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In digital control, the one-sample delay for computation which is often ignored in 50/60 Hz cases has great impact on the performance of a 400 Hz ground power unit (GPU). Taking the delay into account, a detailed proof that it is very hard and even impossible for 400 Hz GPU to maintain a low THD content in the output voltage using the conventional inductor, capacitor or load current feedback based single-loop or double-loop control is given. A new primary-plus-resonant-controller method suitable for fixed-point DSP without modifying the existing single-loop or double-loop control topology is presented. With the new approach, the computation delay is not compensated, which means that complex computation will be avoided. Meanwhile, the relationships between the discrete realization method in other literatures and the zero order hold method for resonant controllers have also been discussed. Two types of tests on a 90-kVA 400 Hz GPU prototype based-on a 16-bit fixed-point DSP show satisfactory performance of the new method feeding linear / nonlinear load.