Methods and apparatus to improve power factor at light-load

#This disclosure relates generally to power factor correction, and, more particularly, to methods and apparatus to improve power factor at light-load.

#In electric power systems, power consuming loads are connected to power generating devices. While ideal loads are purely resistive, many loads have some level of reactance (e.g., capacitive reactance or inductive reactance.) When a load is reactive, energy storage within the load causes a phase shift between the voltage and current components of the power being provided to the load. This phase shift results in an increase in the current being provided, which in turn results in an increased apparent power supplied to the load compared to the real power that is being utilized by the load. The difference between apparent power and real power is quantified by a displacement power factor. The displacement power factor is one component of the true power factor. Additionally, Total Harmonic Distortion (THD) can contribute significantly to the power factor of a load. THD occurs in nonlinear loads which introduce harmonics into the power drawn from the power generating device. These additional harmonics result in increased apparent power being drawn by the load.

#Purely resistive loads have a unity power factor (i.e., a power factor of one), while reactive loads have a power factor of less than unity. Power companies charge based on apparent power and, therefore, charge more for increased levels of apparent power consumption. Thus, loads with a power factor less than unity may be more expensive to operate than loads with a unity power factor for the same real power input to the load.

#FIG. 1 is a block diagram of an example circuit including a power factor correction system.

#FIG. 2 is a block diagram of the example power factor correction system of FIG. 1.

#FIG. 3 is a diagram of an example power output over time by the power factor correction system of FIGS. 1 and 2.

#Many loads have different power factors. For example, resistive loads such as electric heaters have a power factor close to unity, while reactive loads such as electric motors have a power factor that is less than unity (e.g., 0.8, 0.7, etc.). Typically, devices having a reactance of opposite sign are added to the load to correct the power factor and bring the power factor closer to unity. For example, a bank of capacitors may be added to a large inductive load to bring the power factor closer to unity.

#In some examples, the reactance of the load may be constant over a number of variables. For example, the reactance may be constant over time or power consumption levels. However, not all loads exhibit constant reactance. When the reactance of the load is constant (e.g., the load continually exhibits the same reactance) a reactive element of opposite sign can easily be added to the circuit of the load to correct the power factor. When the reactance of the load is non-constant (e.g., the load does not exhibit the same reactance over time, frequency, etc.), alternate methods of correcting the power factor are required such as, for example, active power factor correction.

#Switching power supplies, like those typically found in computer power supplies, exhibit a non-constant reactive load. For example, the activation and deactivation of individual devices, components, and/or circuitry within the computer and/or computer power supply may vary the reactance of the load while additionally adding harmonics which can contribute to Total Harmonic Distortion (THD). When the reactance of the load varies, the reactive components of opposite sign added to the load must also vary to correct the power factor of the load.