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ADDC02805S Folha de dados(PDF) 11 Page - Analog Devices |
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ADDC02805S Folha de dados(HTML) 11 Page - Analog Devices |
11 / 16 page OBSOLETE ADDC02805S REV. A –11– NAVMAT DERATING NAVMAT is a Navy power supply reliability manual that is frequently cited by specifiers of power supplies. A key section of NAVMAT P4855-1A discusses guidelines for derating designs and their components. The two key derating criteria are voltage derating and power derating. Voltage derating is done to reduce the possibility of electrical breakdown, whereas power derating is done to maintain the component material below a specified maximum temperature. While power deratings are typically stated in terms of current limits (e.g., derate to x% of maximum rating), NAVMAT also specifies a maximum junction tem- perature of the semiconductor devices in a power supply. The NAVMAT component deratings applicable to the ADDC02805S are as follows: Resistors 80% voltage derating 50% power derating Capacitors 50% voltage and ripple voltage derating 70% ripple current derating Transformers and Inductors 60% continuous voltage and current derating 90% surge voltage and current derating 20 °C less than rated core temperature 30 °C below insulation rating for hot spot temperature 25% insulation breakdown voltage derating 40 °C maximum temperature rise Transistors 50% power derating 60% forward current (continuous) derating 75% voltage and transient peak voltage derating 110 °C maximum junction temperature Diodes (Switching, General Purpose, Rectifiers) 70% current (surge and continuous) derating 65% peak inverse voltage derating 110 °C maximum junction temperature Diodes (Zeners) 70% surge current derating 60% continuous current derating 50% power derating 110 °C maximum junction temperature Microcircuits (Linears) 70% continuous current derating 75% signal voltage derating 110 °C maximum junction temperature The ADDC02805S can meet all the derating criteria listed above. However, there are a few areas of the NAVMAT deratings where meeting the guidelines unduly sacrifices performance of the circuit. Therefore, the standard unit makes the following exceptions. Common-Mode EMI Filter Capacitors: The standard supply uses 500 V capacitors to filter common-mode EMI. NAVMAT guidelines would require 1000 V capacitors to meet the 50% voltage derating (500 V dc input to output isolation), resulting in less common-mode capacitance for the same space. In typical electrical power supply systems, where the load ground is eventually connected to the source ground, common- mode voltages never get near the 500 V dc rating of the standard supply. Therefore, a lower voltage rating capacitor (500 V) was chosen to fit more capacitance in the same space in order to better meet the conducted emissions requirement of MIL-STD-461D (CE102). For those applications which require 250 V or less of isolation from input to output, the present designs would meet NAVMAT guidelines. Switching Transistors: 100 V MOSFETs are used in the standard unit to switch the primary side of the transformers. Their nominal off-state voltage meets the NAVMAT derating guidelines. When the MOSFETs are turned off, however, momentary spikes occur that reach 100 V. The present generation of MOSFETs are rated for repetitive avalanche, a condition that was not considered by the NAVMAT deratings. In the worst case condition, the energy dissipated during avalanche is 1% of the device’s rated repetitive avalanche energy. To meet the NAVMAT derating, 200 V MOSFETs could be used. The 100 V MOSFETs are used instead for their lower on-state resistance, resulting in higher efficiency for the power supply. NAVMAT Junction Temperatures: The two types of power deratings (current and temperature) can be independent of one another. For instance, a switching diode can meet its derating of 70% of its maximum current, but its junction temperature can be higher than 110 °C if the case temperature of the converter, which is not controlled by the manufacturer, is allowed to go higher. Since some users may choose to operate the power supply at a case temperature higher than 90 °C, it then becomes important to know the temperature rise of the hottest semiconductors. This is covered in the specification table in the section entitled “Thermal Characteristics”. EMI CONSIDERATIONS The ADDC02805S has an integral differential- and common- mode EMI filter that is designed to meet all applicable requirements in MIL-STD-461D when the power converter is installed in a typical system setup (described below). The converter also contains transient protection circuitry that permits the unit to survive short, high voltage transients across its input power leads. The purpose of this section is to describe the various MIL-STD-461D tests and the converter’s corresponding performance. Consult factory for additional information. Electromagnetic interference (EMI) is governed by MIL-STD- 461D, which establishes design requirements, and MIL-STD- 462D, which defines test methods. EMI requirements are categorized as follows (xxx designates a three digit number): • CExxx: conducted emissions (EMI produced internal to the power supply which is conducted externally through its input power leads) • CSxxx: conducted susceptibility (EMI produced external to the power supply which is conducted internally through the input power leads and may interfere with the supply’s operation) • RExxx: radiated emissions (EMI produced internal to the power supply which is radiated into the surrounding space) • RSxxx: radiated susceptibility (EMI produced external to the power supply which radiates into or through the power supply and may interfere with its proper operation) It should be noted that there are several areas of ambiguity with respect to CE102 measurements that may concern the systems |
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