The input-independent and output-series (IIOS) structure is often used in the high step-up ratio converters required for medium-voltage DC (MVDC) grid interface of photovoltaic (PV) systems, which achieves high voltage gain and ensures high efficiency. However, due to the characteristics of IIOS structure, when some PV arrays are shadowed or malfunction, the generated power of these modules will be mismatched. Mismatched power will result in output capacitor voltages deviating from the rated value, which is not conducive to the standardized multi-module design. Besides, voltages of some PV arrays will deviate from their maximum power point (MPP) value, leading to a decrease in the total generated power. In order to balance the module output voltages of the IIOS structure, this paper proposes bidirectional buck-boost and series LC based power balancing units for PV DC collection systems. Compared with a traditional solution, nearly half of the switches are saved and half of the inductors are displaced by small inductors. Power balance can be achieved through the LC power balancing units (LC-PBU) by controlling the duty ratio and the phase of the pulse-width modulation (PWM) signal. Simulations and experiments are carried out to verify the effectiveness of LC-PBU under power sags.