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Power Electronics

Pulse Withstanding Resistors Handle High Surges

Design engineers dealing with surge events need to understand the significant difference between pulse voltage withstanding and pulse power withstanding resistors.

One of the most significant market drivers in the resistor industry currently is the need for pulse-withstanding resistors. In some cases, design rules or time constraints may require the design engineer to overdesign his circuit to accommodate a pulse event. In many other cases, this presents an opportunity to reduce size, weight and cost, provided the correct pulse-withstanding resistor is chosen.

In the past, axial leaded resistors dominated the market. There were various resistor technologies that were used for pulse applications. Carbon composition was a technology that began many years ago and still exists today. While this technology lacks good precision and stability when compared to other technologies, there are still applications where nothing works as well as a carbon comp resistor. Those applications tend to have extremely high surge energy requirements with a need for low inductance. The application must also be able to tolerate a wide range of resistance shift due to temperature, humidity, or even drift over time.

Wirewounds represent another resistor technology that has been around for many years, and their surge handling characteristics are well known. Wirewounds can be built to withstand very high power pulses for short durations with extremely little resistance shift; and in some cases can also withstand relatively high energy (Fig. 1). Their inductance, however, is much higher than that of a carbon comp device even those that are non-inductively wound will still have inductance that is too high for high speed switching power supplies. Neither technology is ideal for high-voltage pulses.

Ceramic, Axial Leaded Technologies

Ceramic composition resistors are good at handling both high voltage and high-energy pulses; however, they are typically only available in 10% tolerances, have limited sizes, are expensive, and their overall stability is inferior to wirewounds and film-based resistors.

Aside from ceramic composition technology, axial leaded resistor options for high-voltage pulses usually employ a film element. That element is typically a thick-film material that is dipped on to cover the entire ceramic core surface or printed in a serpentine pattern to achieve even higher voltage handling. This technology is inexpensive and is a good choice for applications requiring three-watt power ratings or less.

Thick-film technology has improved significantly in the past few years. While it still remains useful only for applications requiring resistors with continuous power ratings of less than 2 watts, there is significant market demand for smaller, lower power and surface mountable packages. Thick-film surge-withstanding resistors that have no or limited laser calibration trimming are capable of handling more than 4000 watts for a single pulse of less than 100 microseconds (Fig. 2). In addition, 0805 size pulse resistors can now handle more than 100 watts for the same duration (single pulse).

Standard thick-film chip resistors will have a laser trim across the width of the part, which can narrow the effective width of the resistive element by as much as 40%. In addition, the amount of laser trimming can vary greatly from one manufacturing lot to another, and even from one part to another within the same lot. These variations result in a corresponding variability in the pulse handling capability of these resistors and make using standard thick-film chip resistors for pulse handling situations problematic. Pulse withstanding chip resistors are designed to maximize the usable width of the resistive element. This design greatly reduces or completely eliminates the hot spot on the resistor where current crowding occurs. That is the principle weak point and main limiting factor in the pulse handling for chip resistors.

High Surge Handling Capability

The high level of surge handling currently available has made thick-film pulse withstanding chip resistors extremely popular for applications that previously required a much more expensive technology. Because of their relatively low cost and high availability, these thick-film resistors can often be used to replace those more expensive parts, while still reducing the overall PCB area used and the weight of the solution. Despite all these advantages, care must be taken to ensure that the pulse voltage does not exceed the limits of the technology. In addition, film resistors are still not an ideal choice for high-energy applications, which require significant mass to absorb the energy pulses. However, many emerging markets such as LED lighting, fuel cell & alternative energy controls, and electric motor controls have made pulse withstanding film chip resistors one of the fastest growing resistive products today.

A frequent question from design engineers is when to specify a part that can withstand high pulse power or current, and when to use a part designed for high voltage. In the film resistor world, this is a critical issue to understand, because the design of the resistive element for each type is exactly opposite of the other. For high pulse power and pulse current handling, the resistive element width is maximized, but for high-voltage pulse handling, the resistive element is designed to be long and narrow. Currently, there is a good selection of commodity medium voltage chip resistors available from various manufacturers. These chip resistors will have the usual block-style resistive elements replaced with a screen printable serpentine pattern. Extending the length of the resistive element provides a proportional decrease in the voltage stress per unit length, hence allowing higher voltage handling for a given chip size. Chip resistors using this technology can handle up to 3000 continuous working volts and can achieve resistance values up to 100M with 5% accuracy (Fig. 3).

Applications requiring extremely high-voltage handling, pulse voltage handling, high resistance values, high precision or some combination of these require fine-pitch serpentine resistive elements which aren't achievable through screen printing. These resistive elements must be directly written onto the ceramic substrate. This type of thick-film printing technique creates a precise and repeatable resistive trace that has much lower noise and VCR compared to screen printed devices. Chip resistors with this kind of resistive element can achieve resistance values up to 50 Gigohms, and can maintain precise tolerances down to 0.5% and TCRs as low as 25 ppm. This level of precision is rarely seen in a thick-film chip resistor, and certainly not from a high-voltage chip resistor. Chip resistors of this type can handle high-voltage pulses of up to 40KV with proper terminal isolation to prevent arcing.

Wirewound Resistors

Finally, there are applications where resistors with low resistance values are subjected to high-voltage pulses. As stated earlier, ceramic composition resistors are a popular choice here. However, many applications require power ratings or tolerances not currently available from a ceramic composition part. For these situations, an axial leaded or surface mount molded wirewound is a popular choice. Wirewound resistors can be designed using element wire that has a thin coating on it to prevent arcing between windings. Another solution is to select a wire alloy with a specific resistance per foot, such that fewer turns are required to achieve the needed resistance, thereby widening the gap between consecutive windings and increasing the voltage capability. In some cases, an alloy can be utilized which increases the resistance per foot without reducing overall wire mass, yielding a resistor with outstanding pulse voltage handling capabilities without sacrificing its pulse power handling.

Pulse-withstanding resistors are rapidly evolving into a dynamic market. Their widespread use in emerging technology sectors is driving development for higher power handling, higher voltage handling, and better precision. Carbon composition, thick-film, ceramic composition, and wirewound technologies all provide critical performance benefits and will remain the core technologies used for future electrical products.

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