JPH082178B2 - Switching power supply circuit - Google Patents

Description

The present invention relates to a switching power supply circuit, and more particularly to a multi-output switching power supply circuit.

[Prior Art] Conventionally, as a switching power supply circuit of this type, for example, there is one as shown in FIG. This is fed back to the drive circuit 6 of the switching transformer 7 by the voltage comparison circuit 4 and the feedback circuit 5 connected thereto, and the output of the secondary winding of the switching transformer is rectified by the rectifier 9.
And the secondary DC obtained by rectifying and smoothing the primary side circuit 1 that rectifies and smoothes with the capacitor 11 to obtain the primary DC voltage V1, and another secondary winding output of the switching transformer 7 with the rectifier 10 and the capacitor 12. The secondary side circuit 2 for obtaining the supplied DC voltage V3 from the voltage V2 via the stabilizing circuit 8.
The primary DC voltage V1 is stabilized by controlling the drive pulse width of the switching transformer 7 by the voltage comparison circuit 4, the feedback circuit 5, and the switching drive circuit 6. The supply DC voltage V3 is stabilized from the secondary DC voltage V2 through the stabilizing circuit 8. In this switching power supply circuit, when the load of the primary DC voltage V1 is small, it operates so as to reduce the drive pulse width of the switching transformer 7, but the minimum drive pulse width is limited by the delay of internal elements of the switching drive circuit 6 and the like. It Therefore, when the load is light, the pulse width cannot be reduced and the primary DC voltage V1
The voltage value of becomes high. In order to prevent this, conventionally, a resistor 15 is connected to the primary circuit 1 to allow a certain amount of current to flow.

[Problems to be Solved by the Invention] However, in the above-described conventional switching power supply circuit, since the resistor 15 is connected to suppress the voltage value increase of the primary DC voltage V1 during a light load, the resistance of the resistor 15 is reduced. There is a drawback in that the output capacity is reduced by the amount of power consumed by 15.

[Means for Solving the Problems] The means for solving the above-mentioned drawback is to rectify the secondary winding output of the switching transformer by feeding back to the drive circuit of the switching transformer by the voltage comparison circuit and the feedback circuit. A primary side circuit for smoothing to obtain a primary DC voltage, and a secondary DC voltage obtained by rectifying and smoothing another secondary winding output of the switching transformer to obtain a supply DC voltage via a stabilizing circuit. In a switching power supply circuit having a secondary side circuit, a transistor is connected between the primary side circuit of the primary DC voltage and the secondary side circuit of the secondary DC voltage, and the primary side circuit is provided with the primary DC voltage due to the rise. Another voltage comparison circuit for supplying the drive current of the transistor is provided.

[Embodiment] A switching power supply circuit according to an embodiment of the present invention will be described below with reference to the accompanying drawings. As shown in FIG. 1, the switching power supply circuit according to the embodiment feeds back to the drive circuit 6 of the switching transformer 7 by the voltage comparison circuit 4 and the feedback circuit 5 connected to the voltage comparison circuit 4 and the secondary winding of the switching transformer 7. The output is rectified and smoothed by the rectifier 9 and the capacitor 11 to obtain the primary DC voltage V1, and the secondary winding output of the switching transformer 7 is rectified and smoothed by the rectifier 10 and the capacitor 12 and obtained. The secondary side circuit 2 for obtaining the supplied direct current voltage V3 from the secondary direct current voltage V2 via the stabilizing circuit 8. Also,
A transistor 13 is connected between the primary side circuit 1 of the primary DC voltage V1 and the secondary side circuit 2 of the secondary DC voltage V2. The transistor 13 is connected to the primary side circuit 1 by a predetermined rise of the primary DC voltage V1. Another voltage comparison circuit that supplies drive current
14 are connected. That is, the collector terminal of the transistor 13 is connected to the primary side circuit 1, the emitter terminal is connected to the upstream side of the stabilization circuit 8 of the secondary side circuit 2, and the base terminal is connected to the voltage comparison circuit 14.

Therefore, according to the switching power supply circuit of this embodiment, the stabilizing operation when the load of the primary DC voltage V1 is large is performed in the same manner as the conventional one, and the primary DC voltage V1 is switched between the voltage comparison circuit 4, the feedback circuit 5, and the switching circuit. The drive pulse width of the switching transformer 7 is controlled and stabilized by the drive circuit 6. The supply DC voltage V3 is stabilized from the secondary DC voltage V2 through the stabilizing circuit 8.

On the other hand, when the primary DC voltage V1 is lightly loaded, the voltage value of the primary DC voltage V1 tends to increase as in the conventional case due to a delay in internal elements of the switching drive circuit 6. However, the voltage comparison circuit 14 detects this and
In order to turn on 13, the primary DC voltage V1 to the secondary DC voltage V2
A current flows through the first DC voltage V1 and the voltage value of the primary DC voltage V1 rises. When the load current of the primary DC voltage V1 is large, the primary DC voltage V1 is the set value of the voltage comparison circuit 4, so the transistor 13 is OFF.

As described above, according to the switching power supply circuit of the present invention, the transistor is connected between the primary side circuit and the secondary side circuit, and the transistor is provided only when the voltage value of the primary DC voltage becomes high. Since it is operated, there is an effect that it is possible to suppress the rise of the primary DC voltage at the time of light load without reducing the output capacity.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a switching power supply circuit according to an embodiment of the present invention, and FIG. 2 is a block circuit diagram showing an example of a conventional switching power supply circuit. V1: Primary DC voltage, V2: Secondary DC voltage V3: Supply DC voltage, 1: Primary side circuit 2: Secondary side circuit, 4: Voltage comparison circuit 5: Feedback circuit 6: Switching transformer drive circuit 7: Switching transformer 8: Stabilization circuit, 9, 10: Rectifier 11, 12: Capacitor 13: Transistor 14: Voltage comparison circuit, 15: Resistor

Claims (1)

[Claims] 1. A primary side circuit that feeds back to a drive circuit of a switching transformer by a voltage comparison circuit and a feedback circuit to rectify and smooth the secondary winding output of the switching transformer to obtain a primary DC voltage, and another one of the switching transformers. A secondary side circuit that obtains a supplied direct current voltage from a secondary direct current voltage obtained by rectifying and smoothing two secondary winding outputs through a stabilizing circuit, the primary side circuit of the primary direct current voltage And a secondary side circuit for the secondary DC voltage, and a transistor is connected between the secondary side circuit and the primary side circuit, and another voltage comparison circuit for supplying a drive current of the transistor when the primary DC voltage rises is provided. Switching power supply circuit.