“Levels of Protection” (LOPs) are used to define the safety specifications for all electronic equipment, according to XP. Insulation or a protective earth and fuse can provide an LOP. Insulation is defined as one of five types with varying LOP ratings (see the table). XP also says that an earth can be similarly classified as functional or protective, with no protection provided by a functional earth and one level of protection by a protective or fused earth.
According to the brief, the basic principle is to provide two LOPs against electric shock. Each insulation type is defined in terms of air clearance and creepage distances. (For definitions of terms like “creepage,” see the sidebar “A Brief Safety Lexicon: Selected Definitions From EN)
The brief gets into more specifics, dealing with fine-grain differences between EN 60959 and EN 60601-1 and current leakage specs depending on the specific kind of medical equipment in terms of whether it never touches the patient, whether it just touches the outside of the patient, or whether it gets stuck inside the patient.
The brief concludes with a case history that provides a sense of where you can expect a “semi-custom” approach, in which some elements of a power supply are preapproved to work, and to what extent the design remains a work-in-progress through conformity testing. The final power supply in the case history was based on XP’s ECM series of small-footprint supplies designed for rapid qualification in medical applications that can be configured for outputs from 40 to 60 W (see the figure).
The only output used from the supply provides a nominal 13.8 V. That meant the only power elements requiring safety and EMC/EMI testing and approval were the battery charger/monitor and the dc-dc converters that provided +5, +12, and +24 V to run the product itself.
So in addition to your test lab, your power- supply vendor is an excellent source of information about global safety and EMC/ EMI conformity and conformity testing.
UPCOMING CHANGES
Beyond that, there’s a further catch. An upcoming third edition of IEC 60601-1 will introduce several new concepts to the medical approval process later this year or maybe next year (see “Evolving Standards Reshape Medical Power Supplies” at powerelectronics.com/power_systems/switch_mode_power_supplies/medical-power-supplies-standards-0407/index.html).
According to Peter Resca and Dave Murray of Astrodyne, these new concepts first and foremost will include risk management, followed by “essential performance.” New testing and design processes will be required to support these approaches. As a result, the certification process will be more about the manufacturer identifying potential hazards and documenting how they’re addressed and less about type tests with defined limits, though they will still be important, too.
“Essential performance identifies operating characteristics that can impact the safety of operators or patients. This will tie into the risk analysis performed under the risk-management system employed with the new standard,” say Resca and Murray. “The purpose is to allow the manufacturer to identify the appropriate levels to ensure safe medical devices. In some cases, this may be a reduction in limit from the current standard, but in many cases it will require additional protection or analysis.”
But wait—there’s more. The new standard introduces the concept of means of protection, which describes the isolation protection between the electrically charged circuitry and any equipment that may come in contact with the device. Resca and Murray also note that isolation protection includes the creepage/clearance distances, insulation, and protective earth connections.
Resca and Murray further separate the means of protection into means of operator protection (MOOP) and means of patient protection (MOPP). These classifications provide greater protection for patients who may be more vulnerable to the medical device in use. MOOP is more closely aligned to the traditional IEC 60950, the standard of protection for information technology equipment (ITE).
In 2007, Resca and Murray noted that we were as many as three years away from adopting the third edition of IEC 6061-1, though MOP, MOOP, and MOPP have likely evolved in the meantime with regard to risk management.
PRACTICAL DESIGN FOR SAFETY
Of course, meeting these safety standards is critical. But what about making the design actually safe?
There is a document available online that meshes with these standards. Yet as I’ve been at pains to point out, simply following it won’t guarantee that you will be able to sell your product in the markets you wish, though following its advice is the practical counterpart of understanding the bureaucracy. So additionally, you should check out a Texas Instruments seminar known as “Safety Considerations in Power Supply Design.” It’s the work of TI’s Bob Mammano and UL’s Lal Bahra, and it’s available online at focus.ti.com/lit/ml/slup227/slup227.pdf.