Power supply device having a switched primary power supply and control means for maintaining a constant off period and a variable on period

A power supply device has improved power efficiency and stability for use in a copier or the like. The primary power supply to the device being switched with a constant turned-off period to achieve variable power supply control.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a power supply device for use as a high 
voltage source in a copier or the like, and more particularly such power 
supply device in which a primary power supply is switched and then 
rectified to obtain a desired DC power supply. 
2. Description of the Prior Art 
Conventionally the turned-on period of the primary coil of an inverter 
transformer is controlled by: 
(i) changing the frequency of a current supplied to a switching transistor 
while maintaining a constant turned-on period thereof; or 
(ii) changing the turned-on period of the switching transistor while 
maintaining a constant frequency in the current supplied thereto. 
The switching wave form of an inverter transformer is known to generally 
show a resonance depending on the inductance of said inverter transformer. 
The method (i) does not result in any loss in the power efficiency if the 
supplied frequency is lower than the resonance frequency since the 
switching takes place when the resonance voltage of the inductance returns 
to zero. However, if the supplied frequency is higher than the resonance 
frequency, the power efficiency becomes deteriorated due to an increased 
loss in the switching transistor, because the switching tends to take 
place before the resonance voltage returns to zero. On the other hand, in 
the method (ii) in which the supplied frequency is maintained constant, 
the power efficiency may be deteriorated because the switching may take 
place before the resonance voltage of the inductance returns to zero for a 
certain turned-on period. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, an object of the present invention is to 
provide a power supply device with improved power efficiency and 
stability. 
Another object of the present invention is to provide a power supply device 
capable, in obtaining a desired power supply by switching a primary 
current, of maintaining a constant turned-off period in said switching for 
achieving power supply control. 
The foregoing and still other objects of the present invention will become 
fully apparent from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be clarified in greated detail by an 
embodiment thereof shown in the attached drawings. 
FIG. 1 is a circuit diagram showing an embodiment of the present invention, 
wherein shown are a differential amplifier Q1 composed of an operational 
amplifier; comparators Q2, Q3 composed of operational amplifiers; an 
inverter transformer T1; a switching transistor Tr1 connected to the 
primary side of said inverter transformer T1; diodes D1-D3; condensers 
C1-C3; resistors R1-R14; a reference voltage A for the differential 
amplifier Q1; a reference voltage B supplied to the resistor R9 through 
the diode D2; a reference voltage C for the comparator Q3; and a load X 
for example a corana charger. Although not illustrated in particular, a 
condenser and a diode are externally connected between the transformer T1 
and the ground, parallel to the transistor Tr1. 
In the following there will be given an explanation on the function of the 
above-described embodiment. 
In response to the actuation of a copy start key for example, the 
transistor Tr1 is turned on and a current flows in the transformer T1 to 
accumulate energy therein. Then, when the transistor Tr1 is turned off, 
the energy accumulated in the transformer T1 is released with a sinusoidal 
oscillation of a constant frequency determined by the inductance L of the 
transformer T1 and the capacitance C of the exteral condenser. The 
amplitude of said oscillation varies according to the turned-on period of 
the transistor Tr1, but the duration of the positive portion in a cycle is 
constant. 
The resistors R4, R5 detect the load current of the transformer T1, and the 
differental amplifier Q1 amplifies the difference between a voltage 
corresponding to said load current and the reference voltage A in such a 
manner as to maintain a constant output load current as will be explained 
later. The output voltage of the differential amplifier Q1 and the 
reference voltage B are supplied, through the diodes D1, D2 in an 
"or"connection, to the non-inverted input port of the comparator Q2, so 
that the level at said non-inverted input port varies according to the 
magnitudes of said output voltage and of said reference voltage B. Said 
comparator Q2 releases an "H"-level or "L"-level signal according to the 
level of said non-inverted input port and that of the inverted input port. 
The condenser C2 is charged and discharged in response to the output of 
said comparator Q2 to generate a sawtooth wave voltage at a point D, as 
shown in FIG. 2(A). The time constant of said sawtooth wave is determined 
by the condenser C2 and the resistor R10. The maximum value of said 
sawtooth wave depends on the level at the non-inverted input port of the 
comparator Q2. The sawtooth wave thus obtained is supplied to the 
non-inverted input port of the comparator Q3 for comparison with the 
reference voltage C, and the base of the switching transistor Tr1 is 
controlled according to the result of said comparison in the comparator 
Q3. 
FIG. 2 is a timing chart showing the functions of various parts in the 
above-described circuit. As shown in the chart (A), the sawtooth wave 
assumes a broken-lined form D for obtaining a constant load current for a 
large load, or a full-lined form B for an ordinary load. The slope of the 
sawteeth is determined by the condenser C2 and the resistor R10 both for 
the broken line D and for the full line B. Such output voltage of the 
comparator Q2 is compared with the reference voltage C in the comparator 
Q3. The chart (B) shows the output square wave of the comparator Q3 
corresponding to the full-lined output voltage B of the comparator Q2, 
while the chart (C) shows the output square wave corresponding to the 
broken-lined output voltage D of the comparator Q2. Such output square 
wave is applied to the base of the switching transistor Tr1 to control the 
turned-on period of the transformer T1, thereby obtaining a constant load 
current. 
In the above-described embodiment, the turned-off periods Tn, Tf of the 
output square wave are always constant a shown in the charts (B) and (C) 
in FIG. 2, because the reference voltage C for the comparator Q3 is 
maintained constant. In this manner the switching transistor Tr1 is 
controlled with a constant turned-off period determined in response to the 
duration of the positive portion of a cycle in the aforementioned 
sinusoidal oscillation and with a varying turned-on period, whereby the 
power efficiency is improved and the power loss is reduced in comparison 
with the conventional methods of varying the frequency while maintaining a 
constant turned-on period or of varying the turned-on period while 
maintaining a constant frequency. 
The present invention is not limited to the foregoing embodiment but is 
applicable also to a chopper-type switched power source by replacing the 
transformer T1 with an inductance and by incorporating the switching 
transistor Tr1 into a power supply circuit. 
As explained in the foregoing, there is obtained a power supply device with 
an improved power efficiency in comparison with the conventional control 
methods since, in obtaining a desired DC power by switching and then 
rectifying a DC primary power, the switching means is controlled with a 
constant turned-off period and with a varying turned-on period. Also in 
case said power supply device comprises a transformer, there is obtained 
an additional advantage of improved stability, particularly in a high 
voltage source, combined with the aformentioned improved efficiency, by 
controlling the turned-on period of the primary side of said transformer 
in such a manner as to obtain a constant secondary output voltage or a 
constant load current in the secondary output circuit.