Troubleshooting DC-DC Converter Noise Issues

⚡ Why EMI/EMC Matters in DC-DC Converters DC-DC converters step down or step up voltages (e.g., from HV battery to 12V system). They switch at high frequencies (50kHz – 500kHz+), which generates significant EMI. Poor EMC design can affect nearby systems (like infotainment, ADAS, BMS) or violate regulatory limits. 🔍 Key EMI/EMC Challenges SourceEMI RiskNotesHigh dV/dt and dI/dt SwitchingRadiated & Conducted EmissionsEspecially from MOSFET/IGBT switchingLayout ParasiticsEmissions/Noise SusceptibilityLoop area, trace impedancePower CablesRadiated EmissionAct as antennas, especially long HV cablesControl SignalsSusceptibilityCAN, PWM signals may be corruptedCommon Mode NoiseEmissions through chassis or groundOften overlooked ✨ EMI/EMC Design Strategies for DC-DC Converters 1. PCB Design Use short, wide traces. Minimize high-current loop areas. Keep power and control grounds isolated with a single-point connection. 2. Filtering Input/output LC filters to suppress conducted noise. Common Mode Chokes (CMC) on power lines. Snubber circuits across switching devices. 3. Shielding Shield the entire converter enclosure (Faraday cage). Shielded cables (especially HV lines). 4. Grounding Use star grounding to avoid ground loops. Isolate noisy and quiet grounds. 5. Component Choices Use soft-switching topologies (ZVS/ZCS) when possible. Use EMI-rated capacitors (X/Y class). 📏 Test Standards Test TypeStandardDescriptionConducted EmissionCISPR 25 / CISPR 32Emission through DC linesRadiated EmissionCISPR 25 / ISO 11452-2Emission from enclosure & cablesConducted ImmunityISO 7637-2Load dump, cranking, burst pulsesRadiated ImmunityISO 11452-4/2External RF susceptibilityESDISO 10605Static discharge events. #EMI #EMC #DC-DC Converters #DC-DC #shielding #filter #AC-DC #RF #RE #RI #HV #LV

Dear Power Electronics Experts, Recently, I designed, developed, and fully fabricated a dual-output buck converter capable of operating from a wide 24–60V input and delivering 5V @ 10A and 12V @ 5A, built around the Analog Devices LTC3891 controller. On LTspice, the converter performed flawlessly—handling dynamic load steps from 0–5A on the 12V rail and 0–10A on the 5V rail without any instability. However, after fabrication, an unexpected issue appeared. At no load, both outputs regulate perfectly at 5V and 12V. But the moment I apply even a slight load—as low as 0.1A to 0.2A on either rail—the output collapses to zero. For current sensing, the LTC3891 allows a maximum sense voltage of 75 mV. For the 12V rail, I used 3 × 40 mΩ (0805) resistors in parallel, giving 13.33 mΩ, so OCP triggers at around 5.6A. For the 5V rail, 3 × 17 mΩ resistors result in 5.66 mΩ, giving an OCP threshold of ~13.2A. At no load, the IC stays in voltage mode, but under load it should transition to current-mode control. The problem is that at light loads, the sense voltage is extremely small—for example, 0.5A across 13.33 mΩ gives only 6.6 mV, nowhere near the current-mode threshold. Because the design originally used Burst Mode, the controller forces a minimum inductor peak current of ~25% of the max limit, causing pulse skipping, which seems to prevent proper transition into current-mode control at light load. The datasheet mentions that a feedforward capacitor (Cff) across the feedback resistor can improve loop response, but it provides no direct calculation method. I would appreciate insights from engineers who have tuned Cff on the LTC38xx family. During troubleshooting, I diagnosed multiple issues: 1-Noise in the voltage and current feedback paths, likely due to PCB layout. The VFB resistors were originally connected before the output capacitors, introducing significant noise—so I re-routed them externally, but the issue persisted. 2-There was no RC filter on the VFB divider initially; adding one externally did not resolve the problem. 3-No gate resistors or bleeding resistors were included initially; Vth of the MOSFET was 5V, so I added 10k bleed resistors externally. I am now revising the PCB layout and have already switched from Burst Mode to Continuous Conduction Mode, but I would appreciate any further recommendations—especially related to compensation design, noise-resistant routing, feedforward capacitor selection, and any LTC3891-specific quirks. Looking forward to hearing insights from experienced power electronics engineers who might have faced similar light-load instability issues.

Me today dealing with some EMC issues… 🧙♂️🪄🐉 EMC might feel like black magic sometimes, but it’s not all spells and wand-waving. Here’s the checklist I worked through today to troubleshoot: 1️⃣ 𝗕𝗲 𝘄𝗮𝗿𝘆 𝗼𝗳 𝘄𝗶𝗿𝗶𝗻𝗴 𝗮𝗰𝘁𝗶𝗻𝗴 𝗹𝗶𝗸𝗲 𝗮𝗻 𝗮𝗻𝘁𝗲𝗻𝗻𝗮. Anything with wiring can pick up noise and radiate it—even cables that seem unrelated to your core system. If the cable isn’t critical, remove it and retest to isolate the problem. If you can’t remove it, try adding a ferrite ring to the cable as close to the board as possible On the PCB, ferrite beads or chokes can also help suppress noise if you’ve got space to add them. 2️⃣ 𝗦𝗹𝗼𝘄 𝗱𝗼𝘄𝗻 𝘆𝗼𝘂𝗿 𝗠𝗢𝗦𝗙𝗘𝗧 𝗴𝗮𝘁𝗲 𝗱𝗿𝗶𝘃𝗲 𝘀𝗶𝗴𝗻𝗮𝗹𝘀. This is one of the top culprits for EMI on motor drive boards. Increasing both the turn-on and turn-off resistors for your MOSFET gate drive slows the rise and fall times of the signal, which directly cuts down on emissions. 3️⃣ 𝗥𝗲𝗱𝘂𝗰𝗲 𝗣𝗪𝗠 𝗳𝗿𝗲𝗾𝘂𝗲𝗻𝗰𝗶𝗲𝘀. We had a 250kHz PWM signal driving a battery charger boost converter. The lab results weren’t happy, so we made some changes: - Dropped the frequency to 75kHz. - Increased the inductor value to match the new frequency. - Slowed down the MOSFET rise time (see point 2). This got us under the threshold—barely (around 2dB). We’ll reduce the charge current by about 15% to get a little more breathing room. 4️⃣ 𝗖𝗵𝗲𝗰𝗸 𝘆𝗼𝘂𝗿 𝗿𝗲𝘁𝘂𝗿𝗻 𝗽𝗮𝘁𝗵𝘀. High-current or high-frequency signals need clean return paths—no exceptions. In our case, we were stuck with a 2-layer PCB (budget constraints, of course), and the ground return path for the low-side MOSFET gate drive signal ended up being pretty big. I spotted a way to reduce the loop area by adding a via. We drilled a quick hole in the board and connected it with a wire. Not pretty, but it worked! The layout will need redoing, but this hack let us verify the solution at the test lab. If you haven’t already, check out 𝗔 𝗛𝗮𝗻𝗱𝗯𝗼𝗼𝗸 𝗼𝗳 𝗕𝗹𝗮𝗰𝗸 𝗠𝗮𝗴𝗶𝗰 𝗯𝘆 𝗛𝗼𝘄𝗮𝗿𝗱 𝗝𝗼𝗵𝗻𝘀𝗼𝗻. It’s the go-to resource for high speed digital electronics theory, and will let you analyse EMC issues way more effectively. What are your favorite resources for EMC troubleshooting? Drop them below—I’m always on the lookout for more tools/knowledge to add to my wizarding arsenal! 🪄 ------------- 🔔 Follow Ryan Dunwoody for more hardware chat 🚀 ♻️ Repost if you're an EMC wizard (or would like to be) 🧙♂️