Don’t get zapped when buying a new high-voltage DC power supply. Understand the specifications first.
Lasers, linear accelera-tors, X-ray machines, defibrillators, radar, and numerous other applications require high-voltage power supplies. In many cases, the supply performs a well-defined role and can be treated as a simple system component. Often, it is an encapsulated or completely closed assembly with only the wiring terminals exposed. In contrast, because laboratory high-voltage DC supplies must be suitable for a wide variety of experiments and tests, they have a comprehensive control panel.
The basic specifications of 10 supplies are presented in the comparison chart that accompanies this article. To be included, a power supply must be in production and representative of a particular product line or series. For example, few supplies with the highest or lowest output voltage or the highest power were chosen.
Not all chart columns will be explained separately here because many headings are self-explanatory. Short-circuit output protection and digital control, for example, are very descriptive phrases. However, other specifications require more explanation.
Although virtually all specifications claim a range from 0 V to some maximum, the actual voltage that can be accurately controlled begins somewhat above ground. For example, the Brandenburg Alpha III Series has a minimum 20-V output on a 5-kV model, increasing to 30 V for the model in the chart.
Some supplies provide two sets of controls to give a coarse- and a fine-setting capability. Single-turn controls can be used in this arrangement. The alternative is to use 10-turn potentiometers with lockable counting dials. For the models listed, 10-turn controls are the preferred voltage and current setting method. A few supplies provide 10-turn controls for both coarse and fine adjustment.
Float Off Ground
In supplies that allow the load to be floated off ground, the low or earth side of the output is isolated from ground. If it’s not practical to connect the load to ground, these supplies permit the low end of the output to be floated as much as 300 V off ground.
The supply chassis remains connected to the safety ground, and compared to the actual output, which can be as high as 300 kV, an offset of
300 V is very small. However, it may be a sufficient margin to avoid distortion of the current flow within a circuit under test, which would have resulted from direct grounding.
To anyone who has used a simple low-voltage supply, changing the output polarity is trivial. Not so for high-voltage supplies. Although the means of changing the polarity vary in detail, they fall into two groups: use of a front-panel switch or manual reversal of connections at the supply output.
Reversing connections usually means exchanging one polarity of a high-voltage multiplier for the other or unplugging the multiplier, turning it around, and plugging it in again. One manufacturer has mounted the multiplier in a separate drawer to improve access although a manual operation still is required.
Obviously, if you need to reverse polarity often, a supply with a front-panel switch is a great convenience. However, this benefit goes beyond mere ease of use and significantly improves operator safety. Also, with no mechanical changes being made, connector wear and possible damage are avoided, resulting in reduced maintenance.
The output voltage of a supply is regulated to remain at a user-set level with minimal deviation. AC input-voltage variation, usually specified as line variation, is a disturbance that can affect the output value. The column for the output voltage line regulation in the comparison chart lists the relevant limit on this deviation, usually for a ±10% change in input voltage.
The other major cause of output variation is a changing load. Accurate control of the output level as the load changes depends on the supply’s capability to deliver current to satisfy both static and dynamic demand. Load regulation describes static performance.
Both line and load regulation often are specified in relation to the maximum output voltage, not to the value you may set. Some data sheets define the maximum voltage as the rated or nominal voltage. This means that for an output voltage only 10% of the maximum, the effective regulation percentage will be 10 times greater when referred to the actual output level.
Load regulation specifies the accuracy with which the output voltage is maintained some time after a load change has occurred. Transient response is a measure of how quickly the output voltage will return to the preset or programmed level after the load disturbance.
Of the supplies listed in the comparison chart, Glassman High-Voltage and F.u.G. share the fastest transient response time of about 1 ms. To provide even faster response, you may need to use a high-voltage amplifier. Typical suppliers include TREK and WME. As an example of what can be achieved, the TREK Model 5/80 has a 1,000-V/µs slew rate, a 5-kV maximum output voltage, and a 50-kHz large signal bandwidth.
Typically, ripple will include energy at 60 and 120 Hz from the input AC power and faster components from 20 kHz to several megahertz which are the switched-mode supply fundamental frequency and its harmonics. Unless stated otherwise, the ripple specification refers to a percentage of the maximum output value regardless of the actual output level you may have set. Adding a large output capacitance may reduce ripple but increase the stored energy and the transient response time.
Output Current Regulation
As with voltage regulation, separate specifications exist for the effects of line and load variations on current regulation. However, supplies vary greatly in their current-limit and constant-current mode behaviors.
The front-panel current control typically sets the current-limit value. This limit eventually will be reached as the output load increases and more current is drawn to maintain the desired output voltage. When the current limits, many supplies automatically switch from a constant voltage to a constant-current mode of operation. If the load increases further and no additional current is available, the output voltage reduces. LEDs often are used to indicate the operating mode.
The same effect occurs in supplies that do not switch to a constant-current mode, but in this case, the output current is not well regulated as the output voltage drops. For example, the Bertan supply provides automatic current limiting at 110% of the maximum rated output current. The supply’s single meter can be switched to read output voltage or current, but there is only one potentiometer, and it is used to set the output voltage.
As can be seen from the comparison chart, most of the manufacturers provide arc protection. Its implementation varies widely. The most basic protection simply withstands arcs. The next level recognizes that an arc has occurred and equates the event with a probable high-voltage fault condition external to the supply. The supply output is shut off for several milliseconds to allow the arc to extinguish, a so-called arc quench delay, and then restarts.
The most sophisticated arc protection counts the number of arcs, provides a quench delay after each, but shuts down the supply after a preset number of arcs. Because the assumption has been made that something serious is wrong, the operator must manually reset the supply after this type of shutdown.
During short-circuit or flashover conditions, the internal filter capacitors will be discharged very quickly. The output current impulse is limited only by internal safety resistors and can reach 500× the nominal value.
When using a high-voltage DC power supply for testing pulsed loads, an external load capacitor must be connected to the power supply. Failure to do so may result in serious damage to the supply.
Adding a capacitor limits the repetitive voltage drop across the supply’s output resistor to less than 10% of the maximum output. A continuous drop higher than 10% causes high power losses in the output resistor, which may lead to its burnout and further supply damage. Single, nonrepetitive pulses can produce up to a 100% voltage drop without damage.
Just as low-voltage bench power supplies use switched-mode technology, so too do many high-voltage supplies. High-frequency switching requires much smaller energy storage components, and its use results in lighter-weight supplies.
Typical of this type of design is F.u.G.’s HCL Series. The rectified line voltage powers a constant frequency square-wave generator that drives the primary of a high-voltage transformer. The transformer output is rectified and filtered. Regulation is via pulse-width modulation of the square wave.
An older but still very effective design approach steps up the 60-Hz line voltage through a transformer that drives a high-voltage multiplier. Regulation is accomplished using thyristors. Although this type of supply usually is heavier than a switched-mode design, it can be very robust and give long service life. Disadvantages in addition to weight include higher stored energy, today considered an operator safety hazard.
The type of insulation used in the high-voltage section also contributes to a supply’s weight. Traditionally, transformer oil and potting compounds have been used, and many manufacturers continue to do so. Glassman is not alone in using air in place of these materials, but as a result of the company’s extensive advertising, air insulation has become synonymous with Glassman. In addition to being lightweight, air-insulated high-voltage circuitry is easily inspected and repaired.
Yet another technology is used in some supplies. For example, the American High-Voltage HR Series uses a series resonant inverter. A pulse driver converts a duty-cycle modulated square wave into a sinusoidal waveform by means of a high-Q resonant tank circuit. The resulting amplitude-modulated waveform is stepped up by a high-frequency, high-voltage transformer that, in turn, drives a high-voltage multiplier. The DC output from the multiplier then is regulated.
Input and Output Considerations
Shielded output cables often are used to avoid electrostatic interference with nearby circuitry. That’s a good reason, but ensuring a proper mechanical termination of the shield to the connector is critical to avoid arcing.
Output discharge time is an important safety specification. Before alterations to output cabling or adjustments to connected equipment can be made, the output of the supply must be allowed to decay to zero after it has been turned off. High-voltage DC supplies contain capacitors that may require several minutes to completely discharge.
Output stability, or deviation over time, is not included in the comparison chart but may be important to your application. Generally, at least a 30-min warm-up period must be allowed before the stability specification will be met. Qualifying conditions vary. For example, after a 30-min warm-up period, the Genvolt Europa range will exhibit <0.01% per hour deviation and <0.02% in eight hours.
Stability is quoted assuming line voltage, load, and all other conditions are held constant. There is a separate output voltage temperature coefficient that must be taken into account if the supply is exposed to significant temperature fluctuations. As an example, the Bertan supply specifies a temperature coefficient of 50 ppm of maximum per °C. For the Model 210-10R, this is equivalent to a deviation of 0.5 V/°C.
Many of the supplies listed in the comparison chart have been in production for several years. There’s nothing wrong with continuing to use proven designs, and Jeremy Simon, director of sales and marketing at Glassman High-Voltage, confirmed that applications haven’t changed too much in the last several years.
However, he went on to qualify the comment: “The major changes in the market are driven by efforts to improve the cost, performance, reliability, and size of a supply. In an increasing number of cases, standard products just won’t do, and modified or custom offerings are required.”
Mr. Simon chronicled high-voltage supply development from the original 60-Hz designs to more recent high-frequency switched-mode supplies. Critical to their development were the many improvements concurrently made to switching semiconductors such as field effect transistors (FETs) and insulated gate bipolar transistors (IGBTs), high-voltage capacitors, and many types of power supply control ICs.
The incorporation of new technologies hasn’t stopped. An exciting view of the future of power supply design has been presented by Mariano Moran, vice president of engineering and technology at Del High-Voltage.1 Rather than use several separate analog control loops for an X-ray system’s cathode, anode, filament, and bias supplies, Del has developed a DSP-based controller subassembly for high-voltage systems.
The DSP-CORE/X-RAY Module contains all the electronics necessary to control a complete X-ray system including the regulators, interface, overload, and interlock circuits as well as the computer interface. Features such as an arc counter, digital filtering, and pulse-width modulation are accomplished via DSP techniques.
Today, there’s little economic necessity for this kind of technology in relatively simple laboratory supplies. But if you want to make certain that you’re buying the latest technology in a high-voltage supply, ask the vendor to specify the clock rate and cache memory depth as well as the maximum output voltage.
1. Moran, M., “Digitally Controlled High-Voltage Power Supplies,” Del High-Voltage Technical Paper, www.delhv.com/del_dsp.html
Thanks to Werner Glier, an application engineer at Magnavolt Technologies, for his help in preparing this article.
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