Multifrequency microgrid (MFMG) is a new type of microgrid which has unique advantages over traditional microgrids. Each microgrid contains of a bus, different sources and different loads. The DC/(DC+AC) converters are required to connect the sources and loads to the multifrequency (MF) bus in a multifrequency microgrid. It is a unique converter which can generate MF currents and voltages from a DC source. Here the converter generates a mixture of DC, 25 Hz and 50 Hz voltages from a DC voltage source. MFMG has a MF bus for which one DC/AC+DC converter needs to act like a grid forming (GF) converter. For a GF converter, the output voltage should be properly controlled. For this purpose, a proper PI controller needs to be designed. This paper focuses on the controller design of the DC/(DC+AC) converter with voltage mode control (VMC). The small-signal model (SSM) in open-loop is required to interpret the dynamics of the system. Here, the buck converter SSM is chosen as the base model, and the duty ratio to output voltage transfer function (TF) is calculated. Based on this TF, a PI controller is designed with VMC for DC/(DC+AC) converter. Here, the step by step controller design technique is explained and finally, the SSM and controller design are verified.