Opportunities and Challenges in Very High Frequency Power Conversion

Abstract

T HIS paper explores opportunities and challenges in power conversion in the VHF frequency range of 30-300 MHz. The scaling of magnetic component size with frequency is investigated, and it is shown that substantial miniaturization is possible with increased frequencies even considering material and heat transfer limitations. Likewise, dramatic frequency increases are possible with existing and emerging semiconductor devices, but necessitate circuit designs that either compensate for or utilize device parasitics. We outline the characteristics of topologies and control methods that can meet the requirements of VHF power conversion, and present supporting examples from power converters operating at frequencies of up to 110 MHz.

High-frequency isolated ac-dc converter with stacked architecture

“…The isolated bus converter provides galvanic isolation, voltage transformation while delivering combined power to the converter system output. With the stacked ac-dc architecture and the feedback control of the regulating converters, the isolated converter operates from a low / narrow-range input voltage, and does not need to provide regulation capability; thus the isolated converter can be designed to be very compact operating at high frequency [11] using a capacitively-aided isolated bus converter [16]. The selected capacitively-aided isolated bus converter maintains ZVS condition for all the primary and secondary switches independent of the power level, which thus provides high efficiency with high frequency operation.…”

“…However, these approaches are not amenable to converter miniaturization because: 1) it is hard to greatly reduce the volume of the converter through high-frequency operation owing to loss limits, large inductance (i.e., high characteristic impedance level), and large parasitic capacitance levels (e.g., large output capacitance of the switch) [11]- [13]; 2) the volume of the energy buffer capacitor is large, and 3) for a boost front end, the following dc-dc converter must operate at high voltage when it is tied to the stepped up (high) voltage after the boost PFC converter, and has a large step-down voltage conversion ratio (especially for low output voltage application), so that it is again difficult to be realized at high frequency with small volume.…”

Two-stage power conversion architecture for an LED driver circuit

“…To attain greater miniaturization, increases in switching frequency are necessary because the values of inductors and capacitors vary inversely with switching frequency. However, the sizes of passive components do not necessarily decrease monotonically with frequency, owing to magnetic-core loss, voltage breakdown, and heat transfer limits [2]- [7]. Consequently, achieving substantial miniaturization through high frequency operation further relies upon appropriate passives design and careful selection of circuit topology to minimize the demands placed upon the passive components, especially the magnetic components.…”

“…The buck converter is "inverted" in the sense that it is designed with "common positives" so that the active switch is referenced to a constant ground potential. Referencing the switch in this way is of great practical importance in achieving extreme high-frequency operation [2].…”

“…A third consideration relates specifically to operating characteristics at high voltages and low power levels. Converters operating at high-voltages and low-currents operate at high impedance levels, and consequently utilize relatively large inductors and small capacitors (e.g., characteristic impedance Z 0 = L/C scales as V /I [2]). Furthermore, inductor and capacitor values scale down with increasing resonant frequency (e.g., ω 0 = 2πf = 1/ √ LC).…”