DC-Link Capacitor Sizing Considerations for HEV/EV e-Drive Systems

Dc-link capacitor sizing considerations are discussed for HEV/EV e-Drive systems. The capacitance value of the dc-link in HEV/EV e-Drive systems affects numerous factors. Some of the most significant are the system stability and the maximum tolerable dc-bus transient voltage with operating point change or with worst-case energy dump into the capacitor. Also requiring attention is the equivalent series resistance and inductance of the capacitor module. The former affects thermal behavior of the capacitor module and the latter affects voltage spikes occurring at every turn-off of a power semiconductor switch. In addition, these factors are dependent on other power-stage component parameters, control structures and controller gains. Also such effects and cross-couplings are operating-point dependent. This makes the dc-link capacitor sizing for HEV/EV e-Drive systems a complex task and it looks as if the task is done primarily by empirical manner with possibly more than enough safety margin. It appears accordingly that no systematic step-by-step methodology for the capacitor sizing has been discussed in the public domain. This paper attempts to present a list of the items to be considered for such a systematic dc-link capacitor sizing, and a detailed description follows for each item. The above-mentioned cross-coupling matters and the resulting trade-offs are briefly discussed. Potential issues caused by the use of emerging wide-bandgap semiconductors are mentioned as well. The target system taken as an example is an HEV e-Drive system, consisting of two electric machine drive inverters and a dc-dc converter, which share a common dc-link.