Selecting Critical Components: MOSFET Q1

The discussion of transformer design has been concluded, and so we begin discussion of component selection, centered on peripheral components of the BD7682FJ-LB power supply IC. We present excerpts of the peripheral circuits of components being explained, but the entire circuit must be studied as well, and when doing so this resource may be used.

Selecting Critical Components：MOSFET Q1

The MOSFET Q1 is the transistor that drives the primary side of the transformer, and is a “SiC MOSFET”, one recurring theme of this design.

In MOSFET selection, the maximum drain-source voltage, peak current, loss due to the ON-resistance (Ron), maximum allowable power dissipation for the package, and other factors must be considered.

As the input voltage becomes lower, the MOSFET ON-time becomes longer and heat generation due to Ron losses becomes greater. A low Ron is one feature of SiC MOSFETs, and this conduction loss is also small. However, the mounting board and the state in which the product is incorporated must always be checked, and a heat sink or other means of heat dissipation should be provided when necessary.

The rating for I_D is selected with Ippk×2 as a rough guide. Here Ippk was determined to be 0.66 A in ② of the section on transformer design.

Vds is calculated using the following equation.

Vspike is hard to calculate numerically. Hence on the basis of an empirical rule, addition of a snubber circuit is assumed, and a MOSFET with a Vds of 1700 V is selected. In this calculation example, a ROHM SiC MOSFET, the SCT2H12NY (1700 V, 1.15 Ω, 4 A, 44 W), or the SCT2H12NZ (1700 V, 1.15 Ω, 3.7 A, 35 W), is selected.

Below, the maximum ratings of the SCT2H12NY are shown as an example. Please refer to the relevant data sheet for other parameters and further details.