Power Supply Voltage

[mrtech2017]

I am working on this power supply and curious as to how the DC voltage of the rectifier output is increased to what the manual indicates it should be. The manual says the DC output of BD901 should be 320vdc when off and 400vdc when turned on. Now I know the output of the rectifier is only around 170vdc by itself and this is about what I have now, way less then what the manual says it should be and the supply is not working. How does this circuit increase the DC line to 400vdc? My thought was the switching of the mosfet,Q902 but I am not an engineer. Thanks.

[mariushm]

Re: Power Supply Voltage

see active power factor correction

Application Note 42047 Power Factor Correction (PFC) Basics



Power Factor Correction (PFC) Handbook Choosing the Right Power Factor Controller Solution

https://cdn.badcaps-static.com/pdfs/...25ddb0846b.pdf


CIRCUITS FOR POWER FACTOR CORRECTION
WITH REGARDS TO MAINS FILTERING

https://cdn.badcaps-static.com/pdfs/...94077c90c3.pdf

[eccerr0r]

Re: Power Supply Voltage

The explanation in the first post seems a bit wrong. BD901 voltage shouldn't increase much when turned on, however voltage after the coil L903/Q902 drain indeed should go higher when on. So, it means the PFC IC IC902 is not turning on your transistor Q902. Supposedly this should be an automatic world sense (i.e. can detect 120 or 240VAC mains) so that 170V you're getting at BD901 is about right for the US, but the voltage after the coil should go up to that 400V mark when turned on.

[mrtech2017]

Re: Power Supply Voltage

Ok. So Q902 mosfet has to be running for the circuit to boost the DC to 400v. Correct?

[budm]

Re: Power Supply Voltage

When the 'STANDBY' pin receives the ON command signal (>2VDC) the switched transistor is turned on to supply the VCC voltage to run the PFC Controller IC.
Do you have better pictures that show the whole page of the schematic?
Upload the service manual will be better.

[mrtech2017]

Re: Power Supply Voltage

The complete manual was to large to upload so I pulled out the PS page and attached it.

This supply had some damage. ic903 was blown apart and a couple other components in that circuit. Q902 was also damaged so it and ic902 were also changed. ic901 was pulling down the vcc line so it was changed.

Now when plugged in I have app. 4.75v on 5v line. When jumped to standby it comes up to the stated 5v. Q922 is switched on and I have 5v at the vcc of ic902 and ic901. But neither ic seems to be running.

But I think I found out why. In looking at the datasheets for both of these ic's I believe they both need app. 12v to turn on. So I am looking into this now. You are welcome to verify my thoughts with attached datasheets links.

https://cdn.badcaps-static.com/pdfs/...d35c2983cc.pdf Page 7

https://cdn.badcaps-static.com/pdfs/...49ac344075.pdf Page 8

[budm]

Re: Power Supply Voltage

That is a lot of blown parts, so what DCV do you have on the Emitter pin if switched VCC Transistor Q922?
You are working in the primary side so the GND ref. for your meter will be the negative leg of the main DC filter cap in the primary side.
BTW, can you give the link to the site so I can download the manual?
And what the model number of the TV?

[mrtech2017]

Re: Power Supply Voltage

It is a Sharp LC-42SB45UT

Complete manual here:

DC is 5v on e and c of Q922 when switched on. 5vdc on e when off.

Yes I am using hot gnd when testing hot side and cold gnd when testing cold side.

[budm]

Re: Power Supply Voltage

5V? The VCC for running the SMPS and the PFC IC is generated by the AUX winding of the standby power supply transformer T904, D908, C965.
So I would check the Voltage of R976 of each end to GND (negative leg of the main filter cap when TV is in standby and when STANdby command is present..
"hot gnd" I hope you mean the negative leg of the main filter cap C907.

[mrtech2017]

Re: Power Supply Voltage

I believe you mean C969.
I have tested on both sides of R976 (5v) And tested D908, replaced C969 and checked winding for open.

Hot gnd meaning any location with an open triangle, neg side of C907 or anywhere else that has an open triangle. All hot gnd..

[budm]

Re: Power Supply Voltage

If you lift one leg of R976 do you then still get about 5V on C969?

[mrtech2017]

Re: Power Supply Voltage

Yes, this was how I found out that ic901 was bad. At first I only had @ 2vdc on this line when Q922 was on and after disconnecting most other components on this line it then came up to the 5v when ic901 was removed. After new ic was installed it held at the 5v when Q922 was turned on.
Last night I lifted 1 side of R976 and connected my external supply and set it at 12vdc and jumped Q922 and I had a good 12v on my vcc's at the ic's. But ic902 was still dead. I have another new ic but have not installed it yet.

I figured that once I see if the PFC runs normal with the 12v supply I will go back and see why there is not 12v on this line.

[budm]

Re: Power Supply Voltage

Yes, that is puzzling since you do have about 5VDC standby on the cold side, and there is not much to that circuit at all. I expect the cold side 5V to be really low too since both windings are driven by the same primary drive.

[mrtech2017]

Re: Power Supply Voltage

Well i changed out ic902 and with 12v on vcc still no 400vdc. So in looking at the datasheets some more and the schematic it looked like it might need more then 12v to turn on. So I started turning up the external P/S and when it hit app. 12.5vdc the PFC came on and I had 400vdc and the 12 and 24 volt supplies. I think it actually should be around 15vdc on the vcc.
So back to T904 I rechecked voltage at D908 and had 5.85vdc. Lifting R976,R917,R991,R921 it dropped to 3.1vdc at D908. Both D908 and C969 test good and I do not have replacements.

What voltage do you think I should have on T904, pins 4 and 5?

[redwire]

Re: Power Supply Voltage

I think more than 5V should be at R976 as IC902's start-up happens at 11-13VDC. The over-voltage max. is ZD910+6V or 21VDC.
I would guess around 16V expected on C969.
Hard to tell if the TNY277 regulation is off (is 5V output OK?) or another issue.

[rickou]

Re: Power Supply Voltage

what voltage do you have at pin one of Q922 only in stby? have you check R974 R975 R977. then simulate the stby command and check the voltage at pin 12 ic901 and pin 8 ic902.