UCC28C40: UCC28C40 OUTPUT Abnormal waveform AT FULL LOAD (12V6A)

Hi Loly,

Does this behavior happen at lower current loads, 2A, 4A, etc?

What is the input voltage when this happens?

Jitter like this could be caused by several things.  Here is a list, from most likely to less likely.

1) Current ramp signal very low (Rcs is presently only 8.3 mohms, or 120A OCP threshold)

2) Control loop instability (can they measure the loop response?)

3) Transformer saturating (what is the current rating of the transformer?)

4) Noisy CS signal due to layout

5) Input voltage fluctuation

6) Too much or not enough slope compensation (depends on Vin when this occurs)

Regards,

Eric

Reducing R7 decreased the loop gain and the abnormal waveform disappeared.  So it is a control loop issue.  As Vin or the load changes the instability might return.  I recommend they measure the bode plot with a network analyzer to determine the gain margin and phase margin across all expected input voltages and loads.

Regards,

Eric

Hi again Loly,

It's easiest and most accurate to measure the loop response.  If they do not have a network analyzer in the lab the customer can model their system and compensation using TI's Power Stage Designer Tool.  See first picture below.

Choose FLYBACK > LOOP CALCULATOR.  For the compensation network, use TYPE II ISOLATED (W/ INNER LOOP) but set Rbias to a large value (like 1 Mohm).  See second picture below.

Regards,

Eric