Driving High-Voltage MOSFETs in Automotive Solid-State Relays Without a Second Power Supply

Automotive and industrial system designers are increasingly adopting higher-voltage architectures to improve energy efficiency, reduce system weight, and support higher power levels. Battery management systems, onboard chargers, and other automotive electronic systems now operate at 400 V, with 800 V platforms fast becoming more prevalent in next-generation EVs. Similar trends are playing out in energy storage systems and industrial power-control equipment.

As operating voltages rise, the need for high-voltage switching methods that reliably maintain crucial electrical isolation between low-voltage control circuitry and high-voltage power stages also increases.

Mechanical relays have long been a widespread option for many high-voltage switching applications, as they provide galvanic isolation while handling substantial loads. However, these components also have several well-known limitations, including contact wear, arcing, audible operation, and finite switching life. As a result, designers are increasingly turning to solid-state relays (SSRs).

SSRs eliminate the concerns of their mechanical counterparts by replacing mechanical contacts with semiconductor switches, typically power MOSFETs. However, MOSFET-based SSRs introduce a different challenge: gate drive. To switch a high-side or back-to-back MOSFET across an isolation barrier, designers must provide a gate voltage referenced to the MOSFET's source rather than the controller's ground.

The common solution is an isolated gate driver paired with a separate isolated power supply on the secondary side. While effective, this approach adds cost and component count, consumes board space, and increases design complexity, particularly in designs with multiple switching channels.

A photovoltaic-output photocoupler offers a different approach. Instead of simply transferring a control signal across an isolation barrier, it generates the gate-drive voltage directly on the isolated side, eliminating the need for a separate isolated bias supply. Toshiba offers one such device, targeting automotive and industrial MOSFET switching applications.

How the TLX9920 simplifies isolated MOSFET Gate Drive

Toshiba’s TLX9920 is a photovoltaic-output photocoupler that provides an isolated gate-drive voltage for external power MOSFETs. Unlike conventional photocouplers that simply transfer a control signal across an isolation barrier, the device generates the gate-drive voltage. Inside the TLX9920, an infrared LED shines onto a series-connected photodiode array. When the LED is energized, the photodiodes generate a DC voltage in much the same way a solar cell converts light into electrical energy. This voltage can then be used to directly charge a MOSFET gate, allowing the device to provide isolated gate drive without requiring a secondary power supply.

The TLX9920 (Figure 1) produces a minimum open-circuit voltage of 13.5 V at a 10 mA input current, providing sufficient gate-drive voltage for many power MOSFETs used in solid-state relay designs. Because the voltage is generated internally, this architecture can eliminate the isolated bias rail and supporting circuitry typically required by conventional isolated gate-drive solutions, further reducing component count and simplifying PCB design.

Figure 1: The TLX9920 combines galvanic isolation and gate-voltage generation in a single package. (Image source: Toshiba)

Toshiba has also addressed one of the traditional drawbacks of photovoltaic couplers. The TLX9920 incorporates a control circuit on the receiver side, eliminating the need for an external discharge resistor and improving switching performance. As a result, the device achieves typical turn-on and turn-off times of 0.6 ms and 0.1 ms, respectively.

While these switching speeds are not intended for high-frequency power conversion applications, they are well suited to relay-style switching tasks where isolation, simplicity, and reliability are often more important than switching speed.

The device's architecture also gives designers greater flexibility than self-contained photorelays. Because the TLX9920 drives external MOSFETs, engineers can select switching devices based on voltage rating, current capability, RDS(on), thermal performance, or system topology. This enables the construction of SSRs capable of handling higher voltages and currents than many integrated photorelays. It is particularly useful in back-to-back MOSFET configurations commonly used for bidirectional blocking and switching in automotive and industrial systems.

The TLX9920 is housed in a low-profile SO6L package measuring 3.84 mm × 10.0 mm × 2.1 mm, and provides a minimum isolation voltage of 5000 Vrms and a creepage distance of more than 8 mm. These characteristics help support reinforced insulation requirements frequently encountered in 400 V and higher systems.

For automotive applications, the device is AEC-Q101 qualified and supports operating temperatures up to 125°C (absolute maximum), making it suitable for demanding environments such as battery management systems, onboard chargers, and high-voltage power-distribution architectures.

Conclusion

As automotive and industrial systems continue to shift toward higher-voltage architectures, photovoltaic-output photocouplers, such as Toshiba’s TLX9920, offer designers a practical way to drive power MOSFETs across isolation barriers without adding unnecessary components or complexity. The TLX9920 combines galvanic isolation and gate-voltage generation in a single package, helping simplify MOSFET-based SSR designs by eliminating the need for a separate isolated gate-drive supply. For applications where isolation, simplicity, and design flexibility are priorities, it provides an effective alternative to more complex isolated gate-drive approaches.