Leakage inductances

L = leakage inductance of the circuit in henry (H ) and C = lumped leakage capacitance of the circuit in farad (F) (L and C being the circuit parameters). 

Other considerations enter into the operation of the magnetic element, such as the desired range of leakage inductance for each winding, electrostatic shielding, etc. (see Appendix D). These factors affect the size and cost of the final magnetic element. 

Other considerations are important during the physical winding process and involve winding losses, leakage inductance, shielding, and physical space. They do not need to be considered now. 

Another method to meet the safety specifications is the use of triple-insulated wire for the secondary windings. Its use can reduce the transformer s size and leakage inductance compared to the border-tape method. Triple-insulated wire has three distinct, approved layers of insulation and it may be wound directly in contact with the primary windings. Whether Mylar tape is needed in or around the transformer depends upon the insulation system used in its construction. This type of transformer can be seen in Figure 3-25. 

The leakage inductance (represented by a small inductor in series with a winding) causes some flux not to couple with the core, but to escape into the surrounding air and materials. Its behavior is not governed by its associated transformer or inductor, hence any reflected impedance to the winding in question does not affect the behavior of the leakage inductor. 

The parasitic loss caused by the leakage inductance can be harnessed as seen later. 

The load (or instmment) impedance is connected in parallel with Z2 and should be very much larger than Z2 if accuracy is to be maintained. If D.C. voltages are involved then the impedances can be purely resistive and, in the steady state, any leakage inductance or capacitance has no... 

You can argue—but 20nH per inch is just a rule of thumb How can we even confirm what the effective leakage inductance really increases on the primary side (as a result of that) ... 

Figure 5-4 How to, and How Not to, Measure Effective Primary Side Leakage Inductance in Flyback...

Measure the in-circuit primary-side leakage inductance Llk in Henries. 

Also, if your clamp is too hot, check that the leakage inductance of the transformer is better than about 1 to 2% of the primary inductance. If not, you need to first reduce the leakage inductance. That calls for improving the coupling between the Primary and (main)... 

In particular, for explosion Group IIC the inductance values permissible for higher currents are considerably lower than 0.1 mH. So, leakage inductances arising everywhere shall be considered during planning and installation. 

In high-power offline Flybacks, the trace inductances on the secondary side reflect on to the primary side, and can greatly increase the effective primary-side leakage inductance and degrade the efficiency. The situation gets worse when we have to stack several output capacitors in parallel, just to handle the higher RMS currents. Long traces seem inevitable here. This has been discussed in detail previously. 

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