Abstract
Constant duty cycle controlled discontinuous conduction mode (DCM) flyback power factor correction (PFC) converter has the advantage of high power factor (PF) and the disadvantage of low efficiency. While, constant on-time (COT) controlled critical conduction mode (CRM) flyback PFC converter has the exact opposite features, besides its switching frequency varies in a line cycle, and the variation range is very large, which complicates the electromagnetic interference (EMI) design. In order to obtain both benefits of these two control methods, an adaptive off-time (AOT) control technique for DCM flyback PFC converter is proposed in this paper. By utilizing the output voltage and the amplitude of line voltage to adjust the off-time of the main switch, the magnetizing current of transformer exactly operates in CRM when the rectified input voltage gets the peak. Thus, the root-mean-square (RMS) current of the main switch and the diode, as well as the conduction loss can be effectively reduced, and high efficiency can be obtained. The proposed control technique also can achieve theoretical unity PF over universal input voltage range of 90~264VAC. Moreover, its variation range of switching frequency is greatly reduced compared to that of COT control. A 60W prototype has been fabricated and tested in the laboratory and experimental results are presented to verify the effectiveness of the proposed method.
Highlights
Power factor correction (PFC) converters have been widely used in AC to DC power conversions to achieve high power factor (PF) and low total harmonic distortion (THD) of input current [1]–[6]
By changing the duty cycle according to the rectified input voltage and output voltage, more energy can be transferred to the load within each switch period than that of constant duty cycle control, so the peak current and the RMS current are significantly reduced, the input current suffers from serious distortion, and the PF is low
This paper proposes an adaptive off-time controlled discontinuous conduction mode (DCM) flyback PFC converter
Summary
Power factor correction (PFC) converters have been widely used in AC to DC power conversions to achieve high power factor (PF) and low total harmonic distortion (THD) of input current [1]–[6]. The on-time of the main switch decreases as the increase of the transient rectified input voltage, so that the switching frequency can be fixed in each half-line period These two aforementioned control methods can reduce or fix the switching frequency for PFC converter operating in CRM, they all suffer from low PF and serious distortion of input current. According to (17) and (27) and with the aforementioned key circuit parameters, Fig. 3 shows the switching frequency of AOT controlled DCM flyback PFC converter and COT controlled CRM flyback PFC converter over a half-line period. Fs_AOT changes with the input voltage and of output power, the variation range is greatly reduced compared to that of COT control
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