Primary-side inductance

Note that when we come to designing off-line transformers, for various reasons like reducing high-frequency copper loss, reducing size of transformer, and so on, it is more common to set r at around 0.5. So the primary-side inductance must then be (from the L x I rule)... 

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...

This is also true because any trace inductance here gets multiplied by the square of the turns ratio, and reflects into the primary side, as discussed previously. This greatly increases the dissipation in the primary-side RCD/zener clamp and severely degrades the converter efficiency. We have to really struggle to minimize secondary-side inductances, especially for low output voltage rails, that is, those with higher turns ratios. 

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

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. 

Ipk is the peak switch current, and Llkp is the primary-side leakage. That certainly is the energy residing in the leakage inductance (at the moment the switch turns OFF), but it is not the entire energy that eventually gets dissipated in the zener clamp on account of the leakage. 

Why did we use the symbol Llk in the dissipation equation above Why didn t we identify it as the primary-side leakage ( Llkp ) The reason is that Llk represents the overall leakage inductance as seen by the switch. So, it is partly Llkp — but it also is influenced by the secondary-side leakage inductance. This is a little hard to visualize, since by definition, the secondary-side leakage inductance is not supposed to be coupled to the primary side (and vice versa). So how could it be affecting anything on the primary side ... 

Measuring the Effective Primary-side Leakage Inductance... 

The magnetization current component is not coupled by transformer action to the secondary. In that sense, it is like a parallel leakage inductance. We need to subtract this component from the total switch current, and only then will we find that the primary and secondary currents scale according to the turns ratio. In other words, the magnetization current does not scale — it stays confined to the primary side. 

Transformers are devices that are used to increase or decrease electrical current flow. Transformers also operate on the principle of magnetic induction. However, a transformer has electrical windings on both sides (Figure 13-23). The electron flow through the primary side of the transformer causes electron flow on the transformer s secondary side. The ratio of the number of windings from each side of the transformer determines the electrical current flow produced. Numerous home appliances use transformers to reduce electron flow, whereas power companies use transformers to increase electron flow. 

The leakage inductance of both coils has been modeled by an inductor in series with the load, since the current in the coils also produces leakage flux. These inductances are labeled L p and Lj, respectively. Notice that the leakage inductance for the secondary side has been divided by the turns ratio n, squared because it was reflected to the primary side. Resistors labeled Rp and Rj have also been placed in series with the load to represent the resistance of the conductors used to wind the coils. Again, the secondary resistance has been divided by the square of the turns ratio, since it was reflected. 

The BIL of the interconnecting cables and the terminal equipment on the secondary must be at least equal to the capacitive and Inductive transferences of the primary surges as determined above. If it is not so, the of the primary arrester must be re-chosen or an arrester also provided on the secondary side. 

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