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Planar integrated matrix transformer (PIMT) is widely used in unregulated full-bridge LLC dc transformer (LLC-DCX) converters due to low profile and excellent current-carrying capacity. The full-bridge LLC-DCX converter has a natural advantage in common-mode (CM) noise cancellation due to its inherent noise sources with equal magnitude and 180-degree phase shift. Symmetrical PIMT can provide consistent CM path impedances and contribute to the cancellation of CM noise. However, the complex core structure poses great challenges to the design of symmetrical PIMT. Due to the differences in inductance between the left and right half turns of each winding, the PIMT with UI-type core composed of two element transformers is asymmetrical even if the windings have been symmetrically arranged. To counteract this issue, a UI-type core with two side legs is improved in this article. By introducing an air gap in the upper magnetic plate, the symmetrical PIMT design can be achieved within the frequency range where the CM impedance is capacitive. To further enhance symmetry beyond the capacitive frequency range, a parallel primary winding layout is proposed. The experimental results from a full-bridge LLC-DCX converter utilizing PIMT with the improved core and primary winding layout validate the reduction of CM noise in the conducted frequency range.