PCB Proofing Design Thermal Design of Surface Mount Power Device

PCB Proofing Design Thermal design of surface mount Power DevICe

PCB proofing design: heat dissipation design of surface mount power supply device. This paper introduces how to work normally when only one printed circuit board copper and platinum is used as the heat sink. First, understand the circuit requirements.

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. 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%=4.9VPD=(VIN(MAX)-VOUT(MIN))+IOUT+(VIN(MAX) × I)=[9V-4.9V] × 700mA+(9V × 15mA)=maximum value of 3W 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 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 ℃ SOT-223 thermal resistance θ JC=15℃/W θ CS=0 ℃/W (directly welded on the circuit board).

6. 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.

The above design results can be used as a rough reference. In the actual design, it is necessary to understand the thermal characteristics of the circuit board to obtain more accurate results that meet the actual design. PCB manufacturers, PCB designers and PCBA manufacturers will explain the heat dissipation design of surface mount power supply devices for PCB proofing design.