{"id":19975,"date":"2026-10-23T16:45:09","date_gmt":"2026-10-23T08:45:09","guid":{"rendered":"https:\/\/www.wehopower.com\/news\/common-mode-current-isolated-dc-dc-capacitance\/"},"modified":"2026-10-23T16:45:09","modified_gmt":"2026-10-23T08:45:09","slug":"common-mode-current-isolated-dc-dc-capacitance","status":"publish","type":"news","link":"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/","title":{"rendered":"Common Mode Current in Isolated DC-DC Converters: Review the Capacitive Path"},"content":{"rendered":"<div class=\"weho-blog-content\" id=\"weho-article-31\" data-weho-article-id=\"31\">\n<p><strong>An isolated DC-DC converter can still pass high-frequency common-mode current through capacitance.<\/strong> Galvanic separation interrupts a direct conductive connection; it does not eliminate the electric-field coupling between input and output. The equipment&#8217;s cables, shields, chassis, and test instruments complete the path that determines where that current flows.<\/p>\n<p>For an OEM connecting isolated power to a sensor or communication interface, the practical question is whether the assembled system meets its noise and functional requirements. A withstand-voltage number alone cannot answer it. This article develops a current-path review and two hypothetical calculations; no WEHO model is assigned an isolation capacitance, noise performance, or safety insulation category.<\/p>\n<details class=\"article-toc\" open=\"\">\n<summary>Article sections<\/summary>\n<nav class=\"toc-links\" aria-label=\"Article sections\"><a href=\"#trace-complete-path\">Trace a complete path across and around the barrier<\/a><a href=\"#weho-sd50-isolation-evidence\">Read SD-50 isolation evidence separately from capacitance<\/a><a href=\"#edge-current-example\">Calculate an edge-current screening example<\/a><a href=\"#sinewave-example\">A sinusoidal component needs a different description<\/a><a href=\"#controlled-test\">Build a controlled connection test<\/a><a href=\"#change-path\">Review changes at the path they affect<\/a><a href=\"#qualification-record\">Release a reproducible interface, not an isolation slogan<\/a><\/nav>\n<\/details>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_86 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table des mati\u00e8res<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Basculer<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewbox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewbox=\"0 0 24 24\" version=\"1.2\" baseprofile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Trace_a_complete_path_across_and_around_the_barrier\" >Trace a complete path across and around the barrier<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Read_SD-50_isolation_evidence_separately_from_capacitance\" >Read SD-50 isolation evidence separately from capacitance<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Calculate_an_edge-current_screening_example\" >Calculate an edge-current screening example<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#A_sinusoidal_component_needs_a_different_description\" >A sinusoidal component needs a different description<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Build_a_controlled_connection_test\" >Build a controlled connection test<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Review_changes_at_the_path_they_affect\" >Review changes at the path they affect<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.wehopower.com\/fr\/news\/common-mode-current-isolated-dc-dc-capacitance\/#Release_a_reproducible_interface_not_an_isolation_slogan\" >Release a reproducible interface, not an isolation slogan<\/a><\/li><\/ul><\/nav><\/div>\n<h2 id=\"trace-complete-path\"><span class=\"ez-toc-section\" id=\"Trace_a_complete_path_across_and_around_the_barrier\"><\/span>Trace a complete path across and around the barrier<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Draw the primary-side return and the isolated-side return as distinct nodes. Then add the converter&#8217;s input-to-output capacitance, signal isolators, shield connections, enclosure capacitance, and any communication cable that reaches other grounded equipment. Include the mechanical arrangement because a metal mounting surface can change the effective capacitance.<\/p>\n<p>Common-mode current is associated with the pair or group of conductors moving together relative to an external reference. Differential load current instead leaves on one supply conductor and returns on its partner. The distinction identifies which path to measure and why a good differential output-ripple result may coexist with a noisy cable interface.<\/p>\n<p><a href=\"https:\/\/recom-power.com\/en\/rec-n-emc-considerations-in-power-supplies-and-dc%21sdc-converters-274.html\" target=\"_blank\" rel=\"noopener noreferrer\">RECOM&#8217;s EMC discussion explains the displacement-current relationship i = C \u00d7 dV\/dt<\/a>. It also distinguishes short current impulses from the spectral or RMS quantities used for other evaluations. A sharp voltage transition can therefore matter even when the coupling capacitance looks small.<\/p>\n<section class=\"weho-model-review\" aria-labelledby=\"weho-sd50-isolation-evidence\" data-weho-model=\"SD-50B-12\">\n<h2 id=\"weho-sd50-isolation-evidence\"><span class=\"ez-toc-section\" id=\"Read_SD-50_isolation_evidence_separately_from_capacitance\"><\/span>Read SD-50 isolation evidence separately from capacitance<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<div class=\"model-review-layout\">\n<div class=\"model-review-intro\">\n<p>Le <a href=\"https:\/\/www.wehopower.com\/fr\/product\/sd-50-single-output-dc-to-dc-converter-power-supply\/\" data-weho-link-kind=\"product_detail\">WEHO SD-50 DC-DC converter product page<\/a> et <a href=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2021\/11\/SD-50.pdf\">SD-50 manufacturer specification<\/a> illustrate the distinction. The PDF lists an <strong>input-to-output withstand-voltage item of 1.5 kV AC<\/strong>, but gives no input-to-output capacitance value. A withstand entry cannot supply the missing capacitance needed for the current-path calculation.<\/p>\n<\/div>\n<figure class=\"model-photo\"><a href=\"https:\/\/www.wehopower.com\/fr\/product\/sd-50-single-output-dc-to-dc-converter-power-supply\/\" class=\"model-product-link\" data-weho-link-role=\"product_image\" data-weho-model=\"SD-50B-12\" aria-label=\"SD-50B-12 product details\" data-weho-link-kind=\"product_detail\"><img loading=\"lazy\" src=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/10\/sd-50b-12.webp\" width=\"420\" height=\"680\" alt=\"Common Mode Current in Isolated DC-DC Converters: Review the Capacitive Path  title=\" decoding=\"async\" common mode current in isolated dc-dc converters: review the capacitive path title =\"\"><\/a><figcaption><a href=\"https:\/\/www.wehopower.com\/fr\/product\/sd-50-single-output-dc-to-dc-converter-power-supply\/\" class=\"model-name-link\" data-weho-link-role=\"product_text\" data-weho-link-kind=\"product_detail\">SD-50B-12 product details \u2197<\/a><br \/>\n<span>SD-50B-12 product photograph for document matching. Isolation capacitance and the applicable electrical ratings require evidence for the exact supplied assembly.<\/span><br \/>\n<a href=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/10\/sd-50b-12-original.jpg\" target=\"_blank\" rel=\"noopener\" class=\"photo-original\" data-weho-link-role=\"view_original\">View original photograph \u2197<\/a><\/figcaption><\/figure>\n<div class=\"model-review-detail\">\n<p>Before reviewing an SD-50 assembly for an isolated interface, reconcile the complete model suffix, unit rating label, and applicable specification revision. The public product records and the pictured SD-50B-12 label are not fully aligned. Ask WEHO to confirm the input-voltage range, rated output voltage and current, and revision that apply to the offered unit before assigning electrical requirements. Request input-to-output capacitance with its measurement frequency, terminal grouping, and typical or maximum status, then evaluate the installed power-and-signal path.<\/p>\n<\/div>\n<\/div>\n<p>Le <strong>20 pF and 100 pF values below remain hypothetical<\/strong>, with no value assigned to SD-50. Similarly, the document&#8217;s withstand entry does not establish a continuous working-voltage limit, safety insulation category, or the equipment&#8217;s common-mode noise result. The useful supplier deliverable is an exact assembly record plus the missing capacitance evidence, followed by a controlled connection test.<\/p>\n<\/section>\n<h2 id=\"edge-current-example\"><span class=\"ez-toc-section\" id=\"Calculate_an_edge-current_screening_example\"><\/span>Calculate an edge-current screening example<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Assume a simplified coupling path with <strong>20 pF<\/strong> effective capacitance and a voltage across that capacitance changing by <strong>24 V in 20 ns<\/strong>. These are invented system inputs, not a converter specification. Approximate the edge as a linear ramp:<\/p>\n<p><strong>dV\/dt = 24 V \/ 20 ns = 1.2 V\/ns.<\/strong><br \/><strong>i = 20 pF \u00d7 1.2 V\/ns = 24 mA during the ramp.<\/strong><\/p>\n<p>The associated charge movement per edge is <strong>Q = C \u00d7 \u0394V = 480 pC<\/strong>. If the effective path were instead 100 pF under the same imposed voltage ramp, the screening values would be <strong>120 mA<\/strong> et <strong>2.4 nC<\/strong> per edge. The ratio is five because the capacitance ratio is five.<\/p>\n<div class=\"weho-table-wrap\" tabindex=\"0\" role=\"region\" aria-label=\"Engineering comparison table\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Hypothetical effective capacitance<\/th>\n<th scope=\"col\">Imposed edge<\/th>\n<th scope=\"col\">Ramp current<\/th>\n<th scope=\"col\">Charge per edge<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>20 pF<\/td>\n<td>24 V in 20 ns<\/td>\n<td>24 mA<\/td>\n<td>480 pC<\/td>\n<\/tr>\n<tr>\n<td>100 pF<\/td>\n<td>24 V in 20 ns<\/td>\n<td>120 mA<\/td>\n<td>2.4 nC<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<figure class=\"article-relation\"><picture><source media=\"(max-width:600px)\" srcset=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/10\/31-mobile.svg\"><img fetchpriority=\"high\" src=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/10\/31-desktop.svg\" width=\"1200\" height=\"675\" alt=\"Common Mode Current in Isolated DC-DC Converters: Review the Capacitive Path  title=\" loading=\"eager\" decoding=\"async\" common mode current in isolated dc-dc converters: review the capacitive path title=\"\"><\/picture><figcaption>Original lumped-path concept and hypothetical edge-current calculation. Not a product pinout, installation circuit, RMS leakage value or safety-compliance result.<br \/>\n<a href=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/10\/31-desktop.svg\" target=\"_blank\" rel=\"noopener\" class=\"diagram-original\" data-weho-link-role=\"view_original\">View full diagram \u2197<\/a><\/figcaption><\/figure>\n<p>These are lumped-circuit screening values. Actual voltage across the barrier can differ from the DC output rating, and the real network includes source impedance, inductance, distributed capacitance, and frequency-dependent return paths. Current does not become a real measured result simply because the equation is correct.<\/p>\n<p>Do not compare the 24 mA ramp value directly with an RMS leakage limit or an RCD threshold. Duration, repetition, waveform, bandwidth, measurement network, and current division matter. Protective-conductor or touch-current assessment is a separate task with its applicable method; the calculation here does not establish safety compliance.<\/p>\n<h2 id=\"sinewave-example\"><span class=\"ez-toc-section\" id=\"A_sinusoidal_component_needs_a_different_description\"><\/span>A sinusoidal component needs a different description<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>For a single sinusoidal voltage across an ideal capacitance, the RMS relationship is <strong>I<sub>RMS<\/sub> = 2\u03c0fCV<sub>RMS<\/sub><\/strong>. At a hypothetical 500 kHz, 20 pF, and 1 V RMS across that capacitance, the current is approximately <strong>62.8 \u00b5A RMS<\/strong>.<\/p>\n<p>This is not another way to label the 24 V ramp above. It represents a different assumed waveform and voltage magnitude. A switching waveform contains multiple spectral components; each experiences the actual circuit impedance. Neither example predicts the common-mode voltage or communication error at a receiving device without the rest of that network.<\/p>\n<p>Keep these quantities separate in the project worksheet: capacitance and its measurement conditions; voltage waveform across the boundary; current at a named conductor group; and the functional result at the receiver. This prevents a component-level comparison from quietly turning into a system guarantee.<\/p>\n<h2 id=\"controlled-test\"><span class=\"ez-toc-section\" id=\"Build_a_controlled_connection_test\"><\/span>Build a controlled connection test<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Begin with the intended production arrangement and a stable equipment operating state. Record the converter identity, input and load, signal activity, cable lengths, shield terminations, enclosure, and instrument connections. Choose instruments with the bandwidth, isolation, and common-mode capability needed for that setup.<\/p>\n<p>A current probe around both output conductors together can help examine their net external common-mode current because opposing load currents ideally cancel. It does not capture current in a separate shield or chassis lead that is outside the aperture. Probe imbalance, bandwidth, positioning, and other return paths remain part of the measurement uncertainty. <a href=\"https:\/\/www.tek.com\/en\/documents\/application-note\/making-accurate-current-measurements-power-supplies-oscilloscopes\" target=\"_blank\" rel=\"noopener noreferrer\">Tektronix explains why current-probe sensitivity and waveform bandwidth need deliberate selection<\/a>.<\/p>\n<div class=\"weho-table-wrap\" tabindex=\"0\" role=\"region\" aria-label=\"Engineering comparison table\">\n<table>\n<thead>\n<tr>\n<th scope=\"col\">Configuration<\/th>\n<th scope=\"col\">Question<\/th>\n<th scope=\"col\">Record together<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Power and representative local load<\/td>\n<td>What baseline path exists before external signals?<\/td>\n<td>Pair current spectrum and output behavior<\/td>\n<\/tr>\n<tr>\n<td>Required communication cable connected<\/td>\n<td>Does the remote reference change the current path?<\/td>\n<td>Cable\/shield connections and receiver error counters<\/td>\n<\/tr>\n<tr>\n<td>Production enclosure and mounting installed<\/td>\n<td>Does the physical return network change?<\/td>\n<td>Bonding locations, cable routes, and relevant spectral components<\/td>\n<\/tr>\n<tr>\n<td>Input and load operating corners<\/td>\n<td>Do switching states alter the interference?<\/td>\n<td>Operating mode and repeatable functional metrics<\/td>\n<\/tr>\n<tr>\n<td>Intended test instruments attached<\/td>\n<td>Does the measuring setup change the result?<\/td>\n<td>Probe return paths and any added capacitance<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Keep required protective bonding intact. A diagnostic comparison is not permission to disconnect PE or compromise an isolation barrier. Use manufacturer-permitted configurations and suitable isolated measurements; an ordinary grounded oscilloscope lead can create the very connection being investigated.<\/p>\n<h2 id=\"change-path\"><span class=\"ez-toc-section\" id=\"Review_changes_at_the_path_they_affect\"><\/span>Review changes at the path they affect<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Lower specified isolation capacitance can reduce one coupling path, but other paths may dominate. Compare values under the stated measurement frequency and terminal grouping, and determine whether a figure is typical or a guaranteed maximum. A value measured between one pair of terminals is not automatically the capacitance of the finished assembly. <a href=\"https:\/\/recom-power.com\/en\/dcdc-bok-chapter3-datasheet-parameters.html\" target=\"_blank\" rel=\"noopener noreferrer\">RECOM discusses isolation-capacitance measurement conditions in its datasheet-parameter guide<\/a>.<\/p>\n<p>A deliberate capacitor can redirect high-frequency current locally while increasing coupling across the isolation boundary. <a href=\"https:\/\/product.tdk.com\/system\/files\/dam\/doc\/product\/power\/switching-power\/dc-dc-converter\/wdcatalog\/cc-e_15-25w_e.pdf\" data-weho-link-kind=\"product_detail\" target=\"_blank\" rel=\"noopener noreferrer\">TDK-Lambda&#8217;s CC-E instructions show this tradeoff for specific products<\/a>. Their component guidance belongs to those products and is not a generic capacitor recommendation for a WEHO converter or an assembled machine.<\/p>\n<p>Any cross-barrier part needs review of its intended function, insulation requirements, permitted current, and effect on system performance. Similarly, a common-mode choke or a different shield termination acts on a particular path. Evaluate the changed assembly; do not infer success from the component name.<\/p>\n<p>Preserve the distinction between this coupling problem and <a href=\"https:\/\/www.wehopower.com\/fr\/news\/buck-converter-negative-voltage-reference\/\" data-weho-link-kind=\"guide\">the choice of output voltage reference<\/a>. A deliberate conductive bond can change the system&#8217;s galvanic boundary, whereas capacitance transfers time-varying displacement current. Both belong on the same path drawing, but they are different mechanisms.<\/p>\n<h2 id=\"qualification-record\"><span class=\"ez-toc-section\" id=\"Release_a_reproducible_interface_not_an_isolation_slogan\"><\/span>Release a reproducible interface, not an isolation slogan<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>The qualification record should contain the path drawing, exact module and installation revision, capacitance data with conditions, named operating cases, instrument arrangement, receiver limits, and results before and after any change. Use a limit relevant to the equipment function and the applicable EMC method; a low cable-current reading alone does not prove immunity or compliance.<\/p>\n<p>For the supply assembly itself, the broader <a href=\"https:\/\/www.wehopower.com\/fr\/news\/buck-converter-module-oem-qualification\/\" data-weho-link-kind=\"guide\">OEM module qualification checklist<\/a> keeps electrical interfaces, mounting, and controlled production evidence connected. It does not replace the isolation-path review.<\/p>\n<p><a href=\"https:\/\/www.wehopower.com\/fr\/contact\/\" data-weho-link-kind=\"inquiry\">Send WEHO your isolated power-and-signal interface requirements<\/a> with the intended reference arrangement, operating voltages, load, cables, enclosure, and required evidence. Identify galvanic isolation as a requirement to verify for the exact candidate, then ask separately for available capacitance and EMC documentation. A common-mode test plan gives that discussion a measurable next step.<\/p>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>An isolation barrier interrupts a conductive path but still has parasitic capacitance. Use edge-current calculations and a controlled connection matrix to qualify the complete system.<\/p>","protected":false},"author":2,"featured_media":19914,"parent":0,"menu_order":127,"comment_status":"","ping_status":"","template":"","format":"standard","meta":{"_acf_changed":false,"_joinchat":[],"footnotes":""},"news_category":[284],"class_list":["post-19975","news","type-news","status-publish","format-standard","has-post-thumbnail","hentry","news_category-product-articles","entry"],"acf":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/news\/19975","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/news"}],"about":[{"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/types\/news"}],"author":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/comments?post=19975"}],"version-history":[{"count":0,"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/news\/19975\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/media\/19914"}],"wp:attachment":[{"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/media?parent=19975"}],"wp:term":[{"taxonomy":"news_category","embeddable":true,"href":"https:\/\/www.wehopower.com\/fr\/wp-json\/wp\/v2\/news_category?post=19975"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}