It’s an unfortunate fact that a resistor can fail under pulse loads. When pulse power is dissipated to the device’s resistive element, it generates heat and increases the resistor’s temperature. Overheating then damages the resistive element, leading to a resistance change or even the opening of the device. To avoid this in your design, below are eleven things you should know about resistors and pulse loads when selecting a component:
- RESISTORS CAN WITHSTAND PULSE LOADS HIGHER THAN THEIR RATED DISSIPATION P70 (The resistor’s datasheet specifies the nominal power dissipation at an ambient temperature of 70 oC as P70)
Heat generation and transfer in the resistor take time, so a resistor’s pulse load capability depends on pulse duration.
For short pulses (adiabatic condition), the heat remains in the resistive element but the shortness of the pulse limits its effect on the resistive element temperature, even for high pulse loads. Thus the resistor will withstand peak pulse loads higher than its rated dissipation.
For long pulses, the prolonged heat generation causes a more significant temperature rise in the resistive element, although this will be limited by heat removal already setting in during the pulse. Hence, for extended pulse durations the permissible peak pulse load approaches rated dissipation.
- THE NUMBER OF PULSES COUNT
The difference between a single pulse load and a continuous pulse load is a function of the number of pulses and the time interval between them.
Single Pulse Load: The time interval T between pulses is long enough to allow for cooling of the resistor between pulses. Applicable pulse parameters are P̂ ≤ P̂specified, Û ≤ Ûspecified, and the average pulse power P̄ → 0.
Continuous Pulse Load: The time interval T between pulses is short and prevents cooling of the resistor between pulses. Applicable pulse parameters are P̂ ≤ P̂specified, Û ≤ Ûspecified, and the average pulse power P̄ being P70 ≥ P̄ ≥ 0.
- THE PULSE SHAPE MAKES THE DIFFERENCE
Pulse shapes vary, ranging from rectangular or triangular to the typical exponential decay of a capacitor discharge or the sharp surge pulse.
For energy pulses of low power and long duration, the pulse energy is the limiting parameter, and pulse shapes can be converted to rectangular shape to compare with the resistor’s pulse load diagram, by calculation of the pulse energy and determination of the duration of the rectangular pulse of same energy and peak power.
For sharp surge pulses the pulse voltage is the limiting parameter, and conversion to rectangular shape is not applicable. Instead, common surge pulse shapes are described by standardized transients and referenced in the resistor’s datasheet accordingly. The surge pulse is described by the 1.2/50 and 10/700 pulses, according to IEC 60115-1, 4.27. An electrostatic discharge is described by the human body model, according to IEC 60115-1, 4.38 and IEC 61340-3-1.
- FINDING THE RIGHT RESISTOR IS NO MIRACLE
Finding a suitable resistor for a pulse load application requires determination of the actual pulse condition. Parameters, such as peak power P ̂, pulse duration t or period T need to be identified and compared to the resistor’s specified pulse load capability.
- ENVIRONMENTAL CONDITIONS CAN LIMIT THE RESISTOR’S PERMISSIBLE PULSE LOAD
Pulse load diagrams are typically defined at room temperature. If your component needs to operate in a higher ambient temperature, or if your application calls for additional continuous power loads that increase the resistive element’s temperature, then a resistor with a higher pulse load capability is probably needed.
- WHICH PULSE LOAD SPECIFICATION CAN I TRUST?
Information on a resistor’s capability to withstand pulses is presented in pulse load diagrams. What these diagrams have in common is that they specify a maximum permissible peak pulse power per pulse duration for pulses of rectangular shape. Apart from that, their information value may differ strongly:
Check whether the diagram covers peak power per pulse duration for just a single resistance value or the full available resistance range for the resistor series. Only in the latter case the specified peak pulse power is reliable, as it is defined by the weakest performing resistance value of this range.
The pulse will stress the resistor, and affects its resistance value. The pulse load specification must therefore also state the maximum permissible resistance change, e.g. 0.25 % R, for the pulse conditions given in the pulse load diagram.
- WHAT IS THE BETTER CHOICE: THIN OR THICK FILM RESISTORS
Film resistors are available in different technologies that come with different pulse load capability. Main factors influencing the pulse load capability of film resistors are the resistive film material, the trimming pattern and the available resistive area.
- HOW TO IMPROVE THE PULSE LOAD CAPABILITY OF THICK FILM RESISTORS
The limited pulse load capability of standard thick film resistors is related to the inhomogeneous resistive film material and the simple trimming pattern that limits the available resistive area. There are, however, ways to push the limits further:
Printing the resistive film on both the top and bottom sides of the resistor’s ceramic body allows for distribution of the pulse-induced heat over twice the resistive area and significantly decreases the pulse-related temperature increase in the resistive film. Double-sided thick film resistors are featured in the CRCW-HP series.
Omitting the trimming cut allows for full utilization of the resistive film area for current flow. Thereby distribution of the pulse-induced heat in the resistive film is improved and hot spots are avoided. Non-trimmed thick film resistors are featured in the CRCW-IF and RCS series.
- THE SMD PULSE LOAD CHAMPION: CARBON FILM MELF RESISTORS
The SMD champion in pulse load capability is the carbon film MELF resistor CMB 0207. Its performance is more than an order of magnitude better than equivalent case size resistors, as it combines the most important characteristics for high pulse load capability:
- Features a proven pulse-resistant cylindrical design, offering the largest effective resistive film area
- Helical trimming pattern, avoiding locally enhanced current densities
- Carbon film material with its unrivaled thermal stability
The above diagram shows typical destructive pulse load limits for Vishay film resistors (R = 1 kΩ). Pulses were applied by capacitor discharge, with a pulse length corresponding to a 3 ms rectangular pulse.
In wirewound resistors the resistive element is composed of a metal wire that is wound around a cylindrical ceramic core. Due to the comparably large mass of the wire, a much higher pulse energy of up to 60 kJ can be dissipated in the wire for very short pulse durations. Since the wirewound resistors’ resistance value is adjusted by wire diameter and length, resulting in different wire mass, its pulse load capability is also strongly resistance-dependent.
For long pulse durations, though the energy is still dissipated in the wire, a significant part of the generated heat can escape from the wire during the pulse. Thus, for those durations the pulse energy handling capability of the entire resistor is much higher than that of the wire itself.
Protecting the wire with a vitreous enamel coating, instead of a cement coating, further improves the wirewound resistors’ pulse load capability as it can withstand higher temperatures.
The below diagram shows an exemplary overload limit as a multiple of continuous power for Vishay wirewound resistors for different pulse durations.
A typical Vishay wirewound resistor can handle about 1000 times its rated power for 3 ms. As an example, the G207 is rated with a continuous power of 17 W. Therefore, for a single pulse of 3 ms duration, the resistor is able to withstand a 17 kW pulse without destruction.
A single Vishay wirewound resistor can handle a pulse energy that would correspond to the braking energy of a 1000 kg vehicle when braking from 70 km/h to a standstill within 5 s.
- POWER METAL STRIP® RESISTORS
When compared to other 1 Ω to 0.0001 Ω current sense resistors, technologies, Power Metal Strip resistors provide superior pulse performance for short duration transients because of their large element mass. This is because the all-metal welded construction does not rely on a substrate for support, and so the element is thick enough to be self-supporting, which results in a large resistance element mass that can absorb more energy before it reaches a thermal limit that causes resistance values to change.
The illustration shows a comparison of the resistance element thicknesses of common current sense technologies. Notice that the substrate is a substantial portion of the resistor’s total mass, but the resistive element is a small fraction. Mass = pulse performance. The substrate provides support for the thin resistance element and for a constant transfer of heat energy from the resistance element to the PCB, which does not contribute to fast transient energy events.
As you can see, there are different solutions for the various pulse load applications. This article is intended to support you to make the right choice in line with your application’s requirements.
Dr. Annika Elsen, Product Marketing Engineer, Vishay Intertechnology
Adrian Michael, Manager Product Marketing Automotive, Vishay Intertechnology
Bryan Yarborough, Product Marketing Engineer II, Vishay Intertechnology