Author Topic: Estimating trace inductance for capacitor placement  (Read 1715 times)

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Offline sheitbrinkTopic starter

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Estimating trace inductance for capacitor placement
« on: June 27, 2018, 06:56:29 pm »
I was wondering if anyone had any ideas on the best way to estimate trace inductance on a PCB when it relates to the placement of +10uF bulk caps.

I'm using a ground plane with 13mil spacing up to the top signal layer. I was wondering if I could use the per unit length inductance off of a microstrip impedance calculator to estimate the inductance of the trace. So I would just take this:
 L = (nH/in)*(trace len)
 
This would be used to estimate the impedance ip to 10's of Mhz.

The reason I ask is because experts in the field claim that inductance is a really misunderstood concept. They also throw around words like self, partial, & mutual inductance, and I don't know which one this nH/in one is.

 
 

Offline iMo

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Re: Estimating trace inductance for capacitor placement
« Reply #1 on: June 27, 2018, 08:36:21 pm »
IMHO it has a little sense to mess with a track inductance(s) when talking +10uF bulk capacitor(s) placement.. What is the impedance of a 10uF bulk capacitor at 10MHz??
« Last Edit: June 27, 2018, 08:40:24 pm by imo »
Readers discretion is advised..
 

Offline sheitbrinkTopic starter

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Re: Estimating trace inductance for capacitor placement
« Reply #2 on: June 28, 2018, 03:00:12 pm »
If you look at the following impedance plot:

https://drive.google.com/open?id=1kYZlxtzwTjZC-T8o9qigUJEdqroSlb_L

you can see what the impedance is for a 10uF ceramic capacitor. the reason the plot is V shaped is because of the ESL of the cap. If you crunch some numbers you get about 0.63nF. But, if you had to put a few cm of trace between your load and the cap, the trace inductance would be higher.

Here is a plot with the orginal plot, and 2,4 and 8nH of inductance added:

https://drive.google.com/open?id=1j_3GMYVun_jgU2s5ZuUIKg_KdPUq7crG

you can see you start to get an effect with in a decade of the capacitors new resonance and really takes over after that.

I suppose what was asking was if anyone knew how to estimate this esl on a trace using cheap microstrip calculator, and not spend $$$ to do some sort of EM simulation. 
 


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