Application description and design

AN1897

4/17

DocID10240 Rev 2

Figure 2. VIPer22A-E FB pin internal structure

1.1.5

Primary driver

In a flyback power supply, the transformer is used as an energy tank during the on-time of

the MOSFET. When the MOSFET turns off, its drain voltage rises from a low value to the

input voltage while the secondary diode conducts, transferring to the secondary side the

magnetic energy stored in the transformer. Since primary and secondary windings are not

magnetically coupled, there is a serial leakage inductance that behaves like an open

inductor charged at Ipk, causing the voltage spikes on the MOSFET drain. These voltage

rating. If the peak voltage is higher than this value, the device is destroyed. The RCD clamp

(see Figure 4) is a very simple and cheap solution, but it impacts on the efficiency and on

the power dissipation in standby condition. Besides, the clamping voltage varies according

to the load current. RCD clamp circuits may allow the drain voltage to exceed the

breakdown rating of the VIPer22A-E during the overload operation or during turn-on with

high line AC input voltage. A Zener clamp is recommended (see Figure 5). However this

solution gives higher power dissipation at full load, even if the clamp voltage is exactly

defined.

1.2

Transformer consideration

On the electrical specifications of a multiple output transformer (cross-regulation, leakage

inductance), the main efforts focused on the proper coupling between the windings. A lower

leakage inductance transformer allows a lower power clamp to reduce the input power. It l

leads to lower power dissipation on the primary side. Auxiliary and secondary windings are

swapped in order to decrease the coupling to the primary one. The secondary windings act

as a shielding layer to reduce the capacitive coupling. Fewer spikes are generated on the

auxiliary windings, the primary and secondary windings have better coupling.