Fred C. Lee

A novel resonant gate driver for high frequency synchronous buck converters

This paper introduces a new resonant gate driver for both the top and bottom switches of a synchronous buck converter. A coupled inductor is used to reduce the size as well as to transfer energy between the top and bottom gate driving. A possible semiconductor integration approach is proposed for this resonant gate driver based on a self-adaptive control method. Theoretical analysis, simulation and experimental results prove that the proposed driver can greatly reduce the gate driving loss and that it is well suited to high-frequency applications.

Two-stage approach for 12-V VR

To meet the stringent specifications of the voltage regulator (VR), the two-stage approach is proposed for the 12-V VR. This paper discusses detailed design considerations, which include the design of the first stage, optimization of the intermediate bus voltage, design of the intermediate bus capacitors, and the design of the ultra-high frequency second stage. The analysis shows that the two-stage approach can realize high frequency, thus significantly reducing the output capacitance and therefore decreasing the cost. A 1.2-V/100-A prototype is built to verify the analysis. The second stage runs at 2MHz per phase and the total efficiency is as high as 81%. Compared to the conventional single-stage multiphase buck, the two-stage approach is more cost-effective and more efficient.

Two-stage approach for 12 V VR

To meet the stringent specifications of the voltage regulator (VR), the two-stage approach is proposed for the 12 V VR. This paper discusses detailed design considerations, which include the design of the first stage, optimization of the intermediate bus voltage, design of the intermediate bus capacitors, and the design of the ultra-high frequency second stage. The analysis shows that the two-stage approach can realize high frequency, thus significantly reducing the output capacitance and therefore decreasing the cost. A 1.2 V/100 A prototype is built to verify the analysis. The second stage runs at 2 MHz per phase and the total efficiency is as high as 81%. Compared to the conventional single-stage multi phase buck, the two-stage approach is more cost effective and more efficient.

A family of high power density bus converters

This paper begins by reviewing current bus converters and exploring their limitations. Then a family of inductor-less bus converters is proposed to overcome the limitations. In the new bus converter, magnetizing current is used to achieve zero-voltage-switching (ZVS) turn-on for all switches. The resonant concept is used to achieve zero-current-switching (ZCS) without turn-off loss and body diode loss. Meanwhile, the self-driven method can be easily applied to save drive loss in the synchronous rectifiers. Based on these concepts, a full-bridge bus converter is built in the quarter-brick size to verify the analysis. The experimental results indicate that it can achieve 95% efficiency at 500 W, 12 V/45 A output. Compared with industry products, this topology can dramatically increase the power density.

VRM transient study and output filter design for future processors

In this paper, the transient response of the (voltage regulator module) VRM output voltage when the processor has a fast load change is analyzed. The parasitic parameters play important roles in the transient. The system can be divided into several resonant loops. Each loop can be approximately considered as a decoupled second order system. The transient response is affected by the magnitude of the load change rather than the slew rate of it. Limitations of the present VRM topology for future specifications and output filter design are discussed.

A family of buck-type DC-DC converters with autotransformers

This paper introduces a family of buck-type DC-DC converters with autotransformers, including forward, push-pull, half-bridge, and full-bridge topologies. Compared with an isolated transformer, the autotransformer has a simpler winding structure, and it only needs to transfer part of the input power, resulting in a smaller secondary winding current. Analysis shows that the autotransformer can also help to reduce the voltage stress and current ratings of power devices in the DC-DC converters. For some applications, a simple lossless passive clamp circuit can be implemented to solve the transformer leakage problems, and the gate drive is significantly improved with a simple self-adaptive dead-time-controlled bootstrap gate driver. Simulation and experimental results show that the proposed topologies are very suitable for high-frequency applications.