Select the right AC/DC power supply to meet unique medical requirements
is the unique nature of medical applications and the very real possibility of component or system faults causing patient or operator harm. It’s especially challenging since the patient is often in direct contact with sensors, probes, or other transducers that can conduct current directly into the body, thus posing a greater risk than casual contact. Begin with safety basics Although shock risk is normally associated with higher voltages, there is only an indirect correlation. Patient or user shock is due to current flowing through the body and back to its source. However, if that current has no return-flow path, then there is no risk, even if the person is touching a high-voltage line.
Except for very specialized exceptions, the line-operated AC/ DC power supply has an input-side isolation transformer which can serve two roles: ■ Provide voltage step up/down of the line voltage as needed before it is rectified to DC ■ Provide input/output isolation so there is no path for the flow of current through the user and back to the neutral line. This is critical in the event of a fault that could put voltage and current on the surface of the unit, and thus to and through the operator or patient (Figure 1) With the isolation transformer in place, this current flow cannot happen because the isolation transformer does not have a wire path from the AC-line neutral to Earth, so the current will not flow through the user.
mains – even if a component or wiring failure provides a new current path on the secondary side. However, no transformer in the real world is perfect and there is always some primary-secondary interwinding capacitance (Figure 3). An even more sophisticated model adds additional sources of interwinding capacitance, shown in Figure 4. This undesired capacitance which allows the flow of leakage current is a function of many variables such as wire size, winding pattern, and transformer geometry. The resultant value can range from as low as one picofarad (pF) to a few microfarads (µF). In addition to transformer capacitive leakage, other sources of unintentional capacitances are spacings on printed circuit boards, insulation between semiconductors and grounded heatsinks, and even parasitics between other components. Transformer leakage current due to capacitance is not the only medical-standard power-supply concern. Obviously, basic AC safety and insulation are concerns. Depending on voltage and power levels, these supplies may need a second, independent insulation barrier in addition to the primary one.
Use AC line or batteries? Although untethered, battery- powered, and portable devices have become common and even preferred in many consumer and commercial products, there are still many situations where battery power is either impractical or undesirable. This is especially the case for medical instrumentation where consistent, reliable, and immediate availability is critical. Among the reasons medical systems may prefer or mandate AC line operation are: ■ High power, voltage, or current requirements that may require
Figure 4: There are other transformer capacitances, in addition to Cps1. Image source: Power Sources Manufacturers Association
the two points of contact with the body are located, such as across or through the chest, from an arm down to the feet, or across the head. Transformer isolation and leakage are critical Leakage is current that passes through the dielectric insulation, whether due to physical “leaks” from the imperfect nature of the insulation, or due to capacitive currents that can cross even exceptionally good insulation. Although leakage current is never desirable, it’s a much more serious concern for some medical applications. A simplified model of a transformer shows perfect galvanic (ohmic) isolation between its primary and secondary sides in Figure 2. No current can flow directly from the AC mains to the powered product – thus forming a complete current-flow loop back to the AC
Why worry about current? Standard line voltage (110/230 volts; 50 or 60 hertz (Hz)) across the chest – even for a fraction of a second – may induce ventricular fibrillation at currents as low as 30 milliamperes (mA). If the current has a direct pathway to the heart such as via a cardiac catheter or other kind of electrode, a much- lower current of less than 1 mA (AC or DC) can cause fibrillation. These are some standard thresholds which are often cited for current through the body via skin- surface contact: ■ 1 mA: Barely perceptible ■ 16 mA: Maximum current an average-size person can grasp and “let go” ■ 20 mA: Paralysis of respiratory muscles ■ 100 mA: Ventricular fibrillation threshold ■ 2 A: Cardiac standstill and internal organ damage The levels are also a function of the current-flow path, meaning where
a large, heavy, costly battery system along with recharge management circuitry
■ Many medical sites run 12, 18, and even 24-hour daily shifts due to patient scheduling ■ Even for those systems that can use rechargeable batteries for primary power or emergency backup, those batteries need to be charged while the system is in use, during which time the AC/ DC supply must provide power In principle, any properly sized, standard off-the-shelf (OTS) AC/ DC supply with suitable voltage and current ratings should be a good fit for these systems. Yet while they are adequate in the basic sense, they do not meet the additional standards placed on medical supplies. The rationale for these additional safety and performance mandates
Figure 2: This basic model of a transformer shows there is no current path from primary side to secondary side. Image source: Power Sources Manufacturers Association
Also, many medical products involve very low signal levels (millivolts or microvolts for
Figure 3: A more realistic model shows basic interwinding capacitance (Cps1) between primary and secondary sides. Image source: Power Sources Manufacturers Association
we get technical
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