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ADDC02805SATV Scheda tecnica(PDF) 11 Page - Analog Devices |
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ADDC02805SATV Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 18 page ADDC02803SC/ADDC02805SA REV. A –11– before it falls below 50 V. In both cases, the ADDC02803SC/ ADDC02805SA can be modified to operate to specification up to the 50 V input voltage limit and to shut down and protect itself during the time the input voltage exceeds 50 V. When the input voltage falls below 50 V as the surge ends, the converter will automatically initiate a soft start. In order to survive these higher input voltage surges, the modified converter will no longer have input transient protection, however, as described below. Contact the factory for information on units surviving high input voltage surges. Input Voltage Transient Protection: The converter has a transient voltage suppressor connected across its input leads to protect the unit against high voltage pulses (both positive and negative) of short duration. With the power supply connected in the typical system setup shown in Figure 23, a transient voltage pulse is created across the converter in the following manner. A 20 µF capacitor is first charged to 400 V. It is then directly connected 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 voltage 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 the case temperature 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 case temperatures higher 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 me- chanical heat removal system. The static relationship of these variables is established by the following formula: T C = T A + ( P D × R θ CA ) where: TC = case temperature measured at the center of the pack- age 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 calcu- lated from the efficiency, , given in the data sheets, and the actual output power, PO, in the user’s application by the fol- lowing 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 de- signed to hold its output voltage constant no matter how its input voltage 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, incremen- tally, like 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. |
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