Abstract: A method for driving high-brightness LEDs in a discontinuous mode (DCM) dual-output single-stage flyback power factor correction (PFC) converter is presented. In order to avoid the crossover effect of the two outputs of the converter, the time division multiplexing method is used to realize the independent adjustment of each output branch current, which improves the reliability of the driver. Since this method only uses one magnetic component, two ways can be realized. The output of the stream reduces the cost of the driver; when the converter operates at DCM, fixed frequency, and fixed duty cycle, a higher power factor can be obtained. Finally, the correctness and effectiveness of the research results are verified by simulation and experiment.
1 Introduction
Today, LEDs are widely used in LCD backlights, automobiles, traffic lights, and general lighting. According to IEC 61000-3-2 Class C regulations, Power Factor Correction (PFC) is required for LED general-purpose illumination drivers larger than 25W, so low-cost power factor correction schemes have become a research topic of interest.
The Active Power Factor Correction (APFC) circuit commonly used in AC/DC converters is a two-stage PFC circuit. The first stage circuit is used for power factor correction, and the latter stage circuit is used as a DC/DC converter. Since there are two cascaded power stages, the size and cost of this type of circuit are usually high. Therefore, another type of APFC topology emerges. This type of topology integrates the PFC circuit and the DC/DC converter, and they share An active power switch becomes a single-stage AC/DC converter, which reduces the cost. This APFC circuit is now widely used in ballasts and chargers.
Using a multi-output converter as an LED driver enables a converter to meet multiple different levels of constant current output requirements, thereby reducing the cost of the driver. The traditional multi-output converter, such as transformer coupling mode and weighted feedback adjustment mode, can realize multi-channel constant voltage output, but cannot realize multi-channel constant current output. Based on this, this paper proposes a class of dual-output single-stage flyback PFC topology.
Under the DCM, this kind of topology can realize independent voltage regulation of independent regulation, and can realize independent constant current output of each channel, and realize power factor correction. In order to avoid the cross-effect of the two outputs of the converter, the time division multiplexing method is used to realize the independent adjustment of each output branch current, so that each channel can drive different types of LEDs separately, and one of the drivers will not affect the other. The normal output of the branch improves the reliability of the driver; since only one magnetic component is used in this method, two constant current outputs can be realized, and a large high-voltage storage capacitor is not required after the rectifier bridge, thereby reducing the cost of the driver. The converter operates at DCM, fixed frequency, and fixed duty cycle, and can also achieve higher power factor. Finally, the correctness and effectiveness of the research results are verified by simulation and experiment.
2 Independently adjust the double constant current output flyback topology
Figure 1 shows the topology of the single-stage flyback PFC converter with independent regulation of dual constant current output and its switching timing. Figure 1 (a) shows the independent output winding topology, with the two outputs being provided by two separate windings.
Figure 1(b) shows the shared output winding topology. The two outputs are provided by the same winding time division. Regardless of the independent output winding type or the common winding type, if the two circuits satisfy D1a + D2a < 0.5, and D1b + D2b < 0.5, the two paths can be operated in complementary phases Фa and Фb, by time division. The two channels are multiplexed and controlled by Signal TMS (Time-Multiplexing Signal, TMS). As shown in Figure 1 (c), when Soa = 1, the converter adjusts the output of the A, the primary switching current Ip rises linearly in the D1aT phase, and the current Isb decreases linearly in the D2a T freewheeling phase, D3a T = ( 1 - D1a - D2a) T, the current Isb is zero, at this time, the converter is in DCM mode; when Sob = 1, the converter adjusts the B output, and if the B is working, the converter is also in DCM mode. It can realize two-way cross-influence control.
Figure 1 Independently regulated dual output single stage flyback PFC converter and its switching timing
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