A new planar inductor with ring-connected magnetic core

m and n are the numbers of the column and of the row of the ring cores of the RCMC, po and ps are the permeability of the space and of the magnetic core, respectively, tm is the thickness of the magnetic core. Lsr is men times the inductance of the single ring core inductor with N turns. The inductance corresponding to that of the RCMC inductor with Is=2xro. W h e n H c r / H s r = 2 and me n>>(m+n), L c r / L s r has maximum value of 2. coil RCHC c o i l ring core
Hsr =
ro-ri
Ni
I
h(r)dr
(1)
ri
for the single ring core
Hcr =
ro-ri
2N i
(ro+ri)/2 h(r) dr
J ri
(2)
for the RCMC where N is number of the turns, i is current i n the coils.
ri
A
ro
Fig. 1 Ring-connected magnetic core (RCMC).
Fig.2 shows magnetic field distribution in a single ring core. Fig.3 shows magnetic field distribution in the A - A cross section of the RCMC shown in Fig.1. In this case, Is is assumed to be equal to 2xri. The average magnetic fields H in the cross sections of these cores are approximately expressed as follows;
*Amorphous Magnetic Device Laboratories, Sendai 982 Japan **Hachinohe Institute of Technology, Hachinohe 031 Japan
Abstract- A new type planar inductor employing ring-connected magnetic core has been devised. By connecting small planar ring cores in matrix array, the magneto-resistance of the magnetic circuits in the core is reduced and high inductance, which is about 1.5 times that of the conventional ring core inductance, is achieved. Moreover, by exciting the core with two coils arranged orthogonally, the stray capacitance which restricts the upper limit of the useful frequency band is decreased. The inductor is suitable for micro inductor operating at high frequency. The ring-connected core can be also used as a transformer with high coupling coefficient and low stray capacitance.
2268
IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5 , SEPTEMBER 1990
A NEW PLANAR INDUCTOR WITH RING-CONNECTED MAGNETIC CORE
K.Shirakawa*, S.lshibashi*, Y.Kobayashi*, and K.Murakami** F.Takeda*
INTRODUCTION Planar inductor operating at high frequency is useful for reducing the size of power supplies because of its small size. Several kinds of planar inductors exist [ 1][2][3]. The magnetic circuits of most inductors are open circuits and the cores are not effectively utilized. It is difficult to achieve high inductance proportional to the permeability of the magnetic core. To increase the operating frequency and to achieve higher inductance with small surface area, the magnetic circuits must be small and closed. The stray capacitance must be small too. This paper describes a new planar inductor with ring-connected magnetic core. The magnetic core is composed of many planar ring cores arranged in a matrix array. The ring cores are connected to each other forming closed magnetic circuits with low magneto-resistance. The magnetic core is excited by orthogonally arranged two coils with low capacitive coupling. The new planar inductor has higher inductance and lower stray capacitance per unit winding and is suitable for micro inductor at high frequency. RING-CONNECTED MAGNETIC CORE (RCMC) The ring-connected magnetic core (RCMC) is shown in Fig.1 where ri is the inner radius and ro is the outer radius. Ring cores are periodically arranged and connected. As shown in Fig.1, in the RCMC, two kinds of closed magnetic circuits are formed. One is a circular loop and the other one is square. Therefore, when the circumference IS of the square loop is shorter than that of the outer circle of the ring core, the magneto-resistance of the RCMC becomes smaller than that of a single ring core.
0018-9464/90/0900-2268$01 .OO
0 1990 IEEE
2269
It is well known that
Lsr = N
p o ps (ro-ri) t m
mnHsr i
(6)
h(r) =
27tr
Ni
(3)
The ratio of the average fields i s given by the following equation;
n
z
1 1
STRUCTURE OF THE INDUCTOR
There are two winding methods as shown in Fig.4. One is parallel winding and the other is orthogonal winding. From the orthogonality of the electric and magnetic fields, it follows that there is no coupling between the fields excited by the orthogonally arranged two coils except due to the coupling through the magnetic field of the core. In case of orthogonal winding, the stray capacitance of the coil, which restricts the upper frequency becomes negligibly small.