1. Field of the Invention
This invention relates to a switching power source in which the voltage error of a secondary side DC output is fed back to a switching circuit through an insulating element, thereby to stabilize the output voltage, and more particularly to a switching power source in which a switching circuit is caused to perform an intermittent switching operation with pulses applied to an error detecting circuit.
2. Description of the Related Art
The efficiency of a switching power source is lowered when its load is extremely low. Therefore, a variety of techniques have been proposed in the art to prevent the lowering of the efficiency when the load is low. An example of the techniques has been disclosed in Japanese Patent Unexamined Publication No. Hei. 4-29482. In the technique, a standby control section is provided which connects the base of a switching transistor to the ground (level) and disconnects it from the ground (level). Further, the standby control section is so designed that the connection is made for a half of one cycle of the commercial power source, or for a predetermined period of the same. Hence, every one cycle of the commercial power source, the switching stop occurs for a half cycle, or for a predetermined period. Therefore, the average value of the switching loss is decreased, so that the conversion efficiency is improved.
However, the above-described conventional technique suffers from the following problems: That is, only in the case where the load becomes low, it is necessary to perform the switching operation intermittently. In the case where the load is ordinary, it is necessary to perform the switching operation continuously. Hence, in the case of the ordinary load, it is necessary to generate a stop signal to stop the operation of the standby control section, and to apply the stop signal thus generated to the standby control section. On the other hand, the standby control section is the primary side block, and the stop signal is generated by the secondary side block. Therefore, it is necessary to provide a photocoupler in the path of transmission of the stop signal. However, the photocoupler is expensive, which means that the resultant switching power source is high in manufacturing cost.