AEC-Q200 Automotive Rated Power Resistors

Wirewound resistors are usually rated for continuous power, which is not sufficient for electric vehicle applications. A typical application is the pre-charging and discharging of large capacitors, often referred to as "soft start". Here, the pulse handling capability of the resistor is also very important. Combining theoretical foundations with finite element simulations of thermal performance, this capability can be determined for longer pulse durations. The specific results obtained make it easy to quickly evaluate changing customer needs and provide the right resistor.

1. Allowable pulse load of winding resistor

Wirewound power resistors are generally rated for continuous power. However, due to their high mass and thermal capacity, the resistor components and wire windings can absorb a large amount of energy with only a moderate temperature rise. Therefore, wirewound power resistors are ideal for pulse load applications.

2. Rated pulse load capacity is important

Due to the widespread use of frequency and voltage converters, the rated pulse load capacity is becoming increasingly important. Pulse load capacity is usually specified only symbolically for a certain power or energy and duration of a pulse. It is very rare that the pulse load capacity is specified for several pulse amplitudes and durations. If the duration of the pulse impact on the resistor is not within the range shown in the data sheet and exceeds the range of adiabatic boundary conditions, it is difficult to calculate the maximum permissible pulse load. Theoretical foundations combined with finite element simulations can calculate the power dissipation of the resistor at an almost infinite pulse duration interval, i.e. the thermal performance from very short pulses to continuous pulses.

3. Electric vehicles require pulse load capability

Electric vehicles require high pulse loads, and limiting the capacitor charging and discharging current is a typical application of wirewound resistors in the field of electric vehicles. In order to keep the production process as simple as possible, the preferred method is to solder all electronic devices to the PCB without using "external" resistors. In this case, several small wirewound power resistors can be directly soldered to the PCB to replace a single large wirewound power resistor.

4. Pulse load generates heat

The effect of the electrical pulse load can be evaluated based on the heat dissipation of the resistor. An effective method is to assume that Newton's law of cooling holds true, i.e. the rate of temperature change is proportional to the temperature difference between the thermal resistor and its cooling encapsulation material, which is at a constant temperature. In the case of cement-type wirewound resistors (such as the AC-AT series), the encapsulation material is the cement surrounding the wire. However, the following arguments can also be applied to enameled or wirewound resistors.

5. Pulse load under adiabatic boundary conditions

Assuming that Newton's law of cooling holds true, the instantaneous temperature change of the winding or resistor component is proportional to the maximum temperature, an exponential function describing the temperature change of the winding and resistor over time can be obtained.

Figure 1: Pulse load limits of R = 47Ω ceramic core AC05-AT (blue curve) and resistor wire (red curve). The two curves are often combined: combination 1 (black curve) underestimates the permissible overload (blue dots); combination 2 (green line) overestimates the pulse load limit at the indicated kink (about 0.05 seconds)

In Figure 1, the blue and red lines show the pulse load limits of a ceramic core AC05-AT 47Ω resistor and its windings, respectively. The maximum pulse load capability of the entire resistor is usually a simple combination of the two curves. One approach is a Newtonian cooling exponential function, combination 1 in Figure 1, which is well below the overload rating specified by 10 times the nominal power for 5 seconds, and therefore underestimates the pulse load capability for this pulse duration. The other approach, combination 2 in Figure 1, overestimates the pulse load capability at the kink shown (at about 0.05 seconds) because the heating of the ceramic core is not taken into account when calculating the winding temperature limit.

6. Pulse load FE simulation

By analyzing the heat flow and temperature distribution within the resistor using finite element (FE) simulations, it is easy to see that the entire AC05-AT resistor heats up slowly under a pulsed electrical load. The resistor wire heats up during the pulse and then cools down. The rest of the resistor is also heated up more slowly by the heat pulse. The pulse load duration is not important in FE simulations, as long as the boundary conditions are appropriate. Therefore, it is possible to simulate the temperature of the resistor and the wire for almost any pulse duration, from adiabatic windings (ms range) to nearly continuous loading of the resistor (100s range). The maximum permissible electrical pulse load can thus be determined based on the specified maximum permissible temperature of the winding.

7. Extended summary

By extending the characteristic time of the heat diffusion of the winding, the finite element simulation results of multiple pulse durations can be summarized to determine the correction factor, which, combined with the exponential function, gives the temperature according to Newton's law of cooling.

8. Pulse load under non-adiabatic boundary conditions

The above correction factor can be used to calculate the pulse load limit under non-adiabatic boundary conditions from the winding perspective (Figure 2). However, the pulse load limit of the entire resistor during long pulse duration is not covered. However, if the characteristic time of the thermal diffusion of the entire resistor is extended to a longer pulse duration, the non-adiabatic limit curve can cover the continuous load limit curve (Figure 2).

Figure 2: Maximum permissible pulse load under non-adiabatic boundary conditions from a winding perspective (blue curve), corrected for the corresponding heat diffusion characteristic time (red curve). The common limit curves greatly underestimate the pulse load capability for pulse durations, shown in the figure from 0.1 s to 10 s for reference (black dashed line).

9. For other resistance values ​​and resistors

By appropriate extension, the results of the FEM simulation of the thermal state of a specific resistor (AC05-AT, 47Ω in this case) can be generalized. In this way, the obtained results can be applied not only to AC05-AT of all resistance values ​​(winding configurations), but also to all other AC-AT types of resistors, as their structures are similar.

This method can even be used for all other similar types of resistors, such as the G200 series, without the need for additional FE simulations, making it extremely efficient. The benefit to the customer is that pulse load capability issues can be solved promptly and accurately.