Safety standards for ac line-powered medical electronic systems require galvanic isolation to protect patients and operators from electrically-induced trauma. The direct connection between machine and patient together with the presence of conductive body fluids and gels increase the risk of injury; thus isolators used in these systems must be robust and reliable.
Optocouplers and transformers are commonly used within medical system isolation circuits, and their deficiencies well known to the design community. Optocouplers are notoriously slow and exhibit wide performance variations over temperature and device age. They are single-ended devices, and consequently exhibit poor common mode transient immunity (CMTI). In addition, optocouplers are fabricated in Gallium Arsenide (GaAs) processes, with intrinsic wear out mechanisms that cause permanent reductions in LED emission at elevated temperature and/or LED current. This degradation reduces optocoupler reliability, performance and service life. While transformers offer higher speed and better reliability than optocouplers, they cannot pass dc and low-frequency signals, thereby imposing limits on system timing (e.g. ON-time and duty cycle). Transformers also tend to be large and power-inefficient, and often require additional external components for core reset.
Unlike optocouplers, complementary metallic oxide semiconductor (CMOS) isolators offer substantial gains in performance, reliability, operating stability, power savings, and functional integration. Unlike transformers, CMOS isolators operate from DC to 150Mbps, and consume less space (up to six isolation channels per package) and are more power efficient. These attributes are made possible by fundamental technologies underlying CMOS isolators; specifically:
* Mainstream, low-power CMOS instead of GaAs process technology: CMOS is the most mature, widely sourced process technology in the world. Advanced circuit design techniques and CMOS technology enable isolators having fast 150Mbps data rate, short 10ns propagation delay, low 5.6mW/channel power consumption, and many other industry-leading performance specifications. CMOS also enables an isolator mean time-to-failure (MTTF) of more than 1000 years at maximum operating voltage and temperature; more than 10 times higher than optocouplers.
* RF carrier instead of light: RF technology further reduces isolator operating power and adds the benefits of precise frequency discrimination for superior noise rejection. Device packaging is also simpler compared to optocouplers.
* Fully differential instead of single-ended isolation path: The differential signal path and high receiver selectivity enables CMTI above 25kV/us, excellent external RF field immunity to 300V/m, and magnetic field immunity greater than 1000 A/m for error-free operation. These attributes make CMOS isolators well-suited for deployment in harsh operating environments where strong electric and magnetic fields are present.
* Proprietary EMI suppression techniques: CMOS isolators meet the emission standards of FCC Part B, and tested to automotive J1750 (CISPR) test methods.
From a system point-of-view, medical equipment is divided into individual classes, according to operating voltage. Class I equipment operates from 70V or less, and requires only basic insulation and protective grounding for all accessible parts. Class II equipment operates from voltages above 70V, and requires reinforced or double insulation. Class III equipment is operated from voltage levels below 25V ac or 60V dc, and is referred to as Safety Extra Low Voltage (SELV). Class III equipment does not require isolation.
From a component perspective, isolator package geometry is important in the prevention of electrical arcing across package surfaces. Safety agencies therefore specify package creepage and clearance dimensions as a function of test voltage. As shown in Figure 1, creepage is the distance along the insulating surface an arc may travel, and clearance is the shortest path through air an arc may travel.