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Chassis Mountable
Switching Regulators
PSA Series
Edition 5/5.2000
5/10
Thermal Considerations
When a switching regulator is located in free, quasi-sta-
tionary air (convection cooling) at a temperature
TA = 50
°C
and is operated at its nominal output current
Io nom, the
case temperature
TC will be about 80
°C after the warm-up
phase, measured at the
Measuring point of case tempera-
ture TC (see: Mechanical Data).
Under practical operating conditions, the ambient tem-
perature
TA may exceed 50
°C, provided additional meas-
ures (heat sink, fan, etc.) are taken to ensure that the case
temperature
TC does not exceed its maximum value of
80
°C.
Example: Sufficient forced cooling allows
TA max = 65°C. A
simple check of the case temperature
TC (TC ≤ 80°C) at full
load ensures correct operation of the system.
1.2
1.0
0.8
0.6
0.4
0
0.2
0.4
1.0
1.2
0.2
0.6
0.8
1.4
Uo/Uo nom
Io/Io nom
I o
nom
Io L
05033
Fig. 5
Overload, short-circuit behaviour Uo versus Io.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
40
60
70
80
Io/Io nom
TA [°C]
0.9
1.0
Forced cooling
05032
TC max
50
Convection cooling
TA min
Parallel and Series Connection
Outputs of equal nominal voltages can be parallel-con-
nected. However, the use of a single unit with higher out-
put power, because of its power dissipation, is always a
better solution.
In parallel-connected operation, one or several outputs
may operate continuously at their current limit knee-point
which will cause an increase of the heat generation. Con-
sequently, the max. ambient temperature value should be
reduced by 10 K.
Outputs can be series-connected with any other module.
In series-connection the maximum output current is limited
by the lowest current limitation. Electrically separated
source voltages are needed for each module!
Short Circuit Behaviour
A constant current limitation circuit holds the output cur-
rent almost constant whenever an overload or a short cir-
cuit is applied to the regulator's output. It acts self-protect-
ing and recovers – in contrary to the fold back method –
automatically after removal of the overload or short circuit
condition.
Fig. 4
Output current derating versus temperature
Output Protection
A voltage suppressor diode which in worst case conditions
fails into a short circuit, protects the output against an in-
ternally generated overvoltage. Such an overvoltage could
occur due to a failure of either the control circuit or the
switching transistor. The output protection is not designed
to withstand externally applied overvoltages. The user
should ensure that systems with Power-One power
supplies, in the event of a failure, do not result in an unsafe
condition (fail-safe).