Power Tip GaN Converters Are Transforming Power Supply Design

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GaN converters reduce switching losses and feature innovative circuit designs and better component integration. They also offer improved thermal management and overcome the power limitations of silicon. This Power Tip shows you how it works.

Power supply designs using converter ICs and discrete silicon-based power FETs have dominated power supply design for many years. However, the trend toward ever-higher switching frequencies and greater power density is pushing this technology to its limits and bringing GaN increasingly into focus.

GaN-based power FETs offer better electrical characteristics than silicon devices, and the integration of power FETs, gate drivers, controllers, and passive components into a single compact package in particular opens up new possibilities in terms of efficiency and power density.

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The LMG708B0 (80-V buck converter) and LMG5126 (42-V boost converter) devices are 50% smaller than equivalent silicon solutions without compromising efficiency. These improvements are based on four key factors: lower switching losses, specialized circuit techniques, the integration of multiple components, and packages with improved thermal characteristics.

Reducing switching losses enables a higher switching frequency and the use of smaller passive components. The wide bandgap (WBG) properties and the lateral structure of GaN-based power FETs in enrichment mode enable a lower R_DS(on) value and lower parasitic charges (Q_G, Q_GD, and Q_OSS) compared to silicon power devices. Furthermore, GaN FETs lack the body diode and the associated reverse recovery charge (QRR).

The improved switching characteristics and reduced parasitic effects of GaN converters also lower total power loss. This makes it possible to increase the switching frequency, reduce the size of inductive and capacitive components, and use smaller heat sinks or eliminate them entirely.

Improved Scalability and Efficiency at Low Loads

Innovative circuit techniques improve scalability and efficiency at light loads. The LMG708B0 GaN step-down converter, for example, features intelligent multiphase clock synchronization that communicates frequency and phase position information between phases via daisy chaining. The resulting interleaving reduces input current ripple and allows for the use of smaller EMI filters. Figure 1 shows a sample circuit.

Conventional step-up and step-down converters for voltages ranging from 12 to 80 V and currents exceeding 20 A typically require at least four discrete power devices, including the high-side and low-side FETs. A multi-chip module (MCM) solution, on the other hand, consolidates the entire design into a 22-pin package measuring 4.5 mm x 6 mm x 0.8 mm (0.18 in × 0.24 in × 0.03 in) in FCRLF (Flip-Chip Routable Leadframe), which contains four chips (two GaN FETs, a controller, and a boot-trench capacitor). This reduction in size provides ideal conditions for the steep voltage and current transients characteristic of GaN devices.

The FCRLF technology used in the LMG708B0 and LMG5126 GaN converters supports a thermally optimized package with two heat dissipation paths. The backs of the two GaN FETs are connected to the top of the package, enabling optional two-sided cooling via a heat sink mounted on top. Without a heat sink, most of the heat is dissipated through the bottom into the multilayer printed circuit board and into the environment, whereas with a heat sink in place, the top surface also contributes to heat dissipation (Figure 2).

As the figure illustrates, this creates two parallel heat dissipation paths that reduce the thermal resistance between the junction and the surroundings. The IC can therefore be operated at a lower operating temperature. Alternatively, the operating current can be increased accordingly at a given package temperature. 

*Timothy Hegarty is a member of the Technical Staff for Switching Regulators at Texas Instruments in Dallas, USA.