A REGULATED HIGH VOLTAGE SUPPLY

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 Field of the Invention. The present invention relates to high voltage supplies, and more particularly to a supply which can generate more than one regulated high voltage output from a single high voltage secondary winding of a transformer. 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 United States Patent Number 4,672,276 entitled "CRT Astigmatism Correction Apparatus with Stored Correction Values" filed 29 May 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. U.S. 3,660,672 discloses a regulated supply comprising transformer means for generating a first and a second regulated voltage output, said supply including first regulating means in a primary circuit of said transformer means for regulating said first output and second regulating means for regulating said second output wherein said first and second regulating means interact with each other to correct for fluctuations in said first and second outputs. The present invention provides a regulated voltage supply as disclosed in U.S. 3,660,672 characterised in that said first and second outputs are high-voltage outputs which are provided from different taps of a common secondary winding of the transformer means, and said second regulating means is arranged to sample said second high voltage output the output of said second regulating means being connected to the lower voltage end of said secondary winding, whereby an error signal, if any, is supplied for correcting the fluctuations in the second high voltage output and, by interaction, the first high voltage output. Preferred embodiments are defined in the dependent claims. A preferred embodiment of the invention is described below by way of example only with reference to Figures 1 and 2 of the accompanying drawings. Brief Description of the Drawings Figure 1 is a block diagram of a regulated high voltage supply according to the present invention, and Figure 2 is a schematic diagram of an embodiment of the regulated high voltage supply of Figure 1. Description of the Preferred Embodiment Referring now to Figure 1, a high voltage transformer 10 is energised by a flyback circuit 12 to produce a first high voltage output HV₁. HV₁ 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 HV₁. A second high voltage output HV₂ is taken off a tap of the high voltage transformer 10, HV₂ 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 HV₂. The change in the lower end of the secondary of transformer 10 affects HV₁ which is regulated by the first regulator circuit 14. Thus, the two regulating circuits 14, 16 interact to regulate any fluctuations in either HV₁ or HV₂. As shown in further detail in Fig. 2 the flyback circuit 12 is composed of the primary 20 of the transformer 10, capacitor C₅ connected across the primary and at one end to a voltage V, a switch transistor Q₁ connected to the other end of capacitor C₅, a catch diode D₁ connected across the collector to emitter of Q₁, and a base drive circuit 22 to drive Q₁. The secondary 24 of the transformer 10 is a plurality of windings connected in series, with HV₁ at the top of the secondary and HV₂ at an intermediate tap 26. A sample of HV₁ 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 R₁-C₁ and R₂-C₂. 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 Q₁ and ground. A sample of HV₂ is picked off via a second attenuator circuit R₃-C₃ and R₄-C₄ 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 D₂ 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 HV₂ cause 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, HV₁ 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 Q₁ to drive it harder, restoring HV₁. This introduces a smaller positive error to HV₂ and the loop continues until HV₁ and HV₂ are restored to their respective regulated values. As an example assume that HV₁ is 19.5kV regulated and HV₂ 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 3.5kV supply error and lowering the 19.5kV supply to 19.2kV. The amplifier 30/FET 32 loop increases the voltage to switching transistor Q₁ by the ratio of 19.5/19.2, re-establishing the 19.5kV supply and causing a new error in the 3.5kV supply of the 19.5/19.2 ratio or 56 volts. Both loops repeat the correction step for a new 3.5kV error at 10 volts, then two volts, and then about 0.3 volts error. Thus, a 300 volt initial error in the 3.5kV 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.

Claims: A regulated voltage supply comprising transformer means (10, 12, 14, 16, 20) for generating a first (HV₁) and a second (HV₂) regulated voltage output, said supply including first regulating means (14) in a primary circuit of said transformer means for regulating said first output and second regulating means (16) for regulating said second output (HV₂) wherein said first and second regulating means interact with each other to correct for fluctuations in said first and second outputs, characterised in that said first and second outputs (HV₁, HV₂) are high-voltage outputs which are provided from different taps of a common secondary winding (24) of the transformer means (10, 12, 14, 16, 20), and said second regulating means is arranged to sample said second high voltage output(HV₂), the output of said second regulating means (16) being connected to the lower voltage end of said secondary winding (24), whereby an error signal, if any, is supplied for correcting the fluctuations in the second high voltage output (HV₂) and, by interaction, the first high voltage output (HV₁). A regulated high voltage supply as claimed in claim 1 wherein said first regulating means (14 comprises: means (R₁, R₂) for sampling said first high voltage output (HV₁), said first high voltage output having a higher voltage than said second high voltage output (HV₂); means (30) for detecting fluctuations in said sampled first high voltage output to generate a first error signal; and first correcting means (32) responsive to said first error signal for correcting said first high voltage output for said fluctuations. A regulated high voltage supply as claimed in claim 2 wherein said second regulating means (16) comprises: means (R₃, R₄) for sampling said second high voltage output (HV₂); means (34) for detecting fluctuations in said sampled second high voltage output to generate a second error signal; and second correcting means (36) responsive to said second error signal for correcting said second high voltage output for said fluctuations. A regulated high voltage supply as claimed in claim 3 wherein said transformer means (10, 12, 14, 16, 20) comprises: a flyback transformer (10) including drive means (22) in a primary circuit thereof for producing said first and second high voltage outputs (HV₁, HV₂) in said secondary circuit. A regulated high voltage supply as claimed in claim 4 wherein said first correcting means (32) is arranged to vary the output of said drive means (22) in response to said first error signal. A regulated high voltage supply as claimed in claim 5 wherein said second correcting means (36) responsive to said second error signal for correcting said second high voltage output (HV₂) is arranged to vary the voltage of a low voltage end terminal of said secondary winding in response to said second error signal.

Inventors: THOMAS, WAYNE D.
Applicants: TEKTRONIX, INC.
Application year: 1985
Main classification: G05F1
IPC codes: H02M3, G05F1