Application Note: Conducted Immunity: Quick Guide

Transcription

1 icatin Nte: Cnducted Immunity: Quick Guide (Please refer t AZD052 fr the full applicatin nte) 1 Intrductin: Lng cables are at risk f picking up RF, but t test this belw 80MHz require very large antennas (few metres lng) nt practical S simulate with injected nise currents, as if radiated RF has successfully cupled int cables, with relevant generic internatinal standard: IEC IEC : Cmmn mde nise nise currents flw in same directin tward and nise is f cntinuus nature, with amplitude mdulatin nt pulsing/surges Capacitive sensing may be very sensitive t mde nise, but with prper PCB layut and crrect use f mde impedance immunity may be imprved significantly. design mde impedance very NB fr nise immunity. Real wrld mde and differential mde cnducted nise. Latter typically frm SMPS. Future versins f IEC may include differential mde nise as well 2 Overview f IEC : Advisable t purchase a cpy f the standard belw just an verview. Testing fr immunity t cnducted interference due t intentinal 150kHz - 80MHz transmitters. Three nise injectin ptins: 1) Using Cuple-Decuple Netwrks (CDN s); 2) Using Electrmagnetic Clamp; 3) Using Bulk Current Injectin, with current clamps. Seems as if CDN technique gives mst repeatable and accurate results. Standard requires nise t be amplitude mdulated at 80% depth, accrding t a 1kHz sine, with levels specified as equivalent pen circuit vltages f 1VRMS, 3VRMS r 10VRMS. Cmmercial prduct level is 3V rms immunity. Typical equipment required: Signal Generatr, Amplifier, Spectrum Analyzer and CDN. Fr CDN, calibratin is dne fr 150 lad with applied vltage derived frm pen circuit vltage. If DC supply cable is lng enugh, have t inject n DC side f design, may be harder test. Mst applicatins will be tested as table-tp equipment Once prduct is pwered and functinal, calibratin data is played back, frequency is swept frm 150kHz t 80MHz, minimum dwell time f 0.5s and a max frequency change f 1% per step. Class A pass requires n false Tuch / Prximity, and detecting valid Tuch and Prximity events. IEC is a fairly cmplex standard, easily miss-applied. Fr frmal cmpliance testing, advisable t use a reputable, accredited EMC lab, and t scrutinize test methd and results. IEC nly applies t cmmercial prducts, nt t Medical, Maritime, Avinics, Machinery and Autmtive prducts where malfunctin/failure culd result in lss f life r large scale financial lss. Stringent internatinal / lcal

2 standards cver such applicatins. Further, cnducted immunity tests may invlve hazardus vltages, als nminally in the DUT. Basic electrical safety principles shuld be applied. If unsure, cnsult a certified prfessinal. Gen + Amp: 50 Z ut Cmmn mde nise currents RF-cnnectr CDN Cupling netwrk R2 R1 L C2 C1 L + nise DUT N Decupling LC filter 150 Z surce N + nise DUT mde impedance Figure 1: Injecting mde nise int a DUT with tw AC pwer lines, using a CDN. Injecting n AC side L N CDN L + nise N + nise Z BLOCK SMPS SMPS Gnd + nise Injecting n DC side L N SMPS V DC Gnd CDN Gnd + nise Figure 2: Injecting nise n the AC r DC side f a applicatin 3 Imprving Cnducted Immunity: Cmmn mde cnducted nise immunity f applicatins can be increased by minimizing the flw f nise currents via capacitive sensing electrdes. This may be dne by: 1) Blcking the nise; 2) Shunting the nise; 3) Selectively filtering the nise 4) Burning the nise; 5) Recgnizing the nise Blck the nise: Typically, mde cnducted nise will reach yur design via the pwer cables and can be blcked by mde chkes watch ut fr impedance freq dependence.

3 icatin Energy V SUPPLY 1 + nise V SUPPLY 2 + nise Rest f applicatin Other electrde Cupling with Typically Cmmn mde chke n tridal cre Figure 3: Blcking mde nise currents with high impedance in each line Max Z pint L dminates C dminates Figure 4: Cmmn mde impedance f a typical SM mde chke available cmmercially, shwing dminance f inductance and parasitic winding capacitance. Shunt the nise: currents may be shunted t by increasing vltage rail cpper areas. Any rail may be used, be it Live, Neutral, VDC r lcal grund - mde nise. Lcatin f additinal cpper is NB. If nise currents first encunter the cpper f Cx electrdes, it may fllw this path. Grund rings als wrk well. With Azteq s high sensitivity, and PCC, grund cpper may be placed directly belw electrdes, significantly imprving immunity.

4 icatin + L + nise N + nise L cpper PSU N cpper V DC Gnd Rest f appl V DC cpper Gnd cpper electrde Capacitive cupling with Capacitive cupling with Less nise capacitive cupling with Figure 5: Shunting nise currents with additinal cpper t increase capacitive cupling with icatin DC supply Gnd + nise Gnd cpper electrde Capacitive cupling with currents cuple via Cx electrde, nt mstly via Gnd cpper as intended Figure 6: Physical lcatin f additinal cpper is imprtant Grund ring: Grund belw: DC supply Gnd + nise icatin DC supply Gnd + nise icatin Gnd Gnd currents flw via grund ring, nt via Cx electrde currents flw via grund, nt via Cx electrde Figure 7: Use f a grund ring arund and/r grunded cpper belw Cx electrdes t increase immunity

5 Selectively filter the nise: may cause differential vltage drps. Due t the high cnducted nise frequencies (up t 80MHz/230MHz), simple RC r LC lw-pass filters may be used t prevent failures due t such differential vltages. Nte that blind applicatin f differential filters will typically nt result in an immunity increase. Burn the nise: Anther ptin is t use high resistance values (rder f 1k ) in ne r bth the supply lines twards the IQSxxx device, resulting in nise energy being dissipated. This ptin is based n the extremely lw perating currents f mst IQS devices, typically well belw 400 A. Nte that sme supply vltage drp will be generated, and must be acceptable in the applicatin. 4 Testing alternatives: Belw is a list f sme lw cst alternative test methds. These cannt guarantee cmpliance, but may give qualitative indicatin f immunity. Always try t benchmark with prduct that is knwn t cmply t the relevant standard. Full cmpliance testing at accredited EMC lab still advisable. Signal generatr with basic capacitive clamp: Cables are placed between clamp plates. Take care nt t realize t large clamp capacitance may damage signal generatr utput. Signal generatr directly int lcal grund: Inject nise directly int the lcal grund using a resistr t limit current. The grund reference plane is cnnected t the signal generatr grund, and DUT is 0.1m abve reference plane. Direct injectin: Use DC-blcking capacitr in series with RF-surce utput t inject RF-currents directly int varius cnductrs f the circuit. May result in very high RF-current levels. Please nte that direct injectin is a risky test, fr DUT, RF-surce, and ther equipment nearby. If amplifier is used a current limiting resistr is NB. D nt pursue this alternative methd if yu are unsure, and fllw relevant safety prcedures when wrking with high vltages. In all the abve, care must be taken nt t exceed lcal legal limits fr RF-radiatin. Severe cnsequences, sme fatal, can result if limits are exceeded. If unsure, cnsult an EMC specialist. 5 Differential Mde Cnducted : Real life cnducted nise may be r differential mde Typical surces f differential mde nise are SMPS, arcing cntacts (which may als result in mde nise EFT), LCD screen drivers etc. Advise the fllwing t imprve the immunity t differential mde cnducted nise: Use single r tw stage RC r LC filters n supply lines, especially frm SMPS Be wary f very lw cst SMPS. Use SMPS that emply cntrllers with switching freq dithering. Fllw Radiated Immunity PCB guidelines, as described in AZD015, t minimize detrimental effects. Especially prper decupling f Vdd and Gnd thrugh realizatin f sufficient cpper. Take care when ruting supply r capacitive sensing lines clse t SMPS sectins, r underneath LCD screens.

6 Cmmn Mde Cnducted : Differential Mde Cnducted : Supply + Supply Supply line 1 + nise Supply line 2 + nise icatin Circuit Parasitic and intended C with Supply + Supply line 1 + nise Supply line 2 + nise Impedance f supply and applicatin t plays n part icatin Circuit Differential mde nise currents Figure 8: Cmmn mde and differential mde cnducted nise currents

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