Planning for power entry modules in your electrical design could save you time and expenses when using RFI/EMI filters and other components to reduce leakage current.
A module is a component that offers multiple functions at the point where AC power enters your design. Choosing the components you need in a module—and the right arrangement of capacitors, resistors, and inductors (and adding or removing them) for your specific needs—should start in the design phase versus trial and error. The functions/components available in power entry modules are power inlets, access outlets, switches, fuses, voltage selectors, circuit breakers, RFI/EMI filters (mA), or medical filters RFI/EMI (2-5µA).
Hospital-Grade Power Entry Modules
The components needed in a module will vary industry to industry—the industry will likely determine where you add, remove, or place “X” amount of capacitors, inductors, and resistors. In the medical industry, your module will almost certainly need an RFI/EMI filter due to worldwide medical standards, which may require the medical filter’s leakage current to be measured in microamps (2-5µA) and not milliamps mA for minimal leakage current as patient safety is paramount inside a medical facility. Most equipment in the proximity of the patient is required to limit leakage current below 100mA to prevent human lethality. Yet an exact current number is impossible to determine due to age, the condition of one’s heart or overall health. The best advice is to keep electrical current as far away from the patient as possible. Period.
“There’s always a trade off with using filters,” said Dan Ford, Technical Support Specialist at Interpower. “In medical, you want less leakage current, so you would want capacitors removed since they add to the leakage current. Simply put, a filter provides “clean” electricity to the equipment.
“In regard to medical equipment,” Ford said, “if you go beyond the 12–15 feet length for medical cords, it becomes much more difficult to meet the equipment standard requirements for emissions testing and leakage current for the entire system. You don’t want high levels of EMI/RFI or elevated levels of leakage current in equipment attached to the patient.”
The main circuit components in filters include inductors, resistors, and capacitors. “The difference between a standard filter and a medical-grade filter is that in the medical version the capacitors between the circuit conductors have been removed as to decrease leakage current values,” Ford said. Low leakage is achieved in modules with medical grade filters by removing the y-capacitors (Cy) from between the two signal paths (L and N) and ground, as well as resizing inductor values. Removal of these capacitors drastically reduces leakage current—however, it also has a negative impact on the overall filter performance (insertion loss). Filtering in medical grade modules will always be less effective than standard filters, but that is the trade-off for lower leakage currents.
“Also, the arrangement of inductors, capacitors, and resistors play a vital role in decreasing unwanted EMI as it is shunted away to ground,” Ford said. “This is especially important in medical equipment where leakage current values are tightly controlled by country-specific or international safety standards to ensure patient safety.” Filter performance specifications are typically based on an industry standard 50-ohm test circuit, and certain aspects of an application’s design may alter the way a filter performs when the circuits are connected.
When considering a power entry module:
Know what country-specific/international standards you are complying with to determine leakage current measurement in milliamps 1/1,000 (mA), or microamps 1/1,000,000 (µA).
Enclose your module in an RF shielded metal box to further reduce noise at the source (many of Interpower modules are already constructed inside the metal box).
Obtain samples of filters to test within their applications to ensure they deliver expected current values to find out what arrangement of inductors, capacitors, and resistors best suit the needs of your electrical design.
