TABLE II.

Multi-Coil Inductive Link Specifications for the FoM-Based Optimization for HMD Charging Application

Parameters		Symbols	2-coil	3-coil	4-coil
L1	Inductance (µH)	L1	-	-	0.21
	Outer diameter (cm)	Do1	-	-	3
	Wire diameter (mm)	w1	-	-	1.6
	Num. of turns	n1	-	-	2
	Wire spacing (µm)	s1	-	-	100
	Quality factor*	Q1	-	-	34.6
L2	Inductance (µH)	L2	1.4	1.3	0.52
	Outer diameter (cm)	Do2	12.5	11.7	15
	Wire diameter (mm)	w2	1.6
	Num. of turns	n2	2	2	1
	Wire spacing (µm)	s2	100
	Quality factor*	Q2	116.8	117.5	427
L3	Inductance (µH)	L3	4.6	0.13
	Outer diameter (cm)	Do3	4
	Wire diameter (mm)	w3	0.64
	Num. of turns	n3	9	1
	Wire spacing (µm)	s3	100
	Quality factor	Q3	222.8	553.3
L4	Inductance (nH)	L4	-	40.8	42.4
	Outer diameter (cm)	Do4	-	1.6	1.65
	Wire diameter (mm)	w4	-	0.64
	Num. of turns	n4	-	1
	Wire spacing (µm)	s4	-	100
	Quality factor	Q4	-	1092	1068
L2 and L3 coupling distance (cm)		d23	10
Max. wire diam. of Tx coils (mm)		w1/2,max	1.6(AWG14)
Max. wire diam. of Rx coils (mm)		w3/4,max	0.64(AWG22)
Nominal source resistance (Ω)		RS	0.5
Nominal load (Ω)		RL	5
Power carrier frequency (MHz)		f0	13.56
Power transfer efficiency (%)		η	28.4	36.1	42.8
Power delivery to load** (mW)		PDL	79	92	14.4
Figure-of-Merit (mΩ−1)		FoM	6.4	12	20.9

^*Q₁ in 4-coil and Q₂ in 2- and 3-coil links also include R_s = 0.5 Ω.

^**Delivered power when V_s = 1 V

Grayed rows indicate the design constraints.