Understanding safety standards is important in the selection and design of power supplies for medical healthcare systems, writes Alistair Winning
The design of medical technology is tightly regulated to ensure the safety of patients and operators. When selecting a power supply for any given project, it is important for product designers to understand how the regulations apply to the type of equipment they are building and its intended use.
Any medical device must meet strict product acceptance criteria. In the US requirements are set out by the Food and Drug Administration, while in Europe the Medical Device Directive is a harmonised standard applicable in all EU member states.
The rules encompass design, manufacture and packaging, and cover materials and components used, a suitable quality management system such as ISO 13485, and measures to minimise any risks to the patient, the operator and any others who may be in the vicinity.
As far as electrical equipment is concerned, particularly in devices connected to the AC line, a number of risks are associated with power supply units (PSUs) and other power conversion circuitry. Potential hazards include electric shock, excessive case or component temperature, fire, and the tendency to produce as well as absorb electro-magnetic interference (EMI) capable of causing faults in other parts of the equipment. Power supplies are subject to a number of general and medical-specific regulations designed to minimise or eliminate these effects and thereby maximise safety for patients and equipment operators.
Electrical safety standards
Most types of equipment designed for connection to the mains are required to meet the international safety standard IEC 60950-1, and must therefore be designed to eliminate, reduce or guard against the typical hazards mentioned earlier.
As far as electric shock is concerned, two levels of protection are required to ensure the user’s safety in normal operation or after a single fault. The standard insists that all equipment should include properly designed basic insulation. In addition to this, double insulation or reinforced insulation may also be provided, as well as further measures such as protective earthing or supplementary insulation.
These robust, user-focused safety requirements in fact enable IEC 60950-1 approved power supplies to be permitted in some medical applications where the equipment is not intended to be connected to or operated near a patient. For European medical devices, this applies to equipment operated at least 1.5 metres from the patient. US regulations impose a minimum distance of 1.83 metres.
For patient-connected devices, or equipment intended for use within 1.5m or 1.83m of a patient, a medical-approved unit meeting IEC 60601-1 is mandatory. The IEC 60601-1 specifications call for increased levels of insulation, and also stipulate reduced levels of allowable leakage current.
The insulation levels specified in both the IEC 60950-1 and IEC 60601-1 specifications are defined in terms of test voltages and creepage and clearance distances, which define respectively the minimum distance between components on the circuit board and between the components and the enclosure. The IEC 60601-1 standards impose higher requirements.
Table 1 shows that the air clearance and creepage distances for IEC 60601-1 units are up to 33% greater than for an ordinary unit meeting IEC 60950-1. Given the larger distances and increased insulation requirements, a medical approved power supply is usually physically larger than a comparable non-approved product. Hence, where the requirements of the end application permit – such as in equipment not for use in the vicinity of the patient - designers can achieve a valuable combination of safe performance, small size, high power density and low cost by considering a non-medical IEC 60950-1 PSU.
The IEC 60601-1 specifications also impose lower leakage current limits. Several leakage currents are defined. These include earth leakage current, which refers to the current flowing along the earth conductor, and enclosure leakage current, which refers to the current flowing from the enclosure to earth via the patient.
Patient leakage current, describing the current flowing to earth via the patient from an applied part, is also defined. Table 2 shows the maximum limits for each of these currents in relation to the three main types of application for IEC60601-1 approved power supplies.
It is worth pointing out that regional variations of IEC 60601-1 are in force: Table 2 shows the specifications of EN 60601-1, which is used in Europe. The US variation, UL 60601-1, stipulates a tighter limit of 0.3mA for earth and enclosure leakage current.
Among the equipment categories shown in table 2, type B equipment has no physical contact with the patient, and includes devices such as laser treatment systems. Type BF covers equipment where there is intentional physical contact with the patient, such as ultrasound scanners and monitors such as ECG equipment, and operating tables.
Finally, Type CF includes equipment such as invasive heart monitors, which have intentional cardiac physical contact with the patient.
In the type BF or CF patient-connected applications, an additional level of isolation is required to isolate the patient from earth, signal ports and the PSU output, and so meet the patient leakage current limits and protect against single fault conditions. An equipment designer can consider other items forming part of the equipment, such as plastic probes or tubing, as part of a system-level strategy to provide the required isolation and thereby protect the patient.
Where an electrical connection to the patient is needed it may be acceptable to use an IEC60601-1 approved AC/DC power supply to feed one or more isolated DC/DC converters providing secondary isolation.
Careful selection of the DC/DC converter, choosing from a range such as the XP Power JHM03-06 series of 3W-6W converters (Figure 1), can help to ensure the isolation requirements are met.
Power supply EMI performance
Compared to US regulations, European medical device standards have historically paid more attention to electromagnetic emissions and immunity to interference. Equipment manufacturers, therefore, can enhance safety and maximise market access by ensuring compliance with the European specifications.
The power supply typically contains several sources of EMI, such as switching noise and rectifier noise (in AC/DC PSUs), but can also conduct EMI from other subsystems such as motor drives, relays and high-frequency system clocks.
In addition, the power supply can be vulnerable to external effects such as electrical fast transients (EFT) and voltage surges in the supply line or radio-frequency interference (RFI).
Filtering is commonly used to combat electromagnetic interference, but the tight limits on leakage currents tend to restrict the types of filtering that can be applied in the design of medical power supplies. The low limits can preclude the use of class Y filter capacitors, for example, or may place restrictions on the capacitance values that can be specified.
Minimising stray capacitance to earth, to meet leakage current requirements, can serve to reduce common-mode noise. However, additional screening or alternative filtering techniques such as using a series inductor may be needed to meet the highest specifications for electro-magnetic compatibility (EMC). These can add size and cost to the PSU and must be designed carefully in order to perform as expected at the required frequencies. As a result, PSUs targeting IEC 60601-1 PSUs are often designed to meet EN55022/11 Class A instead of the more demanding Class B EMC specifications.
As far as protection against EFT events is concerned, placing a conventional transient suppressor between the AC line and neutral or ground can create a leakage path, and therefore can be impractical in a medical power supply. The designer must specify a suppressor with very low steady-state leakage current and high breakdown voltage (typically over 500VDC), if available – or select power-supply components capable of withstanding high-voltage transients that cannot be blocked.
Alistair Winning is technical editor, Farnell
