Comprehensive Modeling of a Back-to-Back Diodes-Based Linear Variable Capacitor

The parasitic nonlinear capacitance of diodes is well-known in power electronics literature. This manuscript analytically describes how the nonlinear parasitic capacitance of two back-to-back reverse-biased diodes can act as a linear variable capacitor (LVC) when driven from an ac current or voltage source. An LVC is a bipolar symmetric structure whose conduction principle is based on the displacement current flowing through the junction capacitance of diodes, and no biasing circuit is needed. The manuscript analytically describes the LVC capacitance dependence on the circuit (current, voltage, and frequency) and diode parameters. An in-depth current- and voltage-based LVC capacitance modeling is presented, including the harmonic analysis, safe operation boundary, and the impact of parasitics. SPICE simulations were used to verify the proposed modeling methodology, followed by experiments on three LVCs made of Schottky diodes. The nonlinear capacitance of the diodes is first characterized for differential capacitance, and then, derived parameters are used to verify the proposed LVC capacitance model with current sources at 1 and 2 MHz. The proposed framework can be used to design resonant power circuits with the LVC as a passive component. An integrated LVC can be applied in high-frequency power circuits for load impedance transformation and power control.