Select the right AC/DC power supply to meet unique medical requirements
will typically need to meet the requirements for both two MOOP and one MOPP. There is no single number that can be placed up front on the value required on each parameter, as their maximum values are a function of many factors. They are also defined by whether the overall design uses single or dual MOPs, and whether that MOP is a MOPP or MOOP. The IEC protection classes govern the construction and insulation of power supplies to protect the user from electrical shock. IEC protection Class II power supplies have a two-wire power cord, with two layers of insulation (or a single layer of reinforced insulation) between the user and the internal current-carrying conductors. The first layer of insulation is typically referred to as “Basic Insulation” such as the insulation normally used on wires. Then, a second layer of insulation is often an insulating case enclosing the product (and may be labeled as “double insulated”), such as the plastic case used with wall-mount and desktop power supplies. Make versus buy Basic power supply design is supported by many available components, application notes, reference designs, and more. As a result, designers might be tempted to design and build their own,
can be quite different; an operator has access to a control panel, for example, while the patient may be “connected” via probes. The 3rd edition standard specifically calls out the Risk Management Process described in ISO 14971 that includes a risk management file where fault conditions are identified and assessed. The recently activated 4th edition of this standard goes even further. First, it has added updates to take technology changes into account. Second, it also expands on the risk analysis and addresses a reciprocal concern about EMC affecting both the medical device in question and other devices in the vicinity. In other words, the standard goes beyond saying, “You should do this” or “You should do it this way”, but now requires assessment and even quantification of relevant risks and how to mitigate them. Supplies and MOP Regulatory standards have created protection classes of products that
Figure 5: Class I devices require only basic insulation and a grounded chassis, while Class II devices require additional insulation modes. Image source: CUI Inc.
Figure 7: The SDM65-24-UD-P5 is a 24 V, 2.7 A, Class II AC/ DC supply intended for external use with respect to the device it is powering. Image source: CUI Inc.
power cord. Instead, a second layer of insulation is included for operator protection due to the absence of the grounded chassis (Figure 5). There are different requirements for MOP, such as isolation, creepage, and insulation in IEC 60601-1, including whether the requirement is for MOOP or the stricter MOPP (Figure 6).
are characterized by the means of providing operator protection from hazardous voltages, designated as Class I and Class II. In a Class I product, there will be a conductive chassis which is connected to safety earth ground. Thus, an input power cord with a safety earth ground conductor is required in protection Class I products. In contrast, a Class II product will not have a safety earth ground conductor in the input
IEC 60601-1 – Medical electrical equipment – Part 1: General requirements for basic safety and essential performance, along with its various associated standards. The 3rd edition of IEC 60601-1 extends the patient focus to require an overall MOP that combines one or more means of operator protection (MOOP) and means of patient protection (MOPP). Thus, while the basic provisions of the 2nd edition, which guard against failure remain in place, the 3rd edition recognizes that the potential hazards seen by each user
body sensors, for example), so generated electromagnetic interference (EMI) or radio- frequency interference (RFI) (broadly called electromagnetic compatibility, or EMC) are also concerns. The relevant standards call out the maximum allowed EMI/ RFI generation, as well as their tolerance. Standards and means of protection (MOP) The primary standard governing medical electronics and safety is
The standard defines which classification is required in
various application situations. For example, equipment that makes physical contact with a patient, such as a blood pressure monitor,
Figure 8: Supplies in the SDM65-UD
series are offered with many standard barrel connector options for the DC output connector, as well as stripped & tinned leads. Image source: CUI Inc.
Classification
Isolation
Creepage
Insulation
One MOOP
1500 Vac
2.5mm
Basic
Two MOOP
3000 Vac
5mm
Double/Reinforced
One MOOP
1500 Vac
4mm
Basic
Two MOOP
4000 Vac
8mm
Double/Reinforced
Figure 6: The different means of protection and levels have different mandates on isolation voltage rating, creepage and insulation. Image source: CUI Inc.
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