JPH08205519A - Switching regulator - Google Patents

Abstract

PURPOSE: To suppress the harmonic components of the input current from a power supply constantly even if the fluctuation of the output of the power supply is large and maintain a high power factor. CONSTITUTION: A harmonic current suppressing first choke coil 4a and a series circuit which is provided as the bypass circuit of the first choke coil 4a and which is composed of a harmonic current suppressing second choke coil 4b and a switching TRIAC 5 are connected in parallel to each other and this parallel circuit is connected between an AC power supply 1 and a rectifier 2. The TRIAC 5 is in an OFF-state while the output current of a secondary DC power does not exceed a predetermined value and is turned ON when the current exceeds the predetermined value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator used in office automation equipment such as copiers, printers and facsimiles.

[0002]

2. Description of the Related Art The development of image forming apparatuses such as copying machines, printers and facsimiles, and other office automation equipment is remarkable, and in particular, image forming apparatuses for forming images on plain paper, such as copying machines, are becoming faster and more multifunctional. Therefore, the increase in the DC power supply capacity has been remarkable. For example, the motor that is the drive source
In order to accurately control the rotation state, the AC motor D
Since there are more types of high-voltage power supplies in place of C motors,
The capacity of the DC power supply was 400V, which was around 100VA in the past.
From Class A to 500VA, some models exceed 600VA, and strict voltage stability is required.

As a power source for such equipment, a conventional dropper type power source for lowering efficiency and stabilizing voltage has been used, but there is a problem that the size becomes large and a large amount of heat is generated. Instead, a switching regulator having a high efficiency DC-DC converter that solves these problems and is small in size and generates less heat has begun to be used.

An example of such a switching regulator will be described with reference to the circuit diagram shown in FIG.
The AC power from DC is rectified and smoothed by a rectifying and smoothing circuit of a choke input type which is composed of a diode bridge 42, a smoothing capacitor 43, and a choke coil 44 provided between the diode bridge 42 and the smoothing capacitor 43. Electric power is supplied and this is applied to the DC-DC converter 45. As the rectifying / smoothing circuit, a capacitor input type excluding the choke coil 44 described above is also used.

However, although the choke input type rectifying / smoothing circuit has good power efficiency, it has a drawback in that the choke coil 44 becomes large in size and the weight and cost increase when the capacity becomes large. Further, in the capacitor input type rectifying / smoothing circuit, when the capacity is also large, the output of the diode bridge 42 must be smoothed by the large capacity smoothing capacitor 43, and an excessive charging peak current flows in a short time, There is a drawback that the power factor deteriorates. Thus, DC-
Even if the efficiency of the DC converter is excellent, if the power factor of the rectifying and smoothing circuit to be combined is poor, the product of the efficiency and the power factor, V
A efficiency (output VA / input VA) does not improve.

Therefore, for example, Japanese Patent Laid-Open No. 63-10745
As disclosed in Japanese Patent Publication No. 7, a parallel circuit of an impedance element and a diode whose polarity is aligned in the discharge direction is connected in series to a smoothing capacitor. When the smoothing capacitor is charged, the peak current is suppressed by the impedance element, and when discharging the smoothing capacitor. It has been proposed to improve the power factor by discharging it through a diode.

Further, as disclosed in Japanese Patent Laid-Open No. 63-23561, the DC power factor is improved by directly inputting the full-wave rectified direct current into the DC-DC converter without using the smoothing capacitor. Is also proposed.

[0008]

However, in the former proposal, if the impedance element is formed by a resistance element, power loss occurs during charging, heat is generated and efficiency is reduced. If the impedance element is formed by an inductance element such as a choke coil, Similar to the above-mentioned choke input type rectifying / smoothing circuit, problems occur in size, weight, cost, etc., and all are unsuitable for large capacity power supplies.

The latter proposal requires another auxiliary power supply circuit in order to eliminate the switching instability near the zero cross of the power consumption caused by removing the smoothing capacitor, the circuit is complicated, and the number of parts is small. There are problems such as increase in cost and increase in cost.

The present invention has been made in view of the above-mentioned points, and a choke coil having a large inductance when the output is small, and a choke coil having a small inductance when the output is large are provided according to the output. To provide a switching regulator that can keep a high power factor by suppressing the harmonic current component of the input current of the power supply even if the output fluctuation is large by switching the choke coil. The purpose is.

[0011]

In order to achieve the above-mentioned object, a first means is to rectify and smooth AC power input from an AC power supply by a rectifying element and a smoothing capacitor, and to perform the rectified and smoothed primary DC. The power is turned on and off by a switching element connected in series with the primary winding of the transformer, and the secondary DC power obtained by further rectifying and smoothing the power induced in the secondary winding of the transformer is output. In a switching regulator that stabilizes the output voltage by controlling the duty ratio of the switching element according to the output voltage of the secondary DC power, a first choke for suppressing a harmonic current is provided between an AC power supply and a rectifying element. The coil and the series circuit of the second choke coil for suppressing the harmonic current for bypassing the coil and the changeover switch are connected in parallel and switched. When the switch secondary DC power of the output current is less than a predetermined value are turned off, when it exceeds a predetermined value is obtained so as to turn on.

The second means is, in the above-described premise structure, a first choke coil for suppressing a harmonic current and a harmonic for bypassing the first choke coil between the rectifying element of the AC power supply and the smoothing capacitor. Connect the second choke coil for current suppression and the series circuit of the changeover switch in parallel,
The changeover switch is configured to be turned off when the output current of the secondary DC power is below a predetermined value and turned on when the output current exceeds the predetermined value.

The third means is the above-mentioned structure, further comprising:
The changeover switch consists of a relay.

[0014]

In the first means, the changeover switch is turned off while the output current during standby or normal operation is relatively small. In this state, the input current of the switching regulator is the first current for suppressing harmonic current. It flows through the choke coil. Since the input current at this time is small, considerable inductance is required to suppress the harmonic current. However, by configuring the first choke coil with a high-inductance small-current one, it is possible to achieve low output. However, since the high impedance can be arranged, the harmonic component can be suppressed and the high power factor can be achieved.

When the output increases during operation and exceeds a predetermined value, the changeover switch is turned on, and the input current of the switching regulator is the second harmonic current suppressing second current.
Flowing through the choke coil. Since the input current at this time is large, a small inductance is sufficient to suppress the harmonic current. Therefore, if the second choke coil is configured with a low inductance (low impedance, low resistance) and a large current, a low impedance is arranged at the time of high output, and a harmonic component can be suppressed. As described above, it is possible to provide a switching regulator that can keep harmonic components of the input current of the power supply and maintain a high power factor even if the output current (power) varies greatly.

In the second means, the first choke coil for suppressing the harmonic current in the above-mentioned first means, the second choke coil for suppressing the harmonic current for bypassing the first choke coil, and the changeover switch are provided. The circuit connected in parallel with the series circuit is connected between the rectifying element of the AC power supply and the smoothing capacitor, and the operation is the same as that of the above-mentioned first means. By this second means, the first and second
The choke coil can be arranged at a position different from that of the first means, and the arrangement position of these choke coils can be selected according to the layout of the switching power supply or the like.

In the third means, since the changeover switch is composed of a relay, if the contact of this relay is arranged in the changeover switch part of the primary circuit, the relay can be driven only by the secondary circuit. More simplified than the second means,
The size can be reduced and the cost can be reduced.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The switching regulator of the present invention will be described below with reference to the embodiments shown in the drawings.

[First Embodiment] FIG. 1 is a circuit diagram showing a first embodiment of the present invention. The switching regulator according to the first embodiment uses the AC power from the AC power supply 1 as 1
A rectifying / smoothing circuit composed of a diode bridge 2 which is a rectifying element for converting into the next DC power and a smoothing capacitor 3, and a harmonic current suppressing first provided between the AC power supply 1 and the diode bridge 2. 1 choke coil 4a
And a second harmonic current suppression for bypassing this
Circuit in which the series circuit of the choke coil 4b and the triac 5 which is a changeover switch are connected in parallel, three DC-DC converters 6 for outputting secondary DC power having different output voltages, and the triac 5 And a control circuit 7 for controlling on / off of the switching element 11 of the DC-DC converter 6, and a light emitting section 8a composed of a light emitting diode or the like for triggering the triac 5 according to an on / off signal output from the control circuit 7. The photocoupler 8 includes a light receiving portion 8b formed of a phototransistor or the like.

The DC-DC converter 6 has a primary winding NP.
And a transformer 9 having three secondary windings NS1, NS2, NS3, and a primary DC power input from the smoothing capacitor 3 which forms a series circuit with the primary winding NP of this transformer 9 and is a control circuit. Connected to a switching element 11 composed of a transistor that turns on / off in response to a control signal from 7, a rectifying / smoothing circuit 12 and 13 respectively connected to secondary windings NS1 and NS2 of a transformer 9, and a secondary winding NS3. And the rectifying circuit 14 that has been operated. The rectifying / smoothing circuit 12 includes a rectifying diode D1 and a commutation diode CD1.
And a choke coil L1 and an electrolytic capacitor C1. The stabilized DC power charged in the electrolytic capacitor C1 is further stabilized to DC 5V by the three-terminal regulator 15 and supplied to the load LD1 as a control power supply. Is. The rectifying / smoothing circuit 13 includes a rectifying diode D2, a commutation diode CD2, and a choke coil L2.
And the electrolytic capacitor C2, and the DC power charged in the electrolytic capacitor C2 is
It is supplied to the load LD2 as a driving power source of C24V. This output voltage is fed back to the control circuit 7 as an output voltage signal, and the inter-terminal voltage of the low resistance R2 provided on the ground side of the rectifying and smoothing circuit 13 is fed as an output current signal. The rectifier circuit 14 is composed of a rectifier diode D3 and a limiter resistor R3, and the direct current (gate current) rectified by the rectifier diode D3 is supplied to the gate electrode of the triac 5 via the limiter resistor R3 and the light receiving portion 8b of the photocoupler 8. Are connected to each other.

The control circuit 7 outputs a control signal in which the duty ratio is changed to the transistor 11 in accordance with the fed back output voltage signal, so that even if the voltage of the AC power supply 1 or the load LD2 changes, the control circuit 7 can be used for driving. The output voltage of the power supply is stabilized to 24V, the fed back output current signal is detected, and when the output current is less than or equal to a predetermined value, the light emitting portion 8a of the photocoupler 88 remains off, and the output current is kept at the predetermined value. When it exceeds, the light emitting unit 8a is controlled to be turned on. Although the voltage between the terminals of the electrolytic capacitor C1 slightly changes due to the fluctuation of the ON time of the transistor 11 and the voltage fluctuation of the AC power supply 1 due to the control signal whose duty ratio changes, the output voltage as the control power supply is changed by the three-terminal regulator 15. Stabilized to 5V.

While the output current flowing through the load LD2 of the driving power source is small and the light emitting portion 8a of the photocoupler 8 is off, the light receiving portion 8b thereof is off and the triac 5 remains off. The input current of the switching regulator during this period is a first choke coil 4a for suppressing a harmonic current having a relatively large inductance (large impedance).
Flowing through. When the output current of the driving power source exceeds a predetermined value, the light emitting portion 8a of the photocoupler 8 is turned on, the light receiving portion 8b is turned on and the gate current flows, and the triac 5 is triggered and turned on. The second choke coil 4b for suppressing harmonics, which is arranged in series with the triac 5, has a inductance that is about a fraction of or less than that of the first choke coil 4a, and a capacity that is several times the current rating. Is set to have. In other words, since the impedance of the second choke coil 4b is overwhelmingly small, most of the current flows through the second choke coil 4b.

When the choke coil is used to suppress the harmonic current of the switching regulator to clear the harmonic current regulation, it generally depends on the capacity of the switching regulator (the capacity of the smoothing electrolytic capacitor 3), but in general. It is said that an inductance of 5 to 6 mH is required. This value is only the inductance when the output of the switching regulator is in the rated state. For example, in the case of an analog copying machine, even if the switching regulator during copying operation requires 200 W, most of it is a drive power source and a control source. The power supply is 10% or less of that, and the load current of the control power supply is almost unchanged during standby and copy operation. Therefore, since the capacity of the smoothing electrolytic capacitor 3 is determined by the rated output capacity of the switching regulator, even if the power in the standby state is small, the harmonic current component is large and the harmonic current at this time is suppressed. Requires an inductance of several tens of mH.

On the other hand, when a current is applied to the choke coil, a voltage is generated between its terminals. Ignoring the coil resistance, the voltage is given by:

V = ω · L · I where ω: 2 × π × f (f: frequency Hz) L: inductance (H) I: current (A).

Therefore, the power supply voltage is 100 V and the frequency is 5
When 30 mH is arranged in the first choke coil 4a with 0 Hz, the input power of the switching power supply at standby is 50 W, and the power factor is 0.5, the voltage between the terminals of the choke coil 4a is 9.4 V (= 2 × 3. 14 x 50Hz x 0.03H x
1A). This voltage is drawn from the input 100V and only about 90V is input to the switching regulator. Further, during the copy operation, the second choke coil 4 is operated with the switching power supply input of 200 W and the power factor of 0.5.
When 5 mH is arranged in b, the voltage between the terminals of this choke coil 4b is 6.3 V (= 2 × 3.14 × 50 Hz ×
0.03H × 1A).

On the other hand, when there is no load, that is, when I = 0A, 100V is directly input, so that the input voltage range of the switching power supply is substantially widened.

As described above, when the output current of the switching regulator is below a predetermined value, that is, when the load is light, the first choke coil 4 set to high inductance (small current).
Since the input current of the switching regulator flows through "a" and the second choke coil 4b set to a low inductance (large current) when the value exceeds the predetermined value of the switching regulator, the input current generated by the capacitance of the smoothing capacitor 3 is The phase lead is corrected by the inductances of the first and second choke coils 4a and 4b, the time flowing to the smoothing capacitor 3 is extended, and the power factor is improved.

Generally, the load LD1 of the control power source has a small capacity, that is, a consumed current, and its load fluctuation is small. Therefore, the rectifying / smoothing circuit 12 is a dropper-type stabilizing power source 3
Even if some power loss or heat is generated by the terminal regulator 15, it is insignificant compared with the power consumption of the main body device provided with this switching regulator, for example, the entire copying machine.

On the other hand, the load LD2 of the drive power source is composed of various motors such as a paper transport motor, a scanner motor for driving the document scanning system, solenoids, electromagnetic clutches, etc., and has a large capacity and dynamically changes moment by moment. Tend. Therefore, stabilizing the output voltage of the rectifying / smoothing circuit 13 by the DC-DC converter 6 is effective in improving the overall efficiency.

Here, an example of the fluctuation of the DC power for driving the switching regulator when the switching regulator is provided in the copying machine having the ADF (Automatic Document Feeder) will be described. FIG. 2 is a graph showing an example of fluctuations in DC power for driving a switching regulator provided in a copying machine equipped with an ADF. In FIG. 2, the vertical axis represents output current and the horizontal axis represents time.

Before the time T1, the standby state is established, and only a slight current flows through the cooling fan motor and the like. When the copy start switch is pressed at time T1, the original feeding motor is rotated at a high speed in order to quickly feed the original to a predetermined position, so that the output current of the driving power supply greatly increases and the first peak P1 is reached. Form. After time T2,
The high-voltage power supply such as each charge obtained by boosting from the drive power supply, the scanner forward which moves at a slow speed, and the output current used for paper conveyance are relatively small and stable. At time T3, the exposure ends, and the scanner returns at high speed while continuing the sheet conveyance, so that the output current again increases and enters the second peak P2. At time T4, the power consumption is maximized because the document is rapidly ejected, and the third peak P3 is formed. After time T5 when all the operations are completed, the operation returns to the standby state as before time T1.

As is clear from FIG. 2, the driving power of the switching regulator shows a peak for a short time between time T1 and T2 and time T3 and T5, and it goes without saying that the standby is intermittently used for a long time. Even during the continuous copy operation with a short standby, the stable period with relatively low power consumption from time T2 to T3 is the first to third peaks P1 and P3.
It will be seen that it is much longer than the total time of 2, P3.

Therefore, the first choke coil 4a having a high inductance and a small current is used in the standby state where the power consumption is extremely low and is stable between the times T2 and T3, and when the power consumption is large, the first choke coil 4a having a low inductance and a large current is used. Two
The harmonic current is reduced by the choke coil 4b, so that even if the output power fluctuates, the overall efficiency is not reduced and the power factor can be improved. Further, since it is composed of the two choke coils 4a and 4b of high-inductance small current and low-inductance large current, both can be extremely small choke coils,
There is no need to use a large choke coil, and there is no increase in the size of the power supply. Furthermore, in general, when trying to reduce the size without changing the inductance and current capacity of the choke coil, there is a tendency for loss to increase and heat generation to increase. However, since a small choke coil can be used, this drawback can be eliminated.

When the choke coil is intended for AC power of 50 to 60 Hz which is a commercial frequency, the choke coil is large and heavy, and the cost is high.
The switching frequency of the C converter 6 is several tens to several tens.
Since it is as high as 0 KHz, the transformer 9 and the choke coils L1 and L2 provided on the secondary side of the transformer 9 can be extremely small for the electric power. Due to the effect of miniaturizing the choke coil and the reduction of the peak current by improving the power factor at the time of large current, the reverse voltage withstand and maximum allowable current of the diode bridge 2 are small, and the smoothing capacitor 3
The reactive power flowing through the device is reduced and heat generation can be suppressed.

Next, a second embodiment of the present invention will be described.
FIG. 3 is a circuit diagram showing a second embodiment. The same reference numerals are given to substantially the same components as those shown in FIG. 1, and detailed description thereof will be omitted.

The second embodiment differs from the first embodiment in that in the first embodiment, the first choke coil 4a for suppressing the harmonic current and the harmonic current suppressing for bypassing the first choke coil 4a are used. The second choke coil 4b and the series circuit of the triac 5 for changeover switch are connected in parallel and provided between the AC power supply 1 and the AC input terminal of the diode bridge 2, whereas in the second embodiment, the triac is used. In place of 5, the transistor 21 constitutes a changeover switch, and the first choke coil 4a and the second choke coil 4a
The choke coil 4b and the series circuit of the transistor 21 are provided in parallel between the DC output terminal of the diode bridge 2 and the smoothing capacitor 3. this is,
Since the front side of the diode bridge 2 is the AC circuit side and the rear side is the DC circuit side, the bidirectional triac 5 is used as a changeover switch in the first embodiment, and the unidirectional one is used in the second embodiment. Transistor 21
In addition, a unidirectional thyristor may be used for the transistor 21 which is the changeover switch.

That is, by providing a circuit composed of two choke coils and a changeover switch thereof between the AC power supply 1 and the smoothing capacitor 3, the effect described in the first embodiment can be obtained. It does not matter before and after the diode bridge 2 which is a rectifying element. This is advantageous in that the layout of the switching power supply can be selected according to the mounting position of the choke coil.

Further, as the changeover switch, the changeover may be performed by a contact of a relay. In this case, the secondary winding NS3 of the transformer 9 in the first and second embodiments, the rectifying circuit 14, the light emitting unit 8a and the light receiving unit 8 are used.
The photocoupler 8 including b and the switch for switching the triac 5 and the transistor 21 are not required, and the relay can be driven only by the secondary circuit, and the contact of the relay may be arranged in the switch portion of the primary circuit. By doing so, the circuit can be simplified and downsized, and the cost can be reduced.

In each of the above-mentioned embodiments, the case where the switching regulator is incorporated in the copying machine has been described as an example, but an image forming apparatus for forming an image on plain paper by the electrostatic latent image technique, that is, a digital copying machine, a laser. Of course, it can be applied to a power supply device such as a printer or a facsimile. Other devices can also be used as a DC power supply device for devices in which the ratio of the peak power consumption time is relatively shorter than the normal power consumption time.

[0041]

As described above, according to the first aspect of the present invention, the first choke coil for suppressing the harmonic current is provided between the AC power supply and the rectifying element, and the harmonic for bypassing the first choke coil. The second choke coil for current suppression and the series circuit of the changeover switch are connected in parallel, and the changeover switch is turned off when the output current of the secondary DC power is below a predetermined value, and exceeds the predetermined value. Since it is turned on at the time, it is possible to suppress the harmonic current and improve the power factor under a fluctuating load with a relatively simple configuration without increasing the size of the power supply device. Can also improve VA efficiency.

According to a second aspect of the present invention, a first choke coil for suppressing a harmonic current is provided between the rectifying element of the AC power supply and the smoothing capacitor, and a harmonic choke coil for bypassing the first choke coil. The second choke coil and the series circuit of the changeover switch are connected in parallel, and the changeover switch is turned off when the output current of the secondary DC power is below a predetermined value, and turned on when the output current exceeds the predetermined value. I did so,
As described above, the effect of claim 1 can be achieved to suppress the harmonic current in the fluctuating load and to improve the power factor with a relatively simple configuration without increasing the size of the power supply device, and overall
In addition to the effect that the A efficiency can be improved, either the arrangement according to the invention of claim 1 or the arrangement according to the invention of the present claim can be selected according to the position where the choke coil can be attached. The degree of freedom can be increased.

According to the invention described in claim 3, since the changeover switch in claims 1 and 2 is constituted by a relay,
The circuit configuration can be further simplified or downsized than the invention according to claim 1 or 2, and as a result, the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a switching regulator of the present invention.

FIG. 2 is a graph showing an example of fluctuations in DC power for driving a switching regulator provided in a copying machine with an ADF.

FIG. 3 is a circuit diagram showing a second embodiment of the switching regulator of the present invention.

FIG. 4 is a circuit diagram showing an example of a conventional switching regulator.

[Explanation of symbols]

 1 AC power supply 3 Smoothing capacitor 4a First choke coil 4b Second choke coil 5 Triac (for changeover switch) 6 DC-DC converter 7 Control circuit 8 Photo coupler 9 Transformer 11 Switching element 12, 13 Rectifying smoothing circuit 14 Rectifying circuit 15 3 terminal regulator 15 21 transistor (for changeover switch) LD1, LD2 load

Claims (3)

[Claims] 1. An AC power input from an AC power supply is rectified and smoothed by a rectifying element and a smoothing capacitor, and the rectified and smoothed primary DC power is turned on and off by a switching element connected in series with a primary winding of a transformer. Then, the secondary DC power obtained by further rectifying and smoothing the power induced in the secondary winding of the transformer is output, and the duty ratio of the switching element is changed according to the output voltage of the secondary DC power. A switching regulator that stabilizes the output voltage by controlling the first choke coil for suppressing a harmonic current between the AC power supply and the rectifying element, and the harmonic current suppression for bypassing the first choke coil. The second choke coil for power supply and the series circuit of the changeover switch are connected in parallel, and the changeover switch is connected to the secondary direct current. Switching regulator power of the output current is turned off when the predetermined value or less, when it exceeds a predetermined value, characterized in that so as to turn on. 2. AC power input from an AC power source is rectified and smoothed by a rectifying element and a smoothing capacitor, and the rectified and smoothed primary DC power is turned on and off by a switching element connected in series with a primary winding of a transformer. Then, the secondary DC power obtained by further rectifying and smoothing the power induced in the secondary winding of the transformer is output, and the duty ratio of the switching element is changed according to the output voltage of the secondary DC power. A switching regulator that stabilizes the output voltage by controlling the first choke coil for suppressing a harmonic current between the rectifying element of the AC power supply and the smoothing capacitor, and a bypass coil for bypassing the first choke coil. The second choke coil for suppressing harmonic current and the series circuit of the changeover switch are connected and connected in parallel, and the switching is performed. Said the switch 2
A switching regulator characterized in that when the output current of the next DC power is below a predetermined value, it is turned off, and when it exceeds a predetermined value, it is turned on. 3. The switching regulator according to claim 1, wherein the changeover switch comprises a relay.