Buck-Converter Design Demystified

Jun 1, 2006 12:00 PM
By Donald Schelle and Jorge Castorena, Technical Staff, Maxim Integrated Products, Sunnyvale, Calif.

MOSFET Selection

Selecting a MOSFET can be daunting, so engineers often avoid that task by choosing a regulator IC with an internal MOSFET. Unfortunately, most manufacturers find it cost prohibitive to integrate a large MOSFET with a dc-dc controller in the same package, so power converters with integrated MOSFETs typically specify maximum output currents no greater than 3 A to 6 A. For larger output currents, the only alternative is usually an external MOSFET.

The maximum junction temperature (T_JMAX) and maximum ambient temperature (T_AMAX) for the external MOSFET must be known before you can select a suitable device. T_JMAX should not exceed 115°C to 120°C and T_AMAX should not exceed 60°C. A 60°C maximum ambient temperature may seem high, but stepdown converter circuits are typically housed in a chassis where such ambient temperatures are not unusual. You can calculate a maximum allowable temperature rise for the MOSFET as follows:

Inserting the values mentioned above for T_JMAX and T_AMAX into Eq. 7 yields a maximum MOSFET temperature rise of 55°C. The maximum power dissipated in the MOSFET can be calculated from the allowable maximum rise in MOSFET temperature:

The type of MOSFET package and the amount of pc-board copper connected to it affect the MOSFET's junction-to-ambient thermal resistance (Θ_JA). When Θ_JA is not specified in the data sheet, 62°C/W serves as a good estimate for a standard SO-8 package (wire-bond interconnect, without an exposed paddle), mounted on 1 in.² of 1-oz pc-board copper.

There exists no inverse linear relationship between a Θ_jA value and the amount of copper connected to the device, and the benefit of decreasing the Θ_JA value quickly dwindles for circuits that include more than 1 sq in. of pc-board copper. Using Θ_JA = 62°C/W in Eq. 8 yields a maximum allowable dissipated power in the MOSFET of approximately 0.89 W.

Power dissipation in the MOSFET is caused by on-resistance and switching losses. On-resistance loss can be calculated as:

Because most data sheets specify the maximum on-resistance only at 25°C, you may have to estimate the value of on-resistance at T_JHOT. As a rule of thumb, a temperature coefficient of 0.5%/°C provides a good indicator for maximum on-resistance at any given temperature. Thus, the hot on-resistance is calculated as:

Assuming the on-resistance loss is approximately 60% of the total MOSFET losses, you can substitute in Eq. 10 and rearrange to yield Eq. 11, the maximum allowable on-resistance at 25°C:

Switching losses constitute a smaller portion of the MOSFET's power dissipation, but they still must be taken into account. The following switching-loss calculation provides only a rough estimate, and therefore is no substitute for evaluation in the lab, preferably a test that includes a thermocouple mounted on P1 as a sanity check.

where C_RSS is the reverse-transfer capacitance of P1, I_GATE is the peak gate-drive source/sink current of the controller and P1 is the high-side MOSFET.

Assuming a gate drive of 1 A (obtained from the gate driver/ controller data sheet) and a reverse-transfer capacitance of 300 pF (obtained from the MOSFET data sheet), Eq. 11 yields a maximum R_DS(ON)25°C of approximately 26.2 mΩ. Recalculating and summing the on-resistance losses and the switching losses yields a net dissipated power of 0.676 W. Using this figure, you can calculate for the MOSFET a maximum temperature rise of 101°C, which is within the acceptable temperature range.