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The main function of the TIMER capacitor is to ensure that the MOSFET only remains in current limit for a fixed maximum time to ensure the SOA is not exceeded. A secondary function is a power “on” delay which is significantly larger than the normal circuit breaker delay using the same TIMER capacitor. Operation of the Timer is shown here: 1) Load current exceeds the threshold, 2) The TIMER is activated, 3) The Gate then regulates the MOSFET at this current limit, 4) The MOSFET is disabled once the TIMER voltage reaches the upper threshold, 4) On an auto-retry device the TIMER then ramps down, 5) This down cycle is the cool-down period for the MOSFET, and 6) When the TIMER reaches the lower threshold, the gate comes back up, re-enabling the MOSFET.

PTM Published on: 2008-04-29