SMPS feedback path

[ghabrius]

Hello. I have an idea of building an SMPS, found a nice IC for the half bridge topology but that's not the point.

The system will have two feedbacks; one for over-voltage protection, the other for voltage regulation. Now I see in the datasheet they have given an opto coupler for the voltage regulation feedback but there's a problem - I just hate thyristors. There are already 5 parts I need to order. Is there a way of giving the current to the opto without the use of thyristor, a transistor perhaps or even a series resistor from the supply output?


Some mfgs add like ten additional parts to fb circuitry, I mean how unstable can it be!

[ghabrius]

BTW I wanna have something like a potentiometer to trim the pwm level. Is it possible to implement? The IC controls the PWM acording to the running current through the COMP pin (0mA gives D=49% , 1mA gives D=0%).

[sawreyrw]

There aren't any thyristors in the circuits you posted. Are you sure you can design a SMPS?

[the_manta]

thats not a thyristor, that a variable Zener, typically a TL431. That topology is used in nearly all primary controlled SMPS with a optocoupler. It's easy to understand, the TL431 just gives reference and when the opto is turned on, the pulse width decreases. Why should there be a thyristor ? Are you sure you're able to build something like that ? A failure, wrong calculated inductors etc. will kill your switchers and bear in mind that this works on mains line.

I don't understand why you want to control the PWM level ? The PWM level controls the output voltage because it determines how long the current in the inductor will ramp up. You can't control that because it is simply in the feedback loop and is already controlled.
After a quick look in the datasheet, the COMP Pin isn't really for linear regulation, it's typical Optocoupler pin for the photodiode. Don't connect anything else here. If you want that, use a controller with Error-OP inputs like good ol' TL431

[ghabrius]

Oh I've made many of them but it's the first time I'm making this sort, the other ones I've made were push-pull based around the SG3525 and the PWM there depended on the comparator voltage input. I have transformer just for the task.. BTW the input voltage will be around 28V, not mains.

What the hell that looks just like a thyristor I never knew there were variable zeners.. So the TL431 should work independently on the desired output voltage?