The Importance of Inductor Soft Saturation

One of the most influential forces driving the electronics industry today is power conservation. Power conservation has made significant strides in the past decade but the complexity and proliferation of consumer electronics, data center computing, and more have dwarfed any net savings in today’s power grids. Further development of power conservation techniques will continue to be an important design requirement.

The most common design method for saving power is by gating power and clocking to the subsystems that can be turned off when non-operational. This effectively makes the total power being supplied to the system very dynamic in nature where inrush and transient power requirements can be tricky to characterize on a system level. As a result, brownout can occur if the power supplies are not designed by considering the full effect of dynamic power. Figure 1 shows an example of a power envelope illustrating inrush and transient demand.

Figure 1: Example of Inrush and Transient Current Demand

Switching mode power supplies (SMPS) are much more common today than linear regulators. SMPS circuitry is more complex but provides benefits by delivering >90% power efficiency when using high efficiency components on the output stage. These components are the output load capacitor and power inductor. Selecting a low loss inductor with soft saturation and a low ESR load capacitor are critical to designing a low ripple, stable power supply.

The ripple voltage must be carefully controlled to minimize possibilities of conductive EMI issues as well as extending component longevity. Excessive output ripple current can cause the output capacitor to heat and possibly fail over time. Selecting the right power inductor can alleviate this problem.

Abracon recommends power inductors that exhibit “soft” saturation as illustrated by the blue line in Figure 2 below. Inductors will lose their effective inductance as the Irms current increases. The inductors with “hard” saturation shown as the ferrite inductor or red line, lose their effective inductance very quickly.

Fig. 2: Effective inductance loss as a function of load current

As the effective inductance reduces, less energy is stored in the inductors magnetic field which has an adverse effect by increasing the output ripple voltage. The limit of hard saturation decreases the usable range of an inductor like a shielded ferrite. However, the soft saturating inductors made from composites have a much wider usable range of current without posing a threat of saturation and excessive output ripple voltage.

In summary, the best inductor for today’s dynamic SMPS power systems is an inductor that has a soft saturation. Soft saturation provides the power supply designer with wider operating ranges, more design flexibility and margin for the dynamic loads of today’s power sensitive systems.