Strange rectifier setup in LED driver - Page 1
Forum section: electronics / repair

author: reodor
author_rank: Contributor
timestamp: February 25, 2025, 01:44:06 pm
content: I have been asked to "take a look at" six Osram LED drivers. They have all failed, but not in identical ways. I have reverse engineered it as best I can, and now I am trying to understand how it works. There are two things that puzzle me:1. The mains rectifier appears to have two sets of diodes on the negative side (D3, D4, D6 and D7), in addition to an extra diode in series with each pair (D5 and D8). The half-bridge capacitor (C6) is not connected to the negative side of the smoothing capacitors, but in stead to one of the diode pairs. I cannot see how current is supposed to flow when Q1 is conducting. How does this work?2. The control chip's data sheet shows that it has a built-in switch that takes current from the RC pin and charges the auxilary capacitor during startup. In this design, it seems they have used extra resistors to provide startup power supply, even though it should not be needed. Why the clunky resistor setup on the RC and Vcc pins?Schematic and datasheet for the control chip are attached.

author: reodor
author_rank: Contributor
timestamp: March 08, 2025, 03:17:39 pm
content: Here are some pictures of the visible damage.

author: reodor
author_rank: Contributor
timestamp: March 13, 2025, 01:15:38 pm
content: It seems they have realized that the product was failure prone, because they are now selling a revised version, the G2. The full name is "OT FIT 55/220-240/1A0 CS L G2". Presumably they are using better capacitors, but there are also several design changes.The rectifier has been changed, but the half bridge rectifier is still connected to the AC line, and not to the rectified negative, as the datasheet shows.Is this some clever trick to get a good power factor? Are they "riding the sine wave" somehow? The unit is marked with a lambda of 0.98, which is most excellent.Reverse engineered schematic attached.

author: tom66
author_rank: Super Contributor
timestamp: March 13, 2025, 04:01:30 pm
content: Given the two series 300V caps, I wonder if they are boosting the DC link voltage up to around 600V... Though, I have no idea why they would do this for an LED lamp!

author: reodor
author_rank: Contributor
timestamp: March 13, 2025, 10:29:05 pm
content: It is common to use a boost converter to facilitate power factor correction (PFC), and although this unit has near perfect power factor, it looks to me like the rectified mains goes directly to charge the two caps. I cannot see any boost mechanism at all.But there are a few things about the circuit I don't understand, so anything is possible.I have not started measuring anything yet. I think I will need to order some capacitors first. Several of the film capacitor are visibly damaged.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 14, 2025, 08:31:40 pm
content: In your second photo, with the burnt windings, would that be the common-mode choke?I cannot see the PWB's reference designation.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 14, 2025, 09:37:00 pm
content: The 300 volt caps are Y2, line filtering "Safety Caps"Look up Y2 capacitor

author: thm_w
author_rank: Super Contributor
timestamp: March 14, 2025, 09:43:00 pm
content: Interesting that C2 and C1 are both bulged out.Not sure what the purpose of D5/C5/C3 is.Quote from: elonthefelon on March 14, 2025, 08:31:40 pmYes its L3 in the schematic.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 14, 2025, 10:31:42 pm
content: Quote from: thm_w on March 14, 2025, 09:43:00 pmIndeed, this is an interesting clue. Is this an indication of a long term over-stress? Were they a secondary failure? Can't see how that could cause the transistors to fail.Just for the sake of curiosity, do you think you could remove one of the caps, disassemble one of the caps and see what happened to the plastic film itself? A failure on the film would be disconcerting, as these caps are supposed to be self healing.

author: reodor
author_rank: Contributor
timestamp: March 27, 2025, 12:30:09 pm
content: Quote from: UnijunctionTransistor on March 14, 2025, 08:31:40 pmYes, that's the common mode choke. On this unit, it looks like one of the rectifier diodes were shorted, so that the current was too high, but not so high that the PCB fuse popped immediately.

author: reodor
author_rank: Contributor
timestamp: March 27, 2025, 12:37:44 pm
content: Quote from: thm_w on March 14, 2025, 09:43:00 pmYes, I think so, too. This is the unit where one of the rectifier diodes were shorted. I have no idea how that could affect the X class caps. Maybe there has been a serious overvoltage condition where both X capacitors and the diode blew?None of the other units have the same symptom.

author: reodor
author_rank: Contributor
timestamp: March 27, 2025, 12:45:49 pm
content: Quote from: UnijunctionTransistor on March 14, 2025, 10:31:42 pmMy apologies for the confusion. I have six of these units. On the one where the X caps have bulged, the transistors do not have visible damage. One of the diodes has shorted, though.Quote from: UnijunctionTransistor on March 14, 2025, 10:31:42 pmI will give that a go.Although they are self healing, they can only heal so many times before they have to give up. It is curious, though, that on this unit, both X caps have bulged, while on all the other five units, both X caps look fine.

author: reodor
author_rank: Contributor
timestamp: March 27, 2025, 01:19:45 pm
content: The manufacturer of the control chip has several application notes here:https://www.redisem.com/support.htm#ApplicationNotesAnchorI have not read them all, but it is already clear to me that I am in over my head here. It looks like the additional diodes and some of the caps are part of a voltage boost feature. Other caps are part of the resonance circuit.On the unit I was using for reverse engineering, C6 is a reddish brown colour, while the other caps all are yellow. As a result, I assumed it would be a metal film capacitor, and that it was the half-bridge capacitor. My assumption now seems to be incorrect. Some of the other units have a yellow cap in the C6 position, and the reddish brown one does not have any "CBB" marking on it.I have tried starting up one of the units. It keeps trying to start, but I may not have the proper load connected. I can, however, see that the voltage across the smoothing caps C10 and C12 does increase every time it makes a start attempt, so there is clearly some kind of boost feature here.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 29, 2025, 09:46:08 pm
content: RediSem: yet another Chinese semiconductor manufacturer who is kicking ass with advanced consumer-grade semiconductors.I have commented in other forums, the US government is extremely concerned with Chinese bleeding edge semiconductors, and rightly so. But flying under the radar there are a myriad of lo-key semiconductor companies which are dominating the ultra-high volume consumer markets.