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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

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.
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.