This paper presents an investigation of Infineon's Monolithic AC GaN switch and its application in uninterruptible power supplies (UPS). This new Monolithic AC GaN switch has a novel solderable top cooling surface, which significantly simplifies its PCB layout and improves it thermal performance. The operation of this new device is characterized in four quadrant operation for comparison with its predecessor, then it is evaluated in a practical 1.5 kW Vienna rectifier for UPS application. Its electrical and thermal performance is examined under various operating conditions. The result shows that, in its new package, this AC GaN switch exhibits excellent ability to minimize stray inductance and maximizing thermal dissipation in practical application, in addition to existing known advantage of AC GaN switches. This Monolithic AC GaN switch is maturing enough to be adopted by the power industry.