JPS58123367A - High voltage power source - Google Patents

Abstract

PURPOSE:To obtain accurate limiting function in a high voltage power source by allowing an output voltage control loop which limits an output voltage at the prescribed value at the time when a load impedance increases to relate to a control loop which maintains the load current constant. CONSTITUTION:When the output power of an attenuator 20 exceeds the prescribed value, and the output of a comparator OA2 is inverted to turn a transistor Tr1 ON, thereby interrupting the input to the primary side power feeding circuit of a transformer T1 by the output of a differential amplifier OA2. Charge stored in a condenser C3 is discharged upon interruption of this input, the output of the comparator OA2 is again inverted to the original state, thereby turning the transistor Tr1 OFF. When the output of the transformer T1 tends to exceed the limit value which is determined by the Zener voltage of a Zener diode ZD1, the transistor Tr1 repeats ON and OFF and maintains the output of the transformer T1 at the limiting value.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high voltage power supply, and more particularly to a constant current output high voltage power supply 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, it is common to use the photosensitive drum and charger to their full insulation strength, especially in the case of a constant current control method.
In order to control the load current at a constant level, the applied voltage often exceeds the limit of insulation strength, adversely affecting the image, and furthermore, pinholes are formed in the drum, often shortening the life of the drum.

Therefore, in conventional constant current output high voltage power supplies, a constant limiter is applied to the input voltage of the step-up transformer, and the voltage applied to the load is suppressed using the constant voltage characteristics of the transformer itself. It is configured so that the output voltage does not exceed a certain level even when the circuit is open. However, this system has drawbacks such as reduced power efficiency when a transformer with a large amount of leakage flux is used to improve usability. Another conventional example is to provide a winding for output voltage detection on the secondary side of the transformer in order to suppress the voltage applied to the load, and operate a limiter on the input side based on the voltage value detected by this winding. However, since the voltage induced in the output (secondary side) winding of the transformer is high,
There were drawbacks such as the inability to increase the degree of coupling with the detection winding, and the accuracy of the limiter was significantly reduced.

:): The present invention has been made in view of the above points, and an object thereof is to provide a high-voltage power supply device having a highly accurate voltage limiter function that does not reduce power efficiency or fluctuate due to variations in transformers. In order to achieve this objective, the present invention makes it possible to keep the load current constant based on the detected voltage of the load current, and to keep the output within a predetermined range based on the divided voltage on the secondary side of the transformer. It is characterized by controlling the primary side of each transformer.

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 the high-voltage power supply device according to the present invention, and OAl is an operational amplifier constituting a differential amplifier circuit 10, which responds to a reference input from an input terminal P1 and a load current to be described later. The detected voltage is differentially amplified. Here, Dl,
D2 is a protection diode, R, IR2, R are resistors, C
l=C2 is a capacitor; 12 is a Darlington-connected transistor Tr2 that amplifies the output of the differential amplifier circuit 10;
．． This is an amplifier circuit consisting of Tr3. Note that R4 is the output resistance of the operational amplifier OA1, and in the amplifier circuit 12, R6 is the resistance D.

D6 is a diode, and C3 is a capacitor. Further, the base of the transistor Tr2 in the input stage of the amplifier circuit 12 is connected to the collector of a transistor Tr1 that operates based on a reference input from the input terminal P2 and a detected value of a divided voltage on the secondary side of the transformer, which will be described later. There is.

Here, R7, R8, and R are resistors, and D, , and D4 are diodes. 14 is a tank circuit connected to the emitter of the transistor Tr3 in the output stage of the aforementioned amplifier circuit 12;
It is composed of a resistor R1 (1+capacitor C4). Also, T1 is a converter transformer, and a self-excited switching converter 16 is constructed by this transformer T and a transistor Tr4 connected to the tank circuit 14 via a diode D. Here, RI2 is a resistor, C6
is a capacitor. 18 is a rectifier circuit composed of a rectifier diode D8 and an output resistor R13;
A high-voltage direct current of +7 to +10V obtained as the output of the output terminal P3 is supplied to a charger (not shown) through the output terminal P3. Further, 20 is an attenuation circuit that attenuates the output of the transformer T to a predetermined division ratio using voltage dividing resistors R, 4, R, and 5. Here, C0 is a capacitor, R16 is a sampling resistor for the load current, and the voltage detected by this resistor R16 and
Differential amplification is performed between the reference voltage applied to the input terminal P1 and the aforementioned operational amplifier OA1. 22 is a comparison circuit composed of a comparator OA2 consisting of an operational amplifier;
A2 connects the secondary output voltage of the transformer T to the resistor R141R.
15 at a predetermined voltage division ratio and a reference voltage created by a constant voltage diode ZD. In addition,
This comparator OA2 has a resistor R1 to eliminate vibration.
Positive feedback is applied to give a slight hysteresis to the comparison level.

Here, D. is a protection diode* RIO
and R1g to R20 are resistors, and C7 to C0 are capacitors. In addition to being supplied with the power supply potential, the negative side of the floating power supply VC is connected to the load current detection point, that is, the intersection of the reference side of the output winding of the transformer T1 and the resistor R16. As a result, the resistance R14°RI5
The voltage conversion portion of the load current due to the resistor RI6 added to the dividing point can be completely eliminated.

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

It is assumed that a predetermined reference input is applied to the input terminals P, P2, and a load such as a charger is connected to the output terminal P3. The load current at this time is detected by the sampling resistor RI6, this detected voltage and the reference input from the input terminal P are differentially amplified by the differential amplifier circuit 10, and this differential amplified output is further amplified by the amplifier circuit 12. is applied to the switching converter 16 via the tank circuit 14. The switching converter 16 controls the voltage applied to the primary side of the transformer T so that the load current is always constant, and a high-voltage direct current obtained by rectifying the output voltage of the transformer T according to the voltage in the rectifier circuit 18 is applied to the charger. be done. In this way, the load current is controlled to a predetermined value, but if the load impedance of the charger connected between the output terminal P3 and the ground becomes larger than the rated value, the output voltage will be lower than the load current. If the impedance increases to □, it will cause dielectric breakdown of the charger or create pinholes in the photosensitive drum, so output voltage control is required to place a high-precision limit on the output voltage.

For this purpose, the voltage divided by the aforementioned attenuation circuit 20 at a predetermined division ratio and the reference voltage of the constant voltage diode ZD are compared by the comparator OA2. When the output voltage of this attenuation circuit 20 exceeds a predetermined value, the output of the comparator oA2 is inverted, turning on the transistor Tr, and inputting the input from the output of the differential amplifier OA to the primary side power supply circuit of the transformer T1. Cut off. By this input cutoff, the electric charge that had been charged in the capacitor C8 of the amplifier circuit 12 is discharged, and when the applied voltage on the primary side of the transformer decreases and the output voltage of the attenuation circuit 2o on the secondary side of the transformer decreases, the comparator The output of OA2 is again inverted to its original state, turning off the transistor Tr. As a result, the output of the differential amplifier OA is again changed to the transformer T1.
is input to the primary side power supply circuit. In this way, when the load impedance increases and the output of the transformer T1 attempts to exceed the limiter value determined by the Zener voltage of the constant voltage diode ZD1, the comparator OA.

The output level of the transformer T2 is inverted, and the output of the transformer T2 maintains the limiter value while the transistor Tr repeats on and off and contains some ripple. When the load impedance is within a predetermined tolerance range (2), the output level of the comparator o4 becomes "H", and the transistor Tr
l is off and the primary voltage of the transformer T is the differential amplifier OA,
controlled by the output of

One embodiment of the present invention is as described above, and is constructed by associating an output voltage control loop that applies a constant limiter force IC to the output voltage when the load increases with a control loop that keeps the load current constant. b, it is possible to realize a very highly accurate limiter function.

As a result, the allowable range of the load impedance of the charger is widened, and the setting range of the load current can also be widened. In addition, unlike the conventional example described above (there is no need to provide a winding for detecting the output voltage in the Nidorance T1), it can be implemented with a simple circuit configuration that only requires an attenuation circuit consisting of a voltage dividing resistor that divides the output voltage. This eliminates the need to reduce the efficiency of the transformer by maintaining constant voltage characteristics over a wide load range in order to suppress the output voltage during no-load conditions, as in the conventional example.

In the above embodiment, since the potential difference between the primary side and the secondary side of the transformer is very large, the load current detection signal and the transformer T
It is desirable to isolate the output voltage detection signal of , and use it as a control signal. Further, although the floating power supply Vc is supplied from the outside, a separate winding may be added to the transformer T so as to guarantee the voltage conversion of the resistor for detecting the load current. Further, although the differential amplifier OA and the comparator 08 are constructed from an active amplifier, they may be constructed from a transistor, an FET, or the like. Furthermore, the switching converter 16 may be a separately excited type instead of a self-excited type.

FIG. 2 is a circuit diagram showing another embodiment of the high voltage power supply device according to the present invention. This embodiment is characterized by eliminating the floating power supply ■c of the embodiment shown in FIG. 1 by making the division of the output voltage by the attenuation circuit 20 sufficiently larger than the voltage detected by the load current detection resistor RI6. The same reference numerals as in FIG. 1 indicate the same parts, and the explanation thereof will be omitted. In order to realize the above-mentioned characteristics, in the embodiment, the comparator OA2 of the comparator circuit 20 is composed of a FETTr having a very large input impedance and a transistor Tr6. Here, R21 to R3 are resistors having required resistance values. The operation in this case is similar to that of the embodiment shown in FIG. 1, and when the output voltage of the transformer T1, that is, the output voltage of the attenuation circuit 20 is within a predetermined range, the FETTr is OFF, so the transistor Tra is also OFF, and the comparator OA2 is turned off. The output level is °
It is L'. On the other hand, when the output voltage exceeds a predetermined value, Tr5. Tr6 is turned on, and the output level of comparator OA2 becomes 'H', and transistor Tr6 is turned on.

is turned on to cut off the output from the differential amplifier 10 to the primary side power supply circuit of the transformer T1, and the output of the comparator OA2 is used to switch the transistor Tr,
on/off control.

Therefore, in this embodiment, the voltage dividing resistor R1 of the attenuation circuit 20
41RI51 Load current detection resistor R16 and resistor R
If the values of 21 to R26 are set as shown in Figure 2, the load current will be at a maximum of 1 in terms of load impedance.
mA, the terminal voltage of the resistor RI6 is 1 V or less, whereas if the output voltage is set to 10, for example, the resistor R,
The voltage at the dividing point of 4,R, , is very large at 100V, so it can be ignored. Also, the current that does not pass through the load is 100V/1
Since it is as small as 00MΩ=1μA, this can also be ignored.

In this way, according to the other embodiment, there is no need to separately provide a floating power supply VC as in the above-mentioned embodiment, so that in addition to the above-mentioned effects, the circuit configuration can be further simplified.

Since the present invention is as described above, it is possible to obtain a high-voltage power supply device with a constant current output that has a highly accurate voltage limiter function that does not fluctuate due to variations in transformers without causing a decrease in power efficiency. It is extremely effective when applied to vessels, etc.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of one embodiment of the high voltage power supply device according to the present invention, and FIG. 2 is a circuit diagram of another embodiment in which the floating power supply shown in FIG. 1 is not used.
:1 10... Differential amplifier circuit 16... Self-excited switching converter 2o... Attenuation circuit 22.
... Comparison circuit T, ... Converter transformer vc ... Floating power supply

Claims (3)

[Claims] (1) Load high voltage output via a transformer; In a high voltage power supply device that outputs two voltages, a means is provided to detect the voltage divided by the transformer secondary (Il+ and the load current: two paired voltages, and the load current The primary side of the transformer is controlled so that the load current is constant based on the detected voltage (based on two), and the output is within a predetermined range (based on two) based on the detected divided voltage. 2. [] A high-voltage power supply device characterized by performing load current control and output voltage control. (2) The load current control is to continuously control the voltage applied to the primary side of the transformer (double twist), and the output voltage control is to control the energization time to the primary fA11 of the transformer. 2. The high-voltage power supply device according to claim 1, wherein: (3) The output voltage control means includes a comparison means for comparing the divided voltage with a reference voltage, and a floating power supply that applies a predetermined bias to the comparison means, and the floating power supply is used to control the transformer. 3. The high-voltage power supply device according to claim 1, wherein the voltage difference between the reference side of the secondary side and the reference voltage is always constant.