8
TDK-Lambda
PFE700SA Series
INSTRUCTION MANUAL
TFR1 : 10Ω - 100Ω
By connecting thermal fuse resistor across R and +BC terminals as shown in Fig. 5-1, in-rush current
during line throw-in can be suppressed. Failures due to in-rush current such as melting of external fuse,
welding of relay or switch connecting joints or shutdown of No-Fuse Breakers (NFB) can occur.
Therefore, be sure to connect this external thermal fuse resistor of 10Ω or more.
The allowable resistance value is limited by the external bulk capacitance value of shown in fig.5-4.
Note that power supply will not operate if this external resistor is not connected
.
●
Selection Method of External Resistor
(1) Calculating Resistance Value for External TFR1
Resistance can be calculated by the formula below.
)
(
Irush
Vin
R
Ω
=
(Formula 5-2)
R
: Resistance Value for External TFR1
Vin
: Input Voltage converted to DC value = Input Voltage (rms) x
2
Irush
: Input surge current value
(2) Required Surge Current Rating
Sufficient surge current withstand capability is required for external TFR1.
Required Surge Current Rating can be selected by I
2
t. (Current squared multiplied by time)
)
s
A
(
R
2
Vin
Co
t
I
2
2
2
´
´
=
(Formula 5-3)
I
2
t
: Current-squared multiplied by time
Co
: Boost Voltage Bulk Capacitance
Vin
: Input Voltage converted to DC value = Input Voltage (rms) x
2
R
: Resistance Value for External TFR1
(3)
Resistance limitation Value for External TFR1
External TFR1 is limited as shown in drawing below.
Drawing below shows resistor value at 25
℃
Base-plate Temperature. Input Surge current might vary
due to temperature. Therefore, verify characteristics by performing actual evaluation.
10
40
70
100
300
600
900
1200
R
es
is
ta
nc
e
V
al
ue
[Ω
]
Bulk Cap[uF]
390
Tbp:25
℃
Fig.5-4
Resistance Value v.s. Boost Voltage Bulk Capacitance