The present invention relates to a power supply circuit which intermittently produces a DC power source voltage by periodically switching a switching element and smoothing the thus produced voltage, thereby obtaining a DC voltage of a predetermined level.
There are two types of DC-DC converters, namely, the self oscillation type and the separate excitation type. The self oscillation type has a simple construction but an obtained voltage does not show satisfying stability. The separate excitation type shows a stable operation but a control circuit thereof is complicated.
Accordingly, an object of the present invention is to provide a novel DC power supply circuit.
Another object of the present invention is to provide a DC power supply circuit which has the advantages of the self oscillation type and the separate excitation type.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a switching system is employed wherein the self oscillation type operation is conducted at the start of circuit operation, and, thereafter, the separate excitation type operation is conducted to stabilize the output voltage.
In a preferred form, a switching transistor is driven, at the start of the circuit operation, in the self oscillation mode. An output voltage level is compared with a preselected reference level so that an ON period of the switching transistor is shortened when the output voltage level is higher than the preselected reference level, and the ON period of the switching transistor is lengthened when the output voltage level is lower than the preselected reference level. Therefore, the output voltage level is stabilized and the ripple is minimized. Moreover, the circuit construction is simplified as compared with the conventional separate excitation type circuit.