Super capacitor management using a Split-Pi symmetrical bi-directional DC-DC power converter with feed-forward gain control

A method for the interfacing of a super capacitor in a hybrid vehicle power system is demonstrated. The use of the Split-Pi bi-directional DC-DC power converter is proposed to overcome the problems of interfacing large voltage swing super capacitors to narrow input voltage range motor controllers. The Split-Pi DC-DC power converter topology and operational characteristics are presented. Results from using the Split-Pi DC-DC converter for super capacitor terminal voltages below and above the motor controller input voltage range in drive mode and regeneration mode are presented. Gain control of the Split-Pi DC-DC converter is demonstrated using feed forward control by direct computation of the required converter gain.

1 Introduction

Hybrid vehicular technology has received much attention in recent times. Electric motor controllers are used to connect the energy store of the vehicle to the electric drive motor as shown in Fig. 1. Motor controllers are generally designed to accept a narrow input voltage supply range, usually adjusted to the battery chemistry's voltage range. The motor controller's input voltage range is adapted to the specific battery discharge characteristics to prevent over discharge of the battery by automatic supply voltage cut-off, which present problems when interfacing to the large voltage ranges of super capacitors. Super capacitors offer a number of attractive characteristics allowing them to be considered as an alternative energy store in hybrid applications [1]. Fig 1.

Schematic diagram of a parallel drive hybrid vehicle with super capacitor energy store.

Fig. 2.

A comparison between the state of charge as a function of terminal voltage of a super capacitor and typical lead acid battery. Below 90% of the terminal voltage only 30% charge remains in the lead acid battery compared to 80% of charge remaining in the super capacitor.