Better resilience against current peaks
Due to their multilayer structure, chip bead ferrites are sensitive to current peaks, which typically occur when switching on power supplies or electric motors. However, an optimized layer design improves their resistance to short-term high current surges.
BY Markus Holzbrecher and Mohamed Koobar
Chip bead ferrites are multilayer inductive components used mainly for EMC filtering. They consist of a nickel-zinc ferrite body with very thin internal silver conductor layers. This multilayer structure enables compact size and useful impedance behavior, but it also makes standard SMD ferrites sensitive to short current surges above their rated current. Such peaks typically occur during switch-on events in power supplies, motor drives, or lamp ballasts. If the current exceeds the permissible limit, the component can be damaged immediately. For conventional multilayer ferrites, the rated current in the data sheet therefore also represents the maximum permissible current amplitude for short-term loading.
Optimized layer design
The WE-MPSB series [1] from Würth Elektronik was developed specifically for applications in which short current peaks exceed the continuous rated current. The acronym stands for Multilayer Power Suppression Bead. Compared with conventional multilayer ferrites, the internal layer design is optimized for higher current capability, significantly lower DC resistance, and high impedance over a broad frequency range. Depending on the type, the DC resistance can be reduced by up to 75 percent. This lower resistance helps the component tolerate higher pulse currents while maintaining effective EMI suppression.
A common use case is an inline filter at the input of a power circuit (Figure 1). During switch-on, the input capacitor may draw a very high inrush current because of its low ESR. As a result, the ferrite briefly sees a current far above the normal operating current. In the example described in Application Note ANP028 from Würth Elektronik [2], a 600 Ω WE-MPSB type with a rated current of 2.1 A is exposed to a single current peak of about 19 A for roughly 0.8 ms before the current decays to the steady-state value. This illustrates why standard ferrites can fail in such applications and why a dedicated pulse-current specification is useful.
Figure 1: Typical application with peak current at switch-on (5 A/DIV | 100µs/DIV).
Test procedure for pulse load capacity
Current peaks of this kind occur in many real circuits, but they vary strongly in shape and duration. To create reproducible data sheet values, Würth Elektronik developed a test method based on the fuse test approach for determining the melting integral I2t. In that method, the component is stressed with defined pulses and sufficient cooling time between pulses. For WE-MPSB ferrites, a rectangular pulse shape was selected for testing because it represents the highest energy input over a given pulse duration and is therefore a conservative worst-case load. Standard tests cover pulse durations from 0.5 ms to 8 ms. A pause between pulses allows the component to cool before the next load cycle.
Although the fuse standard provided a useful starting point for a test routine, multilayer ferrites do not behave like fuses. Their pulse-current capability cannot be predicted reliably with a universal I2t formula. Measurements demonstrate this with a 600 Ω WE-MPSB part. If an 8 ms pulse capability is used to calculate the allowable current for a 2 ms pulse via a constant I2t assumption, the result differs significantly from the measured data sheet value. In practice, the maximum pulse current must therefore be determined empirically for each component type and test condition. The relevant values in the data sheet are the result of long-term measurements, not a simple theoretical conversion. Figure 2 shows the maximum permissible peak current for the respective pulse durations tested. The tested range extends from 0.5 ms to 8 ms.
Figure 2: Representation of the current as a function of the pulse duration and the number of pulses at 8 ms.
Permissible pulse current
Three parameters mainly determine pulse-load capability:
First, pulse duration: the longer the pulse, the lower the permissible peak current.
Second, number of pulses: repetitive stress reduces the allowed pulse amplitude compared with a single pulse. Data sheets therefore distinguish between single-pulse and repetitive-pulse conditions.
Third, temperature: as temperature rises, the DC resistance increases, which further reduces the allowable pulse load. These parameters are interdependent, and the cooling pause between pulses also has a strong influence. For this reason, pulse-current capability must always be considered as a system of boundary conditions rather than as one isolated number.
The development target for the WE-MPSB series was to achieve impedance behavior comparable to the established WE-CBF ferrite family while increasing current robustness. Conventional WE-CBF components are generally not intended to withstand substantial overcurrent peaks above rated current. In contrast, WE-MPSB parts are designed for applications with short-term current surges during switch-on. Because of their lower resistance, they also offer higher rated current capability for comparable impedance values. The pulse-load capacity of an MPSB 600 Ω type is significantly higher than that of a comparable CBF type in the same package size (Figure 3).
Figure 3: Comparison of the pulse load capacity of the WE-CBF and WE-MPSB series.
Wide range of components
The WE-MPSB family covers impedance values from 8 Ω to 600 Ω at 100 MHz and is available in package sizes from 0603 to 3312. Depending on the type, rated current values range from 2.1 A to 10.5 A, while typical DC resistance spans roughly 1.0 mΩ to 80 mΩ. The specified usable frequency range extends from 1 GHz to 3 GHz, with extension toward 8 GHz under qualification. For higher currents, especially above 5 A, PCB layout becomes increasingly important because conductor width, copper thickness, and thermal behavior of the tracks can limit the overall current capability even if the ferrite itself is suitable.
Multilayer ferrites are indispensable EMI suppression elements, but conventional chip bead designs can be vulnerable to inrush and other short overcurrent events. The WE-MPSB series addresses this limitation with an optimized internal structure that combines lower resistance, strong impedance performance, and significantly improved pulse-current capability. Because the allowable peak current depends on pulse duration, repetition, temperature, and cooling conditions, practical design work should use the empirical pulse-current data in the data sheet rather than simplified I2t calculations. For input filters and other circuits with pronounced current peaks, this makes WE-MPSB ferrites a more robust option than conventional multilayer beads.
