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UCC28070: We are verifying current variations in channel A and channel B

Kaji@PAN
Mastermind 7330 points
Part Number: UCC28070
Other Parts Discussed in Thread: OPA172, OPA2172

Hi,

I previously asked in the thread below about current variations in Channel A and Channel B.
After that, the symptoms improved by adjusting the constants according to the actual inductor value for the settings of RSYN, RIMO, CAOA/CAOB.
However, again, the lighter the load, the more variation between channels will occur.
So I would like to ask you a question.
The first is the layout.
In the layout I designed, the length of the wiring to the current sense resistor and the operational amplifier is different between channel A and channel B.
Does this difference in wiring length affect the current variation between channels?
The second is about operational amplifiers.
The reference design operational amplifier used OPA172.
I used OPA2172 for cost reasons.
I am attaching the waveform of the output of a constant load op amp, but the output seems to be affected by another channel.
Do you think this waveform is affected by the OPA2172 crosstalk?
There seems to be no mention of crosstalk in the OPA2172 data sheet.
Also, does crosstalk affect current balance?

Best regards,


  • 0
    TI__Mastermind 34465 points

    Hello ,

    Yes of course the layout is extremely important if you wish to accurately detect the shape and value of the MOSFET current if 
    you are using a current shunt type of method.
    Because the shunt is placed directly in the power circuit and directly connected to power ground then it is very susceptible to noise pickup.
    Additionally you are measuring millivolts of a signal in a high dV/dt environment .
    For these reasons the reference design I sent you has:
    (1) Separate high speed OPAMPS with differential inputs positioned directly on the sources of the boost MOSFETS.
    For this reason there is no crosstalk between OPAMPS
    (2) Careful and balanced layout from the OPAMP outputs to the CSA,CSB inputs.
    These issues are usually not a problem with current transformers because the CT provides an isolated signal between power and signal grounds.
    The CT also provides a magnified signal without the use of an electronic amplifier

    Noise pickup will result in inaccuracy and poor performance but it can be controlled by careful selection of components and a careful pcb layout.

    Regards

    John