Voltage Mode vs Current Mode Direct Comparison: Transient & Frequency Response

In this video, Dr. Seyed Ali Shirsavar from Biricha Digital compares Voltage Mode control against Current Mode control under varying input voltage conditions.

Two nearly identical asynchronous buck power stages are compared under experimental conditions. The first converter is stabilized using Voltage Mode control, and the second uses Current Mode control.

We then vary the input voltage and demonstrate how the frequency response and transient response of these two popular control strategies behave, discussing the results.

Note: The plant transfer function of a standard Voltage Mode converter (without voltage feedforward) contains our input voltage (Vin) as a variable. This means that our plant gain and therefore our crossover frequency change directly as Vin changes, as we show in this video. This is, of course, undesirable.

The plant transfer function of a Current Mode controller does not "directly" contain Vin. However, there is an "indirect" dependence. The Current Mode transfer function is dependent on the slope of the inductor current, the slope of the compensating ramp, and the duty cycle (as well as other variables). The slope of the inductor current and duty cycle, of course, change with Vin. The slope of the compensating ramp may or may not change with Vin, depending on how it is generated.

However, fortunately, these three variables often change in such a way that the impact of variations in Vin appears minimal on the Current Mode transfer function. This is, of course, serendipity, but it does not mean that we will be lucky all the time, every time.

As always, it is best to measure the transfer function under various input voltages to be sure.

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