Accuracy and Precision in High-Voltage Power Supplies

­Power rails for high-voltage applications often require tight load and line regulation with low noise. This article discusses how ‘precision’ and ‘accuracy’ are separate considerations and how they are typically achieved in a power converter design.

The terms ‘accurate’ and ‘precise’ are, well, not always accurately and precisely used in everyday language, according to their formal definitions. In the world of science, they do have separate and different meanings and are used carefully when evaluating the performance of equipment or the results of an action or measurement. ‘Accuracy’ is defined as how close an action or quantity is to the ‘true or accepted’ value and ‘precision’ is how close to each other consecutive actions are. The two parameters can be quantified typically as variances in percentages or absolute values but can be entirely independent. The usual example given is shots on a target; if all are grouped evenly around, but away from the center, that is accurate but not precise. If all are off-center but grouped closely together, that is precise but not accurate. Figure 1 gives the idea.

A marksman would only be happy with the top right, but when the context is provision of a high voltage power supply rail, any of the different combinations could be acceptable depending on the application. For example, an electrostatic chuck used in semiconductor processing to pick up wafers needs relatively low accuracy in its high voltage supply (within a few percent of target value) but needs consistency, implying high precision.

When high accuracy and precision are needed

There are some applications for power supplies that need outputs to be precise and accurate over load and line changes, over multiple units, time, and changing environmental conditions. An example is a part with a high voltage output powering sensitive equipment such as photomultiplier tubes (PMTs) used for scanning electron microscopes, mass spectrometers, and medical imaging. A PMT may need 1200V at a few microamps and amplifies voltage by typically 100 million times to detect photoelectrons at very low levels. With the operating voltage directly affecting the PMT output and ultimately the sharpness of focus of the image, its absolute value (accuracy) and consistency of measurements over ambient temperature changes and between calibration intervals (precision) is important.

Mass spectrometers similarly need high performance power supplies for valid measurements. Static values of power supply output voltage and long-term stability are critical in these applications, but any superimposed noise can also be problematic, ‘drowning out’ a PMT signal for example. Additionally, repeatability using different equipment and over time and temperature variations (precision) is vital for consistent and meaningful results. High voltage supplies are often associated with highly sensitive measurements and operations by their nature and there are many more examples, such as power for e-beam lithography applications in semiconductor manufacturing and piezoelectric drives for lens/mirror positioning, etc.

The challenge for the power supply designer

General purpose power supplies (both AC-DC and DC-DC) have typical specifications that have been accepted by end users over decades. These might be an initial voltage accuracy of +/-2%, regulation of 0.5% from 10% load to full output, and 0.1% output change for a variation of line voltage from minimum to maximum. Ripple and noise together are very often specified as a maximum of 1% of output peak-to-peak, measured in a 20MHz bandwidth. The values stem from the use of practical components in the power supply design that are low cost and easily available, which benefits both the manufacturer and the user. For example, output set voltage is defined by an internal voltage reference and a resistor divider chain from the output. With tolerances added, even components for a standard product must be typically within +/- 0.5% and so are more expensive than the usual +/-1% types. At logic-level voltages, this produces deviations measured in millivolts, but if the output is 2kV, it is tens of volts, representing a problem for sensitive equipment.

Acceptable values for high voltage applications such as e-beam microscopy or e-beam lithography are more like 0.02% - 0.0001% (200ppm – 1ppm) for line and load regulation and perhaps 0.0005%/5ppm of rated voltage for peak-to-peak ripple and noise representing, for example, 100mVpp in 2kVDC.

These figures can only be obtained from a careful power supply design using precision temperature-compensated voltage references, laboratory-quality resistors, and conversion topologies that are inherently low noise, such as power oscillators or resonant types.

Internal converter transformer design is also particularly important to achieve high isolation and low noise. Careful placement of often multiple screens is necessary for low noise and the screens themselves pose isolation and insulation challenges. Even the relative position of the ‘starts’ and ‘finishes’ of a winding can be important to achieve a degree of self-shielding. Modular converters will typically be in metallic enclosures to provide screening with guaranteed safety isolation both from input to output and from input and output to the enclosure.

Practical considerations at high voltage

With high voltages, high sensing impedances are necessary to minimize dissipation. Therefore, leakage current must also be carefully controlled as the slightest contamination from a fingerprint on a PCB could be enough to affect an output or even initiate arcing across the surface. Creepage and clearance distances must scale to the high voltage for compliance with safety standards and PCB tracking must be routed without sharp corners to avoid points of high electric field intensity and consequent breakdown. At extremely high voltages, the PCB material itself needs careful consideration, with FR4 typically replaced by BT epoxy, phenolic-cured rigid laminates, or high voltage Teflon offering better dielectric strength.

High voltage modular power supplies will often be encapsulated to ease the potential problem of arcing from contamination and to provide user safety. Encapsulation can, however, locally increase electric field strength in some circumstances so it must be carefully implemented. Epoxies and silicone materials can be used with their different trade-offs of mechanical strength, moisture resistance, chemical resistance, adhesion, rework-ability, and temperature range. In practice, types are often limited by safety agency pre-approved insulation systems.

All these considerations are necessary if your power converter is going to be cost-effective, very compact and efficient. Poor design choices could result in a product which doesn’t meet market demands for miniature, cool-running products with high reliability and long life. Additionally, protection against short circuits, overloads, and over-voltages is required, while monitoring of output status is often requested along with remote programmability of the output voltage and current limit from zero to full rated value.

Specialists have done the hard work already

Specialist power conversion companies have done the hard work and offer standard products that meet the high voltage requirements. XP Power, for example, has the HCP range (Figure 2) that supplies up to 300kV at 350W, featuring ripple and noise down to 1ppm, with line regulation at 0.001%/10ppm and load regulation at 0.02%/200ppm for 0-100% rated load. Setting resolution is typically less than 0.001%/10ppm via its ‘fine’ potentiometer. The output voltage and current are programmable 0-100% with analog 0-5V control inputs, and efficiency is typically 90%. Other products are available in various packages including encapsulated PCB mount, bench top and rack mount solutions with voltage ratings up to 600kV and output power to 600kW.

Click image to enlarge

Figure 2: XP Power’s HCP range features ripple and noise down to 50ppm, with line regulation at 0.001%/10ppm for +/-10% change and 0.02% load regulation

Conclusion

Achieving both the precision and accuracy needed in a DC power supply design for demanding high voltage applications is a tough challenge. A bought-in, pre-certified part is normally the most cost-effective solution, with minimum design risk and the fastest time to market for an end product.

XP Power