# Inrush current in DC DC converter

I am in the process of learning about DC DC converters. What I am trying to figure out is whether inrush current is needed to maintain the input voltage at the DC-DC converter input while getting its normal operation. Say, if I have 4-5A inrush current expected for a few tens of microseconds in my setup, can I use my power supply clamps to limit the inrush current,without impeding DC-DC converter operation? I have a few feet of cable from my power supply to my fixture to test the DC-DC converter, with a Kelvin connection on my power supply. I don't yet know my load capacitance seen by the power supply at the input of the DC/DC converter. Understanding that the few long cable from power supply to the DC/DC converter has some series inductance, how can I calculate the response time of the power supply to an overcurrent situation? Please help.

• Neglecting the few feet of cable (which really can't be neglected, but that would be tmi for just a comment), a good starting point might be if you define your inrush as an acceptable voltage drop (dt) for your expected current transient (di). You can then calculate a minimum output capacitance size required to maintain your output voltage as desired: C > (2*di)/(f*dv) where f is your switching frequency. This also neglects the ESR of the output capacitor and assumes very low resistance pretty much everywhere. – Los Frijoles Jun 11 '17 at 17:21
• For a typical DC-DC converter, the inrush current is due to the input cap. So if the upstream power supply limits the current, that just slows down the capacitor charging rate. In most cases, that should be fine, especially if the DC converter has proper under-voltage lock out. If the upstream power supply has over current protection scheme that shuts down the ouput, then it may not be fine. It depends on what information are available for estimating the response of the power supply. For example, if the power supply has a step response waveform chart, you can just use that as the estimation. – rioraxe Jun 12 '17 at 4:10