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Old 04-13-2017, 04:22 PM   #1
cmg
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Question LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

Hi All.

I have a problem which is hopefully relevant for other readers of the forum and other monitors as it is concerning the PSU and LED backlight driver. The symptoms are that as the brightness is reduced using the front panel controls, the screen starts to flicker quite annoyingly. I believe this is a fault in the PFC circuit. The output voltage from the PFC circuit pulses in correlation with the flickering. This is on an HP ZR2740W 27" monitor.

The PFC controller is a 28051 (see http://www.ti.com/lit/ds/symlink/ucc28051.pdf), so I'm referring to the reference circuit for this. I don't have the schematic for the PSU but the components I have checked are identical to the reference circuit. The PFC circuit for this monitor is discussed in thread http://www.badcaps.net/forum/showthread.php?t=42498

Observations:
  • no noticeable flicker when the monitor is at high brightness, and normal operation overall
  • as the brightness is lowered it starts to pulse periodically, initally at about 10Hz
  • as the brightness is lowered the flickering gets more pronounced and the frequency gets lower, to about 2Hz.
  • measuring voltage of the PFC output (C802/C807 in this PSU, C_OUT in the reference circuit) with meter (can't use 'scope - not isolated circuit)
  • PFC o/p measures 400VDC when the monitor is on (expected voltage)
  • when brightness is stable (high brightness), there is only approx 2V AC ripple on the 400V line
  • when brightness is unstable, this rises up to 25V AC. The meter has a bargraph which shows me that the AC is a slow pulsing oscillation coinciding with the backlight flicker.

In this circuit the output power cap is two in series (C802, C807). I noticed the DC & AC voltages were quite unbalanced between these caps so I thought the caps were the problem and replaced them with nice low ESR ones, and added 100k shunt resistors. Well.. now the voltage is nice and even between the two caps but the flicker is the same

I'm not familiar with these circuits so not sure where to check to what the cause of this is. For a start I'm not sure if the fluctuation in the PFC output voltage really indicates a problem in the PFC circuit or the load (the LED driver). I haven't worked out the circuit between the PFC and the stage which drives the LEDs. I'm also not sure how the brightness control works. Does anyone have a schematic for another monitor PSU which might be similar?

Any suggestions on how to proceed?

thanks!

Last edited by cmg; 04-13-2017 at 04:25 PM..
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Old 04-13-2017, 05:09 PM   #2
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Default Re: LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

You can start by uploading good clear pictures of the boards and show how they are connected together.
'In this circuit the output power cap is two in series (C802, C807)' Where are these caps?
' I'm also not sure how the brightness control works' LEDs are driven by constant current power supply, less current = less lights.
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Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

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TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
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Old 04-13-2017, 06:07 PM   #3
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Default Re: LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

Thanks for the fast response Bud. The boards are the same as in
this thread and this one. In fact i see you drew a helpful diagram of the LED driver a while ago.

I've attached a couple of pics of my PSU board. (note:
- wires soldered over C807 was to measure voltage on meter without holding it.
- Cap over D803 was because C818 is a bit chipped. This is the VCC decoupling cap for I801. Didn't help.
- pic shows replacement caps for C802, 807. I put tape on the top because they are wider than the originals and i thought may be close to the chassis. They are nichicon low-ESR ones for switch-mode PSU. 220uF/250V, same as orig.)

In the PSU circuit I see:
- the PFC - gives 400VDC regulated. I don't believe this is controlled by the brightness control (?). I801 is the PFC chip.
- an LED driver circuit on the live side. Presumably chops the 400 VDC again to AC. I811 is the chip for this. Not sure what model of IC this is or how the circuit works.
- transformers isolating the LEDs T802, T804, T805 followed by rectifiers and smoothing. There are 3 transformers and 6 DC outputs to the LEDs. No further control circuitry on this side of the transformers - just a rectifier.

What i meant was that I'm not sure how/where the front-panel brightness control adjusts the LED current. The control comes into this board as a voltage level in the range 0-5V - i checked that.

I have observed the 6 LED strips with a scope and i see that the voltages all oscillate the same. Hence, i thought the problem was further upstream, and not the smoothing in this final stage. See pic for scope trace. It shows the oscillation is a pulsing effect. This is @ 50ms/div so its about 10Hz at the time the pic was taken.

Thanks!
Attached Images
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File Type: jpg 20170314_234105.jpg (529.9 KB, 15 views)
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Old 04-14-2017, 03:08 AM   #4
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Default Re: LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

Ah.. made some progress. The controller IC I811 is a TI 25600 type, so now i can see that this is where the brightness control is implemented.

However, the whole circuit (PFC plus this DC-DC converter) will all be pulsing so I'm not sure how best to work out where the cause is.

Thanks!
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Old 04-14-2017, 06:05 PM   #5
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Default Re: LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

.. bit more progress.

Firstly i looked at the AC output of the transformers which are driven by the DC-DC converter. When the monitor becomes unstable, the frequency is modulated and ultimately drops out. It starts off at about 150kHz. Because the frequency is unstable my view is that the problem is in the DC-DC converter or its control.

I traced the control through the Opto-Isolator I812. This in turn is controlled by another TI chip - I810, a Z103WA. This is a dual op-amp with a voltage reference inbuilt. I guess the reference circuit in the datasheet is going to be similar to what is used here.

The input to the Opto Isolator I812 IS oscillating (i.e. at around 10Hz) when the monitor is unstable.

Next i think i will interrupt the feedback from the regulated output voltage to I810/I812 and see if that stops the instability. Maybe it will give some clue as to where it is starting. I suppose it means I need to trace the circuit around I810 as well... I won't have a chance to proceed for a week though.

In the meantime if there are any ideas.. much welcome.
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Old 04-30-2017, 05:53 PM   #6
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Default Re: LED blacklight flickers at low brightness, PFC output voltage unstable. Help!

Finally! Got it

It was the PFC controller IC. Will post some details later.
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