Tackling EMC in medical power supplies requires innovative design

The leakage current requirements of the IEC60601-1 are difficult to achieve while, at the same time, meeting EMC compliance requirements. The maximum permissible normal condition earth leakage is 300µA for worldwide approvals, but this figure applies to the end-product as a whole, not just the power supply. To allow for additional leakage in other components it is highly desirable for the power supply to have an even lower leakage current.

The capacitors in the EMI filters allow a small amount of leakage current to flow and the more effective the filter at suppressing the interference, the more leakage it is likely to produce. It seems, therefore, that there is a trade-off between EMC performance and leakage current.

This leads to an interesting challenge since EMC performance is another crucial issue for medical power supplies. For conventionally designed switched-mode supplies this is indeed true, but EMC performance can be improved by methods other than simply providing more filtering. A better approach is to minimise the amount of interference that the power supply generates in the first place.

Improving efficiency, reducing pollution

Economic and environmental factors are playing an ever increasing part in the drive to improve the efficiencies of, and reduce pollution from, electronic equipment. Towards this goal, a number of voluntary and mandatory initiatives have been established. Among these are the Energy Star, 80+ and ErP Eco-design Directive efficiency improvement programs: more and more manufacturers of end systems are starting to ask power supply manufacturers, not only for the headline efficiency of products but also for high efficiency levels over the typical operating load range of their equipment, as well as for low standby power consumption. Even when their own equipment does not fall within the scope of any standards or Directives today, there is a real desire to produce environmentally responsible products on a voluntary basis.

There are also the expanding RoHS (controls hazardous substances) and WEEE (waste controls and recycling) Directives to consider. Up to now ‘medical devices’ and ‘monitoring and control instruments’ have been exempt from the RoHS and WEEE Directives. Although these exceptions, for many medical devices and monitoring/control instruments, will be removed, many medical equipment manufacturers have already completed or are in the process of modifying their products to comply with these directives.

Improving medical power supplies

The FETs (field effect transistors) used as electronic switches in modern switched-mode power supplies are usually configured to switch as quickly as possible because this helps to minimise losses. Unfortunately the faster the FETs switch, the more interference the switching circuit generates.

Some of the best modern power supply designs, therefore, deliberately slow down the switching operation using special ‘zero-voltage switching’ or ‘ZVS’ circuits so that the power supply’s efficiency is not compromised. ZVS circuitry still allows relatively fast switching of the transistors while achieving voltage transitions (rise and fall times) that are much slower — in of the order of 100ns (nano seconds) compared to 20ns in conventional hard-switching power supplies.

In turn, the amount of electromagnetic interference generated is greatly reduced and therefore only a small EMI filter is needed for these supplies to meet the EMC requirements of even the most demanding medical applications. With only a modest amount of filtering needed, leakage currents can also be kept to a minimum, satisfying another important requirement.

A further benefit is that the ZVS circuitry eliminates the need for an inter-winding shield within the transformer, another technique which was traditionally employed to improve EMC performance. Eliminating this shield not only allows a physically smaller transformer to be used, thereby reducing the overall size of the power supply, but also further increases the efficiency.

The majority of switched-mode power supplies designed over the last 10-20 years use hard switching pulse-width modulated circuits; the latest generations however are using resonant and multi-resonant circuits to achieve the highest possible efficiencies. Some manufacturers also offer power supplies with a number of leakage current options to allow OEMs to find the best trade-off between EMI and earth leakage current for their application.

Digital control

The most recent advancement in medical power supply design is the implementation of digital control technologies. A number of manufacturers are replacing analogue ‘housekeeping’ circuits (under-voltage lockout, fan speed control, customer signals, etc.) with microcontroller based solutions to achieve a reduction in parts count and circuit complexity. Some are also introducing new products that incorporate full digital control of the power supply, which enables improved characteristics, such as a significant peak power rating under all input voltage conditions, further parts count reduction and greater reliability.

The use of digital control (microcontroller-based) allows these power supplies to be smaller in size and much more efficient, consistent with the trend towards environmentally friendly products. So, in addition to medical equipment, these digitally controlled power supplies can be incorporated in industrial and commercial designs where space is limited, providing a smaller and cooler operating end-product.

Some of these new designs include other space saving techniques that also improve efficiency such as integrated magnetics, where multiple transformer and inductor windings are wound on the same magnetic core. The digital control element of these supplies employs a very small microcontroller that replaces bulky and less efficient analogue circuits, which are needed to regulate the DC outputs and handle the housekeeping routines that are intrinsic to all power supplies.

TDK-Lambda’s EFE range of digital power supplies include active PFC (power factor correction) which ensures EN61000-3-2 compliance and operation from a wide-range input from 90 to 264Vac for global applications. Normal condition earth leakage current is less than 300µA at up to 264Vac input, fully complying with most medical safety requirements. Other EMC improving design features include the use of low-loss Silicon Carbide (SiC) Schottky diodes in the PFC circuit which combined with a ZVS topology for the DC:DC conversion ensure Curve B EMC performance with a significant margin. These new digital power supplies are of course certified per the IEC, EN, UL, CSA and ANSI/AAMI 60601-1 standards for medical equipment, and are also be approved to IEC/EN/UL/CSA 60950-1, and designed to meet IEC/EN/UL/CSA 61010-1. The EFE300M digitally controlled power supply meets the rigorous international safety standards for medical equipment, making it suitable for use in B and BF type medical applications.