ELEC-E8421 Components of Power Electronics MOSFET 2015-10-04
Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) Vertical structure makes paralleling of many small MOSFETs on the chip easy. Very low R DS (< 10 mω) can be achieved. MOSFET control is easy due to capacitive gate Fast switching as no recombination is needed at turn off. In n-type majority carrier devices only electrons carry the current. Drain Gate Cross-section view of vertical MOSFET Source Circuit symbol
Detailed cross-section view of vertical MOSFET Note the parasitic components due to vertical structure! Source area Gate Contact area
Problems due to parasitic components 2nd breakdown Parasitic npn transistor may cause 2nd breakdown if it becomes active due to body diode's reverse recovery current and/or high du/dt. The 2nd breakdown voltage may be only half of the normal U DS rating. The activation of body diode can be prevented by adding two diodes, external parallel freewheeling diode and another in series with the MOSFET. Today problems with body diode are mostly solved and especially with SiC MOSFETs the body diode properties are so good that no external diodes are needed G R G du/dt current C GD C GS D R D C GS R b Diode's reverse current Body diode = parasitic transistor's BC p-n junction S R b value should be low enough to prevent 2nd breakdown!
Problems due to parasitic JFET fixed The increased resistance due to parasitic JFET was solved with trench gate and charge compensated structure (super junction, CoolMOS). With trench gate the channel is also vertical. Thus the route between drain and source is shorter and utilization of chip area is better. Super junction structure increases depletion region in off-state and thus increases voltage rating. It allows also higher n-epi doping that reduces resistance in on-state.
Avalanche breakdown capability Today many MOSFETs can tolerate avalanche when switching inductive loads Test circuit
Thermal runaway P Silicon MOSFET's on-resistance H increases significantly with temperature There is a risk of thermal runaway if the transistor is not cooled efficiently (slight increase of R thja or T A may cause the stable intersection point to vanish) SiC MOSFETs have smaller resistance increase and are thus not prone to runaway. Heat sink characteristic P H sink = (T J T A )/R thja T A Stable Unstable Runaway P H fet > P H sink MOSFET power loss P H fet = R DS (T J ) I D 2 T J
Switching characteristics Parasitic capacitances in datasheets: C iss = C GD + C GS C oss = C GD + C DS C rss = C GD Miller effect current path C GD LOAD Miller effect C GS C DS
SiC MOSFETs First SiC MOSFETs were made in 2008 Today (2015) voltage ratings in catalogs are up to 1700 V and module current ratings up to 300 A. Thus about ten times the power transfer capability than with silicon MOSFETs SiC MOSFETs R DS increases much less with temperature than the silicon one's resistance SiC MOSFET's internal body diode has much lower recovery losses than the silicon one's diode, comparable often with the Schottky diode. Thus often no external freewheeling diode is used. Due to the SiC body diode's higher U F it is common to keep the channel conducting also current flowing from source to drain. SiC MOSFETs junction temperature can be today up to 200 C when silicon ones usually cannot tolerate more than 150 C. Higher gate voltage, 15 18 V is recommended for SiC MOSFETs than for silicon ones that typically need only 10 15 V gate voltage SiC MOSFET reliability issues such as gate trapped charges and body diode's crystal stacking defect propagation seem to have been solved but long time reliability is still naturally unknown.
Datasheet parameters of MOSFET Voltages V DSS (drain-source), V DGO, V GS Currents I D, I DM, I GM Power loss P D Temperatures T J, T STG Thermal resistance R thjc Transconductance g FS = ΔI D / ΔV GS (unit siemens S, sometimes even mhos or used!) Gate theresold voltage V GS(th) On-state resistance R DS(on) Capacitances C iss, C oss, C rss Switching times Turn-on delay time t d(on), Rise time t r Turn-off delay time t d(off) Fall time t f
MOSFET datasheet diagrams Drain current vs. drain-source resistance characteristic Gate-source voltage vs. drain current characteristic
MOSFET datasheet diagrams Drain-Source voltage vs. capacitance characteristic
Galvanic isolated gate control circuit Example of one gate driver circuit from an application sheet Note that this may need careful design with voltage levels!
Synchronous rectifier Instead of diodes MOSFETs can be used as very low voltage drop rectifiers. For example, if R DS = 10 mω, the voltage drop at 10 A is only 0,1 V.