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MIC2590B-2YTQ Datasheet(PDF) 20 Page - Micrel Semiconductor |
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MIC2590B-2YTQ Datasheet(HTML) 20 Page - Micrel Semiconductor |
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Micrel, Inc. MIC2590B September 2008 20 M9999-091808 the MOSFET drain. 2. Since the rating for the part is given as “for 10seconds,” derate the maximum junction temperature by 35°C. This is the standard good practice derating of 25°C, plus another 10°C to allow for the time element of the specification. 3. Airflow, if available, works wonders. This is not the place for a dissertation on how to perform airflow calculations, but even a few LFM (linear feet per minute) of air will cool a MOSFET down dramatically. If you can position the MOSFET(s) in question near the inlet of a power supply’s fan, or the outlet of a processor’s cooling fan, that’s always a good free ride. 4. Although it seems a rather unsatisfactory statement, the best test of a surface-mount MOSFET for an application (assuming the above tips show it to be a likely fit) is an empirical one. The ideal evaluation is in the actual layout of the expected final circuit, at full operating current. The use of a thermocouple on the drain leads, or in infrared pyrometer on the package, will then give a reasonable idea of the device’s junction temperature. MOSFET Transient Thermal Issues Having chosen a MOSFET that will withstand the imposed voltage stresses, and be able to handle the worst-case continuous I 2R power dissipation which it will see, it remains only to verify the MOSFET’s ability to handle short-term overload power dissipation without overheating. Here, nature and physics work in our favor: a MOSFET can handle a much higher pulsed power without damage than its continuous dissipation ratings would imply. The reason for this is that, like everything else, semiconductor devices (silicon die, lead frames, etc.) have thermal inertia. This is easily understood by all of us who have stood waiting for a pot of water to boil. In terms related directly to the specification and use of power MOSFETs, this is known as “transient thermal impedance.” Almost all power MOSFET data sheets give a Transient Thermal Impedance Curve, which is a handy tool for making sure that you can safely get by with a less expensive MOSFET than you thought you might need. For example, take the case where tFLT for the 5V supply has been set to 50ms, ILOAD(CONT,MAX) is 5.0A, the slow-trip threshold is 50mV nominal, and the fast-trip threshold is 100mV. If the output is connected to a 0.60Ω load, the output current from the MOSFET for the slot in question will be regulated to 5.0A for 50ms before the part’s circuit breaker trips. During that time, the dissipation in the MOSFET is given by: [] 2V 5A(0.6ΩA 5V E I E P MOSFET = − = × = ( ) 50ms for 10W 5A 2V PMOSFET = × = Wow! Looks like we need a really hefty MOSFET to withstand just this unlikely—but plausible enough to protect against—fault condition. Or do we? This is where the transient thermal impedance curves become very useful. Figure 10 shows those curves for the Vishay (Siliconix) Si4430DY, a commonly used SO-8 power MOSFET. 10-4 10-3 10-2 10-1 1 10 100 600 2 1 0.1 0.01 0.2 0.1 0.05 0.02 Single Pulse Duty Cycle = 0.5 Normalized Thermal Transient Imperance, Juction-to-Ambient 1. Duty Cycle, D = 2. Per Unit Base = RqJA = 67°C/W 3. TJM – TA = PDMZqJA (t) 4. Surface Mounted t1 t2 t1 t2 Notes: PDM Square Wave Pulse Duration (sec) Figure 10. Si4430DY MOSFET Transient Thermal Impedance Curve |
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