Parallel analog and digital timers in power controller circuit breaker
A circuit breaker for a power controller is described where an analog timer and a digital timer are provided in parallel. The digital timer is implemented entirely on the power controller chip so adds no cost to the power system. The digitaltimer provides a maximum delay during an overcurrent condition to ensure the transistor will not be damaged. The analog timer allows the user to select an external capacitor or resistor to provide a delay time that is shorter than the time provided bythe digital timer. The user may disable the analog timer by not using a capacitor and connecting a terminal to ground.
A conventional power controller integrated circuit controls a power transistor to couple or decouple a power supply from a load to be powered. The power controller typically detects overcurrent faults, detects voltage levels, detectsexternally-generated control signals such as an enable signal, and senses other conditions to determine whether to couple or decouple the power supply to or from the load.
It is important that the delay time during which the overcurrent exists does not exceed a time that will damage the transistor or damage the load or other components. Accordingly, a maximum time for the delay must not be exceeded.
An autoretry circuit is also included in the power controller which prevents the duty cycle from exceeding a maximum. The autoretry timer is a digital timer that uses the same oscillator as the digital timer for the circuit breaker. Bothdigital timers are counters, with the autoretry counter having a higher count than the circuit breaker digital timer before triggering. Since the autoretry timer is a fixed time and runs off the same oscillator as the circuit breaker digital counter, amaximum duty cycle of the transistor is enforced which cannot be altered by the user.
When using an analog timer, the user may err and use a capacitor that provides too much delay and damages the transistor in an overcurrent condition. Similarly, the user may use a capacitor in the analog timer for the autoretry that results inthe transistor exceeding its maximum duty cycle for the overcurrent.
The delay circuits in the circuit breakers and autoretry circuits may be analog or digital. Analog delay circuits set the delay time by selecting the value of a capacitor that is charged between an initial voltage (e.g., 0 volts) and athreshold voltage. A larger capacitor increases the tripping time. Such capacitors are external to the power controller package and provide flexibility to the user. The analog delay may also be set by selecting an external resistor in addition to thecapacitor to set the time constant. Digital timer may be integrated on the same chip as the power controller and are thus inexpensive, but provide no flexibility to the user since the delay time is fixed.
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