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A Constant-Current ZVS Class-E DC-DC Converter with A Loosely Coupled Transformer with PCB Windings

air-core transformer
Fig. 1. The drawing of the air-core transformer and the air-core transformer.
  Current source is used in gate drivers, battery chargers, and wireless power transfer. Class-E converter is a good candidate for the application because only one switch is required and ZVS is achieved. This work proposed an isolated class-e dc-dc converter with constant-current (CC) output and ZVS over a wide load range with finite input inductance. An air-core transformer is used for the isolation stage for lower profile and elimination of core loss. The air-core transformer with PCB winding has two planar layers. The radius is 5 mm. The drawing of the air core transformer and the hardware is shown in Fig. 1.
  The converter can be separated with a class-e inverter and a compensation network. The Class-e inverter can be equivalized with a voltage source in series with a capacitor. The fundamental output voltage can be constant if the capacitor is compensated at a certain switching frequency and the load is resistive. The CC output is realized by adding a CLC network to the Class-E inverter. Capacitor C1 and C2 compensate the self-inductance at the primary side and secondary side to realize a resistive load. For duty ratio (D) equal to 0.5, the switching frequency needs to be 1.29 of the resonant frequency of Lin<\sub> and Cin<\sub>. The leakage inductances Ll1<\sub> and Ll2<\sub> need to be large enough to suppress high-order harmonics. In this work, the self-inductance is 600 nH and mutual-inductance is 360 nH. The switching frequency is 6.78 MHz and the output current is 700 mA.
  Fig. 2. shows the testing results with 10 V input and different load-resistances. ZVS is always maintained within the load range and output voltage increases linearly with the load-resistance, which is the characteristics of CC output. The peak efficiency of the converter is 80%.
  In summary, this work demonstrated the principle of realizing CC output on the Class-e dc-dc converter with an air-core transformer. The work can be a good candidate for high-temperature and low-profile applications with the elimination of magnetic cores. The efficiency can be further improved by optimizing the air-core transformer and will be discussed in the future.
Testing waveforms
Fig. 2. Testing waveforms and outptut characteristic of the converter.

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