Load control system having an overload protection circuit

A load control system for delivering power from an AC source to a load, said AC source having a fundamental frequency, said load control system comprising:

• a zero cross detector that monitors said AC source;
• at least one switching element that selectively connects said AC source to said load;
• a sensing circuit that senses an instantaneous ON-state parameter of said at least one switching element and produces an output;
• an overload circuit that receives said output and determines an integrated value of said ON-state parameter and compares said integrated value to a threshold indicative of a maximum average power dissipation of said at least one switching element to determine if said at least one switching element is in an overload condition, wherein said overload circuit comprises:
• an integrating circuit which integrates said ON-state parameter of said at least one switching element over a predetermined period of time that is synchronized to a zero crossing of said fundamental frequency, said integrating circuit outputting an integrated value of said ON-state parameter;
• a threshold generating circuit which generates a first threshold that varies in accordance with a resistance of said switching element when said switching element is ON and said ON-state parameter;
• and
• a comparator circuit which compares said first threshold and said integrated value and turns OFF said switching element when said integrated value exceeds said first threshold;
• a short circuit protection circuit that also receives said output to determine if said load is short circuited;
• and
• a controller that controls said load control system, said controller receiving information from said zero cross detector and outputting a gate drive signal to turn said at least one switching element ON,
• wherein said overload protection circuit reduces the ON time of said at least one switching element when said at least one switching element is determined to be overloaded and said short circuit protection circuit reduces the ON time of said at least one switching element when said load is determined to be shorted circuited.

A method of protecting a switching element connected between an AC source and a load from dissipating power in excess of a predetermined amount comprising:

• measuring a parameter of the switching element;
• integrating the measured parameter over a predetermined time period to produce an output;
• comparing the output to a variable threshold;
• producing a signal when the output exceeds the variable threshold;
• holding said integrated circuit OFF during a period of time said electronic component is normally OFF;
• filtering the output to produce a smoothed signal;
• and
• turning OFF the switching element in response to said smoothed signal.