Primary-secondary coupling capacitor in SMPS

What is the function of the primary-secondary coupling capacitor (eg

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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

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Reply to
Franc Zabkar
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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.

Reply to
Ancient_Hacker

Franc Zabkar a écrit :

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.

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Thanks,
Fred.
Reply to
Fred Bartoli

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

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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Reply to
Franc Zabkar

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

Quite likely.

Yes it will do !

That cap is needed to conform with EMI regulations btw.

Graham

Reply to
Eeyore

No.

See my reply.

Graham

Reply to
Eeyore

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

Reply to
Eeyore

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

Reply to
Eeyore

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.

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

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Thanks,
Fred.
Reply to
Fred Bartoli

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