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ADDC02812DA датащи(PDF) 11 Page - Analog Devices |
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ADDC02812DA датащи(HTML) 11 Page - Analog Devices |
11 / 20 page OBSOLETE ADDC02812DA/ADDC02815DA REV. 0 –11– It is then connected directly across the converter’s end of the two meter power lead cable through a 2 Ω on-state resistance MOSFET. The duration of this connection is 10 µs. The pulse is repeated every second for 30 minutes. This test is repeated with the connection of the 20 µF capacitor reversed to create a negative pulse on the supply leads. (If continuous reverse volt- age protection is required, a diode can be added externally in series at the expense of lower efficiency for the power system.) The converter responds to this input transient voltage test by shutting down due to its input overvoltage protection feature. Once the pulse is over, the converter initiates a soft-start, which is completed before the next pulse. No degradation of converter performance occurs. THERMAL CHARACTERISTICS Junction and Case Temperatures: It is important for the user to know how hot the hottest semiconductor junctions within the converter get and to understand the relationship between junction, case, and ambient temperatures. The hottest semiconductors in the 100 W product line of Analog Devices’ high density power supplies are the switching MOSFETs and the output rectifiers. There is an area inside the main power transformers that is hotter than these semiconductors, but it is within NAVMAT guidelines and well below the Curie tempera- ture of the ferrite. (The Curie temperature is the point at which the ferrite begins to lose its magnetic properties.) Since NAVMAT guidelines require that the maximum junction temperature be 110 °C, the power supply manufacturer must specify the temperature rise above the case for the hottest semi- conductors so the user can determine what case temperature is required to meet NAVMAT guidelines. The thermal charac- teristics section of the specification table states the hottest junc- tion temperature for maximum output power at a specified case temperature. The unit can operate to higher case temperatures than 90 °C, but 90°C is the maximum temperature that permits NAVMAT guidelines to be met. Case and Ambient Temperatures: It is the user’s responsi- bility to properly heat sink the power supply in order to maintain the appropriate case temperature and, in turn, the maximum junction temperature. Maintaining the appropriate case tem- perature is a function of the ambient temperature and the mechanical heat removal system. The static relationship of these variables is established by the following formula: TC = TA + (PD × Rθ CA ) where TC = case temperature measured at the center of the package bottom, TA = ambient temperature of the air available for cooling, PD = the power, in watts, dissipated in the power supply, Rθ CA = the thermal resistance from the center of the package to free air, or case to ambient. The power dissipated in the power supply, PD, can be calculated from the efficiency, h, given in the data sheets and the actual output power, PO, in the user’s application by the following formula: P D = PO 1 η –1 For example, at 80 W of output power and 80% efficiency, the power dissipated in the power supply is 20 W. If under these conditions, the user wants to maintain NAVMAT deratings (i.e., a case temperature of approximately 90 °C) with an ambi- ent temperature of 75 °C, the required thermal resistance, case to ambient, can be calculated as 90 = 75 + (20 × Rθ CA ) or Rθ CA = 0.75 °C/W This thermal resistance, case to ambient, will determine what kind of heat sink and whether convection cooling or forced air cooling is required to meet the constraints of the system. SYSTEM INSTABILITY CONSIDERATIONS In a distributed power supply architecture, a power source provides power to many “point-of-load” (POL) converters. At low frequencies, the POL converters appear incrementally as negative resistance loads. This negative resistance could cause system instability problems. Incremental Negative Resistance: A POL converter is designed to hold its output voltage constant no matter how its input volt- age varies. Given a constant load current, the power drawn from the input bus is therefore also a constant. If the input voltage increases by some factor, the input current must decrease by the same factor to keep the power level constant. In incremental terms, a positive incremental change in the input voltage results in a negative incremental change in the input current. The POL converter therefore looks, incrementally, as a negative resistor. The value of this negative resistor at a particular operating point, VIN, IIN, is: R N = –V IN I IN Note that this resistance is a function of the operating point. At full load and low input line, the resistance is its smallest, while at light load and high input line, it is its largest. Potential System Instability: The preceding analysis assumes dc voltages and currents. For ac waveforms the incremental input model for the POL converter must also include the effects of its input filter and control loop dynamics. When the POL con- verter is connected to a power source, modeled as a voltage source, VS, in series with an inductor, LS, and some positive resistor, RS, the network of Figure 25 results. LP CP –|RN| ADI DC/DC CONVERTER LS RS VS INPUT TERMINALS Figure 25. Model of Power Source and POL Converter Connection The network shown in Figure 25 is second order and has the following characteristic equation: s2(LS + LP )C + s (LS + LP ) –|RN| + R SCP +1 = 0 |
Аналогичный номер детали - ADDC02812DA_15 |
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Аналогичное описание - ADDC02812DA_15 |
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