More details about how startup Sarda Technologies heterogeneously integrated power stage module combines CMOS power drivers with GaAs output transistors in the same QFN package.
The idea seems so intuitive that you wonder why someone hasn’t suggested this before: Use gallium arsenide (GaAs) as the power driver for modules in compute servers and communications switching stations. Traditionally used for basestation transmitters, GaAs transistors automatically improve switching power supply frequencies: This enables higher densities for server power transmission applications, reduces heat dissipation, and helps shrink the size of power chain bricks and modules.
What Sarda Technologies calls its heterogeneously integrated power stage (HIPS) module combines CMOS power drivers with GaAs output transistors in the same QFN package. The use of GaAs increases voltage regulator switching speed by 10x and reduces switching losses by 30%, says Robert Conner, Sarda’s CEO and co-founder. Switching power supplies using Sarda’s modules have been routinely clocked at 5MHz, dropping 12 volts to 1.2, and delivering up to 14 amps with 94% efficiency.
The Holy Grail
Faster switching speed has become the touchstone for modern power supply design. Power MOSFET transistors are widely used for power pulse generation, but their switching activity (their turn-on/turn-off behavior) is sluggish under high-current loads. Thus, a majority of switching power supplies and point-of-load (POL) converters are making do with 100- or 250-kHz clock rates. Even without dramatic increases in energy transfer efficiency, advocates claim, an increase in switching frequency would allow the use of smaller inductors and capacitors, and would dramatically decrease the size of DC-DC converter modules and POL supplies.
Gallium Nitride (GaN) transistor developers like EPC and Navitas are demonstrating power bricks and modules with 2MHz switching speeds, 96% efficiencies and power densities on the order of 500W-per-cubic inch. Sarda’s modules will clock at 5MHz and demonstrate a density of 4,000 watts/in3. Electron mobility favors GaAs with a 8,500 cm2/Vs; GaN on silicon has a 2,000 cm2/Vs mobility figure, compared with 1,400 for silicon MOSFETs
Traditionally, GaAs transistors for RF applications have been depletion mode devices, which turn-on rapidly but need a negative gate drive voltage to be turned off. Although Sarda’s GaAs transistors are presently depletion mode devices, Bob Conner says, Sarda’s custom CMOS driver provides the negative gate drive voltage, an input voltage switch and power sequencing logic. The result is Sarda’s power stage electrically resembles a silicon DrMOS (driver MOSFET) taking its input from an external pulse width modulator (PWM) controller while providing two outputs and operating at higher switching frequency (see Figure 1).
The specialized drivers are fabricated in 0.35-micron CMOS by ams. The GaAs FETs are fabricated by Qorvo with 0.25-0.50 micron geometries. The low-parasitic packaging—which integrates capacitors and inductors, as well as semiconductors in a 5mm x 5mm x 1mm QFN—is a product of the UTAC Group in Taiwan.
The power stage will draw a price premium, in comparison with silicon MOSFETs, but the reduction in capacitor and inductor values with the higher switching speeds will provide a substantial cost and space savings, Conner says.