Datasheet

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
estimate, consider the IC quiescent current indicated in the SRK2000 datasheet (I
q
) and the
energy required for SR MOSFET driving (E
ZVS
).
Equation 9
Where VGS is the gate driver high level, VM is the MOSFET turn-on threshold, and Q
g
, Q
gd
,
and Q
gs
are the charges associated to MOSFET gate driving and are specified in the
SRK2000 datasheet.
A detailed explanation on the calculating energy required to drive MOSFETs in ZVS is
reported in Appendix A of the AN2644 application note (see Reference 5).
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
heatsink is R
th(c-hs)
=1 °C/W. As a consequence, each diode rectifier needs a heatsink with a
thermal resistance of:
Equation 13
E
ZVS
1
2V
GS
V
M
()
--------------------------------- -
2V
GS
2
V
M
2
+()Q
g
Q
gd
()V
GS
2V
GS
V
M
+()Q
gs
[]765 n==
P
SRK2000
I
q
Vcc()2E
ZVS
f
sw
⋅⋅()159 mW=+=
PΔ 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==