Need IC for flyback SMPS isolated feedback 1

[potcore]

Dear Engineers,

I have designed a Flyback SMPS and V(out) needs to be regulated to 5V. Feedback of output voltage needs to be isolated and so I proposed to use the LM3411-5 precision secondary regulator feeding into an opto-coupler (4N27) which links up with the comp pin on the controller IC (LM2587-ADJ) on the primary side.

However, LM3411-5 is not available on RS or Farnell and I have been told to get IC's from there.

Therefore, I am wondering if any reader could propose any alternatives?

Also, please could you check out my "home brew" regulator (using LM324 op-Amp as a comparator) pictured here...

....any thoughts on my "home brew" design would be much appreciated. (The transient response of the feedback control does not have stringent requirements here).

 

[logbook]

I suppose you want constructive criticism on your "home brew". Phew that's a tough one!

If you use an extra winding on the transformer you can use this for the feedback. It is not an exact mirror of the output, but unless your load variations are large, it should be ok. This is much better than using opto feedback.

Your "home brew" is using digital feedback. The comparator will thrash about at some indeterminate frequency, beating with the switcher frequency and causing havoc. You could use a proper linear opto feedback system, but I would recommend using the extra winding if possible. Oh and without wishing to cause offence, ditch the home brew and steal a circuit from an app note (unless you want to risk ridicule and unemployment).

 

[ScottyUK]

So why not use another supplier if RS and Farnell are out of stock? This has the feel of homework / class assignment - "... I have been told to get IC's from there".


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Sometimes I only open my mouth to swap feet...

 

[potcore]

Hi,

Thanks for your replies. The power supplies that i'm making are just for in-house use in my company -since the bought-in supplies that we have keep going missing.
It's not for an assignment, i graduated in 2002 (Bachelors, Electronics).

I've had too much difficulty getting a few pieces of IC samples when i can't give a mass production date, so rely on RS and Farnell.

I have at my disposal Marty Brown's "Power supply cookbook" 2nd Ed. On page 1, i find that i am suggestedly not alone in my lack of power supply expertise, for Brown states that..."All too frequently, the designer assigned to the power supply has very little experience in power supply design"...also, that with regard to the power supply in a system, "nobody wants to touch it because everybody wants to design more exciting circuits".

I have a sneaking suspicion that there is some shortage of expert SMPS engineers. In fact, i've been offered an interview next week with an SMPS company. (i guess they must be desparate(?)) -Any tips on magnetic design most gratefully appreciated as i know they'll ask all about this.

 

[Skogsgurra]

That ("nobody wants to touch it because everybody wants to design more exciting circuits") was true back in the sixties and seventies when PSU design was all linear and cooling was the biggest problem. But now, designing a fast, efficient and quiet switcher in a small volume is somewhat of a challenge that can be a lot more "exciting" than any video stage or opto receiver. (What's so exciting about them? BTW).

I am not so sure that I understand why you need to design a PSU. Because the ones you buy get stolen? Is that what you say?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

I found Marty Brown's original book extremely useful.

http://www.amazon.com/Practical-Switching-Academic-Professional-Technical/dp/0121370305

 

[Madcow]

I started with the UC3845 chip.Lots of manufacturers make
a version of this chip. Then on the secondary side a TL431
and an opto. Fairchild make a 431 and opto in one package.
Check out:
National Semi
TI
Fairchild
Microchip
Linear
Analog devices
onsemi
ST
ECT.

Lots of app. notes.
TI and Microchip have some nice digital controlled
stuff for a little more excitement.

 

[itsmoked]

Buy another supply and then label it with a big ugly label [red]BAD[/red].

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[debodine]

itsmoked:

Are you military or former military? In my unit we protected a lot of critical equipment that way. Of course this was in the 70's before digital tracking of assets with bar code scanners was possible...

In any case, I think your idea deserves a star!

debodine

 

[itsmoked]

Thanks debodine.

I was in the military for 20 years. (Army brat.)
I just know the average PS thief would be quickly put off by such a sign.

I had a $3.5k emulator for a 4bit uP powered by a lab supply. Some a-hole disconnected it used it and returned it, re-connected it. I came in, sat down, and flipped it on. PffffT!

He left it set to 12V, it had been set to 5V.

If he'd just left it on my chair or disconnected..

So I started thinking about how to circumvent that kind of behavior.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Madcow]

I hook my equipment to the
Hi-pot tester when not in use.

 

[Madcow]

Heres some magnetics info.

http://www.smps.us/magnetics.html