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.

UCD3138: HOW TO ADAPT UCD3138HSFBEVM-029 FOR 48V OUT WITH RECTIFIER DIODES

Part Number: UCD3138

We use the UCD3138HSFBEVM-029 evaluation board to design our own DCDC converter working with same input voltage but at 48Vdc outputvoltage instead the original 12Vdc.

We replaced some control components for this new output voltage and power components like main transformer, output DC capacitors and sync rectifiers mosfet. The rectifier mosfets have been replaced by high speed switching diodes.

Doing that the system doesn't work properly. The duty cycle of input primary voltage is not stable. It seems that the duty cycle changes continuously making the operation unstable (discontinuous mode).

Thefirmware uses "feed forward Vin sensing", but in the originl design the Vin_CT signal is an square waveform clean and clear, but using diodes instead of Syc Rectified mosfets, the waveform is really unstable. Thats why we think that the input voltage is not well measured and we cannot load 100% of lad.

We suspect that this is because the output PWM signals depend on Vin_CT.

Can you please recommend the required modifications on the original firmware to make it possible 48V output voltage and rectifier diodes on the secondari side instead of Mosfet?

Thanks for your comprehension!!!

  • Hello,

    since there is no waveform here, I can only imagine what is going on. Frist of all, the vin-ct is feedback signal for Feed-forward control Loop. It must stabilize and maintain the voltage below 1.6 V. System current continues when using the SR. However, when using a diode, the system current is dis-continued. This may be a square waveform that you can see using the SR. But you see the very small duty with diode.  To verify if the problem is caused by the VIN-CT, you can place an external DC voltage on the VIN-CT and adjust the voltage level of 0.7V-1.56V-0.7V to check if the loop is stable. If this verification is stable, you may want to consider using an SR. 

    thanks