Design and Optimization Techniques of Over-Chip Bond-Wire Microtransformers With LTCC Core

A Survey on Microtransformers

“…This area has been mentioned in almost 40% of the papers analyzed in this work [15], [22], [24], [31], [39], [40], [42]- [47], [50], [52], [54], [58], [59], [62], [66], [67], [70]- [73], [79]- [81], [84]- [91], [93]- [99], [101], [105], [116], [120]- [122], [131]- [134]. Some of the examples of specific applications in this area are DC-DC converters, energy harvesting, and power systems on chip.…”

“…Among the different structures, solenoid-type microtransformers can be made with higher turn ratios, with 1:50 being the highest ratio reported [42]- [47]. Regarding the operating frequencies, most of the articles indicate application at frequencies lower than 1 GHz, mainly due to the use of solid cores.…”

“…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].…”

“…Regarding the performances, only a few authors determined the k and Q parameters for microtransformers using the racetrack geometry. Some articles reported coupling coefficients for racetrack solenoid devices to be in the 0.5-0.9 range, but in most cases custom-made core materials were used, instead of those commercially available [43], [47], [61], [70], [83]. For the coplanar devices with racetrack geometry, k values up to 0.93 have been reported by Wang et al [71], [78], but inductance values were relatively low (just a few dozens of nH).…”