AN3303

Power losses and thermal design

Doc ID 18164 Rev 2

19/27

In addition, the power consumption of the SRK2000 must be taken into account: for a rough

Equation 9

SRK2000 datasheet.

A detailed explanation on the calculating energy required to drive MOSFETs in ZVS is

Equation 10

Finally, at full load, the total power saving obtained by implementing SR with respect to

diode rectification is calculated as follows:

Equation 11

A power saving of 7.05 W corresponds to a 4.7 % efficiency boost on a 150 W converter.

5.2

Thermal design consideration

The improvement in efficiency obtained by implementing SR allows dramatic squeezing of

the converter secondary side. This becomes evident when comparing the heatsink required

in case of diode rectification with that required if SR is employed.

Considering the diode rectification, the maximum junction temperature of the selected diode

is 150 °C. Consider 125 °C as the maximum tolerable temperature keeping some margin to

improve system reliability. Supposing an ambient temperature of 60 °C, the maximum

allowed thermal rise is 65 °C. Considering the power dissipation per diode calculated in

Equation 7, the maximum junction-to-ambient thermal resistance allowed is:

Equation 12

It is possible to assume that the thermal resistance between the TO-220FP case and the

thermal resistance of:

Equation 13

E

ZVS

1

2V

GS

V

M

–

()

---------------------------------- 2V

GS

2

V

M

2

+

() Q

g

Q

gd

–

() V

V

M

+

()Q

gs

–

[] 765 n

=

=

P

SRK2000

I

q

Vcc

⋅

()

2E

ZVS

f

sw

⋅⋅

() 159 mW

=

+

=

P

Δ

2P

Diode

2P

MOS

P

SRK2000

+

⋅

() 7.05 W

=

–

⋅

=

R

th j

amb

–

()

65

°C

P

Diode

-----------------

17

°CW

⁄

=

=

R

th hs

amb

–

()

R

th j

amb

–

()

R

th j

c

–

()

–

R

th c

hs

–

()

15

°CW

⁄

=

–

=