Unexpected inverter board shutdown

[Leo Meyer]

Hi everybody,

I've got a strange problem that really puzzles me, concerning an HV
inverter board for the CFL lamps in a noname flat panel monitor with
Hyundai boards and components.

Problem: Inverter board shuts down after generating HV for about half a
second.

Details: Two outputs on the board for altogether four lamps, two
transformers, two XA something switchmode controller ICs, one
unidentified small IC, four power transistors, a couple of discrete
transistors and diodes, all SMD.

What I did to locate the fault:

1. Tested outside the monitor with known good voltage supply. Same.
2. Tested without lamps. Same.
3. Hooked lamps to one output at a time only. Same.
4. Checked all discrete semiconductors. All seem ok.
5. Replaced the only electrolytic cap. Same.
6. Checked transformer operation using an oscilloscope probe on top.
Pure sine on both transformers. Independent of whether lamps are
connected. No distortions. Frequency about 42 kHz.

I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe
anyone knows something about the shutdown logic used in such boards?

Thanks for your help!

Regards, Leo

 

[Art]

Over Current/Voltage protection. Without the lamps connected prob O/V, with
lamps prob O/C. Would need the service manuals fo the specific circuit,
including voltage and current measurements. Nominal is to just replace the
invertors and lamps at one time.

 

[James Sweet]

Hi everybody,

I've got a strange problem that really puzzles me, concerning an HV
inverter board for the CFL lamps in a noname flat panel monitor with
Hyundai boards and components.

Problem: Inverter board shuts down after generating HV for about half a
second.

Details: Two outputs on the board for altogether four lamps, two
transformers, two XA something switchmode controller ICs, one
unidentified small IC, four power transistors, a couple of discrete
transistors and diodes, all SMD.

What I did to locate the fault:

1. Tested outside the monitor with known good voltage supply. Same.
2. Tested without lamps. Same.
3. Hooked lamps to one output at a time only. Same.
4. Checked all discrete semiconductors. All seem ok.
5. Replaced the only electrolytic cap. Same.
6. Checked transformer operation using an oscilloscope probe on top.
Pure sine on both transformers. Independent of whether lamps are
connected. No distortions. Frequency about 42 kHz.

I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe
anyone knows something about the shutdown logic used in such boards?

Thanks for your help!

Regards, Leo

It's probably detecting bad or no lamps, there's a resistor on many of
those that can drift up in value making that detection too sensitive.

 

[[email protected]]

Leo:
Some inverter boards use a ceramic capacitor across the primary of
the high-voltage transformer. I have had a few inverters where this
capacitor
was bad causing the inverter to shut down very quickly. I think the
value of the capacitor was about 0.1Uf at 100 volts.
Hope this helps...
John