Technical Articles About High Voltage Power Supplies & HV Electronics
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POWER SUPPLY USE AND APPLICATION
By Cliff Scapellati
High Voltage Power Supplies for Analytical Instrumentation
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By Marcus Foxwell
Electron Beam Microscopy: Spellman's Role and Applications
Spellman's electron beam microscopy solutions provide the necessary voltages for imaging while maintaining extreme accuracy and precision. Electron microscopes are used across a range of industries and have everyday applications, while playing a role in all our lives. Fields such as materials science, medicine, and forensics have all been revolutionized by advancements in the use of these microscopes. In cancer research, for example, electron beam microscopy (EBM) helps doctors spot abnormal cell structures early, improving detection and treatment. In materials science, scientists can explore and understand nanostructures, driving innovations in electronics and energy-efficient technologies. In forensics, a detailed analysis of residues and fibers plays a crucial role in solving crimes. Spellman is the world’s leading supplier of precision high voltage solutions for electron microscopy. Our EBM solutions provide the necessary voltages to achieve the imaging using an electron beam while maintaining extreme accuracy and precision. Units range from 5 kV up to 120 kV depending on the customer's application. Most of Spellman’s products consists of bespoke solutions for customers; however, a standard range is offered. Powerful Techniques for Highly Detailed Imaging. Scanning electron microscopes (SEM) are widely acclaimed for their remarkable precision in providing highly detailed, three-dimensional images. They use a high-voltage beam of electrons to interact with the surface of a sample and detect emitted and backscattered electrons, resulting in a high-resolution image. The main components of SEM include electron beam generation, beam focusing, sample interaction, image detection, and formation. Electron beam generation is achieved using an electron gun such as a Spellman EBM power supply unit (PSU). The electrons are accelerated toward the specimen using typical voltages between 5 kV and 30 kV. This is called acceleration voltage, the main high voltage output on Spellman's EBM PSU. Voltages in a SEM can reach up to 75 kV to increase the resolution and penetration depth into the sample. The typical magnification range of this microscope is 20 times to 1 million times. For reference, a normal light microscope has a magnification of 2,000 times. The process of focusing the beam is of utmost importance in achieving high-resolution image generation. A beam is meticulously focused using a series of electromagnetic lenses. The beam then scans across the sample, with some electrons and radiation being absorbed, deflected, or emitted. Detectors capture these emitted electrons, known as secondary and backscattered electrons. A computer then uses the captured electrons to generate an image of the specimen. Transmission electron microscopes (TEM), on the other hand, focus on imaging the internal structure of materials at near-atomic resolution, unlike SEMs, which provide three-dimensional images. TEM produces two-dimensional images by passing an electron beam through the sample, which is then captured by a CMOS (complementary metal-oxide-semiconductor) camera. Unlike SEM, which detects deflected electrons, TEM captures those that pass through the sample, offering detailed insights into internal composition and morphology. These systems require voltages in the hundreds of kilovolt range while maintaining an extremely high level of accuracy. The typical magnification range of this type of microscope is 50 times to 50 million times. .
By Cliff Scapellati
High Voltage Power Supplies for Electrostatic Applications
ABSTRACT High voltage power supplies are a key component in electrostatic applications. A variety of industrial and scientific applications of high voltage power supplies are presented for the scientist, engineer, specifier and user of electrostatics. Industrial processes, for example, require significant monitoring of operational conditions in order to maximize product output, improve quality, and reduce cost. New advances in power supply technology provide higher levels of monitoring and process control. Scientific experiments can also be influenced by power supply effects. Contributing effects such as output accuracy, stability, ripple and regulation are discussed. I. INTRODUCTION The use of high voltage in scientific and industrial applications is commonplace. In particular, electrostatics can be utilized for a variety of effects. Broadly stated, electrostatics is the study of effects produced by electrical charges or fields. The applications of electrostatics can be used to generate motion of a material without physical contact, to separate materials down to the elemental level, to combine materials to form a homogeneous mixture and other practical and scientific uses. By definition, the ability of electrostatic effects to do work requires a difference in electrical potential between two or more materials. In most cases, the energy required to force a potential difference is derived from a high voltage source. This high voltage source can be a high voltage power supply. Today's high voltage power supplies are solid state, high frequency designs, which provide performance and control unattainable only a few years ago. Significant improvements in reliability, stability, control, size reductions, cost and safety have been achieved. By being made aware of these improvements, the user of high voltage power supplies for electrostatic applications can benefit. Additionally, unique requirements of high voltage power supplies should be understood as they can affect the equipment, experiments, process or product they are used in. II. OPERATIONAL PRINCIPLES OF HIGH VOLTAGE POWER SUPPLIES A simplified schematic diagram of a high voltage power supply is shown in Fig. 1. The input voltage source may have a wide range of voltage characteristics. AC sources of 50Hz to 400Hz at less than 24Vac to 480Vac are common. DC sources ranging from 5Vdc to 300Vdc can also be found. It is critical for the user to understand the input voltage requirement as this will impact overall system use and design. Regulatory agencies such as Underwriters Laboratory, Canadian Standards Association, IEC and others are highly involved with any circuits connected to the power grid. In addition to powering the main inverter circuits of the power supply, the input voltage source is also used to power auxiliary control circuits and other ancillary power requirements. The input filter stage provides conditioning of the input voltage source. This conditioning is usually in the form of rectification and filtering in ac sources, and additional filtering in dc sources. Overload protection, EMI, EMC and monitoring circuits can also be found. The output of the input filter is typically a dc voltage source. This dc voltage provides the energy source for the inverter. The inverter stage converts the dc source to a high frequency ac signal. Many different inverter topologies exist for power supplies. The high voltage power supply has unique factors which may dictate the best inverter approach. The inverter generates a high frequency ac signal which is stepped up by the HV transformer. The reason for the high frequency generation is to provide high performance operation with reduced size of magnetics and ripple reduction storage capacitors. A problem is created when a transformer with a high step up ratio is coupled to a high frequency inverter. The high step up ratio reflects a parasitic capacitance across the primary of the high voltage transformer. This is reflected as a (Nsec:Npri)² function. This large parasitic capacitor which appears across the primary of the transformer must be isolated from the inverter switching devices. If not, abnormally high pulse currents will be present in the inverter. Another parameter which is common to high voltage power supplies is a wide range of load operations. Due to the presence of high voltage, insulation breakdown is commonplace. The inverter robustness and control loop characteristics must account for virtually any combination of open circuit, short circuit and operating load conditions. These concerns as well as reliability and cost, must be addressed in the High Voltage Power Supply Inverter topology. The high frequency output of the inverter is applied to the primary of the high voltage step-up transformer. Proper high voltage transformer design requires extensive theoretical and practical engineering. Understanding of magnetics design must be applied along with material and process controls. Much of the specific expertise involves managing the high number of secondary turns, and the high secondary voltages. Due to these factors, core geometry, insulation methods and winding techniques are quite different than conventional transformer designs. Some areas of concern are: volts/turn ratings of the secondary wire, layer to layer insulating ratings, insulating material dissipation factor, winding geometry as it is concerned with parasitic secondary capacitance and leakage flux, impregnation of insulating varnish to winding layers, corona level and virtually all other conventional concerns such as thermal margins, and overall cost. The high voltage multiplier circuits are responsible for rectification and multiplication of the high voltage transformer secondary voltage. These circuits use high voltage diodes and capacitors in a "charge pump" voltage doubler connection. As with the high voltage transformer, high voltage multiplier design requires specific expertise. In addition to rectification and multiplication, high voltage circuits are used in the filtering of the output voltage, and in the monitoring of voltage and current for control feedback. Output impedance may intentionally be added to protect against discharge currents from the power supply storage capacitors. These high voltage components are typically insulated from ground level to prevent arc over. The insulation materials vary widely, but typical materials are: air, SF6, insulating oil, solid encapsulants (RTV, epoxy, etc.). The insulating material selection and process control may be the most important aspect of a reliable high voltage design. Control circuits keep all of the power stages working together. Circuit complexity can range from one analog I.C. to a large number of I.C.’s and even a microprocessor controlling and monitoring all aspects of the high voltage power. However, the basic requirement which every control circuit must meet is to precisely regulate the output voltage and current as load, input power, and command requirements dictate. This is best accomplished by a feedback control loop. Fig. 2 shows how feedback signals can be used to regulate the output of the power supply. Conventional regulation of voltage and current can be achieved by monitoring the output voltage and current respectively. This is compared to a desired (reference) output signal. The difference (error) between the feedback and reference will cause a change in the inverter control device. This will then result in a change of power delivered to the output circuits. In addition to the voltage and current regulation, other parameters can be precisely regulated. Controlling output power is easily accomplished by an (X) (Y) = Z function, (V I = W), and comparing it to the desired output power reference. Indeed, any variable found within Ohm's law can be regulated, (resistance, voltage, current and power). In addition, end process parameters can be regulated if they are affected by the high voltage power supply (i.e. coatings, flow rates, etc.). III. HIGH VOLTAGE REGULATION The importance of a regulated source of high voltage and/or constant current is critical to most applications involving electrostatics. Variations in output voltage or current can have direct effects on the end results and, therefore, must be understood as a source of error. In high voltage power supplies, the voltage references that are used to program the desired output can be eliminated as a source of significant error by the use of highly stable voltage reference I.C.s. Typical specifications of better than 5ppm/°C are routine. Similarly, analog I.C.s (op amps, A/D D/A's, etc.) can be eliminated as a significant source of error by careful selection of the devices. There remains one component, unique to high voltage power supplies, which will be the major source of stability errors: the high voltage feedback divider. As seen in Fig. 1, the high voltage feedback divider consists of a resistive divider network. This network will divide the output voltage to a level low enough to be processed by the control circuits. The problem of stability in this network results from the large resistance of the feedback resistors. Values of >100 Meg Ohms are common. This is to reduce power dissipation in the circuit and reduce the effects of temperature change due to self heating. The large resistance and the high voltage rating requires unique technology specific to high voltage resistors. The unique high voltage resistor must be "paired" with a low value resistor to insure ratio tracking under changes of temperature, voltage, humidity and time. In addition, the high value of resistance in the feedback network means a susceptibility to very low current interference. It can be seen that currents as low as 1 X 10-9 amps will result in >100ppm errors. Therefore, corona current effects must seriously be considered in the design of the resistor and the resistor feedback network. Also, since much of the resistor technology is based on a ceramic core or substrate, piezoelectric effects must also be considered. It can be demonstrated that vibrating a high voltage power supply during operation will impose a signal, related to the vibration frequency, on the output of the power supply. IV. AUXILIARY FUNCTIONS INVOLVES WITH THE HIGH VOLTAGE POWER SUPPLY . In many applications of high voltage, additional control functions may be required for the instrument. The power supply designer must be as familiar with the electrostatics application as the end user. By understanding the application, the power supply designer can incorporate important functions to benefit the end process. A typical feature that can be implemented into a high voltage power supply is an "ARC Sense" control. Fig. 3 shows a schematic diagram of an arc sense circuit. Typically, a current sensing device such as a current transformer or resistor is inserted in the "low voltage side" of the high voltage output circuits. Typically, the arc currents are equal to: I = (E/R) where I = Arc current in amperes. E = Voltage present at high voltage capacitor.. R = Output limiting resistor in ohms. The arc current is usually much greater than the normal dc current rating of the power supply. This is due to keeping the limiting resistance to a minimum, and thereby the power dissipation to a minimum. Once the arc event is sensed, a number of functions can be implemented. "Arc Quench" is a term which defines the characteristic of an arc to terminate when the applied voltage is removed. Fig. 4 shows a block diagram of an arc quench feature. If shutdown is not desired on the first arc event, a digital counter can be added as shown in Fig. 5. Shutdown or quench will occur after a predetermined number of arcs have been sensed. A reset time must be used so low frequency arc events are not accumulated in the counter. Example: A specification may define an arc shutdown if eight arcs are sensed within a one minute interval. A useful application of the arc sense circuit is to maximize the applied voltage, just below the arcing level. This can be accomplished by sensing that an arc has occurred and lowering the voltage a small fraction until arcing ceases. Voltage can be increased automatically at a slow rate. (Fig. 6). Another feature which can be found in the high voltage power supply is a highly accurate current monitor circuit. For generic applications this monitor feature may only be accurate to milliamperes, or microamperes. However, in some electrostatic applications accuracy down to femtoamperes may be required. This accuracy can be provided by the high voltage monitoring circuits. However, the user of the power supply usually must specify this requirement before ordering. V. GENERATING CONSTANT CURRENT SOURCES In many electrostatic applications, a constant current created by corona effects is desirable. This can be accomplished in a number of unique ways. A constant current source can be broadly defined as having a source impedance much larger than the load impedance it is supplying. Schematically it can be shown as in Fig. 7: Practically stated, as R2 changes impedance there is negligible effect on the current through R1. Therefore, R1 and R2 have a constant current. In a single power supply application, this can be accomplished two ways. The first is to provide an external resistor as the current regulating device. The second is to electronically regulate the current using the current feedback control as shown in Fig. 2. In applications where multiple current sources are required, it may not be practical to have multiple power supplies. In this case, multiple resistors can be used to provide an array of current sources. This is typically used where large areas need to be processed with the use of electrostatics. Fig. 8 shows this scheme. CONCLUSION This paper presented information useful to electrostatic applications using high voltage power supplies. The high voltage power supply has concerns which differentiate it from conventional power supplies. The designer of high voltage power supplies can be a key resource for the user of electrostatics. Significant control features can be offered by the high voltage power supply. In addition, safety aspects of high voltage use require important attention. High voltage sources can be lethal. The novice user of high voltage should be educated on the dangers involved. A general guideline for safety practices is found in IEEE standard 510-1983 "Recommended Practices for Safety in High Voltages and High Power Testing [4]". REFERENCES: [1] C. Scapellati, "High Voltage Power Supplies for Analytical Instrumentation", Pittsburgh Conference, March 1995. [2] D. Chambers and C. Scapellati , "How to Specify Today's High Voltage Power Supplies", Electronic Products Magazine, March 1994. [3]D. Chambers and C. Scapellati, "New High Frequency, High Voltage Power Supplies for Microwave Heating Applications", Proceedings of the 29th Microwave Power Symposium, July 1994. [4]IEEE Standard 510-1983, IEEE Recommended Practices for Safety In High Voltage and High Power Testing. Click here to Download article PDF.
By Spellman High Voltage Electronics Corporation
Bulletin STP-783, Standard Test Procedure for High Voltage Power Supplies (circa July 1983)
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By The Institute of Electrical and Electronics Engineers
IEEE Std 510-1983 IEEE Recommended Practices for Safety in High Voltage and High Power Testing
1. SCOPE . Excerpts from IEEE Standard 510-1983 have been listed in this section in order to caution all personnel dealing with high voltage applications and measurements and to provide recommended safety practices with regard to electrical hazards. Considerations of safety in electrical testing apply not only to personnel but to the test equipment and apparatus or system under test. These recommended practices deal generally with safety in connection with testing in laboratories, in the field, and of systems incorporating high voltage power supplies, etc. For the purposes of these recommended practices, a voltage of approximately 1,000 volts has been assumed as a practical minimum for these types of tests. Individual judgement is necessary to decide if the requirements of these recommended practices are applicable in cases where lower voltages or special risks are involved. . • All ungrounded terminals of the test equipment or apparatus under test should be considered as . energized. • Common ground connections should be solidly connected to both the test set and the test specimen. . As a minimum, the current capacity of the ground leads should exceed that necessary to carry the . maximum possible ground current. The effect of ground potential rise due to the resistance and . reactance of the earth connection should be considered. • Precautions should be taken to prevent accidental contact of live terminals by personnel, either by . shielding the live terminals or by providing barriers around the area. • The circuit should include instrumentation for indicating the test voltages. • Appropriate switching and, where appropriate, an observer should be provided for the immediate deenergization . of test circuits for safety purposes. In the case of dc tests, provisions for discharging . and grounding charged terminals and supporting insulation should also be included. • High Voltage and high-power tests should be performed and supervised by qualified personnel. . 2. TEST AREA SAFETY PRACTICES • Appropriate warning signs, for example, DANGER – HIGH VOLTAGE, should be posted on or near the . entrance gates. • Insofar as practical, automatic grounding devices should be provided to apply a visible ground on the . high-voltage circuits after they are deergized. In some high-voltage circuits, particularly those in . which elements are hanged from one setup to the next, this may not be feasible. In these cases, the . operator should attach a ground to the high-voltage terminal using a suitably insulated handle. In . the case of several capacitors connected in series, it is not always sufficient to ground only the highvoltage . terminal. The exposed intermediate terminals should also be grounded. This applies in . particular to impulse generators where the capacitors should be short-circuited and grounded before . and while working on the generator. • Safe grounding of instrumentation should take precedence over proper signal grounding unless other . special precautions have been taken to ensure personnel safety. 3. CONTROL & MEASUREMENT CIRCUITS Leads should not be run from a test area unless they are contained in a grounded metallic sheath. and terminated in a grounded metallic enclosure, or unless other precautions have been taken to . ensure personnel safety. Control wiring, meter connections, and cables running to oscilloscopes fall into this category. Meters and other instruments with accessible terminals should normally be placed . in a metal compartment with a viewing window. Temporary Circuits • Temporary measuring circuits should be located completely within the test area and . viewed through the fence. Alternatively, the meters may be located outside the fence, . provided the meters and leads, external to the area, are enclosed in grounded metallic . enclosures. • Temporary control circuits should be treated the same as measuring circuits and housed . in a grounded box with all controls accessible to the operator at ground potential. 4. SAFETY RULES A set of safety rules should be established and enforced for the laboratory or testing facilities. A copy of these should be given to, and discussed with, each person assigned to work in a test area. A procedure for periodic review of these rules with the operators should be established and carried out. 5. SAFETY INSPECTION . A procedure for periodic inspection of the test areas should be established and carried out. The recommendations from . these inspections should be followed by corrective actions for unsafe equipment or for practices that are not in keeping with the required regulations. NOTE: A safety committee composed of several operators appointed on a rotating basis has proven to be effective, not only from the inspection standpoint but also in making all personnel aware of safety. . 6. GROUNDING & SHORTING] • The routing and connections of temporary wiring should be such that they are secure against . accidental interruptions that may create hazard to personnel or equipments. • Devices which rely on a solid or solid/liquid dielectric for insulation should preferably be grounded . and short-circuited when not in use. • Good safety practice requires that capacitive objects be short-circuited in the following situations: • Any capacitive object which is not in use but may be in the influence of a dc electric field . should have its exposed high-voltage terminal grounded. Failure to observe this . precaution may result in a voltage included in the capacitive object by the field. • Capacitive objects having a solid dielectric should be short-circuited after dc proof . testing. Failure to observe this precaution may result in a buildup of voltage on the . object due to dielectric absorption has dissipated or until the object has been . reconnected to a circuit. NOTE: It is good practice for all capacitive devices to remain short-circuited when not in use. • Any open circuited capacitive device should be short-circuited and grounded before being . contacted by personnel. 7. SPACING • All objects at ground potential must be placed away from all exposed high voltage points at a . minimum distance of 1 inch (25.4 mm) for every 7,500 Volts, e.g. 50 kV requires a spacing of at least . 6.7 inches (171 mm) • Allow a creepage distance of 1 inch (25.4 mm) for every 7,500 Volts for insulators placed in contact . with high voltage points. 8. HIGH-POWER TESTING • High-power testing involves a special type of high-voltage measurement in that the level of current is . very high. Careful consideration should be given to safety precautions for high-power testing due to . this fact. The explosive nature of the test specimen also brings about special concern relating to . safety in the laboratory. • Protective eye and face equipment should be worn by all personnel conducting or observing a highpower . test where there is a reasonable probability that eye or face injury can be prevented by such . equipment. NOTE: Typical eye and face hazards present in high-power test areas included intense light . (including ultraviolet), sparks, and molten metal. • Safety glasses containing absorptive lenses should be worn by all personnel observing a high-power . test even when electric arcing is not expected. Lenses should be impact-resistant and have shade . numbers consistent with the ambient illumination level of the work area but yet capable of providing . protection against hazardous radiation due to any inadvertent electric arcing. 9. GENERAL • All high-voltage generating equipment should have a single obvious control to switch the equipment . off under emergency conditions. • All high-voltage generating equipment should have an indicator which signals that the high-voltage . output is enabled. • All high-voltage generating equipment should have provisions for external connections (interlock) . which, when open, cause the high-voltage source to be switched off. These connections may be . used for external safety interlocks in barriers or for a foot or hand operated safety switch. • The design of any piece of high-voltage test equipment should include a failure analysis to determine . if the failure of any part of the circuit or the specimen to which it is connected will create a hazardous . situation for the operator. The major failure shall be construed to include the probability of failure of . items that would be overstressed as the result of the major failure. The analysis may be limited to . the effect of one major failure at a time, provided that the major failure is obvious to the operator. Click here to Download article PDF.
By Paul Treglia and Clive McNamara
Next Generation in High Voltage Power Feed Equipment
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By Derek Chambers and Cliff Scapellati
Specifying High Voltage Power Supplies
Introduction In specifying a regulated high voltage power supply for a particular application, it is important to bear in mind that recent advances in power supply technology have made the latest designs smaller, lighter, and more efficient than was possible just a few years ago. New designs generally operate at high frequencies in the range of 20kHz to 100kHz, and industry-wide, have virtually replaced all units operating at line frequency, even at high power levels. All high voltage power supplies must be operated by personnel familiar with the dangers of high voltage. High voltage sources can be lethal! A general guideline for Safety Practices is found in IEEE Standard 510-1983 "Recommended Practices for Safety in High Voltage and High Power Testing." The two primary factors which have led to these developments are: • The availability of key power components which have low losses while operating at high frequency; • The development of advanced resonant power conversion techniques; Key Power Components include: • Faster switching devices (e.g. transistors, power MOSFETS, IGBTs, SCRs); • Low loss ferrite and powdered iron core materials for choke and transformer cores; • Capacitors with low dissipation factors; • Ultra fast rectifiers which have a low forward voltage drop. • Advanced Conversion Techniques include: • Zero current switching series and parallel resonant inverters (discontinuous mode); • Zero voltage switching LCC resonant inverters (continuous mode); • Soft switching and phase controlled resonant inverters; • Quasi-resonant flyback and push-pull inverters. Compared with line frequency operation, high frequencies offer the following advantages in regulated high voltage power supplies: • Smaller size and weight; • Faster response time; • Lower stored energy; • Higher efficiency; High-voltage supplies such as this multiple-output model use more efficient and higher-performance components and power conversion techniques to reduce weight and improve performance. Technology The heart of any high frequency power supply is the oscillator (or inverter) used to drive the output transformer. The specific designs used in the high voltage power supply industry are too numerous to cover in this article since each manufacturer has developed their own proprietary power switching circuits. However, there is one factor, unique to high voltage power supplies that must be considered in the choice of the oscillator or inverter topology. Specifically, the capacitance which exists across the secondary . . winding of the step-up transformer must be isolated from being reflected directly across the power switching semiconductors. This isolation can be achieved in a number of ways, including: • Using a flyback circuit; • Using an inductor or a series resonant circuit between the switching devices and the transformer; • Including sufficient leakage inductance between the primary and secondary windings of the transformer; • Operating as a self resonant oscillator. The choice of oscillator topology is also influenced by the power level of the supply. For instance, a low power unit for a photomultiplier application could use a flyback or self resonant oscillator, while higher power models (e.g. over a kilowatt) would be more likely to use a driven inverter feeding the output transformer through an inductor or a series resonant circuit. The transformer may also be designed to form part of the resonant inverter power circuit. Properly designed resonant converter designs offer the following desirable characteristics: • Zero current switching, which improves efficiency and minimize the switching losses in the high power switching devices; • Sinusoidal current waveforms in the power inverter circuit, which greatly reduce RFl interference normally associated with pulse width modulation techniques; • Simple paralleling of the supplies to obtain higher output power; • Inherent current limiting and short circuit protection of series resonant inverters. Specification Considerations Probably the most common mistake engineers make in defining a high voltage power supply is to over specify the requirements for output power, ripple, temperature stability, and size. Such over specification can lead to unnecessarily high cost, and can also lower reliability due to increased complexity and greater power density. If a particular parameter in the catalog specification is inadequate for the application, the factory should be consulted. Understanding Specification Parameters The specifications provided by the power supply manufacturer generally include information on the input and output voltages, the output regulation, ripple, and output stability. Often, more detailed information would be useful to the user. In the following sections, power supply parameters are discussed in greater detail than is normally possible on a standard data sheet, and includes definitions and descriptions of requirements encountered by users of high voltage power supplies. The specification parameters are covered in the following order: • Input Voltage • Output Voltage • Output Current • Ripple • Stability • Stored Energy • Pulsed Operation • Line Regulation • Load Regulation • Dynamic Regulation • Efficiency Higher-power high-voltage supplies, like Spellman's series SL which are rated up to 1,200W, operate from ac line power. I. INPUT VOLTAGE . The input power source specified for a particular model is determined by a number of factors including the output power capability of the supply and the form of power available in the application. In general, low power high voltage supplies having outputs between 1W and 60W typically use a dc input voltage of 24Vdc or 28Vdc, while higher power units operate from the ac power line. DC Input In many OEM applications, the high voltage supply is just one part of an electronic system in which dc power sources are already available (e.g. 24Vdc, 390Vdc). These existing dc supplies can also be used as the input power source for a high voltage supply. This arrangement is convenient and economical for modular high voltage supplies operating at low power levels. AC Input Most high power modules over 100W, and rack mounted models are designed for operation from an ac line source. These power supplies are designed to accept the characteristics of the power line normally available at the location of the user, and these can vary significantly in different parts of the world. . In the United States and Canada, the standard single phase voltage is 115/230Vac at 60Hz, while in Continental Europe and in many other parts of the world, the standard voltage is 220Vac at 50Hz. In the UK, the standard is 240Vac at 50 Hz , while in Japan the voltage is normally 100V at 50 or 60Hz. Most power supplies include transformer taps to cover this range, while some new designs cover the range 90Vac to 130Vac and 180Vac to 260Vac without taps. All countries in the European Economic Community will eventually standardize at 230V at 50Hz. Power Factor correction and universal input at power levels below 3kW can be specified for most off-the-shelf high voltage power supplies. Higher power units require custom engineering. II. OUTPUT VOLTAGE . High voltage power supplies are generally designed for continuous operation at the maximum output voltage specified in the data sheet. Laboratory bench models and high power rack units are normally adjustable over the complete voltage range from zero to the maximum specified output voltage. In these models, output voltage is indicated on either digital or analog meters, as specified. Modular supplies, on the other hand, may have either a preset output voltage, or a narrow adjustment range, and include monitor terminals instead of meters for measuring the voltage. It is not generally cost effective to specify a power supply with an output voltage greater than 20% over the maximum voltage actually needed in a particular application. III. OUTPUT CURRENT . Power supplies are normally designed for continuous operation at the full current specified in the data sheet. Current limiting is normally built into the design to prevent overload current from increasing beyond about 110% of the rated maximum value of output current. Overload trip out can usually be specified to disable the power supply when the normal output current is exceeded Current regulation is available on most high power racks and modules. This allows the output current to be controlled by a front panel potentiometer or from a remote source, and provides automatic crossover to voltage regulation when the load current is lower than the programmed value. IV. RIPPLE . Ripple may be defined as those portions of the output voltage that are harmonically related to both the input line voltage and the internally generated oscillator frequency. In high frequency switching designs it is the combined result of two frequencies, namely, the line frequency- related components and the switching frequency related components. Total ripple is specified either as the rms, or the peak-to-peak value of the combined line frequency and oscillator frequency components, and is normally expressed as a percentage of the maximum output voltage. to peak in photomultiplier, nuclear instrumentation and TWT applications) to several percent when the output can be integrated over time, such as in precipitators and E-beam welding. The high frequency ripple may generally be reduced by adding capacitance across the output. On the other hand, when there is a fast response time requirement, the value of output capacitance may have to be reduced. In critical cases, the trade off between slew rate and ripple should be worked out between the customer and the manufacturer of the power supply. V. Line frequency ripple: . When operating from an ac input source, line frequency ripple can represent a significant part of the total peak to peak ripple. Typically, the power supply is designed to have equal amounts of high frequency and line frequency ripple when operating at full output power. It should be noted that, in most designs, the magnitude of the line frequency ripple is attenuated and controlled by feedback in the regulation circuits, which normally have bandwidths to include the line ripple frequency. VI. Switching frequency ripple: In regulated supplies operating from a dc input, line frequency ripple does not exist, and the ripple frequency is simply related to the switching or oscillator frequency of the supply. To reduce switching frequency output ripple, additional filtering components, or sometimes electronic ripple canceling circuits, may be used. When filtering components, such as shunt capacitors or series resistors or inductors, are added to reduce the ripple, they introduce a delay in the control loop circuits which adversely affects the response time of the supply to changes in input or output conditions. The values of the components which control the phase of the signal in the feedback loop are then changed at the factory to maintain stable operation. If an application requires particularly small values of either high frequency or line frequency ripple, it is usually possible to provide a lower ripple at one of these frequencies at the expense of increasing the ripple at the other. In these special cases, the requirements should be discussed with the factory before an order is placed. VII. STABILITY The following factors affect the output stability of a regulated high voltage power supply: VIII. Drift in the reference voltage; IX. Offset voltage changes in the control amplifie X. Drift in the voltage ratio of the feedback divid XI. Drift in the value of the current sense resist All these variations are a function of temperature. Stability in a properly chosen reference device is generally less than 5ppm, and offset errors can be virtually eliminated by careful choice of the control amplifier. This leaves the voltage divider and the current sense resistor as the critical items affecting stability in the output voltage and current. Since these components are sensitive to temperature variations, they are selected to operate at a fraction of their power capability, and are located away from hot components. However, as the power supply warms up and the ambient temperature around the components increases, there are small changes in the ratio of the voltage divider and the value of the current sense resistor which could affect stability. The values for stability are usually given after a specified warm-up period (typically 1/2 hour). Good stability is achievable by using a divider with a low value of temperature coefficient, although this becomes more costly. XII. STORED ENERGY . The stored energy at the output of a high voltage power supply can be dangerous to operating personnel, particularly at the higher voltages since its value is a function of the square of the voltage and the value of the capacitance across the output. Certain types of loads, such as X-ray tubes, are also easily damaged by excessive stored energy in the high voltage power supply when an arc occurs. With power supplies operating at high frequency rather than at line frequency, much smaller values of smoothing capacitance can be used, and the dangers of electrocution are thereby reduced. However, it should be noted that low ripple power supplies which include additional filtering capacitance across the output have correspondingly higher amounts of stored energy. Compared with a power supply operating at line frequency, a switching supply operating at 60kHz could have a fraction of the stored energy of an equivalent line frequency supply, since the value of the output capacitance could be reduced by 1000. XIII. PULSED OPERATION While some power supplies are designed for dc operation, others can be used in pulsed power applications. In most cases, an energy storage capacitor located inside or external to the supply provides the peak pulse current, and the power supply replaces the charge between pulses. The supply operates in the current mode during the pulse and recharging parts of the cycle, and returns to the voltage mode before the next load current pulse. Pulsed loads generally fall into one of three categories: XIV. Very narrow pulses (1usec to 10usec), with a duty ratio of < 0.01 to <1%; XV. Longer pulses (100usec to 1msec), with a duty ratio between 0.05 and 0 XVI. Very long pulses (50msec to 5sec), with a duty ratio between 0.1 and 0 XVII. The first category includes pulsed radar applications in which narrow pulses, having durations in the microsecond range, are generated at typical repetition rates between 500Hz and 5kHz. Compact high power module delivers to 350 watts CW or 600 watts pulse for projection television and CRT testing. 1kV to 70kV with voltage and current programming and monitoring. The second category covers a broader range of applications such as pulsed electromagnet supplies or cable testing where most of the pulse load current is still provided by a capacitor connected across the output. Some modifications to the output and control circuits are usually needed for reliable operation in these applications, and the details of the load characteristics should be discussed with the factory to ensure reliable operation in the customer's system. The third category requires a power supply specifically designed to provide more current than its average rated value for relatively long periods. Typical applications are medical X-ray systems, lasers and high voltage CRT displays. It is essential that the actual load conditions are completely specified by the user before placing an order. XVIII. LINE REGULATION Line regulation is expressed as a percentage change in output voltage for a specified change in line voltage, usually over a ?10% line voltage swing. Measurement is made at maximum output voltage and full load current unless otherwise stated. Line regulation of most high voltage power supplies is better than 0.005%. XIX. LOAD REGULATION Load Regulation is specified at full output voltage and nominal line voltage and is expressed as a percentage change in output voltage for a particular load current change, usually no load to full load. Typical load regulation of most high voltage supplies is better than 0.01%. Click here to Download article PDF. .
By Clifford Scapellati and Paul Treglia
High Voltage Power Supply Technology for Use in Power Feed Applications
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By Boris Sasic, Sam Pindrys and Jim Willsey
Functional Role of X-ray Generators in Industrial Applications
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By Cliff Scapellati
A Product Development Process For High Voltage Power Supplies
ABSTRACT: Applications requiring high voltage power sources are growing at a healthy rate. In most cases the high voltage power supply must be custom designed for a particular application. In addition, market pressure for reduced cost, increased power, and higher reliability require significant research for new, innovative approaches. . The intent of the paper is to familiarize the user and specifier of high voltage power supplies to the development process, thereby improving future development programs. A typical development time for these new designs will be less than one year. An analysis of this development process is discussed. The development process must include: specification of the product, material and labor cost goals, vendor and component issues, process control analysis, electrical/mechanical/material engineering, definition of experiments, stress testing, safety analysis, regulatory requirements, prototype construction and testing, production documentation, design review milestones, and production start up. These requirements are presented with real world applications involving high voltage insulation systems, packaging concepts, high voltage testing, and electronic designs. I. INTRODUCTION: The foundation of any specific product development process is its ability to apply the general methods of project management. Project management tools will allow the successful execution of the process. In general, project management will coordinate all resources required to define, plan, execute, and evaluate the project. The decision to undertake a project may be complex. However, once the decision is made to move forward on a project, the decision to apply methods of project management is easy. By definition, a project signifies that important strategic goals are at stake. Without proper project management the goals will not be achieved. . In the area of high voltage power supply product development, a rigorous and detailed process has been defined and executed with a high success rate. Many areas of work, experimentation, and testing have been proceduralized specifically for high voltage. II. PRODUCT DEFINITION AND CONCEPTUALIZATION: Typically, high voltage power supplies are specified by the next level system designer. Rarely are marketing specifications the basis of the product definition. This greatly simplifies the task of finalizing specifications and getting approval to start the project. The next level system can be defined as the equipment the high voltage power supply will be used in. The system design team will be required to work closely with the engineers designing the high voltage power supply. In most cases, technical discussions can yield a sufficient specification in a matter of days. Other contract issues may cause delayed start to projects and need to be given proper attention. A. Product Conceptualization: In parallel with the technical and specification discussion, a conceptual approach will take form. Initially, relaying on existing platform technologies is the best method to reduce risks. Risk reduction analysis at this phase can save significant cost and time further into the project. Risk analysis needs to be considered for the benefit of both parties. Neither party will benefit by unwarranted and unnecessary risks. However, it requires great discipline to overcome the lure of conceptualizing an approach that may seem novel and exciting. Many engineers will fail this test and may pay a price by having program delays, reliability problems and cost overruns. Of course basic research and development cannot be sacrificed and must be carried out separately from the development efforts. The product development process can be crippled if new R&D needs to be accomplished during its phase. B. The Iterative Design Process: As stated above, using an existing platform product is the best way to minimize risks. However, even with platform products, some new design concepts will be needed. These will typically involve: new mechanical packaging, new interface circuitry, auxiliary power requirements (filament or grid supplies), etc. Initially, these new ideas will be the starting point for design details. However, if not continuously updated, these "first approach" ideas will invariable not yield the best solution. . Simply Stated: Never go with your first idea. It will become outdated quickly once the conceptual design starts to take form. The technique which is best used during these early phases is an iterative design process. Whereby initial concepts are continuously updated as the details take form. From an outside vantage point, the iterations may seem to cause project delays. But in the long run, this process will result in a more solid foundation to insure the strength of the project in its final phases. . At every critical iteration, the user or specifier must participate in the design change. This further insures the validation of the product. III. PROJECT PLANNING: Clearly defining the scope of a project is as important as the conceptualization and design of the product. Without a clear understanding of the "who, what, where, when, why, and how", a project can go off course. This can basically be seen as the business side of the project management. "Business" can be seen as a taboo subject to some technical people. This is perfectly understandable and needs to be factored into the decision making process used by the technical design team. Here, the project manager must have full understanding of the strategic business goals associated with the success of the project. The project manager must continually weigh business issues with technical issues. Difficult judgments and decisions will have to be made. It is here that the defined scope of the program will help guide in decision making. In all cases, the project manager must attempt to impart the business strategy and scope to all team members. In many cases, this will allow "buy in" when judgments are made, or a strategic course change is required. In some cases, team members will not relate to the business strategy and scope of the project. This is natural and must be managed. A. The Work Breakdown Structure: The work breakdown structure (WBS) is a concept routinely used in classical project management. The WBS clearly defines, in a hierarchal manner, the work to be performed. In larger projects, the details of work may not find their way into a formal WBS analysis. However, in small to moderate sized projects, (such as the development of a high voltage power supply), all WBS details should be made visible. In larger projects the WBS tasks may be assigned to groups or departments, but in the small to moderate sized projects, tasks should always be clearly assigned to an individual. Examples of this type of detail would be: printed wiring board design, magnetics design, experimental definitions and analysis, parts list creation, etc. An example of a WBS for a printed wiring board is shown below: 1.0 CONTROL PWB DESIGN . 1.1 Electrical Design . 1.1.1 Controller EE . 1.1.2 Diagnostics . 1.1.3 Interface . 1.2 PWB Layout Design . 1.2.1 Mechanical Area Study . 1.2.2 Component Symbols Created . 1.2.3 Routing Etc. Based on the WBS outline, the individual or group can now pursue their assigned task by organizing the time and resources required for completion. B. Resource Allocation: It is a requirement of the development process that qualified resources be assigned. Invariably, the quantity and capabilities of the team members will determine the success or failure of the project. Insufficient resources, or the unavailability of assigned resources will result in the delayed completion of WBS tasks. Even if sufficient resources are available, capability limits of the individual may also delay task completion. . When assigning resources to tasks, it is critical to specify the project and task goals. They must be specifically defined, assigned clearly to an individual who will be responsible, and with a time base for completion.. . Other influencing factors may effect resources and cause delays. Outside services such as consultants, subcontractors, or vendors can seriously hamper progress if their performance is not acceptable. When individuals are responsible for multiple products or projects, unexpected conflicts will occur. For example, a product that has completed its development phases suddenly requires a redesign or changes. This type of unexpected resource loading is typical, but very difficult to manage. Whenever possible, product support engineers should be used to support non-development activities. C. Project Schedules: The project schedule is another critical tool for managing the project. A number of project scheduling systems can be used.(1) In this specific process, a project master schedule is implemented using a project planning bar chart or GANTT chart. Here tasks are indicated in order with a sequential time base. The order of the tasks can follow the WBS. This helps to keep the WBS and schedule in one data base for easier management. Once again, as in the case of the WBS, it is important to include as many detailed tasks as practical into the project schedule. Otherwise, these tasks can easily be forgotten. Examples of these types of tasks are: Design Review Milestones and Preparation Material and Cost Tracking . Material Ordering . Process Documentation . Shipping Packaging Design . Test Equipment and Procedures . ESS Testing . Manufacturing Tooling . Manufacturing Drawings Release . Etc. When creating the project schedule it is important to have the project team understand and agree on the time allocations assigned to a task. If the time estimates are not credible, the team members may reject ownership and the task will not be completed. In addition to the team members, senior management should be informed, and individual projects should be loaded into a long term department master schedule. IV. DESIGN REVIEW GUIDELINES: The design review forum is a critical part of a project. During these forums, a project review is undertaken in order to inform concerned parties, who are not directly associated with the project team, on the progress of the project. It is important that these design reviews reinforce and amend the progress of the team. In no way can the design review replace daily and weekly project management. By their nature, design reviews occur only at critical phases of a project. Project delays will occur if important decisions are delayed until the design review milestones. A successful technique used for short term review is weekly team meetings. In this forum, the critical team members meet weekly and resolve issues quickly. This group is typically 8-12 people and consist of: project manager, electrical engineers, mechanical engineers, lab personnel, quality control, sales/marketing, and representatives from manufacturing departments. . In the process used for the high voltage power supply development, specific requirements for each design review are required and a checklist is used to insure completion of these requirements. Important design reviews milestones are defined and it is very useful when the end user of the equipment attends design reviews. These milestones occur at the following phases of the project: A. Conceptual Design Review: The conceptual design review occurs early in the project. At this stage, product concepts are reviewed along with the specification requirements. Some of the specific requirements of the conceptual design review are: Design Compatibility with Specifications . Mechanical Design Concepts . Mechanical Outline Drawings. Electrical Design Concepts . Heat Dissipation Concepts . Software/Hardware Architecture . Reliability and Environmental Stress Screening (ESS) . Manufacturability . Technical and Cost Risks . Testing and Maintenance . Program Schedule . Material and Labor Cost Estimates Each of these are discussed and reviewed. Inevitably, new tasks are required as questions are raised. These tasks are tracked as "Action Items", and are assigned to an individual along with a completion date. All action items are reviewed at the weekly meetings. This helps to insure prompt attention to these tasks. B. Critical Design Review: The critical design review occurs mid-way in the project. Here, detailed design data, experimental data, and breadboard hardware review takes place. Many topics covered in the conceptual design review will be reviewed again. However, at this phase the level of detail should be such as to clearly define and identify the product. These details can be described as: . Preliminary Performance Data (to the specification) . Mechanical Design Detailed Drawings . Electrical Schematics . Heat Dissipation and Efficiency Data . Software Specifications . ESS Test Plan . Engineering Acceptance Test Procedure (ATP) . EMC Test Plan . Breadboard Demonstration . Actual Material Costs and Project Expenditures . Once again, action items are assigned. Previous action items from previous design reviews are discussed and hopefully all issues resolved. C. Final Design Review: At this point in the project, verification of the product is reviewed. A completed acceptance test procedure is made available and any open performance or reliability issues are discussed. As before, items from previous design reviews are discussed and hard evidence of completion is presented. V. ISO9000 STANDARDS: The process for high voltage power supply design described here operates under the umbrella of the ISO9000 quality system. Specifically, this process was required to be proceduralized to sections 4.3, Contract Review, 4.4, Design Control, and 4.5, Documentation and Data Control, of the ISO9001 International Standard. It can be demonstrated that all parts of the development process address the ISO standards. Contract review is established early on during the technical specification and project conceptualization phase. Since the high voltage power supply has been defined as a customer driven requirement, the customer is involved in all aspects of the initial review. Changes throughout the product life impact the customer and supplier manage the changes. Design control adherence will naturally occur if the project planning, design review, and resource allocation are followed and properly documented. Design verification and design validation requires special attention. Many items covered in the design reviews will document the design verification. Design validation can be accomplished by in house testing to recreate the end user's conditions, or by receiving successful detailed test reports from the end user. . Although documentation and data control may not directly be required during a product development project, important critical documents are created and need to be controlled early on in the project. This will minimize uncertainty when the product release to manufacturing is done. Click here to Download article PDF. .
RESEARCH PAPERS
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Behavior of HV Cable of Power Supply at Short Circuit and Related Phenomena IEEE Transactions on Dielectrics and Electrical Insulation
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Accurate Measurement of on-State Losses of Power Semiconductors
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