An insulation-type switching power supply comprising: a flyback converter circuit ; and a feedback control circuit configured to perform pulse width modulation, PWM, control of a switching element (Q1) in the flyback converter circuit based on an output voltage (Vout) of the flyback converter circuit , wherein the feedback control circuit includes a PWM control circuit configured to generate and output a first PWM control pulse signal (PWM_PULSE1) and a second PWM control pulse signal (PWM_PULSE2) respectively by alternately extracting pulses of a PWM control pulse signal of the switching element, a transformer , a bidirectional excitation circuit configured to excite a primary winding (L21) of the transformer in a forward direction using the first PWM control pulse signal and to excite the primary winding of the transformer in a reverse direction using the second PWM control pulse signal, and a switching circuit configured to generate a pulse signal by inverting a negative pulse of a pulse signal, induced in a secondary winding of the transformer , to a positive pulse and to switch the switching element by using the generated pulse signal, characterised in that the secondary winding of the transformer includes a first secondary winding (L22) and a second secondary winding (L23), the switching circuit includes a first rectifier circuit configured to half-wave rectify and output a pulse signal induced in the first secondary winding, and a second rectifier circuit configured to half-wave rectify and output a pulse signal obtained by reversing a polarity of a pulse signal induced in the second secondary winding, and the switching circuit is configured to generate a pulse signal to be a logical sum of an output signal of the first rectifier circuit and an output signal of the second rectifier circuit, and to switch the switching element by using the generated pulse signal. a flyback converter circuit ; and a feedback control circuit configured to perform pulse width modulation, PWM, control of a switching element (Q1) in the flyback converter circuit based on an output voltage (Vout) of the flyback converter circuit , wherein the feedback control circuit includes a PWM control circuit configured to generate and output a first PWM control pulse signal (PWM_PULSE1) and a second PWM control pulse signal (PWM_PULSE2) respectively by alternately extracting pulses of a PWM control pulse signal of the switching element, a transformer , a bidirectional excitation circuit configured to excite a primary winding (L21) of the transformer in a forward direction using the first PWM control pulse signal and to excite the primary winding of the transformer in a reverse direction using the second PWM control pulse signal, and a switching circuit configured to generate a pulse signal by inverting a negative pulse of a pulse signal, induced in a secondary winding of the transformer , to a positive pulse and to switch the switching element by using the generated pulse signal, characterised in that the secondary winding of the transformer includes a first secondary winding (L22) and a second secondary winding (L23), the switching circuit includes a first rectifier circuit configured to half-wave rectify and output a pulse signal induced in the first secondary winding, and a second rectifier circuit configured to half-wave rectify and output a pulse signal obtained by reversing a polarity of a pulse signal induced in the second secondary winding, and the switching circuit is configured to generate a pulse signal to be a logical sum of an output signal of the first rectifier circuit and an output signal of the second rectifier circuit, and to switch the switching element by using the generated pulse signal. <EOS>