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TPS40210-HT: TPS40210-HT COMP Pin

Asier Portillo
Prodigy 90 points
Part Number: TPS40210-HT
Other Parts Discussed in Thread: TINA-TI, TPS40210

Tool/software:

Good afternoon,


My name is Asier, and I'm working on a simulation using the TPS40210-HT chip for a flyback. I'm using the TINA-TI simulation environment.

I'm having trouble designing the capacitor and resistor connected to the chip's COMP pin. The Application Note that appears in the chip PDF isn't helpful, since I'm using a flyback. Even so, I've done the calculations and tried the simulation, without getting the desired result.

I've also looked at your other Application Notes, but nothing is clear about how to design this capacitor and resistor.

Thanks in advance,
Asier

  • 0
    TI__Mastermind 31690 points

    Hi Asier,

    Thanks for using the e2e forum.
    Are you using an isolated Flyback topology (FB pin shorted to GND), or a non-isolated topology (FB connected to AUX winding)?
    For the first case, the compensation is placed on the secondary side at the optocoupler.

    I can recommend to use our quickstart calculator tool find the correct placement and values for the compensation:
    https://www.ti.com/tool/download/SLURB14

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 90 points

    Good morning,

    First of all, thank you for your response.

    The system is isolated, but due to internal issues, I can't use an optocoupler. I'm using a transformer for that type of isolation. After the transformer, I have a diode, a capacitor, and a voltage divider from which I get the 0.7 V needed for the chip's FB pin.

    That's why I can't use the calculations you sent me in Excel.

    Best regards,
    Asier

  • 0 in reply to Niklas Schwarz
    Prodigy 90 points

    I'd like to let you know that I'm also having issues stabilizing the output voltage.

    This voltage rises above the reference and then begins to drop. The voltage on the COMP pin behaves linearly, gradually increasing until it reaches a saturation voltage, which consequently saturates the duty cycle.

    Best regards,

  • 0 in reply to Asier Portillo
    TI__Mastermind 31690 points

    Hi Asier,

    I am slightly confused by your answer.
    You say you use a isolated design, but use a diode, capacitor and voltage divider to feed into the FB pin, which would then be a non-isolated design.
    Maybe I am not fully understanding your design implementation.
    Would it be possible to share the schematic for better clarity?

    If you cannot use the calculation tool and have problems with finding a stable compensation, you can also fine tune the compensation on bench.
    For this, you start with a very slow compensation network (just one 1nF capacitor at the COMP pin, nothing else).
    This makes the compensation very slow, but stable in most cases. From there you can measure the bode plot and start shaping the curves by adding the RC element to the network.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 90 points

    Good morning,

    I perform the oscillation with a transformer. I can't use an optocoupler due to internal design issues.

    At the transformer output, I have a voltage divider connected to the chip's COMP pin, with a maximum voltage of 700 mV to reach the reference.

    I'd like to send you the schematic, but due to internal issues, I can't.

    I'd also like to tell you that I've also been running tests connecting a 1 nF capacitor between the FB and COMP pins, but the simulation keeps giving me a convergence error. As I increase the capacitor value, the system stops displaying errors, but the Duty saturates before reaching the desired output voltage.

    I have another question. Should the COMP pin be connected to the FB pin or directly to ground? I mean, if I put a capacitor and a resistor on the COMP pin, do I then connect that wire to ground or connect it to the 700 mV of the FB pin? Some application notes for similar chips connect it directly to ground.

    Thank you very much.
    Regards,
    Asier

  • 0 in reply to Asier Portillo
    TI__Mastermind 31690 points

    Hi Asier,

    If you are not allowed to share the schematic on an open forum, you can also send it to me via the e2e private chat.

    If you optocoupler is used, I would recommend to connect the voltage divider from the transformer output to the FB pin, and not directly to COMP.
    If FB pin is floating, the regulation may become unstable.

    Here is an app note that presents several feedback loop methods for TPS40210 flyback:
    https://www.ti.com/lit/an/slvag06/slvag06.pdf

    Best regards,
    Niklas