-
- 视频加载中。。。
- 课程目录
- 课程笔记
电源设计小贴士27:压降式并行电源供应
In this Power Tip, we will look at a simple method to parallel supplies. Some of the key care concerns when paralleling supplies are:
1) no single point failures added by the droop circuit;2) as a corollary to #1, no master-slave operation;
3) minimum interconnect;
4) no adverse impact on efficiency;
5) good voltage regulation; and
6) preserved load dynamics.
The droop method provides a simple way to meet many of these requirements. It works by allowing the power output voltage to sag as a function of load current. As shown in Figure 1, paralleled power supplies tend to equalize output currents because of this load line.

Figure 1: Drooping power supply output voltage enables current share.
(Click on image to enlarge)
This figure shows the output voltage versus load characteristics of three power supplies. Because of component tolerances, the three power supplies have slightly different V-I characteristics. For a given load condition, a horizontal line represents the output voltage when all three supplies are connected in parallel. The intersections of the horizontal line with the load lines represent the output currents of the individual supplies. This method obviously degrades the system voltage regulation.
There is a trade-off between how well the currents balance and the voltage regulation. The first step of this trade is to determine the regulator tolerance; i.e., how far does the worst case design depart from the nominal. The important items to establish are reference accuracy over temperature and divider tolerance (see Power Tip 18).
While how close to nominal you can set your output voltage by picking resistors that affect your accuracy, it will not impact the current share. You then are ready to pick either your slope or your allowable deviation, and calculate the other. If you assume that the slope is relatively constant, the variables are simply related as:
Where:
SPA = Set point accuracy in percent
D = Voltage droop from no load to full load in percent
LE = Load extreme or how far the loads could be off, in percent
In doing the calculations, you will find the short coming of this method. It takes extreme accuracy in setting the output voltage and significant voltage droop to get a reasonable current share. For instance, as shown in Figure 1, with 3.5% tolerances and 20% voltage droop, you can have 35% current mismatch. This amount of droop may be acceptable in high-voltage systems, but will not be in low-voltage supplies.
The first thought about implementing voltage drop might be to put a big resistor in series with the output voltage, until you look at the tolerance issues and the resulting losses. In our previous example, we would loose over 20 percent of the output power in this resistor.
The next thought is to measure the output current of the power supply, amplify and use this measurement to offset the output voltage setting circuit. This works for voltage mode control, but with current mode control, a much simpler method is available. By limiting the DC gain of the control loop, you have built a synthetic resistor. Appendix 1 below (after the About the Author box) goes through the simple math for calculating the output impedance based on Figure 2.
The result is that the output impedance of this system is equal to the negative inverse of the compensator gain, times the power stage gain. Most power supplies contain an integrator in the compensation, which results in a very large DC compensator gain. By adjusting the DC gain to a specific value, a desired droop can be obtained. Usually this is quite easy to implement, just add a resistor across the error amplifier.

Figure 2: Voltage droop is easily implemented with current mode control.
(Click on image to enlarge)
Please join us next month when we will discuss tips for hot swap controllers.
推荐帖子
- TI CC2540 USB CDC Serial Port驱动安装失败原因及解决方法
- 买了款具有iBeacon基站功能的USB串口适配器,设备是基于TI公司CC2540芯片的。 一般来说,安装这设备的驱动就像我们用无线鼠标那样,插入USB接口后windows就自动安装了,但是新买的这设备没有,足足弄了好几天(win7 32位,msdn版),才发现是windows update自动更新被我关闭的原...
-
fish001
无线连接
- MSP-EXP430F5529LP开发板005-PWM库函数+时钟配置
- 从32转到MSP430最让我头大的就是它的时钟配置了,参考了一些网上的资料,看了几天终于大概了解了一点。 上面这6点是关键,在后面的时钟初始化时要参考。 本次实验目的是要实现P2.0口输出10kHz的PWM,这也是应用中电机控制的常用工作频率。要输出准确的频率,了解清楚各个时钟是非常必要的。...
-
火辣西米秀
微控制器 MCU
- CC2530/CC2540/CC2541常用寄存器
- 1.访问模式 符号 访问模式 R/W 可读写 R 只读 R0 读0 R1 读1 W 只写 W0 写0 W1 写1 ...
-
Jacktang
无线连接
- 单片机延时的实时性解决方法
- 1、如果单片机采用delay函数来进行延时,即采用空等待方式来延时,但是单片机只有一个内核,不能分身去处理别的事情。采用这种方式延时,单片机就只能等待延时时间到达之后,才能去处理其他事情,如果对程序的功能产生影响,就不能采用这种方法。 2、可以采用定时器来记录延时时间,在定时器来计算时间的期间,单片机可以处理其他事情,等计时时间到了之后,再来处理延时之后的事情。如果程序中有多个地方需要用到延...
-
Aguilera
微控制器 MCU