Making the choice should pay attention to the key components: feedback transformer B1 selection of high permeability MnZn amount of 10 mm × 6 mm × 5 mm ring, the winding N1 B1, N3 with a diameter of 0.41 mm enameled double winding parallel wear around 10 turns, N2 with a diameter of 0.55 mm enameled wire around three turns. B2 with EE19 core transformer primary with a diameter of 0.19 mm enameled wire wound 110 turns, secondary with a diameter of 0.31 mm enameled wire around the 16-turn, as the center tap. B3 is a high-frequency choke, can reduce the radiation interference, with a diameter of 0.55 mm enameled each around 10 turns. Precision electronic switching power supply circuit: