Power Electronics: APM: Notes

The APM is a DC/DC converter that enables energy transfer between the two voltage levels of the hybrid car. One voltage level is the high voltage electrical system with approx. 300 V and the other is the familiar 14 V vehicle electrical system. The DC/DC converter replaces the alternator, which was previously used for the voltage supply of the 14 V vehicle electrical system. Thus the electrical voltage supply of the 14 V vehicle electrical system while driving is no longer dependent on the engine speed of the combustion engine.

The APM control unit is used in the E72 only. It is designed as a bidirectional converter. This means that the APM transfers the electrical energy in both directions between the high voltage electrical system and the 14 V vehicle electrical system. The APM has been developed as part of the hybrid co-operation along with GM, DaimlerChrysler (later Daimler and Chrysler) and is based in part on the DC/DC converter of a predecessor project in which BMW did not participate. The developer and manufacturer of APM is Delphi Electronics&Safety.

The APM is activated by the HCP, which is part of the PEB. The APM does not switch on the voltage conversion on its own.

The HCP sends the following commands to the APM:

• Switch conversion on or off
• Conversion direction (high voltage to 14 V or 14 V to high voltage)
• The nominal voltage

The APM then decides, based on the data from the self-diagnosis and the values it has detected itself, whether the conversion can be switched on. During operation, the APM attempts to adjust the nominal voltage to the corresponding voltage level by increasing the current up to the technically possible limit value. The APM cannot decrease the voltage in the vehicle electrical system, for example by lowering the voltage in the 14 V vehicle electrical system to 11 V. However, if the current voltage in the respective voltage level is higher than the nominal voltage of the APM, the APM reduces the current to 0 A. Thus no energy transfer takes place. The APM has a passive discharge circuit that discharges the capacitors in the APM to a voltage value of less than 60 volts within five seconds. If a fault is detected, the APM switches off the conversion automatically.