MXPA99007295A - Supply of high voltage energy for vi deployment appliances - Google Patents

Abstract

The present invention relates to a high voltage power supply of a video display apparatus providing a ultor voltage (U) and a focus voltage (F) to a cathode ray tube. A low voltage power supply (20) provides a variable voltage (HV_B +) to a primary coil (1) of a return flight transformer (IHVT) of the high voltage power supply (40). The magnitude of the variable voltage responds to first (VFB1) and second (VFB2) feedback signals provided by the first (FB1) and second (FB2) negative feedback trajectories. The first feedback signal is derived from the ultor voltage, and is used to regulate the ultor voltage. The second feedback signal is provided by the low voltage power supply by means of a path that derives the high voltage power supply, and is added with the first feedback signal, to ensure that the focus voltage remains at or above a minimum voltage previously determined

Description

SUPPLY OF HIGH VOLTAGE ENERGY FOR VIDEO DEPLOYMENT DEVICES This invention relates generally to the field of high voltage power supplies for video display apparatus, and in particular, to the field of voltage regulation developed for cathode ray tubes used in video display apparatus, such as televisions , computer monitors, and the like. An electron gun in a cathode ray tube generates an electron beam that is used to explore information on the cathode ray tube screen. The electron gun uses a focus voltage applied to a focus grid to adjust the diameter, also referred to as the "dot" or "dot size" of the electron beam. After focusing by the focus voltage on the focus grid, the electron beam is accelerated towards the cathode ray tube screen by the ultor voltage, also referred to as the anode voltage. The ultor voltage is applied to the cathode ray tube on the anode button, which is located on the bell-shaped portion of the cathode ray tube. The ultor voltage in a video display apparatus is normally generated by the horizontal deflection system, which comprises a horizontal deflection circuit and a return flight transformer. This horizontal deflection system is conventional and will not be described further. In this approach, a horizontal retrace pulse voltage generated by the horizontal deflection circuit during its retrace operation mode is applied to a primary coil of the return flight transformer. The horizontal retrace pulse voltage is stepped upward by a high voltage coil of the return flight transformer, and this stepped up voltage is rectified and then filtered to provide the ultor voltage. The filtration is carried out by a ultor capacitance, which can be provided by the capacitance formed between the internal and external conductive coatings of the cathode ray tube. An alternative approach to generating the ultor voltage is to use a dedicated high-voltage power supply. For example, in a video display apparatus that can support a range of horizontal scanning frequencies, it may be convenient, for reasons related to the complexity of the circuit design and the cost of the materials, to use a high voltage power supply separated to generate the ultor voltage. An example of a return-type high-voltage power supply that can be used in a video display apparatus is disclosed in U.S. Patent No. 4,531,181, entitled HIGH VOLTAGE POWER SUPPLY, and issued to Herz and collaborators. The ultor voltage at the output of the high voltage power supply is filtered by a ultor capacitance, which can be provided by the capacitance formed between the inner and outer conductive coatings of the cathode ray tube. The high voltage power supply disclosed in U.S. Patent No. 4,531,181 uses negative feedback to regulate the ultor voltage. A resistor divider network divides the ultor voltage to provide a feedback signal that varies in proportion to the changes in the ultor voltage level. This feedback signal is used to control a device that regulates the input voltage B + to the high voltage power supply. Accordingly, if the ultor voltage decreases in response to the cathode ray tube extracting a higher beam current, the input voltage B + is increased, thereby increasing the ultor voltage. Conversely, if the ultor voltage is increased in response to the cathode ray tube extracting a lower beam current, the input voltage B + is decreased, thus decreasing the ultor voltage. Then a grid voltage, such as a focus voltage or a screen voltage, can be generated from a so-called "focus screen" assembly, which is energized by the high-voltage coil of the return flight transformer, to generate the focus and screen voltages for the cathode ray tube. The focus screen assembly can include a network of fixed resistors, variable resistors and capacitors. A chain of resistors in the focus screen assembly generates the required focus and screen voltages for the cathode ray tube. The component resistors of the focus screen assembly can be deposited on a ceramic substrate, and the assembly is completely enclosed and isolated. The means for adjusting the variable resistors to set the screen and focus voltages are accessible from outside the focus screen assembly box. The Patent of the United States of North America Number 5,602,447, entitled CATHODE RAY TUBE FOCUS SUPPLY, and issued to Smith, discloses three approaches to energize an assembly of four screens: the approach of the resistor divider network, the detected peak approach, and an inventive combination of approaches of resistor divider network and peak detected. The high voltage power supply disclosed in U.S. Patent No. 4,531,181 uses the resistor divider network approach. This approach, shown in Figure 1, involves energizing a plurality of resistors connected in series with the complete ultor voltage. Some of the plurality of resistors may comprise variable resistors. Then the required focus and screen voltages are established by adjusting these variable resistors. The detected peak approach could also be used to generate the desired grid voltage. This approach, shown in Figure 2, involves energizing a plurality of resistors connected in series from a bypass of the high-voltage coil of the return flight transformer. For example, the shunt for the focus voltage is normally selected such that the focus voltage is about one third of the ultor voltage; in other words, the "focus ratio" is equal to approximately one third. The voltage in the shunt must be greater than the required source voltage, so that an adjustment range is available. Again, some of the plurality of resistors may comprise variable resistors, and then the required focus and screen voltages are established by adjusting these variable resistors. It has been empirically determined that, when the detected peak approach is used to generate voltages in a dedicated high voltage power supply, the ultor capacitance may adversely affect the regulation of these voltages at the lower end of the beam current condition range. . Specifically, at lower levels of screen brightness, and consequently, lower levels of beam current, the focus voltage may have a tendency to fall below an acceptable minimum level, resulting in artifacts on the beam tube screen. cathode, caused by the point defocusing of the electron beams. In addition, at these low levels of beam current, the screen voltage may also tend to drop, thus resulting in a change in the cut-off voltage of the cathode ray tube, also referred to as its "black level". . For the above reasons, there is a need for a high voltage power supply that provides regulation of the voltages generated by the detected peak approach, to ensure that these voltages do not fall below a predetermined minimum level. The present invention relates to a high voltage power supply that satisfies the aforementioned need to regulate the voltages generated by the detected peak technique, in order to ensure that these voltages do not fall below a predetermined minimum level. In accordance with a feature of the configurations of the invention described herein, an energy supply for generating a plurality of voltages for a cathode ray tube comprises: an element for generating a variable voltage; a transformer having a first terminal of a primary coil coupled with the variable voltage, and a secondary coil for providing the plurality of voltages; a switching element coupled to a second terminal of the primary coil, and which commutes at a periodic speed, in such a way that energy is stored in the primary coil when the switching element drives, and the energy is transferred from the primary coil to the secondary coil when the switching element does not conduct, to provide the plurality of voltages; a first feedback path for coupling a first feedback signal that is representative of a first of the plurality of voltages, to the generator element, to vary the variable voltage, such that the first of the plurality of voltages is regulated; and a second feedback path for coupling a second feedback signal that is representative of the variable voltage, to the generating element, to keep the voltage variable at, or above, a previously determined minimum level, such that a second voltage is regulated of the plurality of voltages. The first and second feedback signals can be added to an input to the generating element. The secondary coil may comprise a coil of the split diode type. In accordance with another feature of the configurations of the invention described herein, a high voltage power supply for a video display apparatus comprises: an element for generating a variable voltage in response to a pulse width modulated signal; a transformer having a first terminal of a primary coil coupled with the variable voltage, and a secondary coil for providing an output voltage; a switching element coupled to a second terminal of the primary coil, and which commutes at a periodic speed, so that energy is stored in the primary coil when the switching element drives, and energy is transferred from the primary coil to the secondary coil when the switching element does not conduct, to provide the output voltage; a first feedback path for coupling a first feedback signal that is representative of the output voltage, with the generator element, to vary the duty cycle of the modulated pulse width signal in response to the output voltage; and a second feedback path for coupling a second feedback signal that is representative of the variable voltage, with the generator element, to prevent the duty cycle of the pulse amplitude modulated signal from falling below a previously determined minimum level. The first feedback signal can be used to regulate the output voltage, and the second feedback signal can be used to prevent the variable voltage from falling below a predetermined minimum level. The first and second feedback signals can be added to an input to the generating element. According to still another feature of the configurations of the invention described herein, a configuration for providing power for a video display apparatus comprises: a first power supply in switched mode to generate a variable voltage as a function of a cycle of working of a signal modulated in pulse amplitude, the first power supply in switched mode comprising a first feedback path to prevent the variable voltage from falling below a previously determined minimum level; and a second switched mode power supply for generating a plurality of voltages for a cathode ray tube of the video display apparatus, the second switched mode power supply comprising a transformer having a primary coil coupled with the variable voltage, and a secondary coil for providing the plurality of voltages; wherein a second feedback path couples a feedback signal representative of one of the plurality of voltages with the first power supply in switched mode, to regulate the voltage of the plurality of voltages, varying the variable voltage, and the operation of the The first feedback path to prevent the variable voltage from falling below a predetermined minimum level serves to regulate at least one of the others of the plurality of voltages. The second feedback path can be coupled with the first feedback path at an input to the first switched-mode power supply. The above and other characteristics, aspects, and advantages of the present invention, will become clearer from the following description, read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. In the drawings: Figures 1 and 2 show configurations of the prior art for generating a focus voltage for a cathode ray tube. Figure 3 is a diagram, in block form and in schematic form, of a high voltage power supply according to the configurations of the invention described herein. Figure 4 shows a schematic diagram of the high voltage power supply of Figure 3. Figures 5 and 6 show useful voltage waveforms to explain the configurations of the invention described herein. Figure 3 shows a configuration of a power supply system for a video display apparatus that can support a range of horizontal scanning frequencies. A main power supply 10 provides a regulated VPRINCIPAL voltage, equal to about 70 volts, for the video display apparatus. The MAIN voltage is provided to different power supplies inside the video display device. For example, the VPRINCIPAL voltage is fed to the voltage rise regulator 20 formed by the pulse amplitude modulator circuit 30, the switching element Ql, the inductance Ll, the diode DI, and the capacitor Cl. voltage 20 converts the voltage VpRINCIPAL into the voltage HV_B +, which is the input voltage to the high voltage power supply 40, in a manner that is well known to those having ordinary experience in this field. The voltage HV_B + varies between approximately 110 volts and approximately 200 volts over the scanning frequency range and load conditions of the high voltage power supply 40. The high voltage power supply 40 operates in a manner that is well known to those skilled in the art. that have an ordinary experience in the matter, to generate the ultor voltage U, which is equal to approximately 32 kV. The return flight transformer IHVT is an integrated high-voltage transformer, with a high voltage coil of split-diode type 5, comprising the coil segments 2, 3 and 4 separated by the diodes D2 and D3. The focus voltage F is derived from a shunt branch at the junction Jl of the diode DI and the coil segment 3 of the return flight transformer IHVT. The variable resistor R3 of the focus screen assembly 70 is adjusted to provide the desired focus voltage F. In a similar manner, the variable resistor R14 of the focus screen assembly 70 is adjusted to provide the desired screen voltage S. The regulation of the ultor voltage U is realized by a first negative feedback path FBI from the output of the power supply of high voltage 40 to the impulse amplitude modulator circuit 30. The ultor voltage U is divided between the resistors Rl and R2, and the first resultant feedback signal VFB1 at the junction J2 of the resistors Rl and R2 is applied to the input of the error amplifier EA of the impulse amplitude modulator circuit 30, through the impedance matching network 50. The first feedback signal VFB1 controls the duty cycle of the output pulses of the impulse amplitude modulator circuit 30, thereby controlling the conduction time of the switching element Ql, and consequently, the magnitude of the voltage HV_B +. The ultor voltage U varies in response to, and in the direction of, the voltage HV_B +. Therefore, the ultor voltage U increases as the voltage HV_B + increases, and vice versa. If the brightness of the cathode ray tube screen is low - for example, if the screen is black - the beam current that is required to be supplied by the high-voltage power supply 40, it is commensurately low. Therefore, the ultor CULTOR capacitance, the ultor voltage U, and the first feedback signal VFB1 discharge very slowly. As a result, the pulse amplitude modulator circuit 30 reduces the duty cycle of its output pulses to a minimum level, eg, about 2 percent, within three switching cycles of the high voltage power supply 40. Accordingly, the switching element Ql only conducts for a minimum portion of its commutation cycle and, consequently, the voltage HV_B + drops. As is well known by those who have ordinary experience in this field. The peak amplitudes of the voltages induced in the coil segments 2, 3 and 4 by the return flight action of the return flight transformer IHVT, are proportional to the magnitude of the voltage HV_B +. Accordingly, as the voltage HV_B + drops, the peak amplitudes of the voltages induced in the secondary coils 2, 3 and 4 also decrease, until those peak amplitudes are no longer sufficient to activate the diodes D2 and D3. For example, the focus voltage F falls, as shown in Figure 5, because it can not be filled by the energy from the coil segment 2. The drop in the focus voltage F may be, for example, order of several hundred volts. That drop in the focus voltage F, when the brightness of the cathode ray tube screen is low, can create a noticeable defocusing of the information displayed on the screen. Figure 5 shows the voltage waveforms representative of the ultor voltage U, the focus voltage F, the voltage HV_B +, and a vertical deflection voltage from the vertical deflection yoke (not shown) of the video display apparatus. The ultor voltage U is displayed at 2000 V / DIV; the focus voltage F is shown at 500 V / VID; the voltage HV_B + is shown in 20 V / DIV; and the vertical deflection voltage is shown at 20 V / DIV. The time base is 5 milliseconds. The upper area HB of the waveform representative of the voltage HV_B + represents a condition of high beam current, and conversely, the lower area LB represents a condition of low beam current. Figure 5, therefore, shows that, in the transition from the low to high beam current condition, on the left edge of the HB area, the focus voltage F falls sharply, while the voltage HV_B + is increased to compensate a slight decrease in the ultor voltage U. In the transition from the high beam current condition to the low one, on the right bank of the HB area, the voltage HV_B + falls below the LB area in response to an increase in the ultor voltage U During the time when the voltage HV_B + is below the area LB, the focus voltage F is at a level which is insufficient to properly focus the electron beam, and the voltage HV_B + is insufficient to induce in the coil segment 2 of the IHVT return flight transformer, a voltage having a peak amplitude that is suitable for activating diode D2 to fill the focus voltage F. Referring to FIG. 3, in accordance with a characteristic of the present invention, a second negative feedback path FB2, provided from the junction J3 of the resistors R4 and R5, conveniently couples a second feedback signal VFB2, which is representative of the voltage HV_B +, to the impulse regulator circuit 30, through of a feedback resistor R6. The resistors R4 and R5 divide the voltage HV_B + to provide the second feedback signal VFB at junction J3 of the two resistors. The second feedback signal VFB2 is coupled to the input of the error amplifier EA of the pulse amplitude modulator circuit 30 by the resistor R6. The first feedback signal VFB1 and the second feedback signal VFB2 are summed at the input of the error amplifier EA. The second negative feedback path FB2 serves to prevent the voltage HV_B + from falling below a previously determined minimum level in a low beam current condition, for example, a beam current equal to about 50 μA. If the voltage HV_B + tries to lower, the second feedback path FB2 causes the impulse amplitude modulator circuit 30 to increase the duty cycle of its output pulses. In turn, this causes the switching element Ql to increase its driving time, and in this way the voltage HVJ3 + is increased. In this way, the second feedback path FB2 conveniently prevents the voltage HV_B + from decreasing below a predetermined minimum level, as shown in Figure 6. The selection of the appropriate values for the resistors R4 and R5 is determined by the choice of the previously determined minimum level, below which the voltage HV_B + will not fall. By maintaining the voltage HV_B + at or above a predetermined minimum level, the peak amplitude of the voltage induced in the coil segment 2 remains at, or above, a predetermined minimum level. As a result, the focus voltage F is prevented from being lowered to a sufficiently low level that the defocusing of the electron beam point becomes a problem, as shown in FIG. 6. In a similar manner, it is prevented from decrease the screen voltage S to a level sufficiently low that the "black level" of the cathode ray tube is significantly affected. The peak amplitude of the induced voltage in the coil segment 2 remains sufficiently high so that the energy in the coil segment 2 is sufficient to activate the diode D2 to fill the focus voltage F and the screen voltage S, when not driving the switching element Q2 of the high voltage power supply 40. The focus voltage F and the screen voltage S, therefore, do not decrease during a low beam condition of the video display apparatus . At the same time, the peak amplitude of the induced voltage in the secondary coil 5 is insufficient to activate the diode D4, or the peak amplitude is only sufficiently high for the diode D4 to barely conduct. Either way, as shown in Figure 6, the ultor voltage U does not increase while the diode D2 is conducting to fill the focus voltage F and the screen voltage S. A currently preferred embodiment of a power supply of high voltage in accordance with the configurations of the invention described herein, is shown in Figure 4. Resistors Rl and R2 divide in ultor voltage U to provide the first feedback signal VFB1. The high voltage resistors Rl and R2 are sized to divide the ultor voltage U by a factor of approximately 3000. The first feedback signal VFB1 is applied to the pulse amplitude modulator circuit 30 through the impedance matching network 50, that in this preferred embodiment it is implemented with a unit voltage gain buffer zone amplifier using the operational amplifier with industrial part number LM358. The intermediate amplifier 50 is required to couple the impedance of the source of the resistors Rl and R2 with the input impedance of the pulse amplitude modulator circuit 30. The output impedance of the intermediate amplifier 50 is equal to about 500 •. The low output impedance of the intermediate amplifier 50, compared to the impedance of the feedback resistor R6, ensures that the influence of the second negative feedback path FB2 is significant only in a low beam current condition. At higher beam current levels, the first negative feedback path FBI dominates the second negative feedback path FB2.

The output voltage of the intermediate amplifier 50 is divided between the resistors R9 and RIO, to provide the first feedback signal VFB1 to the impulse amplitude modulator circuit 30. The effect of the tolerances of the circuit components in the first feedback cycle FBI negative on the first feedback signal VFB1 are canceled by the resistors R7 and R8. In the exemplary embodiment shown in Figure 4, the pulse amplitude modulator circuit 30 is implemented using an integrated current mode controller circuit with industrial part number UC3842. The first feedback signal VFB1 is applied to peak 2 of the integrated circuit controller UC3842, which is the inverting input of the error amplifier. The frequency response of the error amplifier of the impulse amplitude modulator of the circuit 30 is defined by the compensation network, formed by the resistors Rll and R12 and the capacitors C2 and 03, provided between the peaks 1 and 2 of the integrated controller circuit of UC3842 current mode. In the embodiment of Figure 4, the compensation network provides high gain at low frequencies, and then brings the gain to approximately 10 dB per decade, starting at approximately 2 kHz. The compensation network provides a unit gain crossing at a frequency equal to approximately 16 kHz. The non-inverting input of the UC3842 controller integrated circuit is internally biased at approximately 2.5 volts direct current. In this way, the UC3842 responds to the first feedback signal VFB1 by changing the duty cycle of its output pulses at the peak 7, and consequently, the conduction time of the switching element Ql, in such a way that the U ultor voltage to maintain the first feedback signal VFB1 equal to approximately 2.5 volts. The current flowing through the inductor Ll and switching element Ql is detected by the resistor R13. The resulting voltage developed through the resistor R13 is coupled with the peak 3 of the integrated circuit controller of the current mode UC3842, and is used to terminate the conduction of the switching element Ql, when the peak current flowing through the inductor Ll exceeds at a threshold level. The voltage HV_B + is divided between the resistors R4 and R5 to provide the second feedback signal VFB2. The second feedback signal VFB is coupled to the non-inverting input of the error amplifier in peak 2 of the current mode controller UC3842 by resistor R6. The first feedback signal VFB1 and the second feedback signal FB2 are summed in peak 2 of the current mode controller UC3842. Referring again to Figure 4, in the case of a loss of the ultor voltage U, either by design - such as when an X-ray protection circuit is activated - or by a fault in the high-voltage power supply 40 , the first negative feedback path FBI will cause the voltage HV_B + to try to increase to compensate for the decrease in the ultor voltage U. Under these conditions, an overvoltage regulator circuit 80 is activated. The increase in the voltage HV_B + causes the voltage to rise. voltage at junction J4 of resistors R5 and R15 and voltage regulator VR1, from a value of approximately 1.7 volts to a value of approximately 2.5 volts. In this way, the voltage regulator VRl will operate in its active range, and the transistor Q3, consequently, will start to conduct current to the peak of the current mode controller UC3842. The voltage at peak 2 remains at 2.5 volts, as described above, and the voltage HV_B + remains limited at a higher value which is equal to approximately 200 volts. In the currently preferred mode, the voltage regulator VR1 is implemented using an active circuit, for example, an adjustable precision bypass regulator with industrial part number TL431. The use of an active circuit in the overvoltage regulator circuit 80 conveniently allows a much narrower regulation of the upper voltage value HV_B + than is available with a conventional zener diode. The improvement in regulation may be equal to about 2.5 percent, and this improvement is conveniently achieved without the drawback of the overvoltage regulator circuit 80 which partially conducts under a normal, but high peak beam current condition, which may occur in modes in which the video display apparatus operates at a horizontal scanning frequency that is higher than the standard horizontal scanning frequency NTSC. Having described the preferred embodiments of the invention with reference to the accompanying drawings, it should be understood that the invention is not limited to these precise embodiments, and that different changes and modifications may be made thereto by one skilled in the art, without departing of the scope or spirit of the invention, as defined in the appended claims.

Claims (14)

1. A power supply for generating a plurality of voltages for a cathode ray tube, this power supply comprising: an element (Q1) for generating a variable voltage (HV_B +); a transformer (IHVT) having a first terminal of a primary coil (1) coupled with the variable voltage, and a secondary coil (5) to provide the plurality of voltages; a switching element (Q2) coupled with a second terminal of the primary coil, and which commutes at a periodic speed, in such a way that energy is stored in the primary coil when the switching element is driven, and the energy is transferred from the primary coil up to the secondary coil when the switching element becomes non-conductive, to provide the plurality of voltages; a first feedback path to be coupled with a first feedback signal (VFB1) that is representative of a first of the plurality of voltages to the generator element, to vary the variable voltage, such that a first (U) of the plurality of voltages; characterized by a second feedback path to be coupled with a second feedback signal (VFB2) which is representative of the variable voltage (HV_B +) to the generating element, to keep the voltage variable at or above a predetermined minimum level, in such a manner that a second (S) of the plurality of voltages is regulated.

2. The high voltage power supply of claim 1, wherein the first (VFB1) and the second (VFB2) feedback signals are summed at one input (EA) to the generator element (Q1).

3. The power supply of claim 2, wherein the first of the plurality of voltages comprises a ultor voltage (U) for the cathode ray tube. The power supply of claim 3, wherein the second of the plurality of voltages comprises a grid voltage (S) for the cathode ray tube. The power supply of claim 4, wherein the grid voltage comprises one of a focus voltage and a screen voltage. The power supply of claim 5, wherein the secondary coil (5) comprises a coil of the split diode type. 7. A high voltage power supply for a video display apparatus, this high voltage power supply comprising: an element (20) for generating a variable voltage (HV_B +) in response to a signal modulated in pulse amplitude; a transformer (IHVT) having a first terminal of a primary coil (1) coupled with the variable voltage, and a secondary coil (5) to provide an output voltage (U); a switching element (Q2) coupled with a second terminal of the primary coil (1), and switching at a periodic speed, in such a way that energy is stored in the primary coil (1) when the switching element drives, and energy is transferred from the primary coil to the secondary coil (5) when the switching element becomes non-conductive, to provide the output voltage; a first feedback path to be coupled with a first feedback signal (FBI) that is representative of the output voltage (U) to the generator element, to vary the duty cycle of the modulated pulse width signal in response to the voltage of departure; characterized by a second feedback path to be coupled with a second feedback signal (FB2) which is representative of the variable voltage (HV_B +) to the generating element, to prevent the duty cycle of the pulse amplitude modulated signal from falling below 'a previously determined minimum level. The high voltage power supply of claim 7, wherein the first feedback signal (VFB1) is used to regulate the output voltage (U). 9. The high voltage power supply of claim 8, where the second feedback signal (VFB) is used to prevent the variable voltage from falling below a predetermined minimum level. 10. The high-voltage power supply of claim 9, wherein the first (VFB1) and second (VFB2) feedback signals are summed at one input (EA) to the generating element. 11. A configuration for providing power for a video display apparatus, this configuration comprising: a first switched mode power supply (20) for generating a variable voltage (HV_B +) as a function of a duty cycle of a modulated signal in pulse amplitude, this first power supply in switched mode comprising a first feedback path (FB2), to prevent the variable voltage from falling below a previously determined minimum level.; and a second switched mode power supply (40) for generating a plurality of voltage for a cathode ray tube of the video display apparatus, this second switched mode power supply comprising a transformer (IHVT) having a primary coil (1) coupled with the variable voltage, and a secondary coil (5) to provide the plurality of voltages; characterized because: a second feedback path (FBI) is coupled with a feedback signal representative of one of the plurality of voltages to the first switched mode power supply (20) to regulate one of the plurality of voltages by varying the variable voltage (HV_B +); and the operation of the first feedback path (FB2) to prevent the variable voltage from falling below a predetermined minimum level serves to regulate at least one of the others of the plurality of voltages. The configuration of claim 11, wherein the second feedback path (FBI) is coupled with the first feedback path (FB2) at an input (EA) to the first switched-mode power supply (20). The configuration of claim 12, wherein one of the plurality of voltages comprises a ultor voltage (U) for a cathode ray tube. The configuration of claim 13, wherein at least one of the others of the plurality of voltages, comprises one of a focus voltage (F) and a screen voltage (S).