16.7 A 20V 8.4W 20MHz four-phase GaN DC-DC converter with fully on-chip dual-SR bootstrapped GaN FET driver achieving 4ns constant propagation delay and 1ns switching rise time

Abstract

Recently, the demand for miniaturized and fast transient response power delivery systems has been growing in high-voltage industrial electronics applications. Gallium Nitride (GaN) FETs showing a superior figure of merit (R_{ds, ON} X Q_g) in comparison with silicon FETs [1] can enable both high-frequency and high-efficiency operation in these applications, thus making power converters smaller, faster and more efficient. However, the lack of GaN-compatible high-speed gate drivers is a major impediment to fully take advantage of GaN FET-based power converters. Conventional high-voltage gate drivers usually exhibit propagation delay, t_delay, of up to several 10s of ns in the level shifter (LS), which becomes a critical problem as the switching frequency, f_sw, reaches the 10MHz regime. Moreover, the switching slew rate (SR) of driving GaN FETs needs particular care in order to maintain efficient and reliable operation. Driving power GaN FETs with a fast SR results in large switching voltage spikes, risking breakdown of low-V_gs GaN devices, while slow SR leads to long switching rise time, t_R, which degrades efficiency and limits f_sw. In [2], large t_delay and long t_R in the GaN FET driver limit its f_sw to 1MHz. A design reported in [3] improves t_R to 1.2ns, thereby enabling f_sw up to 10MHz. However, the unregulated switching dead time, t_DT, then becomes a major limitation to further reduction of t_de!ay. This results in limited f_sw and narrower range of V_IN-V_O conversion ratio. Interleaved multiphase topologies can be the most effective way to increase system f_sw. However, each extra phase requires a capacitor for bootstrapped (BST) gate driving which incurs additional cost and complexity of the PCB design. Moreover, the requirements of f_sw synchronization and balanced current sharing for high f_sw operation in multiphase implementation are challenging.