Our first objective is to find out how to design a current-loop compensator Gci in the current control loop of an average current mode controlled converter. But to do so, we first need to understand transfer functions of average current mode controlled converters. In particular, we need to understand how changes or variations in duty cycle, d, affect variations or perturbations in the current that we are interested in controlling. How do we actually establish the transfer functions of average current mode controlled converters? We can rely on our knowledge of converter transfer functions with a duty cycle as the input. So d hat represents a small-signal perturbation in duty cycle, and through converter transfer functions that we had studied earlier, we can find out how that d hat perturbation affects perturbations in the output voltage or, for example, inductor current. Now to close the current control loop around converter transfer functions that are based on the duty cycle d hat as the small signal control input, we sense the inductor current through an equivalent current sensing resistance Rf. The sensed signal is then compared to a reference, or control input we call vc. And the difference between the two is processed by the current loop compensator Gci. The output of the current loop compensator Gci is a signal that we call vm at the input of the pulse width modulator. In a small signal model, the model of a pulse width modulator is simply a gain, the gain being 1 over V sub M, where VM is the amplitude of the waveform in the pulse width modulator. Finally, the output of the pulse width modulator is the duty cycle perturbation d hat. So this is in a block diagram form, a small signal model of a switching power converter that has average current mode control loop around it. Notice that inside the control loop, the control to output transfer function is no longer Gvd, as we had in the voltage mode control converter, but instead is Gid, the response from d hat to the current, in this case, iL hat. So Gid(s) plays the role of a control to output transfer function, and that is the transfer function around which we will design the control loop, including the current loop compensator Gci. So here is the complete control loop around an average current mode control converter. Let's look at the loop gain. We will refer to that loop gain as Ti, i referring to that current control loop, to distinguish it from a voltage loop that we would refer to as Tv later on. To find loop gain Ti, let's identify the control loop. So the control loop goes through the current loop compensator, the pulse rate modulator, the control to output transfer function, Gid, then through the sensor gain Rf. And back to the point where we compare the sensed value to the control input, v sub c hat. So T sub i, here's the loop gain for this control loop. And we can write down Ti as the product of the gains along the feedback loop that is identified here. So we will have Rf, Gic, 1 over V sub m, and Gid. So in this control loop, the loop gain is the one that we will use to design for a desired cross-over frequency and desired stability margins such as a phase, margin, for example. Here's the feedback loop and the identified loop gain. So to design the current control loop, the loop gain that we had just identified will play the central role. The first step will be to find the duty cycle to current transfer function, Gid being d hat input, iL hat output, or if we have some other current, other than the inductor current, that would be in general case some i hat, the current of interest. Then, we would proceed as we did earlier in designing voltage control loops. The plot magnitude and phase responses of the uncompensated current loop gain. The uncompensated current loop gain is simply the loop gain identified here with the Gci, the current loop compensator set to 1. So the uncompensated loop gain is simply the product of Rf, the gain of the pulse width modulator, and the control to output transfer function Gid. Based on the shape of the uncompensated current loop gain, we will determine the required shape of the current loop compensator transfer function, Gci(s), and we will do so in order to achieve desired cross-over frequency and phase margin. In the next lecture we will illustrate the process using an example.