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ADDC02815DA Fiches technique(PDF) 11 Page - Analog Devices |
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ADDC02815DA Fiches technique(HTML) 11 Page - Analog Devices |
11 / 20 page ADDC02812DA/ADDC02815DA REV. A –11– Input Voltage Transient Protection: The converters have a transient voltage suppressor connected across their input leads to protect the units against high voltage pulses (both positive and negative) of short duration. With the power supply con- nected in the typical system setup shown in Figure 17, a tran- sient voltage pulse is created across the converter in the following manner. A 20 µF capacitor is first charged to 400 V. 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. |
Numéro de pièce similaire - ADDC02815DA |
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Description similaire - ADDC02815DA |
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