improve reliability, many applications now use semiconductor switches instead of mechanical relays due to a longer life-span of the semiconductor switches. Use of semiconductor switches has the additional benefit of reducing the size of the circuit as well as reducing the weight.

Use of semiconductor switches necessitates a different style of controller than the controller used for mechanical relay switches. Existing semiconductor switch controllers are expensive to construct and maintain, are larger than desirable for use in many systems, such as aircraft power systems, and do not readily scale for different sized loads and trip currents.

SUMMARY OF THE INVENTION

In one exemplary embodiment a power distribution circuit includes a semiconductor switch, a current shunt connected to said semiconductor switch, a current limit circuit connected to a control input of the semiconductor switch and configured to limit a current through the current shunt, an on status detector connected to said semiconductor switch and an on status output line such that a semiconductor on signal is output on said on status output line when said semiconductor switch is on, a controller connected to an input power source and to a control input of said semiconductor switch, and an overcurrent detection circuit connected to said input power source, said semiconductor switch, and an overcurrent status circuit such that said overcurrent detection circuit is capable of detecting an overcurrent fault in said semiconductor switch.

An exemplary method for controlling a switching circuit including the steps of detecting an overcurrent status of said at least one switching component, limiting a current through the at least one switching component in response to detecting an overcurrent status of said at least one switching component using a current limit circuit, and overriding a detected on status of said at least one switching component when an overcurrent status is detected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

When controlling power to multiple DC loads using multiple semiconductor switches in a DC power distribution system, it is necessary for a controller to be able to monitor the state of each semiconductor switch and appropriately respond to any overcurrent conditions.FIG. 1illustrates a box diagram of an electric circuit capable of controlling and monitoring a semiconductor switch150. The circuit ofFIGS. 1 and 2could be adapted to control and monitor multiple semiconductor switches using methods known in the art.

The example ofFIGS. 1 and 2have a controller110connected to a first input power line112and to a neutral line114. The controller110additionally has a control output116which connects to an overcurrent detection circuit120, a current limiting circuit130, and a semiconductor switch150. The overcurrent detection circuit120is connected to the input power line112, a current limiting circuit130, a current shunt circuit140, and an overcurrent status circuit170. The current limit circuit130is also connected to the input power line112. The semiconductor switch150has a gate input which accepts the control signal116and has a power input connected to the current shunt140, and a power output connected to the output voltage signal118and to an on status circuit160.

The solid state controller110ofFIG. 1controls the state of the semiconductor switch150by using a control signal, an output from the solid state controller110on the control signal output line116. The control signal line116voltage is either 10-15V below the input power line112voltage, placing the semiconductor switch150in an on state, or the control signal line116voltage is equal to the power line112voltage placing the semiconductor switch150in an off state. When the control signal line116voltage is 10-15V below the input power line112voltage, the semiconductor switch150is turned on, and power is allowed to flow from the current shunt140through the semiconductor switch150to the output voltage118. Additionally, power simultaneously flows through the semiconductor switch150to the on status detection circuit160.

When the on status circuit160detects power flowing from the semiconductor switch150, the on status circuit160begins outputting a semiconductor switch on signal which can used to monitor the status of the semiconductor switch system or for any other purpose. When the semiconductor switch150turns off, and thus ceases allowing power to flow through the semiconductor switch150, the on status detection circuit160will no longer detect power, and therefore will cease sending the semiconductor switch on signal. InFIG. 2, the on status circuit160has an additional connection162to the power line112. The connection162allows the isolated on status circuit160to receive power for the optical isolation.

When an overcurrent condition occurs, the semiconductor switch150, the current shunt140, and the output wiring are subject to potential damage if the overcurrent condition is allowed to continue indefinitely. To protect the semiconductor switch150, the current shunt140, and any output wiring from damage, the example ofFIG. 1includes an overcurrent detection circuit120and an overcurrent status circuit170.

The overcurrent detection circuit120detects the current through the current shunt140. When the overcurrent detection circuit120detects a current that exceeds a preset threshold, it determines that an overcurrent condition is present in the circuit. When an overcurrent has been detected, the overcurrent detection circuit120clamps the control signal line116to about 1.5V less than the power line112voltage causing the semiconductor switch150to be latched off. After detecting an overcurrent condition, the overcurrent detection circuit120also outputs a signal to the overcurrent status circuit170. Once the overcurrent status circuit170receives a signal from the overcurrent detection circuit120, the overcurrent status circuit170switches on and begins outputting an overcurrent detected signal.

The overcurrent detected signal can be used to monitor the status of the semiconductor switch system and detect when an overcurrent event has occurred. The semiconductor switch150will remain latched off by the overcurrent detection circuit120until the input to the control circuit110is removed. Once the input signal has been removed, the overcurrent latch is removed, and the control circuit110can be commanded to turn the output on again. If the overcurrent condition still exists, the overcurrent detection circuit120will again detect the condition and latch the semiconductor switch150off.

The current limit circuit130detects the current through the current shunt140. When the current limit circuit130detects that the current reaches a preset threshold, the current limit circuit130changes the voltage on the control signal line116to control and limit the current through the semiconductor switch150. The preset threshold for the current limit is higher than the preset threshold for the overcurrent detection circuit ensuring that if the current limit is reached, the overcurrent detection circuit will latch off the semiconductor switch150.

Existing current limit circuits that can be incorporated inFIGS. 1 and 2as the current limit circuit130include high tolerance ranges that increase the maximum worst case current limit. For some applications, this can result in a maximum worst case current that is well above nominal current limits and can require the usage of excessively large electronic components (e.g. MOSFETs, current sense resistors, output diodes, etc.) that can be exposed to the worst case scenario current.

With continued reference toFIGS. 1 and 2,FIG. 3schematically illustrates a circuit200that can be utilized as the current limit circuit130within the circuits ofFIGS. 1 and 2. The circuit200includes an operational amplifier (Op-Amp)210and provides precision current limiting. A positive reference voltage is provided to the operational amplifier210via a connection211to the input power line112. A pair of resistors229,230are connected to the negative input212of the operational amplifier210and set a reference voltage of the operational amplifier210. The first resistor229in the pair of resistors229,230connects the first input power line112to the negative input212of the operational amplifier210via a node131. The second resistor230of the pair of resistors231connects the negative input212of the operational amplifier210to a −VCC node in the overcurrent detection circuit120via a connection137. In some examples the −VCC node in the overcurrent detection circuit120is the ground114.

The reference voltage in turn determines the magnitude that the circuit200will limit the current to (referred to as the current limit value of the current limiting circuit200). Voltage from the current shunt140is provided back to the positive operational amplifier210input214via a connection133.

In order to prevent or minimize glitches and noise, an output filter240connects the output216of the operational amplifier210to a control input of transistor250. In the illustrated example, the output filter240includes a resistor242directly connected to the output, and a capacitor244connected to the control input. The transistor250buffers the operation amplifier210and is connected to the control output116. The control input of the transistor250is connected to the input power line112via a resistor252and to the connection133via a resistor254. The resistors252,254further assist in stabilizing the operational amplifier210.

By utilizing the operational amplifier210based current limit circuit, a precise current limit is applied to the control circuits ofFIG. 1 or 2. In one example, the current limit is approximately 11.5 A can provide a precision within a range of +/−1.5 A. In another example, the current limit provided is approximately 28 A, and can provide a precision within the range of +/−3 A. Alternatively, these ranges can be expressed as a percentage deviation from a nominal (i.e. targeted) current limit. In one example, the precision can be at most 20% of the nominal current limit resulting in a current limit within the range of the nominal current limit value plus or minus 20%. In another example, the precision can be at most 15% of the nominal current limit resulting in a current limit within the range of the nominal current limit value plus or minus 15%. In each of these examples, the worst case tolerances of the circuit is substantially reduced, relative to previous configurations.

In an alternative example to that ofFIG. 3, the current limit circuit can utilize a comparator in place of the operational amplifier210but is otherwise configured in the same manner. In practical implementations a comparator is cheaper and faster than the operational amplifier210but sacrifices some of the precision of the overall current limit circuit200.

It is known that alternate designs could be used for the on status circuit as well as for the overcurrent status circuit, and fall within the above disclosure. Additionally 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.