1. Field of the Invention
The present invention relates to a charging circuit and, more particularly, to a capacitor charging circuit with a soft-start function, applicable to effectively charge photoflash capacitors of digital still cameras or other capacitive loads over a wide voltage range.
2. Description of the Related Art
In photoflash systems of digital still cameras, a battery voltage source of approximately 3 volts is supplied to charge a photoflash capacitor through a transformer for raising the capacitor voltage from zero to several hundreds volts required for the excitation of a photoflash lamp. U.S. Pat. No. 6,411,064, U.S. Pat. No. 6,417,649, U.S. Pat. No. 6,518,733, and U.S. Pat. No. 6,636,021, all incorporated herein by reference, have already disclosed a variety of capacitor charging methods and circuits for charging capacitive loads such as photoflash capacitors over a wide voltage range. However, several problems are encountered in the prior art charging circuits and still need to be cured with further improvements and innovations.
At first, during an initial period of a charging process, a large amount of inrush current flows into a primary winding of the transformer, resulting in a significantly large amount of energy to be stored in the transformer. However, it takes an extremely long time for delivering the stored energy to the capacitive load through a secondary winding of the transformer since the terminal voltage across the capacitive load is almost zero at that time. As a result, the charging process is adversely affected by a very low efficiency during the initial period.
Each time when the power switch coupled to the primary winding performs ON/OFF switching operations, both of the primary winding current and the secondary winding current inevitably fluctuate up and down with a great proportion to their DC mean values. Because the primary and secondary winding currents are detected in order to determine the ON/OFF timing of the power switch, such fluctuant noise that casts an incredulous shadow over the current detecting results may destroy the control mechanism of the charging circuit.
After continuously supplying energy in the photoflash system for a long time, the battery voltage source may be subjected to a significant drop. The lower the battery voltage, the longer the primary winding current increases to a predetermined upper limit which controls the ON-to-OFF switching operation of the power switch. Consequently, each charging cycle is prolonged, resulting in large output ripples and low charging efficiency.