Opto-couplers in smps part 2

[Steve]

Hi again,
I like to continue the discussion regarding opto-couplers as isolating
element between secondary sided error amp and primary side pwm
control.
Some people mention it's possible to use kind off pwm-style to
interface the error signal.
I realize how this may be done on the secondary side, but how is the
pwm signal actually integrated to become an analouge voltage for pwm
control.
This should require a pwm carrier frequency many times above the main
converter operating freq.

Any advice is appreciated.

Stefan

 

[Jamie]

Hi again,
I like to continue the discussion regarding opto-couplers as isolating
element between secondary sided error amp and primary side pwm
control.
Some people mention it's possible to use kind off pwm-style to
interface the error signal.
I realize how this may be done on the secondary side, but how is the
pwm signal actually integrated to become an analouge voltage for pwm
control.
This should require a pwm carrier frequency many times above the main
converter operating freq.

Any advice is appreciated.

Stefan

just pass the primary side pulse from the OpTo to a T network
of a R and CAP. use a unity gain Transistor to buffer it.
make sure PWM to this network has a common sink to discharge
the reference as well..
Or am i not in the same ball park as you are?

 

[ian field]

Hi again,
I like to continue the discussion regarding opto-couplers as isolating
element between secondary sided error amp and primary side pwm
control.
Some people mention it's possible to use kind off pwm-style to
interface the error signal.
I realize how this may be done on the secondary side, but how is the
pwm signal actually integrated to become an analouge voltage for pwm
control.
This should require a pwm carrier frequency many times above the main
converter operating freq.

Any advice is appreciated.

Stefan

The most common arrangement is the opto-LED in series with an adjustable
zener such as a TL431 across one of the rectified and smoothed secondary
supplies.

 

[Eeyore]

Hi again,
I like to continue the discussion regarding opto-couplers as isolating
element between secondary sided error amp and primary side pwm
control.
Some people mention it's possible to use kind off pwm-style to
interface the error signal.
I realize how this may be done on the secondary side, but how is the
pwm signal actually integrated to become an analouge voltage for pwm
control.

With an integrator circuit maybe ?

This should require a pwm carrier frequency many times above the main
converter operating freq.

Not really, the control loop bandwidth is usually not more than several kHz.

Graham

 

[Steve]

With an integrator circuit maybe ?


Not really, the control loop bandwidth is usually not more than several kHz.

Graham

Hello,

The 0dB point in the loop response is set to 3kHz. Would this be the
requirement?

 

[Eeyore]

Hello,

The 0dB point in the loop response is set to 3kHz. Would this be the
requirement?

It depends what you mean by the 0dB point. I'd expect the bandwidth to roll-off
after that.

Graham

 

[Genome]

It depends what you mean by the 0dB point. I'd expect the bandwidth to
roll-off
after that.

Graham

I think he meant 'The 0dB point in the loop response'.

It's similar to.....

'That cow pat you are about to step in'.

'Well, it depends on what you mean by cow pat'.

Splurge

'Oh, that'll be those Americans again'.

DNA

 

[John Larkin]

I think he meant 'The 0dB point in the loop response'.

It's similar to.....

'That cow pat you are about to step in'.

'Well, it depends on what you mean by cow pat'.

Splurge

'Oh, that'll be those Americans again'.

DNA

Cow-chip-throwing capital of America: Beaver, Oklahoma

Since 1969, Beaver's claim to fame has been its annual dried-
cowpat-throwing contest. Last year's male and female champions were
James Pratt and his daughter Terri, prompting the local newspaper to
comment, "The 2005 toss was a family affair."




John

 

[Genome]

Cow-chip-throwing capital of America: Beaver, Oklahoma

Since 1969, Beaver's claim to fame has been its annual dried-
cowpat-throwing contest. Last year's male and female champions were
James Pratt and his daughter Terri, prompting the local newspaper to
comment, "The 2005 toss was a family affair."

John

That might be one of these cultural differences. I could be wrong but I'll
make this up.

As a result of climate.....That's good enough.

The Cows/Beef eat the lush British Grass and produce Milk, Succulent Beef
and soft patties. The patties don't dry too quickly...... it's the climate.

That's why us lot wear Wellies and throw them instead.

'Oklahoma! where the something or the something or the something.
I'm doing fine Oklahomaaa, Oklahoma something something'.

Doris Day?

DNA

 

[Genome]

That might be one of these cultural differences. I could be wrong but I'll
make this up.

As a result of climate.....That's good enough.

The Cows/Beef eat the lush British Grass and produce Milk, Succulent Beef
and soft patties. The patties don't dry too quickly...... it's the
climate.

That's why us lot wear Wellies and throw them instead.

'Oklahoma! where the something or the something or the something.
I'm doing fine Oklahomaaa, Oklahoma something something'.

Doris Day?

DNA

"Deadwood Stage! Take 42, all quiet!"

'Look, are you sure she promised you a shag? I'm only asking because she's
pretty handy with that whip on the chorus and my mates are starting to
complain about it not being a wonderful day.'

DNA

 

[JeffM]

John Larkin wrote

'Oklahoma! where the something or the something or the something.
I'm doing fine Oklahomaaa, Oklahoma something something'.
http://www.google.com/search?q=wind-comes-sweeping-down-the-plains+waving-wheat

Doris Day?

http://www.google.com/images?q=Shirley-Jones+Oklahoma

 

[Genome]

John Larkin wrote

http://www.google.com/images?q=Shirley-Jones+Oklahoma

I would try and argue with you but, unfortunately, you are right..... Who is
this man or person?

Doris Day, Calimity Jane.... (had something fixed about Annie Get Your Gun)
This much I remember (maybe) but I have no reason for remembering
Oklahoma..... unless someone else did a piss take of it and I saw that
one....creeping memory.

That can't be right, the song is too fixed in my head. I must have seen the
film.

Sheep Farmers?

I give up.

DNA

 

[Eeyore]

The most common arrangement is the opto-LED in series with an adjustable
zener such as a TL431 across one of the rectified and smoothed secondary
supplies.

He's concerned about the gain of that arrangement varying with CTR.

Graham

 

[Terry Given]

ian field wrote:




He's concerned about the gain of that arrangement varying with CTR.

Graham

but it doesnt matter, the TL431 has bags of Aol. just design for the
absolute lowest possible CTR, over the lifetime of the product. easy.

And all opto's are not created equal. Others have referred to this, but
decent opto's come binned for CTR.

IME a far bigger problem is the bandwidth of the opto. it needs to be at
least an order of magnitude higher than the closed-loop bandwidth. I've
seen (and corrected) numerous designs using optos that add 20,30, even
50 degrees of phase shift. That makes closing a loop hard!

Cheers
Terry