Integrated Inductive Switch-mode Power Supply. Fundamentals Feedback Loops : Voltage and Current Mode Controllers

There are many different strategies for controlling the power stage of a static switch-mode converter. Here, we consider switch-mode power supplies, with high integration thanks to semiconductor technologies, and with the objective of serving a regulated DC supply voltage to a consumer (a load). Regulating the output voltage involves taking a measurement of an electrical quantity, comparing it with a reference quantity and using the deviation to control the operation of the power stage in order to correct the output voltage.

The simplest approach, described below, is to measure the output voltage and compare this measurement with a reference voltage; often the reference voltage will be the value of the voltage to be obtained at output, or a fixed fraction thereof. The comparison, if linear (analog), will produce an error voltage. This error voltage will be used to drive the power stage through a modulation operation. This is known as voltage-mode control.

It is also possible, by the same means, to regulate the current in the load, if the current measurement produces an equivalent voltage (e.g. across a shunt). Current-mode control is another technique. Within the power stage, transistor switching produces voltage pulses across an inductance [D3182] . As a result, the current in the inductor varies linearly in segments. In steady-state operation, the inductance current increases linearly for a fraction of the time, then decreases for another fraction of the time, returning to the same initial value. The variation in inductance current reflects the energy transferred to the load. For a given load, the current supplied to the load translates into a voltage. This voltage (i.e. the converter's output voltage) can be regulated by controlling the inductance current. If the output voltage drops, the inductance current must be increased, and vice versa. This operation can be performed at a fixed switching frequency, but is of no particular interest compared to voltage control. It will be explained later that variable-frequency operation offers a more interesting degree of freedom. In this case, the comparison between the current image in the inductor and a reference value is non-linear. The "discrete" nature of current control immediately differentiates it from voltage control.

Approaches can be combined to provide additional performance to closed-loop control. For example, a first closed loop could control the current in the inductor...