The　totem-pole　bridgeless　power　factor　correction　(PFC)　rectifier　has　a　simpler　topology　and　higher　efficiency　than　other　boost-type　bridgeless　PFC　rectifiers.　Its　promising　performance　is　enabled　by　using　high-voltage　gallium　nitride　(GaN)　high-electron-mobility　transistors,　which　have　considerably　better　figures　of　merit　(e.g.,　lower　reverse　recovery　charges　and　less　switching　losses)　than　the　state-of-the-art　silicon　metal-oxide-semiconductor　field-effect　transistors.　Cascade　GaN　devices　in　traditional　packages,　i.e.,　the　TO-220　and　power　quad　flat　no-lead,　are　used　in　the　totem-pole　PFC　boost　rectifier.　But　the　parasitic　inductances　induced　by　the　traditional　packages　not　only　significantly　deteriorate　the　switching　characteristics　of　the　discrete　GaN　device　but　also　adversely　affect　the　performance　of　the　built　PFC　rectifier.　A　new　stack-die　packaging　structure　with　an　embedded　capacitor　has　been　introduced　and　proven　to　be　efficient　in　reducing　parasitic　ringing　at　the　turn-off　transition　and　achieving　true　zero-voltage-switching　turn-on.　However,　the　thermal　dissipation　capability　of　the　device　packaged　in　this　configuration　becomes　a　limitation　on　further　pushing　the　operating　frequency　and　the　output　current　level　for　high-efficiency　power　conversion.　This　paper　focuses　on　the　thermal　analysis　of　the　cascade　GaN　devices　in　different　packages　and　the　GaN-based　multichip　module　used　in　a　two-phase　totem-pole　bridgeless　PFC　boost　rectifier.　A　series　of　thermal　models　are　built　based　on　the　actual　structures　and　materials　of　the　packaged　devices　to　evaluate　their　thermal　performance.　Finite　element　analysis　(FEA)　simulation　results　of　the　cascode　GaN　device　in　a　flip-chip　format　demonstrate　the　possibility　of　increasing　the　device　switching　speed　while　maintaining　the　peak　temperature　of　the　device　below　125　degrees　C.　Thermal　analysis　of　the　GaN-based　power　module　in　a　very　similar　structure　is　also　conducted　using　the　FEA　method.　Experimental　data　measured　using　the　fabricated　devices　and　modules　validate　the　simulation　results.　The　developed　new　package　in　a　flip-chip　configuration　enhances　the　thermal　dissipation　capability　of　the　cascode　GaN　device　in　the　stack-die　format.　The　GaN-based　power　module　built　using　the　same　packaging　structure　also　demonstrates　desirable　thermal　performance.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.