1. Field of the Invention
The present invention relates to a fault detection circuit for use with a power control device to detect a short circuit in the power control device. Particularly, the fault detection 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.
2. Description of Related Art
Solid state power switches used as power control devices tend to fail in shorted states, which is often unacceptable in power switching applications. Various fault detection 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.
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'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's temperature or current rating in order for the circuit to be opened.