Choosing a Better Transformer for Gate Drivers

Applications like automotive traction inverters, on-board chargers, and solar inverters are expected to handle more power than ever before yet fit into an ever-decreasing footprint. As power density increases, every circuit on a PCB needs to be efficient.

One such circuit is the gate driver, which controls the ON/OFF switching of a high-power transistor. The gate driver requires its own power supply that utilizes an isolation transformer. Incorrectly designed, this transformer can cause a system to fail electromagnetic interference (EMI) tests.

Things become even more challenging as applications shift toward faster-switching transistors made from silicon carbide (SiC) or gallium nitride (GaN). More efficient than the silicon components they’ve replaced, these transistors switch at frequencies fast enough to generate electrical noise, which can adversely affect the transformer in the gate-driver supply.

In this article, we’ll explore why LLC resonant converters have become the topology of choice for gate-driver supplies, and how planar transformers compare to conventional wirewound ones.

The complexities of LLC resonant converters

LLC resonant converters have become the most common circuit design for isolated gate-driver power supplies due to their efficiency. An LLC circuit is tuned to operate at a specific frequency, where each switching transition happens at or near zero volts. This reduces wasted energy from heat generation, resulting in a power supply that runs efficiently at high switching frequencies.

What makes LLC resonant converters unusual is the role the transformer plays in keeping the switching frequency stable. Every transformer is imperfect in two ways. For one, some magnetic energy put into the primary side doesn’t get coupled to the secondary side. This “trapped” energy is called leakage inductance. Secondly, even though the two sides of a transformer are physically separated by an isolation barrier, a small amount of common-mode current can still leak across the barrier as interwinding capacitance.

In most power supply designs, leakage inductance is just a nuisance to be minimized. In an LLC circuit, the trapped energy is actually part of how the circuit resonates, so it needs to be a specific, predictable amount—not necessarily low, but consistent. Interwinding capacitance, by contrast, remains purely unwanted and needs to be kept low.

The problem with wirewound transformers

Most transformers in use today are wirewound, with copper wire wound around a ferrite core. They’ve worked well across power electronics for decades, and with careful design, they can achieve low interwinding capacitance and reasonable leakage inductance control.

The difficulty is that the winding process is difficult to control during volume production. Wire tension, layer placement, and winding geometry all influence the final electrical characteristics, and these variables shift between production runs. A transformer that’s perfectly in tune on the bench may come out slightly different at scale, impacting consistency in each LLC circuit. Wirewound transformers also tend to be tall, which causes problems in dense PCBs where real estate is at a premium.

Where planar transformers come in

A planar transformer solves the wirewound problem by getting rid of the winding process entirely. Instead of copper wire wound around a core, the windings are copper traces printed onto a circuit board, the same way any other circuit on a PCB is made. The flat ferrite core is assembled around the planar conductors.

Because the transformer is now made the same way as the rest of the PCB, its characteristics are controlled by the same precise, automated processes and controls. This makes the leakage inductance and interwinding capacitance much more consistent from unit to unit; it’s not dependent on how tightly a wire was wound on a given day. The tighter control also means planar transformers produce less conducted EMI than their wirewound equivalents.

Another big benefit is height. A planar transformer sits very close to the board surface, helping to maintain creepage and clearance requirements in applications like traction inverters and on-board chargers where multiple high-voltage domains need to coexist within a compact enclosure.

Bourns SM9127xAL series

Bourns makes two versions of planar transformers: the SM91270AL and SM91271AL (Figure1). Built on the same platform and designed for low power consumption, both target the same isolated gate-driver supply applications.

Figure 1: Bourns SM9127xAL series planar transformers. (Image source: Bourns)

The main difference between the two is their turns ratio—i.e., the ratio of windings on the primary side to the secondary side, which determines the relationship between input and output voltage. The SM91270AL has a 1:1 ratio, so the input and output voltage are equal. The SM91271AL has a 7:8 ratio, meaning the output is slightly stepped up relative to the input.

Both have been developed in conjunction with specific gate-driver ICs from TI, MPS, and ADI/Maxim, so engineers working with those chips have a valid starting point rather than designing around the transformer from scratch.

The SM9127xAL series is automotive-grade, rated for a working voltage of up to 850 V_DC with reinforced insulation and a Hi-Pot isolation voltage of 6,000 V_DC. The operating temperature range of -40°C to +125°C covers the full automotive thermal envelope. Both models carry AEC-Q200 and RoHS compliance, with a maximum output power reaching up to 5.4 W. The materials are also corrosion-resistant, contributing to a longer service life.

These planar transformers sit less than 3.5 mm above the PCB with no base structure beneath them, and are supplied on 800-piece, 13-inch tape-and-reel packaging for automated assembly. Manufacturing is certified to ISO 14001, which requires facilities to minimize their environmental impact, including energy use. According to Bourns, the packaging is designed with eco-logistics in mind.

Conclusion

As gate-driver power supplies operate at higher frequencies and with space-constrained board layouts, the transformer in the resonant circuit becomes a critical design element. In an LLC converter, its leakage inductance helps define the resonant tank, while its interwinding capacitance can provide a path for high-frequency noise across the isolation barrier.

Planar transformers are well suited for power system designs due to their intrinsic advantages over traditional wirewound transformers. PCB-based construction brings the same process precision to the transformer, producing more predictable electrical characteristics across production volumes.

The Bourns SM9127xAL series applies these advantages to automotive-grade isolated gate-driver bias supplies, delivering controlled leakage inductance and low interwinding capacitance in a low-profile, automotive-grade package.

To learn more about the SM9127xAL series, visit Automotive Gate Driver Planar Transformers SM91270AL and SM91271AL.