Abstract:
A regulated high voltage supply derives two regulated high voltages from a single winding of a transformer. The higher voltage supply is primary side pass regulated, and the lower voltage supply is secondary side offset regulated. The two regulating circuits for the two regulated high voltage supplies interact with each other to correct an error in either high voltage supply.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high voltage supplies, and more particularly to more than one regulated high voltage supply from a single high voltage winding of a transformer. 
     2. Description of the Prior Art 
     The use of a flyback transformer to produce a single high voltage output for the anode of a cathode ray tube is well-established. However, for special requirements, such as for dynamic focusing as described in co-pending United States patent application, Ser. No. 614,613 entitled &#34;CRT Astigmatism Correction Apparatus with Stored Correction Values&#34; filed May 29, 1984 by Conrad J. Odenthal and Barry A. McKibben, a regulated intermediate high voltage supply is desired. Merely adding a second flyback transformer high voltage supply has the disadvantage of increased space requirements as well as increased cost. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a regulated high voltage supply which derives two regulated high voltages from a single high voltage winding of a transformer. The higher voltage supply is primary side pass regulated, and the lower voltage supply is secondary side offset regulated. The two regulating circuits for the two regulated high voltage supplies interact with each other to correct an error in either high voltage supply. 
     Therefore, it is an object of the present invention to provide two regulated high voltage supplies using a single transformer. 
     Other objects, advantages and novel features will be apparent from the following detailed description when read in conjunction with the appended claims and attached drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a regulated high voltage supply according to the present invention. 
     FIG. 2 is a schematic diagram of an embodiment of the regulated high voltage supply of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 a high voltage transformer 10 is energized by a flyback circuit 12 to produce a first high voltage output HV 1 . HV 1  is sampled by a first regulator circuit 14 and an error signal, if any, from the first regulator circuit adjusts the drive of the flyback circuit 12 to correct for such fluctuations of HV 1 . A second high voltage output HV 2  is taken off a tap of the high voltage transformer 10, HV 2  is sampled by a second regulator circuit 16 and an error signal, if any, changes the voltage at the lower end of the secondary of the high voltage transformer 10 to correct for such fluctuations in HV 2 . The change in the lower end of the secondary of transformer 10 affects HV 1  which is regulated by the first regulator circuit 14. Thus, the two regulating circuits 14, 16 interact to regulate any fluctuations in either HV 1  or HV 2 . 
     As shown in further detail in FIG. 2 the flyback circuit 12 is composed of the primary 20 of the transformer 10, capacitor C 5  connected across the primary and at one end to a voltage V, a switch transistor Q 1  connected to the other end of capacitor C 5 , a catch diode D 1  connected across the collector to emitter of Q 1 , and a base drive circuit 22 to drive Q 1 . The secondary 24 of the transformer 10 is a plurality of windings connected in series, with HV 1  at the top of the secondary and HV 2  at an intermediate tap 26. 
     A sample of HV 1  is picked off by line 28 and is passed to the inverting input of a first error amplifier 30 via a first attenuator network of R 1  -C 1  and R 2  -C 2 . The noninverting input of the error amplifier 30 is grounded. The output of the error amplifier 30 is input to the gate of a first field effect transistor (FET) 32. The source and drain of FET 32 are connected between the switch transistor Q 1  and ground. 
     A sample of HV 2  is picked off via a second attenuator circuit R 3  -C 3  and R 4  -C 4  and passed to the noninverting input of a second error amplifier 34. The inverting input of the error amplifier 34 is grounded. The output of the error amplifier 34 is input to the gate of a second FET 36. The source and drain of FET 36 are connected between the bottom of the secondary 24 and ground. Diodes D 2  and D z  (a Zener diode) limit the voltage at the bottom of the secondary 24 to between approximately ground and the Zener voltage of D z . 
     In operation a positive error in HV 2  causes the input of the second error amplifier 34 to go positive. The output of the error amplifier 34 is positive driving the output of FET 36, and thus the bottom of the secondary 24, negative. With the bottom of the secondary 24 going negative, HV 1  decreases, a negative fluctuation. This negative error is input to the first error amplifier 30 to produce a positive output to FET 32. The output of FET 32 goes negative increasing the voltage across switching transistor Q 1  to drive it harder, restoring HV 1 . This introduces a smaller positive error to HV 2  and the loop continues until HV 1  and HV 2  are restored to their respective regulated values. 
     As an example assume that HV 1  is 19.5 kV regulated and HV 2  is nominally 3.5 kV, but is actually 300 volts high. The amplifier 34/FET 36 loop will lower the bottom of the secondary 24 by 300 volts, thus correcting the 3.5 kV supply error and lowering the 19.5 kV supply to 19.2 kV. The amplifier 30/FET 32 loop increases the voltage to switching transistor Q 1  by the ratio of 19.5/19.2, reestablishing the 19.5 kV supply and causing a new error in the 3.5 kV supply of the 19.5/19.2 ratio or 56 volts. Both loops repeat the correction step for a new 3.5 kV error at 10 volts, then two volts, and then about 0.3 volts error. Thus, a 300 volt initial error in the 3.5 kV supply requires approximately 368 volts of movement at the bottom of the secondary 24 to regulate the output. 
     Thus the present invention provides a high voltage supply with two regulated high voltages from a single high voltage secondary of a transformer. Although the invention is described in terms of a flyback transformer, those skilled in the art will recognize that the present invention applies equally well to other power supply topologies.