I'm not sure from your description if the symptom you're experiencing is 'flux walking', but it may be. It is very difficult to avoid having to AC couple full-bridge converters, even with current mode control. Flux walking is the tendency toward a DC bias developing due to any imbalances in the primary waveform. Current feedback control theoretically prevents this, but, in practice, it fails to prevent it under any circumstances in which feedback gain is insufficient. I suggest that you investigate this possibility. Slope compensation is normally used to counteract the feedback control instability associated with duty factors near and above 50%. The high DF means that the system might have insufficient off-time to respond to input perturbations linearly, resulting in a Right Half Plane zero in the control loop transfer function. One way to implement slope compensation is to lower primary inductance, thereby increasing magnetizing current, which adds to slope compensation, since primary current feedback includes magnetizing current. Study this topic carefully. Many designers don't really understand what they're doing in this area. Paul Mathews
- Vote on answer
- posted
19 years ago