9.2.4.9 Power MOSFET and Output Rectifier Selection

The initial design target proposed the use of a 100-V Schottky rectifier. The secondary-side reverse voltage stress can be verified using the final transformer design sh own in Equation 52.

Equation 52.

The value derived from Equation 52 is close to the original design target of 85 V.

For 65-W load, the average DC output current is 3.35 A for 19.5-V output. However, to reduce losses, a much higher current rated diode is typically used, to yield a much lower forward voltage drop V_RECT. As shown in Figure 44, a 30-A rated diode D7 is used in this case, with a forward drop of approximately 0.45 V at 3.5 A, 100°C.

For the primary-side MOSFET, the peak voltage stress can be estimated using Equation 53.

Equation 53.

An allowance of at least 100 V must be added to this figure to account for the leakage inductance spike at turn-off. This voltage spike depends on the transformer implementation and the amount of leakage inductance, as well as the specific design of the snubber. A more aggressive snubber may reduce the voltage spike, but at the expense of higher losses in the snubber. A voltage rating of at least 600 V is recommended for the power MOSFET to allow for leakage.

The MOSFET rms current at low line, rated load, can be estimated using Equation 54.

Equation 54.

As can be seen in Figure 44, the chosen MOSFET Q1 is a 13-A, 600-V device.