Current Mode Control : Ideal Circuit

Current mode control of a SMPS is a popular technique built into several PWM integrated-circuits nowadays. This topic should have been covered in the previous page, but has been held over and presented now.

The block diagram of the current-mode control is similar to that described the previous page on the ideal step-down SMPS circuit. The PI controller illustrated in Fig. 22 of the previous page can used be as it is. In the previous page, closed-loop control was effected by comparing the output of the PI controller with a ramp signal (Fig. 19). For current-mode control, the output of the PI controller should be compared with the signal reflecting the current though the inductor L2, with this signal suitably scaled. This signal is compared with the PI controller output and when the signal corresponding to inductor current tends to exceed the PI controller output, the MOSFET is turned off for the rest of the output cycle. At the start of each cycle, the MOSFET should be turned on.

With just this type of control, the pulse-width tends to fluctuate from one cycle to the next, leading to oscillations in output. Ideally duty cycle should be equal to the ratio of desired output voltage to the source voltage, but the intersection of the PI controller output and the signal reflecting inductor current may not always occur after a time lapse from the start of a cycle that corresponds to the desired duty cycle. To overcome this problem, a component proportional to the time elapsed from the instant the cycle starts can be added to the signal corresponding to the inductor current and this sum can be compared with the output of the PI controller. Let us say that the maximum of the PI controller output be 10 V and let the signal corresponding to inductor current be 10 V when the inductor current is at nominal rated value. Let also a ramp voltage be generated such that it rises from 0 V to 10 V from the start to the end of a cycle. Then a fraction of the ramp voltage can be added to the signal reflecting current, the fraction being equal to the duty cycle which in turn is the ratio of desired output voltage to the source voltage. When the source voltage tends to vary over an input cycle, this fraction computed as the ratio of desired output voltage to the source voltage automatically gets adjusted and the correction to the signal reflecting the inductor current becomes the right adjustment.

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APPLET FOR PWM WITH CURRENT CONTROL