Opto-couplers in smps...

[Steve]

Hi all,
Here we go again with another smps question.
As you may have read earlier, I'm trying to compensate a smps-design.
The error amp is located on the secondary-side. The error signal is
interfaced to primary side through an opto-coupler. And this opto is
what makes me concerned. Since optos has a wide coupling ratio, I'm
afraid, at some point, end up with excessively low loop gain and
eventually makes the loop crossover frequency say hello to the filter
resonance and the thing will go up in smoke. Can this be the case? Do
I have to worry about this.
How to minimize the effects of "wild optos"?

Best regards,
Stefan

 

[Anirban]

Hi all,
Here we go again with another smps question.
As you may have read earlier, I'm trying to compensate a smps-design.
The error amp is located on the secondary-side. The error signal is
interfaced to primary side through an opto-coupler. And this opto is
what makes me concerned. Since optos has a wide coupling ratio, I'm
afraid, at some point, end up with excessively low loop gain and
eventually makes the loop crossover frequency say hello to the filter
resonance and the thing will go up in smoke. Can this be the case? Do
I have to worry about this.
How to minimize the effects of "wild optos"?

Best regards,
Stefan

Sureshot way : Use a pulse width modulator to transfer the amplitude
information across the optocoupler; average it out on the other side
or better still, use the time domain information. That way the
optocoupler is always operating either at full power or zero. And
hence highly INsensetive to varying gains. Or you could use a Hall
effect DC current sensor (also called a DC Current Transformer / DCCT)
to Ohmically isolate the two sections and still transmit the required
data.

--- Anirban

 

[Paul Mathews]

Hi all,
Here we go again with another smps question.
As you may have read earlier, I'm trying to compensate a smps-design.
The error amp is located on the secondary-side. The error signal is
interfaced to primary side through an opto-coupler. And this opto is
what makes me concerned. Since optos has a wide coupling ratio, I'm
afraid, at some point, end up with excessively low loop gain and
eventually makes the loop crossover frequency say hello to the filter
resonance and the thing will go up in smoke. Can this be the case? Do
I have to worry about this.
How to minimize the effects of "wild optos"?

Best regards,
Stefan

Simply specify an optocoupler with a more tightly spec'd coupling
ratio. They exist for just this reason.
Paul Mathews

 

[Spehro Pefhany]

Simply specify an optocoupler with a more tightly spec'd coupling
ratio. They exist for just this reason.
Paul Mathews

It changes with time.


Best regards,
Spehro Pefhany

 

[Joerg]

Sureshot way : Use a pulse width modulator to transfer the amplitude
information across the optocoupler; average it out on the other side
or better still, use the time domain information. That way the
optocoupler is always operating either at full power or zero. And
hence highly INsensetive to varying gains. Or you could use a Hall
effect DC current sensor (also called a DC Current Transformer / DCCT)
to Ohmically isolate the two sections and still transmit the required
data.

Way to go, at least when riding close to the cliff in terms of loop
stability.

Another simple method is to use a little RF transformer instead of the
opto coupler and transfer a carrier where the amplitude is proportional
to the input signal. I often use 13.56MHz for this kind of stuff, less
worries at the EMC lab

 

[Paul Mathews]

It changes with time.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
[email protected] Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com- Hide quoted text -

- Show quoted text -

Of course, I agree that you have to design around the non-ideal
characteristics of optocouplers, but they are out there in the world,
functioning reliably in the billions of units. The drift due to aging
is toward less coupling, and is one of many factors that the designer
must consider when deciding how much margin to require.
Transformer isolation is a good alternative, and it is often a
requirement in space and avionics electronics, with optos not allowed.
In addition to using various pulse width feedback schemes, there are
plenty of examples of amplitude feedback thru transformers, including
one in the TI/Unitrode Seminar Notes, IIRC.
Paul Mathews

 

[Steve]

Of course, I agree that you have to design around the non-ideal
characteristics of optocouplers, but they are out there in the world,
functioning reliably in the billions of units. The drift due to aging
is toward less coupling, and is one of many factors that the designer
must consider when deciding how much margin to require.
Transformer isolation is a good alternative, and it is often a
requirement in space and avionics electronics, with optos not allowed.
In addition to using various pulse width feedback schemes, there are
plenty of examples of amplitude feedback thru transformers, including
one in the TI/Unitrode Seminar Notes, IIRC.
Paul Mathews- Dölj citerad text -

- Visa citerad text -

Does anyone of you have an example of how to implement a pwm feedback
using opto?
How to integrate the pulses to achieve a correct "copy" of the
original error signal.
This must require a carrier frequency more than 10 times the switching
frequency of main converter?

I appreciate any advice.