Active Clamp Forward Principle and Design Example

The single-ended forward converter has the advantages of simple structure, reliable operation, low cost, electrical isolation of input and output, and easy multi-channel output, and thus is widely used in small and medium power conversion occasions. However, it has an inherent disadvantage: during the shutdown of the main switch, a reset circuit must be added to achieve a magnetic reset of the high frequency transformer to prevent saturation of the transformer core. Traditional magnetic reset techniques include the use of a third reset winding technique, lossless LCD clamping technology, and RCD clamping technology. Although these three reset technologies have certain advantages, they also have some defects.

(1) The third reset winding technology adopts the third reset winding technology. The advantage of the forward converter is that the technology is relatively mature, and the transformer energy can be fed back to the grid.

The disadvantage of its existence is that the third reset winding makes the design and fabrication of the transformer more complicated; the transformer core is not bidirectional symmetric magnetization, so the utilization rate is low; the primary side main switch tube is subjected to a large voltage stress.

(2) RCD clamp technology adopts RCD clamp technology. The advantage of forward converter is that the circuit structure is relatively simple and the cost is low.

Its disadvantage is that during the magnetic reset process, most of the magnetization energy is consumed in the clamp network, so the efficiency is low; the core is not bidirectional symmetric magnetization, and the core utilization is low.

(3) LCD clamp technology uses lossless LCD clamp technology. The advantage of the forward converter is that the magnetic field energy can be fully fed back to the grid, which is more efficient.