Guidelines for MOSFET Package Selection

Second, the size of the system limitations

Some electronic systems are limited by the size of the PCB and the internal height, such as communication systems, modular power supply due to height limitations usually use DFN5 * 6, DFN3 * 3 package; in some ACDC power supply, the use of ultra-thin design or due to the limitations of the shell, the assembly of the TO220 package of the power MOSFET feet directly inserted into the root of the height limitations can not use the TO247 package. Some ultra-thin design directly bending the device pins flat, this design production process will become complex.

Third, the company's production process

TO220 has two kinds of package: bare metal package and full plastic package, bare metal package thermal resistance is small, heat dissipation ability is strong, but in the production process, you need to add insulation drop, the production process is complex and costly, while the full plastic package thermal resistance is large, heat dissipation ability is weak, but the production process is simple.

In order to reduce the artificial process of locking screws, in recent years, some electronic systems using clips to power MOSFETs clamped in the heat sink, so that the emergence of the traditional TO220 part of the upper part of the removal of holes in the new form of encapsulation, but also to reduce the height of the device.

Fourth, cost control

In some extremely cost-sensitive applications such as desktop motherboards and boards, power MOSFETs in DPAK packages are usually used because of the low cost of such packages. Therefore, when choosing a power MOSFET package, combined with their company's style and product features, and consider the above factors.

Fifth, select the withstand voltage BVDSS in most cases, because the design of the input voltage of the electronic system is relatively fixed, the company selected a specific supplier of some material number, the product rated voltage is also fixed.

The breakdown voltage BVDSS of power MOSFETs in the datasheet has defined test conditions, with different values under different conditions, and BVDSS has a positive temperature coefficient, in the actual application of the combination of these factors should be considered in a comprehensive manner.

A lot of information and literature often mentioned: if the system of power MOSFET VDS of the highest spike voltage if greater than the BVDSS, even if the spike pulse voltage duration of only a few or tens of ns, the power MOSFET will enter the avalanche and thus damage occurs.

Unlike transistors and IGBT, power MOSFETs have the ability to resist avalanche, and many large semiconductor companies power MOSFET avalanche energy in the production line is the full inspection, 100% detection, that is, in the data this is a guaranteed measurement, avalanche voltage usually occurs in 1.2 ~ 1.3 times the BVDSS, and the duration of the time is usually μs, even ms level, then the duration of only a few or tens of ns, much lower than the avalanche voltage spike pulse voltage is not damage to the power MOSFET.

Six, by the drive voltage selection VTH

Different electronic systems of power MOSFETs selected drive voltage is not the same, AC / DC power supply usually use 12V drive voltage, the notebook's motherboard DC / DC converter using 5V drive voltage, so according to the system's drive voltage to select a different threshold voltage VTH power MOSFETs.

The threshold voltage VTH of power MOSFETs in the datasheet also has defined test conditions and has different values under different conditions, and VTH has a negative temperature coefficient. Different drive voltages VGS correspond to different on-resistances, and in practical applications it is important to take into account the temperature

In practical applications, temperature variations should be taken into account to ensure that the power MOSFET is fully turned on, while at the same time ensuring that the spike pulses coupled to the G-pole during the shutdown process will not be triggered by false triggering to produce a straight-through or short-circuit.