Thermal Design of PCB Surface Mount Power Devices

1. System requirements:

VOUT = 5.0V; VIN (MAX) = 9.0V; VIN (MIN) = 5.6V; IOUT = 700mA; Operating cycle = 100%; TA = 50 °C According to the above system requirements select 750mA MIC2937A-5.0BU regulator, Its parameters are:

VOUT = 5V ± 2% (worst case when overheated) TJ MAX = 125°C. Using TO-263 package, θJC=3°C/W; θCS≈0°C/W (directly soldered on circuit board).

2. Preliminary calculation:

VOUT(MIN)=5V-5×2%=4.9VPD=(VIN(MAX)-VOUT(MIN))+IOUT+(VIN(MAX)×I)=[9V-4.9V]×700mA+(9V×15mA) = 3W maximum value of temperature rise, ΔT = TJ(MAX) - TA = 125°C - 50°C = 75°C; thermal resistance θJA (worst case): ΔT/PD = 75°C/3.0W = 25°C/W.

Thermal resistance of the heat sink, θSA=θJA-(θJC+θCS); θSA=25-(3+0)=22°C/W (maximum).

3. Determine the physical dimensions of the radiator:

A square, single-sided, horizontal solder mask with a copper foil heatsink and a heat-dissipating copper foil covered with a black oil paint, with a 1.3 m/sec air cooling solution, is better than the latter. 

With the solid-line approach, a conservative design requires 5,000mm2 of heat-dissipating copper foil, a 71mm x 71mm (2.8 inches on each side) square.

4. Thermal requirements for SO-8 and SOT-223 packages:

Calculate the thermal dissipation area under the following conditions: VOUT=5.0V; VIN(MAX)=14V; VIN(MIN)=5.6V; IOUT=150mA; Duty Cycle=100%; TA=50°C. Circuit board production equipment is more likely to handle dual-in-line SO-8 packaged devices under permissible conditions. Can SO-8 meet this requirement? With the MIC2951-03BM (SO-8 package), the following parameters can be obtained:

TJ MAX=125°C; θJC≈100°C/W.

5. Calculate the parameters using the SO-8 package:

PD = [14V-5V] x 150mA + (14V x 8mA) = 1.46W; elevated temperature = 125°C - 50°C = 75°C; thermal resistance θJA (worst case):

ΔT/PD = 75°C/1.46W = 51.3°C/W; θSA = 51-100 = -49°C/W (maximum).

Obviously, SO-8 can not meet the design requirements without refrigeration. Consider the MIC5201-5.0BS regulator in the SOT-223 package. The package is smaller than the SO-8, but its three pins have good heat dissipation. Select MIC5201-3.3BS, its related parameters are as follows:

TJ MAX=125°C The thermal resistance of OT-223 is θJC=15°C/WθCS=0°C/W (directly soldered on the board).

6. Calculate the result of using SOT-223 package:

PD = [14V-4.9V] × 150mA + (14V × 1.5mA) = 1.4W rising temperature = 125°C - 50°C = 75°C; thermal resistance θJA (worst case):

ΔT/PD=75°C/1.4W=54°C/W; θSA=54-15=39°C/W (maximum). According to the above data, referring to Figure 1, the use of 1,400 mm2 heat-dissipating copper foil (a square with a side length of 1.5 inches) can meet the design requirements.

The above design results can be used as a rough reference. The actual design needs to understand the thermal characteristics of the circuit board and obtain more accurate results that meet the actual design.