防止超级电容器电源的振荡

在嵌入式系统中使用时，将功能添加到操作系统（例如Linux）非常容易。但是，这些系统中所需的功能之一就是足够的时间在删除系统功率后安全地关闭它们。不允许正确的关闭可以导致数据损坏，以便在重新启用电源时可能不会重新启动系统。

为系统关闭的时间提供时间的一种方法是将超级电容器（有时称为超球门）设计为备用电源系统。这样，在削减电源电源后，可以将处理器的电源保持长达几分钟，以便有时间编写任何打开的文件以磁盘并正确关闭剩余的任何进程。

几家主要供应商制造了控制超级电容器充电和排放的IC。由于这些电容器的电压在充电和放电时会有所不同，因此供应商还提供DC-DC转换器，这些转换器将不同的电压转换为适合为系统其余部分供电的稳定电压。这些现成的电路继续提供稳定的电压，直到超级电容器电压下降到一定水平以下。但是，当电压达到此水平时，商业电路函数就不足。

在电容器termi电压的测量nal is a function of the stored charge and the voltage drop due to its internal resistance. When the capacitor voltage falls below the shutoff threshold, the dc-dc converter and other loads are removed. The voltage drop that was created by the load current across the internal resistance is also removed, which increases the voltage at the capacitor terminals.

如果系统上的电流绘制足够高，则卸下的结果可能是电容器电压再次变得足够高以使DC-DC电路重新打开。负载重新连接，电压下降，然后再次关闭DC-DC电路。这种关闭/开/关骑自行车自行重复，其结果是系统电力网络的连续振荡。

To prevent this power oscillation, a circuit with a controllable amount of hysteresis was needed so that it will not oscillate, even during worst-case load conditions. In addition, this circuit had to work from 0 to 5 V. Several op-amp circuits were considered for this application, but even the low-voltage op amps have undefined behavior when their V_CC导轨从0 V带到V_最小。

这figureshows a circuit that can address this problem. The main circuit's power is provided by the supercapacitor voltage. As the capacitors charge from 0 V, very little current flows through Zener diode D1. As the capacitor voltage reaches approximately 4.3 V, current starts to flow though D1 and into R1. Once 150 μA of current flows and generates about 0.7 V across R1, Q1 will start to turn on. Q2 would have been turned on up to this point, but when Q1 starts to conduct through R3, it will pull the base of Q2 low and turn it off. This pulls the base of Q3 high, allowing it to start conducting current.

这current from Q3 is directed through R8. Some of this current will flow in Q1 and some will flow through R1. Therefore, rather than the voltage across R1 being dictated solely by the current through D1, it will now increase to account for the current from Q3. It’s this additional current that provides the needed hysteresis and causes Q1 to turn off at a lower supply voltage compared to what it turned on beforehand.

Note that the voltage at the collector of Q2 will turn Q4 on, which turns Q5 off, and subsequently turns Q6 on. The voltage drop across R13 provides a signal to the DC-DC circuit that the voltage on the supercapacitors is high enough that it should turn on.

如果现在删除了主电源，并且系统开始从超级电容器中汲取电流，则该电路的主电压导轨也将开始降低。但是，由于R1处的电压是由于通过D1和Q3的电流总和引起的，因此电压必须在关闭Q1之前降至2.3 V左右。此操作将打开Q2，Q4关闭，Q5打开，最后关闭Q6。发生这种情况时，没有电流会流过R13，这也会向DC-DC电路发出信号，也应关闭它。

一旦去除了DC-DC电路的负载，主电压导轨将有所增加。但是，只要它不超过2 V，电路就不会再次打开，从而阻止了电力线振荡。可以通过更换D1和更改R8的值来实现不同的转闭和开关电压。

里克·博斯马is an electronics engineer in charge of product design atDairy Cheq Inc.(Waterloo, Ontario, Canada). He has an Electronics Technologist Diploma from RCC College of Technology (Toronto) and a Bachelor of Engineering degree in electronics from Lakehead University (Thunder Bay). He spent seven years in the Product Engineering group at Honda Motor Company before joining Dairy Cheq in 2009.