Power supply device having control transistors connected in parallel with output voltage terminals

A power supply device converts an input voltage to a transformer to high output voltages at a secondary side. A transistor is connected in parallel with each of two or more series-connected high voltage output circuits of different polarities on the secondary side. The base currents of the transistors paralleled with the output circuits are controlled so that the high voltage output can be continuously varied.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a power supply device, and more 
particularly to a power supply device of a high output voltage adapted for 
use in an image forming apparatus, such as a copying apparatus or a 
printer, utilizing the electrophotographic process. 
2. Related Background Art 
FIGS. 5 and 6 are circuit diagrams showing different examples of 
conventional power supply devices. 
In an example shown in FIG. 5, there are provided a high-voltage 
transformer 181; transistors 182, 183, 184; an operational amplifier 185; 
drivers 186, 187 respectively for the transistors 182,183; load current 
detecting means 189; and a control circuit 188 for supplying the 
operational amplifier 185 with a level signal and the drivers 186, 187 
with clock signals of inverted phases, in response to a signal from the 
current detecting means 189. There are also provided high-voltage diodes 
190,191; high-voltage capacitors 192, 193; a load 194; and a high-voltage 
relay 195. 
The conventional example mentioned above functions in the following manner. 
This circuit effects constant-current control on high output voltages of 
positive and negative polarities. When the transistor 183 is turned on, a 
positive high voltage is generated at the secondary side of the 
transformer 181 to turn on the high-voltage diode 191, thereby charging 
the high-voltage capacitor 193 with a positive high voltage. On the other 
hand, when the transistor 182 is turned on, a negative high voltage is 
generated at the secondary side of the transformer 181 to turn on the 
high-voltage diode 190, thereby charging the high-voltage capacitor 192 
with a negative high voltage. 
The polarity of the high voltage applied to the load 194 is switched by the 
high-voltage relay 195. Then, depending on the signal from the current 
detecting means 189, the control means 188 varies the signal supplied to 
the operational amplifier 185, thus regulating the voltage applied to the 
transformer 181 through the transistor 184. Thus the current in the load 
194 is controlled at a predetermined value in this manner. 
In the circuit diagram in FIG. 6, showing another conventional example, 
there are provided high-voltage transformers 201, 202; transistors 203, 
204; oscillators 205, 206; control means 207 for terminating the 
oscillation of the oscillator 205 or 206 according to the polarity of the 
high output voltage; high-voltage diodes 208, 209; high-voltage capacitors 
210, 211; resistors 212, 213; a load 214; and diodes 220, 221. 
Referring to FIG. 6, the transformers 201, 202 respectively generate 
positive and negative high voltages. The polarity of the high voltage 
supplied to the load 214 is switched according to whether the control 
means 207 causes oscillation in the oscillator 205 or 206. 
The conventional example shown in FIG. 5 requires the expensive 
high-voltage relay 195 for switching the polarity of the output high 
voltage, thus leading to a significantly elevated cost. Also if there are 
required different voltage levels at the positive and negative sides, 
there are required not only the high-voltage relay but also plural units 
of expensive high-voltage transformers. Consequently such structure is 
unfavorable not only in terms of the cost but also in terms of the space 
required. 
On the other hand, the conventional example shown in FIG. 6 does not 
require the high-voltage relay for the polarity switching, but always 
requires plural high-voltage transformers even in case of generating 
positive and negative high voltages of a same magnitude. Consequently the 
structure becomes complex and bulky, leading again to an elevated cost. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a power supply device 
capable of generating high voltages of both polarities without the 
above-mentioned drawbacks of the prior art. 
According to an embodiment of the present invention, the power supply 
device, for elevating an input voltage at the primary side with a 
transformer to provide high output voltages at the secondary side, 
comprises transistors connected in parallel manner between high-voltage 
output terminals of plural high-voltage output circuits of different 
polarities, formed at the secondary side of a same transformer, and base 
current control means for controlling the base currents of said 
transistors, wherein said plural high-voltage outputs are connected in 
series. 
The above-explained configuration enables supply of a high voltage, as 
plural high-voltage output circuits of different polarities, formed at the 
secondary side of the same transformer, are connected in series. In 
addition, the high-voltage output can be continuously varied from positive 
side to negative side, by means of transistors connected in parallel 
between the high-voltage output terminals of the plural high-voltage 
output circuits and the base current control means for controlling the 
base currents of said transistors. 
Other objects of the present invention, and the features thereof, will 
become fully apparent from the following detailed description, which is to 
be taken in conjunction with the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the power supply device of the present invention will be clarified by 
embodiments thereof. 
[1st embodiment] 
FIG. 1 is a circuit diagram of a first embodiment, wherein provided are a 
high-voltage transformer 61; photocouplers 62, 63; high-voltage 
transistors 64, 65; a transistor 66; high-voltage diodes 67-70; diodes 71, 
72; high-voltage capacitors 73-78; capacitors 79, 80; a variable resistor 
81; resistors 82-93; an oscillator 94; an operational amplifier 95; and a 
load 96. 
The above-mentioned circuit functions in the following manner. The on-off 
operation of the transistor 66 in response to a signal from the oscillator 
94 generates a high-voltage rectangular wave at the secondary side of the 
transformer 61. The diode 72 is provided for stabilizing the voltage 
generated at the secondary side of the transformer 61 when the transistor 
66 is turned off. The two coils in the primary side of the transformer 61 
have a same number of turns. 
In the secondary side, a positive voltage generating circuit is constituted 
by the transistor 64, photocoupler 62-a, high-voltage diodes 67, 68, 
high-voltage capacitors 73, 74, 75 and resistors 83, 84. Also a negative 
voltage generating circuit is constituted by the transistor 65, 
photocoupler 63-a, high-voltage diodes 69, 70, high-voltage capacitors 76, 
77, 78 and resistors 85, 86. Each of said circuits is composed of a 
voltage doubling rectifier circuit employing high-voltage capacitors and 
high-voltage diodes, and a voltage control circuit employing a transistor, 
resistors and a photocoupler. The resistor 93 is provided for separating 
the AC components, in order to avoid mutual interference of the voltage 
doubling rectifier circuits of positive and negative voltages. 
In the following there will be explained the control circuit on the 
secondary side. The variable resistor 81 is provided for setting the 
current to be supplied to the load, and the resistors 90-92 constitute a 
load current detecting circuit. A circuit composed of the operational 
amplifier 95, resistors 82, 87 and capacitor 80 constitutes an inverting 
amplifier for controlling the output current to the load, according to the 
output voltage Va of the variable resistor 81, thus providing the load 
with a negative or positive current respectively if Va is large or small. 
For example if the detected voltage Vb is larger than Va, the output 
voltage of the operational amplifier becomes larger as the non-inverted 
input thereof is larger. If the photocoupler 62 is turned on in this 
state, the forward current of the photocoupler 62-b increases further, 
whereby the collector current of the transistor 64 decreases and the 
positive high voltage increases. On the other hand, if the photocoupler 63 
is turned on, the forward current of the photodiode 63-b becomes smaller, 
whereby the collector current of the transistor 65 increases and the 
negative high voltage decreases. In either case, the high output voltage 
increases in the positive direction, so that the load current also 
increases in the positive direction and Vb becomes smaller. Thus the 
circuit always functions in such a manner that Vb becomes equal to Va. 
As will be apparent from the circuit diagram, only either one of the 
photodiodes 62-b, 63-b can be turned on at a time and only a high voltage 
corresponding to the turned-on photodiode is released. 
If the turn-on current of the photodiode increases, the base current of the 
corresponding high-voltage transistor decreases, so that the collector 
voltage of the high-voltage transistor increases and the corresponding 
high-voltage output increases also. On the other hand, if the turn-on 
current of the photodiode decreases, the base current of the corresponding 
high-voltage transistor increases, so that the collector voltage thereof 
decreases and the corresponding high-voltage output decreases also. 
On the other hand, as there is no current in the phototransistor 
corresponding to the turned-off photodiode, the corresponding high-voltage 
transistor is completely turned on. Consequently the corresponding high 
output voltage is shortcircuited by said high-voltage transistor. 
As explained in the foregoing, the present embodiment provides a 
high-voltage power supply device, capable of varying the output current in 
continuous manner from positive side to negative side, in easy and 
inexpensive manner, employing a single transformer and with the 
high-voltage relay. 
[2nd embodiment] 
FIG. 2 is a circuit diagram of a second embodiment, wherein components same 
as or equivalent to those in the first embodiment are represented by same 
numbers. 
In contrast to the first embodiment for constant current control, the 
present second embodiment is designed for effecting constant voltage 
control. 
Referring to FIG. 2, resistors 90-92 constitute an output voltage detecting 
circuit, providing a detection voltage signal Vd. A variable resistor 81 
is provided for setting a target value Vc for the constant voltage 
control. Photodiodes 62-b, 63-b are connected oppositely in comparison 
with the first embodiment, because the detected voltage Vb in the first 
embodiment becomes smaller as the high output voltage increases, while the 
detected voltage Vd in the present second embodiment becomes larger as the 
high output voltage increases. Other parts will not be explained further 
as they are same as those in the first embodiment. 
As explained in the foregoing, also in the constant voltage controlling 
configuration, there can be provided a high-voltage power supply device, 
capable of varying the output voltage in continuous manner from the 
positive side to the negative side, in easy and inexpensive manner, 
employing only a single transformer without the high-voltage relay. 
[3rd embodiment] 
FIG. 3 is a circuit diagram of a third embodiment, wherein components same 
as or equivalent to those in the first or second embodiment are 
represented by same numbers and will not be explained further. 
In contrast to the first or second embodiment which employs photocouplers 
for controlling the base currents of the transistors, the present third 
embodiment employs pulse transformers for this purpose. 
Referring to FIG. 3, there are provided an oscillator 101; a driver 102; 
operational amplifiers 103, 104; pulse transformers 105, 106; diodes 107, 
108; capacitors 109-112; variable resistors 113, 114; and resistors 
121-133. 
A control circuit for controlling the positive high voltage is constituted 
by the operational amplifier 104, pulse transformer 105, diode 107, 
capacitors 109, 111, variable resistor 113, resistors 121-126 and 133. 
Also a control circuit for controlling the negative high voltage is 
constituted by the operational amplifier 103, pulse transformer 106, diode 
108, capacitors 110, 112, variable resistor 114, and resistors 127-132. 
Also an output voltage detecting circuit is constituted by the resistors 
121-123 and 127-129. The variable resistor 113 is provided for setting the 
positive output voltage, while the variable resistor 114 is provided for 
setting the negative output voltage. 
The above-explained circuit functions in the following manner. 
For releasing the positive high voltage, the variable resistor 114 has to 
be shifted to the high-side end, whereby the output voltage of the 
operational amplifier 103 becomes higher to increase the voltage supplied 
to the pulse transformer 106. The pulse transformer 106 is switched by the 
driver 102, based on a signal from the oscillator 101, thereby generating 
a voltage also on the secondary side, thus turning on the transistor 65. 
Thus the negative high output voltage is shortcircuited through the 
resistor 85 and is therefore not released. 
If the variable resistor 113 is adjusted to a desired level in this state, 
the operational amplifier 104 effects control in such a manner that the 
detected value of the output voltage detecting circuit composed of the 
resistors 121-123 becomes equal to said adjusted value. 
For example, if the detected value is larger than the set value, the 
non-inverted input of the operational amplifier 104 becomes larger so that 
the output voltage thereof increases. In response the input voltage to the 
pulse transformer 105 is elevated, thereby increasing the voltage 
generated at the secondary side thereof and also increasing the base 
current of the transistor 64. Since the positive high output voltage is so 
connected as to be shortcircuited by said transistor through the resistor 
83, the collector voltage (positive high output voltage) of the transistor 
64 becomes smaller. In this manner the high output voltage is controlled 
to the desired value. 
For releasing the negative high voltage, the variable resistor 113 has to 
be set at 0. In this state the inverted input of the operational amplifier 
104 becomes smaller than the non-inverted input, thereby increasing the 
output voltage of said operational amplifier 104. In response the voltage 
at the secondary side of the pulse transformer 105 increases to turn on 
the transistor 64. Consequently the positive high output voltage is 
shorted circuited by the transistor 64 through the resistor 83 and is no 
longer released. 
If the variable resistor 114 is adjusted to a desired value in this state, 
the negative high output voltage is controlled in the same manner as in 
case of the positive high output voltage. 
As explained in the foregoing, the present embodiment also provides a 
high-voltage power supply device, capable of varying the output voltage in 
a continuous manner from the positive side to the negative side, in an 
easy and inexpensive manner, employing only one transformer without the 
high-voltage relay. 
[4th embodiment] 
FIG. 4 is a circuit diagram of a fourth embodiment, in which components 
same as or equivalent to those in the first to third embodiments are 
represented by same numbers and will not be explained further. 
In contrast to the foregoing first to third embodiments which are designed 
to continuously vary the output voltage, the present embodiment is only 
capable of switching positive or negative output. However, since high 
voltages of different magnitudes (absolute values) are provided by a same 
transformer, the positive high output voltage is regulated by voltage 
division with resistors 83, 135, while the negative high output voltage is 
regulated by voltage division with resistors 85, 134. The basic function 
will not be explained further as it is same as that in the foregoing 
embodiments. 
A very simple configuration as in the present embodiment can be employed if 
only a constant output voltage is required. 
As explained in detail in the foregoing embodiments, the present invention 
can provide a high-voltage power supply device by serial connection of 
plural high-voltage output circuits of different polarities, formed on the 
secondary side of a same transformer. In addition, the output current can 
be varied in a continuous manner from the positive side to the negative 
side, by means of transistors connected in parallel between the 
high-voltage output terminals of the plural high-voltage output circuits, 
and base current control means for controlling the base currents of said 
transistors. 
In this manner there can be provided a high-voltage power supply device, 
capable of supplying high voltages of both polarities, in an easy manner 
with a single transformer and without the high-voltage relay, and said 
power supply device is adapted for use in an image forming apparatus with 
limited space and cost, such as a copying machine or a printer. 
The present invention is not limited to the foregoing embodiments but is 
subject to various modifications within the scope and spirit of the 
appended claims.