Initial spike, and how to get rid of it? - Page 1
Forum section: electronics / projects

author: paul_allsopp
author_rank: Newbie
timestamp: April 03, 2015, 04:38:36 am
content: Hey guys/gals,I am building a dual rail power supply for a small rover robot and came across an issue that I'm hoping someone can help me with. As you will see from the image I have a voltage spike in the first 0.4ms of my design and not sure of the best way to resolve it. I am thinking that I could do this in a couple of ways: either using a discrete component or two (I am fairly new to electronics, so not sure how), or I can use the attiny84 I was planning to use for the pulse, to delay the power output on the higher voltage rail until I have it steady.I know I could use something like an LM7805, but I'm interested in learning how to modify input sources at a lower level.Thanks for any and all help, much appreciated.PaulMy Schematichttps://drive.google.com/file/d/0B1uriJhzSGi-Y3M2MVppUkNBa2M/view?usp=sharingMy outputhttps://drive.google.com/file/d/0B1uriJhzSGi-NC0xdFdGOFlVejg/view?usp=sharing

author: nuno
author_rank: Frequent Contributor
timestamp: April 03, 2015, 05:24:05 am
content: You should assemble a prototype and test. If you're going to control it with a uC, you can slowstart. I see a few "odd" things with your circuit but don't know enough to provide a rationale. Anyways I suggest you prototype it; you'll probably hit your first "pain" right away on the switch control from a uC.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 03, 2015, 05:38:21 am
content: Slowstart by having a lower initial duty cycle.Or Faster pulse rate, to get less overshoot.Or Soft start via a resistor which is switched out once voltage is established.Or add resistance to give critical dampness.Also Remember there are some non ideal components in the circuit which may help limit overshoot a bit. V1, L1 C1.

author: Phoenix
author_rank: Frequent Contributor
timestamp: April 03, 2015, 05:58:59 am
content: Classic second order underdamped response. Mostly because your circuit still needs a lot more to be effective.It looks like you're designing a buck converter, this will need output voltage feedback for a stable output (or else it will change with load). There is a plethora of ICs out there that will help you do this. Check out some of the big guys websites, they have power regulator design tools. You can also do the control in your micro, but it will be a steep learning curve.

author: paul_allsopp
author_rank: Newbie
timestamp: April 03, 2015, 09:30:29 pm
content: Thanks for the comments.@Phoenix, I will certainly add in a reference from the output to the microcontroller for the control loop part of the PWM.You mention that the circuit needs to be more effective...anything you can point me in the direction of for this?

author: Phoenix
author_rank: Frequent Contributor
timestamp: April 05, 2015, 09:56:09 am
content: Quote from: paul_allsopp on April 03, 2015, 09:30:29 pmA lot of the big players have online power regulator design tools (i.e. TI's WEBENCH). Closed loop control will allow you to tune the response (and will significantly change the startup characteristic).