​Circuit Board Design - Thermal Design of SMT Power Device

Circuit Board Design - Thermal Design of SMT Power Device

Circuit board design and circuit board processing manufacturer explain circuit board design - heat dissipation design of SMT power components

1. System requirements:

VOUT=5.0V； VIN(MAX)=9.0V； VIN(MIN)=5.6V； IOUT=700mA； Operation cycle=100%; TA=50℃

Select 750mA MIC2937A-5.0BU voltage regulator according to the above system requirements, and its parameters are:

VOUT=5V ± 2% (worst case of overheating)

TJ MAX=125℃。 TO-263 package, θ JC=3℃/W；

θ CS ≈ 0 ℃/W (directly welded on the circuit board).

2. Preliminary calculation:

VOUT(MIN)=5V-5 × 2%=4.9V

PD=(VIN(MAX)-VOUT(MIN))+IOUT+(VIN(MAX) × I)=[9V-4.9V] × 700mA+(9V × 15mA)=3W

Maximum temperature rise, Δ T=TJ(MAX)-TA = 125℃-50℃=75℃； thermal resistance θ JA (worst case): Δ T/PD=75℃/3.0W=25℃/W。

Thermal resistance of radiator, θ SA= θ JA-( θ JC+ θ CS)； θ SA=25 - (3+0)=22 ℃/W (maximum).

3. Determine the physical size of the radiator:

A square, one-sided, horizontal copper foil heat dissipation layer with solder mask is used, compared with a heat dissipation copper foil covered with black oily paint, and 1.3 m/s air heat dissipation is used, the latter has the best heat dissipation effect.

The solid line scheme is adopted, and the conservative design requires 5000mm2 heat dissipation copper foil, i.e. 71mm × A square of 71 mm (2.8 inches on each side).

4. Heat dissipation requirements for SO-8 and SOT-223 packaging:

Calculate the heat dissipation area under the following conditions: VOUT=5.0V; VIN(MAX)=14V； VIN(MIN)=5.6V； IOUT=150mA； Duty cycle=100%; TA=50℃。 Under allowable conditions, circuit board production equipment is easier to handle devices packaged with dual SO-8. Can SO-8 meet this requirement? Using MIC2951-03BM (SO-8 encapsulation), the following parameters can be obtained:

TJ MAX=125℃； θ JC≈100℃/W。

5. Calculate the results of SOT-223 encapsulation:

PD=[14V-4.9V] × 150mA+(14V × 1.5mA)=1.4W

Rising temperature=125 ℃ - 50 ℃=75 ℃;

thermal resistance θ JA (worst case):

Δ T/PD=75℃/1.4W=54℃/W；

θ SA=54-15=39 ℃/W (maximum). According to the above data, 1400 mm2 heat dissipation copper foil (a square with a side length of 1.5 inches) can meet the design requirements.

6. Calculate the parameters of SO-8 package:

PD=[14V-5V] × 150mA+(14V × 8mA)=1.46W；

Elevated temperature=125 ℃ - 50 ℃=75 ℃;

thermal resistance θ JA (worst case):

Δ T/PD=75℃/1.46W=51.3℃/W；

θ SA=51-100=- 49 ℃/W (maximum).

Obviously, SO-8 cannot meet the design requirements without refrigeration. MIC5201-5.0BS voltage regulator packaged with SOT-223 is considered. The package is smaller than SO-8, but its three pins have good heat dissipation effect. MIC5201-3.3BS is selected, and its relevant parameters are as follows:

TJ MAX=125℃

Thermal resistance of SOT-223 θ JC=15℃/W

θ CS=0 ℃/W (directly welded on the circuit board). Circuit board design and circuit board processing manufacturer explain circuit board design - heat dissipation design of SMT power components.