Isolated FET Drivers Deliver Power System Performance

Wide bandgap gallium nitride (GaN) circuits continue to push the boundaries for power density in high-voltage applications. Allegro Microsystems and Transphorm have formed a collaborative initiative to expand the application space for these high-power solutions.

GaN circuits enable high-power EV charging systems. Image used courtesy of Allegro

Allegro and Transphorm are working together to develop GaN power circuits using Allegro’s AHV85110 isolated gate driver paired with Transphorm’s high-power SuperGaN FETs. The partnership aims to create GaN-based solutions that are more robust and efficient, with higher power densities for use in servers, data centers, renewable energy, and electric vehicles.

The FDA117 from Littelfuse is a general-purpose optical isolated driver that introduces a new level of performance for simple, optically isolated drive circuits, with sufficient output voltage and current capabilities to drive standard MOSFET and IGBT devices without additional circuitry.

FET Driver Impacts GaN Circuit Performance

Wide bandgap technologies, like GaN, offer many performance advantages over traditional MOSFET switches, particularly in high-power applications, due to their better voltage withstand capabilities and faster switching speeds. However, the same structural characteristics that enable higher voltage withstands make them more challenging to operate.

The AHV85110 is a high-voltage, transformer-isolated gate driver with an integrated drive supply and separate pull-up and pull-down outputs for improved FET driving performance. Integrating an isolated output bias supply on the chip eliminates the need for additional drive supply circuitry, saving space while reducing solution cost and complexity.

AHV85110 functional block diagram. Image used courtesy of Allegro

Powered from a single 12 V nominal supply, the isolated drive outputs of the AHV 85110 are rated up to 13.2 V, with the ability to source currents up to 2 A and sink currents as high as 4 A. With a short propagation delay time (50 ns), the driver can respond quickly to control signals and keep pace with fast-switching GaN circuits, all across an isolation boundary rated to 5,000 V_RMS.

AHV85110 typical application circuit. Image used courtesy of Allegro

According to Tushar Dhayagude, Vice President of Worldwide Sales and FAE at Transphorm, the AHV85110 GaN FET driver will allow designers working with Transphorm’s SuperGaN FETs to significantly reduce the number of external components in their drive circuits, reducing power stage footprints by up to 30% for data center, electric vehicle, and other applications.  

Allegro driver paired with Transphorm SuperGaN FET. Image used courtesy of Transphorm

A new evaluation board that pairs the AHV85110 driver with Transphorm’s recently developed TOLL packaged GaN FETs (35, 50, and 72 mΩ on-resistances) is available to designers to assist with power circuit designs.

Higher Performance Optical Driver

For applications that are not as challenging as driving fast, high-voltage GaN FETs and optically isolated drivers can be simpler and less expensive solutions while offering high isolation levels.

FDA117 four-pin isolated optical driver. Image used courtesy of Littelfuse

Using optical techniques (LED and photodetector), the FDA117 from Littelfuse provides 5 kV_RMS of input-to-output isolation in a four-lead, surface-mount package measuring 9.5 mm x 4.5 mm, with a height of 3.3 mm.

Limited by the low drive capability of their photodetectors, optical isolators are often used for signal-level isolation applications rather than for driving loads directly. However, the FDA117 is raising the performance bar for optical isolated drivers, and with the ability to deliver up to 60 µA of output current, can drive standard MOSFET and IGBT devices directly without the need for additional drive circuitry.

Driving a MOSFET with an optical isolator. Image used courtesy of DigiKey

The FDA117 is a good option for solid-state relay and isolated switching designs in industrial, energy, building automation, and smart homes.
 

The Right Tool for the Job

As with any tool, one is not better than the other; it is just a question of finding the right tool for the job.

The FDA117 from Littelfuse consumes about 30 mA of LED current at its input to deliver 60 µA of drive current from the PD output. The AHV85110 can deliver up to 2 A of peak drive current (intermittent) at high switching frequencies while consuming only 11 mA of supply current.

However, in addition to pure performance and drive power, designers must also factor in the relative priority of solution size, cost, and complexity when determining which isolated driver technology is best for their application. 

Overdesign can kill a product or project just as quickly as an inadequate design.