UCC25660: UCC25660 Vcc drop

Hi Rock,

 Your query is under review. I will get back to you by 9/2.

Regards

Hemanth

Hi Rock,

I reviewed the schematic. Here is my response.

Debugging and recommendation:

1. Having no information on transformer turns ratio, I could only recommend the following. Along with VCCP, measure the following for debugging.

• Measure LO signal to see whether the converter tries to start. And observe whether the LO pulses do come and stop when the VCCP is at its peak.
• Measure OVP/OTP pin voltage and see whether the voltage gets close to 3.5V (OVP threshold). If OVP is confirmed, modify the OVP circuitry to handle higher voltages. Try replacing R65 with 15kohm (or open to disable OVP with caution) and observe OVP/OTP pin voltage along with VCCP, LO and confirm the startup. 
• Measure differential voltage across C40 with isolated voltage probe. If the circuit operates at no-load and enters burst mode and see the capacitor discharges below 8V, recommend design and use a higher value of bootstrap capacitor.
• Ensure transformer truns ratio is chosen correctly.

Other review feedback:

2. What is the target output voltage? The TL431 (U8) selected is only rated for 20V. If output voltage is close to 20V or higher, either use a higher voltage rated regulator or use a 12V zener clamp at optocoupler's anode to ground. 

3. Recommend to use 10kohm value for R55 to have higher current flowing through the opto primary.

Once you try above and get results, kindly share your observations.

Regards

Hemanth