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UCC1919 Datasheet(PDF) 10 Page - Texas Instruments |
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UCC1919 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 11 page 10 UCC1919 UCC2919 UCC3919 Transient Thermal Impedance During a fault condition in the retry mode, the average MOSFET power dissipation will generally be quite low due to the low duty cycle, as defined by: Pavg IV t tt DISS MAX IN ON ON OFF () = •• + (w/output shorted) (35) (In the latch mode, tOFF will be the time between a fault and the time the device is reset.) However, the pulse power in the MOSFET during tON, with the output shorted, is: P pulse I V DISS MAX IN () =• (w/output shorted) (36) In choosing tON for a given VIN,IMAX, and duty cycle it is important to consult the manufacturer’s transient thermal impedance curves for the MOSFET to make sure the de- vice is within its safe operating area. These curves pro- vide the user with the effective thermal impedance of the device for a given time duration pulse and duty cycle. Note that some of the impedance curves are normalized to one, in which case the transient impedance values must be multiplied by the DC (steady state) thermal re- sistance, θJC. For duty cycles not shown in the manufacturer’s curves, the transient thermal impedance for any duty cycle and ton time (given a square pulse) can be estimated from [1]: () () θθ θ JC JC SP trans D D () =• + − • 1 (37) where D is the duty cycle: t tt ON ON OFF + . and θSP is the single pulse thermal impedance given in the transient thermal impedance curves for the time du- ration of interest (tON). Note that these are absolute num- bers, not normalized. If the given single pulse impedance is normalized, it must first be multiplied by θJC before us- ing in the equation above. This effective transient thermal impedance, when multi- plied by the pulse power, will give the transient tempera- ture rise of the die. To keep the junction temperature below the maximum rating, the following must be true: () () θ JC JC DISS trans TT P pulse () max = − (38) If necessary, the junction temperature rise can be re- duced by reducing ton (using a smaller value for CT), or by reducing the duty cycle using the power limiting fea- ture already discussed. Note that in either case, the amount of load capacitance, COUT, that can be charged before causing a fault, will also be reduced. Safety Recommendations Although the UCC3919 is designed to provide system protection for all fault conditions, all integrated circuits can ultimately fail short. for this reason, if the UCC3919 is intended for use in safety critical applications where UL or some other safety rating is required, a redundant safety device such as a fuse should be placed in series with the device. The UCC3919 will prevent the fuse from blowing for virtually all fault conditions, increasing system reliability and reducing maintenance cost, in addition to providing the hot swap benefits of the device. References [1] International Rectifier, HEXFET Power MOSFET Design- er’s Manual, Application Note 949B, Current Ratings, Safe Operating Area, and High Frequency Switching Perform- ance of Power HEXFETs, pp.1553-1565, September 1993. APPLICATION INFORMATION UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 FAX (603) 424-3460 |
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