A feature of 48V hybrid systems is the integration of motors and inverters. This makes the whole system far more compact than previous technology. The motors are now being located on the powertrain itself for improved performance, but this exposes the electronics to a higher level of vibration and temperature cycling.
DC-Link capacitors are one of the power electronics components affected by this change. They are located between the inverter and the battery and their role is to smooth out the voltage spikes caused by fluctuating demand. It is important that the capacitor performs consistently in a wide range of operating conditions including vibration and temperature. Because the DC-link capacitor governs the voltage of the entire ECU, its performance affects the performance of the whole unit.
Why standard capacitors fail
Research shows that standard multilayer ceramic capacitors (MLCCs) are vulnerable to flex cracks. When a PC board flexes due to vibration or thermal cycling, the stress passes through the solder joint and into the ceramic body. As this body is inherently inflexible, it cracks leading to an immediate drop off in its performance. The close the MLCC is mounted to a board edge or other stress points like push connectors, the more vulnerable it is to failure.
One of the main parameters that describe a DC-Link capacitor’s performance is its equivalent series resistance (ESR). A low ESR enables a capacitor to smooth out high frequency disturbances from the load.