• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 64 subscribers
  • Views 2274 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC2895 Asymmetrical operation

Guillaume Gien
Prodigy 60 points
Other Parts Discussed in Thread: UCC2895

Hello,

I am facing an issue with ucc2895 which corresponds to the problem raised in SLU275.

OUT C and OUT D are assymetrical when COMP is approx. 0.87V. As recommanded in SLU275, I have inserted a 1V DC offset to the ramp with a 2.2k resistor. The ramp now looks like what is shown in SLU275. Now, the operation is correct at 0.87V but the operation is assymetrical again when COMP is approx. 1.82V. Bascically, I have pushed the issue 1V above. Have you experienced a similar situation? (Normally pictures should be enclosed in this post).

PICTURES.zip

  • 0
    Prodigy 60 points

    Hello again,

    To sum up the situation, I am in Peak Current Mode Control and I operate in open loop i.e. EAN=EAOUT and EAP is controlled with an external power supply.

    No 400V input but I only supply VCC to circuit and control EAP with an external power supply. OUT C and OUT D asymmetrical behavior occurs when COMP cross the lower part of the RAMP :

    - When SLU275 is applied (@COMP = approx. 1.82V)

    - When SLU275 isn't applied (@COMP = approx. 0.87V)

     I do not know if the flatness of this area can explain this behavior (see picture below).

    This situation is similar to a Voltage Mode Control with a low RAMP signal, isn't it? So I wouldn't expect this behavior.

    Assymetrical operation at COMP=0.87V.TIF

  • 0 in reply to Guillaume Gien
    TI__Mastermind 31580 points
    Hi Guillaume,

    We will take a look at this, do you have a schematic of the circuit around the controller that you can share?

    Regards

    Peter
  • 0 in reply to Guillaume Gien
    TI__Mastermind 25570 points

    I read the app note, SLUA275 and you seem to have implemented it correctly.

    I don't know how much slope compensation you have applied, normally one adds a ramp whose slope is 50% of the output inductor downslope and in this case I would suggest you increase the amount of slope compensation applied to the RAMP and CS inputs and see if the behaviour improves.

    There is also a possibility that noise is being injected on to the CS or RAMP inputs but I'd check the slope compensation first.

    Let me know how you get on.

    Regards

    Colin

  • 0 in reply to Peter Meaney
    Prodigy 60 points

    Yes Peter. Please find two schematics below:

    1- Original schematic

    2- Schematic when testing with EAP tied to external supply