Where to put current sense resistor in Boost PFC?

[cupoftea]

Hi,
Which of the Boost PFC schems is best? One has the current sense resistor near the controller, and the other has the current sense resistor near the current sense transformer output. There is a decent ground plane under all of it.

Which is best, top or bottom schem?

LTspice sim and jpeg schem attached.

UCC28070A PFC controller datasheet

https://www.ti.com/lit/ds/symlink/ucc28070a.pdf?ts=1655880717342

 

[Rahul_d]

If you split your CT burden resistor (increase value by 2 & then parallel them at 2 different locations) & put one close to CT & the other one close to controller will give you better noise immunity, also there should be separate return track just below the positive CS track in the layout to prevent any impedance mismatch / noise

 

[cupoftea]

Thanks yes, that sounds good advice, at the moment, the burden res is at the CST....because putting the burden at the controller would have meant also routing the ramp signal back there aswell....i hope we dont get issues...we do have a light rc filter at the controller.

Do you think we will get noise problems?

I didnt want to route the burden res and ramp sig back, as it will eventually be a 4 phase PFC, and routing all that lot back to the controller will take up quite some room, and complicate the layout.

 

[mtwieg]

Between the two options I'd go for putting the burden resistor close to the CT. But I would also implement a star GND for the current sense transformer and burden resistor which connects to GND only near the pwm controller. Still two traces instead of one, but that's likely the only way to mitigate ground bounce.

 

[Rahul_d]

Thanks yes, that sounds good advice, at the moment, the burden res is at the CST....because putting the burden at the controller would have meant also routing the ramp signal back there aswell....i hope we dont get issues...we do have a light rc filter at the controller.

Do you think we will get noise problems?

I didnt want to route the burden res and ramp sig back, as it will eventually be a 4 phase PFC, and routing all that lot back to the controller will take up quite some room, and complicate the layout.

If you lay your CT tracks one over other (no other switching track should overlap or cross these tracks) you can use one burden resistor near controller & feed slop comp ramp into it.