Giant MLCC cap in CCFL HV supply, what could be its capacitance? - Page 1
Forum section: electronics / repair

author: capslock
author_rank: Contributor
timestamp: January 01, 2025, 07:42:06 pm
content: I tried to kill the high pitch noise eminating from Bosch/Blaupunkt Nav Radios used in 2010-2014 Ford S-Max, Galaxy and Mondeo. My prime suspect was an MLCC cap that seems to connect two transistor collectors (they are marked 435ZDG) near the transformer. Unfortunately, I didn't measure the cap in circuit, and in desoldering, one of the contacts started sliding off.It's a raw MLCC stack, so no molded case. It is 7.0 mm long, 6.38 mm wide and 4.8 mm high. This does not seem to match any known SMD case. Any idea what the casee might be called and who made those?For context, here's a thread in a car forum and a picture of the PCB in question that sits on the back of the LCD touch screen.https://www.motor-talk.de/forum/navi-nerviges-fiepen-t4268623.htmlhttps://www.motor-talk.de/bilder/navi-nerviges-fiepen-g77078414/004-i208918361.html

author: inse
author_rank: Super Contributor
timestamp: January 01, 2025, 07:53:11 pm
content: It seems to be the resonant capacitor in the HV inverter for the backlight.Unfortunately you screwed it, the inverter will probably not run stable without the right replacement.Ceramic capacitors do have a piezo electric effect which creates audio noise when excited electrically.Even with the original component you will not get rid of the noise.You would have to shift the operating frequency into a non audible range.Without schematic there is little chance to get the inverter back running.You could try hacking in a separate backlight inverter search term ccfl inverter those are available with various specs - good luck!

author: capslock
author_rank: Contributor
timestamp: January 01, 2025, 08:12:51 pm
content: Interesting, probably something like this:https://www.google.com/url?sa=i&url=https%3A%2F%2Fdanyk.cz%2Fccfl_en.html&psig=AOvVaw0hYbeKMLc9HeLMEtglxoz9&ust=1735848170088000&source=images&cd=vfe&opi=89978449&ved=0CBEQjRxqFwoTCNjEifaK1YoDFQAAAAAdAAAAABAEWhy do they use a 160 V rated cap in this example when the operating voltage is 12 V? Resonance of the LC circuit or even back-induction on switching transients?It also means I was on the wrong track looking for a high capacitance, low voltage cap. It also begs the question why Bosch did not use a proper MKT cap that would have been acoustically quieter.I guess I'll just try some film capacitor of similar size. If I change the resonant frequency, that would actually be welcome.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2025, 08:21:12 pm
content: Size is probably not the proper selection criterium..Are you from Germany?You can have this one, for 7 inch TFT, automotive

author: capslock
author_rank: Contributor
timestamp: January 01, 2025, 09:01:09 pm
content: Yes, I am, but unfortunately, that is not the board. In your pic, those also seem to be MLCC caps, but they are greenish-gray rather than light brown.

author: capslock
author_rank: Contributor
timestamp: January 02, 2025, 01:57:53 pm
content: Ok, replaced the cap with a molded Siemens film cap 200 nF / 350 V that measured 205 nF salvaged from an old fluorescent light bulb. Turns on right away, maybe a tad brighter than before. High-pitched noise seems a lot better. Cannot hear it from driver's position but then it is raining all day. Noise is still there when I bring my ear close to the radio. It seems a little more broadband now.Measuring with a phone app and the phone held right into the CD slot, the main components of 7.9 kHz and 15.8 kHz are still there (about 40 and 30 dB above noise level). I remember they were much stronger and could be measured from 10 - 20 cm away. Gone is the sporadic peak at 9.9 kHz, but instead there is a whole carpet of changing peaks between the fundamental and the 2nd harmonic.Now, unless I accidentally chose exactly the same capacity as before, why didn't the inverter frequency change? I cannot find any IC that injects a frequency near this power supply.

author: capslock
author_rank: Contributor
timestamp: January 02, 2025, 01:58:25 pm
content: With the new cap in place

author: capslock
author_rank: Contributor
timestamp: January 02, 2025, 02:00:45 pm
content: And this is the spectrum, vertical lines are 20 dB apart

author: inse
author_rank: Super Contributor
timestamp: January 02, 2025, 02:19:13 pm
content: Well, fingers crossed you are right.Maybe it’s not oscillating in a clean way, if it’s a resonant, ZVS or what do I know topology and now untuned.Check for the transistors running hot and impact on radio reception.

author: capslock
author_rank: Contributor
timestamp: January 02, 2025, 02:22:34 pm
content: Well, I can't. This Display unit plugs into the radio mainboard, closing the case.I found one IC closeby but I assume it's a supply or control IC for the FPGA to its right:

author: floobydust
author_rank: Super Contributor
timestamp: January 02, 2025, 08:45:02 pm
content: CCFL inverters typically have a HV ceramic cap across the tube(s), at the output side of the transformer. Check for any. They would make acoustic noise.

author: inse
author_rank: Super Contributor
timestamp: January 02, 2025, 08:50:38 pm
content: In the photo in reply #5, there is none…

author: floobydust
author_rank: Super Contributor
timestamp: January 02, 2025, 09:04:11 pm
content: Yes I don't see one unless it's on the bottom side. Hard to follow traces to see if it's Royer or not, or figure out who makes the transformer.I know high voltages on MLCC makes noise but it should be above 20kHz say?If the noise is intolerable, I'd change it over to LED backlighting I think.

author: inse
author_rank: Super Contributor
timestamp: January 02, 2025, 09:18:30 pm
content: Perhaps it’s not the switching frequency that causes the noise but the superimposed dimming.I would expect a lower dimming frequency but my knowledge on this is a couple of years old (~20) but the car is not that fresh as well…