Impedance Model Based Small-Signal Stability Assessment of an Isolated EV Charger Using LLC Resonant Converter

With the advancement in planar magnetic technology as well as wide band-gap (WBG) devices such as Gallium Nitride (GaN), Silicon Carbide (SiC) MOSFET and high electronic mobility transistor (HEMT), the utility of resonant converters has remarkably expanded. These converters can be used in applications requiring power density since they naturally possess zero voltage switching (ZVS) and zero current switching (ZCS) capabilities. Recently, they are gaining popularity in point-of-load (POL) DC-DC converter applications, such as electric vehicle (EV) chargers. As majority of the EV chargers are grid-connected, cascading a front-end (FE) AC-DC power factor corrected (PFC) converter with POLs becomes necessary. It is well-known in the literature that in cascaded systems, the impedance interaction of any FE converter with POL converters may lead to issues with system stability. The individual converters should be designed with sufficient stability margin to meet Middlebrook's stability criterion. Therefore, the knowledge of small-signal impedances becomes a prerequisite for applying the stability criterion. This paper focuses on determining the small-signal impedances of the LLC resonant converter, which are then used to analyze the stability of an EV charger employing the LLC resonant converter as a POL converter. The analysis is experimentally verified in a laboratory-developed prototype of 800W grid-connected EV charger.