JPH0530736A - Switching power supply - Google Patents

Abstract

PURPOSE:To make it possible to turn a high voltage side switching means OFF before the low voltage side output voltage begins to drop by means of a circuit on the secondary of a transformer at the time of turn OFF of main switch. CONSTITUTION:A protective circuit 40 provided on the secondary of a transformer 4 detects the negative side output voltage Vs induced in the secondary winding 4c of the transformer 4 and when thus detected voltage drops below a predetermined level, a switch control circuit 30 functions to turn a DC switch 14 OFF.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in, for example, a facsimile machine, and takes out a relatively high voltage DC output and a relatively low voltage DC output from the secondary side of one transformer. The present invention relates to a switching power supply device of a one-transformer multi-output type.

[0002]

2. Description of the Related Art A conventional example of this type of switching power supply device is shown in FIG.

This switching power supply device is an example in the case of a flyback converter system, in which a DC input obtained by rectifying AC is turned on / off by a switching element (transistor in the illustrated example) 6, and a primary winding 4a of a transformer 4 is provided.
And the stored energy in the transformer 4 when off,
First secondary winding 4b to diode 10, reactor 1
2 and the high voltage V _H via the DC switch 14
As a DC output (for example, 24V), and from the second secondary winding 4c to the diode 20 and the three-terminal regulator 2
It is configured to be taken out as a direct current output of a relatively low voltage V _L (for example, 5 V) via 2. The output on the high voltage V _H side is used for driving a thermal head or a motor in a facsimile, for example, and the output on the low voltage V _L side is used for driving a logic circuit. 2 is a main switch.

The driving and control of the switching element 6 are
Although it is performed by the oscillation control circuit 8, the method may be a self-excited type or a separately excited type. Further, the feedback circuit from the secondary side to the oscillation control circuit 8 is not important here and is not shown.

The DC switch 14 is a switch control circuit 30 provided between the output line on the low voltage side and the ground.
The on / off is controlled by. That is, from the outside (for example, from the logic circuit in the case of a facsimile)
When the applied logic signal LS becomes the ground level, the transistor 31 is turned on, the transistor 32 is turned on accordingly, and a current flows through the exciting circuit of the DC switch 14 to turn it on.

[0006]

In the above switching power supply device, the falling edges of the output voltages V _H and V _L when the main switch 2 is turned off are as shown in FIG. 5 (at time t ₀ , the main switch 2 is turned on). off). In the figure, the output voltages V _H and V _L decrease obliquely mainly due to the influence of the capacitor externally attached to the power supply device.

In this case, for example, in a facsimile, when the output voltage V _L becomes 4.5 V or less, the operation of the logic circuit becomes unstable, and the logic signal LS to be output thereafter becomes indefinite (that is, becomes the ground level,
It may be open or undetermined), and even in that case, if the output voltage V _H remains on the high voltage side, a failure of the thermal head or the like may occur.

Even if the output voltage V _L on the low voltage side is monitored and the DC switch 14 on the high voltage side is turned off while the output voltage V _L drops from 5 V to 4.5 V, the voltage charged in the external capacitor remains. This voltage is applied to the thermal head and the like.

To solve the above problem, when the main switch 2 is turned off, the high voltage side DC switch 14 must be turned off before the low voltage side output voltage V _L starts to decrease. For that purpose, the DC switch 14 may be turned off by using the drop of the input voltage on the primary side of the transformer 4 as a trigger.

For that purpose, an input voltage detection circuit is provided on the primary side of the transformer 4, and a signal from the input voltage detection circuit is transmitted to the secondary side to turn off the DC switch 14.
A predetermined withstand voltage (for example, 3800V in the case of a facsimile) is required between the primary side and the secondary side of the device. For that purpose, the signal from the detection circuit must be transmitted to the secondary side via a photo coupler. However, since the unit price of this photo coupler is high, this is unreasonable in cost.

Therefore, according to the present invention, when the main switch is turned off, the switching means on the high voltage side can be turned off before the output voltage on the low voltage side starts to decrease by the circuit on the secondary side of the transformer. It is a main object to provide a switching power supply device.

[0012]

In order to achieve the above object, the switching power supply device of the present invention detects the negative voltage of the output voltages of the second secondary winding, and detects the negative voltage. It is characterized in that a protection circuit is provided which controls the switch control circuit to turn off the switch means when the voltage drops below a predetermined voltage.

[0013]

In the output voltage of the second secondary winding of the transformer, the positive side voltage is stabilized by the constant voltage control, but the negative side voltage is the primary side voltage in terms of the winding turns ratio. Proportional. Therefore, when the main switch is turned off and the primary side voltage disappears, the negative side voltage instantly drops.

Then, this voltage drop is detected by the protection circuit, and the switch means is turned off. This allows the switch means to be turned off before the output voltage on the low voltage side begins to drop. Moreover, all of these protections can be performed by the circuit on the secondary side of the transformer.

[0015]

1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention. The same or corresponding portions as those of the conventional example in FIG. 4 are denoted by the same reference numerals, and the differences from the conventional example will be mainly described below.

In this embodiment, the following protection circuit 40 is added to the conventional circuit.

That is, this protection circuit 40 is provided with a rectifying diode 41, a smoothing capacitor 42 and a resistor 43, a threshold setting Zener diode 44, and a control transistor 45 in this example. The cathode of the diode 41 is connected to the non-ground side of the second secondary winding 4c of the transformer 4, and the negative voltage V _S- of the output voltage V _S of this secondary winding 4c. (See also FIG. 2). The Zener diode 44 turns on when the voltage across the Zener diode 44 becomes equal to or higher than the Zener voltage (for example, 12V). The transistor 45 is turned on / off by the potential on the cathode side of the Zener diode 44.

The waveform of the output voltage of the secondary winding 4c of the transformer 4 is roughly as shown in FIG. Among these, the positive voltage V _{S +} is stabilized by constant voltage control of the switching power supply device (for example, 6 to 7 V when the output voltage V _L is 5 V), but the negative voltage is V _S +. _- is proportional with the turns ratio of the windings on the primary side of the voltage transformer 4.

Now, consider the case where the main switch 2 is turned on and the logic signal LS is at the ground level.
In this case, a large negative voltage (for example, about −40 V) is generated on the anode side of the diode 41, and a voltage equal to or higher than the Zener voltage is applied to both ends of the Zener diode 44. Is turned on, and accordingly, the transistor 45 is also turned on. Therefore, the transistors 31 and 32 are also turned on, and the DC switch 14 is turned on.

Next, when the main switch 2 is turned off and the voltage on the primary side disappears (for example, at time t _{0 in} FIG. 3), the negative voltage V _S- decreases instantly, and the diode 41 accordingly. Also, the voltage on the anode side of the Zener diode 44 rapidly approaches 0, and accordingly the Zener diode 4
4 is turned off, the transistor 45 is turned off, and the DC switch 14 is turned off (for example, time t _{1 in} FIG. 3). The time between this time t ₀ and t ₁ is instantaneous.

When the DC switch 14 is turned off, the high-voltage side output voltage V _H drops as in the conventional example. However, the output voltage V _L on the low-voltage side is lightly loaded by turning off the DC switch 14 (for example, in the case of a facsimile, the load on the high-voltage side is much heavier. That is, the stored energy of the transformer 4 is released to the low voltage side, and the output voltage V _L starts to decrease, as shown in FIG. 3, the output voltage V _H on the high voltage side becomes completely zero. After.

As described above, by providing the protection circuit 40, when the main switch 2 is turned off, the DC switch 14 is turned off to output the high voltage side output voltage V _L before the low voltage side output voltage V _L starts to decrease. _{Since H} can be completely set to 0, even if the output voltage V _L drops and the logic signal LS becomes indefinite, the output voltage V _H is completely 0 at that time. There is no possibility that the load on the high voltage side will fail.

In addition, since all of these protections can be performed by the circuit on the secondary side of the transformer 4, the DC switch 14 is detected through the photo coupler in the primary side detection described in the conventional example.
It is not necessary to use an expensive photocoupler as in the case of turning off, and therefore the cost can be reduced (for example,
It will be about half that when using a photo coupler).

Further, when the main switch 2 is turned off, the output voltage V _H on the high-voltage side with a heavy load is lowered first, so the output voltage V _L on the low-voltage side with a light load is held for a momentary interruption, resulting in a momentary interruption. It becomes a strong power supply.

Note that the relationship between the first secondary winding 4b and the second secondary winding 4c of the transformer 4 is such that one secondary winding has an intermediate tap in the above example. However,
Of course, it is not limited to this, and another winding may be used.

As the switch means, a semiconductor switch or the like may be used instead of the DC switch 14 shown in the illustrated example.

The switching power supply device is typically used for facsimiles, but needless to say, it may be used for other purposes.

[0028]

As described above, according to the present invention, when the main switch is turned off, before the output voltage on the low voltage side starts to decrease, the switch means on the high voltage side is turned off to completely remove the output voltage on the high voltage side. Can be set to 0, which can prevent the occurrence of a failure in the load on the high voltage side due to unstable control. Moreover, since all such protection can be performed by the circuit on the secondary side of the transformer, it is not necessary to use an expensive photocoupler, and therefore the cost can be reduced. Further, when the main switch is turned off, the output voltage on the high-voltage side with a heavy load is first lowered, so the hold time for the output voltage on the low-voltage side with a light load is long, and the power supply device is resistant to instantaneous interruption.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing an example of an output voltage waveform of a second secondary winding.

FIG. 3 is a schematic diagram showing an example of a state of an output voltage in the power supply device of FIG.

FIG. 4 is a circuit diagram showing an example of a conventional switching power supply device.

5 is a schematic diagram showing an example of a state of an output voltage in the power supply device of FIG.

[Explanation of symbols]

 2 Main Switch 4 Transformer 4a Primary Winding 4b First Secondary Winding 4c Second Secondary Winding 14 DC Switch 30 Switch Control Circuit 40 Protection Circuit

Claims (1)

Claim: What is claimed is: 1. A transformer having a primary winding, a first secondary winding for relatively high voltage output, and a second secondary winding for relatively low voltage output; A switching power supply device comprising: switch means that is inserted in series with a DC output line on the first secondary winding side of a transformer and that connects and disconnects the switch means; and a switch control circuit that controls ON / OFF of the switch means. A voltage on the negative side of the output voltage of the second secondary winding is detected, and when the voltage on the negative side falls below a predetermined voltage, the switch control circuit is controlled to turn off the switch means. A switching power supply device having a working protection circuit.