Datasheet
Power losses and thermal design AN3303
18/27 Doc ID 18164 Rev 2
5 Power losses and thermal design
SR dramatically reduces output rectification power losses enabling the design of more
efficient power supplies and, even more significant, with a considerable reduction of
converter secondary side size.
To get a better idea of the improvement obtained by implementing SR with the SRK2000,
power loss calculation in a 12 V-150 W application (see Reference 2) is illustrated below.
5.1 Power losses calculation
The average output current of a 12 V-150 W power supply at nominal load is:
Equation 4
The average current flowing through each output rectifier is:
Equation 5
and the RMS current is approximately:
Equation 6
To evaluate power losses, a suitable diode and MOSFET part numbers were selected.
In the case of diode rectification the STPS20L45C was selected (see Reference 3). The
power losses associated to each rectifier can be calculated using the formula indicated in
the STPS20L45C datasheet:
Equation 7
In the case of SR, the selected MOSFET is the STL140N4LLF5, which is actually mounted
on EVLSRK2000-L-40.
Capacitive losses associated to the MOSFET turn-on are negligible because each MOSFET
is turned on after its body diode starts conducting. Also losses at turn-off are of minor
concern because, after the MOSFET is turned off, the current goes on flowing through the
diode.
Supposing the MOSFET turn-on and turn-off timing are optimized as described in Section 3,
losses associated to the current flowing through the body diodes can be neglected too. Most
SR MOSFET losses can be summarized into conduction losses:
Equation 8
I
0
P
0
V
0
------
12.50 A==
I
avg
I
0
2
----
6.25 A==
I
RMS
π
4
---
I
0
9.82 A==
P
Diode
0.28 I
avg
0.022 I
RMS
2
3.87 W=⋅+⋅=
P
MOS
R
DS on()
I
RMS
2
265 mW=⋅=