Back electromotive force zero crossing point detection circuit of permanent magnet direct current brushless motor

The invention discloses a back electromotive force zero crossing point detection circuit of a permanent magnet direct current brushless motor. The circuit comprises a floating power supply circuit, azero crossing point comparison circuit and a logic level translation circuit, wherein the floating power supply circuit is connected with the zero crossing point comparison circuit and the logic leveltranslation circuit and is used for providing electric energy for the zero crossing point comparison circuit and the logic level translation circuit, the zero crossing point comparison circuit is used for judging whether a zero crossing time point exists or not and outputting a floating logic level if the zero-crossing time point exists, and the logic level translation circuit is connected with the zero crossing point comparison circuit and is used for converting the floating logic level outputted by the zero crossing point comparison circuit into a logic level output taking a control systemground GND as a reference.