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1. **Introduction**
High power density represents one of the major development trends in switching power supplies. By minimizing internal heat generation and reducing thermal resistance, efficiency can be enhanced. Choosing an appropriate cooling method is fundamental to thermal design in switching power supplies. Apart from electrical stress, temperature is the most critical factor impacting the reliability of switching power supplies. An elevated temperature within the power supply can lead to component failures. Once the temperature surpasses a certain threshold, the failure rate increases exponentially. This can result in catastrophic component failure. The relationship between temperature and failure rate is directly proportional and can be mathematically represented as:
\[ F = Ae^{-E/KT} \]
Where \( F \) represents the failure rate, \( A \) is a constant, \( E \) is the energy, \( K \) is the Boltzmann constant (\( 8.63 \times 10^{-5} eV/K \)), and \( T \) is the junction temperature.
To address this issue, we can adopt two approaches:
1) Minimize losses from the circuit structure, such as implementing superior control methodologies like high-frequency soft-switching, phase-shifting control, and synchronous rectification technologies. Reducing the number of heat-generating devices and improving the width of printed traces can boost power supply efficiency.
2) Employ more efficient heat dissipation techniques such as conduction, radiation, and convection methods. These include radiators, air cooling (both natural and forced), liquid cooling (water or oil), thermoelectric cooling, heat pipes, etc. Forced air cooling is the primary heat dissipation method in large power switching power supplies, making advancements in forced air cooling technology a focal point of research. Proper duct design and introducing turbulence at the radiator's entrance can significantly enhance heat dissipation.
While optimizing the design to reduce heat generation, it’s essential to use heat transfer principles like conduction, convection, and radiation via heat sinks to swiftly release heat generated by the device into the surrounding environment. This reduces internal heat accumulation and lowers component operating temperatures.
2. **Switching Power Supply Thermal Analysis Software**
Currently, researchers in the switching power supply field use FloTherm or Icepak, electronic system thermal simulation analysis software, for modeling and analysis. However, the industry remains in the early stages of traditional manual heat analysis. Using software for thermal design is a relatively recent trend in China. While thermal simulation isn't infallible, the more accurate the data and models, the more closely the results reflect reality, providing primarily guidance. The power industry demands miniaturization, and designing a heatsink based solely on experience is insufficient for modern development needs.
FloTherm is a suite of electronic system thermal simulation analysis software developed by FLOMERICS Software, a pioneer in electronic system cooling simulation software. It is widely used by electronic circuit design engineers and electronic system structural design engineers worldwide, with an occupancy rate exceeding 80%. Its most distinctive feature is offering thermal design component models for electronic devices. Based on these models, engineers can quickly construct electronic devices such as enclosures, subracks, single boards, chip fans, heatsinks, and others.
Developed using proven Computational Fluid Dynamics (CFD) and numerical heat transfer simulation technology, FloTherm combines FLOMERICS' extensive experience and database of heat transfer in electronic equipment with specialized models tailored for the electronics industry. FloTherm can analyze the state of heat dissipation, temperature, and internal fluids from various layers of the electronic system application, including the environmental layer, electronic system layer, board, and component layers, down to the internal structural layer of the chip. It offers highly efficient, precise, and straightforward quantitative analysis. Utilizing advanced finite volume methods, it processes the structure to simulate thermal radiation, heat conduction, heat convection, fluid temperature, fluid pressure, fluid velocity, and motion in the three-dimensional structural model. Heat dissipation in all three states can be analyzed independently. For defense field cooling media (such as localized liquid cooling), outdoor equipment with solar radiation, and high precision of local occlusion between devices, FloTherm’s software solver is particularly adept at handling radiation heat calculations. FloTherm’s powerful pre- and post-processing modules can directly convert geometric models designed in mainstream MCAD and EDA software, reducing the time required to build models. Additionally, they allow intuitive and convenient visualization of data in temperature field planes, equipotential animations, or reports.
The FloTherm software can be divided into three parts: preprocessing, solving, and post-processing. Preprocessing includes the project manager, drawing board, and Flogate. The project manager handles project management, physical parameters, grid parameters, and calculation parameter settings. The drawing board provides a visual tool for constructing cabinet, subrack, veneer, chip geometry models, and computational meshing. Specific building operations can be performed interactively through the project manager and drawing board. Flogate is a data interface module that can import the assembly drawing file (in IDF format) into FloTherm and directly complete the design of the single board. The solver is the Flosolve module, which completes the transient and steady-state temperature and flow field calculations of the model. Post-processing includes Visualization, Flomotion, and Table. Visualization completes the visualization of simulation results. Besides visual displays, the Floative can also be used to create dynamic displays of the flow field. The thermal analysis model can calculate large amounts of data such as the average temperature in a certain area, airflow, etc., which can be queried through the table module.
Icepak is a specialized thermal design software developed by Fluent Corporation through the integration of ICEMCFD’s meshing and post-processing technology for electronic equipment cooling analysis. It has the following advantages:
1) Modeling capability: In addition to rectangular and circular constructions, there are many complex shape models such as ellipsoids, polyhedrons, pipes, and slopes. The model has a thin-conduction model;
2) Grid technology: Structured, unstructured mesh; tetrahedral, hybrid tetrahedral-hexahedral mesh; capable of quickly generating high-quality meshes for complex models; supports structured and unstructured non-conforming grids;
3) Solver: The FLUENT solver can solve a variety of fluid medium problems; can solve structured and unstructured mesh problems, supports grid parallelism.
3. **Basic Principles of Thermal Design**
The basic procedure for switching power supply thermal design is:
1) First, specify the design conditions, such as power consumption, heat generation, allowable temperature rise, equipment dimensions, and environmental conditions for equipment placement;
2) Determine the cooling method of the power supply and check if the temperature conditions are met;
3) Thermal design of components, circuits, printed circuit boards, and chassis;
4) Check according to the thermal design checklist to determine if the design requirements are met.
4. **Thermal Design of Printed Circuit Board Versions**
From the perspective of facilitating heat dissipation, the printed board is preferably installed upright. The distance between boards should not be less than 2 cm, and the arrangement of devices on the printed board should follow certain rules:
1) For devices using free convection air cooling, it is best to arrange integrated circuits (or other devices) in a vertically elongated manner; for devices that use forced air cooling, it is best to arrange integrated circuits (or other devices) in a longitudinally elongated manner.
2) Devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the uppermost flow (at the inlet) of the cooling airflow, while devices with high heat or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the most downstream of the cooling airflow.
3) In the horizontal direction, high-power devices should be placed as close as possible to the edges of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices should be placed as close as possible to the top of the printed board to reduce the impact on the temperature of other devices while these devices operate.
4) Temperature-sensitive devices should be placed in the lowest temperature area (such as the bottom of the device). They should not be placed directly above heating devices. Multiple devices are preferably staggered on a horizontal plane.
5) The heat dissipation of the printed circuit board in the power supply mainly depends on the airflow, so the airflow path should be studied during the design, and the device or printed circuit board should be properly arranged. Airflow always tends to flow where resistance is low, so when configuring devices on the board, avoid leaving large airspaces in certain areas. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.
5. **How to Design the Thermal Design of Electronic Chips?**
First, we can use the chip datasheet provided by the chip manufacturer as the basis for judgment. The following will explain several important parameters related to heat dissipation in the datasheet.
\( P \) - chip power consumption, unit W (watt). Power consumption is the direct cause of heat generation. Chips with high power consumption must also generate a large amount of heat.
\( T_c \) - chip housing temperature in °C.
\( T_j \) - node temperature, in °C. As the junction temperature increases, the performance of the semiconductor device decreases. If the junction temperature is too high, the chip becomes unstable, the system freezes, and the chip eventually burns out.
\( T_a \) - ambient temperature, in °C.
\( T_{stg} \) - storage temperature in °C. The storage temperature of the chip.
\( R_{ja} \) - junction-to-ambient thermal resistance in °C/W.
\( R_{jc} \) - thermal resistance of the junction to the chip case, in °C/W.
\( \Psi_{jt} \) - can be understood as the thermal resistance of the junction to the upper surface of the chip. When the heat of the chip is only partially transmitted through the upper case, the thermal resistance parameter.
\( LFM \) - wind speed unit, feet/minute.
Since the IC package makes the measurement of the junction temperature inaccessible, it is difficult to directly measure the junction temperature of the IC. As an alternative, the junction-to-case thermal resistance (JC) and the thermal resistance (CA) of the enclosure to the external environment can be used to calculate the junction temperature, as shown in Figure 1. Thermal resistance is the most important parameter in determining the junction temperature of the IC: \( J_A = J_C + C_A \).
Figure 1. Thermal state electrical model for calculating IC junction temperature using thermal resistance. With the increasing importance of thermal design, most chip data now provides parameters such as \( T_j \), \( R_{jc} \), and \( P \). The basic formula is as follows:
\[ T_j = T_c + R_{jc} \times P \]
As long as \( T_j < T_{j\text{max}} \) is ensured, the chip can work normally.
After all, as long as we can guarantee that the junction temperature of the chip does not exceed the maximum value given by the chip, the chip can work normally.
How to determine if the chip needs to increase heat dissipation measures:
1) Collecting the heat dissipation parameters of the chip. Mainly: \( P \), \( R_{ja} \), \( R_{jc} \), \( T_j \), etc.
2) Calculate \( T_{c\text{max}} \): \( T_{c\text{max}} = T_j - R_{jc} \times P \)
3) Calculate the \( R_{ca} \) required to achieve the target: \( R_{ca} = (T_{c\text{max}} - T_a) / P \)
4) Calculate the \( R_{ca}' \) of the chip itself: \( R_{ca}' = R_{ja} - R_{jc} \)
If \( R_{ca} \) is greater than \( R_{ca}' \), there is no need to add additional cooling measures.
If \( R_{ca} \) is less than \( R_{ca}' \), additional heat dissipation measures are needed, such as adding a heatsink, adding a fan, etc.
As mentioned earlier, \( R_{ja} \) cannot be used to accurately calculate the temperature of the chip, so this method can only be used for simple judgments, but not for the final basis.
Such as UC3842A, UC3843A thermal characteristics:
6. **PCB Surface Mount Power Device Thermal Design**
Using Micrel's surface-mount linear regulator as an example, how to work properly when using only one printed circuit board copper plate as a heat sink.
1) System Requirements:
\( V_{OUT} = 5.0V \); \( V_{IN(\text{MAX})} = 9.0V \); \( V_{IN(\text{MIN})} = 5.6V \); \( I_{OUT} = 700mA \); operation cycle = 100%; \( T_A = 50°C \)
According to the above system requirements, select the 750mA MIC2937A-5.0BU regulator with the following parameters:
\( V_{OUT} = 5V \pm 2\% \) (worst case when overheating)
\( T_{J \text{MAX}} = 125°C \). In TO-263 package, \( \theta_{JC} = 3°C/W \);
\( \theta_{CS} \approx 0°C/W \) (direct soldering on the board).
2) Preliminary Calculation:
\( V_{OUT(\text{MIN})} = 5V - 5 \times 2\% = 4.9V \)
\( P_D = (V_{IN(\text{MAX})} - V_{OUT(\text{MIN})}) + I_{OUT} + (V_{IN(\text{MAX})} \times I) = [9V - 4.9V] \times 700mA + (9V \times 15mA) = 3W \)
The maximum temperature rise, \( \Delta T = T_{J(\text{MAX})} - T_A = 125°C - 50°C = 75°C \); thermal resistance \( \theta_{JA} \) (worst case): \( \Delta T / P_D = 75°C / 3.0W = 25°C/W \).
The thermal resistance of the heatsink, \( \theta_{SA} = \theta_{JA} - (\theta_{JC} + \theta_{CS}); \theta_{SA} = 25 - (3 + 0) = 22°C/W \) (maximum).
3) Determine the Physical Size of the Heatsink:
The use of a square, single-sided, horizontal solder resist layer of copper foil and a heatsinking copper foil covered with black oily paint, compared with the air cooling scheme of 1.3 m/s, the latter has the best heat dissipation effect.
With a solid-line solution, a conservative design requires 5,000mm² of heatsinking copper foil, which is a square of 71mm x 71mm (2.8 inches long on each side).
4) Thermal Requirements in SO-8 and SOT-223 Packages:
Calculate the heatsink area under the following conditions: \( V_{OUT} = 5.0V \); \( V_{IN(\text{MAX})} = 14V \); \( V_{IN(\text{MIN})} = 5.6V \); \( I_{OUT} = 150mA \); duty cycle = 100%; \( T_A = 50°C \). Under the allowable conditions, it is easier to handle devices in a dual-row SO-8 package. With the MIC2951-03BM (SO-8 package), the following parameters can be obtained:
\( T_{J \text{MAX}} = 125°C \); \( \theta_{JC} \approx 100°C/W \).
5) Calculate the Parameters in the SO-8 Package:
\( P_D = [14V - 5V] \times 150mA + (14V \times 8mA) = 1.46W \);
Elevated temperature = 125°C - 50°C = 75°C;
Thermal resistance \( \theta_{JA} \) (worst case):
\( \Delta T / P_D = 75°C / 1.46W = 51.3°C/W \);
\( \theta_{SA} = 51 - 100 = -49°C/W \) (maximum).
Obviously, SO-8 can’t meet the design requirements without cooling. Consider the MIC5201-5.0BS regulator in SOT-223 package, which is smaller than SO-8, but its three pins have good cooling effect. Select MIC5201-3.3BS, the relevant parameters are as follows:
\( T_{J \text{MAX}} = 125°C \)
SOT-223 thermal resistance \( \theta_{JC} = 15°C/W \)
\( \theta_{CS} = 0°C/W \) (direct soldering on the board).
6) Calculate the Results of the SOT-223 Package:
\( P_D = [14V - 4.9V] \times 150mA + (14V \times 1.5mA) = 1.4W \)
Elevated temperature = 125°C - 50°C = 75°C;
Thermal resistance \( \theta_{JA} \) (worst case):
\( \Delta T / P_D = 75°C / 1.4W = 54°C/W \);
\( \theta_{SA} = 54 - 15 = 39°C/W \) (maximum).
According to the above data, 1,400mm² heatsinking copper foil (1.5-inch square) can meet the design requirements.
The above design results can be used as a rough reference. In actual design, it is necessary to understand the thermal characteristics of the board and obtain more accurate and satisfactory results.
The following table lists the thermal resistance ratings for common surface mounts. See the data sheet for details.
Table 8 Typical Surface Mount Thermal Resistance (Unit: °C/W)
| Package | \( R_{ja} \) | \( R_{jc} \) |
|---------|---------------|---------------|
| SOD123 | 340 | 150 |
| SOT23 | 55 | 675 |
| SOT223 | 15 | 97.5 |
| SO-8 | 63 | 21 |
| SMB | 13 | |
| SMC | 11 | |
| DPAK | 80 | 6 |
| D2PAK | 50 | 2 |
7. **Analysis of Forced Air Cooling Method**
Under normal conditions, heat transfer includes three modes: conduction, convection, and radiation. Conduction refers to the transfer of heat between directly contacted objects from the lower temperature side of the higher temperature. Convection is the transfer of heat by the flow of fluid, and radiation does not need any medium; it is the heating body directly releasing heat to the surrounding space.
Forced air cooling heat dissipation is more than ten times larger than natural cooling, but the addition of fans, fan power supplies, interlocking devices, etc., not only increases the cost and complexity of the equipment but also reduces the reliability of the system, and also increases noise and vibration. Therefore, in general, natural cooling should be used instead of air cooling, liquid cooling, etc. High-frequency transformers and inductors should use thicker wires to suppress temperature rise. Try to ensure that the magnet loss and the copper loss of the coil are the same, which can minimize the overall power consumption of the high-frequency transformer and reduce heat generation.
In practical applications, heat dissipation measures are typically two ways: using a heatsink and a fan or both. The heatsink transmits the heat of the chip to the heatsink through close contact with the surface of the chip. The heatsink is usually a piece with many blades, a good conductor of heat, whose fully extended surface greatly increases heat radiation, while circulating air can also take away more heat. The use of a fan is also divided into two forms: one is directly installed on the surface of the heatsink, and the other is mounted on the chassis and frame to increase the air flow rate in the entire space. If temperature is equivalent to voltage and power is equivalent to current, the thermal model shown in Figure 1 is similar to Ohm’s law, \( V = IR \) (Ohm’s Law). One of the most basic formulas for heat dissipation calculation is:
Temperature difference = power consumption × thermal resistance
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