MOSFET Analysis: Choosing an Energy-Saving, High-Efficiency MOSFET

Not long ago, Energy Star released the 2.0 version of the external power supply energy efficiency specification. The new specification significantly increases the operating frequency requirements, while further reducing the standby power consumption requirements. For example, in order to meet the new specification, the minimum efficiency of a 2.5W (5V, 0.5A) external power supply must reach 72.3%, and the new specification requires that the no-load power consumption should be less than 300mW, which are significantly higher than the current specifications. Not only external power supplies, but also many handheld products and home appliances are also facing the test of low power consumption. Take mobile phones as an example. As smartphones have more and more functions, low power consumption design has become an increasingly urgent issue.

Faced with the trend of reducing power consumption and improving energy efficiency, design engineers must start from the source and choose energy-saving and efficient devices for their designs as much as possible. High-efficiency power semiconductors can help engineers shorten the development schedule of related products and easily meet the system specification requirements. As a type of power semiconductor, MOSFET is used in many systems, such as portable devices, consumer power adapters, computer motherboards, LCD displays, network communications, industrial control, automotive electronics, and lighting. Especially in DC/DC converters, the choice of power MOSFET will have a key impact on the efficiency of the power supply. Below, we will introduce several MOSFETs used in different fields, which have excellent performance in both on-resistance and switching speed.

Several key parameters of MOSFET

MOSFET is the abbreviation of Metal Oxide Semicoductor Field Effect Transistor, which is translated into Chinese as "Metal Oxide Semiconductor Field Effect Transistor". It is a device made of three materials: metal, oxide (SiO2 or SiN) and semiconductor. According to the different channel semiconductor materials, MOSFET is divided into N-channel and P-channel. The so-called power MOSFET refers to a device that can output a large operating current (several amperes to tens of amperes) and is used in the power output stage.

Among the parameters of MOSFET, there are three main parameters to consider: maximum withstand voltage, maximum current capability and on-resistance. On-resistance (RDSON) is a key parameter. The smaller the on-resistance, the smaller the conduction loss. However, considering only on-resistance is not enough, because there are three main sources of power MOSFET loss: (1) on-resistance causes conduction loss; (2) gate charge causes loss in the drive circuit and switching loss; (3) output capacitance causes power MOSFET energy storage/energy consumption during the cut-off/on process. Therefore, choosing an energy-saving and efficient MOSFET requires considering a variety of reasons and application areas.

In the industry, MOSFET has a universal performance measurement benchmark, namely the quality factor (FOM), which can be expressed as the product of on-resistance (RDS(ON)) and gate charge (Qg), that is, FOM = RDS(ON)×Qg. RDS(ON) is directly related to conduction loss, and Qg is directly related to switching loss. Therefore, the lower the FOM value, the better the device performance.

Several high-efficiency, low-loss MOSFETs

Infineon's 40V, 60V and 80V OptiMOS 3 N-channel MOSFET series in SuperS08 lead-free package have extremely low on-resistance. Its 40V series has a minimum on-resistance of 1.8mΩ, the 60V series has a minimum on-resistance of 2.8mΩ, and the 80V series has a minimum on-resistance of 4.7mΩ. The FOM of these devices is 25% higher than that of similar products using standard TO packages, which can achieve faster switching while minimizing switching losses and gate drive losses, improving power density, and reducing driver heat dissipation. The parasitic inductance of the SuperSO8 package is less than 0.5nH, which is much lower than the 5-10nH inductance of the TO-220 package, which further improves the overall efficiency of the device and minimizes oscillation under switching conditions. OptiMOS 3 40V, 60V and 80V products are suitable for power conversion and power management applications that require high efficiency and power density, including SMPS (switch mode power supply), DC/DC converters and DC motor drivers of many products. These products include computers, home appliances, small electric vehicles, industrial automation systems, telecommunications equipment and consumer electronic devices such as power tools, electric lawn mowers and fans.

STMicroelectronics (ST) also has its own unique technology for MOSFET products. Its STripFET technology uses very high equivalent cell density and smaller cell feature size to achieve extremely low on-resistance and switching loss. STripFET V is the latest generation of STripFET technology. The two MOSFETs based on this technology, STD60N3LH5 and STD85N3LH5, have extremely low conduction loss and switching loss. In a typical voltage regulator module, the reduction of both losses can reach 3W. The product achieves an excellent quality factor FOM. Both products are 30V (BVDSS) devices. The gate charge (Qg) of STD60N3LH5 is 8.8nC, and the on-resistance is 7.2mΩ at 10V, making it an ideal choice for control field effect transistors in non-isolated DC/DC step-down converters. At 10V, the on-resistance of STD85N3LH5 is 4.2mΩ and the gate charge is 14nC, making it an excellent choice for synchronous field effect transistors. Both products are available in DPAK and IPAK packages.

Vishay Intertechnology's 20V N-channel device SiR440DP expands its third-generation TrenchFET power MOSFET family. The device is packaged in a PowerPAK SO-8 package and has a maximum on-resistance of 2.0mΩ at 4.5V gate drive and a maximum on-resistance of 1.55mΩ at 10V gate drive. In DC/DC applications, this MOSFET has an excellent quality factor of 87 at 4.5V. The SiR440DP is used as a low-side MOSFET in synchronous buck converters and secondary synchronous rectification and OR-ing applications. Its low conduction and switching losses will ensure more power-efficient and space-saving designs for voltage regulator modules (VRMs), servers, and many systems using point-of-load (POL) power conversion.