8.7 A 92.7% Peak Efficiency 12V-to-60v Input to 1.2V Output Hybrid DC-DC Converter Based on a Series-Parallel-Connected Switched Capacitor

The battery voltage $(\mathrm{V}_{\mathrm{bat}})$ is becoming higher in automotive applications for a higher efficiency power system. Accordingly, an ultra-low voltage-conversion-ratio (VCR) buck converter is in great demand. Previous ultra-low VCR buck converters use flying capacitors $( \mathrm{C}_{\mathrm{F}}\mathrm{s})$ and power switches between the battery and the inductor, as shown in Fig. 8.7.1 (top-right) [1–6]. In these topologies, $\mathrm{C}_{\mathrm{F}}\mathrm{s}$ reduce the voltage stress applied to switches by dividing the $\mathrm{V}_{\mathrm{bat}}$, which has four major weak points. First, because of the high $\mathrm{V}_{\mathrm{bat}}, \mathrm{C}_{F}\mathrm{s}$ and switches are used as high-voltage (HV) capacitors and LDMOSs, respectively, which are bulky and have larger parasitic components resulting in huge power loss compared to a low-voltage (LV) capacitor and CMOS. If compound semiconductors, such as GaN and SiC, are used, the power loss caused by switches can be reduced [1, 6], however, they are less cost effective than silicon devices. Second, previous converters have a problem of on-duty (D) range. Considering several types of battery and their safety margins in the vehicle, the converter is required to properly operate with a $\mathrm{V}_{\mathrm{bat}}$ range from 12V up to 60V. Previous converters can have the D either smaller than 0.1 or larger than 0.9 when they operate at this $\mathrm{V}_{\mathrm{bat}}$ range and an output voltage $( \mathrm{V}_{\mathrm{o}})$ of 1.2V, as shown in Fig. 8.7.1 (top-left), which makes the converter vulnerable to noise. Third, previous converters have another challenge for the line transient. In the automotive application, $\mathrm{V}_{bat}$ can abruptly vary because of external conditions, as shown in Fig. 8.7.1 (bottom-left). Since the voltage across each $\mathrm{C}_{\mathrm{F}}( \mathrm{V}_{\mathrm{CF}})$ is proportional to $\mathrm{V}_{\mathrm{bat}}$, an abrupt variation of $\mathrm{V}_{\mathrm{bat}}$ results in an abrupt change in $\mathrm{V}_{\mathrm{CF}}$. This induces a huge inrush current through the $\mathrm{C}_{\mathrm{F}}$ or unbalanced inductor currents causing fatal damage to the power switches. Lastly, previous converters suffer from a slow load transient response because the load current $(\mathrm{I}_{\mathrm{o}})$ is only provided by the inductor.