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Offline SMPS with Digital isolator feedback is acceptable?

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Flyback

Well-Known Member
Hello,
Do you see any problems with this (non-optocoupler based) isolated feedback method for offline SMPS’s? Why does nobody do it like this?
The attached schematic shows what surely must be a total revolution in isolated feedback for offline SMPS’s? –It allows the feedback signal to cross the isolation boundary without recourse to an optocoupler. (instead , a digital isolator is used, freeing the engineer from the opto with all its inherent problems such as poor parameter tolerance and ageing problems )
With the error amplifier on the secondary side, the feedback signal takes the form of a variable duty cycle square wave, produced by a simple PWM controller IC. This is passed across the isolation boundary by the digital isolator. On the primary side, the PWM error signal is filtered and then fed to the PWM comparator in the primary side PWM controller IC. –Hence the SMPS output is regulated
There is also the LTspice simulation attached.
What’s wrong with this way of doing it?
Why does nobody do this?

SI8410 Digital isolator datasheet:
https://www.silabs.com/Support%20Documents/TechnicalDocs/si8410.pdf

LT1243 datasheet:
http://cds.linear.com/docs/en/datasheet/1241fa.pdf
 

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Flyback

Well-Known Member
Its more parts than an opto, but far more robust, and is simpler and cheaper than a feedback method using a high frequency transformer, which is the other non-opto alternative.....the transformer may well go obsolete, and you then have a harder job to replace it than just replacing a digital isolator....the extra parts are worth it to avoid an opto...opto's are a big problem for isolated feedback loops, due to their poor temperature performance, ageing, and poor parameter tolerance...those parameters, I hasten to add, are intrinsic to the feedback loops very operation....not good news for optos, optos are bad in the feedback loop of an SMPS.

where did I get it?, well I kind of hacked it from an idea schematic in the Raymond mack book "demystifying switching power supplies"
 
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Flyback

Well-Known Member
thanks, I see your attachment now, you are using bias winding feedback, which is fine , but gives poor tolerance to vout, and poor transient response in vout. Especially in offline flybacks where the bias coil has a significant filter with it so as to filter the leakage spike away.
 

tvtech

Well-Known Member
Most Helpful Member
All I can say is that in the CRT SMPS world.....I have seen more reliability with non optocoupler supplies. A feedback winding on the Chopper Transformer does an adequate job in giving the Voltage regulation we need to keep the picture stable.

However...if a non optocoupler supply fails...it can sometimes be a mission to fix. Strange things go faulty. Like I have had parts on the Primary fail and sort of go intermittent...nothing blows...just it will not Regulate properly. No amount of testing (even with my Analogue YEW) will reveal the fault. A case of changing part for part until it stops it's nonsense. The last was the other day when the 9V1 Zener on the Primary decided to play games with me:arghh:.

With optocoupler supplies....it's like everything blows properly. Either dead short or open. Nothing else. No intermittent regulation problems.....unless of course the opto is damaged too....but hey, they are really cheap. Good practice to change it anyway if you are doing a rebuild.

That's my bit

Regards,
tvtech
 

Flyback

Well-Known Member
I must admit the opto is cheap.
However, on things like a £500 , 150w guitar amplifier, is it worth the makers saving the price of a few components when the amplifier is a quality item which vends at £500?....and after all, guitar amps are not high volume sales items.
I'd say the extra robustmess that one gets by not having an opto is well worth having the extra components that I show.

Im a little confused as to why one of the big seminconductor co's hasn't done an integrated version of what I show above....after all, there is a lot of demand for offline SMPS's with isolated feedback.
 

tvtech

Well-Known Member
Most Helpful Member
We are back on SMPS again

Flyback, I really don't know. My experience with SMPS is based solely on my experience with CRT SMPS. Nothing else.
You see, I can and only will comment on what I know.

People might say SMPS are all different...different designs....different applications etc, etc, etc.
And they are all 100% spot on.

No matter what designers of SMPS think up in the future....I really have no time for them (SMPS).

Unless they are designed by the Masters of that black art...like professionals Delta Electronics.....

I even dabbled with a SMPS design years back. I ordered 100 Viper 50 IC's from America.
My Chopper Transformers were wound and built in New Zealand.
The boards were designed and etched by me right here in South Africa.

I received my Vipers. I received my Choppers. My boards and parts were all here.....

Out of I think four assembled units...only ONE worked properly. It never blew up when the others did???
It could handle what the others could not.

After wasting/blowing up 6 of my Vipers, I stopped. No point in wasting to prove a point if you really don't know the in's and out's of what you are doing...you can read all you want about SMPS....I followed the rules to the letter...and failed.

ST had simulation. They had all the advise on the NET....no go.

Dark Art.....

Different scenario.......People involved and wise to SMPS design..it was all they needed. Many, many Vipers are out there still..found in DVD players and other goods. I have not seen many 50's here though....I believe I still have around 90+ in storage.

So, for me, to repair SMPS is part of my daily job....design and build is another animal altogether :eek:

Regards,
tvtech
 

Flyback

Well-Known Member
I don't know if I would say "dark art", -I once went to a big lighting company who were starting off designing smps's for the led lightbulbs.....they had been failing for weeks because they did not know about interleave winding of their flyback transformers...

I bet if you had chosen say a powerint.com chip you would have got the job done.

They give you demo boards and you can just copy them if you wish.
 

tvtech

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Most Helpful Member
Believe me....to do SMPS design properly is a "dark art"

It could be a little track change here. I could be a little bit of shielding there. It could be a little Ferrite Bead added....
Thing is, if done properly, it has awesome reliability.

I will give you an example...

Common PC supplies. They mostly look the same inside...sure different chips and stuff...but still do the same job...

Somebody in the the know designed the original. And all was copied and made cheaper. Normal story.

Throwing the ball back at you Flyback. Kinda liking your posts :cool:.
You are making me think a little.

Regards,
tvtech
 
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tvtech

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Most Helpful Member
And I might also add...

You know SMPS. Reveal yourself. You try to appear Dumb and ignorant.....you are not.

Big compliment from me :). Sit down, take a breath of fresh air. The World is good. And it will be even better Tomorrow.

Regards,
tvtech
 
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Flyback

Well-Known Member
thanks but I don't feel very clever when I see the dreadful no-load to full-load transient response of this digital isolator SMPS. Here it is running alongside a non isolated SMPS in LTspice..any ideas to improve the dreadful undershoot at no-load to full-load transient?
 

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tvtech

Well-Known Member
Most Helpful Member
thanks but I don't feel very clever when I see the dreadful no-load to full-load transient response of this digital isolator SMPS. Here it is running alongside a non isolated SMPS in LTspice..any ideas to improve the dreadful undershoot at no-load to full-load transient?
LT Spice = cannot help.....
LT Spice is THEORETICAL. Not real world.

I don't know if LT Spice could help me when I have 27KV willing to bite me if I touch the wrong thing...
Or would it warn me??. Just saying.

Different horses for different courses.

Regards,
tvtech
 

alec_t

Well-Known Member
Most Helpful Member
What’s wrong with this way of doing it? Why does nobody do this?
If you look at the datasheet for the digital isolator you will see it's over-printed "Not recommended for new designs". I suspect that's your answer. Perhaps the manufacturers have discovered a problem, even if they are not telling us what it is.
 

Flyback

Well-Known Member
It doesn't really matter about the particular part number of the digital isolator, its the principal of using a digital isolator in this way...there are digital isolators in the market that could be used in this scenario, the question is, why are they not being used as such?
 

alec_t

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Most Helpful Member
Probably costs more than the opto route. Principles don't count when it comes to marketing :)
 
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ronsimpson

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All I can say is that in the CRT SMPS world.....I have seen more reliability with non optocoupler supplies. A feedback winding on the Chopper Transformer does an adequate job in giving the Voltage regulation we need to keep the picture stable.
A CRT monitor/TV set has a limited range from Power max to Power min. Feedback is much easier when load min is 50%.
Those little 5V USB wall worts have very loose voltage regulations specifications. Your job is to get the supply to meat spec. It is not to be better than spec. If you can get the job done with out "feed back" then good for you. Saved time and money.
 

crutschow

Well-Known Member
Most Helpful Member
LT Spice = cannot help.....
LT Spice is THEORETICAL. Not real world.
...................
Of course LTspice is THEORETICAL as are all calculations used in the design of a circuit. But Spice can be very useful tool in predicting the response of the real circuit and discovering problems in the design. It may not perfectly simulate the real circuit but it's usually close enough to significantly help in the design before you go to the actual breadboard.
I don't know if LT Spice could help me when I have 27KV willing to bite me if I touch the wrong thing...
Or would it warn me??. Just saying.
I don't understand what LTspice has to do with warning you about the dangers of a 27kV circuit. How is that different from the real circuit? Certainly it won't warn you when you are about to get bit.
 

tvtech

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Most Helpful Member
Hi

I am so conditioned with working around HV that nothing else really matters....except of course SMPS.
Little joke from me.

LT Spice seems so Mickey Mouse to me when the real world stuff is waiting to bite you with the smallest mistake you might make...

Just me.

Regards,
tvtech
 

crutschow

Well-Known Member
Most Helpful Member
..................................
LT Spice seems so Mickey Mouse to me when the real world stuff is waiting to bite you with the smallest mistake you might make...
Don't see how a complex Spice simulation program that has been refined and used in many of its variants for several decades by many designers can be called "Mickey Mouse"?

One of the uses of Spice is to find some of those "smallest mistakes" before you build the circuit. ;)
 
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