A Quick Guide to Choosing Power MOSFETs

In high power industrial applications, choosing the right power semiconductor is essential for system reliability and longevity. Here’s a quick guide to the selection criteria that separate different MOSFETs.

When choosing a MOSFET for your power system, a good way to narrow down the selection would be to rule out anything that doesn’t have the voltage or current rating that you need. For example, in an inverter circuit that is supplying 250VAC to the grid, 600V MOSFETs are usually specified, in case of high voltage transients.

As well as the overall rated values, MOSFET data sheets also offer safe operating area (SOA) curves, which should be studied closely to determine whether the part you’re looking at will be able to withstand your application safely. SOA curve plots the voltage and current at which the MOSFET can be used safely. Below is an example SOA curve from an IR part.

Figure 1: An example of a MOSFET SOA curve showing the safe operating limits for the part

Aside from the voltage and current ratings, other important characteristics for MOSFETs include the resistance of the MOSFET when it’s switched on, denoted by RDS(ON). Low RDS(ON) means less energy is wasted when the MOSFET is on, contributing to power supply efficiency.

Also have a look at the thermal conditions you expect in your application. RDS(ON) typically rises with temperature, so check this isn’t a deal-breaker. The example curve below, from a Vishay MOSFET, shows the relationship between (normalised) RDS(ON) and junction temperature.

Figure 2: An example of the relationship between RDS(ON) and junction temperature

RθJC is the thermal resistance between the junction of the MOSFET and the case, which measures how good the packaging is at getting heat away from the silicon inside. Today’s high-end packaging technologies do a good job at getting heat away. Consider this power MOSFET from STMicroelectronics – it has an RθJC 0.5°C/W for the D2PAK or TO-220 packages, or 3.33°C/W for the same part in a TO220-FP package. Also listed is the RθJA, the thermal resistance between the junction and the ambient air, which is 62.5 °C/W for both package types.

Of course, these characteristics may be traded off against each other, and cost, so it’s important to remember that different MOSFETs are optimised for different applications. For a power supply load switch in a server application, the MOSFET will be on almost 100% of the time, so it’s switching characteristics aren’t as important as its RDS(ON). However, in a switch-mode power supply (SMPS) where MOSFETs are switching continuously at high speed, switching characteristics will be extremely important.

MOSFETs for SMPS applications are typically evaluated by their figure of merit (FOM), which is a measure of switching characteristics. Efficiency is paramount, so conduction losses and switching losses must both be minimised. RDS(ON) is a good measure of conduction loss, while switching loss is represented by QG, the amount of charge (and therefore energy) it takes to switch the MOSFET. QG is closely related to switching parasitic capacitance. As an example, this ON Semi power MOSFET has a QG of 7.5nC. The figure of merit is calculated by multiplying RDS(ON) * QG. It’s hard for manufacturers to minimise both at the same time, so it’s a good indication of quality.

For best results, the FOM should be compared when the MOSFETs are under the same conditions, that is, the same VGS, VDS and ID, since the characteristics change under different conditions. Consider this one from Fairchild, which advertises RDS(ON)  is 104mΩ at VGS=10V, ID=4.2A, but for a different set of conditions, RDS(ON)  is 156mΩ at VGS=4.5V, ID=3.4A.

Other figures of merit commonly used include RDS(ON)*footprint, since small RDS(ON) in a small board area may mean the same converter can be fitted into a smaller volume, increasing power density.

Even within switching power supply topologies, requirements may differ. For example, in a resonant converter, switching losses are minimised by switching only when VDS or ID is zero (better known as zero voltage/current switching). Conduction losses are therefore disproportionately important and RDS(ON) will play a bigger part in MOSFET selection.

Aside from power supplies, the bridge topologies used in motor control have different requirements too. These MOSFETs won’t be required to switch at high speed, but the body diode needs to have a fast reverse recovery time (trr) in order to protect the device from reverse current caused by an inductive load. Parts with special fast recovery body diodes are available, designed for solar inverters.

Although the ratings, thermal performance, on-resistance and switching characteristics are a good measure of MOSFET quality, which of these characteristics can be traded off against each other will ultimately be determined by the application. Understanding what conditions the MOSFET can expect to experience will determine the most desirable characteristics.