case study # 05

Integrated GaN Cascade High Current Driver Integrated Circuit

A specialty semiconductor company sought to further enhance a Gallium Nitride (GaN) power transistor they had created so it could be used in a wider range of applications –from electric car power systems to chargers for laptops and mobile devices. An important component in realizing this goal was to introduce a CMOS chip to control the transistor, and manage its high voltage and current levels.

Kapik was chosen to design this ASIC based on its strong track record in designing smart analog chips and solving issues relating to power. We needed to address the following considerations:

• Work within several challenging package, cost and engineering constraints: Many power converters require both high and low-side devices, which are either isolated from each other and have separate grounds or have access to a very high-voltage process. In this case, power supply and communication could only be routed through the low-side chip; package and cost constraints restricted access to the high-side chip. There were significant CMOS process limitations (32V tolerant devices) and the design also needed to address issues associated with on-chip currents, transistor limitations and limited pin availability.

• Measure and control GaN transistor operations to ensure safety: The chip we designed needed to measure a variety of operations – including driver and GaN power switch temperature; drain to source current Ids; drain to source voltage Vds; gate voltage; gate current; middle node voltage Vmid for a circuit configuration where the GaN transistor is in a cascode configuration; di/dt slew rate control. It also needed to sense if the voltage across the GaN transistor was safe and have controls to prevent over-voltage spikes for Vds and Vmid, and include protections against over-current.

Kapik took a creative approach to meeting our client’s challenging requirements, delivering a chip that affords considerable user and application flexibility in spite of significant package and cost constraints. We achieved this by:

• Designing an IO ring to address pin bonding constraints: This enabled us to manage issues associated with grounding and signal connectivity with the GaN transistor.

• Resolving power access and signal communication issues for the high-side chip by using high-frequency signaling: The only source of power and signal communication was via the low-side chip, so we developed a design that transmitted power via a high frequency signal from the low-side chip using an off-chip transformer, a rectifier and modulator and demodulator circuits to relay information to the appropriate circuit blocks. To manage electro-migration issues and parasitics, we used multiple IO pads and placed the switching transistors under the pads in a custom IO. A charge pump was also designed to provide the required voltage level for driving the power transistor, after the high-side main power supply was obtained via rectification.

• Ensuring the digital circuitry and driver afforded a high degree of flexibility and programmability: The on-chip digital circuitry includes a 6502 microcontroller, SRAM, ROM, OTP along with other logic and an I2C interface to offer greater control, programmability and communication between high and low-side drivers. The driver allows the user to adjust drive settings and optimize the system based on the application. Users can also modify signal rise/fall times, delays, drive strengths and non-overlap times. We also created a graphical user interface to enable users to easily program settings.

Interested in finding out more about our expertise in mixed signal and smart analog design can help you? Contact us today.

Tags: automotive, power, sensor, consumer, chip design, programmability, interfaces