Server PSU EMI Filter Design Issue? X2 Capacitors Losing Capacitance - Page 1
Forum section: electronics / repair

author: psu13
author_rank: Newbie
timestamp: September 18, 2025, 09:48:34 am
content: Hi all,I have five failed 770 W server power supplies from a well-known and respected manufacturer, all made around 2017. They are the same model, and in each case the PFC stage failed in a similar way. The typical damage includes:Blown input fuse (12 A)One of the two parallel PFC MOSFETs (IPW60R199CP) shortedPSU startup diode 600 V / 3 A (from diode bridge to bulk cap) shortedBurned PFC current-sense resistors (3 × 40 mΩ, 2512 size, in parallel)I attempted to repair one unit.After replacing the damaged components, the supply started working. However, under a 25 A load at the output, an X2 capacitor (275 V, 0.47 µF) exploded next to the Pi-filter choke on the EMI filter board.X2 caps in PSUI noticed that L1 choke was running very hot (over 150 °C), so I decided to investigate why it was overheating and why the nearby capacitor failed. On a second repaired unit, I observed the same choke heating issue.It is evident that the C3 capacitor exploded when heated by the choke.To diagnose further, I measured the input current in neutral wire using a 1 Ω shunt and DSO2512G oscilloscope. Even at a light load of 12 V / 3 A (36 W), there was a large ripple current (~6 A pk–pk) at the PFC switching frequency of 60 kHz.CH1 (yellow) - current (1 V = 1 A), CH2 (green) - input line voltage.Here is a zoomed-in waveform at the peak of the line voltage:It’s clear that the EMI filter is no longer effective, and the choke is heating due to the high-frequency current.I then measured the capacitance of the X2 capacitors in the EMI filter and found severe degradation:Here is EMI filter topology (simplified, no Y2 caps, no common mode choke)AC In┬────┬── Filter board│ │[C1 = 0.1 µF X2] – OK (10–15% capacity loss)│ ││ │ │[C2 = 470 nF X2] – 95% loss│ ││ [L1 = 110 µH] – runs at ~150 °C│ │[C3 = 470 nF X2] – 99% loss│ │┴────┴── Main board│ │[C4 = 1.0 µF X2] – 99% loss│ │[Bridge Rectifier]│→ PFC stageFirst unit used Arcotronics (Kemet) R.46 MKP for the 470 nF X2 caps, and Europtronic (Nantong Jianghai Capacitor Co., Ltd, a major Chinese manufacturer) for the 1.0 µF. [att. 4]Four other units used all capacitors from Europtronic. [att. 3]In all five failed PSUs, the 470 nF and 1.0 µF X2 capacitors had almost completely lost capacitance.I wanted to understand why these X2 capacitors degraded so badly. Was it:high-voltage grid transients,humidity exposure,manufacturing defects,or excessive HF current through the capacitors?(I saw Dave’s EEVblog video on X2 capacitor failures and read up on typical film cap failure modes. I couldn’t dissect mine like Dave on the video — the epoxy potting is extremely hard.)I replaced the failed capacitors with low-cost Chinese jb JFZ 470 nF X2 capacitors. After replacement, the input ripple current dropped to ~200 mA pk-pk at 10 A output load, which seems acceptable.Here is a zoomed-in waveform at the peak of the line voltage:I then measured the AC current through the X2 capacitors using 0.1 Ω shunts (1 V = 10 A), connected at the capacitor terminals to neutral.Continued in the next post due to attachment limit

author: psu13
author_rank: Newbie
timestamp: September 18, 2025, 09:52:27 am
content: The highest current was in C4 (near the bridge rectifier, originally 1.0 µF):Here is a zoomed-in waveform at the peak of the line voltage:~4.5 A pk–pk, ~1.4 A RMSC3 (470nf, the one that exploded. After replacement):~1.6 A pk–pk, ~0.58 A RMSC2 (470nf):~0.145 A pk–pk, ~0.109 A RMSI found datasheets fromKemet R.46 seriesandTDKshowing maximum permissible HF RMS current for standart (non-automotive) X2 capacitors with 15mm lead spacing.Based on those charts, the measured current in C4 is very close to the maximum allowable value in the Kemet datasheet. With original 1.0 µF capacitor, the ripple current could be twice higher (2x capacity, lower impedance).Questions:Could this ripple current (~1.4 A RMS at 60 kHz through C4) explain the rapid capacitance degradation?Once the main 1.0uF capacitor (C4) degrades, does the increased current shift to the others, causing them to fail as well?Could this be a design flaw — i.e. the EMI filter was underdesigned for long-term reliability — leading to X2 cap degradation, loss of PFC stage EMI suppression, and eventually PFC failure?Does anyone know if jb JFZ 0.47 µF X2 capacitors are actually rated for this level of continuous HF ripple current? (The datasheet doesn’t explicitly list RMS ripple current.)Any insight from those with experience in PFC stage or input EMI filter design would be greatly appreciated.

author: tggzzz
author_rank: Super Contributor
timestamp: September 18, 2025, 10:08:31 am
content: You might find it useful to consider two points:how and why X capacitors (and Y capacitors) are designed to fail, and what should happen when they do failwhat is causing your X capacitors to failPlus, of course, whether other capacitors in similar servers are just waiting to fail.Here's a couple of quick non-authoritative referenceshttps://www.capacitorconnect.com/what-are-x-y-capacitors/https://recom-power.com/en/rec-n-class-x-and-class-y-safety-capacitors-142.html

author: JimboJack
author_rank: Regular Contributor
timestamp: September 28, 2025, 11:28:54 pm
content: heat load via the PCBS tracks ?and thermal radiation , there is forced active cooling on the PSU ? be very warm otherwise, upgrade the 40mm fans ?these are the next series up for X2's for harsher env and can take a bit more current.R523I347000P0MR53BI34700000KR523N410050P0Mhttps://content.kemet.com/datasheets/KEM_F3123_R52_X2_310_110C_MIN.pdfhttps://content.kemet.com/datasheets/KEM_F3131_R53B_X2_350.pdfon the JB cap's JFZ'scross reference so they be the same ishhttps://www.jbcapacitors.com/wp-content/uploads/2024/10/JFZ-X2-Metallized-Polypropylene-Film-Capacitors-Cross-Reference.pdfJFZT's have been tested for the same environmental factors as the R52 and R53B series. but no mention of ripple currenthttps://www.jbcapacitors.com/wp-content/uploads/pdf/JFWT-X2-Met-Polypropylene-Film-Capacitor-Temperature-Humidity-Bias-THB-Series.pdfJ.

author: coppercone2
author_rank: Super Contributor
timestamp: September 29, 2025, 01:13:11 am
content: I thought its cheap dielectric that arcs over and each time it arcs it errodes. So its like the voltage rating is not high enough for the environment its in.If the heat is too high, then I guess the breakdown voltage of the dielectric might go down, and it might make nanoscopic holes get bigger because it becomes fluidish and its stretched tight and it wants to settle into a position with less strain.Usually HV arcing over has to do with voids (air) and holes,

author: Swake
author_rank: Frequent Contributor
timestamp: October 01, 2025, 08:17:36 pm
content: For ones those were not RIFA caps.