By Robin Jeffery, Technical Market Analyst at TDK-Lambda EMEA
Emissions take the form of conducted and radiated EMI (Electro Magnetic Interference). Conducted EMI is the electrical noise (0.15 to 30MHz) conducted back to the AC source which can affect other devices operating from that source. Radiated EMI is high frequency electrical energy (30MHz to 1GHz) generated by the power supply that can intrude into cables and signal lines, causing other devices to malfunction.
Immunity covers myriad aspects that could interfere with the power supply’s operating parameters. Unlike the emissions standards which have just two levels, Class A and Class B (more severe), the immunity standards have multiple test levels and performance criteria. The criteria for passing can range from “performance within the specification limits” to “loss of function which is not recoverable”. The latter is not normally acceptable.
ESD testing (electrostatic discharge) ensures that if an operator or patient touches or even approaches a medical device and creates a static spark, the device will continue to perform. With a simple adapter (external) power supply, a failure could result by someone touching the LED indicator on the plastic case of the product. A discharge could pass through the LED and disrupt operation.
Radiated susceptibility tests that the product will not be affected by an external high frequency signal. For home medical devices, such as a dialysis machine, this may be from a nearby mobile phone or Wi-Fi router.
Electrical fast transient bursts could be caused by an air conditioning unit turning off or other devices being plugged in to the same AC outlet. Surge immunity is also tested, which could be caused by electrical storm surges on the AC line. These could be as high as 4kV.
Conducted susceptibility tests immunity to RF signals being injected into the power leads. A magnetic field test will show that the power supply can withstand external magnetic fields. These could even arise from operating a vacuum cleaner in the proximity of the equipment.
The AC supply voltage, although normally continuous, can be affected by outages for a limited amount of time. Voltage dips and interruptions are simulated ranging from 10ms to 5s. Although a typical off-the-shelf power supply is not expected to operate without power for more than a few milliseconds, it is expected to return to normal functionality after the outage occurs. The medical standard now refers to “essential performance”. To clarify, the designer/manufacturer has to determine if a loss of performance or functionality of their medical device product or system will result in an acceptable risk or an unacceptable risk to the patient or operator.
Many power supply products on the market only meet radiated and conducted EMI with external filtering and shielding, which may cause difficulty for the design engineer. This is especially true when the end equipment has to be operated without an earth ground connection (Class II) for applications like home healthcare.
To help ease integration, TDK-Lambda recently launched several power supplies that require no external filtering and are certified for both the industrial and medical markets. For example, the 30 to 1500W CUS-M series complies with the most stringent Class B level for both conducted and radiated EMI. For more information about EMC, please visit: www.emea.lambda.tdk.com/emc