I am debugging a VIPer26-based flyback converter designed, which fails to regulate and is stuck at a lower voltages.
The primary issue is a severely overheating RCD snubber resistor, indicating excessive power dissipation. Paradoxically, the Vds waveform on drain does not show a sharp, high-voltage spike; instead, the waveform appears to be clamped at a high level.
My theory is that the reflected output voltage is much higher than designed. The snubber, which should only be clamping the fast leakage inductance spike, is instead being forced to conduct for a longer duration to clamp the entire primary voltage plateau. This would force it to dissipate an enormous amount of energy, explaining the extreme heat on the snubber resistor.
Additionally, the VDD supply for the VIPer26 is unstable. Instead of regulating at the expected 15V, it oscillates between approximately 10.5V and 13.5V. This behavior, often called 'hiccup mode,' suggests the controller is repeatedly trying to start and then shutting down due to a fault condition that i don't know what it is.
The observed waveforms for both the drain voltage and primary current significantly deviate from application notes and my own simulations.
Crucially, the system fails to exit the soft-start phase correctly, which, combined with the severe snubber overheating and VDD instability, points towards a fundamental power transfer problem (likely high leakage inductance). The strange part, however, is that a simulation built with the same component values—including the primary and leakage inductances measured from the actual transformer—works as expected
