JP2894783B2 - AC high voltage power supply - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC high-voltage power supply for converting a DC voltage into an AC high voltage by an oscillation circuit and a step-up transformer and outputting the converted high voltage, and in particular, an electrophotographic copying machine and the like. The present invention relates to an AC high voltage power supply suitable for applying a high AC voltage to a separation charger or the like in an electrophotographic image forming apparatus.

[Conventional technology]

In an image forming apparatus such as an electrophotographic copying machine or a laser printer, a series of electrophotographic processes such as charging, exposure, development, transfer, and separation are performed to form an image. A high voltage is applied to each of these chargers at a predetermined timing from a high-voltage power supply to cause corona discharge.

Of these chargers, an AC high voltage is applied to the separation charger to weaken the charging of the recording paper.However, if an abnormal current flows due to a short circuit or a decrease in insulation occurring in the separation charger during image formation, the recording paper is discharged. In addition to the inability to separate the recording paper, the occurrence of arc discharge may cause the recording paper to smoke or ignite.

Therefore, in order to cope with such a problem, an AC high-voltage power supply device provided with a protection circuit that detects an abnormal current flowing through the above-mentioned charger (load) and shuts off a high-voltage output when an abnormality occurs is conventionally used. Was.

FIG. 5 is a block diagram showing an example of a conventional AC high voltage power supply for applying an AC high voltage to a separation charger or the like.

In this AC high-voltage power supply device, the transistor Tr is normally turned on and off at a predetermined timing by a control signal of the output control means 1 so that the input voltage (24 VDC) is applied between the emitter and collector of the transistor Tr and the rectifier. The oscillation circuit 3 is supplied through the pull filter 2 to oscillate the oscillation circuit 3, and the oscillation output is applied to the primary side of the step-up transformer T to generate an AC high voltage on the secondary side.

It is supplied from the output terminal 4 to a load such as a separation charger (not shown) via the output resistor R0.

The output current is converted to a voltage by the resistor R1 and monitored by the abnormal current detection circuit 5. However, a corona discharge is usually generated in the separation charger which is a load.
The current value is extremely small.

However, if a short circuit or insulation failure occurs between the charger and the photoconductor or the charger case for some reason, an abnormally large current flows. Exceeds the comparison voltage (reference value) from the comparison voltage generation circuit 6, the output of the comparator 7 is inverted from low level to high level, and the hold circuit 8 holds it and keeps outputting high level "H". Then, the transistor Tr is turned off, the oscillation of the oscillation circuit 3 is stopped, and the high-voltage output operation is stopped.

[Problems to be solved by the invention]

In such a conventional AC high-voltage power supply device, the abnormal current detection circuit 5 constantly detects the output current flowing to the load, and the detected value is compared with a reference value by the comparator 7 to determine whether or not the current is abnormal. Was.

However, even in the normal state, at the time of the rising of each cycle of the AC output voltage, an instantaneous peak current flows as shown by the solid line A in FIG.

Since the magnitude of the instantaneous peak current greatly contributes to the magnitude of the total current during the current output period, it is difficult to determine whether or not the current is abnormal if the level of the entire current waveform is used to determine the abnormal level. Becomes

For this reason, the detection value of the output current is integrated and compared with the reference value to determine whether or not there is an abnormality. Since it greatly contributes to the magnitude of the integral value, it is extremely difficult to optimally set the magnitude of the reference value Vr for determining whether the abnormal current has a waveform as shown by a broken line in FIG. I did.

That is, if the reference value is too high, detection may be delayed or an abnormal current may not be reliably detected. If the reference value is too low, there is a problem that an abnormal current may be regarded as abnormal and erroneously detected.

The present invention has been made in view of the above points, and when an abnormality occurs in an output current due to a load short circuit, insulation failure, or the like, the abnormality is immediately detected, the current is suppressed, and a high voltage output operation is performed. It is an object of the present invention to make it possible to promptly take measures such as stopping, and to prevent the high-voltage output from being uselessly stopped due to an erroneous detection.

[Means for solving the problem]

In order to achieve the above object, the present invention provides an AC high-voltage power supply device that converts a DC voltage into an AC high voltage by using an oscillation circuit and a step-up transformer and outputs the AC voltage. An abnormal current detecting means for detecting an output current value only during a specific period except for a period during which an instantaneous peak current flows through the output circuit of the step-up transformer, and detecting an abnormality by comparing with a predetermined reference value. It is a thing.

The abnormal current detecting means includes a synchronizing pulse generating means for generating a pulse having a width corresponding to the specific period in each cycle in synchronization with the oscillation pulse of the oscillating circuit; An abnormal current detection circuit for detecting an output current value of the step-up transformer, a comparison voltage generation circuit for generating a comparison voltage corresponding to a reference value of the abnormal level, and a current value detected by the abnormal current detection circuit. An abnormal level detection circuit that detects the magnitude of an abnormal current when the voltage exceeds the comparative voltage and exceeds the comparative voltage can be used.

Further, the output current control means for controlling the output current to decrease in accordance with the detected value of the abnormal current by the abnormal current detection means, and the control for decreasing the output current by the means are continuously repeated for a certain period of time. A means for sometimes exhibiting a high voltage output operation.

[Operation] In the AC high-voltage power supply device according to the present invention configured as described above, the abnormal current detecting means synchronizes with the oscillation pulse of the oscillation circuit for DC / AC conversion and outputs the abnormal current to the output circuit of the step-up transformer for each synchronization. For example, in the output current waveform shown in FIG. ₂ , only the specific period such as the period t ₂ to t ₃ except for the period during which the instantaneous peak current flows such as the period t ₁ to t ₂ is hatched in FIG. The current value in the detection area is detected as shown in the figure, and the detected value is compared with a predetermined reference value to detect an abnormality. Therefore, the setting of the reference value is easy, and as shown in FIG. When an abnormal current having a waveform having a certain waveform flows, it can be detected without fail, and a current in a normal state having a waveform shown by a solid line A is not erroneously detected as an abnormal current.

In addition, the abnormal current detecting means is configured as described above, and a synchronous pulse generator for generating a pulse having a width corresponding to the specific period in each cycle thereof in synchronization with an oscillation pulse of an oscillation circuit for DC / AC conversion. If the specific period for detecting the output current value is determined by the means, the period in which the instantaneous peak current flows as shown in FIG. 2 is removed, and the period of the detection region indicated by hatching is detected. It is easy to determine the specific period to be set.

Further, if the output current is controlled to decrease in accordance with the detected value of the abnormal current by the abnormal current detecting means, and if the control is continuously repeated for a certain period of time, the high voltage output operation may be stopped. In addition, it is possible to suppress the continuation of the occurrence of the abnormal current and to restore the normal state, and to stop the output of the high voltage only at the time of a complete and irrecoverable abnormality, thereby ensuring the safety.

〔Example〕

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

FIG. 1 is a block circuit diagram of an AC high-voltage power supply device according to an embodiment of the present invention, and portions corresponding to FIG. 5 described as a conventional example are denoted by the same reference numerals.

Also in this AC high-voltage power supply device, the input voltage (24 VDC) is normally controlled by controlling the PWM controller 11 at a predetermined timing by a control signal from a micro computer (CPU) 10 which controls the sequence control of a copying machine. The pulse width is modulated, supplied to the oscillation circuit 3 through the ripple filter 2 and oscillated, and the oscillation output is applied to the primary side of the step-up transformer T to generate an AC high voltage on the secondary side.

It is supplied from the output terminal 4 to a load such as a separation charger (not shown) via the output resistor R0.

The output current flowing to the load is converted into a voltage by the resistor R1, and the voltage is input to the abnormal current detection circuit 9 to detect the current level. At this time, as shown in FIG. The period during which the instantaneous peak current of the output current waveform flows (t ₁ to
t ₂₎ for detecting a current value of miso specified period (t ₂ ~t ₃₎ except for.

The specific period is determined by a pulse having a width corresponding to the specified period generated by the synchronizing pulse generating means 8 completely in synchronization with the oscillation pulse of the oscillation circuit 3.

Then, the abnormal level detecting circuit 13 compares the current value detected by the abnormal current detecting circuit 9 with the comparative voltage generating circuit 12 so as to correspond to the abnormal level reference value Vr shown by a dashed line in FIG. Compared with the voltage Vr1, if it exceeds it, the magnitude is detected as an abnormal radio wave, and a control signal is output to the PWM controller 11 in accordance with the detected value (for example, only for a time corresponding to the detected value), and the oscillation circuit is output. The oscillation output is temporarily reduced by reducing the duty of the pulse width modulation voltage to be input to 3.

As a result, the output current is reduced, and when the abnormal current stops flowing thereafter, it is possible to restore the normal state.

If the abnormal current continues to flow even after the control to reduce the output current according to the detected value of the abnormal current as described above, such control is repeatedly executed. Control time detection circuit (a type of timer) 14
And outputs a voltage proportional to the duration to the comparator 16.

The comparator 16 compares the voltage with a comparison voltage Vr2 corresponding to a preset fixed time from the comparison voltage generation circuit 15, and when the voltage exceeds the voltage, inverts the output from a low level to a high level. Hold it and keep outputting high level “H” signal.

When that signal is input to the PWM controller 11, the pulse width modulation output is set to zero, the oscillation operation of the oscillation circuit 3 is stopped, and the high voltage output operation is stopped.

At this time, a high-level "H" signal from the hold circuit 17 is also input to the CPU 10, so that the copying machine immediately terminates the execution of the copying process and displays the occurrence of an abnormality on the operation panel. It is good to

As described above, according to the high-voltage power supply device of this embodiment, by detecting the current value only during the period suitable for the abnormality detection in each cycle of the AC output voltage, only when an abnormal current flows through the load, It can be detected reliably.

In addition, since the control for instantaneously reducing the output current is performed based on the detection result, a large abnormal current rarely flows continuously. However, the high voltage is applied only when the abnormal current continuously flows for a certain period of time. By stopping the output, it is possible to prevent the generation of smoke or ignition.

By doing so, the detection time of the abnormal current is not shortened to a necessary abnormal level, and the possibility of stopping the high-voltage output due to erroneous detection is extremely reduced.

It should be noted that the abnormal pulse detecting means is constituted by the synchronizing pulse generating means 8, abnormal current detecting circuit 9, comparative voltage generating circuit 12, and abnormal level detecting circuit 13 in FIG. It also plays a role.

Next, a more specific circuit example of this embodiment is shown in FIG. 3, and its operation will be described with reference to timing charts of waveforms of respective parts shown in FIG.

In FIG. 3, parts corresponding to those in FIG. 1 are denoted by the same reference numerals.

The synchronizing pulse generating means 8 includes a pulse generating circuit 20, a transistor Tr2, an inverter 21, a NAND circuit 22, resistors R2, R3, R4,
And a capacitor C1.

Then, the pulse generation circuit 20 generates a rectangular wave pulse b as shown in FIG. 4 (b) which is completely synchronized with the transmission pulse of the transmission circuit, and is inverted by the transistor Tr2 to be applied to one input of the NAND circuit 22. At the same time, the inverted rectangular wave pulse is again inverted by the inverter 21, and the pulse of the integrated waveform that has passed through the integrating circuit including the resistor R 3 and the capacitor C 1 is used as the other input of the NAND circuit 22.

Was but connexion, a fourth diagram Both inputs from NAND circuit 22 becomes a low level "L" only when (a period corresponding to t ₁ ~t ₂ of FIG. 2) of a high level "H" (c) As shown, a rectangular wave pulse c for determining a specific section is output, and this is output to the abnormal current detection circuit 9 via the resistor R4.

The abnormal current detection circuit 9 includes transistors TR3, TR4, TR5
, Diodes D1 to D3, resistors R5 to R11 and capacitor C
2, one end of the resistor R5 and the transistor Tr4
A current / voltage conversion signal a having, for example, a waveform as shown in FIG. 4 (a) obtained by converting an output current flowing through a load (not shown) such as a separation charger into a voltage by a resistor R1 at a connection point with the emitter. Input through diode D2.

Then, the transistor Tr3 is turned ON only during the period when the rectangular wave pulse c input to the base of the transistor Tr3 is at the high level "H", whereby the transistor Tr4 is also turned ON.
Then, the current / voltage conversion signal a is input, and the resistor R7 and
A detection signal d having a waveform as shown in FIG. 4D divided by R8 is applied to the base of the transistor Tr5.

The detection signal d is detected when a large current flows during a period excluding the instantaneous peak current generation period of each cycle of the output current.

The detection signal d turns on the transistor Tr5.
N / OFF, when ON, a charging circuit is formed from the 5V power supply to the ground via the capacitor C2, the diode D3, the resistor R10 and the transistor Tr5 to charge the capacitor C2,
When it is OFF, the charge stored in the capacitor C2 is discharged through a discharge circuit formed by the resistors R9 and R10.

Accordingly, the output signal e from the connection point between the capacitor C2 and the resistor R11 is, as shown by the solid line in FIG.
When the detection signal d is output, the waveform once lowers from the power supply voltage 5 V in accordance with the magnitude thereof, and then returns to the power supply voltage with a substantially linear slope.

The comparison voltage generation circuit 12 divides the power supply voltage 5V by the resistors R12 and R13, and generates a comparison voltage (a reference value of an abnormal level) Vr1 as shown by a broken line in FIG. 4 (e).

The abnormal level detection circuit 13 includes an operational amplifier 23 and an input resistor R1.
4 and a feedback resistor R15. The output signal e of the abnormal current detection circuit 9 is input to the inverting input terminal via the input resistor R14, and the comparison voltage Vr1 is input to the non-inverting input terminal. A signal f having a triangular waveform as shown in FIG.

This signal f is input to the PWM controller 11 as a control signal, and during the input period, the duty of the pulse width modulation voltage output to the transmission circuit 3 is controlled so as to be reduced in accordance with the magnitude of the signal f. The transmission output of the circuit 3 is temporarily reduced.

As a result, the output current is reduced, and when the abnormal current stops flowing thereafter, it is possible to restore the normal state.

Further, the current control time detection circuit 14
, Diodes D4, D5, resistors R16, R17 and capacitor C3
The output signal f of the abnormal level detection circuit 13 is shaped into a rectangular wave signal g as shown in FIG. 4 (g) by a comparator 24, and a diode D4 is formed by this signal g. Then, the capacitor C3 is charged via the resistor R16, and when there is no signal g, the charge of the capacitor C3 is discharged through the diode D5 and the resistor R17.

Accordingly, the current control time detection signal h, which is the terminal voltage of the capacitor C3, changes as shown by the solid line in FIG. 4 (h).

On the other hand, the comparison voltage generation circuit 15 connects the power supply voltage 5V to the resistor R18.
And R19 to generate a comparison voltage Vr2 corresponding to a predetermined period of time.

The comparator 16 compares the current control time detection signal h with the comparison voltage Vr2, and outputs the output signal i when the voltage exceeds the comparison voltage Vr2.
As shown in FIG. 7I, the signal is inverted from low level "L" to high level "H".

The hold circuit 17 includes a comparator 25 and a transistor
It is constituted by Tr6, a diode D6 and resistors R20 to R23.

Then, the output signal i of the comparator 16 is input to the inverting input terminal of the comparator 25 via the resistor R20, and the comparison signal Vr3 is obtained by dividing the power supply voltage 5V by the resistors R21 and R22.
And outputs a binary signal j (an inverted signal of the signal i) as shown in FIG. 4 (j) to the base of the transistor Tr6.

The transistor Tr6 is turned ON only when the binary signal j is at the low level "L", and changes the control signal k output as the terminal voltage of the resistor R23 to the high level "H" as shown in FIG. To

Once the control signal k becomes high level "H", it is input to the inverting input terminal of the comparator 25 through the diode D6. Therefore, even if the signal i from the comparator 16 subsequently becomes low level "L". And the binary signal j of the comparator 25 is held at a low level "L" by itself, whereby the control signal k also holds a high level "H".

This control signal is input to the PWM controller 11 to make the pulse width modulation output zero, and the oscillation operation of the oscillation circuit 3 is stopped, thereby stopping the high voltage output operation.

At the same time, the control signal is also input to the CPU 10, and if the copying machine is used, all copying processes are immediately interrupted and the occurrence of an abnormality is displayed on the operation panel.

The ripple filter 2 includes a check coil L, a diode D7 and a capacitor C4.

Although the AC high-voltage power supply according to the present invention is suitable for a high voltage power supply for a separation charger of an electrophotographic image forming apparatus such as a copying machine or a laser printer, it can also be used as an AC high-voltage power supply for other equipment. Of course.

〔The invention's effect〕

As described above, the AC high-voltage power supply according to the present invention can immediately detect the occurrence of an abnormality in the output current due to a short-circuit of a load or poor insulation, etc. As a result, it is possible to prevent the high-pressure output from being stopped needlessly.

Furthermore, if the output current is controlled to decrease in accordance with the detected value of the abnormal current, and the high voltage output operation is stopped when the repetition of the control is continuously performed for a certain period of time, the occurrence of the abnormal current may occur. It is possible to suppress the continuation and restore the normal state, and it is possible to stop the output of the high voltage only at the time of a completely unrecoverable abnormality to ensure safety.

Therefore, the safety is extremely high, and it is possible to quickly cope with the occurrence of abnormal current.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram of an AC high-voltage power supply unit showing an embodiment of the present invention, FIG. 2 is a waveform diagram of an output current for explaining the operation thereof, and FIG. 3 is a more specific embodiment of the present invention. FIG. 4 is a timing chart of the waveforms of the respective parts for explaining the operation of the embodiment of FIG. 3, and FIG. 5 is a block circuit diagram showing an example of a conventional AC high-voltage power supply. 3: Oscillation circuit, 4: Output terminal 8: Synchronous pulse generation means 9: Abnormal current detection circuit 10: Microcomputer (CPU) 11: PWM controller 13: Abnormal level detection circuit 14: Current control time detection circuit 16: Comparator, 17 … Hold circuit

Continuation of the front page (56) References JP-A-61-185757 (JP, A) JP-A-61-146957 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02H 3 / 08-3/52 H02J 3/00-5/00 H02H 9/02 G03G 15/00

Claims (3)

(57) [Claims] 1. An AC high-voltage power supply device for converting a DC voltage into an AC high voltage by using an oscillation circuit and a step-up transformer and outputting the converted high voltage, wherein the step-up transformer is synchronized with an oscillation pulse of the oscillation circuit for each period. Detects the output current value only during a specific period excluding the period when the instantaneous peak current flows in the output circuit of
An AC high-voltage power supply device comprising an abnormal current detecting means for detecting an abnormality based on a comparison with a predetermined reference value. 2. An AC high voltage power supply according to claim 1, wherein said abnormal current detecting means generates a pulse having a width corresponding to said specific period in each cycle thereof in synchronization with an oscillation pulse of said oscillation circuit. A synchronous pulse generating means, an abnormal current detecting circuit for detecting an output current value of the step-up transformer only for a period of the pulse width, a comparative voltage generating circuit for generating a comparative voltage corresponding to a reference value of the abnormal level, And an abnormal level detecting circuit for comparing the current value detected by the abnormal current detecting circuit with the comparative voltage and detecting the magnitude of the abnormal current when the voltage exceeds the comparative voltage. Power supply. 3. An AC high-voltage power supply according to claim 1, wherein output current control means controls the output current to decrease in accordance with a detected value of the abnormal current detected by the abnormal current detection means, and an output of said means. Means for stopping the high-voltage output operation when the control for reducing the current is continuously repeated for a predetermined period of time.