Detection distance is a critical specification in automotive driving. The output power of a power amplifier is the bottleneck for the detection range. In this work, a design of 77 GHz power amplifier is presented in a 40 nm CMOS technology with a four-way parallel-series power combing technique for achieving a 20 dBm output power (Psat), which meets a 186-meter detection range in the conditional of 30 mm/h rainfall capacity. In this parallel-series power combing technique, distributed active transformers perform load-pull matching and in-phase power combining. It achieves a compact solution for future on-chip antenna integration. This PA is optimized for maximal power-added efficiency of 21.35% with 20 dBm saturated output power at 77 GHz.