The main internal configuration of the DC/DC converter is shown in Fig. 5. 129. The DC/DC converter consists of the power circuit and the control circuit, for the power conversion. In the power circuit, the switching transistor (MOS-FET is mainly used due to the high speed switching requirement) switches the input DC voltage according to the drive signal from the control circuit, then supplies the AC voltage to the transformer. The transformer changes the AC voltage, and rectifies it through the diodes. In the smoothing circuit, the on-off DC voltage after the rectification is smoothed, and it is then changed in the auxiliary battery.

The control circuit controls the output voltage so as to output the specified voltage even in case there is a disturbed change in the main battery voltage.�@The explanation of the sensing voltage is given here (Fig. 5. 130). The installation of the DC/DC converter and the auxiliary battery and the wiring method are different depending on vehicle models. Therefore, the voltage drop in the charging line from the DC/DC converter to the auxiliary battery is not the same. (In general, the voltage drop is approximately between 0.1V and 0.5V). However, the auxiliary battery requires a precise voltage charge to it. Since the voltage drop not negligible, the terminal voltage of the auxiliary battery is detected directly, and then the output voltage is controlled so that the voltage at the auxiliary battery terminal reaches the specified value. This is so known "battery sensing control". On the other hand, the abnormal sensing control is performed in case of an emergency, such as the disconnection of the battery sensing line. In this case, the sensing point is switched from the auxiliary battery terminal to the output terminal of the DC/DC converter. This is so known "converter sensing control" (or "local control").

In addition to the above, the control circuit has the following functions.

1. Overcurrent control
2. Input overvoltage protection, output overvoltage protection
3. Overheat protection
4. Alarm function