Nexperia Announces ASFET Line of MOSFETs Tailored to Specific ApplicationsNovember 05, 2020 by Gary Elinoff
Because one size doesn’t fit all, Application Specific FETs (ASFETs) allow designers to more easily focus on the specs most relevant to the task at hand
Nexperia’s new targeted ASFET families zero in on motor control, battery isolation, hot-swap and Power over Ethernet (PoE) applications. Among the highlights are maximum current ratings of over 300 amp for motor applications and what Nexperia touts as a three to five time improvement in Safe Operating Area (SOA) for hot-swap applications.
Image courtesy of Nexperia
A World of Changing Priorities
Commented Chris Boyce, Senior Director for the Power MOSFETs Group at Nexperia: “As designers push the boundaries of performance, it is crucial to understand how the MOSFET will be used in the application. There are 100+ parameters on a regular MOSFET datasheet but usually only a few are critical in each project.” He goes on to say that “By keeping individual application requirements front & centre of our thinking, we can choose to optimise the parameters that matter most in a particular use-case, often at the expense of others of less relevance.”
In a separately published article, “Optimizing MOSFETs to fit specific applications”, Boyce mentions the Figure of Merit (FOM), defined as the on resistance times the gate charge, or RDS(on) times QG. He explains how with modern switch mode power supplies (SMPS) operating at ever-faster faster switching speeds, the reverse recovery charge (QRR) is actually more important than OG, making the classical FOM less relevant for some of today’s designs.
Boyce also points out how some of the challenges Nexperia encountered in developing its new ASFETS. For example, enhanced Safe Operating Area (SOA) and low RDS(on) are mutually exclusive in ordinary MOSFETs. Yet the company needed both at a high level for its power over ethernet (POE) and Hot Swap ASSETS.
Nexperia’s new ASFETS come in five broad categories, as summarized below:
ASFETs for Repetitive Avalanche
Nexperia’s ASFETS for Repetitive Avalanch can be employed to power inductive loads such as solenoids. Classical solutions involved a need for additional components to protect the MOSFET when the load current flow continues during switch off.
The company has optimized this family ASFETs to safely handle repetitive avalanche currents while maintaining a junction temperature below 175 °C. Beyond a lower BOM, other results include faster switching and increased efficiency.
ASFETs for Hotswap and Soft Start
Nexperia’s ASFETs for Hotswap and Soft Start sport both enhanced SOA and low RDS(ON). They are proposed to support live board replacement for always on devices such as rack-based computers and storage system
Hot swap for communications. Image courtesy of Nexperia
In these applications, in-rush current must be controlled, requiring a wide SOA. And, once the board powers up and turns on, a low RDS(ON) value is required to keep temperatures down and efficiency up.
ASFETs for PoE
ASFETs for PoE can be applied to the new generation of PoE systems. Based on the new 48V POE standard (IEEE 802.3bt), these devices deliver up to 100 W to powered devices.
These ASFETS must manage inrush current caused by capacitive loads placed on the network through enhanced SOA. They offer better protection, being able to dissipate up to 30 W under +60 °C ambient for up to 20 ms. Additionally, these ASFETs have a small footprint, enabling better overall system density.
ASFETs for Battery Isolation
ASFETs for Battery Isolation protect circuitry when lithium ion batteries (LiB) run out of control due to a fault condition. This application calls for enhanced SOA MOSFETS housed in tough LFPAK packages.
LFPAK package. Image courtesy of Nexperia
ASFETs for DC Motor Control
ASFETs for DC Motor Control need high ID’s for high torque startups and for protection during fault conditions. They will also need VDS’s in the 50 to 55 volt range to accommodate popular 36 volt motors.
The reader who follows the links to the five categories of ASFETs noted above will observe that not all of the devices listed under each category are new. It will also be noticed that a low RDS(ON) is a common virtue across multiple categories.
But, given the high level of emphasis that Nexperia seems to be devoting to the idea of ASFETs, these MOSFETs, tailored to the specific and not the general, may well be an idea whose time has come.