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Switching Power Supply Ramp Compensation
Add time:2009-09-16    Click:336

About switching power supply ramp compensation issues, technical personnel can often be encountered, but essentially non-professional contact, this article is intended to allow the user to understand this problem.
Slope compensation definition: In current mode control, when the current reaches a certain size (set by the error amplifier output) switch off. If the duty cycle exceeds 50%, the rise time of the inductor current is greater than 50% of the whole cycle, cause: when the current mode control duty cycle exceeds 50% of the converter when the inverter switching frequency in the harmonic frequencies point oscillation occurs, precisely half the switching frequency in place, unless you take the slope compensation measures.

For the following reasons, the peak current control slope compensation must be considered.
Stability of the circuit, reducing the peak value / average error suppression harmonic oscillations, ringing the inductor current then the current fall time is less than 50% of a cycle. ' In the short period of time, the current has not had time to return to the static initial value, then the next cycle began again. The next cycle of the initial current larger. In this next cycle, the inductor current is quickly increased to the reference point, the on-time becomes short, the duty cycle becomes narrower; compared to the previous one cycle, the duty cycle is reduced to 50 % or less. But this has led to the off-time is too long, the next cycle of the initial current value is too small, but also makes more than 50% duty cycle once again. This cycle, the current cycle over an interval of a Japanese way too small oscillation occurs.

Operation: For this problem, the slope compensation basically a fixed ramp superimposed on the current signal. Since the slope of superposition is a fixed value, the impact of the current loop can be better suppressed. In fact, the true role of slope compensation is more like the control loop voltage mode control. Can be understood: voltage-mode control with fixed slope ramp and the output of the error amplifier compares the slopes so elected plus more and more time, more and more like the converter voltage mode control, when the slope compensation the magnitude of the current signal amplitude ratio tends to infinity, it turned into a voltage mode control. I just said they can also be explained: The power light load current mode control becomes a voltage mode control.