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Slope compensation
When the duty cycle of a current-mode-controlled converter exceeds 50%, the converter will oscillate at a subharmonic of the switching frequency, actually half of the switching frequency, unless slope compensation is added. The origin of the problem can be understood thus. A current mode control loop works by turning off the switch when the current reaches a certain level (set by the error amp output). If the duty cycle exceeds 50%,the inductor current is being ramped up for more than 50% of the period. This means,obviously, that the inductor current is being ramped down for less than 50% of the period.This smaller time means that the inductor current has not yet returned to its steady-state initial value by the time the next period is getting started, so the current for the next period starts off too high. During this next period, therefore, the inductor current reaches the turn off level too soon, causing the duty cycle to be terminated early; in fact, it’s terminated at less than 50% of the duty cycle. But now the off-time is too long (> 50%) and so the current at the start of the next cycle is too low, causing the duty cycle to again exceed 50%, and so on, oscillating between under- and overshooting the current every other cycle.
Slope compensation fixes this problem basically by adding a fixed ramp to the current signal. Since this ramp has a constant value, the effects of variations in the current signal are better damped. In fact, the 9 effect of slope compensation is to make the control loop somewhat more like a voltage mode control. This makes sense: if you think about it, a voltage mode controller works with a fixed ramp against which it compares the error amplifier’s output. So adding more and more slope compensation brings the converter back closer and closer to voltage mode; if the ratio of slope compensation amplitude to current-signal amplitude went to infinity, you’d be back entirely at voltage mode. This explains the statement above that at low load power, your current mode control converter is back to voltage mode.
References edit
Practical Design of Power Supplies: Ron Lenk
External links edit