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ARH2812SEM Datasheet(PDF) 8 Page - International Rectifier |
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ARH2812SEM Datasheet(HTML) 8 Page - International Rectifier |
8 / 12 page 8 www.irf.com ARH28XXS Series 28V Input, Single Output trade name of Sil-Pad® 4001. This particular product is an insulator but electrically conductive versions are also available. Use of these materials assures optimum surface contact with the heat dissipater by compensating for minor surface variations. While other available types of heat conducting materials and thermal compounds provide similar effectiveness, these alternatives are often less convenient and are sometimes messy to use. A conservative aid to estimating the total heat sink surface area (AHEAT SINK) required to set the maximum case temperature rise (DT) above ambient temperature is given by the following expression: ∆T PP Eff OUT = == − ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ Case temperature rise above ambient Device dissipation in Watts 1 1 where A HEATSINK ≈ ⎧⎨ ⎩ ⎫ ⎬ ⎭ − − ∆T P 80 594 085 143 . . . As an example, assume that it is desired to maintain the case temperature of an ARH2815S at +65°C or less while operating in an open area whose ambient temperature does not exceed +35°C; then ∆T = 65 - 35 = 35°C. From the Specification Table, the worst case full load efficiency for this device is 80%; therefore the maximum power dissipation at full load is given by () P =• − ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ =• = 30 1 80 130 0 25 75 . .. W and the required heat sink area is A = 35 80 7.5 in HEAT SINK 0.85 • ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ −= −143 2 594 318 . .. Thus, a total heat sink surface area (including fins, if any) of approximately 32 in2 in this example, would limit case rise to 35°C above ambient. A flat aluminum plate, 0.25" thick and of approximate dimension 4" by 4" (16 in2 per side) would suffice for this application in a still air environment. Note that to meet the criteria, both sides of the plate require unrestricted exposure to the ambient air. The user should remember that when operating in the vacuum of space, this method does not apply and all heat must be conducted away from the attaching surface. 1Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN As an alternative to application and removal of the DC voltage at the input, the user can control the converter output by providing an input referenced, TTL compatible, logic signal to the enable pin 3. This port is internally pulled “high” so that when not used, an open connection on the pin permits normal converter operation. When inhibited outputs are desired, a logical “low” on this port will shut the converter down. An open collector device capable of sinking at least 100 µA connected to enable pin 3 will work well in this application. Inhibiting Converter Output A benefit of utilization of the enable input is that following an initial charge of the input capacitor, subsequent turn-on commands will induce no uncontrolled current inrush. Figure II. Enable Input Equivalent Circuit Synchronization When using multiple converters, system requirements may dictate operating several converters at a common system frequency. To accommodate this requirement, the ARH28XXS type converter provides a synchronization input port. The sync input port permits synchronization of an ARH converter to any compatible external frequency source operating in the band of 225 to 310 KHz. In the event of failure of an external synchronization source, the converter will revert to its own internally set frequency. When external synchronization is not desired, the sync in port may be left open (unconnected) permitting the converter to operate at its own internally set frequency. |
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