F28M36P63C2: Parallel operation of inverters

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Replies: 20

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Part Number: F28M36P63C2

Dear all,

I have two inverters that work well in stand-alone mode and i want now to operate them in parallel. All the control part is implemented in the Simulink add-on for concerto microcontrollers. The microcontroller i am using is the F28M36P63C2.

The problem i am facing is as follows: when i start operating one of the inverters (inverter 1) with the load and then i want to synchronize the second inverter ( inverter 2), inverter 2 needs to read the voltage at the connection point without connecting (i.e. without closing the relays). Even though at the output of voltage sensor of the 2nd inverter, the waveform is very good, in the CCS graph, every 10-15s this voltage gets very noisy for some seconds and then returns to normal. Thus, it is not safe to try connect.

Maybe the problem is related to the fact that i use the ePWM ADC SOCA for triggering the ADC inputs. Note that when the relays are open, the PWM signals of the second inverter do not correspond to the connection point voltage, which i am trying to read (the pwm signals of the second inverter are not those that create the connection/load voltage i am trying to read, which is created by inverter 1). I am not sure if this exactly is the problem or how i should proceed.

Has anyone faced anything similar before? Any hint would be very helpful.

Thanks in advance.

Alex

20 Replies

  • In reply to Subrahmanya:

    Thanks a lot! 

    Note that i have figures and videos that i can share with you to help you through e-mail or skype.

    Alex

  • In reply to Alexandros Paspatis:

    Update: 5V to GND pins were probed. The uController power supply seems really stable and without any spikes.

  • In reply to Alexandros Paspatis:

    Alex,

    Picking up for Bharathi on this topic.  There is some clarification I'd like to make after reading through the thread.

    1)I'll send you a friend request through the E2E system. We can use that to share files/scope plots privately as you mentioned.  If possible I'd like to see the schematic if you are comfortable sharing.  If you can share just the MCU relevant pages that would be fine as well.  I'd also like to know the PN for the protection device(this may be in the schematic anyway), as well how the two MCUs co-exist in your system, i.e. isolated from one another, etc.

    2)Have you measure the output of the voltage sensor at the ADC pin or just at the voltage sensor itself?  It is very possible we are picking up noise on the trace and the signal is not the same.

    3)You mentioned that using the eCAP yields the same results as the ADC, I inferred you mean you also see the noise on this route as well.  Can you confirm this?

    4)You last post mentioned observing a clean 5V/GND for the chip that give the uC supply.  Can you also look at the 3.3V and 1.8V rails and VREFLO/VSSA signals at the C2000 MCU specifically the analog supply pins?  VREFLO inside the ADC is tied to VSSA, so any ground movement will result in a change in a converted voltage input, even if the input signal is clean

    I think the above is a good start, we can go deeper once I have some of the above info.

    Best,

    Matthew

    Helpful Links:

    Getting Started     

    Application Specific

    More Resources

    C2000 Training Portal Electric Vehicles All C2000 Reference Designs
    C2000 MCU Development Tools Industrial Drives
    C2000 Home Page Power Control

     

  • In reply to MatthewPate:

    Hi,

    Thanks a lot for you time.

    1) I have share information privately. I am waiting for your response.

    2) The output of the sensors is connected to a ribbon. The ribbon ground is connected to the microcontroller ground. I measured the sensor's output pin of the ribbon with regards to the uC ground using a differential voltage probe. Some noise exists in the oscilloscope after 1Mhz but the same noise exists in the 1st inverter (connected) as well which does not exhibit this problem.

    3) Confirmed!

    4) I ll test the 3.3V. Where can i find the 1.8V rail?

    Thanks.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    Any update?

    Alex

  • In reply to Alexandros Paspatis:

    Alex,

    Q4) I ll test the 3.3V. Where can i find the 1.8V rail?

    A4)You'll need to look at the VDD18 pins for this, although after thinking about this some more, I doubt these are causing any issue.  The important thing is that VDDA# pins and VSSA# pins are clean.  VSSA# will also act as the ground reference point for the ADCs.

    Back to the issue with the noise coupling.  From my experience it is often best to place the ADC sample as near the swithcing point of the PWMs to get the max/min voltage levels and a true capture of the system.  However, the switching point of the PWMs is often the noisest condition in a digital power system so we run the risk of sampling that noise into our ADC.

    The observance of the noise coming and going, could either mean we are right on the edge of this switching event with our ADC sample and we "beat" in and out of this over time(which would indicate some amount of sampling jitter); or some element of the topology is coming online only under certain conditions then going away, coming back, etc.

    I would guess we are seeing the former, but you could comment based on what your code is doing if the latter is the case.

    We can try and move the ADC trigger away from this consistently, but to do so I think you will need to bring another PWM online.  In this case the new PWM could be a mirror of the existing one that is controlling the FETs, but place your compare value a few cycles prior to trigger the ADC earlier.  We can keep moving this around to see if we can find a "safe" zone for all conditions.  The drawback here is that the ADC sample won't be as close to the switch in time so there may be some loss in information we get back.

    Another alternative would be to see if we can observe this "noise" getting coupled to the signal or VDDA/VSSA pins and try and damp it/filter it.  This could be some added capacitance near/at the pin.  Something else to check in general is that all decoupling caps you alreay have are as physically close to the device as possible.  This will help reduce the amount of radiated noise we might pick up from the power electronics.

    Look forward to your thoughts on the above.

    Best regards,

    Matthew

    Helpful Links:

    Getting Started     

    Application Specific

    More Resources

    C2000 Training Portal Electric Vehicles All C2000 Reference Designs
    C2000 MCU Development Tools Industrial Drives
    C2000 Home Page Power Control

     

  • In reply to MatthewPate:

    MatthewPate

    We can try and move the ADC trigger away from this consistently, but to do so I think you will need to bring another PWM online.  In this case the new PWM could be a mirror of the existing one that is controlling the FETs, but place your compare value a few cycles prior to trigger the ADC earlier.  We can keep moving this around to see if we can find a "safe" zone for all conditions.  The drawback here is that the ADC sample won't be as close to the switch in time so there may be some loss in information we get back.

    Hi,

    Can you please explain me how to do this on the simulink support package? I already have 3 pwm's (one for each leg). Should i add a 4th with the same switching frequency and just initialise it somewhere else?

    Thanks.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    We tried by creating a 4th PWM block with exactly same settings as the other 3. We just changed the CMPA, which instead of coming from an input it was now specified via dialog. We tried many CMPA values inside our timer period range (i.e. 0-4687) and nothing changed. We had exactly the same issue.

    I have also sent you some extra information in a personal message.

    Let me know if you have any other idea.

    Thanks.

    Alex

  • In reply to Alexandros Paspatis:

    Following up from off forum, would it be possible to add a coupling capacitor across the isolated ground planes in order to provide a ground path for any noise coupling occuring between the two?  Will add others to the chain as well for their input.

    Best,

    Matthew

    Helpful Links:

    Getting Started     

    Application Specific

    More Resources

    C2000 Training Portal Electric Vehicles All C2000 Reference Designs
    C2000 MCU Development Tools Industrial Drives
    C2000 Home Page Power Control

     

  • In reply to MatthewPate:

    Hi,

    Could your please give me more details regarding the connection points of the capacitor and the nature  of the required capacitor?

    Thanks a lot.

    Alex