Topological survey of power factor correction AC/DC converter of the improved single stage
A summary
Pollute the harmonic wave which exchanges the electric wire netting in order to reduce, make the relevant standards (such as IEC1000-3- of limiting the current harmonics outside China 2). So, require the power of ac input to take measures to reduce the electric current harmonic content, improve the power factor. There are two kinds of active power factor correction methods adopted extensively at present, namely two-step PFC and single stage PFC.
PFC grade makes the input current follow the input voltage, enable the sine of input current, improve the power factor, reduce the harmonic content. And then the DC/DC grade connected realizes the fast regulation of the output voltage. Because of adopting the two-step structure, the circuit is complicated, the device is costly, inefficiency. In the low-powered application, two-step PFC is not very applicable. So, studying the single stage PFC and transform technique becomes an important subject in electric and electronic fields.
The intersection of single stage and PFC [2 ] [3 ] share one gas switching tube and one controlling circuit while being with DC/DC grade association PFC grade, realize the facelift and regulation to the output voltage to the input current at the same time. It is different from two-step scheme, the controlling circuit only regulates the output voltage, stability to guarantee output voltage, at steady state, duty ratio invariable, so can require electric current of PFC grade input voltage of the automatic following, though, the input current of the single stage PFC converter is not a sine wave, PF value is not so high as the two-step scheme, but because IEC1000-3-2 only requires for the electric current harmonic content, does not have rigorous requirement to PF value, the input current harmonics of the single stage PFC converter is enough to satisfy IEC1000-3-2. And because of adopting the structure of single stage, the circuit is simple, low cost, high watt density.
So, the single stage PFC converter is widely used in the low-powered occasion. This text mainly topological to go on, analyze, point out the existing problem to the intersection of single stage and PFC, have introduced several kinds of improved topological structure in order to solve these problems.
Analysis and existing problem of isolated Boost PFC circuit of single stage 2
The isolated BoostPFC circuit of typical single stage, this is topological and was made up by boost type PFC grade and forward type DC/DC converter. The active switch S, in order to share the switch, CB, in order to buffer the electric capacity. Through controlling the make-and-break of S, the circuit realizes the facelift and regulation to the output voltage to the input current at the same time.
As everyone knows, the electric current gives up the Discontinuous Conduction mode (DCM) Boost converter,more lower than electric current for automatic following input voltage at duty ratios fixed, so, the PFC grade job can get higher power factor under DCM. However, the great of crest value of the inductive current of import and export, has increased electric current stress and switching loss of the active switch; Inefficiency of the converter; In addition the circuit needs a bigger EMI electric-wave filter. If demand to reduce the voltage, electric current stress of switching element, then need PFC grade job in the serial mode (CCM) of the electric current Leave, can raise the efficiency of the whole converter and reduce EMI at the same time. If add a inductance L1 between a of Fig. 2 and b, can make the PFC grade job under CCM. As to DC/DC converter, in order to raise the efficiency of the converter, generally work under CCM, so, the duty ratio does not vary with load. When load is lightened, output power reduce, and PFC grade input power with the same when being heavily loaded, energy that the capacity of filling with the storage capacitor is greater than taking from the storage capacitor away, cause the voltage build up of the storage capacitor. In order to keep the output voltage unanimous, the voltage feedback loop regulates the output voltage, make the duty ratio reduce, input energy and reduce correspondingly too, this dynamic procedure will not be stopped until after the power balance of import and export. It is that the total line voltage of the direct current obviously rises to support and reduce the consequence brought, that is to say the voltage of electric capacity obviously rises, even reach kilovolt.
Reducing the voltage of electric capacity usually has two kinds of methods: A method is to adopt variable frequency control [4 ], can make the voltage of electric capacity lower than 450V, but the frequency range may be up to tenfold, optimization design of the unfavorable magnetic element; Another kind adopts the coil of the voltage transformer to realize negative feedback. If PFC grade and DC/DC converter work under CCM, while exporting power reduction, though the duty ratio is constant, but the input power will be reduced correspondingly, has inhibited the increase of the voltage of storage capacitor, its efficiency is most high, PF value is reduced to some extent, however, it is very difficult to find one topologically to totally work under CCM, it is relatively complicated too in design. Connect the power flow of the single stage PFC converter in series to pursue, can be found out from the picture, the power is input and conveyed to exporting, having passed variety two times, inefficiency.
So single stage that make it meet IEC1000-3-2 and regulate the output voltage fast, not to input current harmonics in subject problem of converter in PFC, reduce the voltage of electric capacity and raise the efficiency; In addition the single stage PFC converter works in the hard on-off state, the voltage, electric current stress that switching element bears are high, so, the switching loss is very great. So, people have proposed realizing negative feedback with the coil of the voltage transformer, the method of using soft switch technique and connecting PFC in parallel etc. reduces the voltage of electric capacity, switching loss and raising the efficiency. It is improved topological in order to solve these problems to now introduce several kinds. [1][2]
Tags: electric capacity, PFC, The converter, the diode, the switch, Voltage transformer
