Pre-Charging of a DC-Link Capacitor from a High Voltage Battery

Pre-charging of DC-link capacitors limits the inrush current when connecting a power converter to the grid. In its simplest form, this can be realized with a relay parallel to a resistor and a diode as shown in [1] and [2]. This digest proposes an alternative approach for automotive onboard battery chargers that removes any need for such dedicated pre-charging hardware and completely avoids inrush currents. Instead, by exploiting the massive difference in energy storage between the main battery and the DC-link, the energy needed to pre-charge the DC-link capacitor can be reliably obtained using reverse power flow from the main battery through the isolating transformer, even if the main battery were to be almost entirely discharged. Furthermore, this paper shows methods how the battery and DC-link current can be easily limited with variable frequency control or varying duty cycle at constant frequency of the CLLLC output voltage when starting to charge an empty DC-link capacitor. Experimental verification of the control approach is done and it is shown that a charging time smaller than one second is possible.