High Voltage Power Supply

Chapter 1: What is the principle behind high-voltage power supplies?

This section will cover the basics of high voltage power supplies, including their design, construction, and operational principles.

What is a High Voltage Power Supply?

A high voltage power supply is a device that transforms a lower voltage into a much higher voltage, typically ranging from 1kV to 360kV, with some systems extending up to 500kV. These supplies can handle both AC and DC inputs, with DC inputs like 12Vdc and 24Vdc often being more cost-effective.

High voltage outputs are utilized across various fields such as scientific research, industrial processes, medical equipment, and telecommunications. These power supplies are engineered to deliver electrical power to various loads, from lighting in homes and factories to different electrical devices. Examples include power transformers, high voltage capacitors, and specialized units like Cockcroft-Walton and Van de Graaff generators.

High voltage power supplies often feature adjustable outputs, which can be controlled either through a computer interface or directly on the unit. They are employed across various sectors, including but not limited to the medical field, telecommunications, energy spectrum analysis, and other applications. Key characteristics of these power supplies include high output voltage stability, typically within ±1%, efficiency ratings exceeding 70%, and protections against overcurrent and short circuits, with minimal leakage current when shut down.

When handling high voltage power supplies, it is crucial to use specialized output connectors designed to prevent insulation failure, arcing, and accidental contact. For applications exceeding 20kV, standard connectors are available to ensure safety and reliability.

Voltage Classifications

Voltage classifications are generally categorized into high, medium, and low, though the specific thresholds for each category can vary by region. In the United States, the National Electrical Code (NEC) and the National Electrical Manufacturers Association (NEMA) set forth guidelines and standards for these classifications. These standards are adhered to by industries that manage electrical current and voltage.

Furthermore, the American National Standards Institute (ANSI) is responsible for overseeing and regulating these standards. According to ANSI standard C84.1-1989, voltages are classified into five distinct categories:

• High (HV) - 115,000 to 230,000 VAC
• Extra- High (EHV) - 345,000 to 765,000 VAC
• Ultra-High Voltages (UHV) - 765,000 to 1,100,000 VAC
• Medium Voltage (MV) - 2,400 to 69,000 VAC
• Low Voltage (LV) - 240 to 600 VAC

The NEC voltage standards are:

• High Distribution - 1000 to 4160 volts
• Medium Distribution - 50 to 1000 volts
• Low Distribution - 0 to 49 volts

High, Extra-High, and Ultra-High Voltages

High and extra-high voltages are generated at power plants to enhance the efficiency of electricity transmission over long distances.

Medium Voltages

Medium voltage ranges from 1kV to 69kV, though this range can vary by country or region. This voltage is distributed from substations to various end-users, including industrial, commercial, and residential customers. The elevated voltage levels in medium voltage systems facilitate the effective transmission of electricity over extended distances.

Low Voltage

Low voltage encompasses ranges from 0 to 1000 volts in AC systems and from 0 to 1500 volts in DC systems. This voltage is utilized in a variety of applications, including powering computers, home lighting, and even large industrial machinery. It is considered a safe, dependable, and practical source of electricity.

How Does a High Voltage Power Supply Work?

The fundamental operation of a high voltage power supply system revolves around transforming a low input voltage with a low frequency into a high output voltage. This process is achieved using power conversion topologies based on SMPS (Switched-Mode Power Supply) technology. An SMPS utilizes a switching regulator to efficiently convert electrical power.

Power is transferred from an AC or DC source to a DC load using high-frequency switches, such as MOSFETs, and high-frequency transformers. These components are crucial in converting a low input voltage into a high output voltage. During this process, the high-frequency switches rapidly alternate between fully on and fully off states. When a switch is off, power isn't conducted; therefore, energy storage devices like capacitors or inductors provide the necessary power. This continuous switching helps in reducing energy waste.

Essentially, a high voltage power supply is a sophisticated conversion circuit designed to transform filtered DC bus voltages, which are obtained by rectifying the mains input. The input is then converted into high-frequency AC using the aforementioned high-frequency switches. These switches operate with varying duty cycles to manage the power transfer effectively.

The high-frequency AC is further amplified using a high-frequency transformer. After stepping up the voltage, it is rectified, multiplied, and filtered at the secondary stage of the high-frequency transformer to achieve the desired output voltage levels.

Additionally, a high voltage power supply includes circuits that allow users to adjust the output voltage levels. It is equipped with fail-safe protection circuitry to ensure safe operation.

Components of a High Voltage Power Supply

A high voltage power supply system is comprised of various components, each designed to fulfill a specific role. Below are the key components:

Transformer – This device is crucial for transferring electrical energy between two or more circuits while maintaining the frequency. It can either increase (step-up) or decrease (step-down) the AC voltage levels. Additionally, it isolates the electronic components from the AC power source. The primary winding of the transformer is connected to the AC voltage source, generating AC current, while the secondary winding connects to a load. These windings are not electrically connected; instead, voltage is induced in the secondary winding via electromagnetic induction. High-frequency transformers are commonly employed in high voltage power supplies.

Rectifier – This component converts alternating current (AC) into pulsating direct current (DC). The most basic form of a rectifier is the rectifying diode, which operates as a rectifier when in the forward bias direction.

Rectifier circuits can be classified into three types: half-wave rectifier, full-wave center-tapped rectifier, and full-wave bridge rectifier.

Filter – Filters are essential for removing the ripple component from the DC output. They convert the pulsating DC from rectifiers into a smoother DC signal. The two primary types of filters are capacitance filters and resistor-capacitor (RC) filters. Capacitance filters are straightforward and economical, whereas RC filters can block certain frequencies while allowing others to pass through. Common types of RC filters include high-pass and low-pass filters.

Ripple represents the undesired AC component remaining after rectification. This AC part can potentially damage the load, making filters crucial in a power supply to smooth out the signal and reduce the AC component.

Regulator Circuits – The role of a regulator is to maintain a stable DC output. For proper load operation, it is essential to have a consistent output voltage, which is achieved by regulating the voltage despite fluctuations in the input voltage.

Common types of voltage regulators include shunt voltage regulators and series voltage regulators.

Considerations When Choosing High Voltage Power Supplies

When selecting high voltage power supplies, there are several important factors to consider:

Power Supply Style – This refers to the different configurations available, such as internal or open frame, DIN rail, PCB mount, and rack-mounted styles. A PCB mount power supply is designed to be attached to a printed circuit board. Internal or open frame models are intended to be integrated into an instrument or machine and may come with detachable covers. Rack-mounted power supplies are designed to be installed directly onto a rack, while DIN rail power supplies are mounted onto DIN rails.

Input Voltage – High voltage power supplies can be designed for different types of input voltages, including single-phase AC, three-phase AC, or DC input. It is crucial to match the power supply with the appropriate input voltage type to ensure proper operation.

Output Specifications – These specifications include the number of output channels, output polarity, and the maximum values for current, power, and voltage. Output polarity options for high voltage supplies may include negative, positive, or reversible. Outputs can be configured as either single or multiple.

User Interface – High voltage power supplies may feature various interfaces, such as a computer interface, digital front panel, or analog front panel. Some models come with software for performance monitoring and control. The display can range from analog meters and digital readouts to video display terminals. Certain power supplies also offer analog signal monitoring of the output voltage.

Chapter 2: What are the types of high voltage power supplies?

Various categories of high voltage power sources are as follows:

Modular High Voltage Power Supply

These high voltage power supplies feature detachable cables, allowing users to connect or disconnect them as needed. They are designed for OEM (Original Equipment Manufacturer) applications, where integration into larger systems is required.

These modules generally lack a front panel. Control and monitoring are managed via remote analog or digital signals. The input for the high voltage power supply can be either AC or DC, depending on the specific model.

Rack Mount and Bench Top High Voltage Power Supplies

These high voltage power supplies offer output voltages spanning from 500V to 360kV and deliver power from 10W to over 100kW. They find applications in various fields such as plasma ignition, oil well monitoring, ion milling, electron beam evaporation, and more.

Monoblock Type

Monoblock refers to a collection of pre-assembled X-Ray sources that combine high voltage power supplies, filament power supplies, control electronics, and an integrated X-Ray tube into a single, cost-effective unit. It is utilized in X-Ray analysis applications.

Application Specific High Voltage Power Supplies

Application-specific voltage supplies are tailored for particular uses. These high voltage power supplies serve various purposes, including electrostatic chucks, power feed systems, mass spectrometry, image intensifiers, and additional applications.

Triple Output High Voltage Power Supply

One example is the Model 3603, featuring three outputs, a compact design, RS-232 remote control, interlock input, and status LEDs. It is utilized in electron microscopes and measures 25 x 15 x 20 cm. This power supply integrates three high voltage units for scanning electron microscopes (SEM), including filament, bias, and accelerator voltages. These voltages can be remotely programmed via the RS-232 port, and a 3-pin Federal Standard connector links it to an SEM. It operates with a 24V DC input power source.

A C-based demo program enables the control and monitoring of the power supply, displaying the set points for each supply, measured outputs, and the internal temperatures of the individual units.

The bias voltage is derived from the accelerator supply and regulated by an electronically programmable Wehnelt resistor. The acceleration output voltage ranges from -500V DC to -30kV DC, the Wehnelt resistor's bias voltage ranges from 0 to -2kV DC, and the filament output voltage ranges from 0 to 5V DC.

Quad Output High Voltage Power Supply

For instance, the Model 1930 power supply offers four output voltages: an anode voltage of 25kV, a focus voltage ranging from 5 to 8kV, a G1 voltage of -150V, and a G2 voltage that spans from 200 to 800V.

Single Output High Voltage Power Supply

Consider the Model 2591 as an example. This high voltage power supply boasts excellent stability, minimal stored energy, and low ripple. It is specifically designed for focused ion beam systems, with low stored energy at the output to prevent damage to semiconductor wafers during arcing within the ion beam column. Its output voltage ranges from 0.01 to 30kV.

Single Output Programmable Bipolar High Voltage Power Supply

One example is the Model BP2592. This high voltage power supply is designed for electrostatic applications where periodic voltage reversal is needed. It can handle continuous short circuits without damage. A front panel switch allows for output voltage polarity changes. The programmable output voltage ranges from -30 to +30kV DC.

Single Output Unipolar High Voltage Power Supply

For instance, the Model 550 is designed for use in embedded systems. It is robust enough to handle continuous short circuits without sustaining damage and can function independently as a power supply. This cost-effective unit includes a ground leakage detection circuit and offers an adjustable output voltage ranging from 100 to 5000V.

X-Ray Power Supply

For example, the Model XPS50P50 is designed for X-ray generators. It features two outputs: a modulated current for filament power and a constant high voltage output for the anode. The power supply automatically adjusts the filament current to ensure a steady beam current.

The power supply unit offers both local and remote control modes. It includes protections against arcs, overvoltage, and short circuits, and features a safety interlock input for easier integration with safety systems. The beam current and anode voltage can be adjusted either remotely via programming voltages or locally using front panel trimpots. The anode output voltage ranges from +2kV to 50kV.

Flyback Transformers

A flyback transformer accumulates energy in a magnetic field when activated, allowing current to build up. This stored energy is subsequently transferred to the secondary winding. The transformer features a gapped core, which facilitates high energy storage.

Cockcroft-Walton Generators

A Cockcroft-Walton generator converts low AC or pulsing DC voltage into high DC voltage using a voltage multiplier ladder network of capacitors and diodes. This design eliminates the need for heavy cores and bulky insulation. The ladder network can elevate low voltages to very high levels, making Cockcroft-Walton generators popular for their cost-effectiveness and high efficiency.

Leading Manufacturers and Suppliers

Chapter 3: What are the applications and benefits of high voltage power supplies?

This chapter will explore the various applications and advantages of high voltage power supplies.

Applications of High Voltage Power Supplies

High voltage power supplies are engineered to deliver elevated output voltages. They are valuable in the following areas and applications:

• Power Distribution – in order to transmit large amounts of electric power over long distances high voltages have to be used. Voltages used are in the range of tens to hundreds of kilovolts. The power lines may be underground or overhead. Transporting electricity a long distance incurs ohmic losses which are reduced if high voltages are used during the transportation process.
• High voltages are also used to test the performance of electrical insulation of power devices. Power electronic circuits are used in the energy industry for various activities. In order to make sure that the system is reliable and works as it should, it is essential to test the performance of the insulation systems under different types of new stresses from sources that produce fast transients.
• High voltages are also used in electrostatic precipitators which are used for air cleaning.
• They are used in electrospinning and electrospraying in the pharmaceutical and automobile industries.
• High voltages are used to generate electron beams which are used for microscopy.
• High voltages are used in medical imaging.
• High voltages can also be applied in pulse power for water treatment and food processing.
• They can also be used for scientific instrumentation, electron beam lithography, and powering photomultiplier tubes.

Benefits of High Voltage Power Supplies

Maintaining a constant load at high supply voltages, such as those provided by high voltage power supplies, results in lower current. This is crucial for industrial processing machinery, which consumes significant power. Using low voltage in such machinery would require a very high current to achieve the necessary power, leading to the need for large conductors. These large conductors not only waste copper but also incur high installation costs. Conversely, high voltage allows for a lower current to deliver the same power, minimizing conductor size and reducing overall costs.

The size of circuit breakers is based on the amount of current they need to handle. Higher currents necessitate larger, more expensive circuit breakers, which can become bulky and complex to install. By reducing the current required for efficient operation, industries can save on wiring and electrical components, and decrease heat losses. This, in turn, improves the electrical efficiency of the system.

Drawbacks of High Voltage Power Supplies

While high voltages offer several advantages, they also come with certain drawbacks, which are outlined below:

• Insulation for the conductors has to increase as the voltage increases.
• The cost of switching gears and transformers increases.
• The height of the transmission tower increases – as the voltage increases it is essential to increase the height of the transmission tower as well otherwise a phase-to-earth fault would occur. This is an expensive endeavor as a taller tower would require more material and more personnel to build.
• If the voltage becomes too high it can result in the premature failure of electronic components due to overheating. This damage adds up overtime and can’t be undone.
• High voltages can cause fatal injuries and even death if handled incorrectly.

Specifications of High Voltage Power Supplies

The specification parameters for high voltage supplies include:

• Input Voltage – the input power that is specified for a certain model is determined by various factors such as the output power capability of the supply. Generally, low power high voltage supplies which have outputs ranging between 1W to 60W use a DC input voltage of 24Vdc or 28Vdc. A high power module, usually over 100W and rack mounted models operate from an AC line source. They are normally designed to accept power line characteristics usually available at the user’s location. The standard voltage in many parts of the world is 220Vac at 50Hz.
• Output Voltage – High voltage power supplies are designed to operate continuously at the maximum output voltage stated in the data sheet. Laboratory bench models as well as high power rack units are usually adjustable over the voltage range from zero to the maximum output voltage which is specified in the datasheet. However modular supplies might have a preset output voltage or a narrow adjustment range. Generally it is not cost effective to specify a power supply with an output voltage which is greater than 20% over the maximum voltage which is actually required in a particular application.
• Output Current – High voltage power supplies are designed to operate continuously at the current specified in the datasheet. Overload current is prevented from increasing beyond 110% of the maximum rated output current by current limiting, which is usually built into the design. Most high power racks and modules have current regulations available.
• Ripple – The total ripple is usually specified as the rms or the peak-to-peak value of the line frequency and oscillator frequency components combined. It’s expressed as a percentage of the maximum output voltage.
• Stability – Stability is generally less than 5ppm. Careful choice of a control amplifier can eliminate offset errors. This would mean that the voltage divider and the current sense resistor would be the only critical components affecting output voltage and current stability. These two components are sensitive to temperature as a result they are located away from hot components and are selected to operate only at a fraction of their power capability.
• Stored Energy – Stored energy at the output of a high voltage power supply can damage certain types of loads and can be harmful to personnel.
• Pulsed Operation – Some high voltage power supplies are designed for DC operation and some are used in pulsed power applications. Pulsed loads can be divided into three categories: very narrow pulses, longer pulses and very long pulses.
• Line Regulation – The line regulation for most high voltage power supplies is better than 0.005%.
• Load Regulation – The load regulation for most high voltage supplies is better than 0.01%.

Maintenance of High Voltage Power Supplies

Components of a high voltage power supply, such as transformers, switchgears, and switches, require regular maintenance to ensure optimal performance. To extend the lifespan of switchgears, consider the following measures:

Infrared inspections, conducted while equipment is energized, help identify loose bus connections before they lead to system failure. Routine maintenance checks can then address these issues.

Circuit breakers should be operated manually each year to keep contacts clean and ensure smooth operation of mechanisms.

Environmental factors like moisture, heat, and dirt can rapidly deteriorate insulation, conductive materials, and protective devices. Regular cleaning, inspection, tightening, lubrication, and exercising of switchgears are essential. Reconditioning may be necessary if on-site maintenance fails to meet current industry standards.

During reconditioning, the equipment is disassembled, cleaned, and inspected. Damaged parts are refurbished or replaced as needed. Pivot points are lubricated before reassembly and testing to ensure compliance with industry standards.

Monitoring equipment performance, such as circuit breakers, provides insights into its lifespan and helps focus maintenance efforts where needed most.

Upgrading equipment and performing short circuit analysis ensure switchgear components are properly rated to prevent catastrophic damage from improper short circuits.

Predictive maintenance tools use continuous online monitoring to detect potential failures that might be missed in periodic inspections.

For transformers, monthly checks should include monitoring the oil level in the oil cap to prevent system damage. Ensure the silica gel holes are clean for proper breathing action.

Daily checks should include verifying the oil level in the main and conservator tanks, ensuring it remains at the correct level. The silica gel should be inspected and replaced if it turns pink, and any leakage should be promptly sealed.

Annually, inspect oil pumps, air fans, and cooling equipment for the control circuit and transformer. A detailed list of annual checks and activities is outlined below:

• Cleaning all the bushings of the electrical transformer. This must be done using a soft cotton cloth.
• Oil samples from the drain valve must be tested for moisture content and dielectric strength. If the dielectric strength value is low and the moisture content is high the oil must be replaced.
• Space and illumination heaters must be checked and the terminal connections of relay wiring as well as control wiring must be tightened once a year.
• Relays and control switches along with the circuits must be cleaned with the help of an appropriate cleaning agent.
• The resistive value of the earth must be measured once a year using a clamp on the earth resistance meter.

Precautionary Measures in High Voltage Power Supplies

When working with high voltage, it is crucial to ensure that the system can handle the required voltage levels. The withstand voltage is influenced by factors such as creepage distance, insulation distance, and the shape of the electrode. Creepage distance is defined as the length along the surface of the insulator between two conductive parts. Insulation distance refers to the thickness of the insulator when it completely covers the conductive parts. Over time, the withstand voltage may decrease due to factors like humidity and dirt, potentially leading to discharge and leakage as voltage increases. Therefore, selecting appropriate insulating materials is essential for maintaining long-term insulation performance.

Improper handling of high voltages can result in severe electric shock or even fatal outcomes.

Below are some safety precautions to follow when working with high voltages:

• High voltage areas must be covered
• High voltage areas should never be touched when operating equipment
• A ground wire should always be connected
• Power should be turned off before touching equipment
• The input line must be disconnected before touching

Conclusion

A high voltage power supply is a type of power supply that is used to convert a low input voltage to a high output voltage, usually between hundreds and thousands of kilovolts. The output voltage of a high voltage power supply can be adjusted either locally or through the use of a computer interface. High voltage supplies have a variety of uses such as in medical imaging, telecommunications, and power distribution amongst others. When using high voltage supplies it’s essential to make use of a special connector to prevent arcing, insulation breakdown and accidental human contact. The components that make up a high voltage power supply are a transformer, rectifier, filter, and regular circuits.

The factors to consider when choosing a high voltage power supply are: the output specifications that the high voltage power supply is required to have, the power supply style, input voltage, and user interface. There are various types of high voltage power supplies some of them are: Monoblock, rack high voltage power supply, bench top high voltage power supply, DIN rail power supply, X-Ray generator, and many more.

High voltage power supplies are important because they produce a high output voltage that has many important uses, such as in power distribution, testing the performance of electrical insulation of power devices. They are also used in electrospinning and electrospraying in the pharmaceutical and automobile industries. The specification parameters for high voltage power supplies are: input voltage, output voltage, ripple, stability, stored energy, pulsed operation, line regulation, and load regulation. It’s essential to maintain components of high voltage supplies such as switch gears and transformers to ensure proper operation and a long life span. High voltages should be handled with care to avoid fatal injuries.

Leading Manufacturers and Suppliers