| 摘要 |
PROBLEM TO BE SOLVED: To provide a pseudo-resonant flyback switching power-supply device that allows preventing a surge voltage of a rectifier element without depending on the state of an output current and has a simple configuration and high efficiency.SOLUTION: A switching power-supply device includes a main switching element 14 that is an N-channel MOS FET. A control circuit 24 for the switching element 14 includes a pulse generating circuit 26 generating a rectangular pulse V26 in which its high level is higher than an on-threshold voltage Vth of the main switching element 14 and outputting the pulse between gate and source terminals through an output resistor 28. The main switching element 14 has a drive capacitor 34 between the gate and source terminals. The switching power-supply device includes a variable capacitance circuit 32 that connects an additional capacitor 36 in parallel to the drive capacitance 34 when an off-time toff is longer than a time reference value tk. The connection of the additional capacitor 36 moderates the inclination of the voltage waveform passing through the on-threshold Vth when a voltage Vg of the main switching element 14 rises up. |
