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Hydraulic Power Unit (HPU) is a critical component in many industrial and engineering systems that convert mechanical energy into hydraulic energy. There are several types of HPUs, each designed to meet specific requirements and applications. Here's an overview of some common classification.
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1 **Introduction**
High power density represents one of the key development directions in switching power supplies. To minimize heat generation within the power supply and reduce thermal resistance, improving efficiency is essential. Choosing a suitable cooling method is fundamental to the thermal design of switching power supplies. Apart from electrical stress, temperature remains the most critical factor impacting the reliability of such systems. Excessive temperatures inside a switching power supply can lead to component failure. When temperatures surpass a certain threshold, the failure rate increases exponentially, exceeding limits that could result in component damage. The relationship between temperature and failure rate is directly proportional and can be mathematically expressed as:
\[ F = Ae^{-E/KT} \]
Where \( F \) denotes the failure rate, \( A \) is a constant, \( E \) represents energy, \( K \) is Boltzmann's constant (\( 8.63 \times 10^{-5} eV/K \)), and \( T \) is the junction temperature.
To address this issue, improvements can be made from two angles: first, by reducing losses through circuit design, such as adopting advanced control methods like high-frequency soft-switching technology, phase-shifting control, and synchronous rectification. Reducing the number of heat-generating components and enhancing printed line widths can help boost efficiency. Second, implementing more effective heat dissipation technologies is crucial. Techniques include conduction, radiation, and convection methods, such as radiators, air cooling (both natural and forced), liquid cooling (water or oil), thermoelectric cooling, heat pipes, and others. Forced air cooling dominates in large power switching power supplies, making advancements in this technology a focal point of research. Optimizing air duct design and introducing turbulence at the front of the radiator can significantly enhance heat dissipation performance.
While optimizing the design to reduce heat generation, it’s also necessary to apply principles of conduction, convection, and radiation via heat sinks to quickly release heat generated by devices into the surrounding environment. This reduces internal heat accumulation, lowering component operating temperatures.
2 **Thermal Analysis Software for Switching Power Supplies**
Currently, researchers in the field of switching power supplies utilize Flotherm or Icepak, which are electronic system thermal simulation analysis software tools. However, the industry remains largely reliant on traditional human analysis methods. Using software for thermal design is relatively new in China, where thermal simulation provides guidance rather than definitive solutions. More accurate data and models result in more realistic outcomes. The current trend in the power industry demands miniaturization, yet designing a heatsink based solely on experience falls short of meeting modern requirements.
Flotherm is a thermal simulation software suite developed by FLOMERICS Software, a leader in electronic system cooling simulation software. Widely adopted by electronic circuit design engineers and electronic system structural design engineers worldwide, its market share exceeds 80%. Its standout feature lies in providing thermal design component models for electronic devices. These models enable rapid creation of electronic device assemblies, including chassis, subracks, single boards, chip fans, heatsinks, and more.
Built on proven Computational Fluid Dynamics (CFD) and numerical heat transfer simulation technologies, Flotherm integrates FLOMERICS' extensive experience in heat transfer within electronic equipment with specialized models tailored for the electronics industry. Flotherm efficiently analyzes the state of heat dissipation, temperature, and internal fluids across different layers of an electronic system—from the environmental layer, electronic system layer, board, and components down to the internal structure of chips. It employs an advanced finite volume method to process structures, simulating three-dimensional thermal radiation, heat conduction, heat convection, fluid temperature, pressure, velocity, and motion. Heat dissipation in the three states can be analyzed independently. For scenarios involving complex cooling media (like localized liquid cooling), outdoor equipment exposed to solar radiation, and precise occlusions between devices, Flotherm's solver excels in handling radiation heat calculations. Its powerful pre- and post-processing modules allow seamless conversion of geometric models designed in mainstream MCAD and EDA software, reducing model-building time. They also provide intuitive and convenient visualizations of temperature field equipotential lines and fluid motion animations or reports.
Flotherm software can be divided into three parts: pre-processing, solving, and post-processing. Pre-processing includes the Project Manager, Drawing Board, and Flogate. The Project Manager handles project management, setting physical parameters, grid parameters, and calculation parameters. The Drawing Board offers a visual tool for constructing cabinets, subracks, panels, chip geometries, and computational meshing. Specific build operations can be performed interactively through the Project Manager and Drawing Board. Flogate is a data interface module that imports board assembly drawings (in IDF format) into Flotherm, enabling direct single-board design. The Solver is the Flosolve module, which completes transient and steady-state temperature and flow field calculations for the model. Post-processing includes Visualization, Flomotion, and Table. Visualization completes the visualization of simulation results. In addition to visual displays, the Flomotion tool creates dynamic flow field animations. The thermal analysis model can calculate large amounts of data, such as average temperatures in specific areas, airflow, etc., which can be queried through the Table module.
Icepak is a specialized thermal design software developed by Fluent Corporation in the U.S., integrating meshing and post-processing technologies for electronic equipment cooling analysis. It offers the following advantages:
1) Modeling capability: Beyond rectangular and circular constructions, it supports complex shapes like ellipsoids, polyhedra, tubes, and slopes, along with thin-conduction models.
2) Meshing technology: Structured and unstructured meshes; tetrahedral, tetrahedral-hexahedral hybrid meshes; capable of quickly generating high-quality meshes for complex models; supports structured and unstructured non-conforming grids.
3) Solver: FLUENT solver can handle various fluid media problems; solves structured and unstructured mesh problems, supports grid parallelism.
3 **Basic Principles of Thermal Design**
The basic procedure for thermal design in switching power supplies is as follows:
1) First, specify 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 verify whether temperature conditions are met;
3) Perform thermal design for components, circuits, printed circuit boards, and chassis;
4) Verify against a thermal design checklist to determine if design requirements are met.
4 **Thermal Design of Printed Circuit Boards**
From the perspective of facilitating heat dissipation, printed circuit boards should preferably be installed upright. The distance between boards should not be less than 2 cm. Components on the printed circuit board should be arranged according to 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) horizontally.
2) Devices on the same printed circuit board should be arranged as much as possible according to their heat generation and heat dissipation capabilities. 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 upstream (cooling airflow inlet). Devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the downstream end of the cooling airflow.
3) Horizontally, high-power devices should be placed as close as possible to the edges of the printed circuit board to shorten the heat transfer path; vertically, high-power devices should be placed as close as possible to the top of the printed circuit board to reduce the impact of their operating temperatures on other devices.
4) Temperature-sensitive devices should be placed in the lowest temperature area (such as the bottom of the device). Direct placement above heating devices should be avoided. Multiple devices should be staggered on a horizontal plane.
5) Heat dissipation in the power supply mainly depends on airflow, so airflow paths should be studied during design, and components or printed circuit boards should be properly arranged. Airflow tends to flow where resistance is low, so when configuring components on the board, avoid leaving large airspaces in certain areas. The arrangement of multiple printed circuit boards in the entire 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.
Chip power consumption, \( P \), unit W (watts). Power consumption is the direct cause of heat generation. Chips with high power consumption generate a large amount of heat.
\( T_c \)—chip housing temperature in \( ^\circ C \).
\( T_j \)—junction temperature, in \( ^\circ C \). 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 \( ^\circ C \).
\( T_{stg} \)—storage temperature in \( ^\circ C \). The storage temperature of the chip.
\( R_{ja} \)—junction-to-ambient thermal resistance in \( ^\circ C/W \).
\( R_{jc} \)—thermal resistance of the junction to the chip case, in \( ^\circ C/W \).
\( \Psi_{jt} \)-- 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 inaccessible to the junction, 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: \( JA = JC + CA \).
Figure 1. Thermal state electrical model for calculating IC junction temperature using thermal resistance. With the increasing importance of thermal design, most chip data will provide 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 \( T_j < T_{jmax} \) is guaranteed, the chip can work normally.
After all, as long as we can ensure 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) Collect the heat dissipation parameters of the chip. Mainly: \( P \), \( R_{ja} \), \( R_{jc} \), \( T_j \), etc.
2) Calculate \( T_{cmax} \): \( T_{cmax} = T_j - R_{jc} \times P \)
3) Calculate the \( R_{ca} \) required to achieve the target: \( R_{ca} = (T_{cmax} - T_a) / P \)
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, and so on.
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.
For example, UC3842A, UC3843A thermal characteristics:
6 **PCB Surface-Mount Power Device Thermal Design**
Using Micrel's surface-mount linear regulator as an example, how to properly use only one printed circuit board copper plate as a heatsink.
1) System Requirements:
\( V_{OUT}=5.0V \); \( V_{IN(MAX)}=9.0V \); \( V_{IN(MIN)}=5.6V \); \( I_{OUT}=700mA \); operating cycle=100%; \( T_A=50^\circ C \)
Based on the above system requirements, select the 750mA MIC2937A-5.0BU regulator with the following parameters:
\( V_{OUT}=5V\pm2\% \) (worst case when overheating)
\( T_{JMAX}=125^\circ C \). In TO-263 package, \( \theta_{JC}=3^\circ C/W \);
\( \theta_{CS}\approx0^\circ C/W \) (direct soldering on the board).
2) Preliminary Calculation:
\( V_{OUT(MIN)}=5V-5\times2\%=4.9V \)
\( P_D=(V_{IN(MAX)}-V_{OUT(MIN)})+I_{OUT}+(V_{IN(MAX)}\times I)=[9V-4.9V]\times700mA+(9V\times15mA)=3W \)
The maximum temperature rise, \( \Delta T=T_{JMAX}-T_A = 125^\circ C-50^\circ C=75^\circ C \); thermal resistance \( \theta_{JA} \) (worst case): \( \Delta T/P_D=75^\circ C/3.0W=25^\circ C/W \).
The heatsink thermal resistance, \( \theta_{SA}=\theta_{JA}-(\theta_{JC}+\theta_{CS}); \theta_{SA}=25-(3+0)=22^\circ C/W \) (maximum).
3) Determine the Physical Size of the Heatsink:
Using 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 on each side).
4) Thermal Requirements in SO-8 and SOT-223 Packages:
Calculate the heatsink area under the following conditions: \( V_{OUT}=5.0V \); \( V_{IN(MAX)}=14V \); \( V_{IN(MIN)}=5.6V \); \( I_{OUT}=150mA \); duty cycle=100%; \( T_A=50^\circ C \). Under the allowable conditions in circuit board production equipment, 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:
\( T_{JMAX}=125^\circ C \); \( \theta_{JC}\approx100^\circ C/W \).
5) Calculate the Parameters in the SO-8 Package:
\( P_D=[14V-5V]\times150mA+(14V\times8mA)=1.46W \);
Elevated temperature = \( 125^\circ C - 50^\circ C = 75^\circ C \);
Thermal resistance \( \theta_{JA} \) (worst case):
\( \Delta T/P_D=75^\circ C/1.46W=51.3^\circ C/W \);
\( \theta_{SA}=51-100=-49^\circ C/W \) (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:
\( T_{JMAX}=125^\circ C \)
SOT-223 thermal resistance \( \theta_{JC}=15^\circ C/W \)
\( \theta_{CS}=0^\circ C/W \) (direct soldering on the board).
6) Calculate the Results of the SOT-223 Package:
\( P_D=[14V-4.9V]\times150mA+(14V\times1.5mA)=1.4W \)
Elevated temperature = \( 125^\circ C - 50^\circ C = 75^\circ C \);
Thermal resistance \( \theta_{JA} \) (worst case):
\( \Delta T/P_D=75^\circ C/1.4W=54^\circ C/W \);
\( \theta_{SA}=54-15=39^\circ C/W \) (maximum). According to the above data, 1,400 mm² heatsinking copper foil (1.5-inch square) 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 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: \( ^\circ C/W \))
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 to the higher temperature side. Convection is the transfer of heat by the flow of fluid, and radiation does not require any medium; it is the heating body directly releasing heat to the surrounding space.
Forced air cooling dissipates heat more than ten times more effectively than natural cooling, but fans, fan power supplies, interlock devices, etc., are added, which not only increases the cost and complexity of the equipment but also reduces the reliability of the system, and also increases noise and vibration, so 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, the heat dissipation measures are two ways of using a heatsink and a fan or both. The heatsink transfers the heat of the chip to the heatsink through close contact with the surface of the chip, and the heatsink is usually a piece with many blades. The hot good conductor, its fully extended surface greatly increases the heat radiation, while the circulating air can also take away more heat. The use of the fan is also divided into two forms, one is directly installed on the surface of the radiator, the other is mounted on the chassis and the frame to increase the air flow rate in the entire space. If the temperature is equivalent to voltage and the power is equivalent to current, the thermal model shown in Figure 1 is similar to Ohm's law, \( V = I \times R \) (Ohm's Law), the calculation of heat dissipation has one of the most basic formulas:
Temperature difference = power consumption × thermal resistance
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