Battery powered driver for fundamental mode orthogonal fluxgates
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Battery-Powered Driver for Fundamental-Mode Orthogonal Fluxgates. Prepared by: Anton Plotkin Supervisor: 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.

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Battery-Powered Driver for Fundamental-Mode Orthogonal Fluxgates

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Battery powered driver for fundamental mode orthogonal fluxgates

Battery-Powered Driver for Fundamental-Mode Orthogonal Fluxgates

Prepared by: Anton Plotkin

Supervisor: 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

The Aim

Provide the maximum battery life of the driver for

fundamental-mode orthogonal fluxgates.


Contents

Contents

  • Orthogonal fluxgate.

  • Fundamental-mode operation.

  • Current source.

  • Current source with transformer.

  • Full-bridge driver.

  • Comparison.

  • Conclusions.


1 orthogonal fluxgate

vout

iex

1. Orthogonal fluxgate

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

Advantages:

  • Good resolution.

  • Simplicity and small size compared to parallel fluxgates.

    Construction:

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


2 fundamental mode operation

2. Fundamental-mode operation

iex=±80 mA

Vdc=3.5-4.5 V

RL=2…3 W

PL=5…10 mW


3 current source

3. Current source

vex

D/A

iex

RL

Rs

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 %).


4 current source with transformer

4. Current source with transformer

vex

D/A

iex

RL

Rs

iex/n

Advantages:

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

  • Lower amplifier output current.

  • Lower amplifier supply current:Rs = 0.1 RLn2results in a gain of 2 forn=3.

    Disadvantages:

  • The duty cycle of iex should be 50%.

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


4 current source with transformer1

4. Current source with transformer

vex

D/A

vout

Lm

n2RL

Rs

Llk

AOL

Llk

1/b

Dynamic stability:

b=Rs/(Rs+n2RL||jwLm+jwLlk)

  • vout/vex=1, w<w0

  • vout/vex=jwLm/Rs, w0<w<w1

  • vout/vex=(Rs+n2RL)/Rs, w1<w<w2

  • vout/vex=AOL, w>w2

Lm

w0

w1

w2

w

Lm

b

Llk


5 full bridge driver

5. Full-bridge driver

iex

L

iex

RL

t

VDD

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.


5 full bridge driver control

5. Full-bridge driver: control


6 comparison efficiency

6. Comparison: efficiency

Current 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 size

6. Comparison: size

Current 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

6. Comparison: cost ($)

Current source with transformer:

  • Transformer: 20

  • D/A (DAC8830, TI): 7

    Full bridge:

  • Inductor: 15

  • Current transformers: 2×10


7 conclusions

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.


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