Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
1999-02-26
2001-05-08
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S021030
Reexamination Certificate
active
06229721
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to protection of a high voltage power supply circuit.
2. Related Background Art
Generally, since a high voltage power supply apparatus used in an electrophotographic process allows potential adsorption and jumping phenomena for a toner, a circuit obtained by serially connecting an AC high voltage generation circuit and a DC high voltage generation circuit as shown in
FIG. 7
is widely used. In an ordinary monocolor process (charge, development, transfer and cleaning), its current value is relatively small. That is, the value is equal to or lower than 1 mA in a DC high voltage, while the value is equal to or lower than 10 mA in an AC high voltage.
FIG. 7
shows the structure of a general current protection circuit in the high voltage power supply apparatus.
In
FIG. 7
, a drive circuit
100
acts as a complementary circuit of an emitter follower system having such the structure as emitters of transistors
101
a
and
101
b
are connected to each other. Numeral
102
denotes a coupling capacitor which is used in combining AC current. Numeral
103
denotes a high voltage transformer, and numeral
106
denotes an input terminal. Numeral
108
denotes an output terminal from which a high output voltage Vo is generated.
Numeral
120
denotes a protection circuit against an excessive current. Numerals
121
and
122
denote reference voltage generation units composed of ladder resistors for generating a reference voltage used in setting of the output voltage Vo. Numerals
123
and
124
denote detection resistors which detect the output voltage Vo. Numeral
125
denotes a comparator which compares the output voltage Vo detected by the detection resistors
123
and
124
with the reference voltage.
Numeral
130
denotes a bias circuit which generates the DC high voltage, numeral
131
denotes a switching transistor, numeral
132
denotes a flyback transformer, numeral
133
denotes a high voltage rectification diode, numeral
134
denotes a smoothing capacitor, and numeral
135
denotes a current detection resistor.
Numeral
140
denotes a detection circuit which detects abnormality at a load
200
, and numeral
141
denotes a coupling capacitor which passes only an AC component out of high voltage current waveforms detected by the current detection resistor
135
. Numeral
142
denotes a rectification circuit which rectifies the detected AC voltage, numeral
143
denotes a comparator which compares the detected current waveform with the reference voltage, and numerals
144
and
145
denote reference voltage generation units each of which is composed of a ladder resistor for generating the reference voltage. An output terminal of the comparator
143
is connected to an input terminal of the comparator
125
through a diode
151
.
Operations of the circuits connected as above will be described.
A high voltage current from the output terminal
108
flows into the ground through the load
200
. Then, a DC high voltage component feeds back to the smoothing capacitor
134
through the detection resistor
135
. On the other hand, the AC component feeds back to an end of the transformer
103
through the smoothing capacitor
134
.
The DC current and the AC current are overlapped and flow in the detection resistor
135
arranged in a series of current channels including the load
200
. Thus, for example, when a limiter control is performed by detecting the AC current, an AC voltage component in the voltage detected by the resistor
135
is guided into the coupling capacitor
141
used in combining the AC current, whereby only the AC voltage component is detected. The AC voltage is rectified by the rectification circuit
142
, and then compared with a divided voltage being a current limiter start voltage defined by the resistors
144
and
145
, by the comparator
143
.
However, as the current value to be supplied from the high voltage power supply apparatus to the load, e.g., it is sometimes required to supply a larger current value instead of the value equal to or less than 10 mA.
Hereinafter, a concrete example will be explained.
In a color printer or the like, after performing a monochrome development process on a photosensitive drum, at least two colors are synthesized on an intermediate transfer medium called as an intermediate transfer body by using a multiple transfer method. Then, a transfer bias is applied on a printing paper sheet to transfer the synthesized color the sheet. After then, a thermal fixing process is performed by a fixing unit to discharge the sheet. On the other hand, even if a toner on the intermediate transfer body is transferred to the sheet, several percents of toner remains on the body. In case of performing a print sequence for plural sheets, since the remained toner is accumulated one after another. Furthermore, since the accumulated toner is superimposed on following images, stain portions are formed on the image, whereby image quality is deteriorated.
For this reason, in a color process, a cleaning sequence for the intermediate transfer body using a cleaning unit is an extremely important technique.
The intermediate transfer body is generally formed as a belt or a cylinder having a resistive layer. The toner on the photosensitive drum is adsorbed to the body by applying a voltage bias from a high voltage generation unit. The cleaning unit supplies an electrical charge to the toner according to the high voltage generated by a cleaning high voltage power supply unit. After the toner is adsorbed on the photosensitive drum, a cleaning process is performed by a cleaning blade provided on the photosensitive drum.
Since the cleaning high voltage power supply unit uniformly supplies the electrical charge to the toner, a high DC bias voltage and a high AC voltage are required.
The intermediate transfer body is constructed by the resistive layer and a high insulating layer on its surface. In a case where the body contacts with a charge roller to charge it, a relatively large electrostatic capacity is generated between the body and the roller. Although a value of the capacity depends on a printer size, such the value is generally several hundreds of picofarads to 1000 pF. On the other hand, a cleaning high voltage is determined from a printer throughput and a print density, and intensive corona is necessary to charge the toner. For example, a waveform of 2 kHz, 80% duty and 3 kV is required as the waveform of enabling to sufficiently show cleaning capability. As the waveform of an output pulse, a response speed equal to or faster than 50V/&mgr;S is required. In order to apply such the output pulse waveform to the intermediate transfer body, a sufficiently lowered value is required for the output resistance of the high voltage power supply apparatus.
Almost every high voltage loads used in the electrophotographic process are electrostatic capacitors. Particularly, in the high voltage load or the like for cleaning, its flowing current of 90% or more is a current flowing in a dielectric load. On the other hand, in corona discharge and spark discharge between different electrodes and short-circuiting of the load, the flowing current of substantially 100% is an in-phase current flowing in a resistive component.
In a case where the load is a transfer body (i.e., charged member), electrically the load is an equivalent circuit of the capacitor. When the AC voltage is applied, a current corresponding to a known voltage change quantity flows. By such the dielectric current, a desired corona discharge is performed to the toner on the intermediate transfer body, and simultaneously a ground current is flowed through a basic layer (dielectric) of the transfer body itself. The flowing current is relatively large. That is, in case of an applied voltage having pulse waveform, the peak (i.e., peak value) of its current reaches several tens of milliamperes. In a case where an abnormal discharge current flows when a leak discharge is generated due to occurrence of an abnormality of the load such as trans
Mano Hiroshi
Mukaibara Takuya
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Patel Rajnikant D.
Wong Peter S.
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