Measuring the Integrated Circuit Rectifier and Power Link Performance

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CHAPITRE 5 Powering a Pressure Monitor and Micropump

The mouse telemeter available from Millar18 is an example of a wireless, implantable telemeter with supercapacitors for energy storage and inductive power transfer. Though the housing is not suitable for human implantation, and pressure measurement circuitry is not used, the circuitry demonstrates the desired features for a hydrocephalous monitor. Therefore, it has been used to demonstrate the ADSR performance in a telemeter. In addition, a piezoelectric micropump has been used to demonstrate the high power capability of the ADSR.

An Example Telemeter for Hydrocephalus Monitoring

The mouse telemeter is powered wirelessly from a primary that dynamically changes the primary field strength as needed to supply the required power. Field strength is controlled through the primary current via a wireless feedback loop. The telemeter draws approximately 30?? when transmitting and an additional 30 − 50?? when charging the super capacitors. Pressure and temperature data are transmitted to a receiver and can be recorded on a PC. Some key aspects of the telemeter are outlined in the following section.
Pickup and Regulator
The pickup inductor uses a coil similar to the test case coil. It has the same ?0 and ?0 values but triple the number of turns. The voltage is stepped down using a capacitive tap as in Figure 5.1. The combined capacitance ?1?2 (?2 + ?2) ⁄ is tuned to the wireless power resonance frequency. The voltage division ratio changes the effective load such that,
??(??) = ??(?2? + ?2?) ?2?. (5-1)
This provides more freedom in the physical pickup design while still allowing load matching. A full wave diode bridge rectifier is used to convert the sinusoidal voltage into a DC supply that is then regulated with an LDO regulator to provide a stable DC voltage to the circuitry.
Shorting Control
A comparator with two NMOS transistors is used for shorting control across the pickup coil. The combined resistance of the NMOS transistors is approximately 700?Ω. This would result in a 92.75?? power dissipation during shorting control at maximum coupling. This is 3.5 times larger than the calculated 26.5?? dissipated in the IC.
Super Capacitor
A supercapacitor bank is managed with an IC and provides a power buffer in times when the inductive power transfer coupling is too low to power the circuit. The total capacitance is 11.25?? giving approximately 140?? of energy storage when charged to 5V by the supercapacitor management circuit.
Transceiver and Microcontroller
Data storage, transmission and processing all occur on the Nordic19 nRF24LE1 wireless system on chip. It includes an 8-bit CPU, embedded flash and a 2.4GHz transceiver.
Signal Conditioning and Sensor
The telemeter also has biopotential measurement circuitry for monitoring heart rate. It includes a differential amplifier, voltage reference and ADC, similar to what would be required for a wheatstone bridge piezoresistive pressure sensor.

 Adding the ADSR

The diode bridge, shorting control comparator and associated resistors and capacitors were removed from the PCB. The pickup coil and capacitive tap were replaced with the ferrite core test case coil and a single resonant capacitor. A carrier PCB shown in Figure 5.2 was used to attach the IC to the telemeter. The IC was fixed to the PCB using heat curing epoxy, and wire bonds were used to connect the IC nodes to the PCB. The IC and bond wires were then coated in UV light curing epoxy to prevent damage during handling and soldering. PCB was then soldered to the telemeter. Additionally, a supply pad and ground pad were used to connect wires for supplying power to the micropump.

 Powering a Piezoelectric Micro Pump

The air core coil was used to obtain secondary efficiency measurements for powering the micropump as the ferrite core coil encapsulated in the ceramic housing has non-linear core losses with open-circuit voltage. Bartel’s mp6 micropump and micropump controller [39] were powered directly from the ADSR output voltage. The specifications predict the micropump uses less than 200?? and the micropump driver approximately 150?? giving a total power consumption of 350??. The pump was set up to pump water between 5??/??? and 7??/??? according to the product specifications.
Bartel’s mp6 micropump driver requires a supply voltage between 2.5? and 5.5?. The ADSR output voltage stays between 3.3? and 3.6? and could directly power the micropump driver without a low dropout regulator. The supply current was measured with a Tectronix20 TDS2014C oscilloscope across a 250?Ω sense-resistor between the rectifier output supply and the micropump controller input. Using Excel, the current was multiplied with the measured supply voltage to find an actual power draw of 390?? for the micropump and controller. This results in an effective DC resistance of 30Ω at the average supply voltage of 3.45? which is approximately a 15Ω AC load. The secondary efficiency can be found from the plot for the air core coil shown in Figure 5.4 found using equation (2-38).
With the effective 15Ω load, the secondary is expected to be approximately 82% efficent. For a 390?? load this results in a winding loss of approximately 70??. The winding loss was measured as in Section 4.4 over four shorting control cycles and was found to be 66??.
The effective AC load for which maximum power transfer is reached is 100Ω. This is achieved with an approximate 200Ω DC load. With a 3.45? output voltage, the output power at this load would be 60??. The coil losses at the maximum power transfer load are equal to the load power. This result is desirable as the winding losses of 66?? when supplying 390?? are only 17% larger than the winding losses when supplying 60?? at maximum power transfer. Assuming the load power is not dissipated directly as heat, there will be little difference in heating between powering the micropump and telemeter.

Summary

The integrated circuit ADSR was attached to a wirelessly powered bio-potential telemeter for mice and encapsulated in a 20?? x 15?? x 8?? ceramic shell. Leads were attached directly to the ADSR output to supply a micropump. Tests with an air core coils demonstrate the ADSR can supply the required 390?? for the micropump and micropump driver with only 66?? power dissipation in the secondary IPT pickup. The power dissipation is only 6?? larger than the expected losses when supplying a 60?? load.

CHAPITRE 6 Conclusions and Future Work

 General Conclusions

A rectifier and power flow controller suitable for inductively powering an implantable ICP monitor with a micropump or microvalve has been designed, fabricated and experimentally validated.
With the aim of advancing inductive power transfer management to implantable devices, Hydrocephalus was introduced in Chapter 1 as the motivation for this work. The most common treatment options for hydrocephalus patients were outlined and their shortcomings explained. The exceedingly high shunt failure rate, difficulty of shunt failure detection and cost of detection both economically and socially give motivation for the concept of a smart shunt. An implantable electronic device capable of monitoring, active valve adjustment and blockage prevention could greatly improve hydrocephalus patients’ quality of life. It was concluded that batteries could not practically provide power for lifetime implantation based on the concepts of on-demand ICP monitoring, continuous ICP monitoring and active pump or valve adjustment. The suitability of inductive power transfer to provide lifetime power to an implantable device made it the most feasible power supply choice.
The background of IPT was introduced with key concepts such as pickup resonance and single-turn equivalent values. Important differences between series-tuned pickups and parallel-tuned pickups were outlined. A parallel-tuned pickup allowed a more practical coil design with a lower number of turns for the required load range and selected 600??? operating frequency. Furthermore, the parallel-tuned pickup maximum efficiency was shown to occur with a smaller load than the maximum power transfer load. This allowed greater secondary pickup efficiency for times of higher power draw such as during pumping.
Following pickup design, different rectifier topologies were introduced and the active diode synchronous rectifier was analysed in detail. Equations for load power, losses and ADSR efficiency were developed. The practical challenges of limiting current spikes were addressed using a proposed soft turn on technique. Power flow control techniques were also introduced and shorting control was found to be suitable for integration within an IC without any additional components. It was shown that shorting control reduced the effective load resistance seen by the pickup with increased open-circuit voltage. Combined with a paralleltuned pickup matched to the load for maximum power transfer, the decreased load resulted in increased secondary efficiency. Equations describing this result were developed. It was argued that designing an ADSR and shorting control circuit together on a single IC would provide the smallest implant size and highest performance.
A background into integrated circuit fabrication and the CMOS MOSFET was given to provide insight into the challenges faced in IC design. Some basic but important models and circuit structures were explained. An appropriate technology for the rectifier and SC circuit was selected and a background on the IC design process and design tools was also provided. Requirements and a suitable structure were developed for an IC version of the rectifier and SC circuit. Once the ADSR structure was defined, each sub-circuit was developed. The ADSR bridge transistors were sized to provide minimal heating and high rectifier efficiency. An overview of the full ADSR layout was presented with simulations of the efficiency with output power. The rectifier was expected to be over 90% efficient from 30?? to 600??. The peak current limiting soft start technique was investigated and found to keep the current close to 1? without a significant effect on efficiency. Power losses in the shorting control transistors were also simulated and lined up closely with the expected analytical values.
Experimental validation was carried out on fabricated ICs in Chapter 4. Due to the limited number of bond pads, only a limited number of sub-circuits could be tested. The experimental results matched well with the simulated results with the exception of the pass transistor body diodes. Fortunately, the measured body diode forward voltage was significantly lower than expected from simulation giving a higher efficiency during start-up. Measurement setups for the ADSR power conversion efficiency, effect of shorting control on secondary pickup efficiency and shorting control power loss were designed. The experimental results for all setups matched closely with the simulations. The ADSR efficiency was greater than 85% with output powers from approximately 30?? to 600??.
The ADSR was included in a telemeter designed for bio-potential measurements within mice. Except for the pressure sensor, the telemeter demonstrated the small size, necessary components and similar power draw of an ICP monitor. Using the ADSR instead of discrete circuitry removed additional components and allowed a micropump to be included.

 Contributions

The development of an active diode synchronous rectifier and shorting control power flow controller in CMOS has led to contributions not only in integrated circuit power electronic design but also in rectifier and power flow controller analysis, secondary pickup design, and experimental methods for verifying synchronous rectifier performance. The main contributions of this thesis include:
 Designed and fabricated an ADSR integrated circuit suitable for use powering a range of mid-range power active implantable devices including a smart shunt for hydrocephalus.
 The first integrated circuit implementation of shorting control.
 Analysis of the effective load resistance with shorting control (Section 2.7).
 Novel use of the change in effective load resistance from shorting control to increase pickup efficiency with an increase in IPT pickup coupling (Section 2.7 & Section 4.5).
 Novel analysis of current spikes in an ADSR without a DC smoothing inductor (Section 2.5.4).
 Development of a soft-turn-on method to reduce to reduce current spikes in an ADSR rectifier (Section 3.12.3)
 Development of equations characterising the shorting control transistor resistance necessary to remain below a given power dissipation (Section 3.6).
 Improvement of an experimental setup used to calculate the ADSR’s power conversion efficiency providing improved accuracy for measurement results (Section 4.3).
 Development of an analytical equation describing the efficiency and losses in an ADSR (Section 2.5).
 A novel, low-power, band-gap voltage reference circuit was developed (Section 3.10).
 Characterisation of the maximum diode forward voltage drop required for a start-up rectifier to allow ADSR start-up without increased initial field strength or coupling (Section 3.6.1).
 Modification of an existing start-up circuit allowed the design to be implemented with zero static current draw (Section 3.11).
 Implementation of a telemeter with wireless communications and a micropump powered from the designed integrated circuit ADSR (Section 5.1.1).
 Finally, this is the first IC designed, simulated, fabricated and tested at the Auckland Bioengineering Institute. This included, setting up software environment, PDK and identifying the fabrication process appropriate for the ASDR as well as for future use in analogue circuitry to be implemented by the group.

Publications

Journal Papers

Submitted to IEEE Transactions on Circuits and Systems Part I: Robert Gallichan, Ho Yan Leung, David M. Budgett, Rezaul Hasan, Aiguo Patrick Hu and Daniel McCormick, “0-600mW Synchronous Rectifier with Integrated Shorting Control for Transcutaneous Energy Transfer to Medical Implants”
Accepted in Cambridge Wireless Power Transfer Journal: Robert Gallichan, Ho Yan Leung, David M. Budgett, Aiguo Patrick Hu and Daniel McCormick, “Improved Wireless Power Pickup Efficiency using CMOS Synchronous Rectifier with Embedded Shorting Control”

Conference Papers

R. Gallichan, D. McCormick, R. Hasan, P. Hu, and D. Budgett, “Analysis of peak currents in integrated synchronous rectifiers,” New Circuits and Systems Conference (NEWCAS), 2015 IEEE 13th International. IEEE, Grenoble, pp. 1–4, 2015.

 Conference Abstracts

R. Gallichan, D. McCormick, D. Russell, P. Hu, S. Malpas, and D. Budgett, “Miniaturizing wireless power for implantable devices,” in IEEE Instrumentation & Measurement Society Workshop, 2014, p. 1.

Table of Contents
Abstract
Acknowledgement
Table of Contents
Glossary
List of Figures
List of Tables
CHAPITRE 1 Introduction
1.1. Active Implantable devices
1.2. Hydrocephalus
1.3. Monitorin
1.4. The Cost of Hydrocephalus
1.5. A Smart Shunt for Hydrocephalus
1.6. Previous Research into Wireless Power for Implantable Devices
1.7. Objective and Scope
CHAPITRE 2 Structure of the Power Link
2.1. Inductive Power Transfer Fundamentals
2.2. Series and Parallel Pickup Properties
2.3. Parallel Pickup Losses and Efficiency
2.4. Rectifier Topologies
2.5. Active Diode Synchronous Rectifier
2.6. Voltage and Power Flow Control Strategies
2.7. Shorting Control
2.8. Discrete Implementation vs. Integrated Implementation
2.9. Summary
CHAPITRE 3 Integrated Circuit Design
3.1. Notation Conventions for this Chapter
3.2. Integrated Circuit Fabrication
3.3. The Integrated Circuit MOSFET and Common Circuit Structures
3.4. Integrated Circuit Design Process
3.5. ADSR Design for Integrated Circuit Technology
3.6. Power Switches and Start-up Diodes
3.7. Comparator and Bias Circuit
3.8. Bridge-Driver
3.9. Shorting Controller
3.10. Voltage and Current Reference
3.11. Start-up Circuit
3.12. ADSR Design, Simulation and Layout
3.13. Summary
CHAPITRE 4 Measuring the Integrated Circuit Rectifier and Power Link Performance
4.1. Wire Bonding
4.2. Sub-Circuit Measurements
4.3. Measuring the Power Conversion Efficiency
4.4. Multiple Triggering
4.5. Measuring Secondary Efficiency with Shorting Control
4.6. Shorting Control Power Loss
4.7. Comparison with Previous Work
4.8. Summary
CHAPITRE 5 Powering a Pressure Monitor and Micropump
5.1. An Example Telemeter for Hydrocephalus Monitoring
5.2. Powering a Piezoelectric Micro Pump
5.3. Summary
CHAPITRE 6 Conclusions and Future Work
6.1. General Conclusions
6.2. Contributions
6.3. Publications
6.4. Suggestions for Future Work
Appendices
References

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Miniaturising Wireless Power Supplies for Active Implantable Devices