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.

LM25037-Q1: Output voltage drops when system is loaded

Part Number: LM25037-Q1
Other Parts Discussed in Thread: LM25037, TL431, UCC28C43, UCC28C54

Attempting to build an SMPS basically for the isolation feature. I already have a dc-dc buck in front feeding this supply as the input voltage is pretty wide so a two-stage approach was taken.

I'm using a LM25037 with the frequency set to 240 khz, full bridge and a 1:2 transformer. (Note I am not a power supply expert). The transformer is center-tapped on the secondary side. When I power the system on with no load it regulates the 12V Vin to about 11.8V Vout, which is expected given the feedback resistor tolerances and output diode drop. 

Isolated supply using a TL431 and TCMT1100 optocoupler. 

The duty cycle at the gate for the 4 fets is around 47% with no load. Once I apply 10ma of output current the output voltage drops 1V and by the time I am at 100 ma the output voltage is 3.2V.

Tried using the power stage designer tool to aid in the design but it is telling me the duty cycle should be 25.10% whereas I am seeing 47%. Although I could be interpreting it incorrectly.

The gate drive waveforms appear to be okay but the Vds waveforms appear to be incorrect. 

Not really sure if this is slope compensation problem on the ramp pin, a compensation problem in the isolated feedback circuit or a underline design issue. 

Any assistance in pointing me in the right direction would be appreciated. 

Gate drive signal

Signal going into the primary winding

If I move the input voltage to 15V and then back to 12V I some times get the following signal going into the primary side

(schematic can be shared if needed.)