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UCC28220: Boost converter poor regulation feedback

Thomas Liao
Prodigy 10 points
Part Number: UCC28220

Tool/software:

I am building a 6kw boost converter that takes 120Vin and produces 340Vout, however I am having some issues with feedback on the UCC28220 chip. I'm using a 200kHz switching frequency, with 10uH inductors on each phase. For current sensing, I have a ACS758ECB-200U-PFF-T hall-based current sensor.

For some reason, the output voltage varies drastically under load. If the no-load setpoint is 80V, under varying loads, the voltage will sag down to nearly the input voltage. It appears as if the feedback loop is very slow to respond, as when removing the load, the voltage will climb for several seconds before slowly approaching the no-load voltage again. 

To rule out any current sense issues, I have tried disconnecting the current sensors from chip, so the CS pins both have 0V. Below is a screenshot of the schematic - I'd be happy to provide both the full schematic and layout if needed. Please note that I have changed the resistors for UVLO and OVLO, and I can confirm that these pins are within the operating range, and soft-start is present.

If you could suggest a way to fix the voltage variation problem, that would be greatly appreciated.

  • 0
    TI__Guru 73475 points

    Hi,

    It looks IC2 is your feedback loop error amplifier. If so, the first step is to make sure IC2 feedback (pin 2 and 6) follows V_Ref. You need to get this work normally so to make sure the feedback loop can work normally is your first step to check. If it is not following the V_Ref, then the Vout cannot be regulated.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi Hong, I just confirmed that the voltage does indeed follow V_Ref - I have that currently being supplied via an external power supply and I observed that the voltage on pin two and six does follow whatever voltage is being applied.

  • 0 in reply to Thomas Liao
    TI__Guru 73475 points

    Hi,

    IC2 should regulate pin 2 and pin 6 to the same as V_ref. If pin 2 and 6 voltage = V_Ref, but the Vout not constant, then your feedback circuit does not work properly since when Vout drops, pin 2 and pin 6 will drop then IC2 should generate signal to ask bigger duty cycle to make Vout back to regulation point but your test shows Vout not regulated even pin 2 and pin 6 = V_Ref - this means your feedback loop circuit is somewhere not correct - you need to trace back.

    You did not provide your full circuit, so I cannot provide further details how to trace back.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi Hong, thanks for the reply - I've attached my full schematic below, please let me know if that helps! Thank you again for your time.

    InverterBoostConverter.pdf

  • 0 in reply to Thomas Liao
    TI__Guru 73475 points

    Hi,

    Your converter is 120Vin and 340Vout, but you mentioned 80V - what is 80V? Is it Vin or Vout? Can you  set 120Vin and some small load (a small load that can still make Vout close to 340V) then provide the waveforms of Phase1_Gate and Phase2__Gate as well as Vds of Q1 and Q2 so I can have a look to see if they are normal. 

    What is your output current range?

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi Hong,

    Oh I had lowered vout to 80V for testing, however I have resolved the issue now. I was able to fix it by increasing the gain of the error amplifier, by changing R81 and R82 to 1M resistors each, and R34 to 2k, which allowed the op-amp to peg at the positive rail without any potential damage to the chip (the op-amp pegs at 10.5v as it is *not* rail-to-rail). For reference, V_Ref is 2.5v, and i did change the voltage divider resistor (R32) to 40k to achieve the ~340v output. Doing this tightened up the loaded/unloaded output significantly - there is still a +/- ~10v output depending on load but for my application this is acceptable.

    Thanks for your help!