AC-powered medical electronic system safety standards require galvanic isolation to protect patients and operators from the risk of electric shock. Because the conductors are directly connected to the instrument and the patient, the liquid and gel attached to it add to the risk of electric shock; therefore the isolators used in these systems must be durable and reliable.
Optocouplers and transformers are commonly used in isolation circuits for medical systems, but their drawbacks are well known in the design community. It is well known that optocouplers are slow and perform very poorly during temperature and aging changes. They are all single-ended devices and therefore have poor common mode transient immunity (CMTI). In addition, optocouplers are based on gallium arsenide (GaAs) processing, and inherent intrinsic losses result in reduced luminous intensity at high temperatures and/or high LED currents. This attenuation reduces the reliability, performance and lifetime of the optocoupler. While transformers offer higher speeds and reliability than optocouplers, they do not pass DC and low frequency signals, which limits their use in system timing (eg, turn-on time and duty cycle) applications. Moreover, transformers are generally bulky and inefficient, often requiring additional core reset circuitry.
CMOS isolator overview
Unlike optocouplers, complementary metal oxide semiconductor (CMOS) isolators provide better performance, reliability, stability, power savings, and integration. Unlike transformers, CMOS isolators support DC -150Mbps and take up less space (up to 6 isolated channels per package) and are more efficient. These features are achieved by the following CMOS isolator basics:
Mainstream, low-power CMOS processing instead of GaAs: CMOS is the most mature and widely used processing technology worldwide. Advanced circuit design techniques and CMOS technology enable isolator up to 150Mbps data transfer speed, 10ns propagation delay, 5.6mW/channel power consumption, and many other industry-leading performance specifications. The CMOS isolators have a mean time between failure (MTTF) of more than 1000 years at maximum operating voltage and temperature, which is 10 times that of optocouplers.
RF carrier instead of light: RF technology further reduces isolator operating power consumption, high-precision frequency discrimination improves noise rejection, and device packaging is simpler than optocoupler.
Differential isolation instead of single-ended isolation: differential signal path and receive sensitivity allow CMTI to exceed 25kV/us for error-free operation, good external RF immunity to 300V/m, and magnetic field immunity of over 1000A/m. Features make CMOS isolators suitable for harsh operating environments (strong electric and magnetic fields).
Patented EMI suppression technology: CMOS isolators meet FCC Part B specifications and are tested by the Automotive J1750 (CISPR).
safety certificate
From a system perspective, medical devices can be divided into different levels depending on the operating voltage. Class I equipment operates at 70V or less and requires only basic insulation and protective grounding for the accessible parts. Class II equipment operates above 70V and requires enhanced or double insulation. Class III equipment operates below 25 VAC or 60 VDC and is often referred to as Safe Voltage (SELV). Class III devices do not require isolation.
From a component point of view, the isolator package size is important in preventing arcs from crossing the package surface, so the safety mechanism specifies the creepage and clearance distances for a particular test voltage. As shown in Figure 1, creepage refers to the minimum distance that is discharged along the insulating surface. Clearance refers to the shortest distance through the air discharge.
Figure 1: Creepage and clearance
The core of the isolator is an insulator. The dielectric strength determines the voltage level of the isolator. The isolation classification includes “basic†and “enhancedâ€. Basic isolation provides protection against electric shock, but does not consider failsafe conditions (ie, failure does not cause the system to automatically transition to a safe, reliable state); basic isolation devices can be used by users, but must be Included in the system.
The certification test for the basic isolation device is a minimum leakage distance of 4 mm at 1 minute and 1.6 kV RMS. Enhanced isolation provides two levels of protection for compromised security operations and allows user access. The certification test for the enhanced isolation device is a minimum leakage spacing of 8 mm at 1 minute, 4.8 kV RMS. Medical electronic systems almost always require enhanced isolation because they require safe fail-safe features.
The enhanced CMOS isolator complies with the international standard IEC/EN/DIN EN 60747-5-2, and the CMOS isolator also meets the insulation requirements of the IEC-60601-1 medical standard, which requires UL (Underwriters Laboratories) 1577 or IEC-60601. -1 standard certification. IEC-60601-1 specifies dielectric strength test certification guidelines for basic and enhanced isolation, including creepage and clearance limits, as well as voltage and duration. See Table 1 for details.
Table 1: IEC60601-1 CMOS Isolator Safety Standard Requirements
Optocouplers use plastic composite compounds as their primary insulating material and must therefore meet internal spacing specifications, also known as insulation penetration distance (DTI), a term derived from IEC 60601-1. For optocouplers, the DTI is the distance between the LED and the light receiver Die, with a typical minimum distance of 0.4 mm. CMOS isolators use semiconductor oxides as their primary insulating material, which has better dielectric strength and uniformity than composite compound packages and therefore takes up less space. In order to pass the IEC 60601-1 certification, the safety regulator performs the DTI test, even if the CMOS isolator is tested for 10 weeks at 125 ° C and 250 VAC RMS applied voltage, then 1 minute at 4.8 KVAC RMS. Note that DTI evaluation for CMOS isolators is more stringent than optocouplers.
Medical electronic systems must be immune to external disturbances such as local magnetic fields, static electricity, power line disturbances (such as line voltage dips, surges, and transients). Therefore, both optocouplers and CMOS isolators must pass the IEC-61000 standard. The test limits are shown in the IEC 60601-1-2 specification, as shown in Table 2. For example, electrostatic discharge (ESD) is subject to IEC 61000-4-2 and uses the test limits specified in IEC 60101-1-2. The RF Radiation and Power Line Disturbance Test uses the CISPR11 test method, which is a subset of the J1750 automotive specification. (CISPR does not specify test limits, it is a standard for test methods.) Limitations on radiation and power line sensitivity are subject to IEC 60601-1-2.
Table 2: IEC 60601-1-2 anti-interference requirements
Note: The variable U is the AC main voltage before the test application level.
The requirements for passing these tests are very strict: the system cannot have any component failures, parameter changes, configuration errors, or false positives. In addition to field immunity, the test system itself cannot produce significant radio frequency or conducted radiation.
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