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I am learning about control strategies for DC-DC converters.

While I find voltage mode control easy to understand, I struggle to understand current mode control.

enter image description here

Is the idea here to control the output voltage by controlling the inductor current?

If so, what is the relation between them?

JRE
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hakam zoubi
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  • Your diagram is missing a few steps in relation to your text. Perhaps you should expand it to include inputs and outputs? – Tim Williams Mar 28 '23 at 19:43

2 Answers2

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It is a vast subject to discuss but, basically, with voltage-mode control, the error voltage delivered by the compensator directly sets the duty ratio \$D\$. By doing so, you adjust the output power delivered by your converter, according to its dc transfer characteristic like \$V_{out}=DV_{in}\$ for a buck converter:

enter image description here

The duty ratio is elaborated via a pulse-width modulator (PWM) block made of a comparator and an artificial ramp pulsing from 0 to a peak value \$V_p\$. When the ramp intersects with the error voltage (a flat dc level in theory), then toggling occurs, turning the main transistor off. By changing the dc error voltage - the loop does that by monitoring the deviation of \$V_{out}\$ from its target - you directly adjust the duty ratio and ensures regulation. In this mode, you don't need to consider the inductive current \$i_L(t)\$ to operate the converter. You actually implement a current limit but for safety reasons and not for regulation purposes.

In current-mode control, it is different. The sawtooth is replaced by the inductor current which is also a ramp. This current can be directly observed by a current transformer or via a resistive shunt which delivers a voltage image. The error voltage will now set the inductor peak current cycle-by-cycle and will adjust the value based on the operating point: a high peak for a large output power, a low value in light-load conditions:

enter image description here

In this mode, you control the inductor peak current and indirectly the duty ratio \$D\$. If you operate the buck converter in voltage- or current-mode control, \$D\$ will be identical between the two converters for a similar operating point. It is only the way this duty ratio is elaborated that changes between the two. This time, you must monitor the inductor current cycle-by-cyle and it provides inherent protection to the converter.

Below is a quick summary between the two techniques and each bullet is a subject to expand in itself : ) You can have a look at my last small seminar on the subject:

enter image description here

Verbal Kint
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As a simple qualitative explanation, if you put an inner control loop on the inductor current, you turn the inductor into a current source (within the BW of the loop, etc.). The voltage loop "programs" more current in the inductor as the load increases to keep the output voltage constant.
At a high simplistic level, this has the effect of removing the inductor from the LC filter, and making the control-output transfer function a single-pole system, the current source feeding the output capacitor (and load).

On a less simplistic level, you have to add slope compensation to deal with potential subharmonic oscillations, and you wind up with a pair of complex poles at half the switching frequency. The pole due to the inductor actually just moves out to a higher frequency.

But current mode is easier to compensate than voltage mode and has better line regulation and disturbance rejection, among other advantages.

John D
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