AC DC Power Supply
Power supplies are electrical circuits and devices that are designed to convert mains power or electricity from any electric source to specific values of voltage and current for the target device...
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This article will take an in-depth look at high voltage power supply.
The article will look at topics such as:
This chapter will discuss what high voltage power supplies are, their construction, and how they function.
By definition a power supply is a device that is designed to supply electric power to an electrical load. An electrical load refers to an electrical device that uses up electric power. Such a device can be anything from lights in a house or factory to electrical appliances such as a toaster to name a few. Electric power refers to the rate at which an electric circuit per unit time transfers electrical energy. A high voltage power supply is a type of power supply that produces hundreds and sometimes even thousands of volts.
The outputs of a high voltage power supply aren’t always fixed. They can be adjusted either through a computer interface or locally. High voltage power supplies are used in a variety of industries such as but not limited to the medical industry, telecommunications, energy spectrum analysis, and many more. High Voltage Power supplies have a variety of features. Some of the main ones are: a high output voltage stability that is less or equal to ±1%, an efficiency higher than 70%, over current and short circuit protection and a low leakage current at shutdown.
When dealing with a high voltage power supply, it is essential to make use of a special output connector that will prevent insulation breakdown, arcing, and accidental human contact. There are standard connectors which are used for applications that are above 20kV.
The principle operation of a high voltage power supply system involves the conversion of a low input voltage with a low-frequency into a high output voltage. This is done through the use of power conversion topologies built upon SMPS technology. SMPS stands for switched-mode power supply. This is a power supply unit that uses a switching regulator to convert electrical power in an efficient manner.
It transfers power from an AC or DC source to a DC load. High-frequency switches, for example MOSFETs, and high-frequency transformers are used in the conversion of the low input voltage into a high output voltage. During the conversion process the high-frequency switches continually switch between full-on and full-off states. When the switch is in the full-off state power is not conducted, as a result storage devices such as capacitors or inductors are used to supply power. This continual switching is important because it minimizes wasted energy.
A high voltage supply is essentially a complex conversion circuit that transforms DC bus voltages that have been filtered. These DC voltages would have been obtained through rectification of the mains input. This input is transformed into a high-frequency AC through the use of the high-frequency switches stated prior. The switches are controlled by varying the duty ratio. This is done to regulate the power transfer.
A high-frequency transformer is used to amplify the high-frequency AC further. Once the voltages have been stepped-up, they are rectified, multiplied, and filtered at the secondary high-frequency transformer. This is done in order to produce the voltage levels that are required as the output.
A high voltage power supply contains circuitry that enables the user to control the output voltage level. The circuit contains fail-safe protection circuitry.
A high voltage power supply system consists of various components each designed to perform a particular function and these are listed below:
Transformer – this is a device that is responsible for transferring electrical energy from the primary to the secondary winding without affecting the frequency. It can be used to either step-up or step-down the level of the AC voltage. It’s also responsible for isolating the rest of the electronic system from the AC power. The transformer’s primary winding is connected to the AC voltage source, which produces AC current and the secondary winding of the transformer is connected to a load. Both the secondary and primary windings aren’t connected to each other. A voltage is induced in the secondary winding through electromagnetic induction. A high-frequency, high-voltage transformer is used in a high voltage power supply.
Rectifier – this is a device responsible for changing the AC power into a pulsating DC. The most basic rectifier is the rectifying diode. A diode operates as a rectifier in the forward direction.
There are three basic rectifier circuits that can be created using diodes namely: a half-wave rectifier, a full-wave center-tapped rectifier, and a full-wave-bridge rectifier.
Filter – it’s used to prevent the ripple component from showing up in the output. It converts the pulsating DC obtained from rectifier circuits into a smooth DC level. There are two basic types of filters namely: the capacitance filter and the resistor-capacitor filter. The capacitance filter is the simplest and cheapest filter available. The RC (resistor-capacitor) filter, filters by blocking certain frequencies and allowing others to pass through. High-Pass and Low-Pass filters are common types of RC filters.
Ripple refers to the unwanted AC part of the signal which remains after rectification. This AC part can damage the load, so it’s important to have filters in a power supply. The filter is responsible for smoothening the signal and suppressing the AC component.
Regulator Circuits – a regulator’s job is to provide a steady DC output. In order for the load to operate properly it requires a steady output voltage. The output voltage level is maintained despite changes in the input voltage level.
The most common types of voltage regulators used are the shunt voltage regulator and the series voltage regulator.
There are various factors to consider when choosing high voltage power supplies namely:
Power Supply Style – these include internal or open frame, DIN rail, PCB mount, and rack. A PCB mount high voltage power supply is mounted onto a printed circuit board. An internal or open frame high voltage power supply is made to be part of the instrument or machine that includes supplies with detachable covers. A rack mount high voltage power supply is directly mounted on a rack. A DIN rail power supply is mounted in a DIN rail.
Input Voltage – for a high voltage power supply the input voltage can be an AC single phase, AC three phase, or DC input. So it’s important to be aware of what type of input voltage is supposed to go into the power supply in order to select the right one.
Output Specifications – these include the number of outputs, output polarity and the maximum values of current, power, and voltage. When it comes to the output polarity for high voltage supplies the choices are negative voltage, a positive voltage, and reversible polarity. Outputs can be single or multiple.
User Interface – the interface for a high voltage power supply can be a computer interface, a digital front panel or analog front panel. Some high voltage power supplies come with software designed for monitoring and controlling the high voltage power supply performance. The display may be an analog meter, digital readouts, or even video display terminals. Certain high voltage power supplies monitor the output voltage with readable analog signals.
The different types of high voltage supply include:
These high voltage power supplies allow the user to either attach or detach the cables on a need basis. The cables are detachable. These high voltage power supplies are suitable for applications known as OEM (Original Equipment Manufacturer) applications where there’s a need to integrate the high voltage power supply into a higher level system.
These modules typically do not have a front panel. The controlling and monitoring of the power supply can be done through remote analog or digital signals. The input to the high voltage power supply can either be AC or DC depending on the model.
The output voltages of these high voltage power supplies can range from 500V to 360kV and they provide power that ranges from 10W to less than 100kW. They are used in plasma igniters, oil well data logging, ion milling, electron beam evaporation, and other applications.
Monoblock refers to a series of ready to use X-Ray sources made up of high voltage power supplies, filament power supplies, control electronics, and integrated X-Ray tube all packaged in a cost effective and simple assembly. It’s used in X-Ray analysis applications.
Application specific voltage supplies are designed for specific applications. These high voltage power supplies are used in a variety of applications such as electrostatic chucks, power feed equipment, mass spectrometry, image intensifiers, and many more.
An example of such a power supply is a Model 3603. It has three outputs, a compact enclosure, RS-232 remote control and interlock input and status LEDs. It’s useful in electron microscopes. The enclosure is 25 x 15 x 20 cm. This high voltage power supply contains three high voltage power supplies commonly used by SEM (scanning electron microscopes). The three output voltages are filament voltage, bias voltage and the accelerator voltages. These three voltages can be programmed remotely through the RS-232 data port. A 3-pin Federal Standard connector is used to connect the high voltage supply to an SEM. A 24V DC input power source is used.
A demo program written in C allows control and monitoring of the power supply. The set point of each supply as well as measured outputs and internal temperatures of the individual power supplies are displayed.
The bias voltage is obtained from the accelerator supply and is controlled by a Wehnelt resistor. This resistor is electronically programmable. Acceleration output voltage range is -500V DC to -30kV DC, Wehnelt (for the bias voltage) output voltage range is 0 to -2kV DC, and the filament output voltage range is 0 to 5V DC.
An example of such a power supply is a Model 1930. It has four output voltages: the anode voltage at 25kV, focus voltage with a range of 5 to 8kV, G1 voltage at -150V, and G2 voltage in the range of 200 to 800V.
An example of such a power supply is a Model 2591. It’s a high voltage power supply with high stability. It has low stored energy and low ripple. It’s designed for focused ion beam systems. Low stored energy that’s at the output prevents damage to the semiconductor wafer being probed when arcing occurs within the ion beam column. The output voltage is within the range of 0.01 to 30kV.
An example of such a power supply is a Model BP2592. This high voltage power supply is made for electrostatic applications that require output voltage reversal occasionally. It can withstand continuous short circuits without being damaged. A switch on the front panel of the high voltage supply is used to change the polarity of the output voltage. The output voltage is programmable and is within the range of -30 to +30kV DC.
An example of such a power supply is a Model 550. Is designed for embedded systems. It can also withstand continuous short circuits without being damaged. It can operate independently as a power supply. It’s a low cost power supply with a ground leakage detection circuit. The adjustable output voltage is within the range of 100 to 5000V.
An example of such a power supply is a Model XPS50P50. This power supply is intended for X-ray generators. There are two outputs, a modulated current for filament power and constant high voltage output for the anode. The power supply varies filament current automatically to maintain a constant beam current.
The power supply unit has local and remote control modes. It has an arc, overvoltage, and short circuit protection. A safety interlock input makes the integration of safety systems easier. The beam current and anode voltage are controlled either remotely by applying programming voltages or locally through front panel trimpots. The output anode voltage is within the range of +2kV to 50kV.
This chapter will discuss the applications and benefits of high voltage power supplies.
High voltage power supplies are designed to produce high output voltages. These are useful in the following fields and applications:
If the load is kept constant at a high supply voltage, such as the one produced by a high voltage power supply, ensures a lower current. This is of paramount importance in industrial processing machinery because they consume a lot of power. If a low voltage is used in such machinery a very high current would be required to produce enough power for the machine to operate. This high current would require large conductors to be generated which would waste copper and would be expensive to install. However a high voltage would reduce the current required to produce the power necessary to operate the machinery.
The size of circuit breakers is determined by the amount of current that would flow in the system. So the higher the current the larger and more expensive the circuit breakers needed would be. If the protection device increases in size it becomes heavier and its installation becomes very complex. Therefore reducing the current required for optimum operation in industries would save on wiring, and electrical components and reduce the heat losses incurred. Reducing losses increases the electrical efficiency of the system.
Although high voltages have their benefits, they also have a few drawbacks which are listed below:
The specification parameters for high voltage supplies are:
Components that make up a high voltage power supply such as the transformers, switchgears, switches, and other critical components require maintenance to ensure that they operate at optimum level. Some things that can be done to increase the lifespan of switch gears are stated below:
Performing infrared inspections are done while the equipment is energized, it helps in spotting loose bus connections before they cause full or partial system failure. A maintenance routine check can be organized to fix the issue identified.
Circuit breakers must be exercised (operated manually) every year to keep the contacts clean and help operating mechanisms move freely.
Moisture, heat, and dirt amongst other environmental contaminants cause the insulation, conductive materials and protective devices in the equipment to deteriorate fast. This is why switchgears must be cleaned, inspected, tightened, lubricated, and exercised regularly. The equipment can be reconditioned. This should be done when on-site maintenance work is unable to make the equipment meet the current industry standards.
The equipment is disassembled, cleaned, and inspected. The parts that are damaged are either refurbished or replaced, depending on the extent of the damage. Pivot points are lubricated before the equipment is assembled again and tested against the required industry standards.
Keeping track of how equipment, such as circuit breakers, is performing will give a better idea of the life expectancy of the equipment. This helps focus efforts aimed at maintaining the equipment on the areas that need the most attention.
Upgrade the equipment. Perform short circuit analysis to determine if a switchgear is rated properly. If switchgear components have an improper short circuit, damage to the equipment and surrounding area could be catastrophic.
Utilizing predictive maintenance tools employs the use of online monitoring tools that can provide warnings of impending failure. These systems operate non-stop and can detect a condition that may not have been detected during a one time inspection.
For a transformer some of the maintenance checks to be performed are highlighted below: On a monthly basis it’s essential to keep track of the oil level in the oil cap so that it doesn’t drop to a level that may result in the system being damaged. To ensure proper breathing action constantly, the holes in the silica gel must be kept clean at all times.
The oil level of the magnetic oil gauge of the main and conservator tanks must be checked on a daily basis. The oil should always be at the correct level. The silica gel must be checked on a daily basis and replaced if it turns pink. If there’s a leakage it must be sealed immediately.
The oil pumps, air fans, and all equipment used to cool down the control circuit and transformer must be inspected annually. The annual checks and activities that must be done are listed below:
When dealing with high voltage, it’s necessary to ensure it can withstand voltage. Creepage distance and insulation distance of the insulator as well as the shape of the electrode determine the withstand voltage. By definition creepage distance refers to the distance along the surface of the insulator between two parts that are conductive. The thickness of the insulator when the conductive parts are covered by the insulator completely is called insulation distance. The withstand voltage decreases with time due to humidity and dirt. This might cause discharge and leakage to occur as voltage increases. An appropriate insulating material must be chosen so that the insulation can be maintained for a long time.
Failure to carefully handle high voltages could result in electric shock and even death.
Below is a list of safety precautions to observe when handling high voltages:
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.
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