JPS58123365A - High voltage power source - Google Patents

Abstract

PURPOSE:To obtain accurate limiting function in a high voltage power source by allowing a load current control loop which limits a load current at the prescribed level at the time of raising a load impedance to relate to a control loop which maintains the output voltage constant. CONSTITUTION:A load current detected by a resistor R16 is compared by an operational amplifier OA3 with a reference voltage. When the load current exceeds the prescribed value, a transistor Tr2 is turned ON, thereby interrupting the input of the primary side power feeding circuit of a converter transformer T1 by the output of an operational amplifier OA1. When the transistor Tr1 is turned ON, charge stored in a condenser C3 is discharged, thereby again inverting the output of the comparator OA3, and turning the transistor Tr1 OFF. When the load current tends to exceeds the limit value in this manner, the output level of the comparator OA3 is inverted, the transistor Tr1 is, in turn, repeated to conduct ON and interrupt OFF, thereby maintaining the limit value.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high voltage power supply device, and more particularly to a constant voltage output high voltage power supply device for a copying machine.

This type of high-voltage power supply device is configured to apply high-voltage output to a load via a step-up transformer, and is applied, for example, to high-voltage charging of copying machines.

In this case, the insulation strength of the photosensitive drum or charger is often used to its fullest. Especially in the case of a constant voltage control method, the output voltage is controlled at a constant level, so the device may become overloaded due to discharge or other causes. This caused destruction and failed to meet various safety standards. Therefore, conventionally, in this type of constant voltage output high voltage power supply device, the following method has been conventionally adopted as a protection against a load short circuit and a current limiter function. For example, a certain limiter is installed on the input voltage of the transformer that applies the high voltage output to the load (2), and the load characteristics of the transformer itself are used to lower the output voltage during overload, and a high output resistor is inserted to limit the current. The input voltage was limited by detecting the output current with a current transformer.However, with this method, variations in the limiter value occur due to variations in the transformer, and the power loss for current limiting is large. As another conventional method, it has been proposed to insert a current limiter on the primary side of the transformer, but even with this method, the detection accuracy itself is difficult because the output current is relatively small.

The present invention has been made in view of the above points, and an object of the present invention is to provide a high voltage power supply device with low power loss and a highly accurate current limiter function. , the primary side of the transformer is controlled so that the output is constant based on the detected divided voltage, and the load current is within a predetermined range based on the detected value of the load current. It is a feature.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram of an embodiment of a high voltage power supply device according to the present invention. OA is an operational amplifier constituting the differential amplifier circuit 10, which outputs the sum of a reference input from an input terminal P, which will be described later, and a detected voltage based on the load current, and a transformer 2, which will be described later.
Performs differential amplification of the detected voltage according to the next-side divided voltage. Here, D,, D2 are protection diodes, R,, R2,
R3 is a resistor, and C1 is a capacitor. 2 is an amplifier circuit consisting of a Darlington-connected transistor Tr2°Tr for amplifying the output of the differential amplifier circuit 1o. Note that R4 is the output resistance of the operational amplifier OA, and the amplifier circuit 1
2, R6 is a resistor, D3. D6 is a diode, C
3 is condensate □su. Further, the base of the transistor Tr2 in the input stage of the amplifier circuit 12 is connected to the collector of a transistor Tr that operates based on a reference input from the input terminal P2 and a detected value of a load current, which will be described later. Here, R7, R,, R, are resistances, D3. D4 is a diode. Reference numeral 14 denotes a tank circuit connected to the emitter of the transistor Tr3 in the output stage of the amplifying circuit 12 described above, and is composed of a resistor RIOy and a capacitor c4. Further, T is a converter transformer, and a self-excited switching converter 16 is constituted by this transformer T1 and a transistor Tr4 connected to the tank circuit 14 via a diode D7. Here, R12 is a resistor and C5 is a capacitor.

18 is a rectifier diode D8. A rectifier circuit constituted by an output resistor R111, and a DC voltage of +7 to +10 N obtained as an output of the rectifier circuit 18 is supplied to a charger (not shown) via an output terminal P3. Further, 20 is an attenuation circuit that attenuates the output of the transformer T to a predetermined division ratio using a voltage dividing resistor R141RI5. Here, 06 is a capacitor, R16 is a resistor for voltage detection according to the load current, and C2 is a capacitor connected between the voltage dividing point of the voltage dividing resistor R14°RI5 and the ground.

Now, if the output voltage is -VOy, the voltage division ratio of voltage dividing resistor R14 + RI5 is m, and the terminal voltage of resistor R16 is R, then I
Since VOI〉■□, the voltage at the voltage dividing point of resistor R14r R+5 is -vo/m +v, ......
. . . (1), and the detection voltage represented by this equation (1) is supplied to the non-inverting input terminal of the operational amplifier OA mentioned above.

Further, 22 is an amplifier circuit consisting of an operational amplifier OA2, and a detection voltage from a resistor RI6 is supplied to an inverting input terminal of the operational amplifier OA2. Here, R1? S R181R
19 is a resistor, and Do, D, and o are protection diodes.

Further, 24 is a comparator constituted by an operational amplifier OA3 connected to the output stage of this amplifier circuit 22. Here, R201C8 is the resistor and capacitor connected in parallel to the operational amplifier OA, R4P+ R22 is the operational amplifier OA
A voltage dividing resistor R23 that provides a reference voltage to 3 is an input resistor. The output terminal of this operational amplifier OA is a capacitor C7.
, resistor RIO, resistor R0, and diode D to the base of the transistor Tr described above.

Further, 26 is an amplification circuit composed of an operational amplifier OA4, and an input terminal P is connected to the inverting input terminal of the operational amplifier OA4.
1 to a predetermined reference voltage V8 is supplied via a resistor R2□. Here, R24°R25rR26 is a resistance.

One embodiment of the present invention is configured as described above, and its operation will be described next.

Input terminal P1. It is assumed that a predetermined reference input is applied to P2, and a load such as a charger is connected to output terminal P3. The polarity of the detected voltage VR corresponding to the load current by the resistor R16 is inverted to -VR by the operational amplifier OA2, and then added to the reference voltage applied to the input terminal P1 by the operational amplifier OA4.

Now, if each resistance value is R24""R25-R27, the output voltage of operational amplifier OA4 is -Vs + VR... (2)
and is supplied to the inverting input terminal of the operational amplifier OA. At this time, at the non-inverting input terminal of the operational amplifier OA, a detection voltage -V corresponding to the output voltage expressed by the above-mentioned equation (1) is applied.
Jm +VR is supplied.

Therefore, the operational amplifier OA is calculated by the above-mentioned equation (1) and (2).
) formula, i.e. Vs Vo/m ・・・・・・・・・・・・
... (3), and the amplifier circuit 12. By controlling the self-excited switching converter 16 via the tank circuit 14, the primary voltage of the converter transformer T is controlled. by this,
Output voltage■. will always remain constant.

At this time, if the load impedance connected to the output terminal P becomes smaller than the allowable range, the output current increases in inverse proportion to the load impedance. This not only fails to satisfy various safety standards, but also causes damage to the converter transformer T and its own elements. Therefore, in this case, the upper limit of the operating current is often set close to the value specified by safety standards or the allowable value of the thermal strength of the element, so a highly accurate limiter for the load current is required. Become.

In order to apply a limiter to the load current, the load current detected by resistor R16 is reversed by operational amplifier OA2, and then obtained by dividing the power supply voltage -V8 by resistor R211R2□ by operational amplifier oA3. is compared with the reference voltage. When the load current supplied to the operational amplifier OA3, which functions as a comparator, exceeds a predetermined value, the output level becomes cL.
J is reversed to H'', transistor Trl is turned on,
The input of the primary side power supply circuit of the converter transformer T1 is cut off from the output of the operational amplifier OA. This transistor T
The electric charge charged in the capacitor C3 due to turning on of r is D6-D.

-R5-Tr, when the voltage applied to the converter transformer T decreases and the output voltage decreases, the output of the comparator OA3 is inverted again and the output voltage of the converter transformer T is inverted again.

Turn off. In this way, the load impedance becomes small and the load current becomes -■ or R21+R22*R
When trying to exceed the limiter value determined by the value of 16, the output level of the comparator OA is inverted and the transistor Tr,
While repeating on and off, the limiter value is maintained with some ripple in the output.

When the load impedance is within the specified tolerance range,
Since the output level of the comparator OA3 does not reach lH), the transistor Tr is turned off and the primary voltage of the transformer T1 is controlled by the output of the operational amplifier OA.

One embodiment of the present invention is as described above, and is constructed by associating the control loop that keeps the output voltage constant with the load current control loop that applies a constant limiter to the load current when the load impedance increases. A highly accurate limiter function can be realized. As a result, the allowable range of the load impedance of the charger has been widened, and it has become possible to set the output voltage in a wide range. Further, unlike the conventional example described above, there is no need to connect a high resistance in series with the load, so power loss can be reduced.

In the above embodiment, since the potential difference between the primary and secondary sides of the transformer is very large, it is desirable to isolate the output voltage detection signal and the load current detection signal and use them as control signals. In addition, the switching converter 16
may be separately excited instead of self-excited.

FIG. 2 is a circuit diagram of another embodiment of the high voltage power supply device according to the present invention, in which the operational amplifier OA of the embodiment shown in FIG.
This is an example in which the circuit is simplified by using a high input impedance amplifier with an FET input, and the same reference numerals as in FIG. 1 indicate the same components, and the explanation thereof will be omitted. According to this embodiment, operational amplifiers OA2 and OA3 are replaced with an amplifier circuit 28 consisting of an operational amplifier OA, and resistors and capacitors having the same circuit constant values as in the embodiment of FIG. 1 are connected to the operational amplifier OA. At the same time, the voltage division value of the voltage dividing resistor RI4# R15 is further divided by a high resistance R'2+ e R'2□, and an operational amplifier OA4 for adding the detected voltage based on the reference voltage and load current to the input terminal P1 is connected. It is configured by omitting it.

Since the circuit operation is the same as that of the embodiment shown in FIG. 1, it will be omitted.
In this embodiment, the voltage dividing resistor RI41 RI of the attenuation circuit 20
5 load current detection resistor R86, and resistor R'2. .

If the value of y22 is set as shown in Figure 2,
Since the load current is at most 1 mA considering the load impedance, the terminal voltage of resistor R16 is below IV. However, if the output voltage is, for example, 10 KV, the terminal voltage of resistor R14 is
Since the voltage at the dividing point of 1R15 is 100 cm, which is sufficiently large, the terminal voltage of resistor R111 can be ignored. Also,
The current that does not pass through the load, that is, the current that cannot be detected by the resistor R16, is also as small as 100V/100MΩ=1 μA, so it can also be ignored. Therefore, according to this embodiment, the circuit configuration can be extremely simplified compared to the embodiment shown in FIG. 1, as described above.

Since the present invention is as described above, power loss is small and
A high-voltage power supply device with a constant voltage output having a highly accurate current limiter function can be obtained, and the effect is extremely large when applied to a charger of a copying machine, etc.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing one embodiment and other embodiments of the high voltage power supply device according to the present invention.

Claims (1)

[Scope of Claims] (I) A high-voltage power supply device that applies high-voltage output to a load via a transformer, comprising means for detecting a voltage divided on the secondary side of the transformer and a voltage with respect to the load current; so that the output is constant based on the divided voltage
The high-voltage power supply apparatus is characterized in that output voltage control and load current control are performed by respectively controlling the primary sides of the transformers so that the load current falls within a predetermined range based on the detected value of the load current. (2) The output voltage control is performed by continuously controlling the voltage applied to the primary side of the transformer, and the load current control is performed by controlling the energization time to the primary side of the transformer. 2. A high-voltage power supply device according to claim 1, characterized in that: . (3) The reference voltage with which the detected divided voltage is compared is configured such that the voltage difference between the secondary side of the transformer and the reference side is always constant. The high voltage power supply device according to item 1.