Other Parts Discussed in Thread: LM5117
Tool/software:
My converter is designed for 25.2 V @ 20 A using LM5117 from a 40–51.4 V input. With one MOSFET per switch the board runs fine up to 7 A (output falls slowly from 25.2 V no-load to 24.49 V at 7 A). But when I parallel MOSFETs (two per switch) the converter works only up to ~2 A — beyond that output collapses. At 8 A the output drops to < 1 V.
We took design reference from WEBENCH® POWER DESIGNER
Measured output voltages vs load:
No load: 25.20 V
1 A: 25.08 V
2 A: 24.85 V
3 A: 24.76 V
4 A: 24.72 V
5 A: 24.68 V
6 A: 24.62 V
7 A: 24.49 V
8 A: < 1 V (collapse)
I’m posting this to get suggestions about root causes and practical fixes — here’s everything I tried/observed and my recommended troubleshooting list.
Key changes made:
1. Tried giving VCC supply externally
2. Changed C9 from 0.47 uF to 4.7 uF and also C17 from 100 pF to 1 nF
3. Changed switching frequency from 398.529 kHz to 298.028 kHz
The circuit was tested up to 9A at 298.028 kHz, but at 9.5A the output voltage dropped below 1V. However, the output voltage continues to drop across all load currents (eg., 1A, 2A etc.).