In this lecture, we will introduce average current mode control, a very popular method for switching power converters. Let's first review briefly the standard duty-cycle, or as we often say, voltage mode control approach for switching power converters. In the voltage mode control approach, the output voltage or some other voltage of interest is sensed with a sensor that has a gain H, and the sensed value is compared to a reference. The error between the two is processed by an error amplifier, which we also call voltage loop compensator, and the output of the voltage loop compensator, vm, is the input to a pulse feed modulator that produces the control signal, c, for a switch in the switching power converter. The duty-cycle, d, of the control signal, c, is proportional to the input voltage, vm. The design of the voltage loop compensated, Gcv, the Gcv being the transfer function of the voltage loop compensator, is based on small signal models and transfer functions that we have studied in converter controls, of course 3 of this power electronics specialization. So compared to the standard duty-cycle "voltage mode" controlled converter, average current mode control follows exactly the same idea. The same duty cycle control approach is applied, but it is applied to a converter current as opposed to converter voltage. So a general block diagram for an average current mode control converter is shown here. A converter current, i, which could be an inductor current or input current or output current or even a pulsating current through a switch, is sensed with an equivalent current sensing resistance, Rf. The sensed signal is compared to a reference or control input, Vc. The difference between the two, the error between the two, is closest by a current loop compensator with a transfer function Gci(S). The output of the current loop compensator, vm, is the input to a pulse feed modulator and a pulse feed modulator again produces the control signal, c, for a power switch in the switching power converter with a duty-cycle, d, proportional to the input voltage, Vm. So you see that the control loop in the average remote control converter, follows exactly the same pattern as we have seen earlier for standard duty-cycle voltage mode control loop, except that the quantity of interest is a converter current as opposed to converter voltage. The reason the control method is called average current mode control, is that in the process of comparing and processing the error signal between the sense current and the reference current, we employ a compensator that would typically have a low pass nature, is what would filter out high frequency ripple and pulsating components in the sense current. This in effect means that what is really controlled in the current control loop around the switching power converter is the average value of the sense signal. And that's where the name average current mode control comes from. Now, in most cases, in addition to controlling a certain value of the current which by itself can be very useful, we still want to control a voltage, for example, an output voltage. How do we do that? Well, we can simply close an additional outer loop around the current loop control converter. So, a converter that has an average current mode control loop has a convenient input, the reference point, the reference signal for the current control loop, which can be enacted upon by an outer voltage loop that senses the output voltage, compares to reference and produces that control input for the inner current control loop using an outer loop voltage loop compensator Gcv. So, when you look at this blog- the diagram here, you'll see what we call a two loop structure around the switching power converter with an inner current control loop followed by an outer voltage control loop. In summary, in the average current mode control approach, it is the average current, rather than the peak current, that is controlled to follow a reference, which brings up some advantages compared to peak current control; noise immunity is better, the averaging or low pulse filtering in the current control loop means that the control is less sensitive to spikes, ripple or pulsating nature of the sense signal. And the average current mode control can be used in many applications to control the average value of the current precisely, even when that current includes significant ripple or can be even pulsating. The current can be with a converter operating in continuous conduction mode or discontinuous conduction mode. There are some disadvantages in average current control compared to peak current mode control. First, the average current mode control does not provide immediate peak transistor current limit. We can place a limit on the control input to the average mode control loop, and by doing so we can limit the average value of the current. But we don't have a way to limit the peak value of the transistor current. And so, in some cases depending on the application, it may be necessary to include an additional protection feature, where the sense current peak value is fed to a comparator. The output of a comparator can then be used to enact protection features over the converter. The second disadvantage is that the average current mode control does not mitigate transformer saturation issues in push-pull type converters. Applications of average current mode control are very broad. Average current mode control can be, and is applied very often in DC-DC converters. It's also applied in AC-DC rectifiers, so-called low harmonic or Power Factor Correction or PFC rectifiers. And furthermore, it is very often applied in DC-AC inverters. In the lectures that follow, we will see all three applications of the average current mode control approach. To get started, we will first look into the details of how to design the current loop compensator Gci. That's the first task. And that task will in fact apply to all applications including DC-DC, AC-DC rectifiers and DC-AC inverters.
