Switching mode power supply

A switching mode power supply, comprising:

• a transformer comprising an input winding having a first end to which an input voltage is applied, a feedback winding, having a first end which is grounded and to which energy is fed back from a secondary winding, a primary winding, and at least one secondary winding;
• a power transistor having a first electrode and a second electrode connected between the other end of the input winding and a ground;
• at least one rectifier having input ports connected to the other ends of the windings, for rectifying the voltage generated by the secondary windings to direct current;
• filters connected between the outputs of the rectifiers and the ground for filtering the direct current voltage output from the rectifiers and outputting it as an output voltage;
• a feedback portion connected between the output of either the filters or the power transistor controlling portion, for feeding back some of the output voltage to the power transistor controlling portion;
• a power transistor controlling portion, the output port of which is connected to the controlling electrode of the power transistor, for controlling the power transistor in response to a signal output from the feedback portion and a synchronous signal applied from the outside;
• the power transistor controlling portion including a supplementary power source for responding to a signal generated from the feedback portion;
• a first comparator for comparing the voltage applied to a non-inverting input with the voltage applied to an inverting input, when a predetermined voltage generated by the supplementary power source and a synchronous controlling voltage generated by the synchronous signal controlling portion are applied to the non-inverting input and a predetermined reference voltage is applied to the inverting input, and for generating a signal of a high level when the voltage applied to the non-inverting input is higher than the voltage applied to the inverting input;
• an oscillator, the input of which is connected to the output of the first comparator, for generating a pulse signal in response to the output of the first comparator;
• a second comparator for comparing the voltage applied to a non-inverting input with the voltage applied to an inverting input, when the voltage supplied from the supplementary power source is applied to the non-inverting input and the voltage fed back from the power transistor is applied to the inverting input, and for generating a signal of a high level when the voltage applied to the non-inverting input is higher than the voltage applied to the inverting input;
• and a flipflop, the set terminal of which is connected to the output of the oscillator, the reset terminal of which is connected to the output of the second comparator, and the output of which is connected to the controlling electrode of the power transistor, for turning on the power transistor when the output of the oscillator has a high level and for turning off the power transistor when the output of the second comparator has a high level;
• and
• a synchronous signal controlling portion connected between the other end of the feedback winding and the power transistor controlling portion, to which the synchronous signal is applied for turning on the power transistor right before or right after the power transistor is turned on after the power transistor is turned off, in an initial stage in which the synchronous signal begins to be applied in response to the voltage generated in the feedback winding.