WEBENCH® Tools/UCC28070A: Trouble in Replacing CT with current sense resistor

i dont have the current waveform. But i know it's like a sinusoidal but with zero cross at full load(2.2kW) and discontinuous on the positive side of sinusoidal at no load.i have attached the waveform at pin 8 and pin 9 at full loadf 

Hi Peter,

I have added the filter now and i have observed that the duty is varying (on time gate pulse) rapidly for constant load and constant input voltage. i have attached the waveform of gate 1, gate 2  and drain voltage of mosfet 1. Please advice to rectify it, also the PF is very poor in this regard(0.89).

Here i have attached the waveforms for which both the load and voltage are constant, but the duty (gate pulse ON time) is varying rapidly. 

1st channel- Gate 1

2nd channel- Fate2

3rd channel- Drain voltage (mosfet 1)

Thanks

Kumar.V 

Hi peter,

I am applying 230V ac to a bridge rectifier and converting it to 310Vdc and applying it to the interleaved circuit. Constant voltage means 310Vdc here. At lower load (100W i/p) I'm getting only 0.86PF and higher load (say 2300W i/p) 0.96 PF.

Thanks,

Kumar.V

Hi Peter,

I am using current sense resistor (0.012 ohm) at the source of MOSFET instead of CT. I am not using Frequency dither option, instead i grounded ram(2nd pin) and connected CDR Pin to Vref.Would that affect Power factor.Please review and help me to improve power factor, Now i am getting only 0.86 PF.

Hi Peter,

I have attached the input current and voltage waveform. There is a zero cross in the current waveform which affects the Power Factor.

Thanks

Kumar.V

Hi Peter,

I have attached the input current Waveform. The input current waveform has a zero cross in its sinusoidal wave which results in poor Power Factor.

Hi Peter,

I have reduced the capacitor to 2.2nF, also i have enclosed the oscilloscope plots on current sense pins (CSA, CSB).

But my Power Factor is still not Improving (0.94 at full load).Kindly help me in this regard.

These are recorded in the following conditions,

Pin: 2200W

VOut: 376Vdc

Thanks,

Kumar.V

TI UCC28070.pdfHi Peter,

I have attached the updated schematic for your reference. Kindly help me to solve this issue.

Hi Kumar,

I am concerned that the current sense circuit may not have enough bandwidth the INA181A current sensor only has a bandwidth of 350kHz.
This is too low for a 100kHz switching frequency.

You should modify the circuit to use a high speed OPAMP  such as an OPA172IDR  for this application. The High Speed Op-amp will be able to accurately track the PFC switch current. We can then look at optimizing the filtering of the CS signal at the IC pins, currently we have 1k and 1nF, we may be able to reduce this once we get a good quality CS signal.

I see only 10nF decoupling on Vref, please increase to 100nF

It there a decoupling cap on Vcc, you should use 1uF to 10uF, ceramic.

The Rsyn value needs to be calculated using the equation shown in this post

Regards

Peter

I think the main issue is the switching current is not being accurately sensed.

Hi peter,

Thanks for your valuable advice.

I have reduced the offset of the current sense amplifier to 0.1 V Previously it was 1.65V.

Due to this i'm getting 0.99 PF from 50% of my load.

Still i'm getting only 0.87-0.92 PF at loads less than 50%.

Meanwhile i have ordered the OPA172 amplifier as per your recommendation.

Please suggest me to improve power factor at lesser loads<50%.

Thanks,

Kumar.V