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CMOS Isolators: A Critical Safety Measure for Medical Electronics

Fri, 10/01/2010 - 7:13am
Don Alfano, Silicon Labs
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, often requiring additional external components for core reset.

CMOS Isolator Overview
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 150 Mbps, and consume less space (up to six isolation channels per package) and are more power efficient. These attributes are enabled by the following fundamental technologies underlying CMOS isolators: 

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 150 Mbps data rate, short 10 ns propagation delay, low 5.6 mW/channel power consumption, and many other industry-leading performance specifications. CMOS also enables an isolator mean time-to-failure (MTTF) of more than a 1,000 years at maximum operating voltage and temperature and 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 25 kV/us, excellent external RF field immunity to 300 V/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 are tested to automotive J1750 (CISPR) test methods.

Safety Certifications
From a system point-of-view, medical equipment is divided into individual classes according to operating voltage. Class I equipment operates from 70 V or less and requires only basic insulation and protective grounding for all accessible parts. Class II equipment operates from voltages above 70 V and requires reinforced or double insulation. Class III equipment is operated from voltage levels below 25 VAC or 60 VDC and is referred to as Safety Extra Low Voltage (SELV). Class III equipment does not require isolation.

Figure 1. Creepage and clearance distances.From a component perspective, isolator package geometry is important in preventing 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. 

The heart of the isolator is the insulator, the dielectric strength of which determines the isolator’s voltage rating. Isolation classifications include “basic” and “reinforced.” Basic isolation provides a single level of protection against electrical shock and cannot be considered failsafe (i.e. a failure does not cause the system to automatically retreat to a safe, secure state). Devices with basic isolation can be accessible to the user but must be contained within the system.

Certification testing for basic isolation devices consists of applying a stress voltage of 1.6 kVRMS for a period of one minute, with a minimum creepage of 4 mm. Reinforced isolation provides two levels of protection for failsafe operation and allows user access. Certification testing of reinforced isolation devices consists of applying a stress voltage of 4.8 kVRMS for a period of one minute with a minimum creepage of 8 mm. Medical electronic systems almost always require reinforced isolation because of its failsafe protection attribute.

Reinforced CMOS isolators are certified under international standard IEC/EN/DIN (Deutsches Institut für Normung) EN 60747-5-2. CMOS isolators are also certified to the IEC-60601-1 medical standard insulation requirement, which requires UL (Underwriters Laboratories) 1577 or IEC 60747-5-2 certification as a prerequisite. IEC-60601-1 specifies dielectric strength test certification criterion for basic and reinforced isolation, which includes creepage and clearance limits, and stress voltage and duration, as summarized in Table 1.

Table 1. IEC60601-1 Safety Standard Requirements for CMOS Isolators.

Optocouplers use plastic mold compound as their primary insulator and must therefore meet an internal mechanical distance specification referred to as “Distance Through Insulation” (DTI), as referenced in IEC 60601-1. For optocouplers, DTI is the distance between the LED and optical receiver die, typically 0.4 mm minimum. CMOS isolators use semiconductor oxides as their primary insulator, which have greater dielectric strength and uniformity than package mold compound, and therefore occupy less space. To certify to IEC 60601-1, safety regulating agencies perform test for DTI equivalence by thermally cycling CMOS isolators at 125°C for 10 weeks with an applied stress voltage of 250 VACRMS, then post-testing the isolators at 4.8 KVACRMS for one minute. Note the DTI evaluation for CMOS isolators is far more stringent than that of the optocoupler.

Medical electronic systems must be immune to external interference caused by localized fields, static electricity and power line perturbations such as line voltage dips, surges and transients. As a result, both optocouplers and CMOS isolators are safety tested to a number of IEC-61000 standards, using test limits specified by IEC 60601-1-2 as shown in Table 2. For example, electrostatic discharge (ESD) is tested to IEC 61000-4-2 and uses the test limits specified by IEC 60101-1-2. RF emissions and power line perturbations are tested using methods from CISPR11 test methodology, a subset of automotive specification J1750. (CISPR does not specify test limits; it is a test methodology standard.) Limits for emissions and power line sensitivities are specified in IEC 60601-1-2. 

Table 2: IEC 60601-1-2 Immunity Requirements. Note: Variable U is the AC mains voltage prior to the application of the test level.

The criteria for passing these tests are very stringent: the system cannot exhibit any component failures, parametric changes, configuration errors or false positives. In addition to external field immunity, the system under test cannot generate significant radiated or conducted emissions of its own.

Typical Medical Device Applications

Figure 2. ECG front end

ECG Application
Figure 2 shows a block diagram of an electrocardiogram (ECG) front end where analog output from the instrumentation amplifiers is high-pass filtered and then converted to digital format by the serial ADCs. Converted data is transmitted through a reinforced (5 kV) digital isolator to the controller for processing. The digital isolator can operate at throughput rates as high as 150 Mbps per channel for “bottleneck free” data transfer. If parallel output ADCs were used, isolation can be implemented using as few as four six-channel isolators (assuming 16-bit ADCs). 

Figure 3. ISOdriver-Based defibrillator power stage
Defibrillator Application. Figure 3 shows the power stage for a defibrillator where two high-side/low-side isolated gate drivers drive a full-bridge circuit. Note that this circuit requires only two gate drivers with standard high-side bootstrap circuits to implement a full-bridge drive solution. Each gate driver has an on-chip input signal conditioning circuit consisting of Schmitt-trigger inputs, input UVLO protection, output overlap protection, and dead time generator. These features are critical for reliable operation of safety-critical medical systems.

The input stage is followed by a reinforced two-channel digital isolator, the outputs of which connect to the gate drivers, each isolated from the other as well as from the input. Resistors RDT1 and RDT2 determine the amount dead time added within each cycle. If dead time is not required, the DT inputs should be tied to the local source of VDD.

In addition to providing logic input gate drivers, Silicon Labs offers enhanced, function-compatible replacements for optically coupled drivers. These Si822x isolated gate drivers have an input stage that mimics the behavior of an optocoupler LED, allowing direct replacement gate driver products, yet offering lower power operation, better performance across temperature and higher reliability.

Medical Power Supply Application. Figure 4 shows a phase-shift modulated full bridge application typical of power supplies used in large medical systems, such as clinical MRIs. These systems commonly use current-sense transformers, which require external discrete circuits for core reset and special layout considerations. They also tend to have low amplitude output waveforms and often result in problematic EMI performance.

Isolated AC current sensors such as Silicon Labs’ Si850x/1x devices offer integrated reset circuits, high 2 Vp-p full-scale output signals, 5 percent of measurement accuracy, small size and low-power operation. They operate over a frequency range of 50 kHz to 1 MHz (full-scale measurement ranges of 5 A, 10 A and 20 A) and have isolation ratings of 1 kV and 5 kV. These devices have an input resistance of 1.3 mOhm for low power loss and a series inductance of 2 nH for reduced ringing. The current sensor in Figure 4 has a “Ping-Pong” output mode that routes current signals from each leg of the bridge to separate output pin for transformer flux balance monitoring. 

Figure 4. Ping-pong mode in phase-shifted full bridge applications.

Measured current flowing when Q1 and Q4 are on appears on OUT2; and current flowing when Q2 and Q3 are on appears on OUT1. Integrator reset occurs during the current circulation phase (i.e. when Q1 and Q2 are on, or Q3 and Q4 are on).

The examples illustrate how CMOS isolators can be used in electronic medical systems at the circuit level. Other systems may use CMOS isolators for different circuit functions, such as voltage level-shifting or eliminating noise-causing ground loops. Table 3 shows a partial list of medical electronic systems that can benefit from the use of CMOS isolation technology. Isolation requirements in these and other applications result in a virtually open-ended number of CMOS isolator use cases, and CMOS isolator technology will ultimately supplant legacy isolation technologies as the medical electronics market continues to expand.

Conclusion
Electronic medical systems must have integrated robust isolation to ensure patient and operator safety. Stringent international safety regulatory agencies certify medical electronics systems to their specifications for uniform safety. Isolators play a key role in these systems and must be robust and reliable, yet consume minimum space and add negligible system cost. Optocouplers and transformers have been the favored solutions for medical system isolator circuits. However, advances in technology have enabled smaller, more reliable and higher performance isolation devices including single-package, multi-channel digital isolators, AC current sensors and gate drivers. These new isolation products are based on mainstream CMOS technology and offer significant benefits over legacy solutions, including 10 times lower failure rates compared to optocouplers. CMOS isolation products are the ideal solution for many electronic medical systems.

For more information on CMOS isolator emissions, susceptibility and reliability, see the Silicon Labs white paper “RF Isolators Supersede Optocouplers in Industrial Applications” and “Si84xx CMOS Isolators Datasheet,” both available at www.silabs.com/isolation. For more information on isolated gate drivers, download the Si822x ISOdriver Datasheet at www.silabs.com/isolation.

References
• IEC 60601-1, “General Requirements for Basic Safety and Essential Performance”, International Electrotechnical Commission.
• IEC 60601-1-2, “International Standard for Medical Equipment”, International Electrotechnical Commission.
• Designing Medical Devices for Isolation and Safety, Avago Technologies, May 24, 2007 EDN Magazine
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