Bidirectional DC/DC converter and method for charging the intermediate circuit capacitor of a DC/DC converter from the low-voltage battery

A method for charging the intermediate-circuit capacitor (CZK) which is arranged in the high-voltage network (HV) of a DC/DC converter including,

• terminals for a high-voltage battery (UHV) and a low-voltage battery (UNV),
• one or a plurality of transformers (1) for galvanically isolating the low-voltage network (LV) from the high-voltage network (HV),
• an intermediate-circuit capacitor (CZK) in the high-voltage network (HV),
• electronic switches (D1 to D4, M1 to M4) for connecting and reversing the polarity of the winding of the transformer (1) on the high-voltage network (HV) and on the low-voltage network (LV),
• a control device (2) for controlling the electronic switches (D1 to D4, M1 to M4);
• and
• a low-voltage side series inductor (W1);
• and
• a second winding (W2) connected in series with a discharge switch (S) and a discharge diode (D), and selectively connected to the low voltage network (LV) by the discharge switch (S),
• wherein the series inductor (W1) and the second winding (W2) are configured as a discharge transformer (3) for discharging the energy stored inductively in the series inductor (W1), and
• wherein a reverse flow of energy through the series inductor is made possible by the discharge diode connected in series with the second winding discharging the energy in the series inductor,
• wherein the charging with energy from a low-voltage battery (UNV) takes place via a sequence of controlled current pulses, and wherein
• in a first phase, the current pulses are separated from one another by dead times (tT), and the current pulses become continuously longer and the dead times (tT) become shorter, until the dead time of zero is reached, wherein in this phase, the electronic switches (D1 to D4) in the low-voltage network (NV) are open during the dead times (tT), and as a result, the low-voltage battery (UNV) is disconnected from the transformer (1), and wherein the discharge switch (S) is closed, whereby the series inductor discharges;
• and
• in a second phase, the current pulses are separated from one another by short-circuit periods (tK), and the current pulses become continuously shorter and the short-circuit intervals (tK) become longer, until the desired voltage is reached at the intermediate-circuit capacitor (CZK), wherein in this phase, the electronic switches (D1 to D4) in the low-voltage network (LV) are conductive during the short-circuit interval (tK), and short-circuit the winding of the transformer (1), and wherein the discharge switch S is open and the voltage at the series inductor (W) continuously increases.