Author Topic: LED Driver SMPS Flyback Design (Transformers/EMI Filters)  (Read 2646 times)

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Offline David

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LED Driver SMPS Flyback Design (Transformers/EMI Filters)
« on: August 14, 2012, 04:16:00 AM »
Hi all,

I am in the process of designing my very first SMPS flyback transformer. My knowledge on the subject has gone from almost zero to the point where I am confident enough to go ahead with the project (that's what I love about electronics). Now there is one subject that has been bugging myself and giving me sleepless nights for the past couple of weeks (one of the downsides to electronics!), and that is the safety requirements. I am a "young player" in the Electronics industry, as Dave would say, (not long out of university) so please bare with me.

First, a very brief summary of the project:

* The transformer is for an offline flyback (90Vac to 265Vac), constant current LED driver (<9.1W). This is a primary side regulated (PSR) design.
* Due to space constraints I am forced to use a relatively small core (EFD is suited due to low profile).
* At the moment it looks as if an EFD20 will do the trick (however low Ae and small winding width means I will need multiple primary layers which is fairly unavoidable with this design). I have another question on interleaving windings but I don't want to bombard everyone with lots of questions at the moment...
* The transformer must meet the safety requirements of EN61347-2-13.
* The LED driver is a class 2 (double-insulated) safety product.

So here goes the question(s)...

1. I want to use a margin-wound transformer construction to meet the appropriate creepage/clearance requirements. However I have seen many different methods of meeting the requirements. Some application notes/designs use margins on both sides of the bobbin whilst on the other hand some use margin tape purely on the secondary side?

2. If anyone is familiar with EN61347 then I would love to hear from you. I am confused about the actual distances required for the design? Being a class 2 product, I believe I require supplementary insulation? (According to a table I read in EN60598-1)

3. Am I right in saying that you should account for the inherent clearances of the bobbin itself?

4. Where exactly are the clearances/creepage distances measured to and from (in regards to transformers)? Is it purely from primary to secondary windings?

5. Can all of the above be avoided by simply using triple insulated wire (TIW) throughout?

I apologise in advance for all the questions. I would not normally ask in the forums but I think I have exhausted all other options.

Cheers,

Dave

P.S. I am sure some sort of beer payment can be made for any helpful answers! :)
« Last Edit: September 05, 2012, 06:23:32 PM by David »
David
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Offline David

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #1 on: August 15, 2012, 04:58:25 AM »
Bump :)
David
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Offline Neilm

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #2 on: August 15, 2012, 06:50:06 AM »
The first question I have is who is building the transformer? Do they have the equipment to test that the transformer?

I am not too familiar with that standard but I do know EN61010 and have designed SMPS to that standard.

1) As long as there is sufficient creepage and clearance between the primary and secondary any method is OK. Some may be easier to build in certain situations, other constructions will affect the performance of the flyback transformer (for instance increasing the leakage which will increase the turn off spike on the FET)

2) Pass - In 61010 it is 3mm for double clearance.

3) The bobbin will provide some clearance to the pins. The clearance is the shortest distance in air between the primary and the secondary.

4) Clearance and creepage is measured from anything conductive (or whose insulation properties are insufficient) that is connected to the primary, to anything conductive (or whose insulation properties are insufficient) that is connected to the secondary. The clearance is the total distance in air between the two. Please note the phrase "in air". I was once given a unit to approve that the manufacturers said had the correct clearance, but they put a metal screw head in the way so they had less than 10% of the clearance they thought. Practically, this means that if the primary and secondary is connected to the same side of the bobbin, you have to deduct the conductive bits (i.e the pins and associated pads) from the distance between the two.

5) TIW wire might be OK as long as the insulation is sufficient - I don't know what the standard says.

The creepage and clearances will have to be proved by flash testing the transformer. The standard should say what the required test voltage is.

Yours

Neil
Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe. - Albert Einstein

Offline David

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #3 on: August 24, 2012, 04:21:01 AM »
Hi Nelim,

Thank you for your response! I now have a few more answers to my troubles.

There is so much conflicting information on Flyback transformers on the internet, it's hard to know who's using the correct formula! Almost every datasheet and app note has a different approach or method for calculating even 'simple' things such as the primary inductance.

One particular problem I have found is how to calculate the required air gap within the transformer core. Am I correct in saying that this can be achieved by using I2L (Output current squared x Inductance) and then finding a graph plotting this against the air gap? From there I would then select the nearest (or custom) AL value and then I can determine the number of turns required for the primary?

Thanks for the help,

Dave
David
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Electronics Design Engineer

Offline Neilm

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #4 on: August 24, 2012, 05:35:27 AM »
Dave,

I must confess that my knowledge of flyback transformers is more to do with the safety side of the application. The few times I have designed them they didn't come out anything like I expected. Last time I didn't bother and gave it to a guy in the office who is a a magnetics expert and let him design it while I did the control loop.

Yours

Neil
Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe. - Albert Einstein

Offline David

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #5 on: August 24, 2012, 07:20:08 AM »
I must admit, that could be the way this project goes dependant upon time. I'll definetly give it a go though, managed to source some sample cores and bobbins so I will soon start winding and inevitably blowing things up!  :P
David
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Offline AcHmed99

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #6 on: August 24, 2012, 07:45:19 AM »
What you want is magnetic design handbook from TI. There are good books but assuming you are on a deadline go through this

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014

Of immediate interest will be slup127 "Inductor and Flyback Transformer Design"

But get the whole package for future reference.

You typically don't want to put much more then 1mm gap in flyback coupled inductors or the fringing field which is proportional to the gap and large flux swings typical in a DCM flyback will result in significant hotspot to the point the windings will actually fuse. Its all in the slup127 pdf.

I use some on-line tools to get me in intially in the ballpark as well like this

http://schmidt-walter.eit.h-da.de/smps_e/smps_e.html

Just adjust your output power for efficiency; the tool assumes efficiency of unity.

Edit
I see you have your own core's and bobbins. As a heads up you can use sand paper to create a gap. I've used anywhere from 220 grit to 400grit. Use the finer grit for small cores. Its relatively quick for small cores its when you get over ETD29 size it can get tedious. I just put about a 2.2mm gap in an ETD49 core for a choke took about 30 Min's of sanding. For prototyping you can gap the outer leg using spacers like MICA until you verify that that is the gap you want. Just be sure to put half the gap in each outer leg.
« Last Edit: August 24, 2012, 10:15:38 AM by AcHmed99 »

Offline David

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Re: LED Driver SMPS Flyback Transformer Safety (Creepage/Clearance)
« Reply #7 on: August 28, 2012, 03:34:32 AM »
Cheers guys. I already had a good read through the TI notes which were fairly useful. However, they describe yet another method for calculating gaps etc! Transformer design is hard work!
David
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Electronics Design Engineer

Offline David

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Re: LED Driver SMPS Flyback Design (Transformers/EMI Filters)
« Reply #8 on: September 05, 2012, 06:19:39 PM »
Hi all,

Thank you for all the feedback! I am definitely on a steep learning curve here but enjoying it at the same time. I now have a preliminary transformer drawing and have sourced all the parts to start protoyping it soon  :D

I now have a couple of questions regarding EMI filters for flyback supplies...

1. In the flyback design I am working on there is no ground connection (class 2), only Live and Neutral. Am I correct in saying that any noise flowing in the Live will flow back out Neutral, therefore there will be no common mode noise? - Hence removing the need for a common-mode choke?

2. I have seen many different methods of implementing filters to reduce differential mode noise in the hundreds of application notes I have been looking through. The most common arrangement appears to be a simple PI filter, as you may expect. However I have seen two variations of the filter (please see attached image). What is the reason for using two inductors? (It what I would describe as a "stacked" PI filter). Is the inductor required on the Neutral as we are dealing with an AC input? Can only the single inductor on the Live side be used?

Again, apologies for so many questions. I have been scratching my head over this one for a while.  :o

Cheers,

Dave
« Last Edit: September 05, 2012, 07:03:25 PM by David »
David
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Offline David

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Re: LED Driver SMPS Flyback Design (Transformers/EMI Filters)
« Reply #9 on: September 06, 2012, 05:49:36 AM »
I have also seen the use of Common-mode chokes for input filters which utilise the leakage inductance of the choke to filter differential mode noise...
David
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Offline madires

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Re: LED Driver SMPS Flyback Design (Transformers/EMI Filters)
« Reply #10 on: September 06, 2012, 06:29:29 AM »
1. In the flyback design I am working on there is no ground connection (class 2), only Live and Neutral. Am I correct in saying that any noise flowing in the Live will flow back out Neutral, therefore there will be no common mode noise? - Hence removing the need for a common-mode choke?

2. I have seen many different methods of implementing filters to reduce differential mode noise in the hundreds of application notes I have been looking through. The most common arrangement appears to be a simple PI filter, as you may expect. However I have seen two variations of the filter (please see attached image). What is the reason for using two inductors? (It what I would describe as a "stacked" PI filter). Is the inductor required on the Neutral as we are dealing with an AC input? Can only the single inductor on the Live side be used?

For a really good filter you would use a common-mode choke and a LC combination (2 Ls, like the upper part of your diagram) to filter asymmetric and symmetric noise. Since you got two wires (Live & Neutral) you need two inductors to "protect" both wires.

Offline AcHmed99

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Re: LED Driver SMPS Flyback Design (Transformers/EMI Filters)
« Reply #11 on: September 06, 2012, 07:22:38 AM »
Have you found this Fairchild AN4145

"For a balanced filter (an inductor on both the live and neutral
lines), the value is divided by 2, giving 64mH. With the
inductance being so large, two inductors will be used to
reduce its size. Also an advantage to having more inductors
is not only to reduce the inductance, but the Q of the filter
decreases, thereby decreasing the risk of oscillations. "

www.fairchildsemi.com/an/AN/AN-4145.pdf

Onsemi also has an AN on line filters check the product folder for the NCP1200.

You dont really need to calculate the gap just the AL value whoever does the core will know the gap from the AL value. Also just wind the turns you calculated and sand or grind (OR SPACERS) until you hit the targeted inductance for prototyping. Look at typical AL gap tables in the core data sheets try to stay in that range for your AL value preferably 1mm or less gap particularly if the core is used in DCM. You would prefer to stick with a gapped core that is already available.
« Last Edit: September 06, 2012, 07:31:52 AM by AcHmed99 »


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