The impact of grid operating conditions and control parameters on the transient voltage stability of renewable energy sources dominated by the DC voltage control loop was investigated. The mechanistic distinctions between this issue and the Phase-Locked Loop (PLL) dominated transient voltage stability problem were further clarified. To achieve these objectives, a second-order analytical model was established, in which the DC-link voltage and the d-axis current on the AC side were selected as state variables to characterize the transient process governed by DC voltage control. Inspired by the synchronizing torque analysis method for synchronous generators, an analytical approach was developed to assess the influence rules of grid operating conditions and the outer-loop DC voltage controller parameters on transient voltage stability. Furthermore, the stability boundaries for the two distinct instability modes, dominated by the DC voltage loop and the PLL, respectively, were compared using the phase-plane analysis method. Finally, the validity of the theoretical analysis was confirmed through electromagnetic transient (EMT) simulations based on a full-order detailed model. The results demonstrated that under weak grid conditions, voltage disturbances were more likely to induce transient voltage instability dominated by the DC voltage control loop, while the risk of PLL-dominated instability was relatively lower. Consequently, during grid voltage disturbances, the stability risk associated with the DC voltage control loop was more critical than that related to the PLL synchronization stability.