Magnetic saturation of inductor

One of the points to note when selecting an inductor to be used in a power supply circuit is magnetic saturation of the inductor. When magnetic saturation occurs in an inductor, the flowing current rises sharply, causing further magnetic saturation, which causes the inductance component to rapidly decrease, causing a phenomenon in which the current increases suddenly.

Problems when an inductor causes (magnetic) saturation

A monolithic (power FET built-in) switching regulator power supply IC has an internal current limit function and the current limit value is specified in the datasheet. Normally, if the inductor is not saturated, the high-side power FET will be turned off to limit the current if the current exceeds this specified value.
 
However, if the inductor saturates, the current will increase sharply as shown in Fig. 1, and the IC may be damaged if the current limit cannot be controlled in time. Therefore, designers who use many power supply ICs select inductors with saturation current characteristics that are sufficiently larger than the current limit value, assuming overload conditions. Inductors with large saturation current characteristics require inductors with thick wiring to allow current to flow, which makes them large in size, making it difficult to miniaturize inductors and increasing board size.

Analog Devices Solutions

Analog Devices' LT86xx series can safely withstand inductor saturation during output overload or short circuit conditions due to its fast peak current mode architecture (data in Figure 2). See excerpt from the sheet).
　

This series of devices can control the ON time to a very short time on the order of nanoseconds and has a built-in comparator that detects overcurrent conditions at high speed, so even if the inductor saturates, the overload condition can be safely detected. It becomes possible to
　
This allows designers to minimize solution size by selecting inductors based on the output load requirements of the application rather than choosing oversize inductors for overload conditions.

At the end

When selecting an inductor, it is necessary to design a power supply circuit that does not exceed the saturation current regulation, and it may be necessary to select an inductor with a large shape.
　
Analog Devices offers a solution that circumvents this problem and allows for smaller board size designs using smaller inductors. However, it should be noted that the inductor has a rated current based on its own temperature rise, in addition to the saturation current specification. It is important to fully understand the characteristics of both the power supply IC and the inductor when designing the power supply circuit.

Inquiry

In this article, we introduced the reasons (response at saturation, etc.) that allow the inductor (L) to be made smaller in Analog Devices' DC/DC converter ICs. For more information, please contact us below.