[请教高手]为什么负反馈系统相位60度的时候稳定？

workerabc

The relationship between overshoot and phase margin (PM)  is:

overshoot ~ 1+ exp(-PI*PM/(sqrt(10000-PM^PM)));            PI = 3.1415926..

The bigger the PM, the smaller the overshoot. => the less time needed to be settled down.
Wondering could sb run the simulation to verify how accurate it is?
But for the buck regulator, it is hard to run the AC simulation (loop gain) to get the PM. Because the buck regulator is a switching system, the DC points of the circuit are changing periodically with the switching clock. It is impossible to run a AC simulation, which based on the fixed DC status, to get the PM. SS or HB could give the results, but the simulation time is too long. The overshoot could be found via transient simulations and transferred to PM using the above equation. For one second order LDO, the equation gives really accurate results from the simulations (been verified).

61angel

60度是相对比较好，不一定的，太大了稳定，但慢，小了反应快，但稳定性差了，而且过冲厉害

refugee

一般情况下我们不希望向位于度过大或者过小，相位裕度过小往往会有衰减震荡，相位裕度过大会造成截止时间过长。
所以根据应用不同，选择会有不一样。比如在某些大负载电容的情况下，我们希望最好是有一些过冲，这样可以快速对负载充电，且截止时间也较小。所以这类应用60度左右的向位于度就足够了

原帖由 readbook 于 2008-4-8 15:13 发表

                               
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看书的时候还没搞明白为什么相位裕度在60度的时候基本稳定, 和时域有什么关系呢 ？为什么不是70%呢？请高手指点,谢谢！

下面举一个通过阻尼因子和固有频率为指标，将时域和频域的稳定性联系起来的例子

   用阻 ...

kakool

相位裕度选择60度是  信号建立时间与振铃大小的最好折衷~~~~~~~