* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download eet 307 power electronics 2005-2006
Ground (electricity) wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Electric power system wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Three-phase electric power wikipedia , lookup
Power engineering wikipedia , lookup
Electrical ballast wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Power inverter wikipedia , lookup
Shockley–Queisser limit wikipedia , lookup
Current source wikipedia , lookup
History of electric power transmission wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Electrical substation wikipedia , lookup
Semiconductor device wikipedia , lookup
Voltage regulator wikipedia , lookup
Distribution management system wikipedia , lookup
Stray voltage wikipedia , lookup
Opto-isolator wikipedia , lookup
Surge protector wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Power electronics wikipedia , lookup
Alternating current wikipedia , lookup
Voltage optimisation wikipedia , lookup
Mains electricity wikipedia , lookup
Switched-mode power supply wikipedia , lookup
EET 423 POWER ELECTRONICS 2 Q. 4 2007-2008 NOTE: A FULL SOLUTION NOT REQUIRED (a) (i) p-n rectifiers: lack of minority carriers at turn -on Injected carriers from the more heavily doped region become minority carriers in the lightly doped region. At turn on the diffusion time of the minority carries < circuit controlled di/dt hence at the instant of turn-on there are no minority carriers and time is therefore required for charge to build up with an associated drop in forward voltage lack of minority carriers turn on delay forward recovery voltage (Vfr) dependent on device structure lower voltage rated devices have lower Vfr p-n rectifiers: excess minority carriers at turn off During conduction p-n junction rectifiers in addition to the forward voltage drop store charge, proportional to IF, that must be removed before the device is considered to be off. Fast turn off requires the device to be force commutated resulting in the current falling at a rate dependent on the circuit. fast turn off requires the device to be force commutated resulting in the current falling at a rate dependent on the circuit. associated large reverse recovery voltage (4 marks) (ii) Schottky rectifier advantages The two fundamental characteristics of the Schottky that make it a preferred choice over the p-n junction rectifier in low voltage switching power supplies are lower forward voltage drop virtual absence of minority carrier reverse recovery EET 423 POWER ELECTRONICS Q4 solution 1 Prof R T Kennedy 2006-2007 Schottky rectifier disadvantage Reverse leakage current in p-n junctions is not usually a major design factor. Leakage current in Schottky rectifiers, intrinsic to the device, is dependent on reverse voltage and increases significantly with device junction temperature. (3 marks) (iii) Converter Effects p-n devices In addition to on-state losses p-n rectifier turn- on losses: function Vfr, IF tfr fsw p-n rectifier turn- off losses: function of Qrec Vrec fsw shape of recovery characteriistic As switching speeds or frequency of operation increase rectifiers become inefficient and can be the major factor in SMPS efficiency. fast and ultra fast recovery devices have lower losses but introduce EMI problems. peak transistor switch current is increased by the magnitude of the peak reverse recovery current of the rectifier increased transistor power dissipation transistor switch voltage at turn-off) is increased due to the rectifier’s turn-on voltage. Schottky Schottky lower forward voltage drop lower rectification losses higher efficiency and smaller heatsinks the absence of minority carrier reverse recovery ideally zero Schottky switching losses zero reverse recovery reduced switching voltage transients and oscillations reduced EMI smaller less dissipative snubber circuits (3 marks) EET 423 POWER ELECTRONICS Q4 solution 2 Prof R T Kennedy 2006-2007 k : k : 1 SCH1 IR18TQ045 Vout = 5 V L IO =30 A C R EIN SCH2 IR18TQ045 IR18TQ045 Tj,max Rth,j-c Rth,c-s 175 oC 1.5 oC/W 0.5 oC/W IRF 460 (b)(i) REFER TO Appendix 5 Tj,boundary = 139 oC (2 marks) (b)(ii) REFER TO Appendix 5 SCH1 T jn ,boundary T jb 139 oC T jn ,op1 T jb Tsafety 139 15 Pjn ,op1 3.1 W graph 2 Tsink1 T jn ,op ( Pjn .op Rth, j s ) SCH 2 Pjn ,op 2 16.4 W graph 2 T jn ,op 2 150.6 oC graph 2 Pjn ,op1 Pjn ,op 2 3.1 16.4 117.8 50 19.5 3.48 oC / W SCH1 SCH 2 Pjn ,op 1 2 . Tamb 50 oC Rth, s a Tsink Tamb Pjn ,op 1 2 graph 1 124 (3.1 2) 124 oC 117.8 oC 19.5 W (8 marks) (20 marks) EET 423 POWER ELECTRONICS Q4 solution 3 Prof R T Kennedy 2006-2007