Technical Knowledge: 100V MOSFET Devices for Embedded Power Systems

Efficient AC/DC SMPS and DC/DC converters are the backbone of modern power architectures and are used to drive systems such as telecommunications or computers. In order to meet the market demand for these converters, Infineon Technologies has launched a new 100V  MOSFET series of devices . Based on charge balance, this series of devices can significantly reduce the on-resistance. The combination of low gate charge, high switching speed, excellent avalanche resistance and improved body-diode characteristics makes these devices suitable for a variety of different applications.

Introduction

Embedded power systems in telecommunications and computing systems face the challenge of increasing power density. Despite the increasing power requirements, the power system space remains constant. These demands can only be met with higher system efficiency.

Improvements can be achieved at different levels – system, converter and device level. New power architectures can reduce losses at the system level. Optimizing the topology of AC/DC and DC/DC converters can improve efficiency at the converter level. New MOSFET technology can improve efficiency at the device level. MOSFETs are a key component in power converters. Better technology can enable existing topologies to adopt more challenging operating conditions – increasing switching frequency can even change other topologies.

The High Speed ​​Series (HS Series) with ultra-low gate charge can further increase speed by 33%.

Advanced concept of the new OptiMOS2 MOSFET

The concept of power MOSFET compensation was introduced in the 600V CoolMOSTM product launched in 1998. The basic principle of the significant reduction in Rds(on) A compared to conventional power MOSFETs is that the acceptor located in the P column compensates the donor in the N drift region.

Trench field plate MOSFETs are a good choice for applications with breakdown voltages below 200V. The use of a field plate can significantly improve the performance of the device. The device contains a deep trench extending into the majority of the N drift region. An insulated deep source electrode is isolated from the N drift region by a thick oxide layer and acts as a field plate to provide the mobile charges required to balance the drift region donor body under blocking conditions. The thick field plate insulation must be able to withstand the blocking voltage at the bottom of the trench. Accordingly, the oxide thickness in microns must be carefully controlled to avoid too thin oxide in the bottom trench corners and to prevent stress-induced defects. Unlike standard MOS structures, where the electric field decreases linearly and reaches a maximum at the body/drift region pn junction, the field plate provides an almost constant electric field distribution, thus shortening the drift region length for a given breakdown voltage. In addition, the drift region doping level can be increased to reduce the on-resistance. In fact, Rds(on) A can even be reduced below the so-called "silicon limit" - the on-resistance of an ideal p+n- junction at a given breakdown voltage. The combined use of field-plate and trench-gate MOSFETs results in the lowest resistance and fastest silicon switching technology available on the market today.

Application Advantages

In embedded power systems, there are currently three main applications for 100V MOSFETs: AC/DC front-end synchronous rectification switches (output voltage 12V ~ 20V), power switches on 48V wide-range power bus, and primary side main switches of isolated DC/DC converters operating with 48V power bus. Very low Rds(on) values ​​are beneficial to all of the above applications. Other features of 100V OptiMOS2 technology are suitable for some of these specific applications.

The application of charge balance makes OptiMOS2 100V technology highly competitive in most application areas. This technology can achieve benchmark key parameters such as Rds(on), Qg, Qgd, Crss/Ciss ratio and excellent avalanche resistance in a single device. Low on-resistance Rds(on) [12.5mΩ(max)@D-Pak, 5.1mΩ(max)@D2-Pak] plus fast switching capability and excellent avalanche resistance make OptiMOS2 100V the right choice for safe, high-performance and high-power density applications. 1. Avalanche resistance

While inductive loads exist in motor control and similar applications, they do not exist in embedded power systems, so the ability of MOSFETs to safely handle avalanche events is critical. All of the above applications may face failures such as lightning strikes or other unforeseen events that put these devices in an avalanche state. Reliable avalanche resistance ensures safe operation of the system even under these adverse conditions.

At the silicon technology level, there are two mechanisms that can provide charge carriers during an avalanche.

The first mechanism is related to the turn-on of the parasitic npn transistor in the MOSFET. This is a non-thermal destruction because it is caused by the current through the p-base region. As soon as the voltage drop in this region is large enough to bypass the base-emitter barrier with a forward bias, the transistor turns on. This mechanism has an auto-amplification function, which can lead to current-limited avalanche characteristics. For power MOSFETs, this limitation is not favorable because even very low energies are sufficient to destroy the device as long as the critical current level is reached.

The second mechanism is related to the avalanche generation of carriers. The overvoltage on the device is sufficient to accelerate individual free electrons to a level where they can be generated again, resulting in a chain reaction. The energy dissipated in the device is distributed in the drift region. In this mechanism, the device avalanche capability limit is determined by the device's ability to absorb (heat) energy. This failure mechanism is called thermal destruction.

The device fails due to thermal damage and exhibits characteristic characteristics. The extrapolated lines here correspond to the average fault current values ​​at different temperatures. The intersection with the zero current line marks the intrinsic temperature of the device and is a measure of the device's avalanche resistance.

2. Immunity to dynamic conduction

The most effective way to reduce power losses in SMPS is to change the secondary side rectification from a passive system (using diodes) to active synchronous rectification (using MOSFETs). For applications with output voltages of 12V to 24V (depending on the topology), 100V MOSFETs are the right choice for synchronous rectification. Due to the corresponding conduction losses, Rds(on) becomes a key parameter for synchronous rectification.

But MOSFETs used for secondary-side rectification also bring additional risks. The most significant aspect is dynamic turn-on. In hard-switching topologies, when the device starts blocking, there can be very large dv/dt values ​​from drain to source. This dv/dt value is connected to the gate via the capacitive Cgd/Cgs divider and can dynamically turn on the device in question. In this case, a short circuit is formed, resulting in significantly increased losses in the MOSFET and transformer.

3. Lowest FOMg and FOMgd values

The highest demand for efficient power conversion is in the area of ​​DC/DC conversion for telecom and server power supplies and similar systems.

Converters are required to deliver the highest currents in an efficient manner. This is only possible by utilizing state-of-the-art components and topologies and using switching frequencies of 250kHz and above.

For standard 48V wide-range systems, 100V  MOSFETs are often used as the primary side main switch in a half-bridge or full-bridge topology. Due to the very high switching frequency, low on-resistance Rds(on) is required, and low gate charge Qg is also required. FOMG (Rds(on), Qg) becomes a reliable metric for MOSFET selection. In addition, Qgd, which is directly related to the switching losses, is equally important. It is important to note that an increase in overall efficiency of only 1% will mean a 17°C drop in MOSFET switch temperature. Class D amplifiers have similar requirements, in which MOSFETs operate in half-bridge or full-bridge topologies.

The applications for 100V MOSFETs cover a wide range of requirements. The new OptiMOS2 100V series uses advanced MOSFET technology to provide the required characteristics for safe, fast switching and lowest resistance power MOSFET devices.