LT1070 Design Manual (Linear Technology) - 5

Application Note 19
Using Equation 79:
nal data sheet was printed. The old value was greater ( )( ) than – 0.3%/°C, while the new figure is under – 0.1%/°C. The current limit graphs on the new data I ⎛ V ⎞ V V IN OUT I OUT OUT = + N P sheets reflect this improved characteristic. E ⎝⎜ V 2 ( )( ) IN ⎠⎟ + • f V + N V L OUT IN = ∞ 3. Reconsider the necessity of limiting the inductor/ 0.4 A 1 ⎛ V 5 ⎞ transformer current to the manufacturers’ specifica- = + 0.4 0.67 A 4 0.75 ⎝⎜ 6V ⎠⎟ = tion. Maximum current ratings in many cases are determined by core saturation considerations. Allow- The LT1072 is large enough to handle this current, ing the core to saturate does not harm the core. Core yielding; or winding damage occurs only if temperatures rise so far that material properties are permanently altered. ΔIMAX = 2(1.25A – 0.674A) = 1.15A Core saturation used to be considered a “fatal” condi- Using a conservative value of 0.7A for ΔI (note that this is tion for conventional switchers because currents would 56% of the 1.25A Max LT1072 switch current, not 20%), “run away” and destroy switches or diodes. The LT1070 and Equation 77, yields: limits current on an instantaneous cycle-by-cycle ba- sis, preventing current “run away” even with grossly overdriven cores. The major consideration then is the V V ( )( ) ( )6( ) IN OUT 5 L = 145 H μ heating effect of the winding current (I2R). Under I• f V ( +N V 07. 4 k )=( )( 0 )(5 04. )= Δ OUT IN + • 6 short-circuit conditions, winding currents in inductors are nearly constant at the current limit value of the LT1070. Transformer secondary winding currents are
Protecting the Magnetics
nearly constant at 1/N times the LT1070 current limit. A second problem for LT1070 designers has been protec- This assumes that the core is not heavily saturated. If tion of the magnetics under overload or short-circuit the core saturates significantly below the current limit conditions. Physical size restraints often require inductors values, RMS winding current will be significantly lower or transformers which are not specified to handle the full than the current limit. The best way to resolve this current limit values of the LT1070. This problem can be complex situation is to actually measure core/winding handled in several ways. temperature with a thermocouple under overload con- ditions. The thermocouple should be “buried” as deeply 1. Use an LT1071 or LT1072 if full load current require- as possible in the windings and/or core to reflect peak ments allow it. temperatures. The magnetic and electric fields gener- 2. Take advantage of the fact that the LT1070 current limit ated by the switching may affect the thermocouple drops at higher temperatures. The worst-case current meter. If this occurs, simply check the temperature limit values shown on the old data sheets allow for both periodically by turning off power. Consult with the temperature extremes with one specification. New magnetics manufacturer to determine peak allowable data sheets will specify a maximum of 10A for the temperatures, with permanent damage as the criteria, LT1070, 5A for the LT1071 and 2.5A for the LT1072 at not performance specifications. The major failure mode temperatures of 25°C or higher. Be aware that the is winding shorts caused by insulation melting. High temperature dependence of current limit has been temperature insulation is available from most manu- improved considerably on the LT1070 since the origi- facturers. an19fc AN19-5