Vishay Anti-Surge Resistors Aim to Decrease BOM While Improving Reliability

When it comes to reliability in circuit design, one of the major considerations is ensuring that components can operate within the overall system. One aspect of this is device power rating: Can a part handle the amount of power that it will be experiencing?

Unfortunately, achieving a power rating can come at a tradeoff with device size and overall PCB area. Vishay has released a series of anti-surge resistors claiming to increase the power rating in small-package devices to help designers navigate that tradeoff. 

 

Power Ratings and Component Packages in Circuit Design

In the realm of circuit design, the power rating of a component is a pivotal parameter that dictates its performance and reliability. 

 

Different resistor packages impact their power rating (not to scale). Image used courtesy of Synton

 

The power rating, measured in watts, is the maximum power a component can dissipate without exceeding its maximum operating temperature. This is a function of the component's ability to convert electrical energy into another form (like light or heat) and dissipate the heat generated in this process. Should a component experience a power surge greater than its power rating, it will likely burn out, get damaged, and fail.

In general, a major factor that determines a component’s power rating is the component’s package. Within this, power rating is influenced by factors such as the package's size, material, and design. For instance, larger packages like 1210s can typically handle more power than smaller packages like 0805s. 

Even further, designers need to consider more than just normal operations and ensure their components will still work properly in the event of a power surge in the system. 

 

Parallel Resistors for Power Ratings

Often when it comes to power ratings, a designer faces a tradeoff between reliable performance and preserving area. For instance, a small, space-constrained device like a wearable might want to get away with the smallest components possible.

 

Parallel resistors effectively increase overall power rating. Image used courtesy of EE Power’s Dissipation Calculator

 

In a situation where the resistor is the component in question, general practice is to use multiple resistors in parallel instead of a single, larger-package resistor. Consider an example where a designer wants to implement a 5 k resistance to a power rating of ¼ W using only 0805 packages. If the general 0805 package is only ⅛ W, a designer could instead implement two parallel ⅛ W 0805 resistors to functionally achieve a ¼ W 5 k resistor.

In this way, the designer can still save space compared to a larger-package resistor while achieving the desired power rating. Naturally, however, the downside to this approach is the increased BOM. Ideally, there would be a single ¼ W 0805 resistor for the designer to use.
 

Vishay’s New Product

The RCS0805 e3 thick film resistor is a compact, high-performance component designed to optimize board space across a variety of applications. Most notably, the part is designed to excel in handling high and repetitive surge pulses, offering superior pulse load performance and electrostatic discharge (ESD) surge characteristics compared to standard chip resistors. As part of this, the resistor operates at a voltage of 150 V and can withstand temperatures from -55 °C to +155 °C in values ranging from 0 Ω to 10 MΩ.

According to Vishay, the RCS0805 e3, which comes in a 0805 package, can be used in place of four standard parallel 0805 resistors from a power rating perspective. Alternatively, the device can replace two 1206 resistors or a single larger 1210 package. Because of this ability, the product helps designers navigate the reliability versus area tradeoff common in electrical systems.

The part is particularly well-suited for automotive, industrial, telecommunications, and medical sectors, where space is often at a premium, but power and reliability are still concerns.