Why Low ON-Resistance MOSFETs Are Key to Faster, Smaller, and Cooler Charging Designs

Fast charging isn’t just a consumer convenience—it’s a demanding engineering problem. As battery capacities grow in compact devices like smartphones, tablets, and wearables, engineers are under pressure to deliver more current in less space, all while keeping heat and power loss under control. Achieving this balance requires precision in power path design and careful component selection, particularly when it comes to the switching MOSFETs that regulate and protect high-current flow.

Traditional fast-charging designs often rely on dual-MOSFET configurations to implement bidirectional protection and power delivery. These setups meet performance targets but at the cost of board space and thermal headroom. As device form factors shrink and current levels rise, those trade-offs become harder to justify. Engineers need a more compact, integrated solution that doesn’t compromise on electrical performance.

Addressing the Trade-Offs in Power Path Design

Modern fast-charging circuits have strict requirements: ON-resistance typically under 5mΩ, breakdown voltage between 28V and 30V, and current ratings of 20A or more. Meeting these specs while maintaining compact PCB layouts often forces designers to juggle between performance and footprint—sometimes using two large MOSFETs to meet efficiency and protection goals.

One alternative now gaining traction is the use of wafer-level chip-scale packaging (WLCSP) and high-density cell structures to increase performance per unit area. These structural changes offer a path toward lowering ON-resistance while minimizing the physical size of the device, enabling higher power density without additional complexity.

A New Option: ROHM’s AW2K21 MOSFET

ROHM Semiconductor recently introduced the AW2K21, a 30V N-channel MOSFET that combines high current handling with an ultra-compact footprint. The device delivers a typical ON-resistance of just 2.0mΩ in a 2.0mm × 2.0mm package. By integrating two MOSFETs into a single common-source configuration, the AW2K21 supports bidirectional protection in charging and power supply circuits—functionality that usually requires two discrete components.

This allows designers to consolidate the fast-charging protection path into a single part, reducing both board space and routing complexity.

Structural Innovation Behind the Specs

Unlike conventional vertical trench MOSFETs that place the drain terminal on the backside of the die, the AW2K21 locates the drain on the top surface, making it suitable for WLCSP integration. This improves the chip-to-package ratio and further reduces ON-resistance per unit area.

The result is a device that maintains low conduction losses even at high currents—minimizing thermal buildup in dense applications. Compared to using two 3.3mm × 3.3mm MOSFETs, the AW2K21 offers up to 81% reduction in footprint and approximately 33% lower ON-resistance. Even against similarly sized GaN HEMTs, it delivers as much as 50% less ON-resistance, offering a simpler thermal profile and lower power consumption.

Practical Benefits for Portable Devices

This kind of high-efficiency switching in a compact package has clear implications across the fast-charging ecosystem. From smartphones and VR headsets to compact printers, drones, and portable consoles, the ability to shrink power stages without compromising performance opens up new design possibilities.

The AW2K21 also works well as a unidirectional protection MOSFET in load switch applications, thanks to its low ON-resistance and thermal stability. For engineers facing thermal constraints or layout limitations, it provides a flexible option to meet multiple design goals.

ROHM is already working on pushing miniaturization further, with a roadmap that includes an even smaller 1.2mm × 1.2mm version of this technology.

A Step Forward in Power Component Integration

As consumer electronics evolve, the supporting components must evolve with them. Engineers are being asked to do more with less—less space, less power loss, and fewer components. Innovations like ROHM’s AW2K21 help meet that challenge head-on, offering a compact, high-performance solution for today’s fast-charging needs.

Product Snapshot

• Part Number: AW2K21

• Type: 30V N-channel MOSFET (common-source configuration)

• Typical ON-resistance: 2.0mΩ

• Package: 2.0mm × 2.0mm (WLCSP)

• Use Cases: Smartphones, tablets, VR devices, wearables, drones, portable gaming consoles, and more