Embedded Inductors with Laser Machined Gap

Estimated reading time: 14 minutes

Table 2: Inductance data.

Table 3: Winding resistance.

As described earlier, gapping reduces the cores sensitivity to DC currents. Figure 7 shows the inductance versus DC current for a typical 5K permeability core and the gapped samples. Prior to gapping, the embedded 10 turn inductors exhibited inductances in the range of 140 to 170 µH. To demonstrate the response of the un-gapped core, a 10 turns inductor with 158 µH at 0 DC bias current was tested. As DC current is applied, the inductance diminished quickly. At 40 mA of DC current, the inductance of the un-gapped core diminishes to half of the starting value. In comparison, the second plot shows the performance for 5 gapped inductors when a DC bias current is applied. Degradation over the range of 0 to 0.5A is about 20% to 25%.

Figure 7: Inductance vs DC current for un-gapped and gapped ferrite cores.

Additionally, devices were tested over temperature. Figure 8 shows the characteristics for the un-gapped and gapped core from 0°C to 100°C. The ungapped core exhibits variation exceeding 20% over the temperature range while the gapped devices exhibit very little variation over the temperature range and remain within 10% of their value at ambient temperature.

Figure 8: Inductance vs. temperature for un-gapped and gapped ferrite cores.

Finally, a few of the inductors were cross sectioned to inspect the laser cut. Figure 9 shows a photo of a cross sectioned part. The direction of the laser cut was from top to bottom. The laser cut has a width in the 0.20 mm to 0.25 mm range.

Figure 9: Cross section view.

Summary

This paper demonstrates the viability for laser gapping embedded transformers and inductors. The PCB structure of the embedded magnetics brings efficiency to the gapping process. A design example was presented for a 10 µH inductor. After gapping, the device exhibited good stability under varying temperatures and DC bias current. Such device would be suitable for SMPC power converters operating in the 500 kHz to 2 MHz range. The embedded toroid structure is useful for reducing the circuit footprint and cost and is suitable for implementation in either a power conversion module or directly in the system board.

The original objective was to produce a 10 µH inductor using a 6 winding configuration and 0.15 mm wide gap. After experimentation, a 0.2 mm gap was implemented. The larger gap allowed debris to be blow out of the gap during the laser process and provided more consistency between inductance of the gapped devices. The caveat is that the larger gap requires designing the inductor to have a higher initial value. A 10 µH inductor was realized using a 10 turn configuration that had a pre-gapped inductance in the range of 140 µH to 170 µH. Design equations and methodology were presented as a guide for designers who want to pursue this technology.

References

1. Ferroxcube Product Selection Guide 2009.

2. Sanjaya ManikTala, Switching Power Supplies A to Z, 2006 Elsevier Inc., Burlington, MA.

This paper was first presented at the 2018 IPC Apex Expo Technical Conference and published in the 2018 Technical Conference Proceedings.

Jim Quilici is a design consultant at Radial Electronics. You may reach Jim at jquilici@radial-e.com. 

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