Vishay Intertechnology application note on the resistor pulse load characterisation. The article compare different resistor manufacturing technologies from the point of their capability to handle high power pulse load. Carbon film MELF has been identified as the champion – you can read more about the reasons and technology backgrounds.
INTRODUCTION
Many electronic circuits are exposed to high pulse loads. In some applications these occur regularly, such as in pulse width modulated (PWM) devices. In others pulses are incidental, but also inevitable — resulting from electromagnetic interference signals (EMI). Unfortunately, due to the increasing miniaturization or intrinsic limitations of electronic components, their pulse load capability is often insufficient to withstand these pulse loads and they require protection.
Whether pulse loads are regular or incidental, pulse-proof resistors are needed, and MELF resistors are especially well suited to this application due to their excellent pulse load capability — a prominent feature linked to their unique cylindrical design. In addition to standard metal film technology, MELF resistors are available with carbon film, which further enhances their pulse load capability. The largest case size (0207) carbon film MELF not only provides the highest pulse load capability of MELF resistors, but of all SMD film resistors, thus making it the optimum choice for high-pulse-load applications.
This article will help designers in selecting the optimum resistor for protection by identifying the pulse properties — power, duration, and shape — that need to be considered. For this, the effects of pulses on resistors and the factors determining their pulse load capability are explored, and the resulting advantages of carbon film MELF resistors are illustrated.
The destructive pulse load limits of a standard thick film chip resistor, together with advanced thick film chip, thin film chip, and thin film MELF resistors of comparable case sizes (1206 / 0204), and the 0207 case size carbon film MELF, are given in Fig. 7.
The destructive pulse load limit of the standard thick film chip resistor is rather low at 35 W. This is a direct consequence of the technological drawbacks regarding pulse load capability described above: small effective area and hot-spot-creating trimming pattern. In contrast, for the advanced thick film chip resistor the destructive limit is more than 50 % higher. Optimized thick film designs omit trimming or double the resistive area by applying a trimmed film on the resistor’s top and bottom sides. In both cases the destructive pulse load limit is significantly improved. For the latter design it even exceeds the thin film chip resistor’s limit, whose pulse load capability, compared to standard thick film technology, benefits greatly from an advanced trimming pattern, increased effective area, and better thermal stability of the homogeneous film material. Nevertheless, the advantages of the cylindrical MELF design are clearly evident in another increase of the destructive limit by 70 % for the thin film MELF resistor. Of all SMD devices of the same case size, however, it is the carbon film MELF resistor that withstands the highest pulse load of » 500 W.
Considering the largest case size 0207 available for carbon film MELFs, which is of course accompanied by a similarly enlarged effective film area, its destructive pulse load limit is increased to 2 kW — more than an order of magnitude larger than the thick film chip resistor’s limit. This unparalleled high pulse load capability makes the carbon film MELF the SMD champion for high-pulse-load applications.
