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February 1997

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From:
Ralph Hersey <[log in to unmask]>
Date:
Fri, 21 Feb 1997 07:16:14 -0800
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Frederick A Martin wrote:
> 
> Technet,
> 
> I am not a Designer so excuse me if there is a simple answer to this.
> 
> 2 Tracks - side by side on a PCB - FR4 epoxy material
> 
> 8KV running through 1 track
> 
> How far does the 2nd track have to be to have less than 300mA induced current ?
> 
> ***************************************************************************
> ***************************************************************************

  
Hi Fred--

You didn't supply enough information for specific answer, so I'll provide some 
generalizations.

First, as you may know, with high voltage, you have some very serious safety, regulatory 
and liability concerns that need to be addressed.

Based on years of experience, do not use polymeric coatings (solder resist) or 
traditional conformal coatings as "primary" insulations above about 1 kV.  If you are 
not "potting" or seriously encapsulating the conductive patterns, then as a 
rule-of-thumb for the design in normal atmospheric (less than 3 km altitudes) should 
provide a minimum of about 2.5 mm / kV (dc or ac peak) of "air" separation between 
conductors, in the IPC's design spec., D-275, they are more conservative and use about 5 
mm / kV.  Be careful if you reduce electrical spacing and rely on a "conform coating" 
for you primary insulation.  With high voltages, charges will be transfer an electrical 
field through the coating onto it's surface and attract dust/moisture, and eventually it 
will become semiconductive.  This reduces the effective insulation spacing and is a 
latent failure mechanism due to arcing across the surface of the insulator.  Arcing 
leads to conductive leakage paths and ultimate failure and possibly fire.   If you do 
use polymeric coatings, make sure they are compatible with any subsequent conformal 
coatings.

At 8 kV you also will have "corona" discharge in gas (air) design considerations.  
Assuming you have sufficient electrical spacing (air) so as control "corona" and high 
voltage insulative gas failures, then your "induced" current problem becomes one of 
electrical networks.

Currents can be "induced" in another conductive pattern due to direct current (dc) 
electrical leakages or due to alternating or pulsed voltages/currents.

Direct Current (dc) Design Considerations --

For dc applications the 300 mA current could be induced due to the insulation resistance 
between conductors, or could be a transient current due to charging of the capacitance 
between the conductors.  The insulation resistance between (surface) conductive patterns 
consists of three parallel resistance paths; the bulk resistance of the base material, 
the first few molecular layers of insulative material on/at the surface of the base 
material, and the resistance of the gas(air).  A dc leakage current can be induced in 
the conductive patterns by low insulation resistance and can be calculated by using Ohms 
Law, the insulation resistance (surface and subsurface between conductors) R = V/I = 8 
kV/0.3 A = 26.6+.....+67 or ~ 27 k ohms.  Even with dc, there will also be a temporary 
transient current due to the capacitance between conductors and the series impedance of 
the circuit.  This current is exponential, starting with a very high current flow and 
exponentially reducing with time, the current is limited by the impedance of the circuit 
and the applied voltage.

For alternating current (ac) and "Pulsed" applications--

For ac and pulsed applications, in brief,  I would suggest finding a good 
electronics/electrical engineering handbook or one on EMC (electromagnetic compatibility 
which would include EMI, RFI, etc.), and concentrate on the sections focused on "forward 
and backward cross-talk".  The IPC's high speed/frequency design document (I believe 
it's IPC's-D-317) has some info. but I believe it's too brief to provide an insight to 
you questing/problem.

Comments on cross-talk somewhat in brief -

Cross-talk occurs only with changing voltages and currents.  If you have a "continuous" 
ac (alternating current) (like a sine wave electrical power and continuous audio or rf 
waveform) and "pulsed-" or "digital- like" like signals, then you have two factors, the 
capacitive (forward cross-talk) and inductive (backward cross-talk) coupling. 
Simplistically (basic electricity), for capacitive coupling, the mutual capacitance 
between the currents limits the "induced" (actually coupled) current flow between 
conductors and in a function of the ac resistance (called reactance) between conductors. 
 Capacitive (C) reactance (X) is abbreviated as Xc.  Xc is calculated by the formula Xc 
= 1 / (2 * pi * f * C) where pi = 3.14...., f is the operating frequency in Hz, and C is 
the capacitance in Farads.    (Note: As can be seen, the capacitive reactance is 
inversely proportional to frequency, so as the operating frequency goes up, the 
capacitive reactance (resistance) goes down.)  The capacitive coupling current can be 
calculated by the  ac capacitive equivalent of  Ohms Law, which to determine current is 
written in the form of  I = V/R = V/Xc.  As can be seen in the Ohms Law equation, as the 
frequency increases, the Xc is reduced and the coupled current increases.  To conduct a 
300 mA current in the adjacent conductor the Xc would need to be the same ~27 kohms.  If 
you have "pulsed" signals with "fast" or non-sinusoidal wave shapes, the fast rise/fall 
times can be converted to a "Fourier Series" of sine wave harmonic frequencies, and 
coupled currents will vary with frequency.

Inductive coupling is not quite as simple as capacitive, due the complexity of the 
magnetic field couplings which are a function of the time rate (di/dt), and I feel it 
would be too long to post a suitable posting on technet (not intended as a cop-out).  
Inductive coupling is a "transformer-like" coupling, where by a change in current in one 
winding (conductor) (in EMC terms the culprit conductor) induces a voltage in another 
conductor (in EMC terms the victim conductor).  The current induced in the victim or 
secondary conductor is a function of the impedance of the secondary.  The magnetic 
coupling is based on conductor spacing, near by conductive materials, magnetic 
permitivity of near by materials, whether the conductive patterns are edge or broad-side 
coupled, and a whole host of other considerations.  This is why this paragraph of 
response is very brief and we (technet) don't need to post a text book. 

IMHTO (in my humble technical opinion) it will be a definate challenge to induce a 300 
mA current in another conductor with 8 kV electrical spacing.

Hope this helps, if not and you need some more info., help, references, let me know and 
I'll be glad to help.

  
-- 
Ralph Hersey,

Ralph Hersey & Associates
PHN/FAX 510.454.9805
e-mail:  [log in to unmask]


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