Expand the Trigger Control Settings and configure it as shown below. Trigger 1 is triggered 1us after the start of PWM cycle. Trigger 2 is triggered 25us after the start of PWM cycle.
Click the Generate button. Click yes button in the confirmation dialog box and wait until Generation complete is prompted in the MCC output window. In the projects pane, open main. In the PID function, add the highlighted code as shown below. In applications that exhibit fast load steps, current mode converters will show relatively large output voltage fluctuation.
Noise filtering is needed and this results in certain limitations with respect to the minimum ON time of the upper MOSFET, which in turn limits the minimum duty-cycle range of the buck converter. The fixed slope compensation normally also poses a limit on the inductor values that can be used at certain input and output voltage conditions. Current mode converters also have advantages: the internal clock keeps the switching frequency very stable, regardless of the input and output conditions, which in some applications can be an advantage.
The internal clock can also be synchronized to external clock signals, making it possible to run several converters on the same frequency. CMCOT converters also contain a current sense and error amplifier, but now the falling slope of the current is compared to the output of the error amplifier.
Current is therefore sensed in the lower MOSFET which is much easier to do and less prone to noise pick-up, especially in low duty-cycle conditions. The fact that the system does not need to wait for a next clock-cycle makes it possible to react more quickly to sudden step loads; as soon as the output voltage drops and the error amplifier voltage rises above the falling current slope, a new ON time is triggered and converter current rises again.
The current valley must follow the error amplifier output, so gain and speed of the error amplifier affect the control speed. In the CMCOT topology, the maximum bandwidth that can be set by the compensation is related to the inverse of the ON time rather than the switching frequency as in current mode. So CMCOT converters can have higher bandwidth than current mode converters and show less output voltage fluctuations during fast load steps. CMCOT does not suffer from sub harmonic oscillation at high duty-cycles, and therefore does not need slope compensation, which allows a wider choice of inductor values.
In a pure fixed ON time topology, switching frequency would deviate considerably at different input and output voltage conditions. CMCOT also has some disadvantages: Since the converter controls frequency to regulate the output voltage, this makes it impossible to synchronize the converter to an external clock. The frequency control loop also means that the switching frequency can show some variation during load transients.
When the feedback signal falls below the reference, a new fixed ON time is generated and inductor current rises. If the output voltage has not recovered, another ON time is generated after a short blanking period until the inductor current matches the load current and output voltage is at its nominal level again.
Conventional COT converters need some output voltage ripple, in phase with inductor current, to switch in a stable manner. This requires output capacitors with some ESR.
These are summed and then compared with an internal reference. When this voltage sum drops below the reference, the comparator triggers the ON time generator.
A sudden drop in output voltage will immediately result in a new ON time, and the converter can generate successive ON times as long as the output voltage has not recovered. This makes the ACOT topology reaction speed to load transients extremely fast. A special frequency locked loop system will slowly adjust the ON time to regulate the average switching frequency to a defined value.
ACOT converters can exhibit large frequency deviations during fast load transients. When the end application is sensitive to certain switching frequency bands, this sensitivity should be checked with the ACOT converter in dynamic load condition, where the dynamic frequency swing is most pronounced. Key components output capacitors and inductor value in all three applications were identical, so the measurement results and differences are purely caused by the different control topologies, and can be directly compared.
Tags More Cancel. Share More Cancel. Similar topics. This thread has been locked. Expert points. Thank you, Keith. Hello Keith, You can use that circuit with a forward converter which is just another name for a transformer coupled buck converter.
The difficulty with the buck converter is that the switch current is not referenced to ground. As in the drawing below: So the figure you attached is not suitable for this type of buck circuit. Regards John.
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