Excessive heat generated by high temperatures or internal power consumption can alter the characteristics of electronic components and cause them to shut down, work outside of the specified operating range, or even fail. Power management devices (and their associated circuits) often encounter these problems because any power dissipation between the input and the load can cause the device to heat up, so heat must be dissipated from these devices into the PCB, nearby cells. The device or the surrounding air. Even in traditional high-efficiency switching power supplies, heat dissipation must be considered when designing PCBs and selecting external components. When designing a power management circuit, it is helpful to have a basic understanding of heat transfer before looking at heat dissipation issues. First, heat is an energy that is transmitted due to the temperature difference between the two systems. Heat transfer takes place in three ways: conduction, convection, and radiation. Conduction occurs when a high temperature device is exposed to a low temperature device. High-amplitude, high-temperature atoms collide with atoms of low-temperature materials, thereby increasing the kinetic energy of low-temperature materials. This increase in kinetic energy causes the temperature of the high temperature material to rise and the temperature of the low temperature material to drop. In convection, heat transfer occurs in the air surrounding the device. In natural convection, the object heats the surrounding air, which expands to form a vacuum when heated, causing cold air to replace hot air. This creates a circulating air stream that continuously transfers the heat of the device to the surrounding air. Another form is forced convection, for example, the fan actively blows cold air to accelerate the replacement of warm air. Radiation is generated when an object sends electromagnetic waves (thermal radiation) to the surrounding environment. Radiant heat does not require medium transfer (heat can be radiated by vacuum). In PCBs, the primary method of heat transfer is conduction, followed by convection. The following equation gives a mathematical model of conduction heat transfer: Where H is the heat transfer rate (in J/s), K is the thermal conductivity of the material, A is the area, (TH–TL) is the temperature difference, and d is the distance. When the contact area between the interfaces increases, the temperature difference increases, or the distance between the interfaces decreases, the heat conduction speed increases. The heat transfer can be modeled as a circuit by equating the energy source (the heat source or H in the previous equation) with the current source, and the temperature difference between the high temperature device and the low temperature device is equivalent to the voltage drop, (K × A / d) As the thermal conductivity, or the reciprocal (EQ2) is equivalent to the thermal resistance (in °C/W). Typically thermal resistance is expressed as the symbol θ or Rθ or only as RA-B, where A and B are the two devices that undergo heat transfer. Using circuit simulation to rewrite the heat transfer rate equation, the following results are obtained: The simulation can be done in depth to describe another thermal property of the device, called heat capacity. Just as the thermal resistance is modeled as a resistor, the heat capacity (CT, in J/°C) can be modeled as a capacitor. The thermal resistance (ZT) is obtained by connecting the heat capacity in parallel with the thermal resistance. Figure 1 shows a simplified RC model of conducted heat transfer. Energy is modeled as a current source and thermal impedance is modeled as a parallel connection between CT and RT. Consider the discrete MOSFETs mounted on the PCB. The steady state thermal impedance (or thermal resistance RJ-A) is the thermal resistance (RJ-C) from the junction to the device case, the thermal resistance of the device case to the heat sink (RC-S), and the heat sink to air resistance (RS- The sum of A). (RJ-A=RJ-C+RC-S+RS-A). In addition, there can be parallel heat dissipation paths, such as from the MOSFET junction through the device case to the PCB, and from the PCB to ambient temperature. Typically, semiconductor manufacturers will give the junction to the device's case. On the other hand, RC-S and RS-A mainly depend on the properties of the heat sink and PCB. Many factors affect the thermal resistance RC-A or RC-S, including the number of layers of the PCB, the number of vias to the auxiliary surface, the proximity to other devices, and the gas flow rate. Usually RJ-A will be listed in the device data sheet, but this number is derived under specific test board conditions and is therefore only suitable for comparisons between devices measured under the same conditions. Thermal resistance (RJA) is an important parameter for electronic components because it is an indicator of device heat dissipation (based on environmental conditions and PCB layout). In other words, RJ-A can help us estimate the operating junction temperature based on environmental conditions and power consumption. Heat dissipation in switching power supplies For a typical example of thermal considerations in power management circuits, reference is made to the LM3554 circuit provided by National Semiconductor in Figure 2. The device is an inductive boost converter for high power flash LEDs in cellular phone applications. The LM3554 is a good test tool because it is a small device (1.6mm (1.6mm (0.6mm)) and can deliver up to 6W of output power ((1.2A flash current in 5V LED). Even if it is 85% With high efficiency, relatively large output power capability and a tiny 16-bump μSMD package, the device is subject to high operating temperatures. Figure 2. National Semiconductor's LM3554 flash LED driver test circuit.
Figure 1. Simplified thermal impedance model.
In the circuit, each thermal interface has a thermal impedance. Thermal impedance varies with material, geometry, size, and orientation. The thermal impedance of a system (or circuit) has a total thermal impedance to ambient temperature that can be broken down into a parallel and series combination of the thermal impedance of each component in the circuit. For example, in a semiconductor device, the total thermal impedance between the die (also called junction) and the surrounding air (called thermal impedance), ie the thermal impedance (ZJ-A) from junction to environment, will be the structure. The sum of the individual thermal impedances of each individual material.