Dissipation zener clamp

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. 

Note that cleverly designed commercial Flybacks use an RCD clamp instead of a zener clamp. This can reduce dissipation in the clamp by around 20% at low line (where it is... 

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. 

This design strategy could actually have succeeded. But there is a catch. Let us consider what happens if we just remove the input power. By allowing the duty cycle to go up to such a high level, the momentary peak currents actually increase much more now, as compared to a case where the Dmax is set lower. This has severe implications on the transformer, since its size is related to the saturation level. There are also some other subtle issues. For example, it can be shown that the dissipation in the zener clamp can... 

Note that if the zener voltage is too close to the chosen Vor, the dissipation in the clamp goes up steeply. Vor therefore always needs to be picked with great care. That simply means that the turns ratio has to be chosen carefully ... 

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