Author Topic: PCB trace inductance and y capacitors  (Read 1226 times)

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Offline Tor

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PCB trace inductance and y capacitors
« on: May 06, 2016, 07:32:38 am »
Hi semi? beginner here with 1 year as hardware designer.

I work with high power(3,5-10.5kW) hard switched PFCs which generates allot of noise and during layout there is quite allot of different considerations to take into account. This leads me to my question placement of y capacitors and the trace inductance i assume that if i reduce the inductance in the trace i can place the y capacitors in other locations than the optimal since it then (trace+capacitor) would have the same SRF.

These capacitors are intended for this noise return loop : Noise source->metallic housing -> y capacitors-> high power tracks(return to source)

If you double the track width do you reduce the inductance of that track by 50%? This is what basic theory tells me (assuming two separate 1 turn inductors with zero distance between them for the tracks) however when i googled pcb track inductance the full return path seems to play a role and the above basics dont seem to be to correct and you get formulas where track thickness and width do not matter to much for the inductance.

Anyone know if i can calculate the inductance as i think with two separate wires for this case or what basic magnetic theory i have forgotten?

Offline Kleinstein

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Re: PCB trace inductance and y capacitors
« Reply #1 on: May 06, 2016, 08:43:04 am »
It is very hard, to impossible to assign an meaningful inductance to a straight conductor. Inductance is defined in a closed current loop, and a major factor there is the area of the loop, not the length / width of individual traces.

So the trouble with the line width is that you can calculate the local inductance (local field close to conductor) if it is far away from the rest of the loop, but this implies that the local part is only a very small part of the hole system.

The substitution with two wires ignores the interaction between there field or there mutal inductance.

Offline Alex Trofimov

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Re: PCB trace inductance and y capacitors
« Reply #2 on: May 06, 2016, 11:07:30 am »
Try Saturn PCB Toolkit to calculate track's inductance.
If you make the loop wider, then EMI would be worse, while parameters of the filters are still the same.
Sorry, i'm not into PSU design and barely familiar with EMC (no one cares about EMC in Russia, let me tell you)), it's just basic considerations.

Online T3sl4co1l

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Re: PCB trace inductance and y capacitors
« Reply #3 on: May 06, 2016, 02:56:23 pm »
Inductance does vary with trace width, but that's only half the equation.  The other half is determined by the speed of light, which does not vary.

I never found SRF of a capacitor to be a useful property, anyway.

If you need filtering at such frequencies where stray inductance matters, you should probably use a self enclosed power line entry / filter module instead.

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