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.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:
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 think it's there to provide a bit of RF grounding for the secondary
circuits.

Unfortunately it also helps couple any RF noise from the line into the
ground loops in your entertainment setup.

Things I would try:

(1) If you have a scope, CAREFULLY probe the hot and neutral leads for
any spikes. You may want to track down and kill the source of the
spikes. Typical devices that cause spikes-- anything with a SMPS or a
light dimmer or an electronic power switch.

(2) If you don't want to bother doing that, put a spike filter between
the wall and your devices-- a good "surge protector" that has a few
toroid common-mode filters. Or carefully make your own with a few
millihenries of 15 Amp capable toroid and a good line-rated 0.1uF
capacitor.

(3) If you don't want to bother doing that, try bonding your A/V
equipment chassis' together with a thick, say 12 guage stranded wire.
That often surpresses ground-loop voltages enough to make the TV's
watchable. Keep this wire as short as possible. Use Litz wire if
available.

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,
Fred.

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
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:

What is the function of the primary-secondary coupling capacitor (eg
http://tinyurl.com/ha435) in a SMPS?

To dump switching noise on the secondary side back to the nice low impedance of
the reservoir cap on the primary circuit.


I ask this because I suspect it may be the cause of a video "hum" issue.

Quite likely.


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.

Yes it will do !

That cap is needed to conform with EMI regulations btw.

Graham

Ancient_Hacker wrote:

Franc Zabkar wrote:
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 think it's there to provide a bit of RF grounding for the secondary
circuits.

No.

See my reply.

Graham

Fred Bartoli wrote:

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.

I'm afraid the ferrite won't help.

The problem is that the Y cap in that location results in a small ac leakage
current when the secondary circuitry is connected to other grounded equipment.

Depending how well that path is executed you'll get varying results due to
primarily line frequency current.

Graham

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

Eeyore wrote:

Fred Bartoli wrote:

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.

I'm afraid the ferrite won't help.


Well, I didn't take the video "hum" as a pure 50/60 Hz issue.
It's highly unlikely: 1nF, at 50Hz/230V, gives only 70uA leakage
current, which translates to, at most 7uV, in case of a very bad 0.1R TV
to DVD GND path impedance.

I think it's more a CM switching residues problem.
A ferrite toroid with a high AL, could easily improve this by reducing
the CM current between the TV set and the DVD player, and also by
improving the coupling between the signal and gnd path.


The problem is that the Y cap in that location results in a small ac leakage
current when the secondary circuitry is connected to other grounded equipment.


Don't think so. See above.
Anyway, the main frequency can get back there, simply by the means of
modulating the SMPS pulse width.


--
Thanks,
Fred.