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Original question:

UCC28C43-EP: UCC28C43-EP: Higher Output Voltage Ripple

UCC28C43-EP: Continuation for UCC28C43-EP Higher Output Voltage Ripple

Asha C
Intellectual 445 points

Part Number: UCC28C43-EP

Hello John,

In continuation to our previous query:

UCC28C43-EP: UCC28C43-EP: Higher Output Voltage Ripple - Power management forum - Power management - TI E2E support forums

  1. Sense resistor was reduced to 400mohm , around 258mV ripple is observed when ~100mA loaded on 15V. Blue represents output voltage ripple and yellow represents comp pin voltage in the below waveform. Also we observe overshoot on rising edge and ramp down is not smooth. What is the reason for this behaviour.

image.png

2. When C53 was increased to 1uF , with R53 - 4.7k and sense resistor 400mohm, around 3.220V ripple  is observed on 15V output.

image.png

What is the reason for oscillating COMP Pin voltage?

3. Also due to reduction in sense resistor, there is higher primary current due to which secondary current is also increasing and secondary diode2(20V_ISO output) is going bad which is 100V , 2A Rated.

So reducing sense resistor would be challenging in our design since we have to size up other components.

4. We did not see ripple improvement either by reducing R53 to 1k or increasing C53 to 1uF.

Is there any other suggestion?

5. Just as a trial , we removed C72(10nF) , retaining same values as per schematic ie 2ohm sense resistor , having only C53 100pF on compensation circuit ,  we see ripple of maximum 500mV for maximum load of ~350mA. But in this case we see oscillations on COMP Pin as below. Also ramp up on ripple voltage is not smooeth. yellow is output voltage ripple and blue is COMP Pin voltage.

image.png

Can we go with this approach? will there be any issue because of this? Why are we observing oscillations on COMP Pin?

 

Thanks!