Self-excited switching power supply circuit

Abstract

The time constant of the conduction control circuit (12, 23) is set so that the gate voltage of the field effect transistor (3) for oscillation exceeds the threshold voltage VTH in the state where the polarity of the voltage of the feedback winding (2b) is reversed. When the voltage of the winding (2a) becomes lower than the power supply voltage, the oscillation field effect transistor (3) is turned on, and the stray capacitance accumulated between the primary windings (2a) or the parasitic capacitance of the oscillation field effect transistor (3) The charge is discharged slowly. Therefore, the discharge current generated when the oscillation field effect transistor (3) is turned on is reduced, and as a result, energy loss during switching is small and noise is not generated.

Description

technical field [0001] The present invention relates to a self-excited switching power supply circuit, in particular to a flyback type self-excited switching power supply circuit which releases the energy accumulated in the transformer from the secondary output winding when the excitation current of the primary winding of the transformer is stopped. Background technique [0002] As a stable power supply, switching power supply circuits are used in battery chargers or AC adapters, etc. The driving method (switching method) of the switching element is roughly divided into two types: the self-excited oscillation method and the self-excited oscillation method. The self-excited oscillation method uses the voltage generated in the feedback winding of an inductance component such as a transformer as a drive signal to positively feed back to the switching element. The control terminal for oscillation action. [0003] As such a self-excited switching power supply circuit, known such...

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Problems solved by technology

[0021] On the other hand, in the primary winding 2a or FET3, there are stray capacitances between the windings or parasitic capacitances between the drain and the source. Sweep voltage charging, therefore, if the drain of FET3 (primary winding 2a side) voltage does not drop enough, that is, in the state of around 200V, it will be rapidly discharged

[0022] As a result, happens as Figure 5 A discharge current of the magnitude indicated by A in (c) increases the loss of switching elements such as FET3, and at the same time causes noise.

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