Battery-Powered Driver for Fundamental-Mode Orthogonal Fluxgates

1. 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

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

3. Contents • Orthogonal fluxgate. • Fundamental-mode operation. • Current source. • Current source with transformer. • Full-bridge driver. • Comparison. • Conclusions.

4. 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.

5. 2. Fundamental-mode operation iex=±80 mA Vdc=3.5-4.5 V RL=2…3 W PL=5…10 mW

6. 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 %).

7. 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.

8. 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

9. 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.

10. 5. Full-bridge driver: control

11. 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

12. 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)

13. 6. Comparison: cost ($) Current source with transformer: • Transformer: 20 • D/A (DAC8830, TI): 7 Full bridge: • Inductor: 15 • Current transformers: 2×10

14. 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.