JPH0156637B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AC power supply device necessary for the copying process of an electronic copying machine.

In general, electronic copying machines are configured to perform copying through the process of charging, exposing, transferring, and fixing. This is done by applying a high alternating current voltage (for example, 6 KV) of about 100 Hz to cause corona discharge.
The AC high-voltage power supply used for such separation and static elimination uses a so-called PWM switching method to control the output by feeding back current and voltage proportional to the AC output generated on the output side of a fero-resonant transformer. The aim is to stabilize the discharge and make the discharge uniform. However, the electrodes of the charger and drum to which AC high voltage power is applied are wire-shaped for the charger and planar for the drum. On the other hand, the current on the negative side of the AC waveform flows more easily,
As shown in Figure 1, the output waveform is a superimposition of a DC component on AC, and the positive and negative are not balanced, making it impossible to properly separate and eliminate static electricity in the copying process, and as a result, the transfer paper may not come out. However, there are problems in that smooth copying cannot be obtained due to blurred images or the inability to obtain clear images. Therefore, the power supply device for separation and static elimination includes an output control means for stabilizing the output of the AC high voltage, and a DC shift of the output of the AC high voltage to balance the positive and negative of the output waveform to ensure uniform discharge. A device equipped with a shift control means that aims to improve the As this shift control means, a bias winding and an output winding are tightly coupled and wound around a ferro-resonant transformer, and the bias voltage obtained by converting the AC output of the bias winding into DC is applied to the output winding. There are those that control the shift by superimposing it on the AC output of the ferro-resonant transformer, and those that suppress the negative current by inserting a parallel circuit of a diode and a resistor at one end of the output winding of the ferro-resonant transformer.
In the former case, when the output of the output winding is controlled, the output of the bias winding will also change accordingly, so if a constant current type power supply is used, the bias voltage will decrease as the load including capacitance increases. There is a problem that shift control cannot be performed because the specified bias voltage cannot be obtained, and if the winding is selected to obtain sufficient bias voltage even at the minimum rated voltage during constant current output, the number of turns of the bias winding will be Since the number of turns is several times the number of turns of the winding, the bias voltage becomes excessive at light loads, and the capacitors and diode resistors connected to the bias winding must also have high rated values. As a result, losses during light loads increase, power supply efficiency decreases, and stable control becomes difficult, making the device complex, large, and expensive. In addition, in the latter case, since the positive and negative load currents are unbalanced, there is a problem that the iron core is saturated due to DC bias, the input to the iron resonant transformer increases, and the temperature of the winding elements etc. increases significantly. In order to avoid this saturation of the iron core, it is possible to use a transformer with a gap in the iron core, but this increases the excitation current, increases the size of the transformer, and, as mentioned above, increases the input at light loads and causes loss. There is a problem that the efficiency decreases as the amount increases. moreover,
It is possible to make the output waveform of the AC high voltage into a rectangular wave and to vary the duty cycle to achieve a balance between positive and negative, but since the waveform is a rectangular wave and the discharge time is different for positive and negative currents, However, there is a problem in that separation and charge removal by discharge cannot be performed properly.

Taking these problems into account, a power supply section that outputs AC high voltage and a power supply section that outputs DC bias voltage for shifting are provided independently, and the output is controlled to output controlled AC high voltage. There are some devices that shift control the AC high voltage output by superimposing the bias voltage output, but in this case, the output control and shift control can be performed independently, so
Although it has the advantage that shift control is not affected by output control, it requires two transformers and two switching control circuits, making it larger, and in recent years, there has been a demand for higher speed and smaller size. It is difficult to store and arrange the copying machine in the narrow space of a copying machine, and it is also expensive.

The present invention has been made in view of the above-mentioned points, and its purpose is to simplify the shift control of AC high voltage output, to be able to perform it accurately with a structure, and to achieve miniaturization. Our aim is to provide what we have made possible.

In order to achieve the above object, the present invention combines a bias winding with an input winding that interlinks with the magnetic circuit on the input side of a leakage type transformer, and winds the bias winding to form a rectangular waveform proportional to the input winding. By obtaining an output voltage and converting it into DC, the bias voltage is formed so as to be superimposed on the AC high voltage output of the output winding, so that even if the AC high voltage output decreases, it is not affected and is stable. This allows for shift control.

Embodiments of the present invention will be described below with reference to FIGS. 2 to 6. In Fig. 2, 1 is a leakage transformer, and as shown in Fig. 3, two windings interlink with the magnetic circuit on the input side of the iron core, an input winding 1 _a
and shift bias winding _1a , and two windings linked to the output side magnetic circuit via leakage core _1e , output winding _1b and voltage detection winding _1c . It is wrapped. Then, the neutral point of the input winding _1a is connected to the positive terminal of the DC power supply 2, and both ends of the input winding _1a are connected to the negative terminal of the DC power supply 2 through the collectors and emitters of the switching transistors 3 and 4, respectively. By connecting the transistors 3 and 4 to the respective terminals and turning them on and off alternately, the collector current of the input winding _1a is
NC ₁ and NC ₂ are alternately excited to cause the input winding 1 _a to oscillate a rectangular wave alternating current (Fig. 4 1 _a ), which causes the output winding 1 _b to pass through the resonance capacitor 5,
A sinusoidal AC high voltage output corresponding to the turns ratio is generated (FIG. 5, _1b ) and supplied to the load 6. 7 is a switching control circuit configured to alternately send output signals to the bases of the switching transistors 3 and 4 at a constant cycle. This is a standard that sends a voltage (detection signal V _det1 ) proportional to the AC high voltage output of the output winding 1 _b to the non-inverting input terminal, and a reference voltage V _ref1 set from the rated voltage of the load 6. The error amplifier A ₁ inputs the reference voltage V _ref1 of the voltage circuit V _r1 to the inverting input terminal and sends out a signal obtained by amplifying the error of both inputs V _det1 and V _ref1 , and the alternating current flowing through the load 6. The above-mentioned reference voltage V of the reference voltage circuit V r2 is configured to send out a voltage (detection signal V _det2 ) obtained by converting a current proportional to the voltage (detection signal V _det2 ) to the non-inverting input terminal and a reference voltage V _ref2 set from the rated current of the load 6. _ref2 is input to the inverting input terminal, and both these inputs V _det2 and
An error amplifier A ₂ is configured to send out an error amplified signal of V _ref2 , and the output terminals of both amplifiers A ₁ and A ₂ are connected to an inverting input terminal, and a sawtooth waveform output signal is generated at the non-inverting input terminal. Connect the output end of the oscillator OSC to make this oscillator
The sawtooth output signal of the OSC and the amplifier _A1 above
A comparator CP ₁ compares the levels of the higher output signal of A ₂ and A 2 and sends out an output signal in which the pulse width of _the digital signal is modulated. transistor 3,
4 and a driver D _r which alternately sends out an "H" level output signal at a constant period. Reference numeral 8 denotes a voltage detection circuit configured to send a detection signal V _det1 to the non-inverting input terminal of the error amplifier _A1 of the switching control circuit 7. This is done by connecting the input end of a rectifier DB 1, which has diodes bridge-connected, to the voltage detection winding 1 _c of the transformer ₁ , inserting a capacitor C ₁ into the output end of this rectifier DB ₁ via a resistor R ₁ , and connecting the capacitor
Connect the slider of variable resistor VR ₁ to the non-inverting input terminal of error amplifier A ₁ through resistor R ₂ between the terminals of C ₁ , and convert the voltage proportional to the output of output winding 1 _b into DC. This is converted, integrated, and divided to be sent as an AC high voltage output detection signal V _det1 . 9 is a bias circuit, and the bias winding 1 _d
A capacitor C ₂ , a resistor R ₃ , and a photodetector PE consisting of a CDS cell etc. are inserted in series, and a capacitor C ₃ and _a photodetector PE consisting of a CDS cell etc. are inserted between the terminals of the photodetector PE.
Insert a diode whose cathode is connected to the cathode of this diode _D1 , and a diode _D2 whose anode is connected to the cathode of this diode _D1 .
A so-called voltage doubler rectifier circuit is formed by inserting a capacitor C ₄ between the anodes of the capacitor C ₄
Connect the connection point between and diode D ₂ to one end of resonance capacitor 5, ground the other terminal of capacitor C ₄ , and connect it to the ground side terminal of load 6 (drum side in this example) to set the bias voltage. The voltage obtained by differentiating the rectangular wave AC output of the winding 1 _d (Fig. 4 1 _d ) by the capacitor C ₂ and dividing it by the resistor R ₃ and the photodetector PE, and the 1 _d AC output of the bias winding Diode capacitor C ₃ on the negative half cycle of
The above capacitor C ₃ has been charged through D ₁
By charging the capacitor _C4 through the diode _D2 in the positive half cycle by the voltage sum of the charging voltage of
The voltage between the terminals of _C4 is superimposed on the AC high voltage output from the output winding _1b as a DC bias voltage, and when light is incident on the photodetector PE, it is proportional to the amount of incident light. photodetector PE
The resistance value of the bias circuit 9 decreases, and the voltage divided by the resistor R ₃ and the photodetector PE also decreases in a state where it is narrowed down to a large width, so the output of the bias circuit 9 (that is, the bias voltage) also decreases and is superimposed. It's starting to look like this. 10 is a bias voltage detection circuit in which resistors R ₄ and R ₅ are inserted in series between the terminals of the capacitor C ₄ , and the bias voltage is sent out as a detection signal V _B1 from the connection point of resistors R ₄ and R ₅ . ing. Reference numeral 11 denotes a current detector consisting of a shunt or the like inserted between the ground side terminal of the capacitor _C4 and the ground, which converts the current flowing through the load circuit into a voltage and sends it out from its output terminal. 12 is connected from the current detector 11, and the detector 11
This is an AC/DC separation circuit that separates the AC and DC components included in the output of the AC and DC components and outputs the separated components. This means that the load 6 is connected between the output terminal of the current detector 11 and the ground.
The high frequency noise caused by corona discharge between the charger and the drum is sufficiently removed to obtain approximately the effective value.
A so-called low-pass filter is formed by inserting a resistor _R6 with a CR time constant and a capacitor _C5 in series.
Connect the cathode of diode D ₃ through capacitor C ₆ to the connection point of this resistor R ₆ and capacitor C ₅ ,
The anode of this diode D ₃ is connected to the ground side terminal of the capacitor C ₅ , and a capacitor C ₇ is inserted between the cathode of the diode _{D 4 whose} anode is connected to the cathode of the diode D ₃ and the ground. A resistor _R7 and a variable resistor _VR2 are inserted in series between the terminals, and the slider of the variable resistor _VR2 is connected to the non-inverting input terminal of the error amplifier _A2 of the switching control circuit 7. During the positive half-cycle of the AC input, diode D ₃ is conducting, causing capacitor C ₆ to charge to its negative peak value, and then during the positive half-cycle of the AC input, capacitor C ₆ and diode D ₃ The connection point of becomes positive,
Diode D ₄ becomes conductive and capacitor C ₇
and C ₆ are supposed to be connected in series, so
The voltage due to the charge stored in capacitor C ₆ and
The positive voltage of the AC input being applied is the capacitor
By repeating this operation, the capacitor _C7 is charged to a value proportional to the so _- called peak-to-peak value, and this charged voltage is applied to the resistor _R7. Detection signal V _det2 of the output current of alternating current divided by variable resistor VR ₂
and the above resistance
A resistor with a CR time constant that sufficiently removes the alternating current component of the input is connected between the connection point of R ₆ and capacitor C ₅ and ground.
Insert R ₈ and capacitor C ₈ in series, and this resistor R ₈
The connection point between the capacitor C8 and the capacitor _C8 is used as the output terminal, and a detection signal V _B2 of the DC component superimposed on the output current is sent out. 13 is a bias control circuit that controls the bias voltage, and a resistor is connected to the DC power supply 2.
Connect the voltage regulator diode ZD ₁ through R ₉ , and resistor between the cathode and anode of the voltage regulator diode ZD ₁ .
Insert R ₁₀ , variable resistor VR ₃ , and resistor R ₁₁ in series, and output V _s1 set at a voltage corresponding to the shifting DC current (so-called DC bias current) from the slider of variable resistor VR ₃ . The slider of this variable resistor VR ₃ is connected to the inverting input terminal of the error amplifier A ₃ via a resistor R 12, and the AC/DC separating circuit ₁₂ is connected to this inverting input terminal via a resistor R ₁₃ . Connect the connection point of resistor R ₈ and capacitor C ₈ (i.e., the output terminal of the voltage corresponding to the DC bias current),
The non-inverting input terminal of error amplifier A ₃ is connected to the connection point of resistors R ₁₄ and R ₁₅ inserted in series between the cathode and anode of voltage regulator diode ZD ₁ , and the inverting input terminal and output terminal of error amplifier A ₃ are connected to each other. A light emitting device consisting of a resistor R ₁₇ and _{a light emitting diode, etc. is inserted between the cathode of a diode D 5 whose} anode is connected to the output terminal of the error amplifier A ₃ and the ground, with a resistor R ₁₆ and an integrating capacitor C 9 inserted in parallel between them. An error amplifier A for controlling the bias voltage superimposed on the AC high voltage output so that it does not exceed a predetermined value is connected to the anode of _the diode _D6 , in which the element LE is inserted in series and the cathode is connected to the cathode of the diode D5. Connect the output terminal of the error amplifier A ₄ to the inverting input terminal of the error amplifier A ₄ , and connect the resistor R ₁₈ and the variable resistor VR 4 inserted in series between the cathode and anode of the voltage regulator diode ZD ₁ to the inverting input terminal of the error amplifier A ₄ .
Connect the slider of the variable resistor VR ₄ of R ₁₉ via the resistor R ₂₁ , and insert the resistor R ₂₀ and the integrating capacitor C ₁₀ in parallel between the inverting input terminal and the output terminal of the error amplifier A ₄ . , the output terminal of the bias voltage detection circuit 10 is connected to the non-inverting input terminal of the error amplifier A ₄ , and diodes D ₅ and D are made conductive by the higher output of the error amplifier A ₃ and A ₄ . ₆ (i.e. OR circuit) to cause the light emitting element LE to emit light,
As a result, the resistance value of the bias circuit 9 light-receiving element PE is varied according to the amount of incident light, and the capacitor
The bias voltage to be superimposed on the AC high voltage is controlled by narrowing down the voltage differentiated by C ₂ to a large extent by the resistor R ₃ and the resistive voltage division of the light receiving element PE. _{Then ,} the output V _a3 of the error amplifier _{A 3} is expressed _as follows: V _a3 = V _s2 + [ _{−R 16} /R ₁₃ (V _B2 −V _s2 )〕＋
[−R ₁₆ /R ₁₂ (V _s1 −V _s2 )]……(1) Therefore, when setting the shift level depending on the type of load 6, electrode structure, etc., the output V _s2 can be set arbitrarily. By fixing the output V _s1 of the variable resistor VR ₃ to the desired shift level, the output V _a3 of the error amplifier A ₃ can be adjusted to the desired shift level without changing the gain. It has become variable so that In other words, the output V _a3 causes a current to flow through the light emitting element LE so that a desired DC bias current can flow,
The shift level is set by controlling the resistance value of the light receiving element PE. Also, error amplifier A ₄
The output V _a4 of the variable resistor VR 4 is given by V _a4 = R ₀ / R ₂₁ (V _B1 − V _s3 ) (2), but the output V _s3 of the variable resistor VR ₄ is the upper limit of the bias voltage at no load. (That is, the output of the bias circuit 9 when no load is applied) is set based on the above upper limit value and the voltage division ratio of the bias voltage detection circuit 10 that detects this upper limit value. Therefore, under load, the output V _s3 and the output V _B1 of the bias voltage detection circuit 10 are set to have a relationship of V _s3 >V _B1 .

Next, its operation will be explained. Now, when the switching transistors 3 and 4 are turned on and off alternately by the output signal of the switching control circuit 7, the transistors 3 and 4 are connected to the input winding _1a of the transformer 1.
The collector currents of 4 alternately flow to AC excite the transformer 1 (Fig. 4, _1a ), which causes the output winding _{1b to}
A substantially sinusoidal AC high voltage (FIG. 5, _1b ) is generated depending on the turns ratio and the resonance capacitor 5, and is applied to the load 6. At this time, voltage detection winding 1 _c
An approximately sinusoidal AC voltage proportional to the turns ratio (fifth
( _c ) in Fig. 1 is induced, and the voltage detection circuit 8 receives this and rectifies and smoothes the AC input to convert the DC voltage to the resistor _R2.
The voltage divided by the variable resistor VR ₁ is sent to the non-inverting input terminal of the error amplifier A ₁ of the switching control circuit 7 as an AC high voltage output detection signal V _det1 . On the other hand, the load 6 receiving the above AC high voltage
generates a corona discharge between the charger and the drum to separate and eliminate static. At this time, because the electrode structure allows negative current to easily flow, negative direct current is superimposed on alternating current. The current flowing through this load 6 is detected by a current detector 11, and its output is sent to an AC/DC separation circuit 12. The AC/DC separation circuit 12 that receives this is a resistor.
A low-pass filter consisting of R ₆ and capacitor C ₅ removes high frequency noise from the AC input, and during the negative half cycle of the AC input, diode D ₃ is conductive, so that capacitor C ₆ reaches the negative peak value. Then, during the positive half cycle of the AC input, the connection point between capacitor C ₆ and diode D ₃ becomes a positive potential, causing diode D ₄ to conduct.
Since capacitors _C7 and _C6 are connected in series, the voltage due to the charge _stored in capacitor _C6 and the positive voltage of the applied AC input are at the same potential and in phase with each other. By repeating this operation, capacitor C ₇
is charged with a value proportional to the so-called peak-to-peak value of the AC input, and this charging voltage is applied to the resistor.
The voltage divided by R ₇ and the variable resistor VR ₂ is sent to the non-inverting input terminal of the error amplifier A ₂ of the switching control circuit 7 as the output current detection signal V _det2 . At the same time, the AC component of the input is removed by the resistor _R8 and the capacitor _C8 , and is superimposed on the current flowing to the load 6, and the capacitor _C8 is charged by the DC component.
The charging voltage is sent as a DC detection signal V _B2 to the inverting input terminal of the error amplifier A ₃ of the bias control circuit 13 via the resistor R ₁₃ .

The error amplifiers A ₁ and A ₂ of the switching control circuit 7 receive the detection signals V _det1 and V _det2 as follows.
Reference voltage circuit V _r1 , V _r2 input to the inverting input terminal
The outputs obtained by amplifying the errors with respect to the reference voltages V _ref1 and V _ref2 are sent to the inverting input terminal of the comparator CP ₁ , respectively.
Comparator CP ₁ receives this, and error amplifiers A ₁ and A ₂
The level of the higher output of the oscillator OSC is compared with the sawtooth wave output of the oscillator OSC, and a digital signal with a modulated pulse width is sent to the driver D _r . The driver D _r stabilizes the AC high voltage output supplied to the load 6 by alternately turning on and off the transistors 3 and 4 at a constant cycle with a pulse width depending on the input signal.

Next, the shift control operation will be explained. Due to the excitation of the input winding _1a , a rectangular wave AC voltage (FIG. 4 _1d ) is also induced in the bias winding _1d according to the turns ratio, and the bias circuit 9 receiving this
The voltage that is the sum of the voltage differentiated by the capacitor C ₂ and the charging voltage of the capacitor C ₃ charged through the diode D ₁ during the negative half cycle of the AC input is applied to the diode D ₂ during the positive half cycle of the AC input.
This charging voltage is superimposed as a DC bias voltage on _the AC high voltage output from the output winding _1b . This superimposed bias voltage is applied to the resistance of the bias voltage detection circuit 10.
The voltage is divided and detected by R ₄ and R ₅ , and the bias voltage is output to the bias control circuit 13 as the detection signal V _B1 of the bias voltage.
to the non-inverting input terminal of error amplifier _A4 .
Error amplifier _A4 , _which receives this detection signal V _{B1 ,} inverts and amplifies the error with the other input V _s3 and sends out an output V _a4 . has a relationship of V _B1 < V _s3 , so it has a potential of approximately zero V. On the other hand, the error amplifier _A3 receiving the detection signal V _B2 of the AC/DC separation circuit 12 has an input V _B2
The output V _a3 is output by inverting and amplifying the error of both inputs (V _B2 + V _s1 ) and V _s2 , which are the sum _of V s1 and V s1. This output V _a3 is a light-receiving element that obtains the bias voltage necessary to obtain a shift level output that is preset according to the type of load 6 and electrode structure, etc.
This is the voltage that flows the current that causes the light emitting element LE to emit light so as to have the resistance value of PE, and this output V _a3 causes the diode D ₅ to conduct (at this time, the diode D ₆ is non-conducting) and passes through the resistor R ₁₇ . Light emitting element LE
A current is applied to it to cause it to emit light. The light receiving element PE of the bias circuit 9 that receives this light is connected to a capacitor in order to reduce its resistance value according to the amount of incident light.
The voltage differentiated by C ₂ is divided by the resistor R ₃ and the photodetector PE to become a narrowed-down output, which is superimposed on the AC high voltage by controlling the output of the bias circuit 9, that is, the bias voltage, and produces a sinusoidal AC high voltage. Shift control is performed by shifting the voltage to a preset level and balancing the positive and negative voltages. This results in
When the load 6 generates a corona discharge due to the application of an AC high voltage, the state in which negative DC is superimposed on the AC is compensated for, and separation and static elimination are performed.

In the above AC high voltage output control and shift control operations, the transformer 1 has a leakage core between the input winding 1 _a , the bias winding 1 _d and the output winding 1 _b and voltage detection winding 1 _c . Since there is leakage inductance due to 1 _e , the output winding 1 _b
and voltage detection winding _1c become sinusoidal due to resonance, and even if the output (effective value) is controlled and stabilized, the peak value of the output voltage of bias winding _1d will vary by a wide range. There is no significant change, only the pulse width changes (Fig. 4, _1d ), so the peak value of the output of bias winding _1d does not change proportionally due to output control, and the shift The bias voltage required to obtain a DC bias current is a capacitor.
The peak value of the differentiated voltage of C ₂ is connected to the resistor R ₃ and the light receiving element.
Since it is only controlled by the voltage divided by PE, output control and shift control are performed separately and independently. Although the circuit configuration has been described in the above embodiment, as shown in FIG. 6, each of the circuit components described above is arranged in the die space 14 of the transformer 1 and integrally molded with a resin mold. You can do it like this.

According to the present invention, the transformer has an input winding and a bias winding linked to a magnetic circuit on the input side of a leakage type iron core, and an output winding linked to a magnetic circuit on the output side. Since the wire and the voltage detection winding are combined and wound to form a stable DC There is no need to provide a separate transformer to obtain a DC bias voltage as in the conventional case, and it is possible to obtain an AC high voltage output and a DC bias voltage with a single transformer. Moreover, there is no need to install a separate transformer.
A power transistor for exciting this transistor and a switching control circuit for controlling this transistor are also not required, so that the device can be made smaller and more compact. In addition, shift control is performed by a bias circuit connected to a bias winding using a detection signal obtained by extracting the DC component of the output of a current detector that detects the current flowing in the load circuit through a filter element, and a signal obtained by amplifying the error of the reference signal. Since the output of the printer is variably controlled, it can be formed at low cost with a simplified configuration without using expensive power transistors or control ICs, making it suitable for copying machines that require higher speed and smaller size. It can be easily stored and arranged even in the narrow spaces of , and can also improve insulation resistance, making it suitable as a power source for copying machines.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the output waveform when an AC power supply is used for static elimination in a copying machine, etc. Fig. 2 is a block diagram showing an embodiment of the present invention, Fig. 3 is a sectional view of the transformer shown in Fig. 2, Figure 4 is an illustration of the input waveforms of the input winding and bias winding of the transformer in Figure 2, Figure 5 is an illustration of the output waveforms of the output winding and voltage detection winding of the transformer in Figure 2, and Figure 6 is an illustration of the output waveforms of the transformer's output winding and voltage detection winding in Figure 2. FIG. 3 is a schematic layout diagram of FIG. 2; 1: Leakage transformer, 1 _a : Input winding, 1
_b : output winding, 1 _c : voltage detection winding, 1 _d : bias winding, 2: DC power supply, 3, 4: switching transistor, 6: load, 7: switching control circuit, 8: voltage detection circuit, 9: Bias circuit, 1
0: bias voltage detection circuit, 11: current detector,
12: AC/DC separation circuit, 13: Bias control circuit.

Claims (1)

[Claims] 1. A bias winding is coupled and wound around the input winding that interlinks with the magnetic circuit on the input side of the leakage transformer, and this bias winding is inserted into the load circuit connected to the output winding via the bias circuit. , An AC/DC separation circuit is connected to the output end of the current detector inserted in the load circuit, and the AC/DC separation circuit is configured to send out detection signals for the AC and DC components of the current flowing through the load circuit. The present invention is characterized in that the output of the bias circuit is controlled by the error amplified signal of the bias control circuit, which sends out a signal obtained by amplifying the error between the detection signal and the shift reference signal. AC power supply for electronic copying machines.