In the paper a position constraint algorithm is proposed to achieve high power density, based on the thermal resistance model of a half-bridge LLC system under natural convection. An LLC system with resonant frequency of 1MHz, input voltage of 290-330V, output voltage of 48V and power of 1kW is designed by using the third generation of power semiconductor device GaN. The electrical characteristics of the circuit are verified by simulation software. The loss of the system and the heat dissipation path of the LLC system with planar winding are analyzed, and the thermal resistance model of the system is established. Based on the thermal resistance model of the system, the heat dissipation power of each device is reduced, and the simplified thermal resistance model of each device is established. The device position constraint algorithm is established in a complex system without integration and differentiation. The simulation results show that in the LLC system under the position constraint algorithm, each device operates within the required temperature range, the error between the simulated value and the calculated value is within 5%, and the power density reaches 8.23W/cm³.