JPH11178342A - Power supply unit, electronic apparatus, and step-down type rectification/smoothing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply unit, capable of configuring an overvoltage requiring a small amount of power loss and cost-up, in a combination of its step-down type rectification circuit with its post-stage DC/DC converter. SOLUTION: This unit is provided with an overvoltage sensing circuit 117 for outputting a stopping signal CS by sensing the state in which an output voltage of a step-down chopper circuit 112 following the output of a step-down type active filter circuit exceeds its preset value, a DC/DC converter 113 fixes a switching element 113a at an off-state, when th stopping signal CS is received to stop its operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down rectifying / smoothing circuit effective for reducing harmonic components, a power supply device having a DC / DC converter connected to a stage subsequent to the step-down rectifier circuit, and this power supply device. The present invention relates to an electronic device such as a copier or a printer equipped with a computer.

[0002]

2. Description of the Related Art In recent years, problems such as malfunctions of various electronic devices due to a distortion waveform (power supply harmonic distortion) of a commercial power supply line have become more serious. In order to suppress the occurrence of such harmonic distortion, a law regulating the harmonic components of an input current input to an electronic device is being developed.

In general, a power supply input circuit (switching power supply) of an electronic device employs a capacitor input type rectifier provided with a capacitor 52 on the output side of a diode bridge 51 as shown in FIG.

Therefore, only at the phase near the voltage peak of each commercial power supply, current flows from the commercial power supply Vin (AC) to the capacitor 52 through the diode bridge 51,
As a result, the input current flowing from the commercial power supply Vin (AC) into the electronic device has a peak current waveform and contains many harmonic components.

Conventionally, in order to sufficiently reduce such harmonic components, as shown in FIG.
One choke coil 53 is interposed between the diode bridge 51 of the capacitor input type rectifier circuit and
Increasing the conduction angle of the input current, as shown in FIG.
There has been proposed a method of obtaining an input current waveform similar to an input voltage waveform by using a step-up active filter circuit as a power supply input circuit.

In FIG. 8, 61 is a diode bridge, 67 is a flywheel diode (hereinafter simply referred to as a diode), 64 and 65 are choke coils,
62, 63, 68 are capacitors, 66 is an FET, 60
a, 60b, 69a, 69b are resistors, 71 is a current detection circuit, 72 is a first voltage detection circuit, 73 is a second voltage detection circuit, 74 is a multiplier, 75 is an error amplifier circuit, and 76 is PWM.
(Pulse width modulation) control circuit.

However, according to the method shown in FIG. 7, the inductance of the choke coil 53 is
About 10 to 10 mH is required, and to achieve this, there is a problem that the choke coil 53 becomes large and it is difficult to reduce the size and weight of the circuit. This point will be specifically described.

[0008] S1 to S3 in FIG. 9 are output voltages (rectified voltages) Vo (voltages after the rectification) with respect to the input voltage Vin (AC) obtained when the load power Po is 200 W in the power supply input circuit shown in FIGS. DC) and S in FIG.
1 to S3 are input voltages Vin applied to the power input circuit.
Shows the relationship between the output voltage Vo (DC) and the load power Po obtained when the voltage is 230 V.

In both figures, S1 shows the characteristic when the capacitor input type rectifier shown in FIG. 6 is used.
S2 shows the characteristics when one choke coil 53 is added to the capacitor input type rectifier circuit shown in FIG. 7, and S3 shows the characteristics when the boost type active circuit shown in FIG. 8 is used. S4 will be described later.

As apparent from FIGS. 9 and 10, in the circuit in which the choke coil 53 is added, the load power Po
Increases, or when the input current Iin increases due to a decrease in the input voltage Vin, the output voltage Vo decreases significantly due to the impedance of the choke coil 53. For this reason, it is necessary to extend the operating range of the DC / DC converter connected to the subsequent stage to a low input voltage or to increase the capacity of the capacitor 52 in order to cope with an instantaneous power failure. Post-stage DC / D
There has been a problem that the C converter is increased in size.

According to the method shown in FIG. 8, the current .DELTA.I flowing through the choke coil 65 when the transistor 66 is in the on state is represented by Ton during the period in which the transistor 66 is in the on state, and the inductance of the choke coil 65. Is represented by L65, and is represented by the following equation (1).

ΔI = Vin / L65 * Ton (1) As is apparent from equation (1), in order to reduce the load on the transistor 66, it is necessary to increase the inductance of the choke coil 65 (generally, A choke coil of about several mH is used). Therefore, the size of the choke coil 65 or the current capacity of the transistor 66 must be increased. Further, since energy is stored in the choke coil 65 only when the transistor 66 is on, the peak value of the current flowing through the transistor 66 must be increased.

Further, in order to realize a boost type active circuit, a complicated control circuit is required as shown in FIG. Therefore, even if the above method is used, there is a problem that the efficiency is reduced due to the complexity of the power input circuit, the power input circuit is enlarged, the noise is increased, and the cost is significantly increased.

In order to solve these problems, a step-down active filter circuit as shown in FIG. 11 has been proposed. In FIG. 11, the step-down active filter circuit has a diode bridge 110, which is connected to a commercial power supply Vin. The output terminal of the diode bridge 110 is the choke coil 1
11a and a low-pass filter 111 including capacitors 111b and 111c, a step-down converter including a switching element such as an FET, a transistor 112a, a flywheel diode (hereinafter simply referred to as a diode) 112b, a smoothing capacitor 112c, and a choke coil 112d. It is connected to a circuit (not shown) such as a DC / DC converter at the subsequent stage via the mold chopper circuit 112.

In this step-down active filter circuit, the inductance value of the choke coil 112d is:
The choke coil 112d is set to be operable in the discontinuous current mode even at the maximum load.

The output terminal of the step-down chopper circuit 112 is connected to the input terminal of the differential amplifier circuit 114, and the output terminal of the differential amplifier circuit 114 is connected to one connection terminal of the error amplifier circuit 115. The other input terminal of the error amplifier circuit 115 is connected to the reference voltage Vref, and the output terminal is connected to a PWM (pulse width modulation) control circuit 116.

Output voltage Vo of step-down chopper circuit 112
Is input to the error amplifier circuit 115 via the differential amplifier circuit 114, and is compared with the reference voltage Vref. Then, a signal corresponding to the difference between the output voltage Vo and the reference voltage Vref is
The signal is input from the error amplification circuit 115 to the PWM control circuit 116. The PWM control circuit 116 inputs a rectangular wave signal having an on / off ratio according to the output signal of the error amplification circuit 115 to the transistor 112a of the step-down chopper circuit 112. Thus, the output voltage Vo is feedback-controlled so as to take a constant value corresponding to the reference voltage Vref.

In the above configuration, the voltage Vs after rectifying the commercial power supply Vin by the diode bridge 110 (hereinafter referred to as rectified voltage) is input to the low-pass filter 111, and the output of the low-pass filter 111 is supplied to the step-down chopper circuit 112. Is entered. The transistor 112a is controlled at a predetermined frequency sufficiently higher than the frequency of the input commercial power supply Vin and at a predetermined on-duty so that the output voltage Vo of the step-down chopper circuit 112 takes a predetermined value.

With such an operation, the rectified voltage V by the diode bridge 110 as shown in FIG.
s gradually increases, and the current Is shown in FIG. 12B flows during a period during which the rectified voltage Vs is higher than the output voltage Vo, that is, between the time t1 and the time t2 and between the time t3 and the time t4 in FIG. . That is, the conduction angle of the input current is Vs
> Vo, the output voltage Vo is set to an appropriate value (for example, Vo <250 for a 230 V input system).
By setting V), harmonic current can be suppressed.

S4 in FIG. 9 is a relation between the output voltage (rectified voltage) Vo (DC) and the input voltage Vin (AC) obtained when the load power Po is 200 W in the power supply input circuit shown in FIG. S4 in FIG. 10 shows the relationship between the load voltage Po and the output voltage Vo (DC) obtained when the input voltage Vin is 230 V.

As is apparent from both figures, the output voltage Vo
Is controlled to take a constant value even when the load power Po changes, and to take a constant value even when the input voltage Vin is higher than a predetermined value (190 V in the illustrated example). When the input voltage Vin is equal to or lower than a predetermined value, the transistor 112a is controlled to be always on, so that the transistor 112a exhibits characteristics similar to those of the conventional capacitor input rectifier circuit.

The operation of the power supply input circuit shown in FIG. 11 will be described below with reference to FIG. Note that FIG.
FIG. 5 is a diagram showing changes in the output current Is of the diode bridge 110 and the current IL flowing through the choke coil 112d.

At time t1 in FIG. 13, when Vs> Vo, current IL starts to flow through choke coil 112d.
For example, when the transistor 112a is turned on during the period Ton from t2 to t3, the choke coil 112d
The current IL flowing through increases. Transistor 11 at t3
When 2a is turned off, the current IL flowing through the choke coil 112d gradually decreases and becomes zero at t4 when Toff 'has elapsed from t3. At t3, the transistor 11
After turning off 2a, the period Toff (Toff> T
off ′) at time t5, when the transistor 112
a is turned on again, and the same operation as after t2 is repeated.

The choke coil 112 thus changed
The current IL flowing through d is obtained as follows.

[Period of Vs ≦ Vo] Transistor 112
When a is in the ON state, the current IL flowing through the choke coil 112d and the transistor 112a becomes IL = 0. Even when the transistor 112a is off, the current IL becomes zero.

[Period of Vs> Vo] Transistor 112
When a is in the ON state, the current IL flowing through the choke coil 112d and the transistor 112a is expressed as: IL = {(Vs−Vo) / Ld} * T where Ld is represented by the inductance of the choke coil 112d and increases with time T. I do. Its peak value Ip
Is Ip = {(Vs−Vo) / Ld} * Ton.

On the other hand, when the transistor 112a is off, the choke coil 112d and the diode 112d are turned off.
The current IL flowing through b is represented by IL = Ip− {Vo / Ld * T}, and decreases with time T. Therefore, the current I
The time Toff 'until the value of L becomes 0 is represented by Toff' = {(Vs-Vo) / Vo} * Ton.

In the step-down active filter circuit, as shown in FIG.
f ′ <Toff (where Toff is the transistor 11
2a is turned off), that is, the inductance value of the choke coil 112d is determined so that the choke coil 112d can operate in the discontinuous current mode even at the maximum load. As a result, all the energy stored in the choke coil 112d is released in each cycle.

The value of the input current Is is determined by the
a is a value obtained by filtering the current flowing through the low-pass filter 111,

[0030]

## EQU1 ## Is = 1 / T * [(1/2) * Ton * Ip] = Ton ＾ 2 / (2 * T * Ld) * (Vs-Vo)

The input current waveform is the input voltage (rectified voltage)
It is proportional to the difference (Vs-Vo) between Vs and the output voltage Vo.
For example, when the rectified voltage Vs is a sine wave, by setting an appropriate value as the output voltage Vo (Is is
The conduction angle of the input current can be set to any value.

The duty at which the transistor 112a is turned on depends on the choke coil 1 as described above.
It is preferable that the duty is set small enough to discharge all the energy stored in the rectifier circuit 12d, and the duty is set according to the load power applied to the power supply input circuit to which the rectifier circuit is applied, or It is preferable that the setting be made in accordance with the input current to the power supply.

The above-described step-down active filter circuit has the following excellent features.

(1) The conduction angle of the input current can be set only by setting the reference voltage Vref to an appropriate value (only by setting the rectified voltage Vo to an appropriate value).
Harmonics can be suppressed with a simpler configuration than that shown in FIG.

(2) As shown in FIG. 9, when the input voltage is low, the characteristics are the same as those of the capacitor input type rectifier circuit. Therefore, as shown in FIG. 7, only a choke coil is added to the input. In addition, it is possible to prevent the output voltage Vo from decreasing (S2) due to an increase in load power. Therefore, it is not necessary to extend the input voltage range of the DC / DC converter or the like connected at the subsequent stage to the low voltage side, and it is not necessary to increase the capacity of the smoothing capacitor as a measure against an instantaneous power failure.

(3) Since the output voltage Vo is controlled so as to be lower than the peak value of the input voltage, the upper limit value of the output voltage Vo can be set lower than that of the capacitor input type rectifier circuit. The input voltage range of the connected DC / DC converter or the like can be narrowed. Therefore, an element having a low withstand voltage can be used as an element such as a DC / DC converter connected at the subsequent stage, and the element can be reduced in size, reduced in noise, and reduced in cost.

(4) When the step-up type active filter circuit shown in FIG. 8 is used, the voltage applied to the choke coil 65 is Vs, but according to the step-down type active filter circuit, the voltage applied to the choke coil 112d is (V
s-Vo). Therefore, since the inductance value of the choke coil 112d can be reduced, the size and cost of the circuit can be reduced.

(5) The control circuit can be simplified as compared with the boost type active filter circuit.

(6) Since the transistor 112a is a serial element of the rectifier circuit, the rectifier circuit can be used as an inrush current limiting circuit using the transistor 112a. Therefore, a power device such as a triac or a thyristor can be omitted as an inrush current limiting circuit required for a power supply input circuit using a conventional capacitor input type rectifier circuit or a boost type active filter circuit.

(7) Since the configuration of the power supply circuit is simplified,
Noise can be reduced.

[0041]

However, the power supply device using the conventional step-down type active filter circuit has the following problems.

Generally, when a commercial power supply of 230 V is input,
Output voltage of step-down active filter circuit is 200V or more
It is controlled to be about 220V. However, in the step-down active filter circuit, if the switching element is fixed to ON at the time of failure of the control circuit or failure of the switching element, for example, the peak value of the input commercial power supply (for example, 230
In the case of a V input, the current is rectified to 230 * 1.4 = 322 V). Therefore, a measure is taken to protect the DC / DC converter in the subsequent stage from overvoltage.

This will be described more specifically. For example, the output voltage of the step-down active filter circuit is controlled by using the PWM control circuit 116 so that the output voltage becomes 200 V.
Considering the case where the / DC converter 113 is designed as a forward type, a voltage of about 400 V is applied to the switching element of the DC / DC converter during normal operation.
Generally, a device having a withstand voltage of 00 V is used.

However, when the step-down active filter circuit fails as described above, the input voltage becomes 322 V, and the switching element of the DC / DC converter has 64 input voltages.
A voltage of 4 V is applied, and the element with a withstand voltage of 500 V is damaged (generally a short mode failure).

Therefore, there is a method of using an element having a withstand voltage of 900 V as a switching element of the DC / DC converter. However, according to this method, the on-resistance of the switching element is increased. , The power loss increases and the size of the device increases, and the cost is disadvantageous.

It is also conceivable to add a switching element such as a triac in series with the commercial power supply line to turn off the triac at the time of overvoltage. Current flows, causing considerable power loss and disadvantageous in cost.

Further, the conventional step-down active filter circuit has a problem that it is very difficult to design a choke coil. That is, when a choke coil is manufactured by winding a core material such as ferrite, the number of turns is reduced in order to obtain an optimum inductance because the magnetic permeability of the core material is large. Then, when the switching element 112a is turned on, the magnetic flux of (input voltage-output voltage) * (on time of the switching element 112a) is received by the choke coil having the small number of turns, and the magnetic flux density of the core material is increased. As a result, the core loss in the core increases considerably, and the temperature rise of the choke coil increases.

If a core material having a low loss and a low magnetic permeability can be solved, there is no suitable material. Also, even if it exists (the same method of providing a large gap in the air core coil and core), the leakage of the magnetic field will increase,
Current flows in the magnetic field, causing excessive copper loss,
It is often inappropriate, such as causing malfunctions of the circuit (both self and others).

It is also conceivable to connect a plurality of choke coils in parallel with each other and use a choke coil manufactured with a relatively large number of turns to reduce the inductance to 1 / N. In this case, the current between the choke coils is reduced. It is difficult to secure a balance, and it is disadvantageous in terms of cost and size.

The present invention has been made in view of the above-described conventional problems, and provides a power supply device that enables an overvoltage protection configuration with less power loss and cost increase in a combination of a step-down rectifier circuit and a DC / DC converter at a subsequent stage. With the goal. It is another object of the present invention to provide an electronic device equipped with the power supply device. Further, it is another object of the present invention to provide a step-down rectifying / smoothing circuit capable of reducing the size, cost, and loss of the circuit.

[0051]

According to a first aspect of the present invention, there is provided a rectifying means for rectifying an AC commercial power supply, a voltage step-down means for stepping down an input voltage rectified by the rectifying means. An output voltage control means for controlling the output voltage of the voltage step-down means to a predetermined value lower than the peak value of the input voltage;
In a power supply device connected with a converter, the operation of the DC / DC converter is stopped when an output voltage of the voltage step-down means, which is an output of the step-down rectifier circuit, becomes higher than a set value. Things.

In a second aspect based on the first aspect, the voltage step-down means has a choke coil and a switching element connected in series to an output side of the rectifying means, and operates the choke coil and the switching element. And the output voltage control means operates at a frequency higher than the frequency of the input commercial power supply and the choke coil can operate in the current discontinuous mode. A switching element control means for controlling the operation of the switching element with a small on-duty, and a current monitoring means for monitoring a current flowing through the switching element of the voltage step-down means, wherein the current flowing through the switching element is zero. When there is no longer a period in which When the output voltage of the pressure step-down means is higher than the set value, it is obtained by the configuration of stopping the operation of the DC / DC converter.

In a third aspect based on the first or second aspect, the operation of the DC / DC converter is stopped by fixing a switching element in the DC / DC converter to OFF.

According to a fourth aspect of the present invention, there is provided a rectifier for rectifying an AC commercial power supply, a voltage step-down means for stepping down an input voltage rectified by the rectifier, and an output voltage of the voltage step-down means based on a peak value of the input voltage. DC / DC at the subsequent stage of the step-down rectifier circuit having output voltage control means for controlling to a low predetermined value
An electronic device comprising a power supply device connected to a converter and performing a predetermined operation using the power supply device as an operating voltage, wherein the power supply device has an output voltage of the voltage step-down means, which is an output of the step-down rectifier circuit. When the value becomes higher than the set value, the operation of the DC / DC converter is stopped.

In a fifth aspect based on the fourth aspect, the voltage step-down means of the power supply device has a choke coil and a switching element connected in series to an output side of the rectifier means. The output voltage control means is configured to step down the input voltage rectified by the rectification means based on the operation of the switching element, and the output voltage control means has a frequency higher than the frequency of the input commercial power supply and the choke coil operates in the current discontinuous mode. A configuration including switching element control means for controlling the operation of the switching element with an on-duty small enough to be operable,
Current monitoring means for monitoring the current flowing through the switching element of the voltage step-down means, when there is no longer a period in which the current flowing through the switching element becomes zero or less than a predetermined value, and an output of the voltage step-down means When the voltage is higher than a set value, the operation of the DC / DC converter is stopped.

In a sixth aspect based on the fourth or fifth aspect, the operation of the DC / DC converter is stopped by fixing a switching element in the DC / DC converter to OFF.

According to a seventh aspect of the present invention, a rectifier for rectifying an AC commercial power supply, a smoothing capacitor connected between a high side and a low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, And a voltage step-down rectifying / smoothing circuit having a switching element for switching a current flowing through the choke coil, wherein the choke coil of the voltage step-down means has two windings wound on the same core with the same number of turns. The two windings of the common mode choke coil are connected in series so that magnetic fluxes generated by each other cancel each other out.

According to an eighth aspect of the present invention, a rectifier for rectifying an AC commercial power supply, a smoothing capacitor connected between the high side and the low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, And a voltage step-down rectifying / smoothing circuit including a voltage step-down unit having a switching element for switching a current flowing through the choke coil, wherein the choke coil of the voltage step-down unit includes:
First and second windings of a common mode choke coil in which two windings are wound on the same core with the same number of turns.
With a polarity such that the magnetic flux generated by each of the first and second windings in the loop of the first winding, the smoothing capacitor and the second winding cancels out,
The voltage step-down means is connected to the high side and the low side, respectively.

According to a ninth aspect of the present invention, a rectifier for rectifying an AC commercial power supply, a smoothing capacitor connected between a high side and a low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, And a voltage step-down rectifying / smoothing circuit having a switching element for switching a current flowing through the choke coil, and a DC / DC converter connected to a subsequent stage of the step-down rectifying / smoothing circuit. A choke coil is obtained by connecting two windings of a common mode choke coil in which two windings are wound on the same core with the same number of turns in series such that magnetic fluxes generated from each other cancel each other.

According to a tenth aspect, a rectifier for rectifying an AC commercial power supply, a smoothing capacitor connected between a high side and a low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, And a voltage step-down rectifying / smoothing circuit having a switching element for switching a current flowing through the choke coil, and a DC / DC converter connected to a subsequent stage of the step-down rectifying / smoothing circuit. The choke coil includes a first winding, the smoothing capacitor, and a first winding and a second winding, which are two windings of a common mode choke coil in which two windings are wound on the same core with the same number of turns. Each of the first and second loops in the second winding loop
Are connected to the high side and the low side of the voltage step-down means, respectively, with polarities such that the magnetic fluxes generated by the windings cancel each other.

[0061]

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

(First Embodiment) FIG. 1 is a block diagram showing a schematic configuration of a power supply device according to a first embodiment of the present invention. Elements common to FIG. 11 are denoted by the same reference numerals. .

This power supply device is constructed by connecting a DC / DC converter 113 at a stage subsequent to the conventional step-down active filter circuit shown in FIG. 11, and further includes a step-down chopper circuit which is an output of the step-down active filter circuit. 11
2 is provided with an overvoltage detection circuit 117 for detecting that the output voltage Vo has become higher than the set value and outputting a stop signal CS. The DC / DC converter 113
Upon receiving the signal, the switching element 113a is fixed to the off state to stop the operation.

Here, in the step-down type active filter circuit, the set value is set to 200 V to 220 V when 230 V is input.
Since it is generally controlled to the V position, for example, 225
V.

The DC / DC converter 113 includes a switching element 113a, a transformer 113b, a diode 11
3c, 113d, a choke coil 113e, a smoothing capacitor 113f, and a control circuit 113 for controlling a switching operation of the switching element 113a.
g. When the control circuit 113g receives the stop signal CS from the overvoltage detection circuit 117,
The switching element 113a is fixed off.

The case where the switching element 112a of the step-down active filter circuit is always turned on due to a failure or the like and the step-down active filter circuit generates an output overvoltage will be described.

For example, the gate signal of the switching element 112a becomes "H" due to a failure of the PWM control circuit 116 or the like.
When the switching element 112a is fixed at the level and the switching element 112a is fixed at the ON state, the step-down active filter circuit becomes the same as the full-wave rectifier circuit, and the input commercial power supply Vi
A rectification smoothing operation is performed on the peak value of n (AC). That is, in the case of 230 V (AC) input, the step-down active filter circuit outputs a voltage of 322 V (DC), and this voltage is supplied as an input voltage of the DC / DC converter 113 in the subsequent stage.

At this time, the overvoltage detection circuit 117 outputs the stop signal CS because the output voltage Vo has become higher than the set value. Control circuit 11 of DC / DC converter 113
3g, upon receiving the stop signal CS, fixes the switching element 113a in the DC / DC converter 113 to OFF to stop the switching operation.

When the switching element 113a of the DC / DC converter 113 is fixed to the off state, the voltage applied to the switching element 113a becomes the input voltage 322V of the DC / DC converter 113 itself. That is, in a commercial 230 V (AC) input, the switching element 11
Even if 2a is fixed in the ON state, 230 * 1.4
= 322V or more is applied to the DC / DC converter 113
Of the switching element 113a.

Therefore, a general element having a withstand voltage of 450 V to 500 V can be used as the switching element 113a of the DC / DC converter 113. That is, for example, a case is considered in which the output voltage Vo of the step-down active filter circuit is controlled to be 200 V by using the PWM control circuit 116, and the DC / DC converter 113 at the subsequent stage is designed as a forward type. Since a voltage of about 400 V (twice the input voltage Vin + α) is applied to the drain of the switching element 113a of the DC / DC converter 113 during normal operation (when the switching element 113a performs a switching operation), Generally, an element having a withstand voltage of 450 V to 500 V is used as the switching element 113a. Switching element 1
FIG. 2 shows the drain voltage waveform of 13a.

A voltage of 322 V (DC) is applied to the smoothing capacitor 112 c of the step-down active filter circuit. However, a certain type of 250 V withstand voltage electrolytic capacitor is applied with a voltage of about 400 V if the load is light. It is also known that it does not break down. This electrolytic capacitor is used as the smoothing capacitor 112c of the present embodiment.

As described above, the power supply device according to the present embodiment stops the switching element of the subsequent DC / DC converter when the overvoltage of the output voltage of the step-down active filter circuit is detected. Of the switching element and the secondary-side rectifier diode can be prevented.

When the power supply device of the present embodiment is used in an electronic device such as a copying machine, when the output overvoltage of the step-down active filter circuit is detected as described above, D
If the C / DC converter is stopped, 5V for device control
The power goes out and the system goes down. At this time, by automatically turning off the main switch, the failure of the step-down active filter circuit does not spread to other blocks of the device.

In the power supply device of this embodiment, unlike the conventional power supply device, no power loss occurs due to the overvoltage protection circuit, and the cost is hardly increased.

(Second Embodiment) FIG. 3 is a block diagram showing a schematic configuration of a power supply device according to a second embodiment of the present invention. Elements common to FIG. 1 are denoted by the same reference numerals. .

In the configuration shown in FIG. 1, a zero current detection circuit 118 and an AND gate 119 are provided.

The zero current detection circuit 118 monitors the current flowing through the switching element 112a of the step-down active filter circuit, and when there is no longer a period in which the current becomes zero or a period in which the current becomes smaller than a predetermined value, the "L" level is obtained. Is output. When the “L” level signal CG from the zero current detection circuit 118 and the “H” level signal (at the time of overvoltage detection) from the overvoltage detection circuit 117 are input, the AND gate 119 controls the control circuit 113g of the DC / DC converter 113. To output a stop signal CS. Thus, the switching element 113a of the DC / DC converter 113 is fixed to off.

That is, the drain current of the switching element 112a of the step-down active filter circuit is monitored, and when the current becomes continuous (that is, when the switching element 112a is fixed to ON), When the output voltage Vo becomes equal to or higher than a predetermined value, an abnormal voltage is generated in the output of the step-down active filter circuit (for example, the transistor 112).
a is fixed (ON state of a), and the DC /
The switching element of the DC converter 113 is fixed to off.

As a result, an increase in the voltage of the smoothing capacitor 112c of the step-down active filter circuit can be detected promptly and reliably, so that the switching element 113a of the DC / DC converter 113 at the subsequent stage can be detected even instantaneously. Abnormality can be detected and stopped without applying overvoltage. Further, the overvoltage detection point can be set near the step-down rectified voltage during normal operation (for example, about 205 V) without worrying about the malfunction of the detection circuit 117.

As described above, in this embodiment, as a means for detecting an output overvoltage, a method for detecting that the period during which the current flowing through the switching element of the step-down active filter circuit becomes zero disappears, and a method for detecting the step-down active filter circuit. Since the method is combined with the method of directly detecting the output voltage, the response of the overvoltage protection means can be made quick and no malfunction occurs.

(Third Embodiment) FIG. 4 is a block diagram showing a schematic configuration of a power supply device according to a third embodiment of the present invention. Elements common to FIG. 11 are denoted by the same reference numerals. .

In the present embodiment, in the step-down active filter circuit shown in FIG. 11, the common mode choke coil 12 in which two windings are wound with the same number of turns on the same core as the choke coil of the step-down chopper circuit 112 is used.
1, two windings are connected in series such that magnetic fluxes generated by each other cancel each other.

That is, as the common mode choke coil 121, a common mode choke coil using a high magnetic permeability core material (for example, for power line noise suppression) is used. The inductance utilizes the leakage inductance, and the amount of the leakage magnetic field is made smaller than that of the air-core coil.

Usually, a common mode choke coil is
Two windings (two in a single-phase system, three in a three-phase AC system) are wound on the same core by the same number of turns. As a general application, as shown by 120 in FIG. 4, respective windings are inserted in series to the L / N poles of the commercial power supply line, the inductance value is almost zero in normal mode, and large in common mode. It is used as a noise filter by connecting it to a polarity where inductance can be seen.

However, the two windings are not 100% coupled through the core, and the inductance of the normal mode component slightly remains. This is called leakage inductance. The leakage inductance has a very small impedance with respect to the frequency of the commercial power supply in the case of the above-mentioned filter, but is sufficient for use in a step-down active rectifier circuit at about 100 KHz or more.

Also, as a feature of the leakage inductance,
(1) It is several times larger than the air core inductance obtained by removing the core from the common mode choke coil,
(2) Since the magnetic flux for the normal mode is canceled in the core, the iron loss is almost zero. (3) Since it is a closed magnetic circuit, the leakage magnetic field is smaller than an air-core coil or a coil using a large gap core.

In the present embodiment, since the common mode choke coil having such characteristics is used as a choke coil in the step-down active filter circuit, equivalently low loss (less iron loss of the core) and low magnetic permeability In addition, the choke coil has a small leakage magnetic field and exhibits ideal characteristics. Furthermore, in the case of a common mode choke coil using a core material having high permeability, the number of turns of the windings can be reduced to obtain sufficient leakage inductance, and copper loss in the windings is reduced.

(Fourth Embodiment) FIG. 5 is a block diagram showing a schematic configuration of a power supply device according to a fourth embodiment of the present invention. Elements common to those in FIG. 11 are denoted by the same reference numerals.

In the present embodiment, in the step-down active filter circuit shown in FIG. 11, as the choke coil of the step-down chopper circuit 112, the common mode choke coil 13 in which two windings are wound around the same core with the same number of turns.
0 is used.

This common mode choke coil 130
The magnetic flux generated by each of the first and second windings in the loop of the first winding, the smoothing capacitor 112c and the second winding is divided into two windings, the first and second windings. Step-down chopper circuit 112 with polarity that cancels out
These are connected to the high side and the low side, respectively.

By inserting the common mode choke coil 130 as described above, an effect as a common mode filter such as a DC / DC converter at the subsequent stage is generated, and the amount of a noise filter to be inserted into a commercial power supply circuit can be reduced. it can.

[0092]

As described above in detail, according to the power supply device of the first invention, the overvoltage protection means for the case where the switching element of the step-down rectifier circuit fails or the output voltage control means malfunctions is reduced. It can be realized at low cost and low cost.

According to the power supply devices of the second and third inventions, overvoltage can be quickly and reliably detected. As a result, it is possible to detect an abnormality and stop without applying an overvoltage to the switching element of the DC / DC converter at the subsequent stage even at an instant.

According to the electronic devices of the fourth to sixth aspects, it is possible to prevent the failure of the step-down active filter circuit from spreading to other blocks of the device.

According to the step-down rectifying / smoothing circuit and the power supply device according to the seventh to tenth aspects of the present invention, the choke coil of the voltage step-down means is constituted by using the common mode choke coil. A choke coil with a small leakage magnetic field can be obtained, and the circuit can be reduced in size, cost, and loss can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of a power supply device according to a first embodiment of the present invention.

FIG. 2 is a drain voltage waveform diagram of a switching element in a DC / DC converter.

FIG. 3 is a block diagram illustrating a schematic configuration of a power supply device according to a second embodiment of the present invention.

FIG. 4 is a block diagram illustrating a schematic configuration of a power supply device according to a third embodiment of the present invention.

FIG. 5 is a block diagram illustrating a schematic configuration of a power supply device according to a fourth embodiment of the present invention.

FIG. 6 is a circuit diagram showing a conventional power supply input circuit (capacitor input type rectifier circuit).

FIG. 7 is a circuit diagram showing a conventional power supply input circuit.

FIG. 8 is a circuit diagram showing a conventional power supply input circuit (boost type active filter circuit).

FIG. 9 is a characteristic diagram of a conventional power supply input circuit.

FIG. 10 is a characteristic diagram of a conventional power supply input circuit.

FIG. 11 is a circuit diagram of a conventional step-down active filter circuit.

FIG. 12 is a waveform chart showing an operation of the circuit of FIG. 11;

FIG. 13 is a diagram showing changes in an output current Is of a diode bridge and a current IL flowing through a choke coil.

[Explanation of symbols]

 Reference Signs List 110 diode bridge 111 low-pass filter 112a switching element 112d choke coil 112 step-down chopper circuit 113 DC / DC converter 113a switching element 114 differential amplifier circuit 115 error amplifier circuit 116 PWM (pulse width modulation) control circuit 117 overvoltage detection circuit 118 zero current Detection circuit 121 Common mode choke coil

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H02M 3/155 H02M 3/155 H

Claims (10)

[Claims] 1. A rectifying unit for rectifying an AC commercial power supply, a voltage step-down unit for stepping down an input voltage rectified by the rectifying unit, and a predetermined value lower than a peak value of the input voltage to output voltage of the voltage step-down unit. And a DC / DC converter connected to a subsequent stage of the step-down rectifier circuit having output voltage control means for controlling the output voltage of the step-down rectifier circuit. Above the DC / DC value.
A power supply device characterized in that the operation of the converter is stopped. 2. The voltage step-down means has a choke coil and a switching element connected in series to an output side of the rectification means, and an input rectified by the rectification means based on the operation of the choke coil and the switching element. The output voltage control means controls the operation of the switching element at a frequency higher than the frequency of the input commercial power supply and with an on-duty that is small enough to allow the choke coil to operate in the current discontinuous mode. A current monitoring means for monitoring a current flowing through the switching element of the voltage step-down means, wherein a period during which the current flowing through the switching element becomes zero or a period during which the current flowing through the switching element becomes smaller than a predetermined value is provided. When the output voltage is reduced, and the output voltage of the voltage step-down unit is higher than a set value. 2. The power supply device according to claim 1, wherein the operation of the DC / DC converter is stopped at the time. 3. The power supply device according to claim 1, wherein the operation of the DC / DC converter is stopped by fixing a switching element in the DC / DC converter to OFF. 4. A rectifying unit for rectifying an AC commercial power supply, a voltage step-down unit for stepping down an input voltage rectified by the rectifying unit, and an output voltage of the voltage step-down unit to a predetermined value lower than a peak value of the input voltage. An electronic apparatus comprising: a power supply device configured by connecting a DC / DC converter to a subsequent stage of a step-down rectifier circuit having output voltage control means for controlling the power supply device; and performing a predetermined operation using the power supply device as an operating voltage. When the output voltage of the voltage step-down means, which is the output of the step-down rectifier circuit, becomes higher than a set value,
An electronic device, wherein the operation of the converter is stopped. 5. The voltage step-down means of the power supply device has a choke coil and a switching element connected in series to an output side of the rectification means, and the rectification means operates based on the operation of the choke coil and the switching element. The output voltage control means is configured to step down the rectified input voltage at a frequency higher than the frequency of the input commercial power supply and with an on-duty that is small enough to allow the choke coil to operate in the current discontinuous mode. A switching element control means for controlling an operation of the element; a current monitoring means for monitoring a current flowing through the switching element of the voltage step-down means; and a period during which the current flowing through the switching element becomes zero or a predetermined value. When there is no longer a period during which the voltage decreases, and the output voltage of the voltage 5. The electronic device according to claim 4, wherein the operation of the DC / DC converter is stopped when the value is higher than a set value. 6. The electronic apparatus according to claim 4, wherein the operation of the DC / DC converter is stopped by fixing a switching element in the DC / DC converter to OFF. 7. A rectifier for rectifying an AC commercial power supply, a smoothing capacitor connected between a high side and a low side on an output side of the rectifier, a choke coil connected to the smoothing capacitor, and the choke coil. And a voltage step-down rectifying / smoothing circuit having a switching element for switching a current flowing through the choke coil of the voltage step-down means.
A step-down rectifying / smoothing circuit comprising: two windings of a common mode choke coil having one winding wound with the same number of windings connected in series so that magnetic fluxes generated by each other cancel each other. 8. A rectifier for rectifying AC commercial power, a smoothing capacitor connected between a high side and a low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, and the choke coil. And a voltage step-down rectifying / smoothing circuit having a switching element for switching a current flowing through the choke coil, wherein the choke coil of the voltage step-down means has two windings wound on the same core with the same number of turns. First and second windings of the mode choke coil
With a polarity such that the magnetic flux generated by each of the first and second windings in the loop of the first winding, the smoothing capacitor and the second winding cancels out,
A step-down rectifying / smoothing circuit connected to the high side and the low side of the voltage step-down means. 9. A rectifier for rectifying AC commercial power, a smoothing capacitor connected between a high side and a low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, and the choke coil. DC step-down rectifying / smoothing circuit having a voltage step-down means having a switching element for switching a current flowing through
In a power supply device configured by connecting a DC / DC converter, the choke coil of the voltage step-down means mutually generates two windings of a common mode choke coil in which two windings are wound on the same core with the same number of turns. A power supply unit connected in series so that magnetic fluxes cancel each other. 10. A rectifier for rectifying an AC commercial power supply,
A smoothing capacitor connected between the high side and the low side on the output side of the rectifier, a choke coil connected to the smoothing capacitor, and a voltage step-down unit having a switching element for switching a current flowing through the choke coil. At the subsequent stage of the step-down rectifying and smoothing circuit
In the power supply device configured by connecting a C / DC converter, the choke coil of the voltage step-down means is a common mode choke coil in which two windings are wound on the same core with the same number of turns. The first and second windings are polarized with a polarity such that the magnetic flux generated by each of the first and second windings in the loop of the first winding, the smoothing capacitor and the second winding cancels out, A power supply unit connected to a high side and a low side of the voltage step-down unit.