On Thu, 14 Sep 2006 16:51:30 +0200, Fred Bartoli
r_AndThisToo put
finger to keyboard and composed:

Franc Zabkar a écrit :
What is the function of the primary-secondary coupling capacitor (eg
http://tinyurl.com/ha435) in a SMPS? I ask this because I suspect it
may be the cause of a video "hum" issue.

I have a new DVD player that gets along just fine with my 34cm TV but
produces severe hum in the picture of my 80cm TV. The same TV works
well with my other DVD player. I am considering removing the subject
cap from my TV (the DVD is under warranty), but I'd like to know if
there is any reason why I shouldn't. BTW, the TV is a gut-buster so
I'd rather not move it unless I have to.

FWIW, the symptom looks like common mode noise.


SMPS, depending on how the transformer is wound tend to generate some
fair amount of common mode current at switching frequency. (due to
parasitics capacitance between primary and secondary and voltage
distribution between layers and turns).

This capacitor, connected between GND_secondary and "GND_primary"
provides a short path to this current, thus preventing this HF common
current flowing other ways and creating EMI issues.

Removing this cap from your TV set, might or might not solve your pb,
(it might even increase it) but it'll almost surely make your TV not FCC
compliant anymore (if it were not needed the manufacturer would have put
it there).

In case your interference pb is due to your TV residual SMPS CM current,
one way to reduce it further is to add some ferrite ring on one of your
devices mains cable, or on the video signal cable. Several turns help more.

Thanks to both for your suggestions.

I've compared the circuits for both TVs and have found no significant
difference in the area of interest. Both have a parallel RC between
the primary and secondary sides of the SMPS.

The DVD recorder (not player, as in my OP) has a ferrite filter
clamped around the mains cable at the point of entry, so that accounts
for one of the suggestions. The only difference that I can find
between the new DVD and the others is that the new one does not have
the two RF bypass caps between the A and N inputs and the metal
chassis. The PCB has provision for these but the locations are
unpopulated. I think I'll put my warranty concerns aside and
experiment with a couple of 0.001uF Y-class caps.

As for the suggestion re an earth strap, I'm not sure that this would
help given that there are no metal anchor points on the TV.

One other observation that I have regarding this unit is that the
impedance looking into the component inputs is 75 ohms, but the
S-video chrominance and luminance impedances are 51 ohms and 38 ohms,
respectively. The composite input impedance is also 38 ohms. Inside
the box I see three smt resistors, 150R, 75R, and 75R, located near
the sockets. It appears that each of the inputs is double terminated.
For example, the chrominance input appears to be terminated with a 150
ohm resistor in parallel with an additional 75 ohm resistance
elsewhere on the PCB, possibly in the video ADC. This would account
for the measured value of 50 ohms. Similarly, the luminance and
composite inputs would each measure 37.5 ohms. Or am I
misunderstanding something? FWIW, I see no 75R termination resistors
near the composite inputs.

- Franc Zabkar
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