DC/DC converter output current check: Evaluation

Select fault pattern based on the measuring results described in the following:

1. Output current remains less than 10 A, trigger signal does not activate. Set trigger signal mode to "continuous" and check output current.

   Check the EME fault memory. If one of the specified faults is currently present, an internal fault is present in the DC/DC converter.

   Replace electrical machine electronics.

2. Output current greater than 10 A, trigger signal activates, output voltage at least temporarily approx. 14 V.

   If the output current exceeds approx. 120 A, component protection is activated due to overload current (output current and output voltage are limited). There is a fault in the 12 V vehicle electrical system that is responsible for the overload current.

   Check the following lines at the EME for short circuit: Battery voltage, 31.

   Check consumers in the 12 V vehicle electrical system for short circuit (especially those with high power).

   Check the 12 V battery for battery cell short circuit.

   Replace faulty lines/components or repair if permissible.

3. Output current greater than 10 A, trigger signal activates, output voltage at least temporarily approx. 14 V.

   If the output current remains below 50 A at a low level, switch trigger signal to "continuous" mode. Then gradually switch on consumers in the 12 V vehicle electrical system (especially those with high power) and monitor the output current.

   If the output current exceeds approx. 120 A on switching on a consumer, there is a fault in the consumer last switched on. Check and repair/replace lines and consumers.

   If the output current remains below approx. 120 A on switching on all consumers, no fault can be determined. No action required.

The following illustration shows an example of the characteristic of the measured variables for the case: DC/DC converter OK, overload current caused by fault in 12 V vehicle electrical system.

Courtesy of BMW OF NORTH AMERICA, INC.

The output voltage measured before activation of the DC/DC converter corresponds to the voltage of the 12 V battery. In this period, the output current is 0 A. The DC/DC converter is then activated when the high-voltage electrical system starts. This occurs e.g. on waking the control units (terminal 30B on) or on switching on terminal 15. In the example shown, the output voltage initially increases to approx. 14 V after activation of the DC/DC converter. As the output current quickly increases to impermissibly high values, however, the output voltage is automatically reduced shortly after (component protection). In this example, the DC/DC converter in the electrical machine electronics is therefore OK. The DC/DC converter attempts to supply the 12 V vehicle electrical system with energy and supplies an output current to do so. As there is a fault in the 12 V vehicle electrical system leading to overload current, however, the DC/DC converter reduces the output power.

If, in another case, the cause is an internal fault in the DC/DC converter, no DC/DC converter output current would be measurable on starting the high-voltage electrical system.