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TIDM-02008: RMS Calculation Error when running open loop voltage source inverter into PFC

Part Number: TIDM-02008

Hello,

I am using the TIDM-02008 in open-loop voltage source inverter mode. Our DC input is 380V and AC output is 240Vrms.

When connected to a specific load that has an EMI filter and PFC front end, the system triggers an AC Over-Voltage after running for a few cycles. I modified a bit of SW to capture the vSqrSum value used in the "power_meas_sine_analyzer.h" file's POWER_MEAS_SINE_ANALYZER struct. It shows that in the final cycle that reads an RMS value over our threshold of 270Vrms the vSqrSum value is not being reset at the beginning of the measurement, instead starting with the previous value of around 41-42. This causes the final RMS value to be read as 330Vrms.

Is there any hardware situation that may cause this issue to begin occurring?

Are there any known issues with the POWER_MEAN_SINE_ANALYZER struct that may cause this? 

Please let me know if more information is needed.

  • Hi,

    Have you seen this under the resistive load as well? and when it triggers OVP, do you see 330 Vrms in the actual measurement?

    Best,

    John 

  • We have not seen this under resistive load. The over-voltage protection only triggers when we connect to this new load. On the oscilloscope we do not see the RMS value reach beyond 240V.

  • First of all, I don't think open loop voltage source is not a good choice. This inverter mode was designed for grid tied application and the open loop was provided for validation purpose. If you need the standalone mode inverter, you may consider some other reference design.

    Did you see any distortion in the inverter output voltage? sine analyzer is based on the ac voltage and if it is distorted, the calculation can be wrong.

  • We are aware of the limitations of the open-loop control system and are working on modified SW that will close a current and voltage loop. 

    On the oscilloscope we do not notice any distortion when running a resistive load vs running our new PFC load. Pictures below show operation with and without over-voltage protection. The yellow and blue waveforms show output voltage and current, respectively. The output current is heavily distorted by the load, but to us this does not explain why the over-voltage protection would trip.

    Inverter disabling itself with over-voltage protection enabled:

    Inverter running with over-voltage protection disabled:

  • Try with the modification shown below

    TTPLPFC_ac_volRms_sensed_Volts  = TTPLPFC_vRmsAvg_pu * TTPLPFC_VAC_MAX_SENSE;

    Again, this is not the operating condition that we support and therefore, my support can be limited.