Linear relation in optical isolation

[GeoMX]

I'm designing a control for a switching power supply (buck), and I need to isolate the feedback loop from my control (PWM), but I need this isolation to give the same voltage it receives.

I've found the linear optocoupler IL300, but it's impossible for me to get it, so I'm trying to work using a 4N25, I've seen some other peopĺe who have worked with this one but I just don't know how to set it to have a linear behaviour (aproximately). Hope you can help me with this.

Regards.

 

[tunedwolf]

I would say the 4N25 would be a poor choice in a switchmode application, choose a superior optocoupler and save yourself a whole lot of grief.

 

[GeoMX]

Well, I must say you've just confirmed what I've been thinking about the 4N25 for my purposes . But, which one would you recommend? It's almost impossible for me to get the IL300 (haven't found a store where to buy it).

Thanks.

 

[audioguru]

The IL300 has feedback so it copes with the very high tolerance of the coupling of optical couplers and the intensity of its LED fading with time.

We don't look for special electronic parts in stores. We just order them and they are delivered the next morning.

You don't say where in the world you are, but Newarkinone in the US has thousands of them in stock. They are owned by Farnell who is worldwide so they probably have them all over the world.

 

[Papabravo]

Well, I must say you've just confirmed what I've been thinking about the 4N25 for my purposes . But, which one would you recommend? It's almost impossible for me to get the IL300 (haven't found a store where to buy it).

Thanks.

There are virtually no stores remaining that you can purchase components from anymore. The overhead is simply too high and the volume too low for this activity to be profitable. Digi-Key ships worldwide AFAIK.

I don't know of any claim that a 4N25 has a linear characteristic. We use them only for switches and seldom at that.

 

[GeoMX]

Well, I've ordered two HCNR200 from Newark, dunno how long it will take for them to arrive (I live somewhere in the state of Veracruz, Mexico , I tried local online stores but no luck), these are cheaper than the IL300.

In the meantime, I'll try to work with the 4N25, there should be a kind of relation between the input current and the transistor base current, but maybe it'd need some correction/adjustments. Thank you so much for all of your comments.

 

[Papabravo]

It's called the CTR or current transfer ratio. In a single family of optocouplers it can range from 20% to 800%. It degrades over time. Normally in the ordering information on the datasheet there are one or more suffixes documented which bracket the CTR to smaller ranges e.g.

A - 20% to 80%
B - 80% to 150%
C - 150% to 300%
and so on