Download eet 307 power electronics 2005-2006

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