Aiming　at　the　efficient　application　of　pulse　frequency　modulation　(PFM)　and　fixed-frequency　phase　shift　modulation　(FF-PSM)　hybrid　modulation　strategy　in　LLC　resonant　converters,　a　complete　design　method　of　LLC　resonant　cavity　based　on　maximum　voltage　gain　and　efficiency　optimization　is　proposed.　Firstly,　based　on　the　characteristic　of　hybrid　modulation　strategy　working　at　fn≤1,　the　relationship　between　its　peak　voltage　gain　and　normalized　parameters　k,　Q　was　analyzed.　At　the　same　time,　the　k,　Q　constraint　equations　were　constructed　by　realizing　full　range　soft　switching　under　set　conditions.　Based　on　the　influence　of　k,　Q　on　the　normalized　turn　off　current　and　on　current,　the　transfer　efficiency　function　of　k,　Q　was　derived　to　obtain　the　constraint　equation.　Then　the　relationship　between　resonant　frequency　and　efficiency　was　analyzed　by　establishing　a　loss　model,　and　the　remaining　parameters　were　obtained.　Through　simulation,　the　optimized　parameters　of　resonant　tank　were　obtained　by　comparing　the　candidate　parameters　that　meet　the　requirements　with　a　fixed　step　size.　Finally,　a　600　W　experimental　prototype　was　built　to　verify　the　voltage　gain　characteristics　and　the　efficiency　of　the　proposed　method　is　higher　than　the　traditional　one.　©　2023　Editorial　Department　of　Electric　Machines　and　Control.　All　rights　reserved.