Mail*Link(r) SMTP FWD>RE>FW: DES:FAB:ASSY: Breakdown
Welcome to the interesting world of high-voltage electrical/electronics
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Norm started with:
HV breakdown is a current topic here as well. In a basic double sided
application that has a continuos 7KVAC with frequencies up to 30 MHz, is
there any advantage to using the more expensive polyimide laminate in place
of FR4?
Is there any truth to the rumor that polyimide is more 'arc tolerant'
because of it's higher Tg?
Norm Dill
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Then Lou Hart responded with:
Date: 8/13/96 12:04 PM
From: [log in to unmask]
Response to Norm Dill's message...Years back I spent a week at the
Biddle School in Plymouth Meeting, PA, supposedly learning how to detect
partial discharges (arcs that do not bridge conductive electrodes) in
insulators. Partial discharge (sometimes called, erroneously, "corona")
is what is responsible for long term destruction of insulation, in many
cases. Most resistant materials are inorganics, less resistant are
natural organics (paper), and least resistant are synthetic
organics (epoxy encapsulants, for example).
Lou Hart Compunetics
--------------------
And I'd like to add the following:
Arc Resistance is the ability of an insulating material not to develop an
conducting path (carbon or organic material) due to a plasma like arc between
a pair of electrodes. Per the standard tests, the time is reported in
seconds. For most epoxy's, the arc resistance is in the range of 120-180
seconds. To my knowledge, polyimides are similar.
Partial Discharges -- generally, partial discharges are a "limited charge"
local breakdown of a gas due to electrical stress. Partial discharges are
generally a precursor to corona. Partial discharges are the electrical
breakdown of a gas and follows Paschen's Law, and generally occur in a gas
void in a solid/liquid dielectric. For convenience, a sketch should simplify
the condition
CCCCCCCCCC_____
dddddddddd __V1
ddd ddd
ddd ddd V2
ddd ddd ____
dddddddddd __V3
CCCCCCCCCC
V1 and V3 are the voltages across a dielectric "d" between a conductor and the
"top" (V1) and the "bottom" (V3) of a void , which is the hole (blank space)
in the middle of the "d's". V2 is the voltage across the void itself.
Now what happens:
With continuous dc (direct current) voltages, some very small current will
flow due to the "seriesed" insulation resistances. The voltage drops for V1,
2 and 3 will be porportional to the current flow and their individual
resistances. At some applied voltage (or as a function of time), the voltage
across the "void" V2 will exceed the Paschen minimum for the gas contained in
the void and an "arc" will occur between the top and bottom of the void, what
happens is the "top" and "bottom" surfaces of the void form capacitor
electrodes, when the partial discharge occurs, it is the discharge of the
capacitance of the void (from one side of the void to the other) that occurs.
Repeated partial discharges will degrade the dielectric and ultimately cause
"treeing" in the dielectric, then a short (conductive filament growth) and
ultimate failure. For most "small" voids in dielectrics, the partial
discharges are generally less than 100 pC (pico Coulombs), more frequently
about 10.
Voids are worse with ac (alternating currents). With ac, we no longer have a
simple resistive voltage divider. Instead we have three capacitors in series.
Let's assume all three capacitors have the save cross-sectional area, and
have capacitances of C1, 2, and 3 corresponding to V1, 2 and 3. Assume the
dielectric thickness's are equal and the gas dielectric constant Dk = 1, and
the solid Dk = 5. Then capacitors C1 and C3 are each 5X the capacitance of C2
just due to the Dk of the dielectrics -- all else being equal. The capacitive
reactance (the ac electrical resistance of a capacitor) is inversely
porportional to capacitance. (The foumula is Xc = 1 / ( 2 * pi * f * C),
where f = frequency, and c = capacitance in Farads). What this means is that
due to the "seriesing" of the the capacitors, there will be 5X as much voltage
across C2 as there is across each of the other capacitors (C1 and C3) --- talk
about a stress concentrator. Now Paschen's Law comes into effect -- and
you've got zappooo, increased partial discharge breakdown. Yes, there are
some effects due to variations of gas breakdown as a function of frequency.
More details will be avoided at this time, but the above is the basis for the
caution statement in the A-600 E on the acceptance of "measles" for "high
voltage" applications, meaning at STP >240 Vac, rms or 340 Vdc or peak pulsed
amplitude.
Hope this is of interest and/or helps to understand the problems in
"high-voltage" applications.
Ralph Hersey
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Date: Tue, 13 Aug 1996 14:39:15 +0400 (EDT)
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Subject: Re: FW: DES:FAB:ASSY: Breakdown
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