设计绝缘安全的医疗设备

　 根据声音设计实践，光耦为医疗设备提供有效绝缘，保护病人远离潜在的漏电流危险。

　　

      使用交流供电的医疗诊断、测量和治疗设备，由不合适的接地和电绝缘产生漏电流，潜在的将病人甚至医疗人员暴露在电击、烧伤、内器官损伤和心律不齐的危险之中。体液的电导、各种电导液的存在和病人时用的凝胶体，使治疗环境存在更大危险。使用凝胶体减少皮肤正常高于50Ω的电阻值。第二个重大危险来自设备间的辐射，它会降低附近其他设备性能。所以，代理商沿用了US FDA（食品药物管理局）、EU（欧盟）和其他安全部门的规定，确保这些医疗设备遵守安全标准。

　　

　 IEC（国际电工委员会）60601-1标准规定医疗设备电安全以保护病人、操作者和环境为条件。其他标准规定了更多安全必备条件。例如，IEC 60601-1-x间接标准系列处理例如EMC（电磁兼容）、X射线保护和可编程医疗系统的问题。EMC确实是个重要的标准，因为设备不能成为EMI（电磁干扰）源。它会阻止其他设备准确运行，并且必须对操作环境中潜在的EMI免疫。自2005年11月以来，医疗设备不得不遵守最新的IEC 60601-1-2:2001 EMC标准。

　　

　 医疗设备传递数据的部分，设计者用光耦或变压器方法，从高压环境中隔离敏感电路或者病人。基于光连结器的技术过去只支持有限的数据速率，导致变压器隔离方法普遍使用。这种方法提供必备的数据速率，但是一般需要更多器件，占据了PCB（印刷电路板）的更多空间和更复杂的设计。随着光耦更高速数据速率能力和改良的时间特性的引入，这个情形得到改变。

　　

　 增强型电绝缘

　　

　 不同于功能性绝缘，增强型绝缘保护电击和确保设计自动防故障装置（参考文献1）。这个特点很关键，例如ECG（心电图仪）系统或电击去纤颤器（图1和图2）。提供增强型绝缘的光耦用IEC/EN/DIN EN 60747-5-2鉴定，这个是光绝缘半导体器件的国际标准。

　 为满足IEC-60601-1标准需要，光耦必须有UL 1577或IEC 60747-5-2证明，且满足依靠接口绝缘水平的漏电、外部间隙和测试电压需要。这个证明规定了漏电距离为沿最短表面路径穿过固态电解质，两绝缘导体之间的距离。外部间隙是穿过空气的最短距离或者两个电绝缘导线的“视线”距离。

　　

　 对于增强绝缘水平应用，操作电压高于50V有效值（71V峰峰值）或直流时，DTI最小为0.4mm（参考文献2）。DTI是绝缘设备中导线间的内部间隙距离，例如光耦合器或光隔离器中LED和探测器之间（图3）。表1总结了IEC 60601-1来证明医疗设备的特殊规格需要。表2显示了满足IEC 60601绝缘条件的光耦合器指标。

　 为满足IEC 60601-1-2的EMC条件，医疗设备必须和附近引发故障的发射器和电源波动的ESD、RFI免疫（表3）。除了这些要求，设备本身通过导线或放射线散射，干扰了许可的通信源和其他设备，因此应该最小化（表4）。设备必须有高达8 kV击穿空气或6 kV接触的 ESD保护。他们必须对频率80 MHz到2.5 GHz 的非生命支持设备3V/m、生命支持设备10V/m的RF电磁力免疫。这些测试是基本性能鉴定，不能有任何不合格。光耦合器对EMI隔离效果更好，例如变压器，因为光耦合器通过LED光源和光电二极管之间的辐射传输信号。光耦合器测试证明具有抵御高达11 kV ESD电压的能力。光耦有效的使差模信号通过，并阻止共模电流，导致地回路电流产生地偏置电压。

　 综上所述，可利用的光耦合器明显的满足IEC 60601-1的一般医疗安全规定。此外，光耦合器提供极好的EMI，且不辐射电磁波，这是目前医疗设备鉴定的重要标准。

　　

      英文原文：

　　Designing medical devices for isolation and safety

　　Optocouplers, along with sound design practices, provide effective isolation for medical equipment and protect patients from potentially dangerous leakage currents.

By Yeo Siok Been, Jamshed Namdar Khan, and Derek Chng Peng Hui, Avago Technologies -- EDN, 5/24/2007

　　The use of ac-line-powered medical diagnostic, measurement, and treatment equipment potentially exposes patients andeven caregivers to the risk of electrical shock, burns, internal-organ damage, and cardiac arrhythmias directly due to leakage current resulting from improper grounding and electrical isolation. The electrical conductivity of body fluids and the presence of various conductive solutions and gels in the patient care make the treatment environment even more potentially dangerous. The use of gels substantially reduces the normally high resistance of the skin—greater than 50Ω. A second significant 
hazard results from potential electrical emissions among the diagnostic and treatment devices, which can degrade the performance of other nearby medical devices. As a result, many regulations from agencies ranging from the US FDA (Food and Drug Administration), the EU (European Union), and other safety and regulatory bodies ensure that these medical devices comply with the safety standards.

　　Standard IEC (International Electrotechnical Commission) 60601-1 defines medical-equipment electrical-safety conditions necessary to protect patients, operators, and the surroundings. Other standards define more than just safety requirements. For example, the IEC 60601-1-x collateral-standard series deals with issues such as EMC (electromagnetic compatibility), X-ray protection, and programmable electrical medical systems. EMC is indeed an important criterion for medical equipment because the equipment cannot be a source of EMI (electromagnetic interference), which could prevent accurate operation of other medical equipment and must be immune to potential EMI in the operating environment. Since November 2005, medical equipment has had to comply with the updated IEC 60601-1-2:2001 EMC standard.

　　In the portions of the medical equipment transmitting digital data, designers can isolate sensitive circuitry or patients from high-voltage environments using optocoupler- or transformer-based approaches. Optocoupler-based techniques have in the past been able to support only limited data rates, which led to the use of transformer-based isolation. This approach provided the requisite data rates but generally required more components taking up more space on a PCB (printed-circuit board) and a more complex design. This situation has changed with the introduction of optocouplers capable of higher data rates and improved timing characteristics.

　　Reinforced galvanic isolation

　　Unlike functional isolation, reinforced isolation both protects from electric shock and ensures that a design 
is fail-safe—a mode of system termination that automatically leaves system processes and components in a secure state when a failure occurs or when a system detects a failure (Reference 1). This feature is critical for medical equipment, such as an ECG (electrocardiograph) system or a defibrillator (figure 1 and figure 2, respectively). Optocouplers providing reinforced isolation are certified under IEC/EN/DIN (Deutsches Institut für Normung) EN 60747-5-2, which is an international standard for optically isolated semiconductor components.

　　To meet the IEC-60601-1 medical standards’ insulation requirements, optocouplers must have UL (Underwriters Laboratories) 1577 or IEC 60747-5-2 certification and must meet component-creepage, external-clearance, and test-voltage requirements depending on the insulation level of the interface. The certification defines creepage distance as the shortest surface path over a solid dielectric between two galvanically isolated conductors. The external-clearance distance is the shortest distance through air, or “line-of-sight” distance, between two galvanically isolated conductors.

　　When operating at working voltages higher than 50V rms, 71V peak, or dc and for reinforced-insulation-level applications, the DTI (distance through isolation) must be at least 0.4 mm (Reference 2). The DTI is the internal-clearance distance between conductors inside an insulation device, such as that between the LED and the detector inside an optocoupler or optoisolator (Figure 3). To illustrate the typical specification requirements for medical equipment, Table 1 summarizes IEC 60601-1. Table 2 shows specifications of optocouplers that meet the IEC 60601 requirements for insulation. The Type 1 requirement targets devices operating at less than 70V, which require only basic insulation, and the Type 2 requirement targets equipment that operates at voltages greater than 70V and thus requires reinforced insulation or a similar level of protection.

　　To me 
et EMC requirements in IEC 60601-1-2, medical devices must be immune to ESD (electrostatic discharge), RFI (radio-frequency interference) from nearby transmitters and other sources, and power disturbances that cause device malfunctions (Table 3). In addition to these requirements, the device’s own emissions, through either conduction or radiation, may interfere with licensed communications sources or other equipment and thus should be minimal (Table 4). The devices must have ESD protection as high as 8 kV through air and 6 kV on contact. They must be immune at frequencies of 80 MHz to 2.5 GHz and 3V/m of RF electromagnetic force for non-life-supporting equipment and 10V/m for life-supporting equipment. These tests are essential performance criteria, in that they cannot have any component failures, changes in programmable parameters, resetting to factory defaults, changes of operating modes, or false alarm. Properly designed optocouplers are more immune to EMI than are other isolation devices, such as transformers, because optocouplers transmit signals through optical radiation between the LED light source and the photodiode. Tests on optocouplers have demonstrated their ability to withstand ESD voltage as high as 11 kV (Reference 1). Optocouplers are effective in passing the intended differential-mode signals and blocking the unintended common-mode currents and resulting ground-offset voltage that can result from ground-loop currents.

　　In summary, available optocouplers clearly meet the general medical-safety requirement that IEC 60601-1 defines. In addition, optocouplers provide excellent EMI and emit no electromagnetic waves, which is now an important measure for medical-equipment certification.