EPM Series Application Notes
Rev.3.1 May, 2013
TAMURA CORPORATION
6 / 17
1.8 Surge current prevention（TH101)
When input power is applied, an in-rush current which charges the input smoothing electrolytic capacitor flows.
An in-rush current that is too large will cause the power supply voltage to become unstable and may affect the
devices which share the power supply. The fuse and rectifying diodes used may also be damaged.
When using a power thermistor to suppress the surge current, select it by paying attention to the following:
①Be sure that the temperature is within the operating temperature range of the thermistor.
②Since the characteristic is such that a rated current drop accompanies a temperature rise, check the highest
temperature of the thermistor and that the load current (power supply input current) is the rated current or
less at the highest temperature.
③With a power thermistor, the relationship between the maximum value of the usable input voltage (rms) and the
capacitance of the input smoothing electrolytic capacitor are set. Check that the relationship between the input
voltage (rms) and input smoothing electrolytic capacitor is not a problem with the thermistor used.
④Since a power thermistor has a large thermal time constant and its resistance value remains low after the input
is tripped, when the input is repeatedly turned on and off, the surge current cannot be controlled. When using
under this condition, check that the effect, etc. on the fuse, bridge diode, and input power supply are no
problem.
The surge current calculation expression is:
As the calculation result is only a reference, conduct a thorough check using actual equipment.
・AC input ：
Inrush Current
（
peak value
） ：
)
R
R
(
2
)
rms
(
Vin
L
101
TH
+
⋅
I
2
t ：
)
R
R
(
102
C
)
rms
(
Vin
L
101
TH
2
+
⋅
・DC input
Inrush Current
（
peak value
） ：
)
R
R
(
)
dc
(
Vin
L
101
TH
+
I
2
t ：
)
R
R
(
2
102
C
)
dc
(
Vin
L
101
TH
2
+
⋅
R
L
：Line impedance (
Ω
)
R
TH101
：Thermistor resistance (
Ω
) at 25℃
C102：electrolytic capacitor（F）