Y/X EMI caps and a case for a DIY power supply

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I am planning to build a simple and reliable power supply for a projetc with MCU (atmega88), tons of RGB leds (20mA), IR receiver, FETs and stuff.

The input is 220V/50Hz mains with EP (L,N, EP). Output is 5V, 1A at max.
The PS must work 24hours a day, 365days a year because MCU is always standby mode waiting for IR signal or mechnical switches to be switched.

I already have a torroid transformer 220V->12V 1.5A, so, i will use it.
After the rectifying bridge i will put LM2576 witch is a 52Khz 3A 5V buck converter. And voila, i have a pretty good PSU.

However, as i look at certified PSUs i see that they use X and Y filtering caps on mains side. I am sure there is good reason for it. Do i need to use it here to suppress EMI? If so, which parameters should i use for X and Y caps? Also, usually PSUs have metal case and the case is grounded. Should i also use metal case and ground it or i may safely leave it plastic and connect Y caps to the input PE wire?
How it all supposed to be done if one want to certify PSUs? I want this level of EMI suppression and safety.

Thanks you in advance.

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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The easiest way is to use a ready-made power filter. You can get them with an inlet connector that takes a regular IEC cordset.

Another option is to use an external power supply module (brick or wallwart style) and keep the high voltage and switching noise outside of your project. That would also make certification easier.
/mike

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This is no way to learn anything. Besides such thing costs about $10 here, but if i but separate X/Y caps it will like $3 and it can mount them on the pcb (which i do anyway). I don't want to have a bleed resistor, no need for it.
I am not sure that i need to have Ls there, but apparently it is an integral part of any normal EMI filter, so, i will put a couple chokes too. Which is actually ONE device with two coupled chokes, as i understand it. + $1. (for 10mH, 0.7A).

The power supply itself will be separated from the device by about 1M of wire. They are on separate pcbs.

Also, this thing (shown filter) is huge. The PSs will be pretty compact. I am not planning to certify it, i am just asking how to build one properly.

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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Those caps on the mains side are for EMI reduction. If you use a linear regulator, than that is not much of an issue. Same if it is for your personal use, only.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk, i use LM2576 and as i wrote:
"LM2576 witch is a 52Khz 3A 5V buck converter"
so, it's not a linear regulator, it's a switcher.

I tend to disagree that if it is for personal use tben no need for filters. Filters exists for a reson, i believe. Since i did some power supplies for my DIY stuff and plann to do more i can end up with many power suplies without EMI filters generating tons of different EMI in my apartment. Does not seem like a good idea. So, i'd like to do it the right way, more or less.

Can you suggest anything on the original question?

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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Maybe i should ask a bit different question fist.
There are two option: use PE and not use PE.
Maybe i should NOT use PE connector at all, put the PSU into a plastic box and make a EMI filter from X2 CAP + a choke.
When should i bother with PE connection?

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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I'm not an expert...

The devices which use only two leads from the Mains power, (Hot & Neutral), are "double insulated". They do not have a metal case, and no exposed metal, (nuts & bots), can come in contact with any of the circuitry. The output power for a power supply in this case must/usually floats, and is isolated, as you do not have a power ground/earth ground/physical ground available to you if you only bring in two leads.

There was a thread a while back about "isolated" wall wart power supplies, with extensive testing done by Nard.

If you use a metal case then it has to be grounded for safety. You do not want any single failure within the box to ever possibly make the case live. If it was live and someone touched it they could sustain an electrical shock which could be fatal.

You need to check carefully what specifications you wish to design to. It varies from country to country and based upon the specific application.

As an example, the grounded electrical power cord on medical devices has to have the green, ground lead longer than the hot and neutral leads within the plug. This is so that if the cord restraint failed, and the user repeatedly pulled on the wire, and the wires came loose, the ground wire would be the last wire to become disconnected.

Rules go no and on ...

JC

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Actually, good question. The answer is: it depends. If I would use a metal box, I would ground the box with PE, and would never connect it to ungrounded outlet without PE.

Also the EMI filter design depends if you have PE or not, and if you want the output terminals floating or grounded to PE.

For example, PC power supply EMI filters are designed to have PE and the output ground (black lead) is connected to PE. Now if you connect this kind of device to ungrounded outlet, the EMI filter keeps the PE (=case and ground too) floating at half of the mains voltage. The current is not hazardous but if you touch grounded PC and ungrounded PC it tingles a bit or may even hurt. Plus connecting grounded PC and ungrounded PC together may cause something to break or at least the other PC is grounded only through the other PC.

Modern laboratory power supplies require grounded input, and the output terminals are still isolated from ground, so you can connect for example two 12V lab power supplies together to get either 24V or +12V and -12V supplies, and offers you the PE terminal too so you can ground any output point you want with PE.

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Okay, if i consider that the power supply can be flooded with water (it might be the case that a glass of drinkable liquid is spilled over upper surface and it drips onto the power supply) then what solution will be batter? Let's consider i cannot provide 100% assurance that the PSU is water sealed. If it is metal and grounded it will quickly short and shutdown (fuse will work either in the PSU on in the input of the apartment). If it is plastic and not grounded that we might hope that internal fuse will melt. Or maybe not, but cam mains voltage to to the secondary side, burn everything and make secondary wires "HOT" ?

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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Your using a low frequency transformer from mains to low voltage dc. You probably don't need any X or Y caps for this application provided you follow good grounding, decoupling and layout techniques for your dc:dc switcher.

X and Y caps can help quite a lot for offline SMPS converters where the switch is operating non-isolated from the mains input.

oddbudman

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Good point oddbudman, mains is not directly the input of SMPS, there is a linear PSU before it.

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If you don't use PE in a commercial setup that
needs certification your transformer als must
fulfill strong isolation requirements ("double
isolation").

Flooding with water is an interesting issue. Think
about the small power supplies without PE. I don't
think they have protection against looding with water.

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Thank you all!

oddbudman, i think you made some thigs clearler to me.
As i now understand there are OFF LINE SMPS which do not have a transformer on the imput side, but instead rectify the AC voltage and directly apply it to a switching DC-DC converter which works on 50Khz-1Mhz range and is not isolated from the mains, so, all this noise from the switcher can and do go into the main. Hence the input EMI filter is 100% needed.

In my case I use a transformer on the input which operates on AC frequency and a decoupled switcher on the secondary side of the transformer. So, as you say the 52Khz noise from the switcher will not go into the mains. Is it really so? Does transformer really block this noise? I am also sure that some of the 16Mhz noise from the MCU will be on the secondary side. And several KHZ noise from PWM channels for the leds (those delivery about 3W of power to the leds).

Also, can this type of power supply (transfomer + dc-dc buck on the secondary) be called a SMPS or it is actually some other type of power supply. I need to know, so the terminology is right.

Thanks again!

Artem Kuchin.
Electronics hobbyist. Born in 1976.

Last Edited: Thu. Mar 17, 2011 - 04:35 PM
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You will need some minimalistic filtering at transformer mains input.
Buck converter shall have filters at output in order to smooth load voltage. Good design will exhibit low PWM noise component on Vcc.
Your supply is SMPS anyway - just using transformed mains for input.
A hint - You may employ leakage relay at mains socket. Will be safe if liquid spills over or alike.

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Switcher noise may get into mains, but remember that you surely have a big reservoir cap after the transformer, and you may even put LC filter after the reservoir cap too. And the switcher input side must have some very low ESR capacitors so they should keep the EMI from reaching the mains transformer even.

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So, it IS SMPS :)

So, i will not use PE. Make floating output. Put an X2 cap on the mains input side. Put a low ESR cap on the switcher input side + big cap after the recifier.
Right?

Do you suggest tantalum low ESR on the switcher input?
Maybe a combination: big electrolitic like 2200uF and a tantalum in parallel?

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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For switcher, you should always follow what the switcher chip documentation suggests.

It seems it says minimum of 100uF aluminium electrolytic or tantalum, plus a bunch of other requirements, like formulas for RMS or peak current ratings of the cap.

For rectifier bulk capacitor, you must know what is the largest average current the switcher is going to take, and what is the minimum allowed voltage ripple on switcher input (minimum voltage switcher needs in your system and what is the voltage the transformer charges it to, taking into account some 10-20% of tolerance in mains voltage etc).