interesting Half bridge design in Plasma TV power supply

[mook johnson]

Gents,

My Plasma TV died on me (2 years out of warranty) and pride just wouldn't
let me throw it out or call the local repair man. From the online reports
I read about these plasmas it appears Panasonic has problems with their 2007
and 2008 models failing at the 3-4 year mark. Mine was a 58PZ800u if
any of you have ever fixed one successfully.

Anyway I managed to get a copy of the service manual for my model and I
noticed something interesting. There is a power supply that takes in a PFC
regulated AC mains voltage (boosted to ~200VDC) and put it through a half
bridge converter to make a variable 160 - 190VDC supply for the Vsus output.

The topology is slightly different than I have seen for half bridge before.
One side of the transformer is tied to the typical Mosfet totem pole, but
the other side just goes to a single capacitor and to ground. The value of
the cap was something like .033uF.

I suppose this would work since you're essentially AC coupling the
transformer but why does a "standard" half bridge topology have two
capacitors in series across Vin and the transformer tied to the middle?

The disadvantage I see to this one is that the first pulse will put full Vin
voltage across the primary until the cap charges up to 1/2 the supply
voltage.

The advantage if the standard topology is also that the capacitor bridge
also serves as an input filter even when the supply is fed by a DC source.
When it is supplied by an AC source they serve double duty as bulk 120Hz
filtering and the capacitive center point for the half bridge transformer.

Any of you ever ran a half bridge that way?

 

[mook johnson]

Gents,

My Plasma TV died on me (2 years out of warranty) and pride just wouldn't
let me throw it out or call the local repair man. From the online reports
I read about these plasmas it appears Panasonic has problems with their
2007
and 2008 models failing at the 3-4 year mark. Mine was a 58PZ800u if
any of you have ever fixed one successfully.

Anyway I managed to get a copy of the service manual for my model and I
noticed something interesting. There is a power supply that takes in a PFC
regulated AC mains voltage (boosted to ~200VDC) and put it through a half
bridge converter to make a variable 160 - 190VDC supply for the Vsus
output.

The topology is slightly different than I have seen for half bridge
before.
One side of the transformer is tied to the typical Mosfet totem pole, but
the other side just goes to a single capacitor and to ground. The value of
the cap was something like .033uF.

I suppose this would work since you're essentially AC coupling the
transformer but why does a "standard" half bridge topology have two
capacitors in series across Vin and the transformer tied to the middle?

The disadvantage I see to this one is that the first pulse will put full
Vin
voltage across the primary until the cap charges up to 1/2 the supply
voltage.

The advantage if the standard topology is also that the capacitor bridge
also serves as an input filter even when the supply is fed by a DC source.
When it is supplied by an AC source they serve double duty as bulk 120Hz
filtering and the capacitive center point for the half bridge transformer.

Any of you ever ran a half bridge that way?

piccy pls

Should be in alt.binaries.schematics.electronics soon. Its a PDF format.

 

[mook johnson]

Should be in alt.binaries.schematics.electronics soon. Its a PDF format.

Make that

alt.binaries.schematics.electronic (no s). The other one doesn't seem to
work anymore.

 

[Tim Williams]

The cap to ground doesn't make any sense, startup transients as you note.
If it comes up slowly while the chopper is running, it won't make much
difference. I suppose it saves a cap, but a pair of 0.015's aren't any
more expensive than one 0.033.

As for the small value, I'd guess it's a resonant converter (series
resonant, against the transformer magnetizing / leakage inductance).

Tim

 

[mook johnson]

The cap to ground doesn't make any sense, startup transients as you note.
If it comes up slowly while the chopper is running, it won't make much
difference. I suppose it saves a cap, but a pair of 0.015's aren't any
more expensive than one 0.033.

As for the small value, I'd guess it's a resonant converter (series
resonant, against the transformer magnetizing / leakage inductance).

Tim

Did you see the schematic on alt.binaries.schematics.electronic?

the more I look at it it doesn't look quite like a classical half bridge.
There are not output inductors. It looks like this circuit just chops the
PFC input voltage so the transformer can step it down. There are post
regulators in there for the regulated voltages.

Its still a strange configuration unless like you said it is a resonant or
current fed topology with no output inductors....hmmmm.

 

[Tim Williams]

Did you see the schematic on alt.binaries.schematics.electronic?

No, didn't come through here.

At any rate, unless it's running at a very high frequency, and the
transformer has very low leakage, such a small coupling capacitor won't
transfer very much power. Most likely answer, it's rated for a reasonable
voltage (over 400V?) so that resonance carries it through.

the more I look at it it doesn't look quite like a classical half
bridge. There are not output inductors.

With sufficient transformer leakage, none would be needed.

I'm guessing the transformer is either normal, or split bobbin wound so it
has relatively high leakage. Power control by frequency most likely.

Tim

 

[mook johnson]

No, didn't come through here.

At any rate, unless it's running at a very high frequency, and the
transformer has very low leakage, such a small coupling capacitor won't
transfer very much power. Most likely answer, it's rated for a reasonable
voltage (over 400V?) so that resonance carries it through.


With sufficient transformer leakage, none would be needed.

I'm guessing the transformer is either normal, or split bobbin wound so it
has relatively high leakage. Power control by frequency most likely.

Tim

The transformers are E core looking ferrite jobbies (or their round
centerleg counterparts) at about 2.5" square. Can't tell if there is a gap
in them or not but they did not add a shim to the outer legs like seen in
cheaper power supplies.

Actually the build quality of the Panasonic is fairly impressive for
commercial stuff. its just a weird topology that I'm interested in finding
out the pros and cons of as to why they use that instead of a textbook
topology.

 

[mook johnson]

Did you see the schematic on alt.binaries.schematics.electronic? its in
PDF format and too complicated for ascii art.

 

[[email protected]]

No, didn't come through here.

At any rate, unless it's running at a very high frequency, and the
transformer has very low leakage, such a small coupling capacitor won't
transfer very much power. Most likely answer, it's rated for a reasonable
voltage (over 400V?) so that resonance carries it through.


With sufficient transformer leakage, none would be needed.

I'm guessing the transformer is either normal, or split bobbin wound so it
has relatively high leakage. Power control by frequency most likely.

I've not seen the circuit, but is it a valley fill topology?

Poor man's PFC control. Charge caps in series, discharge in
parallel? Looks odd at first view.

Grant.

 

[mook johnson]

I've not seen the circuit, but is it a valley fill topology?

Poor man's PFC control. Charge caps in series, discharge in
parallel? Looks odd at first view.

Grant.

the PFC circuit looks pretty textbook Bridge Rectifier that goes to an
inductor then a FET to ground and a diode to peak charge the output caps
(300uF at 450V). Current is measured at a current sense resistor in the
source of the FET. All the PWM is handled on a little daughter board.

I actually measured the voltage with the TV partially "running" and was
surprised about how much ripple I saw with my multimeter. p - p was in the
40V range with an average of 390Vdc. On DC I got jumpy reading that ranged
from 370 - 410V I switched it to AC and it read ~ 40V but it wasn't rock
steady and wandered a bit. I'm thinking this is the 120Hz ripple but with
the lightly loaded situation it was in I wouldn't expect that much ripple.
The measurements were made with a decent but no-name meter so the AC
measurements can be taken with a grain of salt. I calibrated the DC last
year against my calibrates lab meter and it was on the money.

I measured the capacitance of the PFC and they are typical of electrolytics
and on the low end of the range and still in spec. I borrowed another
power supply that was pulled from a new TV with a broken screen and it
measured the same so I don't think the electros are wearing out.

Hiccup mode at light load is the only thing I can think if. The load is
light because I had to disconnect the screen derivers to get the TV to start
and stay running long enough for me to take a measurement.