Load control system having an overload protection circuit

Details Load control system having an overload protection circuit Load control system having an overload protection circuit

1999-06-21

2002-02-12

Huynh, Kim (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S018000, C323S276000

Reexamination Certificate

active

06347028

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to load control systems, and more particularly, to a lighting control system having an overload protection circuit to limit the power dissipation of a switching element in the control system from exceeding a predetermined maximum level.
BACKGROUND OF THE INVENTION
Phase-controlled lighting controllers are well known and perform dimming functions by selectively connecting an AC power source to a load during each half-cycle. The AC power may be switched using controllably conductive devices such as triacs, anti-parallel SCRs, field effect transistors (FETs) or insulated gate bipolar transistors (IGBT). The amount of dimming is determined by the ratio of “ON” time to “OFF” time of the controllably conductive device. In conventional forward phase-controlled dimming, the controllably conductive device (triac or SCR) is OFF at the beginning of each half-cycle (i.e., at the zero crossing) and turned ON later in the half-cycle. In reverse phase-controlled dimming, the controllably conductive device (FET or IGBT) is switched ON to supply power to the load at or near the zero crossing and is switched OFF later during the half-cycle. For each method of phase-controlled dimming, the ratio of ON time to OFF time is determined based on a user-selected desired intensity level.
Lighting controllers are rated to control a predetermined maximum load. If the controller is overloaded the maximum temperature rating of the controllably conductive device may be exceeded and the device will not last as long as a properly loaded device or fail catastrophically rendering the controller useless. A lighting controller can easily be overloaded by an installer who connects too many lamps to the controller or by a maintenance person who replaces failed lamps with higher wattage lamps.
Another factor that may lead to an elevated device temperature is operating the lighting controller in an elevated ambient temperature. Lighting controllers are rated to operate in an ambient temperature range usually 0 to 40° C. An elevated ambient temperature would cause an otherwise properly loaded device to operate above its safe operating temperature.
Several methods of sensing overload conditions may be found in the prior art. For example, U.S. Pat. No. 5,325,258, to Choi et al., discloses a gate driver circuit that uses sense resistors to determine the current flowing through a low side and high side FET. While the FET is being driven (i.e., ON), a voltage across the sense resistor is compared to a fixed threshold voltage. If the voltage across the sense resistor remains above the fixed threshold for a period of time set by a blanking circuit, the FET is determined to be overloaded and shut down. The blanking circuit is provided to prevent spurious signals from shutting down the FET driver. While Choi et al. prevents overload conditions under certain circumstances, it would fail to detect a short circuit condition during the blanking period. Also, because Choi et al. compares the current passing through the FET to a fixed threshold, the device may not accurately detect overcurrent conditions that occur early in the ON period of each half cycle.
U.S. Pat. No. 5,010,293, to Ellersick, discloses a current limiting circuit for a power FET. A bipolar transistor is connected to shunt the gate of the power FET to the potential at its source when the bipolar transistor is conducting in order to limit the current passing through the power FET. A sense resistor is provided in series with a conductor path for controlling a base element of the bipolar transistor to cause the transistor to conduct when current through the sense resistor exceeds a predetermined amount. However, the Ellersick circuit is limited because it compares the current passing through the FET to a fixed threshold, which may not accurately detect overcurrent conditions early in the ON period of each half cycle and because the power FET becomes active to limit the current which dissipates a lot of power.
U.S. Pat. No. 5,079,456, to Kotowski et al., discloses a current monitoring circuit that includes a smaller sense FET that carries a current proportional to a larger power FET in the device. A comparator senses the voltage across the smaller transistor to indicate if the current in the sense transistor exceeds a predetermined amount equal to a maximum source current of the sense transistor. A second embodiment regulates the source current through the sense transistor in order to regulate the current through the power transistor wherein the sense transistor is operating in the linear region. By modifying the drain to source voltage of the sense transistor the device can regulate the current carried by the power transistor. A particular disadvantage of the Kotowski et al. system is that it requires a separate sense FET to monitor the power FET, which adds to the complexity and cost of the monitoring circuit. Again, the FET becomes active to limit the current which dissipates a lot of power.
U.S. Pat. No. 4,937,697, to Edwards et al., discloses another protection circuit that monitors instantaneous FET drain to source voltage to provide a current sense signal. When the current sense signal exceeds a predetermined reference limit signal, a first control circuit turns the FET OFF instantly. A reference generator provides a reference limit signal having a predetermined temperature variation as a function of the sensed temperature of the FET such that current limits may be set for low device temperatures. A second control circuit is provided to protect against overcurrent conditions created by short circuits by turning the FET OFF when sensed FET current exceeds a predetermined limit after a delay. The delay circuit inhibits operation of the control circuits until a predetermined time after the FET is turned ON. During this time there is no protection.
While each of the systems described above attempts to prevent overloading and overheating of the controllably conductive devices for their particular applications, they require the use of more costly hardware or fail to provide adequate protection over a wide range of operating conditions and environments. In addition, the devices of the prior art function to limit the flow of current through the controllably conductive device in overload conditions by modifying the drain to source voltage, which does not reduce the overall power dissipation in the FET. The load control circuit of the present invention reduces the current flow to a safe operating level while not increasing dissipation in the FET. The present invention provides a solution to these problems.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, there is provided a protection circuit for use in a load control system for limiting power dissipated by an electronic component that switches an AC source to a load. The electronic component may be, e.g., a field effect transistor. The protection circuit includes an integrating circuit which integrates a measured parameter of the electronic component over a predetermined period of time and produces an output value, a threshold generating circuit which generates a first threshold indicative of a maximum average power dissipation of the electronic component, and a comparator circuit which compares the first threshold and the output value. The comparator provides a signal to turn OFF the electronic component when the output value exceeds the first threshold.
In accordance with a feature of the invention, the first threshold may be determined in accordance with an ON-state resistance of the electronic component and the measured parameter. Further, the first threshold may have a variable value that changes during one-half of a period of a fundamental frequency of the AC source. The predetermined period of time may begin when the AC source crosses a zero potential, and have a length no longer than one-half of a period of a fundamental frequency of the AC source.
According to other features of the invention, the protection circui

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