AN1889 Application note ESBT STC03DE170HV in 3-phase auxiliary power supply Introduction The need to choose a high value of the fly-back voltage is well known to power supply designers when efficiency and high duty cycle become important requirements. Three- phase auxiliary power supplies, starting from a bulk voltage of 750 V, require theoretically power transistors with a block voltage capability higher than 1200 V. Practical considerations linked to better efficiency and safe margin may impose to choose devices with even higher breakdown voltage (i.e. 1500 V, 1700 V). Looking at the power switches currently available, while power bipolar transistors are strongly limited in switching frequency operation, Power MOSFETs show a much lower current capability, which may limit their use to low power applications. Recently available in the market, the ESBTs (Emitter Switched Bipolar Transistors) represent a valuable and cost effective alternative for all those applications where high voltage and high switching frequencies are a must. This application note describes the realization of a 50 W 3-phase auxiliary power supply by using the STC03DE170HV as main switch for the fly-back converter. Particular attention has been given to the transformer design as well as to the ESBT driving circuit requirements. August 2007 Rev 3 1/33 www.st.comContents AN1889 Contents 1 ESBT: theory and evolution . 4 2 Application description 7 2.1 Pre-design requisite 8 2.2 Fly-back transformer design . 11 3 Output circuit 14 4 Clamping circuit . 15 5 ESBT driving circuit 16 6 Current transformer core selection 19 7 PWM driver 22 7.1 Primary side regulation 22 7.2 Control loop compensation 22 7.3 Switching frequency and max duty cycle setting . 22 7.4 Current sensing and limiting . 22 7.5 ESBT gate drive 23 8 Start-up network . 24 9 Experimental results 25 10 PCB layout and list of materials 30 11 Revision history . 32 2/33