Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-05
2003-03-04
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021070, C399S066000
Reexamination Certificate
active
06529388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a toner system printing device which performs toners such as a printer or copier, and more particularly to a high voltage generating circuit for a high voltage required for toner system printing.
2. Related Art
In a toner-system printing apparatus such as a laser-printer or copier, toners deposited on a photosensitive drum are duplicated on a printing sheet of paper using a duplicating roller. For this purpose, a high voltage is applied to the duplicating roller. This high voltage is required to be variable in a wide range of −1300 V to +2700 V.
FIG. 4
shows a prior art high voltage generating apparatus satisfying the above requirement. This prior art apparatus includes high voltage generating circuits
81
and
82
. The high voltage generating circuit
81
serves to vary a secondary output voltage continuously within a range of 700 V to 4700 V according to the duty ratio of a PWM signal
84
. The high voltage generating circuit
82
serves to generate a stabilized voltage of about −2000 V. The secondary output of the high voltage generating circuit
82
and the secondary output of the high voltage generating circuit
81
are connected in series. Therefore, the voltage at a high voltage output
83
varies continuously within a range of −1300 V to +2700 V according to the duty ratio of the PWM signal
84
.
The, above high voltage circuit will be explained below in detail. A transistor for oscillation (hereinafter simply referred to “transistor”) Q
81
with a collector connected to a primary coil L
81
and a base connected to an auxiliary coil L
83
carries out self-exited oscillation in a non-saturated range. A voltage control circuit
86
controls the oscillating amplitude of the transistor Q
81
in such a manner that an output created on the basis of a signal
85
representative of a voltage on the secondary output is sent to the auxiliary coil L
83
. Therefore, the voltage of the secondary output is stabilized to a voltage represented by the duty ratio of the PWM signal
84
(this technique is referred to as a first prior art).
One of prior arts related to a switching power source is disclosed in the Unexamined Japanese Patent Application Publication No. Hei11-206116. In this technique, in order to limit a voltage from DC electric power to a voltage (second voltage value) or lower which is lower than a prescribed output voltage (first voltage value), a constant voltage control means is provided for feedback-controlling a switching means on the basis of an induced voltage from an auxiliary winding. The constant voltage control means takes out the induced voltage at the same timing as the timing when the output is taken out from the secondary winding. Therefore, an increase in the voltage from the DC electric power is detected as an increase in the induced voltage from the auxiliary winding. When the induced voltage is just about to exceed a prescribed value, the control of suppressing the increase in the voltage from the DC electric power is performed. Therefore, when the load is abruptly changed from its heavy state into an opened state, an increase in the voltage from the DC electric power is limited to the second voltage value (this technique is referred to the second prior art).
[Problems that the Invention is to Solve]
However, use of the first prior art presents the following problem. The transistor Q
81
performs the self-excited oscillation within a non-saturated range. Therefore, the oscillating waveform at the collector at the transistor Q
81
is a sine wave, but contains distortion. The ratio of distortion varies as hfe of the transistor Q
81
varies. The ratio of distortion also varies when the voltage value at the secondary output and the load current value vary. In short, the ratio of distortion varies for different apparatus. Even with the same apparatus, it varies according its state of use.
FIG. 2
shows an oscillating waveform containing the above distortion. Where the oscillating waveform contains such distortion, the voltage of the coil output rectified and smoothed over period t
1
is higher than that rectified and smoothed over period t
2
.
On the other hand, the secondary output of the high voltage generating circuit
81
is the double-voltage rectified output from the secondary coil. Therefore, the voltage of the secondary output is a sum of the voltage rectified over the period tl and the voltage rectified over the period t
2
. However, the signal
85
representative of the voltage of the secondary output is either one of the voltage rectified over the period t
1
and the voltage rectified over the period t
2
. Therefore, where the voltage rectified over the period t
1
is used as the signal
85
, the voltage represented by the signal
85
is higher than an actual secondary output voltage. Thus, the voltage of the secondary output is controlled to be lower than the voltage directed by the PWM signal
84
. On the other hand, the voltage rectified over the period t
2
is used as the signal
85
, an opposite state occurs. Namely, the voltage of the secondary output is controlled to be lower than the directed voltage. As a result, in either case where the voltage is rectified over the period t
1
or t
2
, the accuracy of the voltage of the secondary output is deteriorated, thereby attenuating the printing quality of the portion printed using toners.
In order to obviate such inconvenience, it has been proposed to correct the deviation of the voltage represented by the signal
85
from an actual voltage of the secondary output with the aid of a variable resistor
87
, thereby preventing deterioration in the voltage accuracy. However, as described above, the ratio of distortion in the oscillating waveform also varies according the state of use. Hence, the quantity of deviation also varies according to the state of use. This presents a problem that the correction by the variable resistor
87
does not always result in suitable correction. In addition, this also presents a problem that the step of adjusting the variable resistor
87
is required.
Meanwhile, in the second prior art, the element on the primary output performs a switching operation. Therefore, in order to take out the voltage corresponding to the voltage of the secondary output from the auxiliary winding, the output must be taken out from the auxiliary winding at the same timing as the output is taken out from the secondary output. Namely, this technique is defined by the theory of switching, but is not adopted for the purpose of increasing the detecting accuracy. Therefore, in a configuration in which since the transistor performs self-excited oscillation within the non-saturated range, the oscillating waveform can be regarded as a sine wave and the voltage corresponding to the secondary output can be taken out when either half-wave of the output sent from the coil is rectified, it is difficult to apply the second prior art to the purpose of increasing the voltage accuracy at the secondary output.
SUMMARY OF THE INVENTION
This invention has been accomplished in order to solve the problems described above. An object of this invention is to provide a high voltage generating apparatus for use in a toner-system printing device capable of improving a voltage accuracy at a secondary output in a configuration in which a transistor performs self-excited oscillation within a non-saturated range, thereby increasing printing quality. In this case, the voltage accuracy at the secondary output can be improved in such a manner that, as a voltage detected signal representative of a voltage of the secondary output, a signal is used which is obtained when the output from a voltage detecting coil is rectified and smoothed at the same timing as the output from a secondary coil is rectified and smoothed by a rectifying/smoothing circuit, thereby removing a bad influence from distortion of self-excited oscillation on the voltage accuracy at the secondary output.
In addition to the above object, another o
Funai Electric Co. Ltd.
Patel Rajnikant B.
Rosenthal & Osha L.L.P.
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