DC grids are recognized as a promising solution for next-generation power distribution systems due to their high efficiency and reduced energy conversion stages. Since modern electronic sources and loads operate with DC power, DC-DC converters serve as key components. In particular, non-isolated DC-DC converters are gaining attention due to their high efficiency and power density. However, the removal of galvanic isolation significantly lowers common-mode (CM) impedance, leading to increased leakage current and electric shock to users. To address these issues, a non-isolated three-switch converter topology is employed, which uses fewer switches while effectively suppressing CM currents. A low-frequency (LF) leakage current controller with a feedforward for enhanced transient response is developed, based on the CM equivalent circuit analysis. The operable range of the converter is analytically derived with consideration of the voltage fluctuations in both bipolar and unipolar DC grids. In addition, a hybrid pulse width modulation (PWM) scheme is introduced to suppress high-frequency (HF) CM currents, along with the DC CMV injection method to enhance the suppression performance. Experimental results in the bipolar and unipolar grids confirm that the proposed methods effectively suppress both LF and HF CM currents, ensuring the user safety. The 11 kW hardware prototype achieves a peak efficiency of 99.38% and reduces the rms leakage current by over 91.35% compared to the conventional designs.