Design Note, V1.0, Mar. 2003 T D A4 8 6 3 DN-PFC-TDA4863-1 TDA4863 Driving MOSFET with large Capacitances Author: Wolfgang Frank http://www.infineon.com/pfc Power Management & Supply N e v e r s t o p t h i n k i n g . TDA4863 Driving MOSFET with large Capacitances Revision History: 2003-03 V1.0 Previous Version: Page Subjects (major changes since last revision) For questions on technology, delivery and prices please contact the Infineon Technologies Offices in Germany or the Infineon Technologies Companies and Representatives worldwide: see our webpage at http://www.infineon.com. CoolMOSTM, CoolSETTM are a trademarks of Infineon Technologies AG. Edition 2003-03 Published by Infineon Technologies AG, St.-Martin-Strasse 53, 81669 München, Germany © Infineon Technologies AG 2003. All Rights Reserved. Attention please! The information herein is given to describe certain components and shall not be considered as warranted characteristics. Terms of delivery and rights to technical change reserved. We hereby disclaim any and all warranties, including but not limited to warranties of non-infringement, regarding circuits, descriptions and charts stated herein. Infineon Technologies is an approved CECC manufacturer. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office in Germany or our Infineon Technologies Representatives worldwide. Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may only be used in life-support devices or systems with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life-support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. TDA4863 Driving MOSFET with large Capacitances 1 DN-PFC-TDA4863-1 Large Capacitances In adapters MOSFET with a lower on-state resistances RDS(on) are often used in order to reduce power losses. But such transistors have typically large capacitances Ciss, Coss, and Crss according to Figure 1. Especially in power factor correction (PFC) preconverters this issue is even more dramatically, because there are points of operation, at which the drain-source-voltage is very low or even zero. At those points, the parasitic drain-gate-capacitance Crss(VDS) (“Miller-capacitance”) increases highly nonlinearly. This can be easily seen in the datasheets of the MOSFET, as it is shown in figure 24 of [2]. Lboost ton toff tosc TP VCC idisplace iL D Crss vDS RG idisplace Gate Coss G Q1 Ciss S vGate RG Figure 1 Equivalent Circuit of a MOSFET with parasitic Capacitors In discontinuous conduction mode (DCM) the drain-source-voltage swings down to zero by system, if the input voltage is lower than 50% of the output voltage even without the MOSFET being switched on. This means that the drain potential also goes down to zero which will cause a capacitive current flowing into the gate pin of the MOSFET and through the capacitor Crss. The larger the capacitance Crss the larger is the amplitude of the capacitive current. This may reverse bias the lower gate drive transistor and may lead to substrate currents in the control IC of the MOSFET and may cause malfunction. Substrate current can be Design Note 6 V1.0, 2003-03 TDA4863 Driving MOSFET with large Capacitances Large Capacitances detected easily by measuring the voltage at the gate drive pin. Substrate currents cause a voltage of about -0,7 V. This effect is well known. Usually schottky diodes are used directly at the gate drive pin to ground according to Figure 2 in order to clamp the gate drive voltage of -0,3 V minimum. MUR460 D5 TDA4863 5 6 Figure 2 7 1 2 4 VOUT R9 33k R4A Q1 SPP20N60 422k C3 R10 R4 422k 12Ω Schottky C8 C2 1µ R2 33k C1 2,2µ R11 0R47|| 0R47 100µF 450V R5 5k1 C1 µ1 GND Gate Drive Design of TDA4863 with Schottky Clamp Diode The rating of the schottky diode depends on the peak value and the rms value of the clamp current. But typically small signal schottky diodes with a forward current capability of approximately 100 mA are already sufficient. Design Note 7 V1.0, 2003-03 TDA4863 Driving MOSFET with large Capacitances Summary of Used Nomenclature 2 Summary of Used Nomenclature Physics: General identifiers: Special identifiers: A .........cross area b, B .....magnetic inductance c, C .....capacitance d, D .....duty cycle f...........frequency i, I ........current l, L .......inductance N .........number of turns p, P .....power t, T.......time, time-intervals v, V......voltage W ........energy h..........efficiency AL ........... inductance factor V(BR)CES .. collector-emitter breakdown voltage of IGBT VF........... forward voltage of diodes Vrrm .......... maximum reverse voltage of diodes big letters: constant values and time intervals small letters: time variant values K1, K2 ..ferrite core constants Components: C .........capacitor D .........diode IC ........integrated circuit L..........inductor R .........resistor TR .......transformer Indices: AC.......alternating current value DC.......direct current value BE .......basis-emitter value CS.......current sense value OPTO..optocoupler value P .........primary side value Pk........peak value R........... reflected from secondary to primary side S .........secondary side value Sh .......shunt value UVLO ..undervoltage lockout value Z..........zener value Design Note 8 fmin ......... value at minimum pulse frequency i ..............running variable in ............input value max ........maximum value min .........minimum value off ...........turn-off value on ...........turn-on value out ..........output value p .............pulsed rip ...........ripple value 1, 2, 3 .....on-going designator V1.0, 2003-03 TDA4863 Driving MOSFET with large Capacitances References 3 References [1] Infineon Technologies AG: TDA4863 - Power factor controller; Preliminary Data sheet; Infineon Technologies AG ; Munich; Germany; 02 / 02. [2] Infineon Technologies AG: SPP20N60C3 CoolMOS - Power Transistor; Data sheet; Infineon Technologies AG ; Munich; Germany; 10 / 02. Design Note 9 V1.0, 2003-03 Infineon goes for Business Excellence “Business excellence means intelligent approaches and clearly defined processes, which are both constantly under review and ultimately lead to good operating results. Better operating results and business excellence mean less idleness and wastefulness for all of us, more professional success, more accurate information, a better overview and, thereby, less frustration and more satisfaction.” Dr. Ulrich Schumacher www.infineon.com Published by Infineon Technologies AG