Patent ID: 7339800
Filing Date: 2008-03-04
Classification: H02M

Abstract:
1. A switching power source device of resonance type comprising: a DC power source; first and second switching elements connected in series to the DC power source; a primary winding of a transformer connected in parallel to one of the first and second switching elements; a resonance capacitor for forming a series circuit together with the primary winding of the transformer; a rectifying smoother connected to a second winding of the transformer for supplying DC power to a load; an output voltage detector for detecting output voltage on the load to produce a detection signal; and a control circuit for applying on-off signals to each control terminal of the first and second switching elements in response to the detection signal from the output voltage detector; wherein said control circuit comprises a frequency control circuit for controlling the on-off operation of the first and second switching elements in response to the level of detection signal from output voltage detector to decide the switching frequency; said frequency control circuit comprises an on-time control circuit which reduces the on-span of one of first and second switching elements more during the light load period in response to the level of detection signal from the output voltage detector than during the non-light load period to control power consumption, and a regulatory capacitor for determining the on-period of the first and second switching elements; said on-time control circuit comprises charging and discharging circuits for charging and discharging the regulatory capacitor; said charging circuit increases charge current toward the regulatory capacitor to shorten the charge time during the light load period, thereby causing the on-time control circuit to reduce the on-period of one of the first and second switching elements more than during the non-light load period; and said discharging circuit decreases discharge current from the regulatory capacitor to expand the discharge time during the light load period, thereby causing the on-time control circuit to extend the on-period of the other of the first and second switching elements more than during the non-light load period.