Advanced　automotive　electrical　systems　employ　multi-voltage　level　ac　and　dc　systems　as　well　as　electrical　mechanism　systems　that　have　unique　characteristic,　dynamic　and　stability　problems　due　to　the　time　dependency　of　converters.　Power　electronic　converters　and　motor　drives　in　tightly　regulated　automotive　systems　often　behave　as　constant　power　loads　which　have　negative　impedance　characteristics.　Classical　control　methods　often　focus　on　the　constant　resistance　load　where　the　linearizing　analysis　is　applying　in　time　domain.　In　this　paper,　based　on　the　DAB　DC-DC　converter,　circular　switching　surface　(CSS)　strategy　which　achieves　step-up　and　step-down　operations　by　controlling　the　four　transistors,　is　described　to　avoid　the　instability　of　constant　power　load.　This　CSS　control　strategy　is　a　nonlinear　method　to　observe　the　switching　actions　of　different　states　in　phase　plane　(Vout,　iL).　This　report　has　described　the　control　law　of　this　proposed　closed　loop　control　method.　Meanwhile,　some　simulation　results　and　experimental　results　have　indicated　the　feasibility　of　CSS　control　strategy.　©　2020　IEEE.