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OhmCraft Resistor Pulse Withstanding Capability
W. M. Mathias
Chip resistors with capability to withstand high voltage pulse stresses
Introduction
Many electronic systems must be designed to operate and survive in the presence of
electrical pulses and transient overloads. These pulses present a transient high energy shock to
the resistor, and/or a high voltage, high energy excursion. This requirement ranges from input
protection network needs to internal failsafe overload and shutdown circuitry in diverse application
such as power supplies, telecommunication networks, and AEDs (Automated External
Defibrillators). Central elements in these designs are chip resistors with the capability to survive
and remain in tolerance in the presence of these harsh conditions. Until recently, chip resistors did
not have the capability to withstand extreme abuse, thus the designer was forced to utilize leaded
devices or multiple chip resistors in series to provide this protection. Due to advances in
construction techniques, chip resistors with substantial pulse withstanding characteristics are now
available in a wide range of sizes, tolerances and values.
Basis of Performance
Resistors are traditionally manufactured using a variety of technologies including thick film,
thin film, wirewound, metal film, and foil. Each of these resistor technologies fills a particular
application niche. Thick film materials, applied in the construction of chip resistors, are well suited
for pulse withstanding applications.
The typical method of construction of thick film resistors is to screen-print the thick film
resistive element on to a substrate. In allied operations including glass overcoat, wrap around
conductors, laser trimming, and singulation are performed yielding finished chip resistors.
OhmCraft pulse withstanding resistors are unique in that in place of screen printing; the resistive
element is directly produced using a MicroPen
direct writing system as seen in the photograph.
Writing 1206 chip resistors:
For this production lot, the resistor
element is a 4 mil wide line.
2
Pulse Voltage Capability
OhmCraft’s direct writing technique is the basis of OhmCraft Fine Film
thick film processing
technique. Fine Film
technology is critical to the robust performance of pulse withstanding
resistors. The major benefit is direct writing’s ability to write narrow lines and narrow spaces, thus
enabling long serpentine, high square count designs. To illustrate this point, the drawings on the
left are conventional thick film resistors and the one on the right is an OhmCraft Fine Film
chip
resistor.
Conventional thick film OhmCraft’s Fine Film
Block style Course serpentine Fine Line serpentine
An electrical pulse can damage and or degrade the actual resistive element (the feature
shown in black in the above drawings) when the voltage per unit length exceeds the design limit of
the base thick film resistive material. This thick film material constraint is defined as a threshold of
‘x’ volts per millimeter of resistor element length. A straight forward way of improving the pulse
withstanding capability of a resistor is to simply increase the total length of the resistive element.
This is the basic principle behind OhmCraft’s Fine Film
pulse withstanding chip resistors.
For example, the block style resistor, shown in the drawing on the left above, has a
relatively low total resistor element length, thus will have a modest working voltage and pulse
capability specification. Most commodity type thick film resistors are based on this style design,
and thus are not particularly suited for, or capable of, reliable operation in pulse withstanding
applications. The middle drawing above shows the resistive element of a conventionally
manufactured course serpentine chip resistor. The total element length is longer and will show
improved pulse withstanding capability. However, the drawing on the right that represents an
OhmCraft Fine Film
pulse resistor, with a long relatively serpentine resistive element is designed
for maximum pulse voltage capability.
Now let’s review a few real application examples.
Case #1
A customer is using a 26.1Kohm, 1% tolerance, 2010 chip resistor in an RF filter circuit. In
normal operation, this resistor must withstand 1.5KV exponential pulses with a time constant of
approximately 4 milliseconds. This customer selected an OhmCraft Fine Film
chip resistor
because competitive chip resistors are not capable of handling these functional requirements.
The inherent high voltage capability coupled with the superior energy absorption of the OhmCraft
resistor proved to be the design solution for this critical application. In this case, the energy
absorbed and peak power is:
E = ½ C*V
2
Peak Power = V
2
/ R
E = ½ (0.12 *10
-6
) * 1500
2
Peak Power = (1500)
2
/ (26100)
E = 0.135 Joules Peak Power = 86 Watts