UCC2897A: Unstable output at higher load

Navin Kumar2

Part Number: UCC2897A

I am using UCC2897A control chip for converting 80-120 V input to 12V output, and a maximum load of 25A. My schematic diagram is shown below: -

My output is going unstable when I am loading 12A at 120V or 15A at 100V. This is happening because the SS_SD pin is discharging and charging as shown below. (Green is the SS_SD voltage and Blue one is the output)

I am not able to understand why my SS_sd pin is even discharging when VDD pin is stable at 12.8V, Vref is stable at 5V. LINEOV is also in the limit. This is happening at different load condition at different input. (120V input and 12A output or 100V input and 15A output).

Please suggest us with probable reasons for this.

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Have you looked at the CS signal ?

Regards

John

0 Navin Kumar2 over 5 years ago in reply to John Griffin1

Intellectual 380 points

Hi John!

I have looked at the CS signal as well at two different sense resistors. Below are the voltage signals for two 17.5mohm and 35mohm sense resistors respectively. None of them is exceeding the limit.

0 John Griffin1 over 5 years ago in reply to Navin Kumar2

TI__Mastermind 34465 points

Hi Navin,

It looks like you have excessive switching noise on the CS input and this may be triggering an overcurrent

Please make sure the filter cap on CS is placed directly across pin 8 and pin 9.

This should eliminate any switching noise interference

Regards

John

0 Navin Kumar2 over 5 years ago in reply to John Griffin1

Intellectual 380 points

Hello John,

CS pin is controlling the current by cycle by cycle current limit or by hiccup mode. But in the Functional block diagram of the control chip, the voltage at the CS pin is not responsible for the discharge of the capacitor at the SS/SD pin. Am I correct?

If I am correct, then what could be the reason for the discharge of capacitor at SS/SD pin?

0 Navin Kumar2 over 5 years ago in reply to Navin Kumar2

Intellectual 380 points

Hello John,

I am waiting for your response.

0 John Griffin1 over 5 years ago in reply to Navin Kumar2

TI__Mastermind 34465 points

Hello Navin,

Activating the hiccup threshold on CS input will cause a restart of the controller prior to a discharge of the SS/SD pin.
And you can see that this is happening on your first scope plot.
Have you tried eliminating any of this high switching noise?
The section of the schematic you attached has nothing noteworthy about it but the noise on the switching waveforms are excessive.
I think the first thing you should do is investigate the source of this and try to reduce this noise interference

Regards

John

0 John Griffin1 over 5 years ago in reply to John Griffin1

TI__Mastermind 34465 points

Hello Navin,

I am closing this thread since there has been no new input in 3 weeks.

Please open a new post if you have any other issue

Regards

John

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UCC2897A: Unstable output at higher load