Hi, I have a high power boost dc-dc converter that is attached below. I supply him through a rechargeable 9Volt battery. In specific, between the battery and the converter I have placed another dc-dc convertet to keep the battery level stable at 11Volts. So, the input of the high power dc-dc converter is the 11Volt output of the smaller boost dc-dc converter. The output of the hig power dc-dc converter is about 65Volts. The output load of the high power dc-dc converter is about 11k ohm. I have noticed that when I turn on the switch there is a big input inrush current. I would like to ask the followings. Firstly, how can I measure that input inrush current? Secondly, how can I limit that current? I have read about NTC thermistors, but which one should I pick and where should I place it in the circuit? Thank you.

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Hi,
The inrush current charges the input capacitors when you first supply the board. You can measure it by placing a small resistor in between the supply and the DC-DC an measure the voltage across the resistor with an oscilloscope. The value of the NTC thermistor in the cold state should be high enough that it limits the current to your desired value and low enough (after the inrush) to not interfere with the the system. Here is a link that explains current limiting using a NTC: https://product.tdk.com/de/techlibrary/applicationnote/howto_ntc-limiter.html

Sometimes a low resistance resitor in the input path is sufficient for the application.