Patent ID: 6236577
Filing Date: 2001-05-22
Classification: H02M,H04N

Abstract:
A power circuit comprising:(a) a transformer including a primary winding, and a secondary winding and a tertiary winding magnetically coupled with the primary winding;(b) a switching element connected between one end and the other end of a DC power circuit, through the primary winding of said transformer;(c) a switch circuit inserted in a first circuit by being connected in series with one end of the secondary winding;(d) a first rectifier-and-smoothing circuit connected in series with the other end of said switch circuit;(e) a second rectifier-and-smoothing circuit connected with the tertiary winding for supplying power to a second circuit;(f) a first output-voltage-detection and error-amplifier circuit for obtaining a signal corresponding to the difference between a detected value of a first output voltage obtained from said first rectifier-and-smoothing circuit and a first reference voltage;(g) a second output-voltage-detection and error-amplifier circuit for obtaining a signal corresponding to the difference between a detected value of a second output voltage obtained from said second rectifier-and-smoothing circuit and a second reference voltage;(h) a microprocessor for activating said switch circuit, and said second output-voltage-detection and error-amplifier circuit such that(1) said second output-voltage-detection and error-amplifier circuit is activated while power is supplied only to the second circuit,(2) power is supplied to the first circuit upon turning ON of said switch circuit, and(3) said second output-voltage-detection and error-amplifier circuit is stopped while said switch circuit is ON;(i) a control circuit including a light-emitting element, responding to at least one of the outputs of said first output-voltage-detection and error-amplifier circuit and second output-voltage-detection and error-amplifier circuit, and a photo-receiving element optically coupled with said light-emitting element,for controlling said switching element such that the first or second output voltage is controlled to be constant in accordance with the change in the resistance value of said photo-receiving element; and(j) a circuit for raising the second output voltage obtained from said second rectifier-and-smoothing circuit by restricting current flowing into said second output-voltage-detection and error-amplifier circuit immediately before said microprocessor renders said switch circuit conductive.