Current-Mode Control for Switching Regulators
This article provides a primer on current-mode control, a widely used alternative to voltage-mode control that provides faster response to changes in input voltage and load current.
Introductory articles on switching regulators sometimes show diagrams that depict only the power stage, though if you’ve been reading my articles on switching regulator techniques and topologies, you know that these circuits require both a power stage and a controller. While it may be true that the power stage is the key to inductor-based voltage conversion, feedback-based switch control is the key to generating a predictable, stable output.
In my primer on closed-loop control, we examined and simulated a voltage-controlled circuit. This time, we’ll discuss a different control scheme: current-mode control, also known as CMC.
Before we jump into our main topic, let’s briefly review the most straightforward means of closed-loop control. It follows the steps below:
- The output voltage is fed back to an error amplifier via a resistive voltage divider.
- The error amplifier produces an error signal proportional to the difference between the scaled output voltage and a reference voltage.
- A comparator uses the error signal and an externally generated ramp signal to produce a PWM waveform that drives the switch.
- The changes in PWM duty cycle influence the output voltage.
When all of this is integrated into a properly compensated feedback loop, the regulator will lock onto the specified output voltage and automatically respond to line and load variations. This is what we call voltage-mode control.
Below, Figure 1 shows a voltage-controlled setup for a generic circuit.
Figure 1. Voltage-mode control scheme.
Though intuitive and effective, voltage-mode control has an inherent limitation: voltage changes are detected at the output, which necessarily changes gradually because of capacitance, and the effect of the primary control variable (the PWM duty cycle) is also observed at the output. The closed-loop control action must therefore propagate from the output all the way around to the output again. This slows the process down, making voltage-mode control a rather laggy method of dealing with line or load fluctuations.
CMC fundamentally modifies the control loop’s transfer function. Its basic premise is that by sampling the current of the inductor Lo inside the power stage (Figure 2) and incorporating this information into the feedback loop, the circuit can regulate the output voltage by way of the inductor current. In other words, the variable under direct control is the power stage's inductor current, and the output voltage adjusts itself as a consequence of adjusting the inductor current.
Figure 2. Example of a DC-DC buck converter power stage.
Compared to voltage-mode control, CMC contributes significant complexity to the design of the control system. Nevertheless, it is a feasible approach that improves response time and simplifies loop compensation without degrading circuit performance in any serious way.
Though the details will vary with the converter topology and the type of CMC being implemented, the diagram in Figure 3 should give you an idea of how current-mode control can be incorporated into a buck converter.
Figure 3. A current-mode-controlled buck converter.
A current-sense resistor (RSENSE) generates a voltage proportional to inductor current. Note that I’m using the term “inductor current” somewhat loosely—the current through the sense resistor isn’t always identical to the current through the inductor. In the diagram above, the sense resistor is on the output side of the inductor and in series with the inductor, and the voltage across RSENSE will always be directly proportional to instantaneous inductor current.
It’s also possible to locate the sense resistor so that it’s in series with the switch in the power stage. This produces a voltage proportional to the inductor current during the switch-on portion of the switching cycle. With a boost converter, however, the inductor is in series with the input supply. To be in series with the inductor, the sense resistor would have to be on the input side of the circuit.
As shown in the diagram, voltage feedback is still present—sensing the inductor current doesn’t replace sensing the output voltage. Instead, these two measurements are combined in a way that allows the loop to respond to output deviations by controlling inductor current. Next, we’ll discuss two distinct ways this can be accomplished.
Peak Current-Mode Control vs. Average Current-Mode Control
Peak CMC and average CMC represent two different ways of controlling inductor current. With peak CMC, inductor current—which is converted into a voltage by RSENSE and an amplifier—is compared to an error signal. From this, a PWM waveform is produced that turns off the switch when instantaneous inductor current reaches a specified magnitude.
With average CMC, the voltage corresponding to inductor current is delivered to an integrated current error amplifier. The output of this amplifier becomes an input to the PWM-generating comparator. An externally generated ramp signal provides the comparator’s other input.
The generic CMC diagram we examined above shows a peak CMC scheme. Average CMC would look more like Figure 4.
Figure 4. A buck converter with an average CMC, as opposed to peak CMC, control scheme.
Average CMC addresses peak CMC’s shortcomings, but it isn’t necessarily superior—as usual, there are pros and cons with each approach. Though average CMC offers significant theoretical advantages, these advantages don’t always translate into noticeably improved performance for physical circuits.
In this article, we reviewed voltage-mode control for switching regulators, explained why current-mode control is a desirable alternative, and went over some introductory information on how CMC operates. Next time, we’ll look at a SPICE schematic that will help us gain greater insight into the electrical behavior of a CMC buck converter.
All images used courtesy of Robert Keim