Selecting a MOSFET requires the engineer to use their expertise in scrutinizing different specifications for individual applications.
In an application such as a load switch in a server power supply, the switching aspects of a MOSFET matter little because the MOSFET is on almost 100% of the time. The on resistance (RDS(ON)) may be the key figure of merit in such an application. Still other applications, including switching power supplies, use MOSFETs as active switches, and cause the engineer to value other MOSFET performance parameters.
MOSFETs may be best known as switching elements in power supplies, but the components can also offer great benefits at the output of a power supply.
The parallel supplies equally share the load and assure the system will continue to operate even if one supply fails. This architecture, however, also requires a way to tie the output of the parallel supplies together while ensuring that a failure in one supply doesn't affect the other power supplies. A power MOSFET at the output of each supply allows the supplies to share the load and still isolates each supply. MOSFETs used in such a role are called "ORing" FETs because they essentially connect the outputs of multiple supplies in a logical "OR" configuration.
Designers working on an ORing FET application clearly need to concentrate on different characteristics of the MOSFET than a designer working on a switching-centric application. In the server example, the MOSFET acts as nothing more than a conductor during normal operation. Therefore, minimal conduction losses are the most important concern for designers in an ORing application.
In addition to RDS(ON), power-supply designers may find several other MOSFET parameters very important in the selection process. In many cases, the designer should closely scrutinize the Safe Operating Area (SOA) curves on the data sheet that describe both drain current and drain-to-source voltage. Essentially, SOA defines the power-supply voltage and current levels at which the MOSFET can be safely deployed.
The SOA curve might come more into play if the design were to implement a hot-swap capability. In the hot-swap case, the MOSFET would need to operate in a partially-on state. The SOA curve defines the current and voltages limits for different pulse times.
Noting the recently mentioned current rating, it's also worth considering thermal parameters because the always-on MOSFETs are subject to heat. Moreover, escalated junction temperatures can result in a rise in RDS(ON). MOSFET data sheets specify thermal resistance parameters which, in effect, define how well the MOSFET package conducts heat away from the semiconductor junction. RθJC, in its simplest definition, is the thermal resistance from the junction to the case.
Expounding on this a bit, in practice the measurement denotes the resistance from the device junction (in a vertical MOSFET this is near the top surface of the die) down to the outside surface of the package, as described in the datasheet.
In the case of PowerQFN packaging, the case is defined as the center of the large drain tab. Therefore, RθJC defines the thermal effect of the die and package system. RθJA defines the thermal resistance from the surface of the die to the ambient air, and is specified, typically via a footnote, relative to the design of the PCB including number of layers and the thickness of the copper plating.
Now let's consider the switching power-supply applications, and how that use demands a different view of the data sheet. By definition, the application requires that the MOSFET regularly switch on and off. While there are dozens of topologies used in switching power supplies, let's consider a simple example.
Clearly, power-supply design is sufficiently complex, and there is no simple formula for MOSFET evaluation. But let's consider some key parameters and why those parameters matter. Traditionally, many power supply designers have used a compound figure of merit - gate charge (QG) multiplied by RDS(ON) - in evaluating or ranking MOSFETs.
Gate charge and on resistance are important because both have a direct effect on the efficiency of a power supply. Losses that impact efficiency come primarily in two forms - conduction losses and switching losses.
Gate charge is a primary factor in switching losses. Gate charge is specified in nanocoulombs (nc) and is the energy required to charge and discharge the MOSFET gate. Gate charge and RDS(ON) are intertwined at the semiconductor design and fabrication process. Generally, devices with lower gate charge specs will have slightly higher RDS(ON) specs.
MOSFET parameters with secondary importance in switching power supplies include output capacitance, threshold voltage, gate resistance, and avalanche energy.
Some specific topologies also change the relative merit of various MOSFET parameters. For instance, traditional synchronous buck converters compared with resonant converters. Resonant converters minimize switching losses by switching the MOSFETS only when VDS (Drain to Source Voltage) or ID (Drain Current) passes through zero. These techniques are referred to as soft switching or ZVS (Zero Voltage Switching) or ZCS (Zero Current Switching). Since switching losses are minimized, RDS(ON) is more significant in such topologies.
Both types of converters benefit from relatively low output capacitance (COSS) values. The resonant circuit in a resonant converter is dictated by the leakage inductance of the transformer combined with COSS. Moreover, the resonant circuit must fully discharge COSS during the dead time when both MOSFETs are switched off. Therefore, resonant topologies value a low COSS.
Traditional buck converters, sometimes called hard-switching converters, benefit from a low output capacitance for a different reason. The energy stored at the output capacitance is lost each cycle, whereas that energy is recycled in a resonant converter. So a low output capacitance is especially important in the low-side switch of a synchronous buck regulator.