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CCS/SFRA: when I change the injection amplitude, the result may vary largely.

Daniel Wang
Prodigy 20 points
Part Number: SFRA

Tool/software: Code Composer Studio

Dear experts:

I have been porting the SFRA Library into my project recently, but there seems to be some problems with the results I got from the GUI when I did the open loop frequency sweep. Would you please be kindly enough to help me fix it.

Our project is a 2.5kW DC/DC converter using the three-level phase-shifted half bridge circuit, and the MCU is TMS320F28032PAG with the 61.1kHz PWM frequency. I’m trying to use the SFRA tools to get the open voltage loop of the converter. The problem is that when I apply different injection amplitude at the same DC point, the results I get may vary largely.

For example,

The bandwidth is 1979.74Hz, the gain margin is 8.70dB and the phase margin is 66.48deg with the injection amplitude being 0.015.

And the bandwidth is 1725.16Hz, the gain margin is 10.62dB and the phase margin is 75.10deg with the injection amplitude being 0.025.

And more over, the open loop curve I get from the GUI is much different from the results I have got by using a hardware FRA. So, I’m wondering is there something wrong with my implement. Please help me.

Thanks.

  • 0
    TI__Mastermind 43730 points

    Hi, 

    This is a tricky topic, the injection amplitude affects the result of the SFRA. There are two factors that play in here 

    1. Higher the injection amplitude, better the reading from an SNR perspective

    2. Higher the injection, more the disturbance to the output and hence it starts deviating from the assumption that only a small signal element it disturbed. 

    I'll be honest, the variation that you see in the reading if within what I would expect of the tool, especially if the injection amplitude is changed. 

    Now addressing the question, related to comparison with the hardware FRA can you elaborate on how much difference there is that you are seeing? 

  • 0 in reply to Manish Bhardwaj
    Prodigy 20 points

    Dear Manish，

    Thanks for your quick reply. I know exactly there must be a trade off when we talk about the amplitude of the injection signal. But the result I got using the SFRA tools is something much different from that by using hardware FRA, as you can see below.

     

     

     

    I am wondering if there is something wrong when I importing the code into my project.

  • 0 in reply to Daniel Wang
    TI__Mastermind 43730 points
    Daniel,

    Are you in open loop ? or closed loop?

    I have done these comparisons for LLC, Inverter, PFC, Vienna rectifier, Buck and many other topology so i do trust what SFRA is producing

    Please note, that if you are comparing Plant , it will be different between HW analyzer and SFRA, however the Open Loop Gain should be similar.

    Maybe you can post what is the code you are running?