New IC controller tech eases power supply design

Electronic News, August 12, 1996 by Mehmet Nalbant

Although this block diagram appears very simple, there are several aspects of the design that must be considered for proper operation. The volt-second withstand capability of the mag-amp is directly proportional to the number of turns and the cross sectional area of the core. As a result, for wide dynamic range control, both a large number of turns as well as larger core sizes must be used. This size problem is compounded when there is a need for an output current limit.

The mag-amp exhibits a residual inductance even after it saturates, and larger number of turns on the core exaggerates the residual inductance. This residual inductance has a tendency to reduce the effective maximum duty cycle that is available for power transfer to the output. All of this means that the power transformer and power switches in the primary side have to be "overdesigned" to endure the higher RMS currents that will result. Three examples of these costly design margins are an increase in the size of the main transformer, primary power switches and an increase in size of the output inductor. In addition, because mag-amps have to withstand the maximum input voltage during short-circuit conditions, a designer must "overdesign" the mag-amp core, typically by a factor of two, for short-circuit protection, thus further increasing the size and bulk of components, impacting the cost associated with the final power supply.

Enter phase-modulated control technology...

Recent advances in analog IC technology have yielded solutions which can significantly reduce the size, cost and complexity of mag-amp designs, while at the same time improving performance. ICs such as the Linfinity LX1571 phase-modulated secondary-side controller regulator have been conceived to solve many of the problems that plague mag-amp-based designs, while also solving some of the problems present in other similar post-regulator approaches.

Using the LX1571 replaces costly mag-amp cores with a low on-resistance MOSFET, resulting in a cost savings of up to 70 percent and a design effort that is easily reduced by a factor of two. In addition, because there is no residual inductance associated with the operation of the MOSFET, the reduction of the effective duty cycle is eliminated, allowing more power to reach the output, or the RMS current at the primary is reduced.

What makes the LX1571 both unique and more efficient is that the gate drive to the MOSFET is actually applied before the secondary voltage of T1 goes positive, termed "Look Ahead Switching" by Linfinity, as contrasted with leading-edge, modulating mag-amps or other types of switching post regulators. This, in turn, makes current-mode control possible. Moreover, the duty-cycle control of the secondary voltage of T1 is accomplished by phase shifting the gate drive waveform of the power MOSFET in relation to the secondary voltage (see Figures 2A and 2B). The advantages of current mode control are fast-transient response and simplified, more stable control loop design, which is not possible with mag-amp designs.