AC/DC Projector Repair - Page 1
Forum section: electronics / repair

author: AcM
author_rank: Contributor
timestamp: October 14, 2022, 07:08:53 am
content: Hi all,I have a problem with a projector, after a long time without using it, I powered it and it worked for a few hours then two big bangs happened.I opened the unit and find a current surge limiter burnt and the ballast board with 2 mosfets in short circuit. So I replaced the broken parts and replaced also the electrolytic capacitors.The power supply board is divided into 3 sections, one produces the 5V standby voltage, the second section produces the 12V for the "logic" board and the last section generates a 385V with a PFC boost circuit for the lamp.Connecting to the mains the board alone without any other connection, I see the 5V standby power but the 12V for the main board and the 385V for the lamp are "intermittent".The 12V circuit and the PFC boost circuit are powered up by the same "auxiliary power" circuit, if I remove the PFC chip the 12V works properly but as I assemble again the PFC chip the 12V again starts to work intermittently.I try lower the lamp output voltage just to understand if the PFC chip is working properly regulating the output and seems to work at lower voltages (I replaced the feedback resistor network to obtain 335V) so in this configuration, I obtained a stable 12V and 335V.At the moment I'm focusing on the auxiliary circuit which has a periodic charge/discharge capacitor behavior, I understand that there's something that absorbs too much, so to check if there's a faulty capacitor on the auxiliary I added a capacitor but nothing changed. The power supply has also a safe overvoltage circuit that "kills" the auxiliary supply if the lamp voltage goes higher than about 450V. I also removed that circuit but the behavior doesn't change so that part is not faulty.Any advice? I'm unable to find schematics and I'm running out of ideas.Thanks in advance for any hints.Regards

author: florentbr
author_rank: Contributor
timestamp: October 14, 2022, 01:14:48 pm
content: QuoteMaybe an overvoltage protection kicks in due to unfiltered transients from the switching PFC.I would monitor the input and output of the PFC with an oscilloscope.Check the capacitance of the High Frequency Bypass Capacitor at the output of the bridge rectifier.Have you tried without or with a lower load on the 12v and 380v ?Power Factor Correction (PFC) Handbook - onsemihttps://www.onsemi.com/pub/Collateral/HBD853-D.PDF

author: AcM
author_rank: Contributor
timestamp: October 14, 2022, 03:22:01 pm
content: Hi florentbr,thanks for your reply!The PFC circuit is in page 77 of the handbook you linked, the only difference is that the Vcccomes from the auxiliari winding of the 12V isolated AC/DC.The high frequency capacitor seems to be fine, I measure 2uF as the marking.The output of the PFC at the startup reaches almost 375V then slowly (no load) fall and then "oscillates" around 352V,the input capacitor has the input voltage rectified but periodically there's a "hole" in the voltage.First the Output Startup

author: florentbr
author_rank: Contributor
timestamp: October 14, 2022, 04:07:42 pm
content: Could be that the inductor took a hit when the mosfet shorted.I don't think PFC are supposed to work without load, check with a low load.Vcc on the last picture doesn't look good.Try to bring your own 12v on the board.

author: AcM
author_rank: Contributor
timestamp: October 15, 2022, 09:31:13 am
content: Hi florentbr,thanks for your reply.I try with 4-5W load on the PFC, and same load on the 12V, the output doesn't change.The only thing that changes loading the PFC is the Vcc, it has a faster period respect to the picture attached in the previous post and the average is about 5-6V but always not enough to supply the 12V DC/DCAt startup the Vcc reach immediately 15V stable, when the PCF starts to work (about 50-60ms from Vcc stable) the Vcc become unstable and not able to support both circuits.So I think there's something going on on the auxiliary supply stage or the PFC requiring too much current.