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Doug Grant

When Will Someone Make a Clean Switcher?

Doug Grant
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Scott Elder
Scott Elder
9/25/2013 2:54:26 PM
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The FCC Spirit
@ Doug - <<...a trick to pass the FCC specs...modulating the switching frequency such that the noise was smeared out over a wider band ...meeting the letter of the law, but not the spirit.>>

Aren't you being a bit harsh, Doug?  Consider that CDMA for cell phones depends upon the same principle and they intentionally radiate 0.25W.  That is how MORE users are added on to a slice of the RF spectrum rather than interfering.

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Bill_Jaffa
Bill_Jaffa
9/25/2013 9:38:30 AM
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Switcher noise
My HP Officejet 4110 printer is an excellent EMI source, I can pick it up in nearby AM radios even 5-10, even 15 feet away. The noise goes up to about 800 kHz--wipes out station below that frequency, much less above that. SO to listen to some sports games, I have to turn off the printer. 

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
9/25/2013 8:55:00 AM
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Master
Re: Ferrites
Vishal,

I agree that these modules are available. My point is, they are protected by those EMI rejection panels and other components employed by the product designer. The LCD modules, as I understand, do not have inherent rejection capabilities.

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Vishal Prajapati
Vishal Prajapati
9/25/2013 8:04:38 AM
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Re: Ferrites
I think the most harsh environment LCDs I have seen is HMIs. They even comes with Capacitive touch display. So, I think for that harsh industrial environment they perform pretty well with no errors seen on the screen. So, I think you should look at it which modules they are using or how they are protecting them.

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Vishal Prajapati
Vishal Prajapati
9/25/2013 7:47:52 AM
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Master
Re: Manufacturer
I am taking a case of LED Drivers. The design notes of LED Drivers are always showing Common Mode Chock at the input stage before Rectifier. But whatever drivers available here in India below 20W doesn't use any type of EMI Filter in the input stage. It is to reduce the cost of entire module. And surprisingly no one cares about EMI feeding to Mains Grid line.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
9/25/2013 6:54:27 AM
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Master
Ferrites
"The noise that escapes the enclosure can often be knocked down by the application of appropriate ferrite material. That's the little doughnut-shaped thing on the cord of your laptop's power supply."

@Doug,

We have used a similar technique by using those split ferrite modules on the cables that interconnect PCBs/modules. That paid rich dividends in passing the tests. We even used it on a 16 wire cable going to the LCD. 

Another important question : All these LCD Modules seem vulnerable to EMI. Is there a manufacturer who designs them for EMI environment? Or it is left to us, designers?

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Netcrawl
Netcrawl
9/25/2013 4:36:33 AM
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Re: Manufacturer
For economic reason manufacturer might not include an EMI filter, its will only add some cost to their current production. Its sounds like an extra works for their engineers and for their design. They want to get their products faster, adding extra stuff like EMI would not help them.  

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Vishal Prajapati
Vishal Prajapati
9/25/2013 4:08:03 AM
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Manufacturer
I never have to deal with such noise issue or probably I have been ignorant about it. But I think there is enough documentation and design notes available from standard switcher companies. Most of the time designer consideres all that things and designs a circuit. But at the end it is up to manufacturer to decide whether to include the EMI filter on the input stage or not. May be for the sake of more profit, they eliminate the EMI filter from BOM for mass production.

 

So, untill the manufacturer doesn't understand it as his responsibility, which is less likely to happen, or some regulatory body which imposes compulsion on manufacturer it is not going to happen.

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