Choosing the Right Driver for Isolated Power Path and Load Management

Drivers are the interface between a controlling functional block such as a microcontroller or analog loop and a load. Some drivers are basic voltage/current boosters, while others incorporate additional features that enhance the performance of the load.

Although they’re usually chosen second, after the selection of a specific load device such as a transformer, power switch (e.g., MOSFET or IGBT), or cable, the right driver can make the difference between a decent outcome and a really good one. It's easy to gloss over their role and reduce it to being merely an intermediary with little added value, but that’s not the case at all.

Consider a power path that needs galvanic (ohmic) isolation (Fig. 1). A transformer is a preferred way to achieve this, yet even such a basic, reliable, relatively simple passive component can benefit from an enhanced driver.

1. Using a transformer is the preferred way to implement galvanic isolation of a power path between power domains.

Nexperia Transformer Drivers Help Boost Performance

Enabling superior performance when driving a transformer is the role of the NXF6501-Q100, NXF6505A-Q100, and NXF6505B-Q100 driver ICs from Nexperia B.V. (Fig. 2).

2. The Nexperia NXF650x(A/B)-Q100 series incorporates advanced features in addition to the basic driver role (left); in an isolated supply, its role is to galvanically separate two power domains (right).

Qualified to the automotive AEC-Q100 Grade 1 standard, these push-pull transformer drivers enable the design of small, low-noise, and low-EMI isolated power supplies for a range of applications. These range from traction inverters, motor control, and DC-DC converters to battery-management systems and on-board chargers in electric vehicles.

The drivers are also suitable for use in industrial applications like telecommunications, medical applications, instrumentation and industrial automation equipment, solar inverters, energy meters, and programmable logic controllers (PLCs).

The NXF650x(A/B)-Q100 series can drive low-profile, center-tapped transformers from a 2.25- to 5.5-V power supply with high output current (1.2 A at 5 V) and high efficiency of up to 90%. To enable small form-factor designs, the devices have a 440-kHz (NXF6501 and NXF6505B) or 160-kHz (NXF6505A) internal oscillator and a gate-drive circuit that produces complementary output signals to drive internal ground-referenced N-channel power switches (Fig. 3).

3. The three members of the NXF650x(A/B)-Q100 series are similar but have distinct differences.

Precision Control, Ultra-Low EMI, and Extended Protection

Alternatively, a clock signal can be applied externally to the NXF6505A and NXF6505B to enable precise control of switching harmonics or enable synchronization of multiple NFX6505x(A/B) -Q100 devices. Additionally, the NXF650x(A/B)-Q100 series employs slew-rate control and spread-spectrum clocking (SSC) to deliver ultra-low noise and electromagnetic interference (EMI).

Comprehensive testing under various operating conditions demonstrates that these drivers meet stringent international EMI standards CISPR 25 (automotive radiated emissions) and CISPR 32 (industrial radiated emissions), ensuring reliable operation in sensitive electronic environments.

The NXF650x(A/B)-Q100 series also includes a comprehensive range of internal protection features such as overcurrent protection (1.7 A) with hiccup mode, undervoltage lockout, thermal shutdown, and break-before-make circuitry to ensure safe operation. In addition, these transformer drivers have a soft-start (~5 ms) feature that prevents high inrush currents during power-up with large load capacitors.

On top of that, these devices incorporate fail-safe inputs that prevent back-powering of the local power supply and eliminate the need for power-sequence management. This list of functions differentiates these drivers from more-basic units that can do the job in concept but have limited features for dealing with the harsh realities of their application environment.

Although drivers are conceptually simple, their actual complexity and list of features mandates solid documentation to ensure they are used properly and with minimal unwanted surprises. For the NXF6501-Q100, this critical documentation is a 29-page datasheet with many graphs, tables, static and dynamic characterizations, and design-in details; for the NXF6505A-Q100 and similar NXF6505B-Q100; their shared 39-page datasheet contains even more information. Other support collateral incudes a detailed and insightful application note TN90008, “Reduce radiated emissions in isolated power designs using NXF650x-Q100.”

Evaluation Boards for All Drivers

It's now standard practice for vendors to offer an evaluation board for almost every active device and many passive ones. The Nexperia parts ease the design-in challenge as each has its own such board—designated NXF6501-Q100, NXF6505A-Q100, and NXF6505B-Q100—and 10-page User Manual that includes the expected schematic, top and bottom board photos, bill of materials, and board layers (Fig. 4).

4. Each member of the family has its own evaluation bord, again with similarities but distinct differences as well. Top to bottom: the NXF6501-Q100, NXF6505A-Q100, and NXF6505B-Q100.

These devices are available in compact industry-standard footprints like the 5-pin SOT8098 (TSOT5) and 6-pin SOT8061 (TSOT6) package, and they safely operate in ambient temperatures ranging between −55 and 125°C.