<p>This paper proposes a boundary-based minimum-injection high-frequency balancing strategy for low-speed standard half-bridge modular multilevel converter (MMC) motor drives to reduce the injected high-frequency balancing voltage and current and the associated electrical stress while maintaining the prescribed submodule capacitor-voltage ripple limit. The proposed method preserves the conventional Korn-type high-frequency injection (HFI) balancing path, but determines the commanded injection level from a constrained minimum-injection condition governed by the residual arm-energy demand and the available electrostatic buffering capability of the capacitor stack. An analytical operating boundary between HFI-assisted balancing and capacitor-based self-buffering is established by comparing the residual-energy requirement with the usable capacitor-voltage headroom constrained by modulation feasibility and the upper submodule voltage limit. Accordingly, the injection coefficient and the average capacitor-voltage reference are jointly coordinated so that the capacitor stack buffers the admissible residual energy, whereas the HFI channel supplies only the remaining balancing component required for ripple regulation. Comparative simulations against fixed HFI and a recent adaptive HFI benchmark demonstrate reductions of 33.0% in the mean compensation coefficient, 29.1% in the representative peak-to-peak submodule capacitor-voltage ripple, 13.7% in the representative arm-current RMS, 28.5% in the representative arm-current peak, and 25.3% in the current-squared loss proxy. The common-mode-voltage benefit is mainly reflected in the RMS and accumulated-burden indices rather than in the reduction of every instantaneous peak. These results verify that the proposed strategy reduces the injected high-frequency balancing content and the associated current- and voltage-side stresses under explicit ripple, modulation, and capacitor-headroom constraints.</p>