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LT1681 Forward controller in offline use? (low current sense threshold of 150mV)

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treez

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The primary current sense threshold of the LT1681 Two Transistor forward controller is 150mV max.
Do you think this is too low (ie due to possible noise issues) for use in offline SMPS’s of 300 to 600W?
Also, could the low sense threshold noise problem be in some way mitigated by using a current sense transformer rather than a sense resistor?

LT1681 datasheet
https://www.analog.com/media/en/technical-documentation/data-sheets/1681f.pdf
 

Won't the built in gate driver also run into problems driving larger fets too? That low side sensing scheme is used in converters in the kw range. The controller has blanking which should help.

I'd look into adding filtering. An RC in the sense line and/or a C in parallel with the shunt to directly squelch fet C at the source (poles set to >5X switching frequency perhaps). I'd see if its possible to join the control ground to the power ground at the location of the shunt and/or place the shunt between the controller and the power stage.
 

Yes i would use an external gate driver...regarding the low current sense threshold...if 150mV is ok, it makes you wonder why so many ICs, (eg the ubiquitous UCC28C4X range) have a 1 volt current sense threshold maximum.
I mean, if 150mV max is acceptable, then why ever use anything higher?, after all, 150mV offers low current sense resistor dissipation.
 

Well nothing is a black and white in this world. The question comes down to how much effort it takes to get it working reliably.

As an aside its like layout. I get annoyed when people say "it fits" or "doesn't fit". No. As space gets tight it goes from fitting in 5 days layout to fitting in 10 days layout (with some design compromises) to fitting in 15 days layout (with more design compromises) etc. It can almost always fit...
 

Thanks, i think we would agree that using a current sense transformer would always reduce noise problems with the current sense pin?
(assuming layout and all other things were equal)
 

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