A figure-of-merit for design of high performance inductive power transmission links for implantable microelectronic devices

Abstract

Wireless power transfer through inductive coupling is used in many applications such as high performance implantable microelectronic devices (IMDs). Power transfer efficiency (PTE) and power delivered to the load (PDL) are two conventional inductive link design merits that determine the energy source and driver specifications, heat dissipation, power transmission range, and risk of interference with other devices. Unfortunately designing the inductive link to achieve a high PTE will drastically reduce the PDL and vice versa. Therefore, we are proposing a new figure-of-merit (FoM), which includes both PTE and PDL with proper weights, to help designers of inductive power transfer links to strike a balance between high PTE and PDL at the same time. Three design examples based on the PTE, PDL, and the new FoM have been presented for IMDs to demonstrate the usage and efficacy of the FoM. Our measurement results show that the inductive link optimized based on the FoM can achieved 1.65 times higher PTE than the one optimized for the PDL (72.5% vs. 44%) and at the same time provide 20.8 times larger PDL compared to the one optimized for the PTE (177 mW vs. 8.5 mW for 1 V driving voltage). The inductive links optimized for the PTE and PDL provide 24% higher PTE and PDL compared to the one optimized based on the FoM, respectively.

Fig. 1

(a) Lumped circuit model of the inductive power transmission link with the PA loss modeled with R_s (b) Equivalent circuit at resonance showing the reflected load from secondary onto the primary loop.

Fig. 2

The amount of PTE and PDL losses in percentage compared to the maximum possible value vs. n when the inductive link is designed to maximize the proposed FoM in (4) as opposed to the PTE or PDL. It can be seen that n = 2 results in similar PTE and PDL losses equals to 25%.

Fig. 3

(a) The PTE and PDL measurement setup using network analyzer with all the coils tuned at the carrier frequency, and both R_s and R_L connected in the setup, (b) The inductive link optimized based on the FoM and used to measure the PTE and PDL. Coil specs are listed in Table I

Fig. 4

Comparison between three sets of 2-coil links optimized for PTE, PDL, and FoM vs. coupling distance, d₁₂. R_s = 0.5 Ω and R_L = 100 Ω. a) The calculated and measured values of the PTE and PDL for V_s = 1 V b) The FoM values defined in (4) for n = 2. The inductive links’ specifications for the nominal distance of d₁₂ = 10 mm are summarized in Table I.