UC3847 current mode control IC doubt on Vref


2008-02-19 6:47 am
Hi All,

I am currently designing a 300W half bridge SMPS, that would give me an output of 25V, switching frequency at 100 Khz. The IC that i plan to use is UC3847.....i have a few doubts :

The Vdc that i convert from the mains is 400V, and this voltage passes through the MOSFET's and primary winding of the transformer and the output of the secondary winding is 25V.

My doubt is when i use the control IC regarding the Vref ,since i need my output is 25V do i need to give an input of 25V to the Vref pin..im not very sure of the function of Vref here, because it says that Vref voltage is 5V. please advise.


2006-05-18 10:16 am
No offense intended ... but your question makes me wonder ... are you sure you're up to this?

Get http://focus.ti.com/lit/ds/symlink/uc3847.pdf and look at the block diagram. Vref is an output from the built-in 5.1V regulator feeding off Vin.

Then look at http://focus.ti.com/lit/an/slua075/slua075.pdf which should show you how to close the regulator loop in a circuit without isolation of secondary to primary(!!!)

If the above does not trigger the famous spark of insight, please do yourself and us a favour and work with 12V DC, not 400V. The stuff is LETHAL - that is legalese for "it will kill you" :D

Oh BTW ... you surely realized that unlike the UC3846, the UC3847 has active-low outputs, right?