JPH0783595B2 - Positive and negative output power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a positive power dual output power source used for supplying respective DC voltages to a charging device and a developing device in a copying machine, a laser beam printer or the like. Regarding the circuit.

(B) Conventional Technology In an electrophotographic image forming apparatus such as a copying machine or a laser beam printer, a charging device that charges a photoconductor and a developing device that attaches toner to an electrostatic latent image formed on the photoconductor are provided. It is provided, and conventionally, a single power supply circuit supplies voltage to the charger and the developer, respectively.

An example of a power supply circuit that obtains two outputs with different voltages is shown in FIG. In FIG. 3, N1 is the primary winding of the transformer,
A switching circuit including the switching transistor Q1 is connected. A rectifying / smoothing circuit including diodes D1 and D2 and capacitors C1 and C2 is connected to the secondary winding N2 of the transistor. The voltage obtained by this rectifying / smoothing circuit is output from the output terminal OUT1 through the resistor R1. Resistance between the output terminal OUT1 and the other terminal (ground) of the rectifying and smoothing circuit
A shunt regulator circuit composed of R2 and the regulator circuit RE is connected, and a predetermined DC voltage is output from the output terminal OUT2.

The power supply circuit shown in FIG. 3 can be configured by a relatively simple circuit, but only two voltages having the same polarity can be output to the two output terminals. Next, FIG. 4 shows an example of a conventional power supply circuit that obtains two voltage outputs having different polarities.

In FIG. 4, N3 is a secondary winding provided separately from the secondary winding N2, which is connected to the rectifying diode D3, the smoothing capacitor C3 and the resistor R3 and has a predetermined direct current from the second output terminal OUT2. The voltage is output. Since the direction of the rectifying diode D3 is opposite to that of the diodes D1 and D2, OUT2 is
And a voltage of opposite polarity is generated.

(C) Problems to be Solved by the Invention In an electrophotographic copying machine, a laser beam printer, or the like, a high-voltage generating circuit that supplies a high voltage to a charger that charges a photoconductor and a bias voltage to a developing device. When the developing bias potential has the same polarity as the charging potential of the photoconductor, the power supply circuit shown in FIG. 3 can be used, but the characteristic process of the photoconductor (reversal development, etc.) is required. Therefore, when the developing bias potential has a polarity opposite to the charging potential of the photoconductor, the power supply circuit of the type shown in FIG. 4 must be used.

However, in order to generate such two voltages having different polarities, conventionally, it is necessary to use a transformer having a secondary winding separately provided, and a rectifying / smoothing circuit also has to be provided separately. There is a problem that the size becomes large and the cost becomes high.

SUMMARY OF THE INVENTION An object of the present invention is to provide a positive and negative output power supply circuit which can generate two DC voltages having different voltages from a single secondary winding of a transformer and solve the above-mentioned problems.

(D) Means for Solving the Problem The present invention relates to a primary winding of a transformer, a switching circuit for connecting and disconnecting a current flowing through the primary winding, and a rectifying / smoothing circuit connected to the secondary winding of the transformer. In the switching regulator, a first load is connected between one output terminal of the rectifying / smoothing circuit and ground, and a resistor is connected in parallel to the first load, and the other output terminal of the rectifying / smoothing circuit is grounded. In between, a zener diode is connected in a direction in which the current flowing through the first load flows as a zener current, and a bias voltage is generated from the zener voltage of the zener diode to generate a bias voltage.
It is characterized by supplying to the load of.

(E) Function FIG. 1 shows the configuration of the positive and negative output power supply circuits of the present invention.
A switching circuit 1 for connecting and disconnecting a current flowing through the primary winding N1 is connected to the primary winding N1 of the transformer, and a rectifying / smoothing circuit 2 is connected to a secondary winding N2 of the transformer. In the figure, 3 is a first load, 4 is a second load, and
The load 3 is connected between one output terminal A of the rectifying and smoothing circuit 2 and the ground, and the resistor R4 is connected in parallel to the first load 3. A Zener diode ZD is connected between the other output terminal B of the rectifying / smoothing circuit 2 and the ground, and a variable resistor R5 is connected in parallel to the Zener diode ZD. The second load 4 is connected to the output of the variable resistor R5.

The Zener diode ZD is a current flowing through the first load 3.
Since IL1 is connected in the direction in which it flows as the Zener current Iz, a Zener voltage is generated between the anode and the cathode of the Zener diode ZD. The voltage is divided by the variable resistor R5 and a bias voltage is applied to the second load 4. Even if the first load 3 is opened, the dummy current Ih flowing through the resistor R4 is
Since the current flows through ZD, the supply voltage to the second load can be stabilized against changes in the current flowing through the first load.

In this way, a single secondary winding of the transformer is used to generate a high voltage for the first load and an intermediate voltage of opposite polarity to generate a bias voltage for the second and the load. It becomes possible.

(F) Embodiment FIG. 2 shows a power supply circuit for an electrophotographic image forming apparatus according to an embodiment of the present invention.

In FIG. 2, Q1 is a switching transistor that interrupts the current flowing through the primary winding N1 of the transformer, and is turned on / off by a rectangular wave signal output from the switching control circuit 5.
Controlled off. Diode in the secondary winding N2 of the transformer
A rectifying / smoothing circuit composed of D1 and D2 and capacitors C1 and C2 is connected, and the first output terminal A is connected via a resistor R1.
Generates a positive high voltage at the output terminal OUT1 of. 3 in the figure
Is a charger (first load), which is connected between the output terminal OUT1 and the ground. Also, a resistor R4 is connected between the output terminal OUT1 and ground. A parallel circuit of a Zener diode ZD and a variable resistor R5 is connected between the other output terminal B of the rectifying and smoothing circuit and the ground. Further, the variable output of the variable resistor R5 is led to the output terminal OUT2, and the developing device (second load) 4 is connected to this terminal OUT2 as a load.

The operation of the circuit shown in FIG. 2 is as follows. The switching control circuit 5 turns on / off the switching transistor Q1.
By performing the off control, the current flowing in the primary winding N1 of the transformer is interrupted to generate an induced voltage in the secondary winding N2 of the transformer. The induced voltage is rectified and smoothed by D1, D2 and C1, C2 to generate a predetermined positive DC voltage at OUT1. As a result, the charger 3 carries out positive corona discharge with respect to the photoconductor, and the discharge current IL1 flows. At this time, the voltage applied to the charger 3 greatly exceeds the Zener voltage of the Zener diode ZD, and the Zener current IZ flows through the Zener diode ZD. This causes a negative voltage (ZD Zener voltage) at point B.
Occurs. This voltage is divided by the variable resistor R5, generated at the output terminal OUT2, and given to the developing device 4 as a negative bias voltage. The developing device 4 is a high impedance load of several 100 MΩ, and the load current IL2 is extremely small.
5 can have a high impedance, and the constant voltage characteristic by the Zener diode ZD can be stabilized by setting Iz> Ir. Since the resistor R4 is provided in parallel with the charger 3, even if the corona discharge of the charger 3 is stopped and the load is opened, the dummy current Ih flows through the resistor R4 and the predetermined value from the terminal OUT2 A pressure output can be obtained. Therefore, by lowering the output voltage of OUT1 to the corona discharge voltage or lower, the charger can be artificially turned off, and a predetermined voltage can be generated at OUT2 even in this state.

According to this embodiment, since it is not necessary to provide a smoothing capacitor in the voltage output section for the developing device, it is also necessary to provide a capacitor charge discharging resistor such as the resistor R3 in the circuit shown in FIG. Absent.

In the example shown above, the variable resistor is connected in parallel to the Zener diode to variably output the intermediate voltage output voltage. However, as shown in the output terminal OUT3 in FIG. 2, a constant voltage is directly output from the Zener diode. You can also get it.

Also, in the embodiment, the output terminal OUT1 is configured to obtain a positive voltage and the output terminal OUT2 is configured to obtain a negative voltage.
It may be configured to obtain a positive voltage on T2.

(G) Effect of the Invention According to the present invention, it is possible to generate two DC voltages having different voltages from the single secondary winding of the transformer and supply the two DC voltages to the first and second loads, respectively. It is not necessary to use a transformer with a secondary winding dedicated to medium voltage separately, and there is no need to separately provide a rectifying and smoothing circuit.
Miniaturization and cost reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing the configuration of the present invention. FIG. 2 is a circuit diagram of a power supply circuit for an electrophotographic image forming apparatus according to an embodiment of the present invention. 3 and 4 are circuit diagrams of conventional power supply circuits. 3 ... Charging device (first load), 4 ... Developing device (second load), Q1 ... Switching transistor, D1, D2, C1, C2 ... Rectifying and smoothing circuit, ZD ... Zener diode.

Claims (1)

[Claims] 1. A switching regulator in which a switching circuit for connecting and disconnecting a current flowing through the primary winding is connected to a primary winding of a transformer, and a rectifying and smoothing circuit is connected to a secondary winding of the transformer. A first load is connected between one output terminal and the ground, and a resistor is connected in parallel to the first load, and the first load is connected between the other output terminal of the rectifying and smoothing circuit and the ground. A Zener diode is connected in the direction in which the flowing current flows as a Zener current, and a bias voltage is generated from the Zener voltage of this Zener diode to generate a second voltage.
Both positive and negative output power supply circuit characterized by supplying to the load.