slope compensation

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

search the TI website under power management. There are plenty of explanations in the application notes on exactly what slope compensation is and how to implement it.

Hi Mook, I've already read some app. notes in this respect and wouldn't have posted if there were not so many important questions unanswered, at least for me. Rolf

Mook Johnson schrieb:

Without slope compensation, the current fully controls the pulse width. When you add slope compensation, you are effectively lowering the gain of the local feed back loop by reducing the amount of control the current has on the pulse width.

In continuous mode, the pulse width this cycle has an effect on the current in the next. This means that there is a phase lag. If you allow the gain to be unity or more when this phase lag reaches 180 degrees, this local loop will want to oscillate. It is near imposible to make the overall loop have enough gain at these frequencies to prevent this oscillation. As a result, you need to lower the gain of the local loop.

In a bucker, the exact same circuit can serve a different purpose. If there is an upwards step on the input voltage of a discontinuous bucker, the output of a current mode bucker will dip. This is because less charge gets through the inductor before the transistor is turned off. In a non-current controlled circuit, the output would rise. You can balance one against the other to make one that is less sensitive to the input voltage changes.

Slope compensation amplitude is applied relative to the slope normally present in the sensed current, which is the di/dt of the main filter/storage inductor. Normally the inductor current ramp is expected to be only a small portion of the peak or average current being sensed. Added slope greater than 1/2 the natural ramp is overkill.

If you are using your controller's absolute peak current limit as a limiter for output current, then any superimposed signal will reduce the output current by that factor, unless it is taken into account.

As this limit is seldom intended as an accurate feature, it is normally set considerably higher than the intended rated output of the circuit, in any event. Close tolerance current limiting requires additional circuitry, and is generally unneccessary in a fault protection feature.

RL