JPH0119591Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to a protection circuit for a switching regulator, and its purpose is to provide a protection circuit that stops oscillation and protects a switching transistor when the output terminal is short-circuited. As shown in Fig. 1, a conventional switching regulator has a rectifying and smoothing section 2 that includes an abnormal voltage absorbing capacitor 1, a rectifying diode 2, a protective capacitor 3, a smoothing electrolytic capacitor 4, and a noise reduction resistor 5.
The oscillation section 30 is composed of a demagnetization capacitor 6, a resistor 7, a switching transistor 8, bias resistors 9, 11, 14, a capacitor 10, a feedback capacitor 12, and a resistor 13. The primary winding 26, the secondary winding 2
7. The pulse transformer 15 is composed of the tertiary winding 28, and is stabilized by the rectifier diode 16, the smoothing electrolytic capacitor 17, the output stabilizing resistor 18, the FET 19, the transistor 20, the resistor 21, the Zener diode 22, and the electrolytic capacitor 23. voltage output section 3
1 was configured. In the switching regulator shown in FIG. A square wave pulse current is passed through the primary winding 26 of the
Square-wave secondary and tertiary voltages are induced in the secondary and tertiary windings. The tertiary voltage is input to the control terminal of the switching transistor 8, and the secondary voltage is converted into a DC voltage by the stabilized voltage output section 31 and output to the output terminals 32-34. However, with this switching regulator,
There is a drawback that when the output terminals 32 to 34 are short-circuited, an excessive current flows through the switching transistor 8, and the switching transistor 8 is easily destroyed. The present invention eliminates the above-mentioned drawbacks, and embodiments thereof will be described below with reference to the drawings. FIG. 2 is a circuit diagram of a switching regulator according to the present invention, and the difference from the conventional circuit shown in FIG. 1 is that the tertiary winding 2 of the pulse transformer 15
8 is connected in parallel with a series resonant circuit 35 including a capacitor 24 and a coil 25. In this switching regulator, the switching frequency of the oscillator 30 is at the rated load.
₁ , but changes to ₂ when the output terminals 32 to 34 are short-circuited. Therefore, the circuit constants are set so that the resonance frequency of the series resonant circuit 35 formed by the capacitor 24 and the coil 25 is ₂ , and as a result, the tertiary voltage induced in the tertiary winding 28 when the output terminal is short-circuited is As a result, the signal is no longer fed back to the control terminal of the switching transistor 8, and the oscillation of the switching transistor 8 is stopped. Next, to explain this series resonant circuit 35 in more detail, FIG.
If the capacitance value of is C ₁ and the inductance of the coil 25 and the tertiary winding 28 are L ₁ and L ₂ respectively, then the impedance Z〓 _AB between AB in the circuit of Fig. 3 is Z〓 _AB = (ω ² C ₁ L ₁ −1)ωL ₂ /ω ² C ₁ (L ₁ +L ₂ )−1
and

When [formula], Z〓 _AB =0. Therefore, by setting the constants of the capacitor 24 and the coil 25 so that the switching frequency ₂ when the output terminals 32 to 34 are short-circuited is equal to the resonance frequency at which Z〓 _AB = 0, the transistor 8 is oscillation can be stopped. As mentioned above, the present invention has a configuration in which the capacitor 24 and the coil 25 connected in series are connected in parallel to the tertiary winding 28 of the pulse transformer 15 to control the drive of the switching transistor 8, so it is simple. This configuration has the remarkable effect of stopping the oscillation of the switching transistor 8 when the output is short-circuited, thereby preventing the switching transistor 8 from being destroyed.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional switching regulator, Fig. 2 is a circuit diagram showing a switching regulator according to the present invention, and Fig. 3 is a circuit diagram showing an equivalent circuit of the resonant circuit of the oscillation section. be. 8...Switching transistor, 15...Pulse transformer, 24...Capacitor, 25...Coil, 26...Primary winding, 27...Secondary winding, 2
8... Tertiary winding, 29... Stabilized voltage output section, 3
2, 33, 34...output end, 35...series resonant circuit.

Claims (1)

[Claims for Utility Model Registration] A pulse transformer in which a pulse current is applied to a primary winding, a secondary voltage is induced in a secondary winding, and a tertiary voltage is induced in a tertiary winding; A switching transistor that generates the pulse current by intermittent DC current by blocking oscillation when a voltage is applied to the control terminal, and a stabilizing voltage that converts the secondary voltage into DC voltage and outputs it to the output terminal. A switching regulator comprising an output section, characterized in that a series resonant circuit that resonates at a switching frequency when the output end is short-circuited is connected in parallel to the tertiary winding. protection circuit.