JP3265448B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply which is incorporated in electronic equipment such as a laser beam printer and a facsimile and is used as a stabilized DC power supply requiring a plurality of outputs.

[0002]

2. Description of the Related Art Conventionally, a switching power supply is incorporated in a laser beam printer whose external shape is shown in FIG. 3 and whose internal block configuration is shown in FIG. And in this laser beam printer,
The commercial power supply 1 is electrically insulated so as not to receive an electric shock even if touched by an operator, and the alternating current from the commercial power supply 1 is converted into a plurality of DC stabilized voltages by the switching power supply 2, and a low A DC stabilized voltage is supplied to a CPU 3 and a memory 4 controlled by a personal computer or the like, and a high DC stabilized voltage of about 24 V is supplied to a paper feed motor 5 and a high-voltage power supply 6, while about 12 V Is supplied to a display 7 such as a liquid crystal.

That is, the switching power supply 2 here
Is used to stably operate the CPU 3, the memory 4, the motor 5, the high-voltage power supply 6, and the display 7 provided in the electronic device.
A DC stabilizing voltage for each of the high, medium and low voltages is generated.

First, the configuration and operation of a switching power supply according to a first conventional example will be described with reference to FIG.

[0005] The switching power supply has input terminals 11 and 11 'of a commercial power supply, first output terminals 12 and 12' for outputting a high voltage DC stabilized voltage, and a second output terminal for outputting a medium voltage DC stabilized voltage. It has output terminals 13 and 13 'and third output terminals 14 and 14' for outputting a low DC stabilized voltage. The input terminals 11, 11 'and the first to third output terminals 12, 12', 13, 13 ', 14, 14'.
A noise filter 15, a primary side rectifying and smoothing circuit 16, a transformer 17, a first secondary side rectifying and smoothing circuit 18 for high voltage, and a second secondary side rectifying and smoothing circuit 1 for medium voltage
9. While the third secondary rectifying / smoothing circuit 20 for low voltage and the output control circuit 21 are connected, the primary rectifying / smoothing circuit 1
A switching transistor 22 is provided, which functions as a quadrature conversion switching element that performs high-frequency switching of the DC obtained in 6 and converts the DC into a single-phase AC.

The noise filter 15 in this case is for preventing the inflow of external noise contained in the commercial power supply and the outflow of noise generated due to switching. The primary-side rectifying / smoothing circuit 16 includes four diodes. It is composed of a full-wave rectifier circuit 23 connected in a bridge and a smoothing capacitor (electrolytic capacitor) 24.

[0007] The switching transistor 22 is connected to a base winding 25 wound around the core of the transformer 17 to generate a base current, and a current limiting circuit 26 for limiting the base current. Circuit 26
Is composed of a base resistor 27 and a capacitor 28 connected in series to the base of the switching transistor 22. Also, this switching transistor 2
For 2, a primary winding 29 wound around the core of the transformer 17 and applied with a DC input voltage is connected in series. Note that reference numeral 30 in the figure indicates a starting resistor.

Further, each of the first to third secondary-side rectifying / smoothing circuits 18, 19, 20 includes a diode 31, 32,
33, and the first to third secondary windings 37, 3 which are wound around the core of the transformer 17 and function for high, medium, and low voltages.
8 and 39. The first output terminals 12 and 1 are connected to the first secondary side rectifying and smoothing circuit 18 for high voltage.
A protection circuit 40 functioning as an overvoltage protection circuit 2 'is provided. The protection circuit 40 includes a thyristor 41 connected between the first output terminals 12 and 12 ′ in parallel with the first secondary winding 37 and serving as a protection circuit switching element, a resistor 42 and a zener diode 43. It is composed of The switching element for the protection circuit here is not limited to the thyristor 41, but may be a transistor, a relay, a triac, or the like.

At this time, the second and third secondary windings 3
8, 39 and the second and third output terminals 13, 13 ',
Three-terminal regulators 44 and 45 are connected between the respective terminals 14 and 14 '. Further, the output control circuit 21 controls the output from the switching power supply in response to the fluctuation of the load, and controls one end of the first output terminals 12 and 12 ′, that is, the first output terminal 12. And one end is connected to the second and third
The other ends of the output terminals 13, 13 ', 14, 14', that is, the second output terminal 13 'and the third output terminal 1
A photocoupler 46 having the other end connected to 4 'and a voltage dividing resistor 47 are combined.

Therefore, AC commercial power applied to the input terminals 11 and 11 'constituting the switching power supply according to the conventional example is input to the full-wave rectifier circuit 23 via the noise filter 15, and After being subjected to full-wave rectification by the full-wave rectifier circuit 23, it is smoothed to DC by a smoothing capacitor 24. Then, the obtained DC is converted into a single-phase AC by high-frequency switching by the switching transistor 22, and the obtained single-phase AC is converted into a desired voltage by the transformer 17.

Further, the first output terminals 12 and 12 'output a high-voltage DC stabilized voltage generated through the first secondary-side rectifying and smoothing circuit 18, and the output high voltage, that is, , 24 V are supplied to the motor 5 and the high-voltage power supply 6. At this time, the intermediate voltage generated through the second secondary-side rectifying / smoothing circuit 19, that is, 1
The DC stabilized voltage of 2 V is output from the second output terminals 13 and 13 ′ and supplied to the display 7, which is a low voltage generated through the third secondary-side rectifying and smoothing circuit 20.
The DC stabilized voltage of V is output from the third output terminals 14 and 14 'and then supplied to the CPU 3 and the memory 4.

However, in the switching power supply according to the first conventional example, two three-terminal regulators 44 and 45 are provided.
The use of a switching power supply according to the second conventional example as shown in FIG. In FIG. 6, the same parts and portions as those in FIG. 5 are denoted by the same reference numerals.

That is, in this switching power supply, the third secondary side rectifying / smoothing circuit 20 for low voltage and the third
The three-terminal regulator 45 connected between the output terminals 14, 14 'is eliminated, and the first and third output terminals 12,
The first output terminal 12 'and the third output terminal 14', which are the other ends of the terminals 12 ', 14, and 14', are shared to form a common output terminal 48, and the smoothing capacitor 34 and the protection circuit 40 Thyristor 41 is connected between the first output terminal 12, which is one end of the first output terminals 12, 12 ', and the common output terminal 48, and the resistor 42 of the protection circuit 40 and the voltage division of the output control circuit 21 are connected. The connection with the resistor 47 is made by a resistor 49 and a zener diode 50.

However, when the above configuration is adopted,
When the output terminal of the first output terminal 12 and the common output terminal 48 is short-circuited, a discharge current flows out of the smoothing capacitor 36, and
A large discharge current of about 4 A flows along the path indicated by the broken line A in FIG. 6 and then flows into the first secondary winding 37. Therefore, the transformer 17 is saturated, and the switching transistor 22 is switched to the ASO (AREA SAFE).
OPERATING) It will cause destruction.

Therefore, in the switching power supply according to the second conventional example, the first output terminal 12 and the third output terminal which are one ends of the first output terminals 12 and 12 'and the third output terminals 14 and 14', respectively. A bypass diode 51 functioning as an output terminal short-circuit protection element is connected between the terminals 14, and when the output terminals of the first output terminal 12 and the common output terminal 48 are short-circuited, as shown by a broken line B in FIG. The discharge current flowing out of the bypass diode 5
1 is configured to flow along a path through. When such a configuration is adopted, only a small current of about 1 A flows into the first secondary winding 37 for high voltage, so that the transformer 17 does not saturate, and Transistor 22 is A
It will not cause SO destruction.

[0016]

By the way, when a short-open test is performed on the electronic components constituting the switching power supply according to the second conventional example, the transformer 1
A of the first secondary winding 37 for high voltage wound around the core 7
If the point 31 or the point b or the diode 31 is opened due to poor soldering or the like, no current flows to the photocoupler 46 constituting the output control circuit 21, and this output control As a result, the output of the switching power supply cannot be controlled by the circuit 21. In particular, the third output terminal 14 and the common output terminal 4
The low voltage output from 8, that is, the DC stabilized voltage of 5V starts to rise.

When the low-voltage DC stabilized voltage continues to rise, the voltage rise is detected by the Zener diode 50, and the thyristor 41 constituting the protection circuit 40 is turned on.
As a result, the third output terminal 14 and the common output terminal 48 are short-circuited through the thyristor 41 and the bypass diode 51, and a short-circuit current flows along a path indicated by a broken line C in FIG. Become. In such a case, as can be seen from FIG. 7 showing the current limiting characteristic of the switching power supply, the forward voltage VF1 by the thyristor 41 and the forward voltage VF2 by the bypass diode 51 overlap each other. As a result, the short-circuit current I ₁ becomes excessively large, so that the thyristor 41 and the bypass diode 51
Will generate abnormal heat, and the board constituting the switching power supply will be burned, possibly causing a fire.

The present invention has been made in view of such inconvenience, and an object of the present invention is to provide a switching power supply capable of reducing a short-circuit current flowing through a protection circuit switching element.

[0019]

According to a first aspect of the present invention, there is provided a switching power supply obtained through a transformer for converting a single-phase alternating current obtained by a quadrature conversion switching element and a first secondary winding. A first rectifying and smoothing voltage and then outputting a high-voltage stabilized DC voltage from a first output terminal;
A secondary-side rectifying / smoothing circuit, rectifying and smoothing the voltage obtained through the second secondary winding, outputting a medium-voltage DC stabilized voltage from a second output terminal, and A second secondary-side rectifying / smoothing circuit in which a three-terminal regulator is connected in series between the other end of the wire and the second output terminal; and a rectifying / smoothing voltage obtained through a third secondary winding. A third secondary-side rectifying / smoothing circuit that outputs a low-voltage DC stabilized voltage from an output terminal, and is connected between the first and third output terminals in parallel with the first secondary winding. Output terminal short-circuit protection element, a common output terminal in which the other ends of the first and third output terminals are shared, and a protection circuit switching element constituting a protection circuit of the first secondary side rectifying and smoothing circuit. Switching power supply, and the protection circuit switching element Or connected in parallel with the third secondary winding and between one end of the second output terminal and the common output terminal, and having its input terminal connected to the input side of the three-terminal regulator. It is characterized by being.

The switching power supply according to a second aspect of the present invention rectifies and smoothes the voltage obtained through the first secondary winding, and a transformer for converting the single-phase alternating current obtained by the quadrature conversion switching element. A first secondary-side rectifying / smoothing circuit for outputting a high-voltage DC stabilized voltage from the first output terminal; and a second output terminal after rectifying and smoothing the voltage obtained through the second secondary winding. And a second secondary-side rectifying / smoothing circuit that outputs a low-voltage DC stabilized voltage from the first and second rectifying / smoothing circuits.
A third secondary-side rectifying / smoothing circuit for outputting a medium-voltage DC stabilized voltage from the output terminal, and connected between the first and third output terminals in parallel with the first secondary winding. Output terminal short-circuit protection element, a common output terminal in which the other ends of the first and third output terminals are shared, and a protection circuit switching element constituting a protection circuit of the first secondary side rectifying and smoothing circuit. The protection circuit switching element is connected in parallel with the first to third secondary windings and connected between one end of the third output terminal and the common output terminal. It is characterized by having.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of the switching power supply according to the first embodiment, and FIG. 2 is a circuit diagram showing the configuration of the switching power supply according to the second embodiment. Note that the overall configuration of each switching power supply at this time is not fundamentally different from that of the conventional example. Therefore, in FIGS. 1 and 2, parts and portions that are the same as those in FIGS. 5 and 6 are denoted by the same reference numerals. However, a detailed description of the function of each electronic component is omitted.

Each of the switching power supplies according to the first and second embodiments is incorporated in a laser beam printer whose external shape is shown in FIG. 3 and whose internal configuration is shown in FIG. Depending on these switching power supplies 2, depending on the switching power supply 2, the AC commercial power supply 1 may be connected to a plurality of DC stabilizing voltages, that is, a 5V DC stabilizing voltage required by the CPU 3 and the memory 4, and a paper feeding voltage. The conversion is performed to a 24V DC stabilized voltage required by the motor 5 and the high-voltage power supply 6 and a 12V DC stabilized voltage required by the display 7 such as a liquid crystal display.

As shown in FIG. 1, the switching power supply according to the first embodiment has input terminals 11, 1 of a commercial power supply.
1 ', first output terminals 12, 12' for outputting a DC stabilized voltage of a high voltage (24V), and second output terminals 13, 13 'for outputting a DC stabilized voltage of a medium voltage (12V). Third output terminals 14 and 14 'for outputting a low-voltage (5 V) DC stabilized voltage, and the other ends of the first and third output terminals 12, 12', 14, and 14 'are provided. A certain first output terminal 12 'and a third output terminal 14' are commonly used as a common output terminal 48. A noise filter 15 and a primary-side rectifying / smoothing circuit 1 are provided between these input terminals 11, 11 'and the first to third output terminals 12, 12', 13, 13 ', 14, 14'.
6. Transformer 17, first secondary rectifying / smoothing circuit 18 for high voltage, second secondary rectifying / smoothing circuit 19 for medium voltage, third secondary rectifying / smoothing circuit 20 for low voltage, output control circuit A switching transistor 22 functioning as a switching element for orthogonal transformation is connected with the switching transistor 21.

The transformer 17 converts the single-phase alternating current obtained by the switching transistor 22 into a voltage. The first secondary-side rectifying / smoothing circuit 18 is wound around the core of the transformer 17. After rectifying and smoothing the voltage obtained through the first secondary winding 37, the first output terminal 1
A high-voltage DC stabilized voltage is output from the first output terminal 12 and the common output terminal 48, which are one ends of the terminals 2 and 12 '. Then, the second secondary-side rectifying / smoothing circuit 19 rectifies and smoothes the voltage obtained through the second secondary winding 38 wound on the core of the transformer 17, and outputs the rectified and smoothed voltage from the second output terminals 13 and 13 '. And a second output, which is the other end of the second secondary winding 38 and the second output terminal 13, 13 'in the second secondary side rectifying and smoothing circuit 19. A three-terminal regulator 44 is connected in series with the terminal 13. Further, the third secondary-side rectifying / smoothing circuit 20 rectifies and smoothes the voltage obtained through the third secondary winding 39 wound on the core of the transformer 17 and then outputs the third output terminals 14 and 14 ′. A low-voltage DC stabilized voltage is output from the third output terminal 14 and the common output terminal 48, which are one ends of the terminals.

On the other hand, the first secondary-side rectifying / smoothing circuit 18 is constructed by using a thyristor 41 serving as a protection circuit switching element, a resistor 42 and a Zener diode 43, and Protection circuit 40 functioning as overvoltage protection for secondary-side rectifying / smoothing circuit 18
Are provided, and a resistor 4 constituting the protection circuit 40 is provided.
2 is connected via a resistor 49 and a zener diode 50 to a voltage-dividing resistor 47 constituting the output control circuit 21 together with the photocoupler 46. The first secondary side rectifying / smoothing circuit 18 is provided with a bypass diode 51 functioning as an output terminal short-circuit protection element. The bypass diode 51 is connected to the first output terminal 1.
The second and third output terminals 14 and 14 'are connected between the first output terminal 12 and the third output terminal 14, which are one ends of the respective terminals.

Further, the first secondary side rectifying / smoothing circuit 18
Is arranged in parallel with the first to third secondary windings 37, 38, 39, and the second thyristor 41 is one end of the second output terminals 12, 12 '. The input terminal of the thyristor 41 is connected between the output terminal 12 and the common output terminal 48. At this time, the second secondary winding 38 and the second output terminal constituting the second secondary side rectifying / smoothing circuit 19 are connected. 13 is connected to the input side of a three-terminal regulator 44 connected in series.
The switching element for the protection circuit here is not limited to the thyristor 41, but may be a transistor, a relay, a triac, or the like.

Therefore, when a short-open test is performed on the electronic components constituting the switching power supply according to the first embodiment, a high-voltage first secondary winding wound around the core of the transformer 17 is used. 37 points a or b,
Alternatively, when the diode 31 is opened, the current stops flowing to the photocoupler 46 constituting the output control circuit 21 as in the conventional example, and the output of the switching power supply can be controlled by the output control circuit 21. Therefore, the low voltage output from the third output terminal 14 and the common output terminal 48, that is, the DC stabilized voltage of 5V starts to increase. When the low-voltage DC stabilized voltage continues to increase, the voltage increase is detected by the Zener diode 50, and the thyristor 41 included in the protection circuit 40 is turned on.

However, the thyristor 41 here
Is a 12V second output terminal 12 and a 5V common output terminal 4.
8 and the thyristor 4
When 1 is turned on, the second output terminal 12 and the common output terminal 48 are short-circuited, and a short-circuit current flows along a path indicated by a broken line D in FIG. The short-circuit current I ₂ flowing at this time is, as can be seen from the current limiting characteristic of the switching power supply shown in FIG.
Not give only becomes large based only on forward voltage VF1 is due, it is smaller than the short circuit current I ₁ in the conventional example. Therefore, the thyristor 41 and the bypass diode 51
No abnormal heat generation occurs, and no fire resulting from such abnormal heat generation can occur.

By the way, in the switching power supply according to the first embodiment, the thyristor 41 is connected between the second output terminal 12 of 12V and the common output terminal 48 of 5V. 41, a third output terminal 14 for outputting 5 V and 5 V
May be connected between the common output terminal 48 and the common output terminal 48. However, with such a configuration,
The forward voltage VF1 of the thyristor 41 will be short-circuit current flows I _3, towards the short-circuit current I ₃ is short-circuit current I
_This is not necessarily a good idea, as it can lead to results larger than ₂ .

Further, in the switching power supply according to the first embodiment, the second secondary-side rectifying / smoothing circuit 19
Outputs a DC stabilized voltage of a medium voltage (12 V) from the second output terminals 13 and 13 ′, and the third secondary-side rectifying / smoothing circuit 20 outputs a low voltage from the second output terminal 14 and the common output terminal 48. Although the DC stabilized voltage of (5 V) is output, the second secondary-side rectifying / smoothing circuit 19 outputs a low-voltage (5 V) DC stabilized voltage from the second output terminals 13 and 13 ′. Alternatively, the third secondary-side rectifying / smoothing circuit 20 may output a DC stabilized voltage of an intermediate voltage (12 V) from the second output terminal 14 and the common output terminal 48. Then, in the switching power supply according to the second embodiment having such a configuration, as shown in FIG. 2, the thyristor 41 functioning as a switching element for a protection circuit outputs a 12V DC stabilized voltage to the thyristor 41. Output terminal 14 and common output terminal 48
A connection is made between.

That is, the switching power supply according to the second embodiment includes a first secondary-side rectifying / smoothing circuit 18 that outputs a high-voltage DC stabilized voltage from the first output terminals 12 and 12 ′, A second secondary rectifying / smoothing circuit 19 for outputting a low-voltage DC stabilized voltage from the output terminals 13 and 13 ', and a third secondary rectifying and smoothing circuit 19 for outputting a medium-voltage DC stabilized voltage from the third output terminals 14 and 14'. And a secondary-side rectifying / smoothing circuit 20.
The thyristor 41 constituting the protection circuit 40 of the first secondary-side rectifying / smoothing circuit 18 is replaced by first to third secondary windings 37 and 3.
8 and 39 and the third output terminals 14 and 1
4 'is connected between the third output terminal 14, which is one end of the terminal 4', and the common output terminal 48. Even if that had been such a configuration would not flow only a small short-circuit current I ₂ by passing it through the thyristor 41.

[0033]

As described above, according to the switching power supply of the present invention, when a short-open test of an electronic component is performed, the first secondary winding for high voltage wound on the core of the transformer. Even when the line or the diode is opened, an excessive short-circuit current flows to the protection circuit switching element of the first secondary-side rectifying / smoothing circuit that outputs the DC / DC stabilized voltage for high voltage. This does not occur, and the short-circuit current flowing through the switching element is greatly reduced. Therefore, there is obtained an effect that the switching element does not generate abnormal heat and there is no possibility that a fire due to the abnormal heat may occur.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a switching power supply according to a first embodiment.

FIG. 2 is a circuit diagram showing a configuration of a switching power supply according to a second embodiment.

FIG. 3 is a perspective view showing an external shape of the laser beam printer.

FIG. 4 is a block diagram illustrating an internal configuration of the laser beam printer.

FIG. 5 is a circuit diagram showing a configuration of a switching power supply according to a first conventional example.

FIG. 6 is a circuit diagram showing a configuration of a switching power supply according to a second conventional example.

FIG. 7 is an explanatory diagram showing current limiting characteristics of a switching power supply.

[Explanation of symbols]

 12 One end of the first output terminal 12 'The other end of the first output terminal 13 One end of the second output terminal 13' The other end of the second output terminal 14 One end of the third output terminal 14 'The other of the third output terminal End 18 First secondary-side rectifying / smoothing circuit 19 Second secondary-side rectifying / smoothing circuit 20 Third secondary-side rectifying / smoothing circuit 41 Thyristor (switching element for protection circuit) 44 Three-terminal regulator 48 Common output terminal

Claims (2)

(57) [Claims] 1. A transformer for voltage-converting a single-phase alternating current obtained by a quadrature conversion switching element, and a high voltage output from a first output terminal after rectifying and smoothing a voltage obtained through a first secondary winding. Rectifying and smoothing circuit for outputting a stabilized DC voltage of the first and second sides, and rectifying and smoothing the voltage obtained through the second secondary winding. A second secondary-side rectifying / smoothing circuit having a three-terminal regulator connected in series between the other end of the second secondary winding and the second output terminal, and a voltage obtained through the third secondary winding. And a third secondary-side rectifying / smoothing circuit for outputting a low-voltage stabilized DC voltage from a third output terminal after rectifying and smoothing the first output. An output terminal short-circuit protection element connected between the first and third output terminals; A switching power supply comprising: a common output terminal in which the other ends of the third output terminals are shared with each other; and a protection circuit switching element constituting a protection circuit of the first secondary side rectifying / smoothing circuit. The switching element is connected between the one end of the second output terminal and the common output terminal in parallel with the first to third secondary windings, and has an input terminal connected to the input side of the three-terminal regulator. A switching power supply characterized by being connected to a switching power supply. 2. A transformer for converting a single-phase alternating current obtained by a quadrature conversion switching element into a voltage, a rectified and smoothed voltage obtained through a first secondary winding, and a high voltage output from a first output terminal. A first secondary side rectifying / smoothing circuit that outputs a stabilized DC voltage, and a rectified and smoothed voltage obtained through a second secondary winding. A second secondary-side rectifying / smoothing circuit for outputting, and a third secondary for rectifying and smoothing the voltage obtained through the third secondary winding and outputting a DC stabilized voltage of an intermediate voltage from a third output terminal. An output-end short-circuit protection element that includes a side rectifying / smoothing circuit and is connected between the first and third output terminals after being arranged in parallel with the first secondary winding; and the other of the first and third output terminals. Common output terminal with common end and protection of first secondary side rectifying and smoothing circuit A switching power supply comprising: a protection circuit switching element constituting a circuit, wherein the protection circuit switching element is arranged in parallel with the first to third secondary windings and at one end of a third output terminal. And a common output terminal.