1. Home
  2. Electronics Design
  3. Opto-couplers in smps...

Opto-couplers in smps...

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

Reply to
Steve
Loading thread data ...

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

Reply to
Anirban

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

Reply to
Paul Mathews

It changes with time.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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 :-)

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Hide 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

Reply to
Paul Mathews

rd"

com

f=2Ecom-Hide quoted 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.

Reply to
Steve

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.