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DC/DC converters can behave in a way that first becomes obvious when you look at the internal construction. If the power source allows it, the DC/DC converter draws a very heavy surge at the moment it is switched on. This surge is known as inrush current, and is the result of the way in which the DC/DC converter starts up. If a DC/DC converter that was previously off is switched on again, the internal oscillator first has to start, the magnetic field has to form around the transformer and the capacitor at the output side has to charge up. The dynamics between the oscillator, the core material in the transformer and the ESR in the output capacitor in a DC/DC converter are decisive in the duration of this inrush current. Inrush current surges can reach a hundred times the static current input, even if they only last a few microseconds. However, this may be enough to blow a fuse or set off power limiters in the power supply. Fuses that occasionally blow or tendencies towards power fluctuation in the power supply occurring when the DC/DC converter is switched on are typical symptoms of inrush current problems. At the worst, the inrush current may even destroy a DC/DC converter - for example, when a high-power DC/DC converter without surge protection is connected to large lead batteries that can produce 200-300A for a short period. A soft-start function in the DC/DC converter may provide a solution. This ensures that the inrush current in the DC/DC converter cannot reach such high levels by reducing the dynamic in the oscillator. However, there is a price to pay in the speed at which the DC/DC converter starts up. Input filters are far more cost-effective. Apart from separating the DC/DC input while filtering out interference produced by the input side of the DC/DC converter, these input filters limit the power surge speed in the inrush current. These EMI filters always contain at least one or two coils in direct current regulation. The retardation coil counteracts the inrush current surge by slowing it down, significantly reducing the power surge in the DC/DC converter while spreading it over time somewhat. This ensures that inrush current cannot destroy the DC/DC converter, even when it is connected to a highly dynamic power supply.
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