Simple, They’re Not: Datacenter Power Supplies Rely on Advanced High-Current Inductors

Inductors, along with their passive device siblings, resistors and capacitors, have a public relations problem. Unlike solid-state devices with their deep physics underpinnings and multibillion-dollar fabs, inductors simply don’t get the attention and respect they deserve.

The general feeling among engineers and other technologists is that their simplicity makes them unworthy of admiration. In reality, though, they have a sophistication of their own, even when compared to active devices.

For many years after the 1830s, when Michael Faraday discovered that a changing magnetic field could induce current and Joseph Henry independently studied “self-induction” (a conductor inducing current in itself), inductance and the inductor remained somewhat of a mystery. It wasn’t until electromagnetics was better understood, using both basic electrical laws and Maxwell’s equations, that the mystery of how merely forming wire into a loop could change its electrical properties was solved.

Part of the public relations dilemma is that the inductor (Figure 1, top, left) is represented by a very ordinary schematic symbol (Figure 1, top, right). Further, the basic concepts taught in engineering school usually depict the inductor as just a piece of wire that has been bent or wound; hence, the term “coil,” an informal name for the inductor in many applications. In truth, they are successfully used across a range of designs for storing energy to filter ripple at the output of a switch-mode power supply (SMPS) (Figure 1, bottom) and for creating an RF-resonant tuning circuit.

Figure 1 : Early air-core inductor coils for crystal radios were formed from wire wound around a hollow cylinder form (top, left), and the corresponding schematic symbol is a simple coil (top, right); the block diagram of a typical SMPS shows the location of the output filter inductor (bottom). (Image sources: United Nuclear, Bourns, and Circuit Basics LLC)

The inductor’s physics is defined by a brief equation that links inductance (L), voltage (V), and the rate of change of current (I): V = L × (dI/dt).

The equation indicates that the inductor does not impede DC currents (apart from its ohmic resistance) but resists (“chokes”) changing currents. For any inductance value, this opposition increases with the rate of change (frequency). Its “quality factor” (Q) is a dimensionless measure of the inductor’s losses, defined as the ratio of its inductive reactance to its effective series resistance, which includes both DC resistance (DCR), ideally close to 0 ohms (Ω), and frequency-dependent losses.

In the early days of radio, do-it-yourselfers built crystal radios using a tuning coil made of many turns of wire wrapped around a rod or cardboard tube just a few inches long. That’s how most of them are shown in basic electronics literature, and the wirewound inductor is still used in specialized cases. However, today’s high-current power supplies need a different type of inductor with low inductance, very low DCR, and high current capability.

Coils no longer suffice

Inductors are essential components in high-current, high-efficiency SMPSs used in datacenters. Nearly every design includes a small inductor for energy storage and to smooth the output ripple, working alongside an output capacitor.

At the required datacenter current levels, even a tiny DCR translates into significant inefficiency and waste heat. The numbers show why: a resistance of just 10 milliohms (mΩ) carrying 100 amperes (A) results in 100 watts of loss (P = I²R). That’s why a DCR below 1 mΩ is vital in high-current situations.

To mitigate losses and dissipate the waste heat, while supporting ever-smaller form factors and surface-mount processes, vendors employ advanced materials, innovative physical designs, and enhanced fabrication techniques.

A good example is the SRP1024HMCT-75NM (Figure 2) from Bourns’ SRP1024HMCT series of shielded power inductors. With a footprint of 0.157 × 0.417 inches (in.) (4 × 10.60 millimeters (mm)) and a very low profile of 0.087 in. (2.2 mm), this high-current inductor provides 0.075 microhenries (µH) of inductance (±20%) with a DCR of just 0.4 mΩ.

Figure 2: The SRP1024HMCT-75NM is a 0.075 µH inductor with a tiny footprint, a low profile, and a DCR of just 0.4 mΩ. (Image source: Bourns)

These specifications tell only part of the story. The inductor is rated for a root-mean-square (rms) current of 50 A and a saturation current of 65 A. Its shielded construction uses a hot press molded process and a core of carbonyl powder around enameled copper wire to counter two problems caused by the fast rise/fall-time switching action of a power supply: audio buzzing noise due to electro-acoustic resonances and electromagnetic interference (EMI). The latter can result in the overall design failing to meet the strict EMI limits on radiated electrical noise.

All inductors that carry more than a negligible amount of current are affected by self-heating. The SRP1024HMCT-75NM supports an operating temperature range of -40°C to 125°C. Within this range, designers need to know and model the effect of temperature on inductor parameters and performance, and factor that information into their power circuit models and subsequent simulations.

For these reasons, Bourns provides a graph (Figure 3) showing the relationship between DC current, associated temperature rise, and the resulting decrease in inductance.

Figure 3 : A graph from the SRP1024HMCT-75NM inductor datasheet shows the relationship among DC current, temperature rise, and effective inductance. (Image source: Bourns)

The datasheet also provides the specific soldering temperature profile for this component. While this information mainly interests the manufacturing and production team, other components on the bill of materials (BOM) might have different requirements, which could lead to changes in the production process or in the BOM component selections.

Conclusion

For datacenters, it’s easy to focus on power-supply topology and overall performance, especially efficiency, and give short shrift to the subtleties of basic two-terminal passive components such as inductors. As shown, Bourns shielded inductors enable SMPSs to support the high currents of datacenters despite associated temperature rise, EMI considerations, and even physical size constraints.