Rethinking the Synchronous Rectifier in QR Flyback Converters

At these loads, the SuperQ device demonstrated an approximate 0.3-W reduction in total power loss, translating to a 0.2% to 0.5% efficiency improvement.

However, the most important observation isn’t the absolute efficiency number, but rather the mechanism by which it’s achieved. The reduction in total system loss can’t be explained solely by improved conduction performance on the secondary side. Instead, the data indicates a redistribution of losses throughout the converter.  The differences become more pronounced at higher input voltage because switching losses increase with both bus voltage and operating frequency.

This behavior is especially significant because it demonstrates that SR optimization affects more than just secondary conduction efficiency. The observed reduction in total converter loss indicates that changes in SR switching behavior alter the resonant energy exchange of the entire power stage, lowering switching-related losses on the primary side as well.

Secondary Optimization Improves Primary Switching Behavior

One of the more significant findings from the evaluation is that the synchronous rectifier directly influences primary-side switching behavior. Although the SR is physically located on the secondary side of the converter, its capacitance and switching characteristics interact with the transformer and resonant energy-transfer process in ways that materially affect primary-side switching loss.

This interaction becomes increasingly important at higher input voltages. In QR flyback converters operating from a universal AC input, higher V_IN conditions typically produce higher bulk bus voltage and a higher switching frequency. Under these conditions, switching losses increasingly dominate over conduction losses. Specifically, the turn-off overlap losses (also called I×V losses) and E_OSS-related turn-on losses become a larger percentage of total converter dissipation.

The measured efficiency data reflects this behavior. At lower input voltages, where conduction losses dominate, the efficiency difference between devices is relatively modest. However, as input voltage rises, the efficiency advantage of the next-generation silicon MOSFET becomes more pronounced because the converter becomes increasingly sensitive to switching-related losses.

The mechanism behind this improvement can be understood by examining how secondary-side capacitance is reflected through the transformer during resonant operation. The SR device contributes capacitance to the resonant network formed by the transformer magnetizing inductance, leakage inductance, and parasitic node capacitances. Devices with slower charge and discharge behavior or higher stored capacitive energy increase the energy circulating within this network during switching transitions.

Using the SuperQ-based SR device, the lower stored charge and faster capacitive discharge behavior reduce the amount of residual energy present during the primary switching transition. As a result, the primary drain-source voltage transitions more rapidly during turn-off, reducing the overlap between voltage and current and thereby lowering I×V switching loss.

Such behavior is shown in Figure 6, which compares the primary switch drain-voltage turn-off transition of a superjunction SR device with that of the SuperQ SR device under identical operating conditions.