Other Parts Discussed in Thread: LM74680
Tool/software:
Setup:
LM74680EVM
AC source: Weakly coupled inductor, 0-30V, 60Hz
Load: 94mF output capacitor + light load
Problem: The LM74680's switching logic does not work appropriately with the large 94mF capacitor. Instead of normal rectification, experiencing sporadic micro-pulse charging behavior.
The AC waveform collapses since both top side MOSFETs are switched on and we see a sporadic spike in the AC input when a MOSFET pair briefly conducts. Regular diode bridge works normally, but LM74680 fails to operate correctly if controller is Enabled. The controller appears unable to handle the combination of weak coupling + large capacitance.
With large loads, the switching works fine up to a certain threshold. As the input power increases at some point, the switching starts to become inconsistent.
Solutions Found:
Option 1: Adding 110μH series inductor before capacitor enables proper LM74680 operation
Option 2: Reducing output capacitance allows switching logic to work appropriately
Questions:
Is there a maximum recommended output capacitance for proper LM74680 operation with weakly coupled sources?
Are there application guidelines for capacitor sizing with ideal diode bridge controllers vs. passive diode bridges?
The switching logic seems sensitive to the load characteristics in ways that passive diodes are not.
