Self-excited switching power supply circuit

Abstract

A self-excited switching power supply circuit is provided which reduces a discharge current generated when an oscillating field effect transistor (3) is turned on, whereby it is possible to reduce energy loss and generation of noise when switching is executed. A time constant of an ON control circuit (12, 23) is set such that when a polarity of a voltage of a feedback winding (2b) has reversed, a gate voltage of the oscillating field effect transistor (3) exceeds a threshold voltage VTH. Accordingly, after a voltage of a primary winding (2a) has become equal to or less than a power supply voltage, the oscillating field effect transistor (3) is turned on, and electrical charge stored in stray capacitance between the windings of the primary winding (2a) and in parasitic capacitance of the oscillating field effect transistor (3) is discharged gradually. <IMAGE>

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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