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,

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.

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

RMS

2

265 mW

=

⋅

=