Thermal design technology of power supply system
The DC-DC power supplies we design generally include components such as capacitors, inductors, Schottky, resistors, and chips. The conversion efficiency of power supply products cannot be 100%, and there must be losses. These losses will be in the form of temperature rise. In front of us, the power supply system will cause accelerated life decline due to poor thermal design. Therefore, thermal design is a particularly important aspect of system reliability design. But thermal design is also very difficult and involves too many factors, such as the size of the circuit board and whether there is air flow.
When we look at IC product specifications, we often see terms such as RJA, TJ, TSTG, TLEAD, etc.; first of all, RJA refers to the chip thermal resistance, that is, the temperature rise of the chip node corresponding to each loss of 1W, and TJ refers to the junction temperature of the chip. Temperature, TSTG refers to the storage temperature range of the chip, and TLEAD refers to the processing temperature of the chip.
First, understand the terms related to temperature: TJ, TA, TC, TT. As can be seen from "Figure 1", TJ refers to the junction temperature inside the chip, TA refers to the ambient temperature where the chip is located, TC refers to the temperature of the pad on the back of the chip or the bottom shell temperature, and TT refers to the surface temperature of the chip. A common parameter that characterizes thermal performance in data sheets is thermal resistance RJA. RJA is defined as the thermal resistance from the node of the chip to the surrounding environment. where TJ = TA + (RJA *PD)
There are two main heat dissipation paths for the heat generated by the chip. The first path is from the node of the chip to the top plastic package of the chip (RJT), through convection/radiation (RTA) to the surrounding air; the second path is from the node of the chip to the back pad (RJC), through convection/radiation /Radiation (RCA) is conducted to the PCB board surface and the surrounding air. For chips without thermal pads, RJC refers to the thermal resistance from the node to the top of the plastic package; because RJC represents the lowest thermal resistance path from the node in the chip to the outside world.
Design example: A certain DC voltage step-down solution has an output of 5V, a current of 1A, a conversion efficiency of eta of 90%, and an ambient temperature of TA of 50°C. The capacitor used has a rated temperature of 100°C and is very close to the chip. The chip TJ temperature is required to be controlled at 90°C.
First, the system loss PD=VOUT*IOUT*(1/η-1)=5*1*(1/0.9-1)=0.56W.
Assuming that all losses are included in the chip, the thermal resistance RJA≤(90℃-50℃)/0.56≤71.4℃/W can be calculated.
1.PCB board size
The thermal resistance of the selected chip should be lower than 71.4℃/W. If the SOP8-EP chip is selected, its RJA is 60℃/W. It is still necessary to design a PCB board or heat sink to transfer heat from the plastic package to the surrounding air. In the case of only natural convection (that is, no air flow) and no heat sink, the circuit board area required according to the rule of thumb for a circuit board covered with copper on both sides can be estimated by the following equation:
The heat sink can effectively reduce the temperature of the chip, but the location of the heat sink is also important. For chip components, the heat sink can be placed directly on top of the chip plastic package, as shown in "Figure 2". However, due to the large thermal resistance of the chip plastic package and poor contact between the heat sink and the heat sink, the performance of the heat sink will be reduced. You can also connect the heat sink to the via hole on the back of the chip to improve the performance of the heat sink.
5. Air cooling
In an environment where the product space is relatively large and is not sealed, airflow can be generated by a low-power fan, which can significantly reduce the overall thermal resistance of the system.
6. Glue filling
For products that require waterproofing, dustproof, and anti-vibration, thermal grease can be poured into the sealed mold so that the power system components can transfer heat to the shell through the thermal grease, thereby dissipating the heat.
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