Design of Low-Voltage Integrated Step-up Oscillators with Microtransformers for Energy Harvesting Applications

Abstract

This paper describes the modeling of startup circuits in battery-less micropower energy harvesting systems and investigates the use of bond wire micromagnetics. The analysis focuses on step-up Meissner oscillators based on magnetic core transformers operating with input voltages down to 100 mV, e.g., from thermoelectric generators. As a key point, this paper examines the effect of core losses and leakage inductances on the startup requirements obtained with the classical Barkhausen criterion, and demonstrates the minimum transconductance for oscillations to occur. For validation purposes, a step-up oscillator IC is fabricated in a STMicroelectronics 0.32 technology, and connected to two bond wire microtransformers, respectively, with a 1:38 MnZn ferrite core and with a 1:52 ferromagnetic low-temperature co-fired ceramic (LTCC) core. Coherently with the proposed model, experimental measurements show a minimum startup voltage of 228 mV for the MnZn ferrite core and of 104 mV for the LTCC core.

A Survey on Microtransformers

“…The high L-values that can be reached using this structure is another advantage. Macrelli et al, for example, built a~4 mm diameter toroidal microtransformer that achieved 315 µH inductance in the secondary winding [46]. Their prototype had a turn ratio of 1:38 and a coupling factor close to 1 (up to 4 MHz).…”

“…The windings were obtained by wire bonding over a micromachined ferrite core bonded to the copper layer in a PCB substrate, enabling the coils to be mounted with a small bond pad pitch. In general, the performance of solenoid-type microtransformers is good, with many of them presenting coupling coefficients higher than 90% [48], [44]- [46], [49]- [60] and with some displaying quality factors higher than 10 [46], [47], [55]- [58], [60]- [66].…”

“…This geometry is exclusively used in solid core solenoid devices and in coreless toroidal microtransformers [42], [44]- [46], [48], [114]- [120]. As a consequence of the solenoid structure, these microtransformers provide the best overall performance parameters, with inductance values up to 400 µH, quality factors higher than 20 and coupling coefficients higher than 90%.…”

Autonomous Energy Harvester Based on Textile-Based Enzymatic Biofuel Cell for On-Demand Usage

“…Our approach is to use the DC output voltage of the biofuel cell as a trigger signal of commercial quartz oscillator (OV-7604-C7, low power crystal oscillator 32.768 kHz, Micro Crystal Switzerland). Referring to datasheet of the quartz oscillator, required minimum DC voltage is 1.2 V. This minimal DC voltage could be made from output of biofuel cell by using a transformer [ 43 ]. In our case, constraint on device size is not a concern as it is based on textile for workout wear and thus multiple stacks of current-generating electrode having corresponding fluidic channel and common reservoir at the end of the channel have been used to create wanted DC output voltage.…”

“…Basically, it is achieved by matching load resistance with the internal resistance (or impedance) of the energy harvesters. For dealing with low input power or voltage of energy harvesters based on biofuel cells, intermediate capacitor for accumulation of enough charge to transfer to output [ 42 ] or micro-transformer for step-up oscillator [ 43 ] have been used. Alternatively, parallel multiple fuel cell elements are proposed for most efficient energy harvesting [ 44 ].…”