Abstract:
A high voltage generating device is provided with a high voltage transformer and a rectifying circuit which is connected to a terminal of the output coil of the high voltage transformer different from the high voltage output terminal of the output coil and which is incorporated in the unit of the high voltage transformer.

Description:
This application is a continuation of application Ser. No. 643,277 filed 8/22/84 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high voltage generating device for example for driving a charger in an electrophotographic apparatus. 
     2. Description of the Prior Art 
     Conventionally, such a high voltage generating device has a structure as shown in FIG. 1, wherein provided are a conventional high-voltage transformer T; a primary coil W1 thereof; a secondary output coil W2 thereof; a smoothing condenser C; a rectifying diode D; and a spark suppressing resistor R. The output high voltage V2 is equal to the sum of the output voltage V1 of the transformer itself and the voltage V3 of the other coil terminal. 
     In case of varying the voltage V3 in order to widen the variable range of said high output voltage V2 in such circuit, there is proposed a method of connecting the rectified output of a rectifying circuit RC as shown in FIG. 2 to the terminal of the voltage V3 of the transformer T and to modify the output of said rectifying circuit RC. In such case, however, the potential V3 is inevitably limited within a certain range due to the insulation or other conditions of said terminal or components or patterns relating thereto which are shown in FIG. 2. Also in case the rectifying circuit shown in FIG. 2 is formed, for example, on a printed circuit board, a relatively large circuit structure is inevitable in order to realize a high insulation voltage by a sufficiently large distances between the components. 
     On the other hand, in case of supplying different plural loads with high voltages, the use of the transformer and control switching element as shown in FIG. 1 for each load leads inevitably to a larger structure and a higher cost. It is therefore desirable to employ a transformer for providing high voltages to plural loads. In such an arrangement, however, independent control of the voltage supply is quite difficult because, in the common use of a transformer for plural loads, the control of a high voltage to be supplied to a load affects the high voltages supplied to other loads. 
     SUMMARY OF THE INVENTION 
     In consideration of the foregoing, an object of the present invention is to provide a compact high voltage generating device with improved high voltage insulation. 
     Another object of the present invention is to provide a high voltage generating device with a relatively wide variable range of the high voltage output. 
     Still another object of the present invention is to provide a high voltage generating device with an inexpensive and simple sturcture capable of providing plural loads with high voltages. 
     Still another object of the present invention is to provide a high voltage generating device capable of independently controlling high voltages to plural loads through the use of a common transformer. 
     The foregoing and still other object of the present invention, and the advantages thereof, will become fully apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional high voltage transformer; 
     FIG. 2 is a circuit diagram of a rectifying circuit; 
     FIG. 3 is a circuit diagram of an embodiment of the present invention; 
     FIG. 4 is a voltage wave form chart showing voltage wave forms at various points of the circuit shown in FIG. 3; 
     FIG. 5 is a circuit diagram showing load connection of the high voltage source shown in FIG. 3; 
     FIG. 6 is a circuit diagram of a single wave rectifying circuit in another embodiment; and 
     FIG. 7 is a circuit diagram showing an example of a constant current control circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. 
     FIGS. 3 and 4 are, respectively, a circuit diagram of an embodiment of the present invention and a wave form chart showing voltages at various points of said circuit. In FIG. 3 there are shown a two-output high voltage transformer 1 with two output coils; a switching transistor 2 for driving said high voltage transformer; rectifying circuits 3, 4, 5 provided in the high voltage transformer unit respectively for positive rectification, positive voltage doubling rectification and negative rectification; a positive high voltage output terminal 6 and a negative high voltage output terminal 9 which are respectively connected to a load requiring an independent control; transformer terminals 7, 8, 10; a constant current control circuit 11 for detecting the potential of said negative high voltage output terminal 9 by means of a sampling resistor 16 and turning on and off the switching transistor 2 in such a manner as to obtain a constant negative high voltage current from said terminal 9; a voltage elevating transformer 12 driven by the on-off operation of a switching transistor 14 to determine the potential of said terminal 7; a control circuit 17 for detecting the high voltage output current from said high voltage output terminal 6 by means of a sampling resistor 15 and controlling the on-off operation of said switching transistor 14 so as to obtain a constant high voltage current from said terminal 6; and a diode 13 for returning excessive energy in the transformer 12 to the power supply. 
     The output current of the transformer 12 is connected to the terminals 7, 8, then is rectified in the rectifying circuit 4 provided in the transformer 1, and is supplied to the output terminal of the transformer providing the positive high voltage. 
     The incorporation of the rectifying circuit 4 in the transformer unit enables a circuit formation without employing excessively large insulating distances among the components constituting said rectifying circuit. At the same time the variable output range of the high voltage output terminal 6 can be widened since the rectifying circuit 4 can operate over a wider voltage range. 
     FIG. 4 shows voltage forms at various points in the circuit shown in FIG. 3, wherein (a) represents the output wave form of the voltage elevating transformer 12 at a point A, (b) represents the output wave form at a point B obtained by rectifying the output of the transformer 12 with the rectifying circuit 4 with voltage doubling, and (c) represents the output voltage at the positive high voltage output terminal 6 obtained as the sum of the outputs of said transformer and said high voltage transformer 1. 
     FIG. 5 shows the method of connection of loads to the high voltage source shown in FIG. 3, wherein the loads R1, R2, for example chargers in an electrophotographic apparatus, are connected between high voltage output terminals 6, 9 and the ground. 
     In the following there will be explained the function of the above-described circuit. The switching transistor 2 shown in FIG. 3 repeats the on-off operation to drive the high voltage transformer 1, thus generating high voltages between the terminals 6, 7 and between the terminals 9, 10. As shown in FIG. 5, loads R1, R2 are connected to the output terminals 6, 9 for high voltage supply. The corresponding currents are detected by the sampling resistors 15, 16 and are supplied to the constant current control circuits 11, 17. 
     FIG. 7 shows an example of the constant current control circuits 11, 17, wherein shown are an error amplifier 21 and a pulse width modulation circuit 22. The value detected by the sampling resistor 17 or 16 is supplied to an input terminal of the error amplifier 21, while a reference value obtained by voltage division with resistors is given to the other input terminal, whereby the error amplifier 21 amplifies the error. 
     The pulse width modulation circuit 22 forms a pulse signal of a duty ratio corresponding to the output of the error amplifier 21, and said pulse signal is used for controlling the on-off operation of the aforementioned switching transistor 2 or 14. 
     The current from the negative high voltage output terminal 9 is detected by the sampling resistor 16, and the control circuit 11 controls the on-off operation of the switching transistor 2 according to the detected value to supply a constant current from the negative high voltage output terminal 9. 
     On the other hand, the current from the positive high voltage output terminal 6 is detected by the sampling resistor 15, and the control circuit 17 controls the on-off operation of the switching transistor 14 according to the detected value to supply a constant current from the positive high voltage output terminal 6. However the variable range of the output voltage from the above mentioned terminal 6 is determined by the variable range of the voltage elevating transformer 12. 
     In this manner plural loads can receive high voltage supplies not from respective exclusive transformers but from a common transformer, with the possibility of independent control of voltage supplied to plural loads. Consequently the present invention allows a reduction in the cost and the space required by the high voltage supply unit. 
     In the foregoing embodiment the rectifying circuit 4 is composed of a voltage doubling rectifier, but it may also be composed of a single rectifier as shown in FIG. 6 or other various rectifying circuits. 
     As explained in the foregoing, the present invention allows one to make the high voltage supply unit compact by increasing the packing density of the transformer, and to improve the reliability by totally enclosing the high voltage generator within the high voltage transformer.