How to Install Optocoupler

A good start is the optocoupler component manufacturer websites as they have data sheets and application notes which give general information on how to use optocouplers in similar applications.

Further information can be found with an internet search using search terms like "SMPS optocoupler".

Thanks. Thats the first thing I did. However what got me confused is that the apps show a 5 V VCC and mine is 12. Not sure if the opto would take that also it seems that 12 v would be applied to the fet driver input. so, I'm confused. cd

5V logic is quite common so is often used as the benchmark for application designs (they cannot show all permutations otherwise the application notes would be the size of a book).

It shouldn't matter but the information should be in the optocoupler data sheet as to what the optocoupler inputs and outputs can handle. If in doubt, check the characteristic operating curves which should cover operation at the different voltage levels.

One of the major advantages of the optocoupler is that the input (control) voltage can be quite different to the output (drive) voltage while still maintaining a linear response.

There is is more than one style of opto coupler. If you have a wander over to Element 14 or RS component websites and trawl through there you will find something to suit I'm sure.

Just remember that they are a close coupled LED and (mostly) photo transistor. The LED's tend to be a tad current hungry for full saturation of the transistor (turning it on) compared to common garden variety indicator LED's.

A CN75GB(E14 # 1328377) has 5kv isolation a maximum of 90Volts C/E for about 100mA current draw with 60mA input drive. Its also pretty common.

Hope that helps.

GA.

Why not just stick a single transistor amplifier between the PWM output and the FETs.

Or have I completely missed the point.

Yes, sorry, you missed the point. I want COMPLETE isolation, not electrically connected. I.E., a opto isolator. No possible loading of the PWM. It's a simple question. How do I connect a optocoupler between my PWM and FET Driver to give the same PWM signal to the fet driver as I am now, using my 12 Vdc VCC ? Thats the question. CD

Can you post a circuit diagram of the area of the circuit where you want the optocoupler to be. that will help the rest of us to give you a more definitive answer.

Thanks Tubo - Sure. I'll post a pic of the circuit this evening but basically both the pwm and fet driver operate off a vcc of 12 Vdc. The output of the pwm TL494 (pin9) goes directly to the fet driver IRS21844 input (pin1). That pwm signal is 275 Khz square wave at 10 Vpp, 50% dc. Thats where I need a optoisolator to keep the fet driver from loading (distorting) the pwm output. It works good now. I just want it better. I think I can put a current limiter resistor between the 12Vdc to the collector of the opto and take the sig off the junction of the CL resistor and collector of the opto and connect that to the fet driver input. But I'm not sure if thats the best way to do it. CD