Replacing an optocoupler in regulation loop of smps?

[treez]

Hello,

I wish to use this pwm controller in an smps, (circuit connection as on page one of datasheet)

UC35705 datasheet:
https://www.ti.com/lit/ds/symlink/ucc35705.pdf

However, i do not need isolation , and do not want a bulky poorly toleranced component like an optocoupler in the circuit, ........

...so what can i use that will behave like an optocoupler, but is not an optocoupler..?

eg, are there current mirror modules that could be used here?

I am looking for the cheapest alternative way.

 

[ted]

Basically, on the right side of the optocoupler (=LED side) the system has essentially a voltage reference (such as TL431) monitoring the output voltage and controlling the LED current accordingly,

You could get the same effect by just using an ordinary transistor instead of optocoupler's output transistor, and regulating the base current of that transistor directly. Because an optocoupler has very low current gain (<<1), and any transistor has much higher current gain (typ.many tens to few hundreds), you have to re-design the voltage monitoring circuit accordingly. Key here is right level of gain in the feedback part for loop stability and good enough regulation. You have also consider, while doing that, the wide range of transistor gains. However, compensating well enough for that gain variation should be reasonably easy.

Good luck!

ted

 

[goldsmith]

Hi treez

However, i do not need isolation , and do not want a bulky poorly toleranced component like an optocoupler in the circuit, ........

Why tolerance ? you can make very high precise SMPS feedbacks with an opto couplers and a TL431 or other types of zeners diodes . but if isolation isn't important for you , you can use two simple resistors .
Best Wishes
Goldsmith

 

[FvM]

UCC35705 is special in so far that the designer omitted the refernce and error amplifier present with industry switch mode controllers. As a first approach you can connect a wire from TL431 cathode to FB input. But the duty cycle will not go down to 0 % with this simple circuit. Depending on your application requirement, a small voltage gain between TL431 and FB may be needed.