Planar Inductors For Differential Chokes

Applications PQC's proprietary planar inductors are used as differential chokes in practically all power topologies in variety of applications. Among those are single or multi-winding filtering chokes in industrial and commercial off-line power supplies, as well as in military DC-DC converters. Other applications include EMI differential filter stages, power inductors for nonisolated topologies such as buck, boost, buck-boost and others. Advanced Construction PQC inductors utilize the same advanced geometry cores, patent pending SMD packages, heavy copper planar windings and flexible tab technology as our transformers. Benefits of PQC planar inductors are similar to those of planar transformers such as small size, high quality, reliability, repeatability, and superior thermal management. PQC inductors cover a wide range of rated currents and inductance values from low power SMD chokes with less than a postage stamp footprint to up to 400 amperes capability and yet smaller than a credit card footprint parts.

Operating Range:

A typical inductance curve of a single-winding inductor as a function of bias current is shown in Fig. 2-1 below.

In region I inductance value drops slightly mostly as a linear function of bias current. PQC specifies the end of region I when inductance drops 10% to Lnom from the value at zero bias current. The current associated with Lnom is identified as I max, which is temperature dependent. PQC characterizes cores at 100°C, which is a reasonable maximum steady state operating temperature of a power inductor. When inductor runs at lower temperature than 100°C, the I max value increases, and the "knee" moves to the right. An opposite happens at higher operating temperatures. PQC recommends that designers run all planar inductors in region I where core losses are low and total losses are dominated by winding losses. In region I inductance value is well defined which helps to keep output ripple within specification, and feedback loop stabilization is more predictable.

If bias current is allowed to rise beyond I max, inductance drops in nonlinear fashion along "the knee". PQC identifies the end of region II when inductance drops 25% from the value at zero bias current. Core losses increase significantly in region II and together with winding losses, which are proportional to Ibias , can bring total losses to a threshold of thermal runaway. Sometimes, designers allow inductor to enter region II for a short time and low duty cycle during current limiting operation with a soft restart.

If bias current is allowed to rise beyond the point of 25% drop in inductance value, the part enters region III, where the core looses permeability fast and saturates. No inductor should operate in this region, because thermal runaway and permanent damage are likely. In case of an unexpected power surge, which may take the inductor in this region for a very short period of time, once the surge is removed, the inductor returns to normal operation without any damage.

Planar Inductor Selection

Every ferrite core has a well defined ampere-turns capacity for a given gap at elevated working temperature. Having just a few standard cores a designer can come up with an infinite variety of inductors for every possible gap/turns combination for inductance/current requirement. To help customers select an appropriate core size PQC presents a matrix of parts in Table 2. The matrix places preselected single-winding planar inductors in cells where their I max value intersects corresponding Lnom values. A limited quantity of cells is filled, but they give sufficient understanding of what is achievable. To meet multi-winding requirements PQC developed an Inductor Prototype Request Form.

Planar Inductor cooling

Depending on current rating and core size planar inductors may run without any air flow for cooling. In this case they must be attached to a base plate with controlled temperature. Inductors operating close to I max and at elevated ambient temperature must be cooled by a combination of conduction and air flow.