23-April-2001
EPR-000008 – 1.2 W Universal Input Non-Isolated TNY254
7 Inductor
7.1 Calculation
The inductor value can be determined using the TNY253-255 flyback transformer
spreadsheet with the following considerations:
1) the frequency value (fL) is ½ the line frequency, 25 Hz for 50 Hz, 30 Hz for 60 Hz, to
account for half wave rectification.
2) Z factor, the ratio between the secondary losses and the total losses, has to reflect the
dominance of the primary losses, as the TinySwitch losses (high Rdson) overshadow the
output diode losses. Z factor does not reflect the extra power loss due to R1 and R2.
The efficiency used in the spreadsheet is only the efficiency of the converter portion of the
supply, it does not include the losses in the input resistor.
3) the output diode rating can be calculated from the formula (VR O PIVS/0.8), where
PIVS=VMAX+VO-VDS. The inductor can be looked at as a transformer with 1:1 turns ratio,
therefore VOR=VO+VD.
4) the output capacitor minimum value is dictated by the output RMS ripple current and the
maximum value by the specified maximum output voltage ripple.
Select the next higher standard “L” value (for the rated “I”) from a qualified vendor
(min. 400 VDC voltage rated inductor) like Chilisin, and the smallest DC resistance.
Power Integrations, Inc.
Page 9 of 20
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