A high-density natural cooling half-bridge power module for a top-cooled 650-V GaN HEMTs is proposed in this paper. Compared with Si and SiC devices, GaN HEMTs has advantages, such as low on-resistance, low drive loss, low switching loss and small size. However, it also have challenges such as large switching oscillations and small heat dissipation area. This paper proposes a PCB layout scheme with low parasitic impedance and bipolar gate drive with +6V/-3V. It keeps GaN devices completely on one side of the PCB while makes the parasitic inductance of the power loop within 1nH. Hence, the installation of heat sinks becomes easier. This paper verifies the above design in a CCM boost converter that widely used in photovoltaic and energy storage systems. A loss evaluation based the spice model of GaN device GS66516T is given. Finally, a 2.5kW experimental prototype with the proposed power module is manufactured. It was tested to get peak efficiency of 99.14% and full-load efficiency of 99.09%. The experimental results show that the temperature rise of GaN HEMTs is limited within 50°C in a windless environment, which effectively proves the rationality of the proposed power module design and loss evaluation method.