The ruggedness of passive components is a topic of frequent discussion, and takes on many meanings depending on the application, as discussed in our recent blog “‘Rugged component’ has many meanings; what’s yours?” The question then becomes how do you obtain the required performance n basic passives such as power resistors and not have to worry that these normally “modest” components will become weak links in your design.
One of the options which designers and component engineers consider when looking for a rugged power resistor is a ceramic substrate-based unit. These resistors are capable of providing reasonably rugged performance for some situations, but not the harshest ones or those with additional design-in constraints. The extreme combination of heat and vibration which these resistors must tolerate means that they are subject to latent cracks and thus fracturing, resulting in an unacceptable open-circuit failure.
Fortunately, there is an alternative which offers superior ruggedness for military, aerospace, and even surprisingly harsh applications such as hybrid and electric vehicles (EV/HEV). Steel-based power resistors can tolerate the thermal cycles and vibration-induced fracturing, for greater long-term reliability. (Note that the term “steel-based” means that the substrate is steel, not the resistive material itself, of course.
While many of the military applications are off-limits to further details, the automotive ones are not. In a Formula Student Germany motor-racing event for electric vehicles, Figure 1, the Elbflorace team from TU Dresden needed to find suitable pre-charge and discharge resistors for the DC link capacitors.
Figure 1: High-power, low-mass, low-profile resistors for pre-charge and discharge of the DC link capacitors were used in the all-electric race car from the Elbflorace team from TU Dresden, running in the Formula Student Germany event.
Both functions are essential to the safe operation of the vehicle: the pre-charge resistors limit the charging current for the capacitors and thus also the load on the battery; the discharge resistors ensure that the DC-link capacitors are discharged within a few seconds of the drive being shut down thus rendering them safe. The component selection is further complicated by the challenge of fitting high-power resistors into a very limited space in an application which is also highly sensitive to mass, as any extra mass degrades vehicle performance and handling.
The team chose the resistors from the TT Electronics WDBR series, because they offered minimal mass in an ultralow-profile solution, along with needed ruggedness. Despite their small size – from about 28 × 32 mm × 0.9 mm thick for the smallest unit up to 100 × 150 × 1.5 mm for the largest – these thick-film on steel components can handle significant power. With a 0.53 °C/W heatsink and no forced-air cooling, they are rated from 500 W (smallest size) to 7 kW (largest size) peak power and 160 to 280 W (continuous). With the addition of modest forced-air cooling at 5 m/s and the same heatsink rating, their power ratings increase to 300 W for the smallest size and to 1500 W for the largest.
What makes the dissipation rating possible for these resistors is their construction and associated low thermal resistance, with a dielectric layer is applied to a machined stainless-steel substrate, Figure 2. the thick-film conductor and resistor patterns are printed and fired, and then protected with a high-temperature overglaze.
Figure 2: Key to the performance of the WDBR resistor series is its multilayer construction, built on a stainless-steel substrate which is highly immune to cracks and fracturing which result from thermal extremes and vibration.
Available resistance values of 12 to 150 Ω and light weight of 6 to 189 grams (depending on size and associated power rating of selected device) makes then a good fit for dynamic braking, inrush-current limiting, and snubber circuits in space and mass-challenged applications which require the extreme in resistor ruggedness. For example, the WDBR2 can dissipate 1 kW for 10 seconds yet weighs just 17 grams. The next time you have the need for ruggedness and low mass in a power-handling resistor, look beyond ceramic substrates and consider steel, as there is a substantial difference.