| 摘要 |
PROBLEM TO BE SOLVED: To solve a problem that under a light load, the voltage of an FB terminal 14d is conventionally reduced by an output feedback circuit 18 as compared with cases under a rated load, and thus both the on-time and the off-time of a main switch 15 are shortened for a short oscillation cycle and increase oscillation frequency; therefore, the proportion of turn-on time and turn-off time during which switching loss occurs to the entire cycle is increased, and this significantly degrades efficiency and makes it difficult to save a power consumed on standby. SOLUTION: A switching power supply unit is provided with a first capacitor 51 and a switch 52 placed between the ZCD terminal 14a and the GND terminal 14b of a critical mode control circuit 14, and with a light load changeover circuit 53 for changing over the switch 52. The light load mode changeover circuit 53 is provided with a VCC voltage detection circuit 54 for detecting the voltage at a VCC terminal 14c and an FB terminal voltage detection circuit 55 for detecting the voltage at an FB terminal 14d.
|
