Franc Zabkar wrote:

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.

Simply a cost-cutting move !

You can happily use 2n2 there btw. Do make sure they're Y rated caps though !

You'll still get the leakage current from the pri-sec cap though.

Graham