Switching Power Supplies A - Z

Appendix 1

returning from the diode and output capacitor. Assume two inches of total secondary side

trace length for example, and remembering the thumb rule of 20 nH/inch, we get say 40 nH

secondary leakage inductance. For a 12 V output, this will reflect to the switch side as an

effective leakage of 40 × (8.75)

2

= 3062 nH or about 3 µH. This will be added to the

existing primary side leakage (typically about 10 µH) to give about 13 µH. The associated

energy will be dissipated in the zener clamp. However for a 5 V output, the same 40 nH gets

reflected as 40 × (21)

2

= 17640 nH or about 18 µH! This will give a total primary side

leakage of 10 + 18 = 28