247 Initial Conditions

The following compromise parameters will be used for this design:

1. Complete energy transfer will be maintained down to 30 V output, giving a full 300 W output to this voltage. Below 30 V, the unit will revert to incomplete energy transfer, resulting in some reduction in the maximum output power, but also a corresponding reduction in the maximum primary current.

2. At 10 V the maximum output current will be limited to 20 A to reduce the stress on the output rectifiers and secondary winding, reducing the maximum power to 200 W.

3. At output powers below 25 W, the minimum pulse width will be fixed, and the repetition rate will be reduced to control the output, giving better control at light loads.

In this supply, power FETs provide considerable advantages. With these devices used as the main switching element, switching times are very short and storage times negligible. Consequently, good switching action can be maintained for very narrow conduction angles.

The transformer design is complicated by the fact that the secondary must deliver both high currents at low voltages and low currents at high voltages. The output rectifiers and capacitors must also be rated for both of these conditions. The modified power characteristic at lower output voltages reduce? these problems.

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