Battery-Powered Driver for Fundamental-Mode Orthogonal Fluxgates

Prepared by: Anton PlotkinSupervisor: Professor Shmuel Ben-Yakov

Department of Electrical and Computer Engineering Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel

28.06.06

The Aim

Provide the maximum battery life of the driver for fundamental-mode orthogonal fluxgates.

Contents

1. Orthogonal fluxgate.

2. Fundamental-mode operation.

3. Current source.

4. Current source with transformer.

5. Full-bridge driver.

6. Comparison.

7. Conclusions.

1. Orthogonal fluxgate

15 mm

1 m

m

20 mmAmorphous wire

Sensing coil

Crimp connectors

iex

Printed circuit board

Advantages:

• Good resolution.

• Simplicity and small size compared to parallel fluxgates.

Construction:

• Core: Co-based amorphous wire, 120-m diameter.

iex

vout

From: Macintyre S.A., Magnetic Field Measurement.

2. Fundamental-mode operation

i ex

, mA

0 2.5 5.0

0

80

-80

4.90.1 2.4

Time, ms

Positive excitation

Negative excitation

2.6

Vdc=3.5-4.5 V RL=2…3 iex=±80 mA

PL=5…10 mW

3. Current source

Advantages:

• Simplicity.

Disadvantages:

• Requires a bipolar supply voltage to obtain a bipolar iex.

• Requires a gain of 10 (Rs ≈ 0.1 RL) to reduce the losses of Rs, which yields a relatively high amplifier supply current.

• The output current of the op-amp is iex.

• The efficiency of the output stage is low (10 %).

D/A

Rs

vex iex

RL

4. Current source with transformer

D/A

Rs

vex

Advantages:

• The maximum efficiency of the output stage is 75%.

• Lower amplifier output current.

• Lower amplifier supply current: Rs = 0.1 RL n2 results in a gain of 2 for n=3.

Disadvantages:

• The duty cycle of iex should be 50%.

• Transformer: core and copper losses, large size, EMI.

iex/n

iex

RL

4. Current source with transformer

D/A

Rs

vex

Dynamic stability:

=Rs/(Rs+n2RL||jLm+jLlk)

• vout/vex=1, <0

• vout/vex=jLm/Rs, 0<<1

• vout/vex=(Rs+n2RL)/Rs, 1<<2

• vout/vex=AOL, >2

n2RLLmLlk

AOL

Lm

Llk

Llk

Lm

vout

5. Full-bridge driver

iex

VDD

RLL

Advantages:

• Any duty cycle of iex with a single VDD.

Disadvantages:

• Triangle-wave current instead of sine-wave one.

• The control requires current measurements.

• The dependence of frequency on L and VDD.

• Inductor: core and copper losses, large size, EMI.

iex

t

5. Full-bridge driver: control

L5

{L*n*n}

1

2

R15400k

0

R14

1k

D4

1N4678

12 L3

{L*n*n}

1

2

Vdd

Vdd

U4A

74HC14

1 2

714

U4B

74HC14

3 4

714

U6SD103A

01

12

PARAMETERS:VDD = 4.2VImax = 0.09Imin = 0.004Rload = 3n = 100L = 10uH

R6200k

R161k

C6

1nD31N4678

12

R17

1k

Vdd

L1

0.5mH

1 2D

pre

Vdd

U10SD103A

01

12

U2SI1539DL

Dn

Gn

Sn

Dp

Gp

Sp

6

21

3

54

C31n

E1

V(%IN+, %IN-)EVALUE

OUT+OUT-

IN+IN-

L2 {L}12

pre

L4 {L}12

R5230k

R240k

0

0

D2

1N4678

12

U3A

TLV2352I/3_1/TI3

2

84

1

+

-

V+V-

OUT

R221k

Vdd

R420k

K K2

COUPLING = 0.99K_Linear

L1 = L4L2 = L5

Vdd

D

U3B

TLV2352I/3_1/TI5

6

84

7

+

-

V+V-

OUT

E2

V(%IN+, %IN-)EVALUE

OUT+OUT-

IN+IN-

U5A

74HC74

3

1

2

4

5

147

6CLK

CLR

D PRE

QVCC

GND Q

U1SI1539DL

Dn

Gn

Sn

Dp

Gp

Sp

6

2 1

3

5 4

U15SD103A

01

12

U9SD103A

0 112

C10.1n

0

K K1

COUPLING = 0.99K_Linear

L1 = L2L2 = L3

U7SD103A

0 112

R340k

R121k

V1{VDD}

R1 {Rload}

6. Comparison: efficiencyCurrent source with transformer: 30 % efficiency

(Irms=50 mA)

• Output stage efficiency: (3.6/4.2) x 75 % = 65 %

• Transformer: Pcopper=6 mW

• Op-amp: Pqs=5 mW

• P(Rs)=1 mW

Full bridge: 40 % efficiency (Irms=60 mA)

• P(RDS on)=9 mW

• Inductor: Pcopper=4 mW, Pcore=1.4 mW

• P(Rc)=1.5 mW

6. Comparison: sizeCurrent source with transformer:

• Transformer: toroid, D=4.83 mm, H=2.54 mm (w/o winding)

Full bridge:

• Inductor: pot, D=7.24 mm, 2B=4.16 mm

• Current transformers: toroid, D=2.54 mm, H=1.27 mm (w/o winding)

6. Comparison: cost ($)Current source with transformer:

• Transformer: 20

• D/A (DAC8830, TI): 7

Full bridge:

• Inductor: 15

• Current transformers: 2×10

7. Conclusions• The maximum efficiency of the driver is 40 %

(the minimum losses are 16 mW).

• The main factors limiting the efficiency of the current source are the supply of the op-amp and the transformer copper losses.

• The main factors limiting the efficiency of the full bridge are the switching losses (either RDSon or gate driving) and the inductor copper losses.

• Both the transformers and inductor should be carefully shielded to reduce the EMI.