benefits for having a high open loop gain- Power Supplies - Page 1
Forum section: electronics / projects

author: chimera_786
author_rank: Regular Contributor
timestamp: May 27, 2013, 10:46:28 pm
content: Hey guys-Ive been looking over some control theory but I cant get my head wrapped around something.What is the advantage of having a high open loop gain at low frequencies. Im not talking specifically in terms of op amps, but rather, I am talking in terms of power supply stability.I would really appreciate it if someone would explain the benefits a having a high open loop gain of the system (assume a buck converter in Voltage Mode Control) in regards to low frequency ripple (not the switching noise) attenuation. What would happen if I dont have a reasoably high open loop gain at low frequencies.I do, however, understand why I need to have low gain at higher frequencies: to prevent the noise at higher frequencies to find its way to the output.Thanks

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 28, 2013, 01:19:14 am
content: Chimera_786 and the group,The general idea of loop gain and stability in a switching power is this:1) The circuit must have a phase change of less than 180 degrees when the gain is 1. The difference between the actual phase and 180 degrees is called the phase margin. It is normal to design for a phase margin of greater than 45 degrees under all conditions.2) The circuit must have less than unit gain when the phase shift in the control loop is 180 degrees. The difference in the gain is normally expressed in dB. A target value of 10dB is used.1 & 2 are requirements for all linear control loops or servo systems.There is a third requirement in switching power supplies that the gain should be less than -10dB at Fsw/2. This is because as you approach the switching frequency the system is no-longer continuous, it is more like a sampled data system. If you have too much gain at Fsw/2 you will have sub-harmonic oscillations.Now, if you have a system that has a single dominant pole, the loop gain will have a slope of -20dB/decade (of frequency).Say, you have a switching frequency of 1MHz and you arrange the crossover frequency to be 120 kHz (Fsw/8). This system now has gain of 1 at 120kHz.At 12kHz you will have a loop gain of 20dB (10x), at 1.2KHz 40dB (100x) 120Hz (1000x).This means that line frequency ripple on the power supply input at 120Hz will be rejected by 1000x. ( line frequency is 2x the line frequency because of rectification).At dc a high gain is desirable because it translates into good line and load regulation.There are lots of advantages to using current mode control. I suggest you do a little research into the differences between voltage and current mode control.Jay_Diddy_B

author: chimera_786
author_rank: Regular Contributor
timestamp: May 28, 2013, 01:36:48 am
content: Jay-I was not aware of the 3rd stability criterion- I assume its associated with current mode control. But containing focus on the initial question (which I believe you answered):High gain at lower frequencies represent the rejection ratio and when the frequency goes in the sub mili hertz range (i.e DC), it essentially just becomes line regulation.A couple of follow up thoughts (feel free to add to):1- The way the control loop of the converter will reject (assume the 120Hz AC line ripple) is that its essentially faster than the ripple frequency and can react to the perturbations and increase/decrease the duty cycle to mantian a tight steady state point. (If I have this backwards, then please provide an explanation as to how the control loop will reject the 120Hz ripple)2- So, to understand a little more about the high open loop gain at DC, a good place would be to read up more on line regualtion (i.e of an op amp) to see how the two are related.Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 28, 2013, 03:36:36 am
content: Hi,The third stability criteria, is true for all switching supplies.It is difficult, in a lot of power supply topologies, with the exception of current mode buck to get enough gain at Fsw/2 to cause a problem. So the criteria is automatically met, but it is still there.Jay_Diddy_B