| 摘要 |
Disclosed is a method and circuit for automatically adjusting the firing angle of a switching element, such as a thyristor or triac, to minimize surge currents produced in reactive loads in AC power applications. A typical application would be in a microwave oven controller when the circuit would be used as an interface between the microwave oven control circuitry and the thyristor driving a magnetron. The circuit is comprised of a digital-control, phase-locked loop oscillator which produces a series of triggering pulses which are synchronized with the AC line voltage; the oscillator oscillates at a known frequency and clocks a counter during one half-cycle of the line voltage, resetting the counter during the opposite half-cycle. A peak fire detection circuit is coupled to the oscillator. An enable signal is generated by the microwave oven control circuitry. Since the oscillator frequency is known, the AC line voltage peak occurs at a known state of the counter. This count is decoded in the peak fire circuit and when the circuit is enabled by the microwave oven control circuitry, an initial trigger pulse is applied to the gate of the thyristor, turning it on. After the thyristor is initially fired under peak fire control, the retriggering is controlled by a zero fire circuit which detects zero current crossings (zero current) through the triac. The triac will enter its non-conductive state when the main terminal current is less than the specified holding current of the device. When the triac is in the non-conductive state, the main terminal voltage will be the AC line voltage. The disclosed circuit will generate a retriggering pulse when the main terminal voltage reaches a predetermined magnitude. This method of generating trigger pulses is used in all subsequent cycles.
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