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Old 13th March 2003, 12:23 PM   #1
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Default Interference with Philips CD recorders

Does anyone of you have such a Philips cd recorder for use in a HiFi chain ? Do you have the same problems with it as I have ?

I have a CDR570 and a CDR765. Both radiate terrible noise on my tuner when used with an aerial. I heard this complaint from a friend that has a CDR570 too.

So I checked the switched PSU of the 765 and discovered that Philips used standard electrolytics that are in fact not the best ones for use in a SMPS. I changed them for Chemicon SXE series ( same values ) just because I thought they were the right ones at the right spot.

When I checked again the level of the interference had changed but it was still there. Since a recording from radio is impossible I decided to check further. These recorders don't have safety earth connections. So I connected it to earth and the noise was gone. In the Netherlands safety earth is not used in living rooms and the recorder doesn't have an earth connection too. Just connected a test wire to central heating radiator and the recorder. But since earth is not available at the spot and the recorder isn't designed to use earthed I decided to investigate some more.

To make a long story short: I discovered that the heatsink of the switching Mosfet in the primary part of the PSU was mounted isolated. In a brave ( but unwise I admit ) attempt I shorted the heatsink with a cap the the case of the device. It was quiet at once. I also tried to couple it capacitive with a 0.1 uF film cap. Same thing: quiet again. For clarity: the Mosfet used is a plastic isolated device ! Yes I had checked that before I did the trick. So the switching noise is transferred to the isolated heatsink.

Is connecting the heatsink to the case allowed according safety rules and is it common practice to ground heatsinks in SMPS ? Or should I connect it to the negative pole of the electrolytic of the primary part ? I haven't done this yet but I am stuck with what is safe and effective. Not into SMPS'es as you can see.
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Old 14th March 2003, 11:51 AM   #2
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No one ?
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Old 14th March 2003, 12:11 PM   #3
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Default Re: Interference with Philips CD recorders

Quote:
Originally posted by jean-paul

To make a long story short: I discovered that the heatsink of the switching Mosfet in the primary part of the PSU was mounted isolated. In a brave ( but unwise I admit ) attempt I shorted the heatsink with a cap the the case of the device. It was quiet at once. I also tried to couple it capacitive with a 0.1 uF film cap. Same thing: quiet again. For clarity: the Mosfet used is a plastic isolated device ! Yes I had checked that before I did the trick. So the switching noise is transferred to the isolated heatsink.

Is connecting the heatsink to the case allowed according safety rules and is it common practice to ground heatsinks in SMPS ? Or should I connect it to the negative pole of the electrolytic of the primary part ? I haven't done this yet but I am stuck with what is safe and effective. Not into SMPS'es as you can see.
Well done JP !

You may directly connect the heatsink to the ground of the primary electrolitic of the SMPS, that is, ASSUMED that the transistor is isolated from the heatsink.

This should have been done first place in the original design..........

all the best
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Old 14th March 2003, 12:22 PM   #4
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Thanks a lot for the advice, Guido ! I will change it ASAP so that I will finally be able to use the machines.

Makes me curious if this has been changed with newer devices of Philips ?

They say you're in-the-know ...

Regards,

Jean-Paul
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Old 14th March 2003, 01:04 PM   #5
tiroth is offline tiroth  United States
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I have seen many computer SMPS supplies where the heatsinks are live.
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Old 14th March 2003, 01:06 PM   #6
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So the mains voltage is on the heatsink ? Is that safe and allowed and were these SMPS'es from a known brand ?
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Old 14th March 2003, 01:24 PM   #7
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Quote:
Originally posted by jean-paul
So the mains voltage is on the heatsink ? Is that safe and allowed and were these SMPS'es from a known brand ?
The heatsinks are usually located inside the PSU so I guess
that is not a problem as long as the usual safety regulations
are obeyed.

In your case, jean-Paul, I understand that the heatsink wasn't
connected to anything at all, so do you or anybody else have
an explanation of the phenomenon? Capacitive coupling
between the transistor and heatsink, perhaps?
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Old 14th March 2003, 01:28 PM   #8
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Quote:
Originally posted by jean-paul
So the mains voltage is on the heatsink ? Is that safe and allowed and were these SMPS'es from a known brand ?

Hi all

That is allowed, as long as they cannot be touched during normal use

Live means that they are connected to the drain / collector of the switching transistor, which is at about 320V or 160V DC (maximum) compared to primary ground, depending on the countries.

The fact that this is allowed, indicates sufficent screening (both safety and RF emission-wise) is obtainded from the cabinet.

best regards
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Old 15th March 2003, 09:11 PM   #9
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In fact, every assembly technique is allowed to be applied into an equipment labeled having a "double isolation". That's why so much plastic is utilised in modern equipment.
Jean - Paul, it is absolutely "safe" to earth the heatsink via a capacitor to ground. But since what you are actually doing is to "drain" the spectral polution to ground, you understand that the ground becomes now more noisy than it was before. If you were asking then a legal authority for approval, you would be faced with a request to additionaly filter the noise injected to the mains. This (the heavier mains filtering) is probably what Philips wanted to avoid in the first place!
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Old 15th March 2003, 09:20 PM   #10
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OK, it's good to see there are more people that know what double isolated/insulated ( never know which word to choose ) means !! A lot of DIY people want their gear ( I agree with tube equipment earthed ) earthed and thus get pollution from earth on their gear and sometimes they get a free groundloop too.

I know what you wrote but nevertheless I wondered if devices with high dV/dt as switching MOSFET's in SMPS had to comply to certain rules. Did a search with Google but could not find a relevant explanation.

The spectral pollution is now radiated in a very strong way. I really wonder how these machines could pass EMC tests. Connecting the heatsinks to ground stops the heavy interference in air but can have the drawback that mains maybe gets polluted slightly.

I doubt if this will really will be the case as a decent common mode net filter with some caps is used to prevent pollution going that way. Nevertheless it is a good test for my new oscilloscope to see if it is like I predict. As I found out the radiation goes via the case. Earthing the case as a test gives good results too. However earthing a double isolated device is against my nature.

It's a matter of choice between two evils although the second evil may be less evil than the other one.
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