EMI considerations for isolated power supplies – how to test, troubleshoot & resolve problems Keith Keller Analog Field Applications Engineer Milwaukee WI TI Information – Selective Disclosure Presentation summary • • • • • Background – EMI standards, test setup, LISN, detector types, etc. Fundamental causes of Differential-mode (DM) & Common-mode (CM) EMI Mitigation options to deal with DM & CM noise Tips for troubleshooting & debug Practical examples throughout • Based on PMP21479 65-W USB-PD Active-clamp-Flyback topology What you’ll learn: • Understand what causes EMI • Understand the sources and causes of parasitic elements in EMI filter component, PCB’s and throughout the board assembly • Learn practical techniques to test, debug & root-cause EMI issues • Learn how transformer construction influences EMI • See real-world examples of how to resolve EMI issues TI Information – Selective Disclosure 2 EMI – where do I start? • I built my prototype PSU, and resolved all functional issues • I ran first-pass EMI scan – and my design fails badly! – How can I fix the EMI? – Where do I start? • This presentation will help to show how you can get to a result like this without necessarily adding a big EMI filter 47 dB TI Information – Selective Disclosure EMI background – standards & test setup TI Information – Selective Disclosure What are EMC & EMI & EMS? • EMC = Electro Magnetic Compatibility • EMI = Electro Magnetic Interference (emission) • EMS = Electro Magnetic Susceptibility (immunity) EMI EMS CS = Conducted Immunity RS = Radiated Immunity ESD = Electrostatic discharge EFT/B = Electrical Fast Transient/Burst Immunity SURGE PFMF = Power Frequency Magnetic fields DIP (Voltage variations) EMC TI Information – Selective Disclosure In this topic only CE = Conducted Emission RE = Radiated Emission Harmonic distortion Fluctuation and Flicker Reference: https://en.wikipedia.org/wiki/Electromagnetic_compatibility 5 EMC standards • Standards for power supplies REQUIREMENT EMI EMS PARAMETERS Emissions (RE & CE) EN55032 (/CISPR 32) Harmonic EN61000-3-2 Fluctuation and flicker EN61000-3-3 ESD EN61000-4-2 RS EN61000-4-3 EFT EN61000-4-4 Surge EN61000-4-5 CS EN61000-4-6 PFMF EN61000-4-8 DIP EN61000-4-11 In this topic only Reference: https://www.us.tdk-lambda.com/media/292070/tdk-lambda-blog-110507.pdf TI Information – Selective Disclosure 6 Conducted Emissions (CE) standards • Summary of main product standards for conducted emissions Product sector CISPR standard EN standard FCC standard Automotive CISPR 25 EN 55025 -- Multimedia CISPR 32 EN 55032 Part 15 ISM CISPR 11 EN 55011 Part 18 CISPR 14-1 EN 55014-1 -- CISPR 15 EN 55015 Part 15/18 Household appliances, electric tools and similar apparatus Lighting equipment Reference: Timothy Hegarty, “An overview of conducted EMI specifications for power supplies”, http://www.ti.com/lit/wp/slyy136/slyy136.pdf TI Information – Selective Disclosure 7 How Conducted EMI is measured • Horizontal & vertical ground planes – (or screened room) Vertical Ground Plane • Equipment Under Test (EUT) on nonconductive table • EUT powered through LISN – (Line Impedance Stabilisation Network) • Measure HF emissions from LISN [1] EN55022, 2010, “Information technology equipment – Radio disturbance characteristics – Limits and methods of measurement” TI Information – Selective Disclosure 8 What is the purpose of the LISN? 50µH L1 L2 Inoise/ITotal Total input currents LISN AC/DC or DC/DC Converter Current for power50µH Load Noise Source 1µF 1µF 0.1µF 0.1µF Noise current 50Ω 50Ω G Ground Plane • LISN measurement output is a high-pass filter • Low frequency power current passes straight through from AC source • High frequency current (noise) is trapped by the LISN capacitor and the amplitude is measured based on the voltage across 50Ω load -> measured by EMI receiver TI Information – Selective Disclosure 9 LISN – Line Impedance Stabilisation Network • • • • Presents stable, consistent & repeatable line source impedance Separation of power source noise “Total” noise levels measured separately on L1 (Live) and L2 (Neutral) Terminated into 50, internal to EMI receiver EMI receiver 50Ω terminator provided by EMI receiver ** Functional equivalent circuit of a LISN, not a complete schematic ** TI Information – Selective Disclosure 10 EMI Receiver – Detector Types Built-in detectors – peak, quasi-peak, average SW Peak Noise Measurement Quasi-peak Noise Measurement V Average Time TI Information – Selective Disclosure 11 EN55022/32 Limit Lines – Conducted Emissions Class A and Class B limits, quasi-peak & average, 150kHz–30MHz Class B Class A (Residential, commercial & light industrial) (Heavy industrial) (PC, notebook adapter) [1] EN55022, 2010, “Information technology equipment – Radio disturbance characteristics – Limits and methods of measurement” TI Information – Selective Disclosure 12 – ~ ~ + LISN + EMI Filter + Power Supply TI Information – Selective Disclosure – ~ ~ + LISN + EMI Filter + Power Supply TI Information – Selective Disclosure – ~ ~ + LISN + EMI Filter + Power Supply TI Information – Selective Disclosure – ~ ~ + LISN + EMI Filter + Power Supply TI Information – Selective Disclosure EMI debug & improvement TI Information – Selective Disclosure EMI Fundamentals – Component Parasitics • Parasitic elements are the cause of EMI • If all components were ideal, there would be no EMI issues • EMI noise is generated by parasitic elements: – ESR (equivalent series resistance) – ESL (equivalent series inductance) – Parasitic capacitances • EMI noise is coupled & propagated through parasitic elements: – Capacitive coupling – Inductive coupling TI Information – Selective Disclosure 18 Why care about Differential-Mode (DM) EMI? • DM noise couples to the AC utility supply network • Long AC distribution cables – act as good dipole antenna • Will inadvertently radiate switching noise and interfere with radio communications Radiated interference Switching PSU L N AC mains distribution cables DM noise source Load Radiated interference TI Information – Selective Disclosure 19 How is DM EMI generated? • Switching ripple current produces ripple voltage across ESR (& ESL) – ~ ~ + • Ripple voltage is the DM noise that needs to be attenuated/filtered TI Information – Selective Disclosure 20 Mitigation options for DM noise • Include EMI at the design phase – Design & component choices to minimise DM EMI signal amplitude to be filtered – Chose frequency, inductance, etc. to minimise pk-pk ripple current Power Stage Designer Software – Choose capacitors with low esr to minimise pk-pk ripple voltage – Good PCB layout important to minimise EMI • Design DM LC filter to reduce the ripple that gets onto the AC line input – DM signal can be easily simulated/analysed and DM filter can be calculated/simulated – ~ ~ + Power Stage Design SW TI Information – Selective Disclosure 21 DM filter design methodology • Time-domain current waveform easy to measure or simulate • Fourier Series expansion of switching current – Convert waveform into harmonic components • Or sample current waveform – Use Fourier Transform to determine harmonics • Establish required attenuation at each frequency – To get sufficient margin below the relevant EMI limit • Design the DM filter to achieve required attenuation – Need to check all frequencies of interest • Filter size is usually defined by the lowest frequency switching harmonic that falls within the conducted EMI band – Typically EMI starts at 150 kHz for AC-DC PSU TI Information – Selective Disclosure 22 DM filter practical considerations • DM filter requires high attenuation over wide bandwidth AC voltage – Example, peak PSU current amplitude typically several amps – At 50 dBV, current in LISN 50-ohm resistor only ~6.3 uA Ipeak (non-PFC) Ipeak (with PFC) • DM inductor passes the full AC line current – Beware inductance roll-off with DC-bias – Must not saturate to be effective – needs high current rating – Consider the peak line current for non-PFC – high crest factor • DM filter located close to switching PSU, with fast changing magnetic fields – beware filter bypassing or noise coupling • Parasitic capacitance across DM inductor is important – reduces effectiveness at high frequency – E.g. to filter 300 kHz component, could set LC freq ~30 kHz -> expect ~40 dB attenuation at 300 kHz, but parasitic cap => more like 30 dB TI Information – Selective Disclosure 23 Why care about Common-Mode (CM) EMI? • Again, AC distribution cables and output load cables act as good dipole antenna • CM noise will radiate from the cables and interfere with radio communications TI Information – Selective Disclosure 24 How is CM EMI generated? – ~ ~ + • Switching voltage across parasitic capacitance causes CM current flow to earth TI Information – Selective Disclosure 25 How is CM EMI generated? • Switching voltage across parasitic capacitance causes CM current flow to earth Vac – ~ ~ + CM EMI Voltage Z EARTH TI Information – Selective Disclosure 26 Mitigation options for CM noise 1. Shielding: reduce flow of HF current to EARTH 2. Cancellation: arrange transformer and power stage for balanced CM 3. Filtering: increase impedance of the EARTH return path and provide alternative routes for the HF current TI Information – Selective Disclosure 27 CM mitigation by shielding 1. Shielding inside the transformer – Internal shields between pri & sec 2. Shielding outside the transformer – GNDed flux-band 3. Shielding of noisy circuit nodes – GNDed heatsinks over/around high-voltage switching nodes 4. Shielding of EMI filter from switching circuit – GNDed shields/enclosures around filter TI Information – Selective Disclosure 28 CM mitigation by cancellation/balance • Single-ended topologies – need explicit additional cancellation elements (flyback & forward) – Generate voltage proportional to CM waveform, with opposite phase – Capacitor to inject cancelling current to balance CM – Can be explicit physical capacitor added to design – Or can use parasitic cap, part of transformer structure • Double-ended switching topologies – inherently balanced for CM (push-pull, half-bridge & full-bridge) Np1 • Single-ended topology alternatives – Can position switches/rectifiers in middle of single-ended windings to improve CM balance – But difficult to level-shift drive, balance parasitic cap, etc. Ns1 Icm1 Icm2 Np2 Ns2 TI Information – Selective Disclosure 29 CM mitigation by filtering • CM filter uses high impedance CM chokes and low-impedance Y-capacitors • CM choke limits the flow of CM current from Earth – ~ ~ + • Y-cap provides low-impedance to keep CM current local to primary GND and away from Earth TI Information – Selective Disclosure 30 How to separate CM & DM noise • Useful to observe CM & DM separately – Just CM issue, or just DM, or both? • Sophisticated method: – LISN with dual output (L & N) or two single-line LISN’s – Use DM & DM power splitters/combiners to attenuate one or other noise type • Simple (low-cost) methods: – Add large X-capacitor upstream of the UUT • Attenuate DM, leave mostly CM – Use large Y-capacitor – attenuate CM, leave mostly DM – Observe the baseline CM being coupled to the secondary side in the time-domain Can be very insightful TI Information – Selective Disclosure 31 Observing the time-domain CM signal at the output • Observe time-domain CM signal on scope, very informative – Remove Y-cap temporarily (maximise observable signal amplitude) – Power UUT through LISN, with resistor loads – Coil several turns of wire around the load cables (sensing/pick-up coil) – Connect scope earth lead to LISN earth – Connect scope tip to sensing coil TI Information – Selective Disclosure 32 Interpretation of time-domain CM signal • Will see “switch-node” shaped waveform – capacitively-coupled to output • Large pk-pk amplitude => bad CM noise • Will require significant CM filtering to suppress • Small pk-pk amplitude => good CM noise • “Balanced” structure for low CM • Will require much smaller CM filter • Residual HF “spikes” => should only need small HF CM choke TI Information – Selective Disclosure 33 CM filter choke practical considerations • Freq response of core material – High- cores –> high L @ low freq, but roll off fast at high freq – High-freq cores –> low-, smaller L value @ low freq, better vs freq – Sometimes need to use 2 CM chokes, 1 for LF & 1 for HF • Split-wound vs bifilar-wound toroid – Split-wound popular, lower cost, “free” DM choke from leakage – Bifilar –> 1-side insulated wire, higher cost, but better noise immunity • Parasitic input-output cap – multi-layer windings – Multi-layer –> more turns, more L –> but also higher parasitic cap • High input-output cap => lower self-resonant freq, worse @ HF – Less layers => lower L, but also lower cap – Sectional bobbins – used to reduce input-output parasitic capacitance TI Information – Selective Disclosure 34 CM filter choke winding example • Split-wound 2 layer: – 25T, 5.1 mH – Excess pass margin @ LF – Low pass margin @ 15 MHz • Bifilar-wound 1 layer: – – – – 14T, 1.1 mH Low input-output cap Lower L at low freq, but better at HF Better balance across frequency span CM choke inductance vs freq – – – – 14T, 1.4 mH Low input-output cap Lower L at low freq, but better at HF Better balance across frequency span L (mH) • Split-wound 1-layer: 6 5 4 3 2 1 0 2L, split 1L, split 1L, bifilar 0 200 400 600 Frequency (kHz) 800 1000 TI Information – Selective Disclosure 35 CM choke example Initial split-wound choke had EMI issues due to asymmetric noise coupling from transformer – causes big difference in EMI on L vs N TI Information – Selective Disclosure 36 CM choke example Changed to bifilar-wound choke, much better EMI – much better noise immunity 12 dB TI Information – Selective Disclosure 37 Transformer internal shielding • Shield added to keep most of CM current local to primary • Shield is 1-turn winding => lower induced voltage, less voltage across capacitance between shield & sec => less CM current flows • Shield must be thin –> minimise induced eddy currents loss; significant as Fsw increases TI Information – Selective Disclosure 38 Transformer CM noise example • Typical interleaved Flyback transformer construction • Primary (green), secondary (blue), auxiliary bias (red) TI Information – Selective Disclosure 39 Transformer CM noise example • Typical connections to the Flyback power stage TI Information – Selective Disclosure 40 Transformer CM noise example • Typical waveforms TI Information – Selective Disclosure 41 Transformer CM balance example • Move auxiliary in-between inner Pri-SEC, add more turns & strands to fill layer • Add foil shield (purple) in-between outer Pri-SEC TI Information – Selective Disclosure 42 Transformer CM balance example • Connections to Flyback power stage • Shield connects to tap on auxiliary bias winding TI Information – Selective Disclosure 43 Transformer CM balance example • Waveforms – now don’t care about primary layers, only the aux & shield • Aux & shield waveforms have same shape, amplitude & phase as secondary → balanced TI Information – Selective Disclosure 44 Transformer “housekeeping” best practices • Centre-leg air-gaps only – outer legs will radiate • “Noisier” windings on inside of layer structure • GND ferrite core to local primary GND – large Cparasitic • Flux-band to minimise stray coupling – GNDing & flux-banding can give >10 dB improvement! • Trade-off between interleaving for low leakage inductance vs. low inter-winding parasitic capacitance • Beware internal construction details: Fluxband + EMI shield ~14 dB – Different parasitic pri-sec capacitance – Different average CM voltage across the pri-sec capacitance – Arrange winding layers to minimise voltage difference • Minimise CM current through parasitic cap between layers TI Information – Selective Disclosure 45 PCB layout tips • Detailed Layout Discussion in Tech Day 2018 • Minimise area of traces with high dv/dt – minimise parasitic capacitance • Minimise area of loops with high di/dt current • Place filter components away from noise sources • No GND plane under EMI filter • CM choke input/output pins do not cross • No GND plane under EMI filter • Wide spacing between traces minimises parasitic coupling capacitance TI Information – Selective Disclosure 46 PMP21479 65-W ACF USB-PD • Improve transformer structure for CM balance • Add transformer grounding & flux-banding + EMI shield • Improve CM choke position/location/type • Improve output cap position/location, PCB orientation – Largely “free” improvements – Little or no added cost or efficiency penalty 47 dB TI Information – Selective Disclosure 47 Summary & Conclusions • Consider EMI right from the start – inherent part of the power supply design • Minimise DM & CM EMI noise at source • DM filter can be designed/calculated/simulated more easily than CM • CM balance important as much as practically possible • Debug to establish if EMI issue is CM or DM or both • Assess CM performance in time-domain • For isolated PSU transformer is most important component – Internal construction details, CM balance, shielding, parasitic capacitance, house-keeping • EMI filter components – be aware of parasitics and HF effects • PSU layout and component placement – be aware of stray coupling paths TI Information – Selective Disclosure 48