Radiator selection and heat calculation
At present, electronic products mainly use chip-type packaged devices, but many high-power devices and some power modules still use a lot of through-hole packages, which can be easily installed on the heat sink for heat dissipation. The purpose of conducting heat dissipation calculations for high-power devices and power modules is to select a suitable heat sink under certain heat dissipation conditions to ensure that the device or module operates safely and reliably. Thermal calculation Any device has a certain loss during operation, and most of the loss becomes heat. Low-power devices have low losses and do not require heat sinks. High-power devices have large losses. If no heat dissipation measures are taken, the temperature of the die can reach or exceed the allowable junction temperature, and the device will be damaged. Therefore, a heat dissipation device must be added. The most commonly used is to install the power device on the radiator, use the radiator to dissipate the heat to the surrounding space, and if necessary, add a cooling fan to strengthen the cooling and heat dissipation at a certain wind speed. In some large-scale equipment power devices also use flowing cold water cooling plate, it has better heat dissipation effect. The heat dissipation calculation is to determine the appropriate heat dissipation measures and heat sink through calculation under certain working conditions. The power device is installed on the heat sink. Its main heat flow direction is from the die to the bottom of the device, and the heat is dissipated to the surrounding space through the radiator. If there is no fan cooling at a certain wind speed, this is called natural cooling or natural convection cooling. The heat has a certain thermal resistance during the transfer process. The thermal resistance from the device die to the bottom of the device is R JC, the thermal resistance between the bottom of the device and the heat sink is R CS, the thermal resistance of the heat sink to dissipate heat to the surrounding space is R SA, and the total thermal resistance R JA = "R" JC + R CS + R SA. If the maximum power loss of the device is PD, and it is known that the allowed junction temperature of the device is TJ and the ambient temperature is TA, the allowable total thermal resistance R JA can be obtained as follows. R JA≤ (TJ-TA) / PD Then calculate the maximum allowable thermal resistance R SA of the radiator to ambient temperature as R SA≤ ({T_ {J} -T_ {A}} \ over {P_ {D}})-(R JC + R CS) In consideration of leaving room for design, the TJ is generally set to 125 ° C. The ambient temperature should also consider the worse situation, generally set TA = 40 ℃ 60 ℃. The size of the R JC is related to the size and packaging structure of the die, and can generally be found in the data sheet of the device. The size of R CS is related to mounting technology and device packaging. If the device adopts thermal grease or thermal pad, and then install it with the heat sink, the typical R CS value is 0.1 0.2 ℃ / W; if the bottom surface of the device is not insulated, and additional mica sheet insulation is needed, the R CS can reach 1 ℃ / W. PD is the actual maximum power loss, which can be calculated according to the operating conditions of different devices. In this way, R SA can be calculated, and a suitable radiator can be selected according to the calculated R SA value. Introduction to radiator The small radiator (or heat sink) is made of aluminum alloy sheet material through stamping process and surface treatment, while the large radiator is extruded from aluminum alloy to form a profile, which is then made by mechanical processing and surface treatment. They have various shapes and sizes for different devices to install and devices with different power consumption. The radiator is generally a standard part, and profiles can also be provided. The user cuts into a certain length according to the requirements to make a non-standard radiator. The surface treatment of the radiator has electrophoretic painting or black oxygen polarization treatment, and its purpose is to improve the heat dissipation efficiency and insulation performance. It can be increased by 10 15% under natural cooling and 3% under ventilated cooling, and the electrophoretic coating can withstand 500 800V. Radiator manufacturers give thermal resistance values ​​or related curves for different types of radiators, and give different thermal resistance values ​​under different heat dissipation conditions. Calculation example A power operational amplifier PA02 (product of APEX) is used as a low-frequency power amplifier, and its circuit is shown in Figure 1. The device is packaged in an 8-lead TO-3 metal case. The working conditions of the device are as follows: the operating voltage VS is 18V; the load impedance RL is 4, the operating frequency can be up to 5kHz under DC conditions, the ambient temperature is set to 40 ° C, and natural cooling is used. According to the PA02 device data, the typical value of the quiescent current IQ is 27mA and the maximum value is 40mA; the typical value of the RJC of the device (from the die to the case) is 2.4 ℃ / W, and the maximum value is 2.6 ℃ / W. The power consumption of the device is PD: PD = PDQ + PDOUT Where PDQ is the power consumption of the internal circuit of the device, PDOUT is the power consumption of the output power. PDQ = IQ (VS + | -VS |), PDOUT = V ^ {2} _ {S} / 4RL, substituted into the above formula PD = IQ (VS + | -VS |) + V ^ {2} _ {S} / 4RL = 37mA (36V) + 18V2 / 4 4 = 21.6W The static current in the formula is 37mA. Radiator thermal resistance R SA calculation: R SA≤ ({T_ {J} -T_ {A}} \ over {P_ {D}})-(R_ {JC} + R_ {CS}}) To leave a margin, TJ is set to 125 ° C, TA is set to 40 ° C, R JC takes the maximum value (R JC = "2" .6 ° C / W), R CS takes 0.2 ° C / W, (PA02 is directly installed in the heat dissipation On the device, there is thermal grease in the middle). Substituting the above data into the formula R SA≤ {125 ℃ -40 ℃} \ over {21.6W}-(2.6 ℃ / W + 0.2 ℃ / W) ≤1.135 ℃ / W The thermal resistance of HSO4 during natural convection is 0.95 ℃ / W, which can meet the heat dissipation requirements. Precautions 1. The maximum power consumption value in the device data can not be taken in the calculation, but should be calculated according to the actual conditions; the maximum junction temperature in the data is generally 150 ℃, leaving 125 ℃ in the design, the ambient temperature is also Can not take 25 ℃ (to consider the summer and the actual temperature of the chassis). 2. The radiator should be installed in a direction that is conducive to heat dissipation, and heat dissipation holes should be opened at corresponding positions on the chassis or cabinet (so that cold air enters from the bottom and hot air escapes from the top). 3. If the shell of the device is an electrode, the mounting surface is not insulated (not insulated from the internal circuit). Mica gasket must be used for insulation during installation to prevent short circuit. 4. The pins of the device should pass through the heat sink, and holes should be drilled on the heat sink. In order to prevent the pins from colliding with the hole wall, a PTFE sleeve should be put on. 5. In addition, different types of radiators have different thermal resistances under different heat dissipation conditions, which can be modified during design. That is, in practical applications, the thermal resistance of these radiators can be calculated with reference to similar structural shapes ( (Cross-sectional area, perimeter) heat sink composed of profiles instead. 6. In the above calculation, some parameters are set, which may be different from the actual value, and the size of the substitute model is not completely the same, so in mass production, a simulation test should be made to confirm whether the radiator is suitable for selection, and if necessary Some modifications (such as the length of the profile or changing the type of the profile, etc.) can only be used for mass production.
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