Part # Application Note Industrial Diode datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN603 APPLICATION NOTE TURBOSWITCHTM IN A PFC BOOST CONVERTER INTRODUCTION STMicroelectronics offers two families of 600V ultrafast diodes (TURBOSWITCH"A" and "B" ) having dif-ferent compromises between the forward characteristics and the reverse recovery characteristics. This paper explains why TURBOSWITCH"B" is a suitable family for PFC boost converters working in dis-continuous mode, and why the TURBOSWITCH"A" should be used for PFC s working in continuousmode. In this kind of application, the main concern for the designer is to evaluate the power losses. For that, STproposes a very powerful tool. A program has been developed in order to calculate the losses in the diodeand in the transistor in a PFC working in continuous mode at a constant frequency. This application notedescribes how the calculations are performed. This software determines clearly that there is an optimumMOSFET turn on di/dt to increase the efficiency of the design and reduce EMI. PARAMETERS DEFINITION The basic circuit of the Figure 1 shows the current, voltage and frequencies notations used in this paper. List of the parameters: Vm: mains voltage VM: peak value of the mains voltageIL: current in the coil ID: current in the diodeIT: current in the transistor IP: peak current in the coil D: duty cycle of the diodeFC: switching frequencyVO: output voltageF: mains frequency REV. 2 June 2004 1/13 AN603 APPLICATION NOTE Figure 1. Boost PFC converter notations IL T = 1/F IP t D ID IL L IT Vm C V O T Vc V c Vm Tc=1/Fc T = 1/F VM t t D Tc TURBOSWITCH IN A PFC BOOST CONVERTER WORKING IN DISCONTINUOUS MODE The discontinuous mode is used for power below 200-300W. In this mode, the current in the diode beforereaching zero A decreases very slowly (less than 1A/µs). The slope is fixed by the coil and is equal to(Vm ­ VO)/L. The low value of this slope generates low values of reverse recovery current (IRM) and there-fore low switch-off losses. For this reason the forward voltage (VF) of the diode becomes the most impor-tant parameter. The best choice is to use a TURBOSWITCH"B" 1-2A/600V. The major part of the losses is the conduction losses (PCOND). They can be calculated with a good ap-proximation by: PCOND = VF (IF(AV)) x IF(AV) The average current in the diode is equal to the output power (POUT) divided by the output voltage VO: IF(AV) = POUT / VO TURBOSWITCH IN PFC BOOST CONVERTER WORKING IN CONTINUOUS MODE In continuous mode (output power higher than 200-300W) the current in the diode decreases very quickly.The (diF/dt)OFF of the diode is fixed by the MOS transistor control and is equal to a few hundred A/µs ((diF/dt)OFF of the diode is equal to the (diF/dt)ON of the transistor).The reverse recovery current of the diode when the transistor switches on flows in the transistor and gen-erates high turn-on losses in the transistor. For this reason the most important parameter of the diode isthe IRM.The TURBOSWITCH"A" family represents the optimum in term of VF/IRM compromise for this type of ap-plication and is recommended.

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