Electrophotography – Machine operation – Having power supply
Reexamination Certificate
2001-09-25
2002-12-31
Tran, Hoan (Department: 2852)
Electrophotography
Machine operation
Having power supply
C363S021010, C399S089000
Reexamination Certificate
active
06501921
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a toner system printer for carrying out printing by using a toner such as a laser system printer or a copying machine, and more particularly to a high voltage generating circuit for generating a high voltage to be required for toner system printing.
2. Description of the Related Art
JP-A-63-87173 has disclosed the conventional art in which a switching operation is changed into an intermittent operation to prevent the breakdown of an oscillating element when an output is short-circuited in a switching power source. In this technique, a resistor for detecting a current is inserted in a current path of an emitter of a switching transistor. When a voltage of the emitter exceeds a predetermined voltage, a current of the switching transistor is controlled to be a constant current. Moreover, a comparator for detecting that a load is short-circuited is provided. When a short circuit is caused, the switching operation of the switching transistor is stopped by the comparator. Furthermore, when a charging period of a capacitor for soft start is passed after the switching operation is stopped, the switching is restarted. In other words, when the load is short-circuited, the switching transistor carries out intermittent oscillation. Therefore, the switching transistor is prevented from being broken when the load is short-circuited (which is a first conventional art).
Moreover, JP-A-9-9616 has disclosed a technique. In this technique, when the short circuit of an output is detected, constant current charging for a capacitor is started. When a terminal voltage of the capacitor exceeds a first voltage, an internal bias source is turned OFF, thereby stopping the driving operation of a switching transistor. Then, when the terminal voltage of the capacitor is raised to be a second voltage which is higher than the first voltage, the capacitor is discharged to have an initial voltage, thereby restarting the driving operation of the switching transistor. In other words, when a load is short-circuited, the switching transistor is intermittently driven. Therefore, it is possible to prevent the switching transistor from being broken when the load is short-circuited (which is a second conventional art).
In a method in which a switching element is brought into a saturation state when it is turned ON as in the switching method according to the technique described above, large output power can cause efficient conversion to be carried out. However, in the case in which slight output power is enough as in a high voltage generating circuit for a toner system printer, a conversion efficiency is deteriorated and an unstable operation is caused. For this reason, the high voltage generating circuit for a toner system printer employs such a structure that a transistor for oscillation which is connected to a primary coil is always carries out self-oscillation in a non-saturation region and an amplitude of oscillation is controlled to stabilize an output voltage. In other words, there has been employed a structure in which an output voltage is stabilized by a method different from the first conventional art and the second conventional art.
FIG. 7
shows the structure described above, that is, a conventional art of a high voltage generating circuit for stabilizing an output voltage by controlling an amplitude of oscillation of a transistor for oscillation. More specifically, a transistor for oscillation Q
1
(which will be hereinafter referred to as a transistor Q
1
) has a collector connected to a primary coil L
1
and a base connected to one of terminals of an auxiliary coil L
3
. Moreover, an output of an OP amplifier
3
for generating a voltage control signal based on a voltage of a secondary output is led to the other terminal of the auxiliary coil L
3
. For this reason, the transistor Q
1
increases an amplitude of oscillation when a voltage of the voltage control signal sent from the OP amplifier
3
is raised, and reduces the amplitude of the oscillation when the voltage of the voltage control signal is dropped. On the other hand, the OP amplifier
3
reduces the voltage of the voltage control signal when the voltage of the secondary output is raised to be higher than a set value, and raises the voltage of the voltage control signal when the voltage of the secondary output is reduced to be lower than the set value. Therefore, the voltage of the secondary output is stabilized to be a voltage set based on a voltage setting signal
22
(the reference numeral
21
denotes a path for applying a reference voltage to set an operating point of the OP amplifier
3
).
In the structure described above, that is, such a structure that the voltage of the secondary output is stabilized by controlling the amplitude of the oscillation, the transistor Q
1
stops the oscillation even if the oscillation is not externally stopped in respect of the relationship in which a load is too increased if an output is short-circuited. On the otherhand, in the case in which the oscillation of the transistor Q
1
is stopped, the voltage of the output (voltage control signal) of the OP amplifier
3
is raised so that a base current of the transistor Q
1
is increased. Therefore, a collector current is increased. More specifically, when the transistor Q
1
is brought into an oscillation stop state, a calorific value becomes greater than that in the oscillation state so that an element is broken more easily. For this reason, the methods according to the first and second conventional arts (the structure in which a switching element is brought into a saturation state during turn—on) cannot be used as a method for carrying out protection when an output is short-circuited. Therefore, another method is used to protect the transistor Q
1
.
More specifically, when the output is short-circuited, the voltage of the voltage control signal sent from the OP amplifier
3
is raised. Therefore, there has been employed such a structure that a resistor R
21
is inserted between the output terminal of the OP amplifier
3
and the other terminal of the auxiliary coil L
3
and a clamp circuit comprising diodes D
5
and D
6
is connected between the resistor R
21
and the auxiliary coil L
3
. Accordingly, the voltage of the other terminal of the auxiliary coil L
3
is controlled to be approximately 1.3 V also when the output voltage of the OP amplifier
3
is raised. As a result, an increase in the base current of the transistor Q
1
is restricted and an increase in the collector current is suppressed. Consequently, the transistor Q
1
can be protected from breakdown (which is a third conventional art).
Moreover,
FIG. 8
shows a structure in which a different method from the third conventional art is employed for protecting the transistor Q
1
. More specifically, a portion comprising the resistor R
21
and the diodes D
5
and D
6
in
FIG. 7
is replaced by a resistor R
22
and a Zener diode D
7
(other portions are identical to those of the structure shown in FIG.
7
). In such a structure, accordingly, the voltage of the other terminal of the auxiliary coil L
3
is shifted to the lower side by a Zener voltage of the Zener diode D
7
with respect to the output voltage of the OP amplifier
3
. Therefore, the voltage of the other terminal of the auxiliary coil L
3
is controlled to be low also when the output voltage of the OP amplifier
3
is raised. As a result, an increase in the base current of the transistor Q
1
is restricted and an increase in the collector current is limited. Consequently, the transistor Q
1
can be protected from breakdown (which is a fourth conventional art).
For the secondary output, moreover, there has been proposed a structure in which a resistor R
31
is inserted in a current path of a secondary output as shown in FIG.
9
A. Alternatively, there has been proposed a structure in which a resistor R
32
is inserted in the ground level side to detect a current value of the secondary output as shown in FIG.
9
B. Moreover, there has been proposed a s
Funai Electric Co. Ltd.
Rosenthal & Osha L.L.P.
Tran Hoan
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