Thyristor driver controller for reactive loads and method of operation thereof

A control circuit for a microwave oven which minimizes surge currents in the AC current produced by an AC voltage applied to a magnetron in the oven, comprising:

• a thyristor to be coupled to said magnetron which may be switched into a conductive and non-conductive mode,
• a digital processor for producing an enable signal when said thyristor is to be turned on,
• peak fire circuit means coupled to said enable signal which generates a series of trigger pulses synchronized with the AC line voltage and generates a zero fire enable signal that corresponds to the first peak of the AC line voltage following initiation of said enable signal, and
• zero fire circuit means coupled to said thyristor and said peak fire circuit for sensing the change in voltage and current of said thyristor such that when a voltage of a predetermined magnitude across said thyristor appears and said zero fire enable signal is present, trigger pulses turn on the thyristor initially at the first peak of the AC line voltage following initiation of said enable signal and thereafter when the AC current through said thyristor approaches zero,
• said digital processor comprising a microprocessor.

A control circuit for a microwave oven which minimizes surge currents in the AC current produced by an AC voltage applied to a magnetron in the oven, comprising:

• a thyristor coupled to said magnetron which may be switched into a conductive and non-conductive mode,
• a digital processor for producing an enable signal when said thyristor is to be turned on,
• peak fire circuit means coupled to said enable signal which generates a series of trigger pulses synchronized with the AC line voltage and generates a zero fire enable signal that corresponds to the first peak of the AC line voltage following initiation of said enable signal, and
• zero fire circuit means coupled to said thyristor and said peak fire circuit for sensing the change in voltage and current of said thyristor such that when a voltage of a predetermined magnitude across said thyristor appears and said zero fire enable signal is present, trigger pulses turn on the thyristor initially at the first peak of the AC line voltage following initiation of said enable signal and thereafter when the AC current through said thyristor approaches zero,
• said peak fire circuit means comprising a frequency-locked loop oscillator for producing a series of trigger pulses synchronized with the AC line voltage, and
• decoder means coupled to said frequency-locked loop oscillator for producing a zero fire enabling gating signal at the peak of said AC line voltage, said frequency-locked loop oscillator comprising an up/down counter coupled to signals representative of the frequency of said AC line voltage and said trigger pulses for determining which of said signals occurs first and generating at least one digital signal indicating whether said trigger pulse frequency should be increased or decreased,
• digital-to-analog converter means coupled to said digital signals for generating increasing or decreasing analog control voltage steps related to the increasing or decreasing digital signal steps,
• a variable frequency control oscillator coupled to said converter for adjusting the trigger pulses upward or downward in order to obtain synchronization between said AC line voltage and said trigger pulses, and
• a counter coupled between said oscillator and said up/down dounter for dividing down said trigger pulses.

A circuit for minimizing the surge currents in an AC load current produced by an AC voltage applied to a reactive load comprising:

• a thyristor coupled to said load which may be switched into a conductive and non-conductive state,
• a frequency-locked loop oscillator for producing a series of trigger pulses synchronized with the AC line voltage,
• decoder means coupled to said frequency-locked loop oscillator for producing a zero fire enabling gating signal at a predetermined time with respect to said AC line voltage, and
• zero detector circuit means coupled to said thyristor, oscillator and decoder means for sensing the change in current and voltage of said thyristor when said thyristor changes states such that, when a voltage of predetermined magnitude appears across said thyristor and said enabling signal is present, a trigger pulse is gated therethrough to said thyristor to switch it into its conductive state, said frequency-locked loop oscillator comprising an up/down counter coupled to signals representative of the frequency of said AC line voltage and said trigger pulses for determining which of said signals occurs first and generating at least one digital signal indicating whether said trigger pulse frequency should be increased or decreased,
• digital-to-analog converter means coupled to said digital signals for generating increasing or decreasing analog control voltage steps related to the increasing or decreasing digital signal steps,
• a variable frequency control oscillator coupled to said converter for adjusting the trigger pulses upward or downward in order to obtain synchronization between said AC line voltage and said trigger pulses, and
• a counter coupled between said oscillator and said up/down counter for dividing down said trigger pulses.

A circuit for minimizing the surge currents in an AC load current produced by an AC voltage applied to a reactive load comprising:

• a thyristor coupled to said load which may be switched into a conductive and non-conductive state,
• a frequency-locked loop oscillator for producing a series of trigger pulses synchronized with the AC line voltage,
• decoder means coupled to said frequency-locked loop oscillator for producing a zero fire enabling gating signal at a predetermined time with respect to said AC line voltage, and
• zero detector circuit means coupled to said thyristor, oscillator and decoder means for sensing the change in current and voltage of said thyristor when said thyristor changes states such that, when a voltage of predetermined magnitude appears across said thyristor and said enabling signal is present, a trigger pulse is gated therethrough to said thyristor to switch it into its conductive state, said circuit being implemented on a single chip utilizing I.sup.2 L technology.