Load control system having an overload protection circuit

摘要

A load control circuit having an overload circuit (36) capable of protecting a switching element (26) from overload conditions over a wide range of operating conditions. The overload circuit limits the maximum average power dissipation of at least one switching element (26) in accordance with a comparison of an average voltage across the switching element over a half-cycle and a variable threshold. The variable threshold is determined based on an ON-state resistance of the switching element (26) and the environment in which the switching elements are operating. The variable threshold accounts for the squared term (V<2> or I<2>) in determining power dissipation and allows power to be determined based on average voltage. The system includes a short circuit protection circuit (32). It also includes a controller (22) which receives information from a zero cross detector (16) and turns on the switching element (26) in response thereto.