PDF《Application应用报告 Report》

I (t) = PEAK VINAC(t) T L ON ? I (t) = AVG VINAC(t) T

2 L ON ? ? Application Report SLUA587CMarch

2011 Fred Wu Power Management Products/Field Applications ABSTRACT This application report describes how to use an external circuit to improve the soft-start performance of the UCC28060 and UCC28061 PFC controllers.

Using a soft-start circuit can reduce a large start-up current and decrease the PFC output overshoot voltage.

1 Introduction With its advantages of high efficiency, low ripple, and good electromagnetic interference (EMI) performance, the UCC28060 and UCC28061 (UCC2806x) interleaved power factor correction (PFC) controllers are well received by many customers. However, if the start-up current is too large, the power supply can easily go into fault protection, or even be damaged. Additionally, the PFC output voltage should be a small overshoot voltage to improve the safety of the switch components and the downturn dc/dc converter circuits. In order to maintain a low overall cost and reduce the number of interconnections, the UCC2806x has no soft-start (SS) pin. As a result, the UCC2806x typically uses a voltage-loop compensation circuit for device start-up. As a result, there is a tradeoff between the loop stability and using the soft-start technique in the usual manner.

2 Theory of UCC2806x Operation The UCC2806x contains the control circuits for two boost pulse-width modulation (PWM) power converters. The boost PWM power converters ramp current in the boost inductors for a time period proportional to the voltage on the error amplifier output. Each power converter then turns off the power MOSFET until current in the boost inductor decays to 0, as sensed on the zero current detection inputs (ZCDA and ZCDB). Once the inductor current decays to 0, the power converter starts another cycle. This on/off cycling produces a triangular current wave, with the peak current set by the on-time and the main power input voltage, as shown in Equation 1. (1) The average line current is exactly equal to half of the peak line current, as shown by Equation 2. (2) With TON and L essentially staying constant during an ac line period, the resulting triangular current waveform during each switching cycle has an average value that is proportional to the instantaneous value of the rectified ac line voltage. This architecture, in turn, results in a resistive input impedance characteristic at the line frequency and a near-unity power factor. The outputs of the two PWMs operate 180° out-of-phase so that the power-line ripple current for the two PWMs is greatly reduced from the ripple current of each individual PWM. This design reduces ripple current at the input and output, and allows a noticeable reduction in both size and cost of input and output filters. All trademarks are the property of their respective owners.

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2011 Using an External Soft-Start Circuit for UCC28060/61 Submit Documentation Feedback ? 2011, Texas Instruments Incorporated 应用报告 ZHCA415-

2011 年3月电源管理产品部/现场应用 摘要 本应用报告说明如何采用一个外部电路来改善 UCC28060 和UCC28061 PFC 控制器的软起动性能.使用软起 动电路能减小大的启动电流并降低 PFC 输出过冲电压.

1 引言 凭借其高效率、低纹波与优良电磁干扰 (EMI) 性能的优势,UCC28060 和UCC28061 (UCC2806x) 交错式功 率因数校正 (PFC) 控制器受到了众多客户的广泛青睐.然而,假如启动电流过大,电源很容易就会进入故障 保护状态甚至受损.此外,PFC 输出电压还应当是一个小的过冲电压,以改善开关组件及下游 dc/dc 转换器 电路的安全性. 为了保持低的总成本并减少互连线的数量,UCC2806x 未提供软起动 (SS) 引脚.因此,针对器件的软起动 UCC2806x 通常采用一个电压环路补偿电路.于是,需要在环路稳定性与按常规方式使用软起动技术之间进 行权衡折衷.