The general purpose of a controller in a power supply is to try and ensure that the output voltage of the power supply follows a reference value.
A conventional arrangement employing such a controller is shown generally in FIG. 1.
In the arrangement shown, the power supply circuit has a switching circuit 10 which provides a regulated output voltage VOUT to a load 8 from an input voltage VIN. As would be familiar to those skilled in the art, the switching circuit, also referred to as the power stage, employs one or more switching devices configured in combination with one or more storage elements (e.g. inductors and capacitors) to convert an input voltage to an output voltage. The switching elements and storage elements are arranged together in a switching topology. Examples of switching topologies include, for example but are not limited to, Buck, Boost and Flyback.
A controller 6 is employed to control the operation of the switching circuit 10. Generally, the controller supervises the switching operation to regulate the output voltage. Internally, the controller employs a feedback loop that compares the actual output voltage VOUT with a desired output VREF to derive an error voltage Verr which is used by the controller to generate switching signals for the switching circuit. These switching signals are conventionally pulse width modulation (PWM) signals although other modulation schemes are known.
There are two main approaches to operating a controller which are referred to respectively as voltage-mode control and current-mode control.
Voltage-mode control is the traditional approach employed. In Voltage-mode control, a feedback loop is provided by feeding the error voltage Verr to a compensator, which for example may be a proportional, integral, derivative (PID) compensator which in turn provides a compensator output which is used directly to generate the PWM signal, i.e. the duty cycle of the PWM signal is proportional to the compensator output. Generally, the compensator output is compared with a generated ramp signal in a comparator to generate the PWM signal.
Voltage mode control suffers from a number of disadvantages including, for example, slow response to line and load variations. In particular, any change in load is only detected by a change in output voltage and then corrected by the feedback loop. The output filter of the feedback loop adds two poles to the control loop requiring a Type III compensation. This usually means relative slow response.
Current-mode control was developed in the 1980's to address the disadvantages of voltage mode control.
Current-mode control operates by introducing a second feedback loop which feeds back a measurement of the inductor current. The fed back measurement of inductor current is provided as one input to a comparator used to generate the PWM signal with the error voltage providing the other input. Thus in simple terms, the fed back measurement of inductor current effectively replaces the generated ramp signal in voltage mode control.
Current-mode control addresses a number of disadvantages of voltage-mode control including, for example, that there is typically just one pole as opposed to two in voltage mode, that Type II compensator may be employed and the power stage dynamics do not change in discontinuous mode conditions, inherent Vin Feedforward and accurate current sharing. It directly controls the output current rather than voltage, and the power stage is only a single pole to the feedback loop. This allows simpler and higher bandwidth compensation (one pole and Type II) over a comparable voltage mode circuit.
The present application is directed at the mechanism by which the measurement of inductor current is provided in a current-mode controller