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UCC28C43-EP: Higher Output Voltage Ripple

Asha C
Intellectual 435 points

Part Number: UCC28C43-EP

Hello,

We are using UCC28C43MDREP in our design.

image.png

 

image.png

Input voltage range : 180V - 320V , Switching frequency : 100kHz

Output voltage1 - 15V , Max Load Current - 0.5A

Output Voltage 2 - 20V_ISO , Max Load Current - 0.120A

We are using a transformer with inductance 2.2mH and turns ratio as

36(Primary) : 3(Output Voltage1) : 4(Output Voltage2)

When tested at below conditions:

At Conditions:

  1. Vin = 320VDC, Output Load1(15V) = ~330mA and Output Load2(20V_ISO) = 80mA . Ripple of 1.182V is observed on 15V Output.
  2. Vin = 180VDC, Output Load1(15V) = ~330mA and Output Load2(20V_ISO) = 80mA . Ripple of 1V is observed on 15V Output.
  3. Vin = 180VDC, Output Load1(15V) = ~50mA and Output Load2(20V_ISO) = 80mA . Ripple of 0.782V is observed on 15V Output.
  4. Vin = 320VDC, Output Load1(15V) = ~50mA and Output Load2(20V_ISO) = 80mA . Ripple of 1.022V is observed on 15V Output.

image.png

Ripple frequency is varying from 200Hz to 2kHz in the above conditions.

Currently we have 47uF(tantulum) + 47uF(tantulum) = 94uF output capacitor on 15V line.

 

To debug the issue:

  1. We replaced one 47uF with 100uF(tantulum) resulting in 147uF output capacitance. No major  improvement observed on ripple. Still ~1V observed on 320VDC Input at ~330mA load on 15V.
  2. Later added additional 22uF Ceramic capacitor on 15V Line with ESR of about 0.2ohm at 200Hz. No major improvement observed. 

3. Later , we modified the compensation circuit connected to FB and COMP Pin, by replacing C72(Ccomp) to 2.2nF( to reduce ripple). No imprvement observed there as well. Also we are observing COMP voltage varying from 4.46V to 2.3V in this condition. Why is it so?

image.png

4. Since no improvement observed in any of the conditions, we removed both C72(Ccomp) and R53 (Rcomp) retaining only C53.  At this condition , Vout ripple reduced to 500mV but not clean. Also Comp voltage varies from 4.720V to 280mV.

image.png

Could you please help us on reducing the ripple as soon as possible.

 

 

 

 

  • 0
    TI__Genius 9715 points

    Hi Asha, 

    There is some immediately apparent instability at the COMP pin, which corresponds to the same fluctuation in output voltage that you are seeing. 

    What is the value of the current sense resistor that you are using? Could you additionally share your transformer turns ratio?

    Thanks,

    John

  • 0
    Intellectual 435 points

    Hello Gomez,

    We are using 2ohm Current sense resistor and Turns ratio is 36(Primary):3(Output1- Sec1):4(Output2 - Sec2) which is 36:3:4.

  • 0 in reply to Asha C
    Intellectual 435 points

    I have attached the schematic as well in the previous reply.

  • 0 in reply to Asha C
    TI__Genius 9715 points

    Hi Asha, 

    Thanks, what is the magentizing inductance of the transformer and what is the value of the capacitor in parallel with the bottom Vout voltage divider resistor?

  • 0 in reply to John Gomez
    TI__Genius 9715 points

    Hi Asha, 

    Your current sense resistor value is likely way too large at 2ohm. It could be causing instability in the control loop.

    Please try changing this resistor to about 500mohm, then re-test, and report back. 

  • 0 in reply to John Gomez
    Intellectual 435 points

    Hello Gomez,

    We are having 2.2mH inductance for the transformer and the capacitor parallel to voltage divider resistor connected to FB Pin is DNP currently.

  • 0 in reply to John Gomez
    Intellectual 435 points

    Hello Gomez,

    In order to limit the primary current to <0.4A , we have set the current sense resistor to 2ohm. 500mohm would set the overcurrent limit to higher value.

  • 0 in reply to Asha C
    TI__Genius 9715 points

    Hi Asha, 

    The voltage threshold for detecting an overcurrent event is 1V. If you wanted to set the overcurrent protection threshold at 400mA, you would have to set your current sense resistor to 1V/400mA = 2.5ohms. 

    However, this is besides the point that I am trying to make. In order to understand why you are seeing significant oscillations at the COMP pin (which are causing oscillations in your output voltage) we need to isolate the root cause of the issue. For this, I am recommending to try a smaller current sense resistor value, to observe if there is any change. 

    Please try the 500mohm CS resistor such that we can alter the characteristic of the entire feedback loop and observe any changes. Please collect the same COMP and output voltage scope shot after you have made this change.

    Thanks,

    John

  • 0 in reply to John Gomez
    Intellectual 435 points

    Hello John,

    Okay , we shall do the suggested modification to current sense resistor and come back to you with the results.

    Thank you!