AC/DC power supplies in medical applicationsFollow article
Professional and home healthcare equipment has never been in greater demand, and it would be rare to find products that do not need electrical power. Naturally, standards apply for the electrical safety, functionality and electromagnetic compatibility of ACDC power supplies.
Safety and essential functionality requirements are specified in IEC 60601-1:2005 and collateral documents. This standard appears as an ANSI/AAMI document in the US, a ‘Euronorm’ in Europe and worldwide with some local deviations. As of January 2021, the latest amendments are A1:2012 and A2:2020, updating the standard to remove out of date references to other standards, correcting errors, making clarifications and to align better with IEC 62368-1, the standard for audio/video, communication and IT equipment. The medical EMC standard IEC 60601-2 is currently at edition 4.
Built-in medical power supplies are only part of product certification
AC-powered medical equipment could have an internal ‘built-in’ power supply or an external adapter at lower powers. If internal, the product designer takes responsibility for the high voltages and energy sources present and will often opt to buy a ‘medically certified’ power supply. However, it is the end product that is ultimately certified for the application, so there are other considerations. For example, labelling, connectors, internal wiring to the power supply and fusing.
Experienced engineers will also know that an EMC-compliant built-in power supply does not guarantee a system pass in testing, due to variations in grounding, external wiring and pick-up from other noise sources. A common practice is to add a further EMI filter at the AC inlet, perhaps incorporated in a connector. However, care should be taken to not exceed the limits for leakage current, which can be as low as 10µA for the most stringent ‘cardiac-floating’ (CF) product rating. Leakage from an external filter directly adds to the AC power supply internal leakage and there is any way a danger that the external filter will interact with the internal one, possibly even making EMI worse. So ideally, a built-in power supply should connect as directly as possible to the AC inlet without extra filtering and if it holds a high medical certification level and meets EMI limits stand-alone, with a good margin, there is a good prospect that the system will be overall safety- and EMC-compliant.
One of the features of the latest version of IEC 60601 is that it recognizes that some medical environments such as laboratories are more similar to commercial than a hospital, and only ‘operator’ protection is required rather than ‘patient’. This means that in theory, power supplies meeting IEC 62368-1 can, with provisos, be used in some medical areas where only measures of operator protection (MOOPs) are required. Similarly, it is possible that a ‘reinforced’ grade IEC 62368 power supply could be used as part of a system with further levels of protection to achieve overall measures of patient protection (MOPPs). However, the fusing of medical equipment has different requirements and a safety assessment would need to be performed to verify that the IEC 62368-1 product is really compliant in all possible medical environments it could be used in, including ‘professional’, ‘home healthcare’ and ‘special’, such as in emergency vehicles. The environment over-voltage category, pollution degree and altitude also need to be considered. Further, the stringent EMC requirements of IEC 60601-2 apply to all medical applications, including operator environments, so commercial or industrial power supplies are unlikely to be compliant.
The safest solution is the best option
For the safest solution, the best option is to specify a power supply with 2 x MOPP certification and compliant EMC performance for the intended application. Power supplies from the RECOM RACM230-G, RACM550-G and recently released RACM-1200V series are examples (Figure 1).
Figure 1: RECOM RACM230-G, RACM550-G and RACM-1200V medically-certified power supplies
These have peak or forced-air power ratings of 230W, 550W and 1200W respectively. When fans are not allowed, as is often the case in medical environments, the products can be baseplate-cooled delivering 160W, 300W and 800W. All products have 2 x MOPP/250VAC rating up to 5000m altitude and feature up to Body/BF (Body/Body Floating) rating for earth leakage (RACM550/1200). The safety barriers in each of the RECOM power supplies are shown in Figure 2.
Figure 2: Safety barriers RACM230/550/1200 series
When a power supply has external signal connections, as is the case with the RACM1200-V, with its PMBusTM interface, these connections are classified as SIP/SOP (Signal Input/Output Part) and must be isolated from the patient connection with 2 x MOPP. This could be done in the signal lines externally to the power supply but alternatively, if the patient-connected power rail is low power, a separate medically-certified DC/DC converter with 2 x MOPPs can be set between the power supply and patient connection (Figure 3). This technique also reduces leakage current to a very low value. For power ratings from 1W to 30W, the REMxx series from RECOM is a good solution.
Figure 3: A 2 x MOPP DC/DC converter provides SIP/SOP isolation
It has become less viable to use AV/IT grade power supplies with add-on modifications, or declarations of restrictions of use in medical equipment. Fully medically-certified power supplies such as those from RECOM are an assured way to meet the required safety and EMC standards for a wide range of medical application environments. It can even be an overall lower cost approach when development and certification costs, along with time to market are factored in.
The RECOM products mentioned are in industry-standard form factors with class-leading power density with the RACM550-G and RACM1200-V models including an isolated +5V 1A/1.2A standby output. All types also feature a ‘smart fan’ output which can control system fan cooling, if needed.
The RACM1200-V with its single 24V or 48V output boasts an efficiency of over 95% and meets EuP Lot 6 requirements for standby losses.
When selecting an AC/DC power supply for a medical application, RECOM is a good choice supported by its distribution partner RS Components, providing local stockholding and access to technical support.