Thermal Data
Two-parameter model
Over Temperature Protection (OTP)
The PKG DC/DC power modules have an internal over temperature
protection circuit. If the case temperature exceeds min +115 °C the
power module will go in to OTP-mode. As long as the case tempera-
ture exceeds min +115 °C the power module will operate in OTP-
mode.
Power dissipation is generated in the components mounted on the
ceramic substrate. The thermal properties of the PKG DC/DC power
module is determined by thermal conduction in the connected pins
and thermal convection from the substrate via the case.
The two-parameter model characterize the thermal properties of the
PKG power module and the equation below can be used for thermal
design purposes if detailed information is needed. The values are given
for a power module mounted on a printed board assembly (PBA).
During OTP-mode the output voltage pulsates between zero and
nominal output voltage, which reduces the power loss inside the
power module. The PKG DC/DC power module will automatically
resume normal operation when the temperature decreases below
min +115 °C.
Note that the thermal resistance between the substrate and the air,
Rth sub-A is strongly dependent on the air velocity.
Tsub = Pd × Rth sub-P × Rth sub-A/(Rth sub-P + Rth sub-A) + (TP–TA)
× Rth sub-A/(Rth sub-P + Rth sub-A) + TA
Where:
Pd
Tsub
TA
: dissipated power, calculated as PO × (1/h-1)
Electrical Data
: max average substrate temperature, » TC
max
: ambient air temperature at the lower side of the power
module
Fundamental circuit diagrams
TP
: average pin temperature at the PB solder joint
Rth sub-P : thermal resistance from Tsub to the pins
Rth sub-A : thermal resistance from Tsub to TA
Single output
v
: velocity of ambient air.
4
9
8
Air velocity in free convection is 0.2–0.3 m/s (40-60 lfm).
2
1
3
20
Control
Rth sub-P = 2.5 °C/W
10
15
Case
Isolated feedback
10
5
0
Dual output
7
0
6
2
4
Air velocity (m/s)
6
9
4
T
8
A
T
A
2
1
R
th sub-A
T
sub
v
Control
R
th sub-A
th sub-P
T
sub
R
th sub-P
P
d
10
3
R
T
P
Case
Isolated feedback
T
P
EN/LZT 146 03 R1A (Replaces EN/LZT 137 R5) ©Ericsson Microelectronics AB, May 2000
4
ERICSSON [ ERICSSON ]
