Gating control for a static switching arrangement with improved conduction angle balancing means

Claim 1

A gating control for an a-c phase controlled static switch comprising first and second thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their nonconducting state preceding the gating of either thyristor, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. first balancing means for comparing the conduction angles of said thyristors on those pairs of successive conduction periods during which said first thyristor conducts preceding said second thyristor and for deriving a first balance-indicating signal having a magnitude indicative of the difference between said conduction angles compared by said first balancing means, d. second balancing means for comparing the conduction angles of said thyristors on those pairs of successive conduction periods during which said second thyristor conducts preceding said first thyristor and for deriving a second balance-indicating signal having a magnitude indicative of the difference between said conduction angles compared by said second balancing means, e. summing means effective while said thyristors are in their nonconducting state immediately preceding the firing of said first thyristor for adding together: i. said effective input control signal, ii. said off-time indicating signal for the period immediately preceding firing of said first thyristor, and iii. said first balance-indicating signal, to provide a first gating-control signal that has a magnitude dependent upon the sum of signals i, ii, and iii, f. means for developing a gating pulse for delivery to said first thyristor when said first gating control signal reaches a predetermined level, whereby said first thyristor is gated into conduction at a gating angle dependent on the magnitudes of said effective input control signal and said first balanceindicating signal, g. said summing means also being effective while said thyristors are in their nonconducting state immediately preceding the firing of said second thyristor for adding together: i. said effective input control signal, ii. said off-time indicating signal for the period immediately preceding firing of said second thyristor iii. said second balance-indicating signal, to provide a second gating control signal that has a magnitude dependent upon the sum of signals i, ii, and iii of (g), (h) and means for developing a gating pulse for delivery to said second thyristor when said second gating control signal reaches a predetermined level, whereby said second thyristor is gated into conduction at a gating angle dependent on the magnitudes of said effective input control signal and said second balance-indicating signal.