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Several methods exist for providing adequate electrical isolation in equipment to be connected to a medical patient. One approach involves the use of ac-dc power supplies designed for medical applications.
Specification IEC60601 defines safety and electromagnetic compliance (EMC) for medical systems. It specifies that when equipment, in normal use, necessarily comes into physical contact with the patient to function, it is classified as a body floating (BF) applied part.
This is an end-equipment requirement, and the connection is not necessarily an electrical one. For example, it could be something like an ultrasound scanner. Typical BF equipment that may have an electrical connection includes patient monitors and surgical equipment. Others that do not necessarily have an electrical connection include infusion pumps and respirators.
The challenge for the designer is to ensure that the connection between the patient and the equipment minimizes leakage current under normal operation, and also protects the patient from mains voltages under fault conditions by isolating the patient from ground. In applications where an electrical connection is needed, the power system becomes the critical factor for meeting these requirements.
It is important to appreciate that the majority of fully approved medical power supplies are not suitable for direct connection to patients, because the output-to-ground isolation meets neither the isolation nor the leakage current requirements set forth in IEC60601. In most power supplies, the output-to-ground insulation is defined as operational, meaning it cannot be relied on to provide a safety isolation barrier. Rather, it merely serves as the insulation needed for the circuit to work properly.
A “basic” level of insulation, however, will provide the required isolation. There are two possible solutions. The first is to provide an additional level of insulation between the output of the power supply and the applied part (Fig. 1). The alternative is to design a power supply with an output that meets the basic insulation requirements.
Isolation with DC-DC Converters
An additional level of insulation can be achieved using an appropriate dc-dc converter between the ac-dc power supply and the applied part. The dc-dc converter needs to have a minimum of basic insulation at mains working voltage and additional double/reinforced insulation at the normal working voltage, defined as the voltage seen across the isolation barrier. This approach has the equally important benefit of reducing patient leakage current. The ac-dc power supply's main transformer provides 4000 Vac isolation but has stray capacitance between its windings. An additional dc-dc converter effectively puts a small capacitance in series with that of the main transformer, reducing the overall capacitance to ground, and hence lowering the leakage current.
In Fig. 1, the term live part refers to the dc output of the power supply in the IEC60601 specification, not the mains. It is the terminology used in IEC60601. The additional insulation provided by isolation barrier 2 isolates the patient from the power supply and, in doing so, isolates the person from ground because that is the only potential connection to ground.
This kind of solution is ideal where the current drawn via the applied part is on the order of microamps or milliamps and the total power consumption is 50 W or less. Typical applications include electro cardiogram (ECG) or other patient-monitoring equipment. While this solution also can be used for higher power applications, it becomes less efficient and more expensive in those cases. Furthermore, there are a limited number of standard dc-dc converters available at higher power levels.
Isolation within an AC-DC Power Supply
Where more power is required, such as in surgical equipment, an ac-dc power supply designed to meet the basic insulation requirements in a standalone configuration is preferred for reasons of size, cost, efficiency and reliability because there are fewer parts in the total power system. In some circumstances, the proven electrical design of a standard ac-dc power supply can be adapted rather than a custom design. This can provide faster time to market, lower costs and shorter approval times for the product.
The main design considerations to meet the BF demands are usually physical as opposed to electrical changes to an existing design. A specific example of component placement in a unit that complies with these requirements can be seen in Fig. 2, which shows the physical design of a 450-W, BF-certified power supply that delivers a single 48-V output at 9.5 A for a surgical drill.
Key design considerations for isolation are:
The layout of the printed circuit board to provide adequate air clearances and creepage distances between the output(s) and ground to achieve basic insulation at the mains voltage.
Any capacitor used between the output(s) and ground to reduce noise needs to be Y-rated.
Every component in the output circuit needs to be physically and electrically separated from ground. For example, heatsinks must be electrically floating, rather than connected to ground or to the power-supply casing. This means that heatsinks may need to be larger than normal to achieve adequate dissipation.
Electrolytic capacitors need to be carefully positioned. Their cases are normally connected to ground but they need to be far enough away from the edge of the printed circuit board to avoid accidental contact with the power-supply housing.
Leakage Current and EMC
The requirements for good EMC performance and low patient leakage current are fundamentally in conflict, because capacitors used to provide good EMC performance contribute to higher leakage current. However, concentrating on minimizing patient leakage current by optimizing the main transformer design can overcome this problem.
Maintaining acceptable creepage and clearance distances to meet medical specifications is an important transformer design issue, and one that can lead to increased size. The power-supply topology, to a large extent, will determine the required creepage and clearance distances. Low-power designs, up to about 100 W, will normally use a flyback topology to minimize costs, but measures are needed to control the relatively high voltage across the transformer isolation barrier if size is to be minimized, too.
One technique to minimize transformer size, adopted in XP's ECM40-60 series of IEC60601 approved power supplies (Fig. 3), is to combine two modes of operation: discontinuous, in which the input inductor current is reduced to zero at the beginning of each cycle, and continuous, in which it is not.
Discontinuous mode confers greater loop stability. However, because the energy stored is a function of the square of the peak inductor current, continuous mode allows more energy to transfer, permitting the use of a smaller transformer. When using this technique, care has to be taken to eliminate the potential loop instability and core saturation problems normally associated with continuous operation.
The ripple voltage on the reservoir capacitors and the stray capacitance of the transformer are the main determinants of patient leakage current (Fig. 4). There is usually a need to minimize the size and cost of reservoir capacitors, but the larger the capacitor, the lower the input ripple and the lower the patient leakage current, so an appropriate compromise has to be made for each design.
Measuring Leakage Current
The patient leakage current can be measured according to IEC60601-1 using the circuit in (Fig. 5). This schematic represents the case where no isolation is provided between the live or ground connection of a secondary ac mains and the patient (as is usually the case with an electric motor; the isolation does not meet the IEC60601-1 standard requirements and so cannot be guaranteed).
The power-supply output is fully loaded, and the leakage current between secondary and ground is measured with a patient-equivalent filter. The resistor, R, is a purely resistive load with a value such that the full power will be drawn from the output. The voltages, both ac and dc, are measured across the filter. Usually, the dc component is negligible compared to the ac component.
The maximum current allowed is 100 µA (˜100 mVac) for BF-type applied part and 10 µA (˜10 mVac) for a cardiac floating (CF)-type applied part, which makes intimate contact with the patient. Examples of both applied part types are shown in the table.
A separate measurement of the patient leakage current must be performed on the fully assembled medical equipment. However, these preliminary measurements give a good idea of what the final value of the patient leakage current will be in the completely assembled unit.