When looking to incorporate MOSFET devices into power system designs, there are numerous issues that should be given careful consideration. Through having a good understanding of each of these, the best fit can be found between the various component options available and the specific design criteria that need to be adhered to. Also the necessary trade-offs between different parameters can be better executed.
One of the key MOSFET parameters that must be examined is on-state resistance (RDS(ON)). Whatever the situation, engineers specifying a MOSFET will generally need to be able to assess what the maximum acceptable RDS(ON) value will be. As this is the major contributor to power loss while the device is active it has the potential to cause unwanted battery drain in low-voltage portable applications, where the power budget is likely to be modest. Therefore, choosing a MOSFET with a low RDS(ON) will be highly advantageous in such circumstances. Care must be taken if comparisons are being made between different MOSFETs. MOSFET datasheets will normally state RDS(ON) at a junction temperature (TJ) of 25°C. With higher operating temperatures RDS(ON) rises. The rate-of-rise of on-state resistance varies between manufacturers and MOSFET technologies. The operating temperature also needs to be taken into account when considering maximum breakdown voltages VDS(max) to ensure that like-for-like evaluations are carried out correctly. The maximum breakdown voltage VDS(max) rises with operating temperature. Since some manufacturers datasheets cite VDS(max) at maximum junction temperature (TJ) rather than room temperature of 25°C, the maximum breakdown voltage appears to be higher than expected.
The gate charge (QG) defines the charge energy required to switch the MOSFET device. It is through the MOSFET’s QG that its switching performance will effectively be established. If the QG is small, then the switching frequency supported can be higher. As the RDS(ON) has a detrimental effect on the QGD, engineers have to find a way to deal these two opposing dynamics, so that a balance can be struck between high switching capacity and low power consumption. The multiplying together of RDS(ON) and QG results in what is known as the MOSFET’s figure of merit (FoM). This gives a valuable guide as to the device’s overall performance.
The current rating (ID) is another key parameter. This denotes DC current that can flow in the forward direction and relates closely to the maximum power losses that stem from the RDS(ON). The threshold voltage (VTH) describes the minimum gate bias that can be applied in order to create a conduction channel between the MOSFET’s source and drain. Once the gate-source voltage (VGS) exceeds this value, then the MOSFET channel will start to conduct an electrical current. The breakdown voltage (VDSmax) is the maximum drain source-voltage. How the MOSFET is constructed will determine these last few parameters. Finally, MOSFET size is a critical factor too. Having increasing space constraints to contend with, greater pressures are being placed on engineers to find MOSFET devices that can attain higher performance benchmarks, while being housed in compact packages.
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