AEC-Q200 rated power resistors for automotive use

Wirewound power resistors are usually rated with respect to continuous power, which can be insufficient for e-mobility applications. A typical application is the pre- and discharge of large capacitors, often referred to as “soft start.” In this case the pulse-handling capability of the resistor is also very important. This capability can be determined over a large range of pulse durations by combining theoretical basics with finite element simulations of the respective thermal performance. The results can be generalized so that changing customer specifications can quickly be evaluated and suitable resistors suggested.

Wire allows for pulse load

Wirewound power resistors are typically specified according to their continuous power rating. However, the resistive element, the wire, can pick up a relatively large amount of energy while undergoing only a moderate temperature rise due to its relatively high mass and heat capacity. This is why wirewound power resistors are the perfect choice for pulse load applications.

Specification of the pulse load capability is important

The specification of the pulse load capability is becoming increasingly important due to the widespread use of frequency and voltage converters. The pulse load capability is often only exemplarily documented by the specification of one pulse of a certain power or energy and duration. It is rarely specified by listing a table of a few pulse amplitudes and durations. Should the resistor be stressed by a pulse for a duration that is not in the range of those given in the datasheet, and which is not in the range of adiabatic boundary conditions, then it is difficult to calculate the maximum permissible pulse load. However, a combination of theoretical basics and finite element simulations allows for calculating the thermal performance of a resistor over a virtually unlimited pulse duration interval, i.e. from very short pulses to continuous power.

e-mobility needs pulse load capability

Limiting the charge and discharge current of capacitors is a typical application for wirewound power resistors in the e-mobility sector because of the respective high pulse load. Soldering all electrical components to a PCB may be preferred instead of using “external” resistors, in order to keep the production process as simple as possible. In this case a number of smaller wirewound power resistors are directly soldered to the PCB, replacing a single large wirewound power resistor. For that type of application and manufacturing, Vishay has put the focus on the AC-AT series [1], which is the first of its kind to be AEC-Q200 automotive qualified.

Pulse load generates heat

We look at the cooling of a resistor in order to be able to evaluate the effect of an electric pulse load. An effective way to do that is to presume that Newtonian cooling prevails, i.e. that the rate of temperature change is proportional to the temperature difference of the hot resistor and its cooler ambient materials, and that the temperature of the latter is constant. In the case of a cemented wirewound resistor (e.g. of the AC-AT series), that ambient material is the cement surrounding the wire. However, the following reasoning can be applied to enameled or sand-filled wirewound resistors, too.

Pulse load at adiabatic boundary conditions

Presuming Newtonian cooling, and hence the proportionality of temporal temperature change and maximal temperature of the wire or resistor, results in an exponential function describing the time-dependent temperature of the wire and resistor [2].

Figure 1
Pulse load limits for the ceramic core of an AC05-AT (blue curve) and the resistive wire for R = 47 Ω (red curve). Both curves are often combined: Combination 1 (black curve) underestimates the permissible overload (blue dot); Combination 2 (green line) overestimates the pulse load limit around the shown kink (at about 0.05 s).

The respective pulse load limits of the ceramic core of an AC05-AT 47 Ω resistor and its wire are shown in Figure 1 in blue and red, respectively. The maximal pulse load capability of the entire resistor is typically a simple combination of both curves. One way is an exponential function of the Newtonian cooling type, combination 1 in Figure 1, which is however far below the specified overload rating of 10 times the nominal power for 5 seconds and is therefore underestimating the pulse load capability in this range of pulse durations. Another way, combination 2 in Figure 1, overestimates the pulse load capability around the shown kink (at about 0.05 s), because the heating of the ceramic core is not taken into account when calculating temperature limits for the wire.

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