Maximizing power density with topology innovations

In addition to component-level innovations, topology innovations can help you simplify power conversion in high-voltage systems. The AC/DC rectifier is a great example of how wide band-gap technologies can elevate well-known topologies to improve power density and reduce design weight. Historically, engineers used a bridge diode rectifier with a capacitor to rectify the AC voltage into the DC voltage, as shown in Figure 7.

The power factor of such a rectifier is generally lower than 0.5, depending on the total impedance of output capacitor and load. This is not energy-efficient, as such a design generates too much unused power (reactive power).

To solve the low-power-factor issue, engineers came up with the idea of an active power factor correction (PFC) circuit. Figure 8 shows a boost PFC circuit, which generally takes a universal AC voltage (90 V_AC to 264 V_AC) and boosts the voltage to a regulated 400-V voltage at the output. With input voltage sensing, the controller regulates the inductor current to follow the AC sinusoidal shape to get an almost unity power factor (0.99).

This type of boost PFC rectifier is able to achieve really high efficiency (>98%) with a superjunction silicon MOSFET and a SiC diode.

The full-bridge diode rectifier in the boost PFC rectifier does consume more than 1% of overall efficiency losses in kilowatt-level high-voltage systems. For example, a more than 20-W loss on a full-bridge diode rectifier is expected in a 2-kW rectifier. It is very difficult to dissipate a 20-W loss from a single device. To reduce losses on the full-bridge diode rectifier, the totem-pole bridgeless PFC shown in Figure 9 presents a good alternative. Because the rectifier function is integrated with the boost converter and has only two additional MOSFETs (instead of four diodes), the total rectifier loss (with the two low-frequency FETs) is much lower than in the original bridge rectifier example.

A continuous-conduction-mode (CCM) totem-pole bridgeless PFC is a hard-switching converter, which is widely applied in high-voltage rectifiers. Therefore, if you apply a silicon MOSFET to a totem-pole bridgeless PFC, the silicon MOSFET will suffer from high switching losses caused by Q_rr. As shown in Figure 10, after the top-left MOSFET body-diode current conduction, Q_rr will generate a reverse-recovery current to charge the bottom-left MOSFET C_oss during the dead time of the left half bridge. Once the bottom-left MOSFET turns on, the Q_rr-induced energy will dissipate into the bottom-left MOSFET. The Q_rr-related loss consumes the loss reduction on the full-bridge diode rectifier.

The existence of wide band-gap FETs can, for the most part, help solve Q_rr-related loss issues with the new totem-pole bridgeless PFC topology. A SiC MOSFET can achieve a 20 times smaller Q_rr than a superjunction MOSFET with the same on-resistance level – and a GaN FET can achieve zero Q_rr. When combining component and topology innovations in the rectifier example (in other words, applying wide band-gap FETs with a totem-pole bridgeless PFC), you can achieve over 99% efficiency (a >1% efficiency improvement), unlocking higher power density and lighter weight in your designs.