FAQ
Q.
How do I limit the input surge current (inrush current) of the DC-DC converter?
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  • DC-DC-Wandler


A. 1. Method for connecting resistors in a series

Figure 1
(Important Points)
・Limit the input surge current by connecting resistors in a series. The required resistance value is calculated using the following formula: R (Ω) =V (V) /Ip (A).
・Since this resistor causes a voltage drop even in a steady state and continues to consume power, it is necessary to pay attention to the power rating of the
resistor and the supply capability of the input power source.


2. Method for connecting thermistors in a series

Figure 2
(Important Points)
・Limit the input surge current by connecting thermistors in a series instead of resistors. When the power is turned on, the thermistor shows a high resistance
value due to the low temperature and limits the input surge current. After that, due to self-heating, the resistance value decreases as the temperature rises, so
the steady-state loss can be reduced compared to when a resistor is used.
・The circuit configuration is simple yet effective. Therefore, it is necessary to carefully consider the usage conditions at the time of selection. In particular, when
used at low temperatures, it is possible that the resistance value may not decrease even if self-heating is applied. On the other hand, when used at high
temperatures, the resistance value is low from the initial state, and it is possible that the expected limiting effect may not be obtained.
・Even if the power source is turned off and then immediately turned on again, the resistance value is low from the initial state and the limiting effect cannot be
obtained.


3. Methods for using resistors and semiconductor switches

Figure 3
(Important Points)
・Input surge current is limited by connecting resistors in a series and a semiconductor switch such as a FET or thyristor in parallel with the resistors. When
the power is turned on, the semiconductor switch is opened and the input surge current is limited by the resistor. After that, the input voltage, etc., is detected
and the semiconductor switch is short-circuited to suppress steady-state loss.
・Input surge current characteristics can be set relatively freely compared to other methods; however, the circuit configuration becomes complicated.