Knowledge is Power - Essential Attributes of Power MOSFETs

One of the key MOSFET parameters that must be examined is on-state resistance (R_DS(ON)). Whatever the situation, engineers specifying a MOSFET will generally need to be able to assess what the maximum acceptable R_DS(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 R_DS(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 R_DS(ON) at a junction temperature (T_J) of 25°C. With higher operating temperatures R_DS(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 V_DS(max) to ensure that like-for-like evaluations are carried out correctly. The maximum breakdown voltage V_DS(max) rises with operating temperature. Since some manufacturers datasheets cite V_DS(max) at maximum junction temperature (T_J) rather than room temperature of 25°C, the maximum breakdown voltage appears to be higher than expected.

The gate charge (Q_G) defines the charge energy required to switch the MOSFET device. It is through the MOSFET’s Q_G that its switching performance will effectively be established. If the Q_G is small, then the switching frequency supported can be higher. As the R_DS(ON) has a detrimental effect on the Q_GD, 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 R_DS(ON) and Q_G 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 (I_D) 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 R_DS(ON). The threshold voltage (V_TH) 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 (V_GS) exceeds this value, then the MOSFET channel will start to conduct an electrical current. The breakdown voltage (V_DSmax) 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.

For more detail on how to maximise the effectiveness of the power MOSFETs selection process, download Toshiba’s latest white paper on the subject: