rev. 0.2 4/13 copyright ? 2013 by silicon laboratories AN520 AN520 cmos a dvanced g alvanic i solators for m edical e lectronics 1. introduction 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; therefore, isolators used in these systems must be robust and reliable. optocouplers and transformers are commonly used within medical system isolation circuits, and their deficiencies are well known to the design community. optocouplers are notoriously slow and exhibit wide performance variations over temperature and device lifetime. they are single-ended devices, which exhibit poor common-mode transient immunity (cmti). in additi on, optocouplers are fabr icated in gallium arsenide (gaas) processes with intrinsic wear-out mechanisms that cause permanent reduct ions in led emission at el evated temperatures and/or led currents. this degradatio n reduces optocoupler reliability, performance, and se rvice life. while transformers offer higher speed and better reliabilit y than optocouplers, they cannot pass dc and low-frequency signals, thus 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. 2. cmos isolator overview unlike optocouplers, complementary metal oxide semiconductor (cmos) isolators offer substantial advantages in performance, reliability, oper ating stability, power savings, and functional in tegration. unlike transformers, cmos isolators operate from dc to 150 mbps, require less spac e (up to six isolation channels per package), and are more power-efficient. these advantages are made possible by the following fundamental technologies underlying cmos isolators. ? standard cmos process technology silicon dioxid e based capacitive isolation barrier instead of gaas process technology cmos is a well understood process technology with 40+ ye ars of learning and offers 5.5 times lower failures-in- time (fit) rate than gaas -based optocouplers. the silicon dioxide is olation barrier offe rs a time dependent dielectric breakdown (tddb) of 60 years compared to less than 15 years in optocouplers. it also offers a mean time-to-failure (mttf) of 87 years. the operating temperat ure range is C40 to +125 c as compared to C40 to +85 c for optocouplers. th is wide operating te mperature range lead s to greater parame tric stability over voltage and temperature, and lower operating power versus optocouplers. ? improved performance ? shorter propagation delay time and pwd, wider operati ng temperature range and greater parametric operating permit greater system stability t han when using optocouplers. ? high frequency 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 selectiv ity enable cmti above 25 kv/s, excellent external rf field immunity to 300 v/m, and magnet ic field immunity greater than 1000 a/m for error-free operation. these attributes make cmos isolators well-suited for deploy ment in harsh operating environments where strong electric and magnetic fields are present, such as in motor-control circuits and medical mri systems. ? proprietary emi suppression techniques cmos isolators meet the emission standards of fcc pa rt b and are tested to automotive j1750 (cispr) test standards. for more information on cmos isolators emissions, susceptibility, and reliability vs. optocouplers, see the silicon laboratories website at www.silabs.com/isolation .
AN520 2 rev. 0.2 3. 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 le vels below 25 vac or 60 vdc 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. therefore, safety agencies specify package creepage an d clearance dimensions as a function of test voltage. as shown in figure 1, creepage is the di stance along the insulating surface that an arc may travel, and clearance is the shortest path through air that an arc may travel. figure 1. creepage and clearance distances 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 fa ilsafe (i.e., a failure does not cause the system to automatically retreat to a safe, secure state). de vices with basic isolation can be ac cessible to the user but must be contained within the system. certificatio n testing for basic isolation devices in 250v rms systems that require 4mm of creepage, consists of applying a stress voltage of 1.6 kv rms for a period of 1 minute. reinforced isolation provides two levels of protection for failsafe operation and allows user access. certificat ion testing of reinforced isolation devices in 250v rms systems that require 8 mm of creepage, consists of applying a stress voltage of 4.8 kv rms for a period of 1 minute. medical electronic system s almost always require re inforced isolation because of its failsafe protection attribute. reinforced cmos isolators are certifi ed under international standard iec/en/din (deutsches institut fr normung) en 60747-5-2. cmos isolators are also certified to th e iec-60601-1 medical standards insulation requirements, which require ul (underwriters laboratories) 1577 or iec 60747-5-2 certification as a prerequisite. iec-60601-1 specifies dielectric strength test cert ification criteria for basi c 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 working voltage insulation creepage clearance test voltage v dc v rms type (mm) (mm) v rms for 1 minute 17 12 basic 1.7 0.8 1600 reinforced 3.4 1.6 3200 34 30 basic 2 1 1600 reinforced 4 2 3200 creepage clearance
AN520 rev. 0.2 3 optocouplers use a plastic mold compound as their primary insulator and must, therefore, meet an internal mechanical distance specification referred to as dist ance through insulation (dti), 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 utilize semiconductor oxides for their primary insulator, which of fer greater dielec tric strength and uniformity than package mold compound s and, therefore, occupy less space. to certify to iec 60601-1, safety regulating agencies perform tests for dti equivalence by thermally cycling cmos isolators at 125 c for 10 weeks with an applied stress voltage of 250 vac rms and post-testing the isolators at 4.8 kvac rms for one minute. note the dti evaluation for cmos is olators is far more stringent than that of the optocoupler. medical electronic systems must be immune to external in terference caused by localized fields, static electricity, and power line anomalies, such as line voltage dips, sur ges, and transients. as a result, both optocouplers and cmos isolators are safety tested to a number of iec-61 000 standards using test limits specified by iec 60601-1-2, as shown in table 2. for example, electr ostatic 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 j1 750 (cispr does not specify test limits; it is a test methodology standard). limits for emissions and power line sensitivities are sp ecified in iec 60601-1-2. 85 60 basic 2.3 1.2 1600 reinforced 4.6 2.4 3200 177 125 basic 3 1.6 1600 reinforced 6 3.2 3200 354 250 basic 4 2.5 1600 reinforced 8 5 3200 table 2. iec 60601-1-2 immunity requirements immunity test standard iec 60601 test level electrostatic discharge (esd) iec 61000-4-2 6 kv contact, 8 kv air electrical fast transient/burst iec 61000-4-4 2 kv (power supply lines), 1 kv (i/o lines) surge iec 61000-4-5 1 kv lines-to-lines (basic), 2 kv lines-to-lines (reinforced) brownouts, voltage dips, interruptions and voltage variations on power supply lines iec 61000-4-11 less than 5% u (>95% dip in u for 0.5 cycle) 40% u (60% dip in u for 5 cycles) 70% u (30% dip in u for 25 cycles) <5% u (>95% dip in u for 5 sec) power frequency (50/60 hz) magnetic field iec 61000-4-8 3 a/m note: variable u is the ac mains voltage prior to the application of the test level. table 1. iec60601-1 safety standard requirements for cmos isolators (continued) working voltage insulation creepage clearance test voltage v dc v rms type (mm) (mm) v rms for 1 minute
AN520 4 rev. 0.2 the criteria for passing these tests are very string ent. 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. 4. typical applications 4.1. 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 converted to digital format by the serial adcs. converted data is transmitted through the 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 are used, isolation can be implemented using as few as four si x-channel isolators (a ssuming 16-bit adcs). in addition to logic input isolators, silicon labs also offers the si87xx series of enhanced, function-compatible replacements for optocouplers. the si87xx series has an input stage that mimics the behavior of an optocoupler led, allowing functional replacement of optocouplers, yet offering lower power operation, better performance across temperature, and higher reliability. for more inform ation, see the si87xx series of isolator data sheets available at www.silabs.com . figure 2. ecg front end silicon labs digital isolator controller a1 a2 a3 filter serial adc diff amp filter serial adc diff amp filter serial adc diff amp vdd1 gnd1 2.7v to 5.5v c1 0.1 to 1.0uf en1 b1 b2 b3 vdd2 gnd2 2.7v to 5.5v c2 0.1 to 1.0uf
AN520 rev. 0.2 5 4.2. defibrillator application figure 3 shows the power stage for a defibrillator where two high-side/low-side isolated dr ivers (isodrivers) drive a full-bridge circuit. note that this circuit requires only two isodrivers with standard high-side bootstrap circuits to implement a full-bridge drive solution. each isodriver has an on-chip input signal conditioning circuit consisting of schmitt-trigger inputs, input uvlo protection, output overlap protecti on, and a dead time generator. these features are essential for the reliable operation of safety-critical medical systems. the input stage is followed by a reinforced two-channel is olator, the outputs of which connect to the gate drivers, each isolated from the other as well as from the inpu t. resistors rdt1 and rdt2 determine the amount of 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 logic input isodrivers, silicon labs also offers the si822x/6x series of en hanced, functi on-compatible replacements for optically-coupled drivers. the si822x/6x series has an input stage that mimics the behavior of an optocoupler led, allowing functional replacement of gate driver products, such as the hcpl-3120, yet offering lower power operation, better performance across temp erature, and higher reliability. for more information, see the si822x/6x series of isodriver data sheets available at www.silabs.com . figure 3. isodriver-based defibrillator power stage si8234bd-c-is isodriver dt vdd1 gnd1 2.7v to 5.5v c1 0.1 to 1.0uf en1 si8234bd-c-is isodriver via vib dt vdd1 gnd1 2.7v to 5.5v c1 0.1 to 1.0uf en1 voa vob 5v to 24v gnda gndb hv vdda vddb voa vob gnda gndb vdda vddb 5v to 24v 5v to 24v 5v to 24v t1 q1 q2 q3 q4 control inputs via vib control inputs rdt2 rdt1 5v to 24v
AN520 6 rev. 0.2 4.3. medical powe r 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 exhibit problematic emi performance. the si850x/1x series of isol ated ac current sensors offers integrated reset circuits, high 2 vp-p full-scale output signals, 5% measurement accuracy, small size, and low-po wer operation. they operate over a frequency range of 50 khz to 1 mhz (full-scale measurement ranges of 5, 10, and 20 a) and available isolation ratings of 1 kv and 5 kv (reinforced). these devices ha ve an input resistance of 1.3 m ? for low power loss and a series inductance of 2 nh for reduced ringing. the current sensor in figure 4 ha s a ping-pong output mode that routes current signals from each leg of the bridge to separate outp ut pins for transformer flux balance monitoring. figure 4. si851x (ping-pong mode) in a phase-shifted full bridge application 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). for more information, see an398: us ing the si85xx current sensors in switch mode power supplies. the examples above 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. iin iout t1 q1 q3 ph1 ph2 r1 out1 gnd vin si851x mode ph4 ph3 q2 q4 r2 vdd vdd out2 out1 out2 ph1 ph3 ph2 ph4 3 - 4 1 - 4 1 - 2 2 - 3 reset reset si85xx state measure trst out1 out2 measure r3 r4 switches turned on vdd
AN520 rev. 0.2 7 table 4 is a summary of silicon laboratories isolation products compliant to medi cal specification iec 60601. table 3. example applications for cmos isolation products in medical systems equipment category system examples cardiology systems blood pr essure, ecg, defibrillator fluid pumps iv pumps, portable drug pumps, fluid evacuation systems lab equipment biomedical test syst ems, centrifuges, warming cabinets ob/gyn equipment fetal monitors, suction pumps, surgical hysteroscopes otoscopes/opthalmascopes power supplies and interface adaptors physical therapy equipment chilling units, ultr asound/ems units, measurement instruments radiology mammography, x-ray systems, mri systems, motorized viewers sterile processing equipment autoclaves, auto mated washers, distille rs, ultrasonic cleaners table 4. silicon laboratories isolation products summary* silicon labs product isolation rating (kv rms ) max working voltage (v rms ) package c r e e p a g e (mm) c l e a r a n c e (mm) iec 60601 compliant si822x 5 125 wb soic-16 8* 8* yes si823x 5 125 wb soic-16 8* 8* yes si826x 5 125 dip8 7 7 yes si826x 5 250 sdip6 8.3 8.3 yes si826x 5 250 lga8 10 10 yes si841x/2x 5 125 wb soic-16 8* 8* yes si850x/1x 5 125 wb soic-20 8* 8* yes si86xx 5 125 wb soic-16 8* 8* yes si87xx 5 125 dip8 7 7 yes si87xx 5 250 sdip6 8.3 8.3 yes si87xx 5 250 lga8 10 10 yes *note: 8 mm creepage and clearance assumes conformal coating is used. creepage in air is 7.6 mm.
AN520 8 rev. 0.2 5. summary electronic medical systems must have robust integrated isolation to ensure patient and operator safety. stringent international safety regulato ry agencies certify medical electronics systems to their specifications for uniform safety. isolators play a key role in these systems and mu st be robust and reliable while requiring minimum space and adding negligible cost to the system. optocouplers and transformers have been the preferred solutions for medical system isolator circuits. however, advances in te chnology have made possible 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 base d on mainstream cmos process technology. cmos is a well understood process technology with 40+ years of learni ng and offers 5.5 times lowe r failures-in-time (fit) rate than gaas-based optocouplers. cmos isolation products are the ideal solution for many electronic medical systems. when comb ined with silicon laboratories mixed-signal mcus, isodrivers , and si85xx-series current sensors, cmos isolators enable highly-integrated, po wer-efficient designs that comply with critical safety specifications for medical systems.
AN520 rev. 0.2 9 n otes :
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