Using HV supplies in space-constrained systems

Using HV supplies in space-constrained systems

Expanding demands in several emerging markets present users of small -form-factor power supplies with some challenging and tough selection choices

BY MIKE JANTO
Emco High Voltage
Sutter Creek, CA
http://www.emcohighvoltage.com

High-voltage supplies for today's markets must be produced in large quantities with design reliability in tact. Homeland security, microfluidics, semiconductor capital equipment, and a drove of scientific instruments are examples of markets where small-form-factor high-voltage power supplies are frequently used. Oftentimes, these power supplies must meet national and international special directives on safety and environmental issues, including RoHS.

Whereas manufacturers are challenged to produce power supplies with very-long-term reliability to be used in hostile environments, system designers are faced with selecting a supplier that can deliver the performance their applications need and has the capacity to produce in high volume with consistent reliability. These conditions must be met in markets with low commodity prices, and failures less than 100 ppm are expected.

HV supplies must deliver, even in hostile environments.

Often, the task for the high-voltage supplier is to meet demands for laboratory precision in these low-cost supplies. Intelligent control circuitry consumes already limited PCB real estate, yet the immutable rules of high-voltage physics must be sustained in these compact packages.

Review fundamentals
Long-term reliability in the high-voltage world is better defined as high-voltage “containment.” High voltage is like a caged animal that never stops looking for ways to escape. Supplier selection must reach beyond the technical expertise to deliver product specifications, especially when precision outputs ranging as high as 40 kV are being generated in packages that fit in the palm of a hand.

Product designs must do more than deliver parametric performance required by an application. They must also serve as the catalyst that activates maximum performance from well-established production processes.

Look for closely linked design for manufacturing (DFM) protocol during the product design. The cornerstone that establishes long-term reliability is, of course, the basic design itself.

Circuit design is the first line of defense for long-term reliability when defined as HV containment. This includes redundancy and protection for primary circuits, immunity to ESD events, immunity to RF, and low open-field-radiated emissions. PWB topography is vital to minimizing the long-term effects of voltaic and electrolytic metal migration while providing high-volt/mil clearances.

Component orientation that adapts to the manufacturing process is also an important aspect for the packaging designer on these products. Test points for in-circuit test and auto insertion rules crowd the available real estate and must be done in close coordination with in-house or out-sourced EMS providers.

In the high-voltage reliability game, it is common for the designer to work with component suppliers to develop custom parts when standard off-the-shelf isn't good enough. Lead-time and cost of custom components may increase, but reliability benefits offset this added value.

Taking a design into production is the next tier in the long-term reliability effort. Due to precision demands, component test at incoming inspection, prior to release to stock, identifies parts to be used in critical circuits. This test activity can range from AQL sampling, to 100% screening for narrow-band tolerances, or thermal conditioning and test of precision parts.

Evaluation of a supplier's production processes, whether they exist in-house or are out-sourced, should emphasize five critical priorities for high voltage. These are cleanliness, moisture management, encapsulation integrity, electrostatic discharge (ESD) control, and automated testing that eliminates operator influence on results.

Environmental and test concerns
Cleanliness and moisture management in high voltage cannot be overemphasized. The same cleanliness controls, specific to HV production employed in the supplier's factory, must be installed and monitored at contract manufacturers in the supply chain. Highly evolved out-source controls should be in place when outsourcing is being considered.

Encapsulation for small-form-factor high-voltage supplies must allow no room for error. This is where the payoff for stringent cleanliness and moisture management exists. It is this process where causes for delayed encapsulation failure such as dewetting and dielectric breakdown are prevented, especially when subjected to thermal cycling.

Look for mature processes under stringent quality control that uses encapsulating compounds designed for the application. This should include UL 94V-0-listed compounds.

Removing human influence from test results and datalogging can only be achieved with automation. This way, pass/fail criteria and data collection are not subject to interpretation or entry errors. This allows the manufacturer to not only set derated go-no-go limits, it also provides a way for Quality Assurance to stop statistical fliers from moving to finished goods.

Networked real-time access to in-process test results by technical staff is important to early detection of yield changes in manufacturing. Test yields throughout the process not only clear units under test to move on, they also indicate overall process health as defined by monitored yield control points.

New markets and high-volume demands have brought the search for high-voltage power supplies in PC-mounted or small form factors to new levels of complexity and reliability. The emphasis on long-term reliability, laboratory precision, and low cost in these devices can make the decision difficult.