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Old 5th September 2011, 08:36 PM   #291
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Originally Posted by thomaspf View Post
Here is a picture of the switching ripple from the SMPS800R. Peak-2-peak this is almost 4V. Any ideas for a filter to reduce by at least an order of magnitude, ideally less than 100mV.
Are-you sure that is not electro-magnetic RFI ? Did-you have put your SMPS in a shielded box ?
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Last edited by Esperado; 5th September 2011 at 08:51 PM.
 
Old 5th September 2011, 09:08 PM   #292
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Yes I am pretty sure. The two power supplies are mounted onto the same heatsink and share a common ground.

The first is picture is taken from the plus output of the SMPS800R

The second picture is taken from the VAS stage power supply which happens to be a Sigma 22 from AMB.

The settings on the scope are untouched.

Cheers

Thomas
 
Old 5th September 2011, 09:41 PM   #293
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Originally Posted by thomaspf View Post
Yes I am pretty sure.
Again, it is absolutely indispensable to enclose the SMPS. There was one thread on a french forum where somebody, as sure as you, had tried everithing to filter this SMPS. Then he had shielded-it, and the real noise was, if i remember well, around 30mv without any additional filtering. Better no additional filtering was able to decrease this noise level, often increased on the contrary. Your noise looks exactly like his one, before the grounding.
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Old 5th September 2011, 09:50 PM   #294
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Okay, so I have common ground and the two power supplies are mounted directly next to each other. The measurements are taken an inch away from each other.

Not very clear how RFI at the order of 2V swing would impact one but not the other.

Coming up with a cage is not so hard, so maybe I try that. Ventilation is going to be an issue though. Do you have a link to that forum post?

Cheers

Thomas
 
Old 5th September 2011, 10:25 PM   #295
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Quote:
Originally Posted by thomaspf View Post
Coming up with a cage is not so hard, so maybe I try that. Ventilation is going to be an issue though. Do you have a link to that forum post?
Projet : Ampli à base de LME49830 - "Gran Torino 150" - Page 49 - Amplification.
He had done it with a simple mesh metallic plate.
It was an amusing thread to read for me, because he was searching a power supply noise problem, when it was evident from the beginning this was an RFI problem on the input line of his amplifier, coming from the SMPS.
Note that he got, with twisted power wires between SMPS and Amplifier: 32mV of noise, not so bad ? JWithout twisted wires, it was 37.5mV .
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Last edited by Esperado; 5th September 2011 at 10:36 PM.
 
Old 6th September 2011, 10:54 AM   #296
Cristi is offline Cristi  Europe
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The real ripple of any power supply can be measured properly in some certain conditions. the most important is the oscilloscope probes quality and the way how is connected to the measuring point, the output connector.
The probe must always use the shortest GND return path, without the clip cable and with the loop made of tip and shield to be not more than 10mm.
Here are some interesting articles http://www.eet-china.com/ARTICLES/20...BM_DA_AN05.PDF
How to Measure Power Supply Output Ripple
Power Tip #6: Accurately Measuring Power Supply Ripple
if the ripple was measured with the GND clip connected on the pcb, then the fringing flux of the transformer induced the noise which can be seen in loop formed by the proble GND cable. the topology used in this power supply, LLC resonant converter lead to strong fringing flux in the transformer, and this flux can be contained by a simple shielding of the board or installing with proper clearance to the small signal stage . i would not recommend to try to shield the transformer alone, this will change some important parameters as leakage inductance and would pose safety problems. the radiated EMI is in plan with perpendicular axis on the pcb and decrease exponentially with the distance.
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Old 6th September 2011, 11:42 AM   #297
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Quote:
Originally Posted by Cristi View Post
The real ripple of any power supply can be measured properly in some certain conditions. the most important is the oscilloscope probes quality and the way how is connected to the measuring point, the output connector.
The probe must always use the shortest GND return path, without the clip cable and with the loop made of tip and shield to be not more than 10mm.
Here are some interesting articles http://www.eet-china.com/ARTICLES/20...BM_DA_AN05.PDF
How to Measure Power Supply Output Ripple
Power Tip #6: Accurately Measuring Power Supply Ripple
if the ripple was measured with the GND clip connected on the pcb, then the fringing flux of the transformer induced the noise which can be seen in loop formed by the proble GND cable. the topology used in this power supply, LLC resonant converter lead to strong fringing flux in the transformer, and this flux can be contained by a simple shielding of the board or installing with proper clearance to the small signal stage . i would not recommend to try to shield the transformer alone, this will change some important parameters as leakage inductance and would pose safety problems. the radiated EMI is in plan with perpendicular axis on the pcb and decrease exponentially with the distance.

Good info to know.

Can you change the SMPS800R output to +-65v?
How much time?
How much extra cost?

Thanks
 
Old 6th September 2011, 03:59 PM   #298
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Quote:
Originally Posted by Cristi View Post
The real ripple of any power supply can be measured properly in some certain conditions. the most important is the oscilloscope probes quality and the way how is connected to the measuring point, the output connector.
The probe must always use the shortest GND return path, without the clip cable and with the loop made of tip and shield to be not more than 10mm.
That is very interesting and I will go and measure directly at the connector with the ground clip off and a 10mm connection to the probe shield.

However for practical purposes in an amp, I will have to have a longer ground connection to the start ground. When I measure the voltage on the amplifier board the ripple is very much there.

In fact if I look at a 1Khz output signal at the speaker outputs the ripple is clearly visible in the waveform.

I guess I will have to try the cage.

Cheers

Thomas
 
Old 7th September 2011, 04:48 AM   #299
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I modified the probe from the scope to get the ground connection with a short piece of wire directly at the tip and connected that setup to the output terminals of the SMPS800R.

The result is indeed identical to what Gilles saw in the thread on the french forum but I am getting a little higher peak-to-peak riple of 270mV. The same procedure for the Sigma 22 gets about 20mV peak-to-peak swing.

While this is progress from the 4V yesterday I still don't understand how to connect wires to the device to reach the star ground and the power connector of the amplifer board without introducing a large amount of ripple into the amplifier board.

This leaves me with a question for Cristi. You suggested that enclosing the power supply in a Faraday cage might interfere with it's operation. On the other hand there seems to be evidence that this dramatically reduces the switching ripple, so is it safe to do it?

Cheers

Thomas
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Last edited by thomaspf; 7th September 2011 at 04:51 AM.
 
Old 7th September 2011, 07:37 AM   #300
alkasar is offline alkasar  France
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Quote:
Originally Posted by thomaspf View Post
This leaves me with a question for Cristi. You suggested that enclosing the power supply in a Faraday cage might interfere with it's operation. On the other hand there seems to be evidence that this dramatically reduces the switching ripple, so is it safe to do it?

Cheers
Thomas
Quote:
Originally Posted by Cristi
i would not recommend to try to shield the transformer alone, this will change some important parameters as leakage inductance and would pose safety problems. the radiated EMI is in plan with perpendicular axis on the pcb and decrease exponentially with the distance.
If I may, cristri said putting the onboard transformer alone in a faraday cage may cause problems. Putting the entire SMPS in a cage can not harm.
 

 


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