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开关DC-DC转换器具有效率高、功耗低、体积小特点,已广泛地应用在便携式电子产品中。DC-DC转换器开始工作时,其输出电压值较小,反馈信号使得误差放大器工作在非平衡状态,功率开关管长时间导通,电感上产生浪涌电流[1-2]。软启动电路可以解决该问题,它通过控制功率开关管导通占空比来限制电感电流,从而消除浪涌电流,使输出电压平稳上升,无过冲[3-6]。
软起动电路通常包括两部分:斜坡电压产生电路和软启动控制电路。传统斜坡电压产生电路利用恒定电流对片外大电容进行充电来实现缓慢上升的斜坡电压信号,增加了芯片的面积和管脚数目[7]。为了提高集成度,出现了各种可片内集成的斜坡产生电路。文献[8-9]采用nA级微小电流对片上电容充电实现缓慢上升的斜坡电压,但是微小电流难以精确控制,容易受工艺影响。文献[10-11]采用数字DAC控制方式来产生斜坡电压,即通过多位计数器控制电流镜阵列流过电阻得到斜坡电压[10]或通过多位计数器控制电流源流过电阻阵列得到斜坡电压[11],其电路较复杂有毛刺出现,并且随斜坡电压精度的提高其电路规模大大增加。传统软启动控制方法是在启动初始阶段用缓慢上升的斜坡信号替代误差放大器的输入参考电压,控制功率开关管的导通时间逐渐增加,电感电流和输出电压缓慢上升,当斜坡信号增大到参考电压值后,控制开关把误差放大器的输入信号从斜坡信号切换到参考电压信号,实现从启动阶段到稳定工作状态的切换[10, 12]。从文献[10, 12]的测试结果可以看出,该方法在切换时刻电感电流会出现明显的波动,过渡不够平滑;同时该方法除了斜坡信号产生电路外还需要额外的电路。
本文提出了一种新颖的软启动电路,其中斜坡电压产生电路采用脉冲吞咽技术和窄脉冲充电技术,以小电容实现了缓慢上升的斜坡电压;只在传统误差放大器中增加两个晶体管便实现了从启动状态自动平滑地过渡到稳定状态的软起动控制,无需开关切换且无额外功耗。整个软起动电路简单易实现,功耗低。
Compact On-Chip Soft-Start Circuit for DC-DC Converters
doi: 10.3969/j.issn.1001-0548.2018.01.005
- Received Date: 2016-08-01
- Rev Recd Date: 2017-06-30
- Publish Date: 2018-01-30
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Key words:
- DC-DC converters /
- error amplifier /
- inrush current /
- ramp-up voltage generator /
- soft-start circuit
Abstract: An on-chip soft-start circuit consisting of a ramp-up voltage generator and an error amplifier with soft-start function is presented. The ramp-up voltage with slow ramp rate is achieved with small capacitors by using techniques of pulse swallowing and pulse current charging. The ramp-up voltage generator shuts down automatically to save power after the start-up has finished. The controlling scheme of start-up is realized by just adding two extra transistors to the conventional error amplifier circuit. In the start-up period, the output voltage of the converter and the current of the inductor rise are slowly controlled by the error amplifier output voltage which follows the ramp-up voltage. When the start-up finishes, a smooth transition from start-up to the steady states is achieved without disturbance to the current of the inductor, which always happens in the conventional method by using transferring switch. The soft-start circuit is applied to a peak-current mode boost DC-DC converter implemented with CSMC 0.5 μm BCD process. Simulation results show that the inrush current during start-up is eliminated effectively, and the output voltage rises smoothly without overshoot. Moreover, a smooth transition to the steady states without disturbance is achieved. The soft-start circuit is compact and suitable for on-chip implementation.
Citation: | YANG Xiao, LI Jing-kun, XU Jin-li, CHEN Guo-yan, LOU Fu-jun, LI Hao. Compact On-Chip Soft-Start Circuit for DC-DC Converters[J]. Journal of University of Electronic Science and Technology of China, 2018, 47(1): 30-36. doi: 10.3969/j.issn.1001-0548.2018.01.005 |