Expedite Transformer Calculations for Flybacks

Jan 1, 2008 12:00 PM
By Kirby Creel, Senior Design Engineer, Data-tronics, Romoland, Calif.

Knowing the nominal primary inductance and the peak current, the design of the transformer can proceed. The transformer design for a flyback circuit does not follow normal transformer design procedures. The flyback transformer can be viewed as two inductors sharing a common core. (Reference 3 given at the end of this article illustrates that selection of a transistor or the turns ratio is the first step. Chapter 7 of the same reference provides general design guidelines. Reference 4 also provides detailed transformer design information.)

The transistor selected for this example is International Rectifier's IRFD220. The drain-to-source breakdown voltage is 200 V. With an output voltage of 600 V and allowing for a margin of 10% below the breakdown voltage (20 V), a minimum of a 60-V switching spike yields 120 V peak. Use of this transistor gives a nominal turns ratio of 600/120 = 5:1.

Selecting a transistor with a high breakdown voltage keeps the turns ratio low, reducing the losses in the switching transistor. Windings are designed to keep leakage inductance and distributed capacitance low with adequate spacing for voltage breakdown without impregnation.

A demonstration transformer was wound with the following design data:

• Core: RM5I/-3F3 with approximately a 0.003-in. gap in the center leg
• Bobbin: RM5 printed-circuit style
• Primary: 60 turns of #31 AWG
• Secondary: 306 turns of #41 AWG (actual ratio = 5.1:1, extra turns completed even layers for low leakage inductance)

The transformer test results included:

• Primary dc resistance 1-3= 0.73Ω
• Secondary dc resistance 4-5 = 36.1Ω
• Primary inductance = 1.305 mH
• Leakage inductance = 5.99 µH
• Distributed capacitance 4-5 = 10.2 pF.

The Fig. 5 circuit is used to test the transformer. The operating frequency was set to 50 kHz by setting the counter to read frequency, holding the discharge switch closed, pressing the start button and adjusting the 1-kΩ potentiometer.

Either the manual or automatic stop must be used to terminate the charge cycle because the output voltage can exceed 600 V and either the capacitor or transistor will fail. Tests using the automatic stop gave an average total pulse count of 415,000. This total pulse count yields a charge time of 8.3 sec. The difference between the result and the design charge time of 10 sec is due to a smaller capacitor (measured at 5.8 µF), a reference diode that was slightly below nominal and, most of all, an efficiency that was better than 50%.

Fig. 6 is the charging waveform. The waveform was made by replacing the output monitor with the 10-MΩ input impedance of a X10 oscilloscope probe. Fig. 7 shows the voltage between the drain lead of the MOSFET and ground just before stop.

When designing a circuit of this type, there are a couple of pitfalls. One is the inherent danger of a high voltage. High voltage requires careful layout with attention to safety. The other common problem with this type of circuit is the storage capacitor itself. Finding a capacitor with low enough leakage at the operating condition can be a difficult task. Capacitor specifications sometimes specify leakage values at one-tenth of the rated voltage. Leakage currents near the full voltage may be many times the given value rather than would be the expected linear change. The capacitor used in the example is a metallized polyester film type.

The information provided will help engineers design capacitor charging circuits. The equations and procedures presented give insight into the interaction of variables. This information can be adapted and used for testing most capacitor charging circuit designs.

References

1. Pressman, Abraham I., Switching Power Supply Design, 2^nd Edition, McGraw-Hill, ISBN 0-07-052236-7, 1998.

2. Linear Technology Corp., LT3468 Data sheet, LT/TP0105 IK Rev A, 2003.

3. Pressman, Abraham I., Switching Power Supply Design, Chapter 4.3.2., 2^nd Edition, McGraw-Hill, ISBN 0-07-052236-7, 1998.

4. Dixon, Lloyd H. Magnetics Design Handbook, Texas Instruments, 2001.

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