31.4 98.7% Efficiency 1200V-to-48V LLC Converter with CC/CV Mode Charging Compliant with EVSE Level 1

With the tremendous and fast-growing demand for electric vehicles (EVs), the efficiency of 48V EV battery charging becomes the crux of the matter. The 1200 V power supply of electric vehicle supply equipment (EVSE) will become the mainstream specification of EV charging base to achieve efficient and fast charging. However, the low voltage-conversion ratio (VCR) of only 4% results in inefficiency and is impractical in conventional step-down chargers. Although many exotic topologies have been proposed, such as multilevel topology and double step-down (DSD) topology, the hard-switching operation during the high-side (HS) switches turn-on accounts for most of the power loss and difficulty breaks through 96% efficiency. Therefore, a resonant LLC converter [1–4] with soft switching characteristics in both HS and low-side (LS) switches is an excellent choice for high efficiency at low VCR operation. However, apart from IGBT and SiC, few technologies can withstand 1200 V stress. While, long off-duration and negative-voltage turn-off requirements lead to long dead-time and complex circuits design for IGBT and SiC power devices, respectively. Therefore, LLC primary-side divider-based converters become more attractive. Although the four-level converter [5] (left top of Fig. 31.4.1) can evenly relieve the voltage stress of each switch to $\mathrm V_{\mathrm{IN}}/3$, it does not fully consider the large conduction loss of the three switches $\mathrm S_{4}$ to $\mathrm S_{6}$ when $\mathrm {V_{x}}$ is pulled low. Thankfully, the 3:1 switched-capacitor buck in [6] (right top of Fig. 31.4.1) reduces conduction losses by using one switch $\left(\mathrm S_{1}\right)$ on the conduction path when $\mathrm {V_{x}}$ is pulled low. However, 1) four capacitors will worsen the power density, 2) large equivalent series resistance (ESR) losses will inevitably occur between the four capacitors, and one of the paths will have as much as 8 ESR losses, and 3) parallel capacitor topology will cause current imbalance because the voltage mismatch of each capacitor requires additional current compensation increasing conduction losses (middle left of Fig. 31.4.1). Therefore, conduction loss and ESR loss should be fixed in this topology. The power loss distribution is shown in the middle right of Fig. 31.4.1, since the LLC converter with resonant characteristics can realize zero-voltage switching (ZVS), the switching loss is much smaller than the conduction loss, and in this work is not shown. This paper proposes that when $\mathrm {V_{x}}$ is pulled low, the equivalent switch and ESR losses on the current path are minimized, which can effectively improve efficiency. When the 48 V battery is charged in constant current (CC) mode, the large current $\mathrm{I}_{\mathrm{LS}}$ on the secondary side passes through the parasitic resistance $\mathrm{R}_{\text {para }}$ between the package and the printed circuit board (PCB) trace, reducing the voltage $\mathrm{V}_{\text {PGND }}$ level (lower left of Fig. 31.4.1). To make matters worse, the LLC topology with an N-fold step-down transformer excites N-fold current on the secondary side. A negative $\mathrm V_{\text {PGND }}$ means that a lower reference voltage $\mathrm V_{\text {REFo }}$ overestimates the charging current (lower right of Fig. 31.4.1), so the reduced current increases the charging time. In this paper, monolithic Gallium Nitride (GaN) based control and low $\mathrm {R_{O N}}$ [7, 8] switch on the primary side are used to minimize conduction losses at high voltage, and silicon based control and synchronous rectifiers (SR) are used on the secondary side for sending back accurate feedback signals to the primary side.