AN2811

Design considerations

Doc ID 14904 Rev 1

7/25

Figure 3.

Modified buck converter

For ease in driving Q1 using a conventional buck converter, a modified buck converter has

been introduced as shown in Figure 3. Such topology is widely used to drive LEDs. With this

modified solution, the MOSFET is no longer floating. In this case the output (Vout) is not

connected to ground, and it becomes quite difficult to sense the output current in the output

stage directly. Compared to a buck converter, the flyback converter may be the better

choice. Figure 4 shows the typical circuit of a flyback converter.

Figure 4.

Flyback converter

The auxiliary winding can be added to the transformer (T1) to provide bias for Q1. Unlike the

buck converter, T1 provides isolation between Vin and Vout. Since such isolation is not

required for this application, a current sense resistor can be placed across the primary

ground and negative polarity of Vout. Thus, Vout shares the same primary ground. In this

topology, the MOSFET is not floating. Thanks to VIPer17 the board is built with a high-

performance low-voltage controller chip with an 800 V avalanche rugged power MOSFET.

Designed with VIPer17, only a few external components are required which allows a smaller

profile in the design.