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Design Ideas |
Planar-magnetics technology is growing in popularity as a design option for dc-dc converters. In addition to providing electrical benefits, planar magnetics offers reduced leakage inductance and improved consistency. It also significantly reduces the cost of magnetics if you integrate it into the printed-circuit board. To use planar magnetics in a dc-dc converter, the switching frequency should be higher than 500 kHz. This high frequency limits the number of turns required for the magnetic element, thus making it easier to implement on the pc board.
In the circuit in Figure 1, a high-frequency PWM circuit (IC1)
operates in current mode. The IC contains an integrated 200V MOSFET and a 200V
preregulator start circuit. Using this circuit, you can implement a standard
forward topology with switching frequencies as high as 1 MHz. The forward
topology shown uses resonant reset. At high switching frequencies, the resonant
frequency set by the output capacitance of the MOSFET and the primary inductance
of the transformer resets the transformer without the need for an additional
reset winding. This self-reset feature simplifies the design of the magnetics.
The design in Figure 1 forms a high-frequency, 10W dc-dc converter that switches at 600 kHz. The circuit accepts input voltages from 36 to 72V and provides a 5V, 2A output. The circuit is fully short-circuit protected and operates at 85% typical efficiency. (DI #1945)
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