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LT3420EMS Datasheet(PDF) 10 Page - Linear Technology |
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LT3420EMS Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LT3420/LT3420-1 10 3420fa COMPONENT SELECTION Choosing the Right Transformer The flyback transformer plays a key role in any LT3420/ LT3420-1 application. A poorly designed transformer can result in inefficient operation. Linear Technology Corpora- tion has worked with a number of transformer manufactur- ers to develop specific transformers for use with the LT3420/LT3420-1. These predesigned transformers are sufficient for a large majority of the applications that may be encountered. In some cases, the reader may choose to design his own transformer or may simply be curious about the issues involved in designing the transformer. The fol- lowing is a brief discussion of the issues relating to trans- former design. Transformer Turns Ratio The turns ratio for the transformer, N, should be high enough so that the absolute maximum voltage rating for the NPN power switch is not exceeded. When the power switch turns off, the voltage on the collector of the switch (SW Pin) will “fly” up to the output voltage divided by N plus the battery voltage (neglecting the voltage drop across the rectifying diodes). This voltage should not exceed the 38V (LT3420) or 50V (LT3420-1) breakdown rating of the power switch. Choose the minimum N by the following formula. N V V LT N V V LT MIN OUT BAT MIN OUT BAT ≥ ≥− 38 3420 50 3420 1 – () – () For an LT3420 design, a 5V battery voltage and a 330V output results in a NMINof10soaturnsratioof10orgreater should be used. Transformer Primary Inductance A flyback transformer needs to store substantial amounts of energy in the core during each switching cycle. The transformer, therefore, will generally require an air gap. The use of an air gap in the core makes the energy storage ability, or inductance, much more stable with temperature and variations in the core material. Most core manufactur- ers will supply standard sizes of air gaps with a given type of core, resulting in different AL values. AL is the induc- tance of a particular core per square turns of winding. To get a certain inductance, simply divide the desired induc- tance by the AL value and take the square root of the result to find the number of turns needed on the primary of the transformer. The LT3420/LT3420-1 detect the output voltage via the flyback pulse on the SW pin. Since this can only occur while the power switch is off, an important criteria is that the value of the primary inductance of the transformer be larger than a certain minimum value. The switch off time should be 500ns or larger for the LT3420 and 350ns or larger for the LT3420-1. The minimum inductance can be calculated with the following formula: L V NN LT L V NN LT PRI OUT PRI OUT ≥ ≥− 500 10 14 004 3420 350 10 10 0015 3420 1 9 9 •• •( . – . ) () •• • ( .–. ) () – – VOUT: Target Output Voltage N: Transformer Turns Ratio Transformer Leakage Inductance The leakage inductance of the transformer must be care- fully minimized for both proper and efficient operation of the part. The DC voltage rating of the SW pin on the LT3420 is 38V while on the LT3420-1 it is 50V. These ratings are for DC blocking voltages only and additional precautions APPLICATIO S I FOR ATIO |
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