Calculate Dissipation For MOSFETs In High-Power Supplies

A Design Example: The CPU core supply shown in Figure 3 delivers 1.3 V at 40 A. Two identical 20-A power stages operating at 300 kHz supply the 40-A output current. The MAX1718 master controller drives one stage, while the MAX1897 slave controller drives the other. This supply's input range spans 8 to 20 V, with the specified maximum ambient temperature of the enclosure at 60°C.

The synchronous rectifier comprises two IRF7822 MOSFETs in parallel, with a combined maximum R_DS(ON) of 3.25 mO at room temperature, and about 4.7 mO at 115°C, the assumed T_J(HOT). With a maximum duty factor of 94%, a 20-A load current, and the 4.7-mO maximum R_DS(ON), these paralleled MOSFETs dissipate about 1.8 W. Supplied with 2 in.² of copper to dissipate that power, the overall T_JA should be about 31°C/W. The temperature rise of the combined MOSFETs will be about 55°C, so this design will work with an ambient temperature of up to about 60°C.

Two IRF7811W MOSFETs in parallel, with a combined maximum R_DS(ON) of 6 mO at room temperature, and about 8.7 mO at 115°C (the assumed T_J(HOT)), make up the switching MOSFET. The combined C_RSS is 240 pF. The MAX1718's (and MAX1897's) 1-O gate drivers deliver about 2 A. At V_IN = 8 V, the resistive losses are 0.57 W, and the switching losses are approximately 0.05 W. At 20 V, the resistive losses are 0.23 W, and the switching losses are approximately 0.29 W. Total losses at each operating point roughly balance, and the worst-case total equals 0.61 W at the minimum V_IN.

Because this level of power dissipation isn't high, we can provide this pair of MOSFETs with under 0.5 in.² of copper area, achieving an overall T_JA of about 55°C/W. This still enables operation up to an ambient temperature of 80°C with a 35°C temperature rise.

The copper areas in this example are required for the MOSFETs alone. If other devices dissipate heat into those areas, more copper area will likely be required. If space isn't available for this additional copper, reduce the total power dissipation, spread the heat to areas of low dissipation, or use active means to remove heat.

Thermal management is one of the most difficult areas of high-power portable design. That difficulty makes the iterative process outlined above necessary. Although this process should get the board designer close to the final design, lab work must ultimately determine whether the design process was sufficiently accurate. Calculating the thermal properties of the MOSFETs and ensuring their dissipation paths while checking those calculations in the lab helps ensure a robust thermal design.