Datasheet LT8315 (Analog Devices) - 14

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APPLICATIONS INFORMATION Table 1. Predesigned Transformers — Typical Specifications TRANSFORMER PART LPRI NUMBER (mH) NP:NS:NT ISOLATION VENDOR TARGET APPLICATIONS
PS16-077 4 24:1:4 Reinforced Sumida 140V–380V to 5V/1.5A PS16-051 4 10:1:2 Reinforced Sumida 140V–380V to 12V/0.6A PS15-195 4 3:1:1 Reinforced Sumida 100V–500V to 12V/0.2A PS16-078 4 5:1:1 Reinforced Sumida 140V–380V to 24V/0.3A 750316022 3.3 24:1:4 Functional Wurth 140V–380V to 5V/1.5A 7508111324 2.75 10:1:1 Reinforced Wurth 140V–380V to 12V/0.6A 7508111518 2.4 2.5:1:0.25 Reinforced Wurth 140V–380V to 48V/0.15A The following equation calculates output power:
Flyback Transformer Modeling
POUT = 0.5 • η • VIN • D • ISW(MAX) A flyback transformer can be thought of as an ideal trans- where former with a parallel magnetizing inductance and series leakage inductances, as shown in Figure 5. η = Efficiency ≈ 80% ( LLEAK(PRI) LLEAK(SEC) V )•N NPS:1 D ≈ OUT + VF PS (VOUT + VF)•NPS + VIN LPRI ISW(MAX) = Max. switch current limit = 100mV/RSNS 8315 F05 IDEAL The calculated power is approximate, and does not take
Figure 5. Transformer Model
into account timing variations caused by circuit parasitics. The actual output power must be evaluated on the bench. The magnetizing inductance, which is the mutual induc- Example: Consider a 12V output converter with a VIN(MIN) tance shared by both primary and secondary windings, is of 250V and a VIN(MAX) of 390V. With a ten-to-one primary- essential for absorbing energy and delivering it to the load. to-secondary winding ratio NPS = 10 and a sense resistor It stores energy in magnetic flux lines that pass through RSNS = 330mΩ, the maximum power output is 11W at both primary and secondary windings. VIN(MAX) = 390V but lowers to 10W at VIN(MIN) = 250V. If the leakage inductances are small, the magnetizing
Selecting a Transformer
inductance can be measured by leaving the secondary open-circuited and measuring the inductance of the pri- Transformer specification and design is possibly the most mary, resulting in an inductance LPRI. The magnetizing critical part of successfully applying the LT8315. In addition inductance can also be measured from the secondary to the usual list of guidelines dealing with high-frequency by leaving the primary open-circuited and measuring the isolated power supply transformer design, the following secondary inductance LSEC. The relationship between the information should be carefully considered. primary-referred magnetizing inductance and secondary- Linear Technology has worked with several leading mag- referred magnetizing inductance is given by the primary- netic component manufacturers to produce pre-designed to-secondary turns ratio NPS as: flyback transformers for use with the LT8315. Table 1 L 2 PRI = LSEC • NPS shows the details of these transformers. The transformer also has leakage inductances, which are parasitic inductances associated with each winding. These 8315fa 14 For more information www.linear.com/LT8315 Document Outline FEATURES APPLICATIONS DESCRIPTION TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS