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2 Using an External Soft-Start Circuit for UCC28060/61 SLUA587CMarch

2011 Submit Documentation Feedback ? 2011, Texas Instruments Incorporated 2.1 UCC2806x 的导通时间控制原理 栅极驱动导通时间随误差放大器输出电压而变化(变化因数被称为 KT),如(3) 式所示. TON = KT (VCOMP -

125 mV) (3) 式中:VCOMP 为误差放大器的输出,125 mV 为调制器偏移. 由(3) 式我们可知:对于给定的 KT,栅极驱动导通时间仅随 COMP 引脚的电压而变化. 2.2 UCC2806x软起动 当连接在 COMP 和AGND 之间的补偿电容器从一个低电荷水平充电至终值时,PWM 逐渐地从零导通时间上 升至标准导通时间.该过程运用的是一种软起动方法,其时间常数由误差放大器的输出电流和补偿电容器的电 容值设定. 采用这种方案时,假如没有可用的外部软起动电路,那么软起动电路既要采用电压补偿电路、同时也受限于电 压补偿电路.如果电路满足了软起动要求,就很有可能导致环路稳定性变差以及可听噪声的增加.另一方面, 如果电路满足了环路稳定性要求,则有可能产生较大的启动电流和较高的过冲电压. 2.3 用于 UCC2806x 的外部软起动电路 为了保证 PFC 转换器的稳定性,我们可以通过增设外部电路轻易地改变软起动电路,以减小启动电流并降低 启动过冲电压,如图

1 所示 . 当PFC 使能信号从低电平变至高电平时,电流将通过 R

1、R

2、C2 和Q1 对电容器 C1 进行充电.请注意, 电容器 C2 远远小于 C1;

它仅用于滤除噪声.在充电期间,C1 两端的电压将从零增加至使能信号的高电平, 而R2 两端的电压则将从一个高分压电压下降至零.当电阻器 R2 两端的分压器电压高于 Q1 接通电压门限 时,Q1 将正常工作.在Q1 导通期间,R3 将分摊 COMP 源电流,这样 COMP 电压将缓慢增加.由(3) 式可 知,由于 COMP 电压也在增加时,因此 TON 缓慢增加.因此,输出功率将缓慢增加.运用这种方法将使启动 电流和输出过冲电压均有所降低. 假如我们把使能信号 (Vcc) 的输入电压定义为 Vs,并将使 Q1 导通的最小电压定义为 VT,则可按 (4) 式计算出 外部软起动时间. ts = (R1 + R2)? C1? In[R2Vs/((R1 + R2)? VT)] (4) 图1:UCC2806x 的外部软起动电路配置 UCC2806x 的工作原理 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 www.ti.com Theory of UCC2806x Operation For example, refer to Figure 1;

here, VS =

5 V, C1 = 2.2 ?F, R1 =

47 k?, R2 =

66 k?, and VT = 0.6 V. We can then calculate the added external soft-start time from Equation

4 to be approximately

393 ms. After the soft-start, the voltage across C1 is the same as the enable voltage, while the voltage across R2 will be

0 V. Q1 will be turned off. R3 will not work after the soft-start process ends. So the voltage-loop compensation circuit is only RZ, CZ, and CP. The stability of the PFC converter will not be affected by the external soft-start circuit when the start-up has finished. Figure 2a is the start-up waveform of the UCC28060EVM at 90-V ac input (the input power is

220 W) without using an external soft-start circuit. Figure 2b shows the start-up waveform of the UCC28060EVM after adding an external soft-start circuit (again, the input power is

220 W). For both images, Ch1 is the PWM drive out waveform;

Ch3 is the PFC output waveform;

and Ch4 is the input current waveform. Figure 2. External Soft-Start Waveforms: Start-Up at 90-V Input We can find that the maximum input current is approximately 16.2 A if there is no external soft-start circuit, while the maximum input current drops to approximately 10.8 A if an external soft-start circuit is added. The lower input start-up current enables the use of cheaper input/output filters and less expensive switching components, while offering better EMI performance and improved safety. Figure 3a shows the PFC output waveform of the UCC28060EVM at 90-V ac input (the output current is 0.32 A). Figure 3b presents the PFC output waveform of the UCC28060EVM with the external soft-start circuit from Figure