PDF《Application应用报告 Report》

2 UCC2806x 的工作原理 UCC2806x 包含用于两个升压脉宽调制 (PWM) 电源转换器的控制电路.升压 PWM 电源转换器使升压电 感器中的电流斜坡上升,其持续时间与误差放大器输出端上的电压成正比.每个电源转换器随后将功率 MOSFET 关断,直到升压电感器中的电流衰减至零为止(在零电流检测输入端 ZCDA 和ZCDB 上进行检 测).当电感器电流衰减至零时,电源转换器将起动另一个周期.这种接通/关断循环产生一个三角形电流 波,其峰值电流由导通时间和主电源输入电压设定,如(1) 式所示. IPEAK(t) = (VINAC(t)?TON)/L (1) 平均线路电流恰好等于峰值线路电流的一半,如(2) 式所示. IAVG(t) = (VINAC(t)?TON)/(2?L) (2) 由于 TON 和L在ac 输入电压周期中基本保持恒定,因此每个开关周期中的最终三角电流波形具有一个与整 流ac 线路输入电压的瞬态值成正比的平均值.反过来,这种架构又将在线路频率下产生一种电阻性输入阻抗 特性以及一个接近于

1 的功率因数. 两个 PWM 的输出异相 180° 操作,因此两个 PWM 的电源线纹波电流相比每个单独 PWM 的纹波电流大幅 减小.这款设计减小了输入和输出端上的纹波电流,并显著地缩减了输入和输出滤波器的尺寸与成本. 所有商标均为其各自拥有者的产权. 为UCC28060/61 使用一个外部软起动电路 ZHCA415 -

2011 年3月提交文档反馈 T = K (V

125 mV) ON T COMP ?

47 k R1 ?

66 k R2 ?

66 k R2 ?

24 k R3 ?

1 nF C2

1 nF CP 2.2 F C ? Z 2.2 F C1 ? 5-V Enable Signal UCC28060/61 COMP t = (R + R ) C ln S

1 2

1 ? ? R V (

2 S T R + R ) V

1 2 ? Theory of UCC2806x Operation www.ti.com 2.1 Theory of On-Time Control of UCC2806x Gate drive on-time varies with the error amplifier output voltage by a factor called KT, as shown in Equation 3. (3) Where: VCOMP is the output of the error amplifier, and

125 mV is a modulator offset. For a given KT, from Equation 3, we can see that the gate drive on-time only varies with the voltage of the COMP pin. 2.2 UCC2806x Soft-Start The PWM gradually ramps from zero on-time to normal on-time as the compensation capacitor from COMP to AGND charges from a low level to the final value. This process implements a soft-start technique, with a time constant set by the output current of the error amplifier and the value of the compensation capacitors. Using this description, the soft-start circuit will both use and be limited by the voltage compensation circuit if there is no external soft-start circuit available. If the circuit meets the soft-start requirement, it may possibly result in worse loop stability and increased audible noise. On the other hand, if the circuit meets the loop stability requirement, it may produce a greater startup current and higher overshoot voltage. 2.3 External Soft-Start Circuit for UCC2806x To secure the stability of the PFC converter, we can easily change the soft-start circuit in order to reduce startup current and decrease the startup overshoot voltage by adding the external circuit, as Figure

1 illustrates. Figure 1. External Soft-Start Circuit Configuration with UCC2806x When the PFC Enable signal goes from low to high, the current will charge capacitor C1 through R1, R2, C2, and Q1. Note that capacitor C2 is much smaller than C1;

it is only used to filter the noise. The voltage across C1 will increase from zero to the high level of the Enable signal while the voltage across the R2 will decrease from a high divide voltage to zero during the charging time. When the divider voltage across resistor R2 is greater than the threshold of the Q1 turn-on voltage, Q1 will operate normally. R3 will divide the COMP source current during the Q1 on-time so that the COMP voltage will increase slowly. From Equation 3, the TON is increased slowly as the COMP voltage also increases. Thus, the output power will be increased slowly. This approach results in a lower start-up current and lower output overshoot voltage. If we define the input voltage of the Enable signal (VCC) as VS, and the minimum voltage to make the Q1 conduct as VT, then the external soft-start time can be calculated as shown in Equation 4. (4)