A　two-level　dual　active　bridge　(DAB)　Converter　cannot　reach　high　efficiency　in　wide　voltage　transmission　ränge　occasions.　Therefore,　a　multilevel　structure　is　applied　to　DAB　to　optimize　Converter　efficiency　and　reduce　the　voltage　stress　of　power　switching.　A　tracking　control　strategy　for　minimum　inductance　current　RMS　value　based　on　three-level　extended　phase　shift　(3-EPS)　control　is　proposed　when　a　large　mismatch　exists　between　input　and　Output　voltages.Firstly,　the　relationship　between　the　phase-shift　ratios　D0-f0(D2)　and　Dx—f^D^　at　several　optimal　extreme　points　in　different　power　regions　is　obtained　by　the　Lagrange　multiplier　method.　Different　extreme　points　can　achieve　different　power　ranges.　The　inductance　current　RMS　value　of　each　optimal　extreme　point　is　compared　in　the　power　overlap　ränge,　and　the　global　Optimum　extreme　points　in　different　power　ranges　are　obtained.　Secondly,　the　optimal　boundary　points　are　analyzed　and　compared　with　global　Optimum　extreme　points.　Because　the　inductance　current　RMS　values　of　global　Optimum　extreme　points　are　always　smaller　than　those　of　boundary　points　in　each　power　ränge,　the　global　Optimum　extreme　points　are　the　final　global　optimal　operating　points.　Finally,　the　positive　PI　Controller　or　negative　PI　Controller　is　chosen　based　on　the　partial　derivative　polarity　of　the　power　to　D2　to　track　the　global　optimal　operating　points.　Besides,　based　on　the　switching　point　current,　the　zero-voltage　switch　(ZVS)　of　most　Switches　can　be　realized　under　ideal　conditions.　However,　the　ZVS　ränge　is　compressed,　considering　that　the　inductance　energy　is　enough　to　discharge　the　parasitic　capacitance　fully.Experimental　results　show　that　when　voltage　transmission　ratio　k　equals　1.6　or　2.6,　the　efficiency　of　optimal　3-EPS　control　is　higher　than　optimal　two-level　extended　phase　shift　(2-EPS)　control　and　Single　phase　shift　(SPS)　control　in　low　to　medium　power　ränge.　However,　the　efficiencies　of　the　above　three　controls　are　very　close　when　the　per　unit　power　reaches　0.8.　When　per　unit　power　equals　0.5　and　k　is　between　1.1　and　1.2,　the　advantage　of　3-EPS　is　not　apparent.　When　k　is　between　1.2　and　2,　the　advantage　is　evident　again.　Compared　with　2-EPS,　when　k　is　larger　than　2,　the　effect　of　3-EPS　decreases　as　k　increases.　When　k　equals　4,　the　efficiencies　of　3-EPS　and　2-EPS　are　very　close.　The　efficiency　of　the　minimum　inductance　current　RMS　value　tracking　control　strategy　is　higher　than　the　peak　current　optimal　control.The　following　conclusions　can　be　drawn　from　analysis　and　experiment　results.　(1)　The　optimal　control　strategy　proposed　achieves　lower　inductance　current　RMS　value　under　a　wide　input　voltage　ränge,　and　the　efficiency　is　higher　than　90%　in　most　power　ranges.　(2)　When　k　is　kept　unchanged,　compared　with　2-EPS　and　SPS,　the　effect　of　3-EPS　decreases　as　the　unitary　power　increases.　(3)　When　power　is　kept　unchanged,　the　effect　of　3-EPS　is　not　apparent　if　k　is　larger　than　1　but　close　to　1　because　the　mismatch　between　input　and　Output　voltages　is　minor.　When　k　is　between　1　and　2　and　away　from　1,　the　effect　of　3-EPS　is　obvious.　(4)　When　k　is　larger　than　2,　compared　with　2-EPS,　the　effect　of　3-EPS　decreases　as　k　increases.　When　k　reaches　4,　the　effect　of　3-EPS　is　almost　vanished.　©　2024　China　Machine　Press.　All　rights　reserved.